{"history":[],"article":{"id":"67bbe5e5-c2bf-436d-92fc-629ce1fb038d","title":"Elastic recoil detection","slug":"elastic-recoil-detection","content":":::summary\n:::\n\n# \n**Elastic [recoil](https://bliptext.com/articles/recoil) detection analysis** (**ERDA**), also referred to as forward [recoil](https://bliptext.com/articles/recoil) scattering or [spectrometry](https://bliptext.com/articles/spectrometer), is an [ion beam](https://bliptext.com/articles/ion-beam) analysis technique, in [materials science](https://bliptext.com/articles/materials-science), to obtain elemental [concentration](https://bliptext.com/articles/concentration) depth profiles in [thin films](https://bliptext.com/articles/thin-films).\nThis technique can be achieved using many processes.\nIn the technique of ERDA, an [energetic](https://bliptext.com/articles/energy) ion beam is directed at a sample to be characterized and (as in [Rutherford backscattering](https://bliptext.com/articles/rutherford-backscattering-spectrometry)) there is an [elastic nuclear interaction](https://bliptext.com/articles/nuclear-interaction) between the [ions](https://bliptext.com/articles/ions) of the beam and the [atoms](https://bliptext.com/articles/atoms) of the target sample.\nSuch interactions are commonly of [Coulomb](https://bliptext.com/articles/coulomb-s-law) nature.\nDepending on the [kinetics](https://bliptext.com/articles/kinetics-physics) of the ions, [cross section](https://bliptext.com/articles/cross-section-physics) area, and the loss of energy of the ions in the matter, ERDA helps determine the [quantification](https://bliptext.com/articles/quantification-science) of the [elemental analysis](https://bliptext.com/articles/elemental-analysis).\nIt also provides information about the depth profile of the sample.\nThe energy of incident energetic ions can vary from 2 [MeV](https://bliptext.com/articles/electronvolt) to 200 [MeV](https://bliptext.com/articles/electronvolt), depending on the studied sample.\nThe energy of the beam should be enough to kick out (“recoil”) the atoms of the sample.\nThus, ERDA usually employs appropriate source and detectors to detect [recoiled atoms](https://bliptext.com/articles/atomic-recoil).\nERDA setup is large, expensive and difficult to operate.\nTherefore, although it is commercially available, it is relatively uncommon in [materials characterization](https://bliptext.com/articles/characterization-materials-science).\nThe [angle of incidence](https://bliptext.com/articles/angle-of-incidence-optics) that an ion beam makes with the sample must also be taken into account for correct analysis of the sample.\nThis is because, depending on this angle, the recoiled atoms will be collected.\nERDA has been used since 1974.\nIt has similar theory to Rutherford backscattering spectrometry (RBS), but there are minor differences in the set-up of the experiment.\nIn case of RBS, the [detector](https://bliptext.com/articles/sensor) is placed in the back of the sample whereas in ERDA, the [detector](https://bliptext.com/articles/sensor) is placed in the front.\n\n# Characteristics of ERDA\nThe main characteristics of ERDA are listed below.\n\n# History\nERDA was first demonstrated by L’Ecuyer et al. in 1976.\nThey used 25–40 MeV [35Cl](https://bliptext.com/articles/isotopes-of-chlorine) ions to detect the recoils in the sample.\nLater, ERDA has been divided into two main groups.\nFirst is the light incident ion ERDA (LI-ERDA) and the second is the heavy incident ion ERDA (HI-ERDA).\nThese techniques provide similar information and differ only in the type of ion beam used as a source.\nLI-ERDA uses low voltage [single-ended accelerators](https://bliptext.com/articles/electrostatic-particle-accelerator), whereas the HI-ERDA uses [large tandem accelerators](https://bliptext.com/articles/electrostatic-particle-accelerator).\nThese techniques were mainly developed after heavy [ion accelerators](https://bliptext.com/articles/particle-accelerator) were introduced in the materials research.\nLI-ERDA is also often performed using a relatively low energy (2 MeV) [helium](https://bliptext.com/articles/helium) beam for measuring the depth profile of [hydrogen](https://bliptext.com/articles/hydrogen).\nIn this technique, multiple detectors are used: [backscattering detector](https://bliptext.com/articles/electron-backscatter-diffraction) for heavier elements and forward (recoil) detector to simultaneously detect the recoiled hydrogen.\nThe recoil detector for LI-ERDA typically has a “range foil”.\nIt is usually a [Mylar](https://bliptext.com/articles/mylar) foil placed in front of the detector, which blocks scattered incident ions, but allows lighter recoiling target atoms to pass through to the detector.\nUsually a 10 [μm](https://bliptext.com/articles/m) thick Mylar foil completely stops 2.6 MeV helium ions but allows the recoiled [protons](https://bliptext.com/articles/proton) to go through with a low energy loss.\nHI-ERDA is more widely used compared to LI-ERDA because it can probe more elements.\nIt is used to detect recoiled target atoms and scattered beam ions using several detectors, such as [silicon](https://bliptext.com/articles/silicon) [diode](https://bliptext.com/articles/diode) detector, [time-of-flight detector](https://bliptext.com/articles/time-of-flight-detector), Gaseous ionization detector etc. The main advantage of HI-ERDA is its ability to obtain quantitative depth profiling information of all the sample elements in one measurement.\nDepth resolution less than 1[nm](https://bliptext.com/articles/nanometre) can be obtained with good quantitative accuracy thus giving these techniques significant advantages over other surface analysis methods.\nAdditionally, a depth of 300 nm can be accessed using this technique.\nA wide range of ion beams including [35Cl](https://bliptext.com/articles/isotopes-of-chlorine), [63Cu](https://bliptext.com/articles/isotopes-of-copper), [127I](https://bliptext.com/articles/isotopes-of-iodine), and [197Au](https://bliptext.com/articles/isotopes-of-gold), with different [energies](https://bliptext.com/articles/energies) can be used in this technique.\nThe setup and the experimental conditions affect the performances of both of these techniques.\nFactors such as [multiple scattering](https://bliptext.com/articles/scattering) and ion beam induced damage must be taken into account before obtaining the data because these processes can affect the data interpretation, quantification, and accuracy of the study.\nAdditionally, the [incident angle](https://bliptext.com/articles/angle-of-incidence-optics) and the [scattered angle](https://bliptext.com/articles/scattering) help determine the sample surface topography.\n\n# Prominent features of ERDA\nERDA is very similar to [RBS](https://bliptext.com/articles/rutherford-backscattering-spectrometry), but instead of detecting the [projectile](https://bliptext.com/articles/projectile) at the back angle, the recoils are detected in the forward direction.\nDoyle and Peercey in 1979 established the use of this technique for hydrogen depth profiling.\nSome of the prominent features of ERDA with high energy heavy ions are:\nERDA can overcome some of the limitations of [RBS](https://bliptext.com/articles/rutherford-backscattering-spectrometry).\nERDA has enabled depth profiling of elements from lightest elements like [hydrogen](https://bliptext.com/articles/hydrogen) up to heavy elements with high resolution in the light mass region as discussed above.\nAlso, this technique has been highly sensitive because of the use of large area position sensitive telescope detectors.\nSuch detectors are used especially when the elements in the sample have similar masses.\n\n# Principles of ERDA\nThe calculations that model this process are relatively simple, assuming projectile energy is in the range corresponding to Rutherford scattering.\nProjectile energy range for light incident ions is in 0.5–3.0 [MeV](https://bliptext.com/articles/electronvolt) range.\nFor heavier projectile ions such as [127I](https://bliptext.com/articles/isotopes-of-iodine) the energy range is usually between 60 and 120 MeV; and for medium heavy ion beams,[36Cl](https://bliptext.com/articles/chlorine-36) is a common ion beam used with an energy of approximately 30 MeV.\nFor the instrumentation section, the focus will be on heavy ion bombardment.\nThe E2 transferred by projectile ions of mass m1 and energy E1 to sample atoms of mass m2 recoiling at an angle ϕ, with respect to the incidence direction is given by the following equation.\nEq.\n1 models the energy transfer from the incident ions striking the sample atoms and the recoiling effect of the target atoms with an angle of ϕ.\nFor heavier ions in elastic recoil detection analysis, if m2/m1 <<1, all recoiling ions have similar velocities.\nIt can be deduced from the previous equation the maximum scattering angle, θ’max, as Eq.\n2 describes:\nUsing these parameters, absorber foils do not need to be incorporated into the instrument design.\nWhen using heavy ion beams and the parameters above, the geometry can be estimated as to allow for incident particle collision and scattering at an angle deflected away from the detector.\nThis will prevent degradation of the detector from the more intense beam energies.\nThe differential elastic recoil cross-section σERD is given by:\nwhere Z1 and Z2 are the atomic numbers of projectile and sample atoms, respectively.\nFor m2/m1 <<1 and with approximation m=2Z; Z being the atomic number of Z1 and Z2.\nIn Eq.\n3 two essential consequences can be seen, first the sensitivity is roughly the same for all elements and second it has a Z14 dependence on the projector of the ion.\nThis allows for the use of low energy beam currents in HI-ERDA preventing sample degradation and excessive heating of the specimen.\nWhen using heavy ion beams, care must be taken for beam-induced damage in sample such as [sputtering](https://bliptext.com/articles/sputtering) or [amorphization](https://bliptext.com/articles/metamictisation).\nIf only [nuclear interaction](https://bliptext.com/articles/nuclear-interaction) is taken into account, it has been shown that the ratio of recoiling to displaced atoms is independent of Z1 and only weakly dependent on the projectile mass of the incident ion.\nWith heavy ion bombardment, it has been shown that the sputter yield by the ion beam on the sample increases for [nonmetallic](https://bliptext.com/articles/nonmetallic-material) samples and enhanced radiation damage in [superconductors](https://bliptext.com/articles/superconductivity).\nIn any case, the acceptance angle of the detector system should be as large as possible to minimize the radiation damage.\nHowever, it may reduce the depth profiling and elemental analysis due to the ion beam not being able to penetrate the sample.\nThis demand of a large acceptance angle, however, is in conflict with the requirement of optimum depth resolution dependency on the detection geometry.\nIn the surface approximation and assuming constant energy loss the depth resolution δx can be written:\nwhere Srel is the relative energy loss factor defined by:\nhere, α and β are the [incidence angles](https://bliptext.com/articles/angle-of-incidence-optics) of the beam and exit angle of the recoiling ion respectively, connected to the scattering angle ϕ by ϕ=α+β.\nIt should be noticed here that the depth resolution depends on the relative energy resolution only, as well as the relative stopping power of incoming and outgoing ions.\nThe detector resolution and energy broadening associated with the measuring geometry contribute to the energy spread, δE.\nThe detector acceptance angle and the finite beam spot size define a scattering angle range δϕ causing a kinematic energy spread δEkin according to Eq.\n6:\nA detailed analysis of the different contributions to depth resolution shows that this kinematic effect is the predominant term near the surface, severely limiting the permitted detector acceptance angle, whereas energy straggling dominates the resolution at larger depth.\nFor example, if one estimates δϕ for a scattering angle of 37.5° causing a [kinematic](https://bliptext.com/articles/kinematics) [energy](https://bliptext.com/articles/energy) shift comparable to typical detector [energy](https://bliptext.com/articles/energy) resolutions of 1%, the angular spread δψ must be less than 0.4°.\nThe angular spread can be maintained within this range by contributions from the beam spot size; however, the solid angle geometry of the detector is only 0.04 msr.\nTherefore, a detector system with large solid angle as well as high depth resolution may enable corrections for the kinematic energy shift.\nIn an [elastic scattering](https://bliptext.com/articles/elastic-scattering) event, the kinematics requires that the target atom is recoiled with significant energy.\nEq.\n7 models the recoil kinematical factor that occurs during the ion bombardment.\nEq.\n7 gives a [mathematical model](https://bliptext.com/articles/mathematical-model) of the collision event when the heavier ions in the beam strike the specimen.\nKs is termed the kinematical factor for the scattered particle (Eq. 8) with a scattering angle of θ, and the recoiled particle (Eq. 9) with a recoil angle of Φ.\nThe variable r is the ratio of mass of the incident nuclei to that of the mass of the target nuclei, (Eq. 10).\nTo achieve this recoil of particles, the specimen needs to be very thin and the geometries need to be precisely optimized to obtain accurate recoil detection.\nSince ERD beam intensity can damage the specimen and there has been growing interest to invest in the development of low energy beams for reducing the damage of the specimen.\nThe [cathode](https://bliptext.com/articles/[cathode](https://bliptext.com/articles/cathode)) is divided into two insulated halves, where particle entrance position is derived from charges induced on the left, l, and right, r, halves of the [cathode](https://bliptext.com/articles/[cathode](https://bliptext.com/articles/cathode)).\nUsing the following equation, x-coordinates of particle positions, as they enter the detector, can be calculated from charges l and r:\nFurthermore, the y-coordinate is calculated from the following equation due to the position independence of the [anode](https://bliptext.com/articles/anode) pulses:\nFor transformation of the (x, y) information into scattering angle ϕ a removable calibration mask in front of the entrance window is used.\nThis mask allows correction for x and y distortions too.\nFor notation detail, the cathode has an ion drift time on the order of a few milliseconds.\nTo prevent ion saturation of the detector, a limit of 1 kHz must be applied to the number of particles entering the detector.\n# [Schematic of ERDA. ϕ, α and β are recoil angle, incident angle, and exit angle, respectively. x is the probing depth. | upload.wikimedia.org/wikipedia/commons/c/c1/ERDA///schematic.png]\n\n# Instrumentation\nElastic recoil detection analysis was originally developed for [hydrogen](https://bliptext.com/articles/hydrogen) detection or a light element ([H](https://bliptext.com/articles/hydrogen), [H](https://bliptext.com/articles/hydrogen)e, [Li](https://bliptext.com/articles/lithium), [C](https://bliptext.com/articles/carbon), [O](https://bliptext.com/articles/oxygen), [Mg](https://bliptext.com/articles/magnesium), [K](https://bliptext.com/articles/potassium)) profiling with an absorber foil in front of the energy detector for beam suppression.\nUsing an absorber foil prevents the higher energy ion beam from striking the detector and causing degradation.\nAbsorber foils increase the lifetime of the detector.\nMore advanced techniques have been implemented to negate the use of absorber foils and the associated difficulties that arise through the use of it.\nIn most cases, medium heavy ion beams, typically 36Cl ions, have been used for ERDA so far with energies around 30 MeV.\nDepth resolution and element profiling of thin films has been greatly advanced using elastic recoil detection analysis.\n\n## Ion source and interactions\n[Particle accelerators](https://bliptext.com/articles/particle-accelerator), such as a [magnetron](https://bliptext.com/articles/cavity-magnetron) or [cyclotron](https://bliptext.com/articles/cyclotron), implement [electromagnetic fields](https://bliptext.com/articles/electromagnetic-field) to achieve acceleration of elements.\nAtoms must be electrically charged (ionized) before they can be accelerated.\n[Ionization](https://bliptext.com/articles/ionization) involves the removal of electrons from the target atoms.\nA magnetron can be used to produce hydrogen ions.\n[Van de Graaff generators](https://bliptext.com/articles/van-de-graaff-generator) have also been integrated with particle accelerators for light ion beam generation.\nFor heavier ion production, for example, an [electron cyclotron resonance](https://bliptext.com/articles/electron-cyclotron-resonance) (ECR) source can be used.\nAt the [National Superconducting Cyclotron Laboratory](https://bliptext.com/articles/national-superconducting-cyclotron-laboratory), neutral atoms have their electrons removed using an ECR ion source.\nECR works by ionizing the vapor of a desired element such as chlorine and iodine.\nFurther, utilizing this technique, metals ([Au](https://bliptext.com/articles/gold), [Ag](https://bliptext.com/articles/silver), etc.) can also be ionized using a small oven to achieve a vapor phase.\nThe vapor is maintained within a [magnetic field](https://bliptext.com/articles/magnetic-field) long enough for the atoms to be ionized by collisions with [electrons](https://bliptext.com/articles/electron).\nMicrowaves are applied to the chamber as to keep the electrons in motion.\nThe vapor is introduced via injection directly into the “magnetic bottle” or the magnetic field.\nCircular coils provide the shape for the magnetic bottle.\nThe coils are found at the top and bottom of the chamber with a [hexapole](https://bliptext.com/articles/hexapole) magnet around the sides.\nA hexapole magnet consists of permanent magnets or superconducting coils.\nThe plasma is contained within the magnetic trap that is formed from electric current flowing in solenoids located on the sides of the chamber.\nA radial magnetic field, exerted by the hexapole magnetic, is applied to the system that also confines the plasma.\n[Acceleration](https://bliptext.com/articles/acceleration) of the [electrons](https://bliptext.com/articles/electron) is achieved using [resonance](https://bliptext.com/articles/resonance).\nFor this to occur, the electrons must pass through a resonance zone.\nIn this zone, their gyrofrequency or [cyclotron frequency](https://bliptext.com/articles/cyclotron-frequency) is equal to the frequency of the microwave injected into the plasma chamber.\n[Cyclotron frequency](https://bliptext.com/articles/cyclotron-frequency) is defined as the frequency of a charged particle moving perpendicular to the direction of a uniform magnetic field B.\nSince the motion is always circular, cyclotron frequency-ω in radians/second-can be described by the following equation:\nwhere m is the mass of the particle, its charge is q, and the velocity is v.\n[Ionization](https://bliptext.com/articles/ionization) is a step-by-step process from collisions of the accelerated electrons with the desired vapor atoms.\nThe [gyrofrequency](https://bliptext.com/articles/gyrofrequency) of an electron is calculated to be 1.76x107 [Brad](https://bliptext.com/articles/binary-radian)/second.\nNow that the vapor of the desired has been ionized, they must be removed from the magnetic bottle.\nTo do this, a high voltage is between the hexapoles applied to pull out the ions from the [magnetic field](https://bliptext.com/articles/magnetic-field).\nThe extraction of the ions, from the chamber, is carried out using an electrode system through a hole in a positively biased plasma chamber.\nOnce the ions have been extracted from the chamber, they are then sent to the cyclotron for acceleration.\nIt is very important that the ion source used is optimal for the experiment being carried out.\nTo perform an experiment in a practical amount of time, the ions provided from the accelerator complex should have the correct desired energy.\nThe quality and stability of the ion beam needs to be considered carefully, due to the fact that only the ions with the correct flight trajectory can be injected in the cyclotron and accelerated to the desired energy.\nDuring ERDA, the idea is to place an ion beam source at a grazing angle to the sample.\nIn this set up, the angle is calculated as to allow the incident ions to scatter off of the sample so that there is no contact made with the detector.\nThe physical basis that has given the method its name stems from the elastic scattering of incident ions on a sample surface and detecting the recoiling sample atoms while the incident ions [backscatter](https://bliptext.com/articles/backscatter) at such an angle, that they do not reach the detector; this is typically in reflection geometry.\nAnother method for preventing incident ions from making contact with the detector is to use an absorber foil.\nDuring analysis of the elastically recoiled particles, an absorber foil with selected specific thickness can be used to \"stop\" the heavy recoil and beam ions from reaching the detector; reducing the background noise.\nIncorporating an absorber into the experimental set up can be the most difficult to achieve.\nThe stopping of the beam using either direct or scattered methods can only be accomplished without also stopping the light impurity atoms, if it is heavier (beam ions) than the impurity atoms being analyzed.\nThere are advantages when using absorber films:\nThe major criterion for absorber foils used in ERDA is whether a recoiling impurity [atom](https://bliptext.com/articles/atom) can be transmitted through the absorber, preferably a commercially available [metal foil](https://bliptext.com/articles/metal-foil), while stopping heavy particles.\nSince the lighter [atoms](https://bliptext.com/articles/atom) leave the absorber with smaller energies, the kinematic calculations do not provide much help.\nFavorable results have been obtained by using heavier ion beams of approximately 1 MeV/ nucleon.\nThe best overall candidate is the [35Cl](https://bliptext.com/articles/isotopes-of-chlorine) ion beam; although, [79Br](https://bliptext.com/articles/isotopes-of-bromine) would give better sensitivity by one order of magnitude compared to the [35Cl](https://bliptext.com/articles/isotopes-of-chlorine) ion beam.\nThe mass resolution, of the detector at θ= 0°, of thin samples is ΔM/Δx ~ 0.3 amu/1000 Angstroms of the profile width.\nWith thick samples, the mass resolution is feasible at θ≤30°.\nIn thicker samples there is some degradation of [mass resolution](https://bliptext.com/articles/mass-resolution) and slight loss of sensitivity.\nThe detector solid angle has to be closed, but the thick sample can take more current without heating, which decreases sample degradation.\n# [Van de Graaff generator coupled with a particle accelerator | upload.wikimedia.org/wikipedia/commons/b/b2/Image///of///a///Van///de///Graaff///generator///coupled///with///a///particle///accelerator.JPG]\n\n## Detectors\nOnce the ion beam has ionized target sample atoms, the sample ions are recoiled toward the detector.\nThe beam ions are scattered at an angle that does not permit them to reach the detector.\nThe sample ions pass through an entrance window of the detector, and depending on the type of detector used, the signal is converted into a spectrum.\n\n### Silicon diode detector\nIn elastic recoil detection analysis, a silicon [diode](https://bliptext.com/articles/diode) is the most common detector.\nThis type of detector is commonly used, however, there are some major disadvantages when using this type of detector.\nFor example, the energy resolution decreases significantly with a Si detector when detecting heavy recoiled ions.\nThere is also a possibility of damage to the detector by radiation exposure.\nThese detectors have a short functional lifetime (5–10 years) when doing heavy ion analysis.\nOne of the main advantages of silicon detectors is their simplicity.\nHowever, they have to be used with a so-called “range foil” to range out the forward scattered heavy beam ions.\nTherefore, the simple range foil ERD has two major disadvantages: first, the loss of energy resolution due to the energy straggle and second, thickness inhomogeneity of the range foil, and the intrinsic indistinguishability of the signals for the various different recoiled target elements.\nAside from the listed disadvantages, ERDA range foils with silicon detectors is still a powerful method and is relatively simple to work with.\n\n### Time of flight detector\nAnother method of detection for ERDA is time of flight (TOF)-ERD.\nThis method does not present the same issues, as those for the silicon detector.\nHowever, the throughput of TOF detectors is limited; the detection is performed in a serial fashion (one ion in the detector at a time).\nThe longer the TOF for ions, the better the time resolution (equivalent to energy resolution) will be.\nTOF spectrometers that have an incorporated solid state detector must be confined to small solid angles.\nWhen performing HI-ERDA, TOF detectors are often used and/or ∆E/E detectors-such as [ionization chambers](https://bliptext.com/articles/ionization-chamber).\nThese types of detectors usually implement small solid angles for higher depth resolution.\nHeavier ions have a longer flight time than the lighter ions.\nDetectors in modern time-of-flight instruments have improved sensitivity, temporal and spatial resolution, and lifetimes.\nHi mass bipolar (high mass ion detection), Gen 2 Ultra Fast (twice as fast as traditional detectors), and High temperature (operated up to 150 °C) TOF are just a few of the commercially available detectors integrated with time-of-flight instruments.\nLinear and reflectron-TOF are the more common instruments used.\n\n### Ionization detector\nA third type of detector is the gas [ionization](https://bliptext.com/articles/ionization) detector.\nThe gas ionization detectors have some advantages over silicon detectors, for example, they are completely impervious to beam damage, since the gas can be replenished continuously.\nNuclear experiments with large area ionization chambers increase the particle and position resolution have been used for many years and can easily be assimilated to any specific geometry.\nThe limiting factor on energy resolution using this type of detector is the entrance window, which needs to be strong enough to withstand the atmospheric pressure of the gas, 20–90 [mbar](https://bliptext.com/articles/bar-unit).\nUltra-thin [silicon nitride](https://bliptext.com/articles/silicon-nitride) windows have been introduced, together with dramatic simplifications in the design, which have been demonstrated to be nearly as good as more complex designs for low energy ERD.\nThese detectors have also been implemented in heavy-ion rutherford backscattering spectrometry.\nThe energy resolution obtained from this detector is better than a silicon detector when using ion beams heavier than helium ions.\nThere are various designs of ionization detectors but a general schematic of the detector consists of a transversal field ionization chamber with a [Frisch grid](https://bliptext.com/articles/frisch-grid) positioned between anode and cathode electrodes.\nThe anode is subdivided into two plates separated by a specific distance.\nFrom the anode, signals ∆E(energy lost), Erest(residual energy after loss), and Etot (the total energy Etot= ΔΕ+Erest) as well as the atomic number Z can be deduced.\nFor this specific design, the gas used was [isobutane](https://bliptext.com/articles/isobutane) at pressures of 20–90 mbar with a flow rate that was electronically controlled.\nA [polypropylene](https://bliptext.com/articles/polypropylene) foil was used as the entrance window.\nIt has to be noted that the foil thickness [homogeneity](https://bliptext.com/articles/homogeneity-physics) is of more importance for the detector energy resolution than the absolute thickness.\nIf heavy ions are used and detected, the effect of energy loss straggling will be easily surpassed by the energy loss variation, which is a direct consequence of different foil thicknesses.\nThe [cathode](https://bliptext.com/articles/cathode) electrode is divided in two insulated halves, thus information of particle entrance position is derived from charges induced at the right and left halves.\n# [Gas ionization chamber, in which positive charges are migrating toward the cathode and the negatively charged ions are migrating toward the sub divided anode through a Frisch Grid. | upload.wikimedia.org/wikipedia/commons/c/c4/Gas///ionization///chamber///%281%29.png]\n\n## ERDA and energy detection of recoiled sample atoms\nERDA in transmission geometry, where only the energy of the recoiling sample atoms is measured, was extensively used for contamination analysis of target foils for nuclear physics experiments.\nThis technique is excellent for discerning different contaminants of foils used in sensitive experiments, such as carbon contamination.\nUsing 127I ion beam, a profile of various elements can be obtained and the amount of contamination can be determined.\nHigh levels of carbon contamination could be associated with beam excursions on the support, such as a [graphite](https://bliptext.com/articles/graphite) support.\nThis could be corrected by using a different support material.\nUsing a Mo support, the carbon content could be reduced from 20 to 100 at.% to 1–2 at.% level of the oxygen contamination probably originating from residual gas components.\nFor nuclear experiments, high carbon contamination would result in extremely high background and the experimental results would be skewed or less differentiable with the background.\nWith ERDA and heavy ion projectiles, valuable information can be obtained on the light element content of thin foils even if only the energy of the recoils is measured.\n\n## ERDA and particle identification\nGenerally, the energy spectra of different recoil elements overlap due to finite sample thickness, therefore particle identification is necessary to separate the contributions of different elements.\nCommon examples of analysis are thin films of TiNxOy-Cu and BaBiKO.\nTiNxOy-Cu films were developed at the [University of Munich](https://bliptext.com/articles/ludwig-maximilian-university-of-munich) and are used as tandem solar absorbers.\nThe copper coating and the glass substrate was also identified.\nNot only is ERDA is also coupled to [Rutherford backscattering spectrometry](https://bliptext.com/articles/rutherford-backscattering-spectrometry), which is a similar process to ERDA.\nUsing a solid angle of 7.5 mrs, recoils can be detected for this specific analysis of TiNxOy-Cu.\nIt is important when designing an experiment to always consider the geometry of the system as to achieve recoil detection.\nIn this geometry and with Cu being the heaviest component of the sample, according to eq.\n2, scattered projectiles could not reach the detector.\nTo prevent pileup of signals from these recoiled ions, a limit of 500 Hz needed to be set on the count rate of ΔΕ pulses.\nThis corresponded to beam currents of lass than 20 particle pA.\nAnother example of thin film analysis is of BaBiKO.\nThis type of film showed superconductivity at one of the highest-temperatures for oxide [superconductors](https://bliptext.com/articles/superconductivity).\nElemental analysis of this film was carried out using heavy ion-ERDA.\nThese elemental constituents of the polymer film ([Bi](https://bliptext.com/articles/bismuth), [K](https://bliptext.com/articles/potassium), [Mg](https://bliptext.com/articles/magnesium), [O](https://bliptext.com/articles/oxygen), along with [carbon](https://bliptext.com/articles/carbon) contamination) were detected using an [ionization chamber](https://bliptext.com/articles/ionization-chamber).\nOther than potassium, the lighter elements are clearly separated in the matrix.\nFrom the matrix, there is evidence of a strong carbon contamination within the film.\nSome films showed a 1:1 ratio of K to carbon contamination.\nFor this specific film analysis, the source for contamination was traced to an oil [diffusion pump](https://bliptext.com/articles/diffusion-pump) and replaced with an oil-free pumping system.\n\n## ERDA and position resolution\nIn the above examples, the main focus was identification of constituent [particles](https://bliptext.com/articles/particle) found in thin films and depth resolution was of less significance.\nDepth resolution is of great importance in applications when a profile of a sample's elemental composition, in different sample layers, has to be measured.\nThis is a powerful tool for materials characterization.\nBeing able to quantify elemental concentration in sub-surface layers can provide a great deal of information pertaining to chemical properties.\nHigh sensitivity, i.e. large detector solid angle, can be combined with high depth resolution only if the related kinematic energy shift is compensated.\n\n# Physical processes of ERDA\nThe basic chemistry of forward recoil scattering process is considered to be charged particle interaction with matters.\nTo understand forward recoil spectrometry, it is instructive to review the physics involved in elastic and inelastic collisions.\nIn elastic collision, only kinetic energy is conserved in the scattering process, and there is no role of particle internal energy.\nMeanwhile, in case of inelastic collision, both kinetic energy and internal energy are participated in the scattering process.\nPhysical concepts of two-body elastic scattering are the basis of several nuclear methods for elemental material characterization.\n\n## Fundamentals of recoil (backscattering) spectrometry\nThe Fundamental aspects in dealing with recoil spectroscopy involves electron back scattering process of matter such as thin films and solid materials.\nEnergy loss of particles in target materials is evaluated by assuming that the target sample is laterally uniform and constituted by a mono isotopic element.\nThis allows a simple relationship between that of penetration depth profile and elastic scattering yield.\n\n## Main assumptions in physical concepts of Back scattering spectrometry\nPhysical concepts that are highly important in interpretation of forward recoil spectrum are depth profile, energy straggling, and multiple scattering.\nThese concepts are described in detail in the following sections:\n\n## Depth profile and resolution analysis\nA key parameter that characterizes recoil spectrometry is the depth resolution.\nThis parameter is defined as the ability of an analytical technique to measure a variation in atomic distribution as a function of depth in a sample layer.\nIn terms of low energy forward recoil spectrometry, hydrogen and deuterium depth profiling can be expressed in the following mathematical notation:\nwhere δEdet defines as the energy width of a channel in a multichannel analyzer, and dEdet/dx is the effective stopping power of the recoiled particles.\nConsider an Incoming and outgoing ion beams that are calculated as a function of collisional depth, by considering two trajectories are in a plane perpendicular to target surface, and incoming and outgoing paths are the shortest possible ones for a given collision depth and given scattering and recoil angles.\nImpinging ions reach the surface, making an angle θ1, with the inward-pointing normal to the surface.\nAfter collision their velocity makes an angle θ1, with the outward surface normal; and the atom initially at rest recoils, making an angle θ1, with this normal.\nDetection is possible at one of these angles as such that the particle crosses the target surface.\nPaths of particles are related to collisional depth x, measured along a normal to the surface.\nFor the impinging ion, length of the incoming path L1 is given by:\nThe outgoing path length L2 of the scattered projectile is:\nAnd finally the outgoing path L3 of the recoil is:\nIn this simple case a collisional plane is perpendicular to the target surface, the scattering angle of the impinging ion is θ = π-θ1-θ2 & the recoil angle is φ = π-θ1-θ3.\nThe target angle with the collisional plane is taken as α, and path is augmented by a factor of 1/cos α.\nFor the purpose of converting outgoing particle in to collision depth, geometrical factors are chosen.\nFor recoil, R(φ, α) is defined as:\nFor forward scattering of the projectile, R(φ,α) is given by:\nPaths of scattered particles are considered to be L1 for incident beam, L2 is for scattered particle, and L3 is for recoiled atoms.\n# [Planar representation of scattered projectile path of an ion beam. This figure is plane representation of a scattered projectile on the target surface, when both incoming and outgoing paths are perpendicular to target surface. | upload.wikimedia.org/wikipedia/commons/8/84/ERDA1.png]\n# [Planar representation of recoiled path of an ion beam. This figure is plane representation of a recoiled ion path on the target surface, when both incoming and outgoing paths are perpendicular to target surface. | upload.wikimedia.org/wikipedia/commons/0/07/ERDA2.png]\n# [Typical geometrical configuration of recoil spectrometry | upload.wikimedia.org/wikipedia/commons/7/7b/3-///d///picture///final.png]\n\n## Energy depth relationship\nThe energy E0(x) of the incident particle at a depth (x) compared to its initial energy E0 where scattering occurs is given by the following equations.\nSimilarly, energy expression for the scattered particle is:\nand for the recoil atom is:\nThe energy loss per unit path is usually defined as stopping power and it is represented by:\nSpecifically, stopping power S(E) is known as a [function](https://bliptext.com/articles/function-mathematics) of the [energy](https://bliptext.com/articles/energy) E of an ion.\nStarting point for the energy loss calculations is illustrated by the expression:\nBy applying the above equation and energy conservation Illustrates expressions in 3 cases:\nHere, and ; S(E) and S_r(E) are the stopping powers for the projectile and recoil in the target material.\nFinally, the stopping cross-section is defined by, where ε is the stopping cross-section factor.\nTo obtain the energy path scale, we need to evaluate the energy variation δE2 of the outgoing beam of energy E2 from the target surface for an increment δx of collisional depth, while E0 remains fixed.\nThis causes changes in path lengths L1 and L3.\nThe variation of the path around the collision point x is related to the corresponding variation in energy before scattering:\nMoreover, particles with slight energy differences after scattering from a depth x undergo slight energy losses on their outgoing path.\nThen the change δL3 of the path length L3 can be written as:\nδL1 is the path variations due to energy variation just after the collision and δL3 is the path variation because of variation of energy loss along the outward path.\nThe above equations can be solved assuming δx = 0 for the derivative dL_1/dE_2 and :\nIn elastic spectrometry, the term [S] is called as energy loss factor:\nFinally, the stopping cross section is defined by, where N is the atomic density of the target material.\nThe stopping cross-section factor is given by:\n\n## Depth resolution\nAn important parameter that characterizes recoil spectrometer is depth resolution.\nIt is defined as the ability of an analytical technique to detect a variation in atomic distribution as a function of depth.\nThe capability to separate in energy in the recoil system arising from small depth intervals.\nThe expression for depth resolution is given as:\nHere, δET is the total energy resolution of the system, and the expression in the denominator is the sum of the path integrals of initial, scattered and recoil ion beams.\n\n## Practical importance of depth resolution\nThe concept of depth resolution represents the ability of Recoil spectrometry to separate the energies of scattered particles that occurred at slightly different depths δRx is interpreted as an absolute limit for determining the concentration profile.\nFrom this point of view, a concentration profile separated by a depth interval of the order of magnitude of δRx would be undistinguishable in the spectrum, and obviously it is impossible to gain accuracy better than δRx to assign depth profile.\nIn particular the fact that the signals corresponding to features of the concentration profile separated by less than δRx strongly overlap in the spectrum.\nA finite final depth resolution resulting from both theoretical and experimental limitations has deviation from exact result when consider an ideal situation.\nFinal resolution is not coincide with theoretical evaluation such as the classical depth resolution δRx precisely because it results from three terms that escape from theoretical estimations:\n\n## Influence of energy broadening on a recoil spectrum\nStraggling is energy loss of particle in a dense medium is statistical in nature due to a large number of individual collisions between the particle and sample.\nThus the evolution of an initially mono energetic and mono directional beam leads to dispersion of energy and direction.\nThe resulting statistical energy distribution or deviation from initial energy is called energy straggling.\nEnergy straggling data are plotted as a function of depth in the material.\nThe energy distribution of straggling is divided into three domains depending on the ratio of ΔE i.e., ΔE /E where ΔE is the mean energy loss and E is the average energy of the particle along the trajectory.\nThe standard deviation Ω2B of the energy distribution is, where NZ2Δx is the number of electrons per unit area over the path length increment Δx.\nAn expression of energy for straggling was proposed by Symon in the region of 0.2 < ΔE/E ≤ 0.5.\nTschalar et al. derived a straggling function, where σ2(E) represents energy straggling per unit length (or) variance of energy loss distribution per unit length for particles of energy E, and E(x) is the mean energy at depth x. The Tschalar's expression is valid for nearly symmetrical energy loss spectra.\n# [Propagation of the energy straggling distribution through an Al foil for protons of 19.6 MeV with different distribution functions fB:Bohr,fS:Symon,fT:Tschalar | upload.wikimedia.org/wikipedia/commons/2/22/Energy///Straggling///Effect.png]\n\n## Mass resolution\nIn a similar way mass resolution is a parameter that characterizes the capability of recoil spectrometry to separate two signals arising from two neighboring elements in the target.\nThe difference in the energy δE2 of recoil atoms after collision when two types of atoms differ in their masses by a quantity δM2 is:\nMass resolution δMR (≡ δE2/ δM2).\nA main limitation of using low beam energies is the reduced mass resolution.\nThe energy separation of different masses is, in fact, directly proportional to the incident energy.\nThe mass resolution is limited by the relative E and velocity v.\nExpression for mass resolution is: ΔM = √(∂M/∂E.∆E)2 + √(∂M/∂v.∆v)2\nΔM = M(√((∆E)/E)2+√(2.∆v/v)2)\nHere, E is the [energy](https://bliptext.com/articles/energy), M is the [mass](https://bliptext.com/articles/mass), v is the [velocity](https://bliptext.com/articles/velocity) of the [particle beam](https://bliptext.com/articles/particle-beam), and ΔM is the reduced [mass](https://bliptext.com/articles/mass) difference.\n\n## Multiple scattering scheme in forward recoil spectrometry\nWhen an ion beam penetrating in to matter, ions undergo successive scattering events and deviates from original direction.\nThe beam of ions in initial stage are well collimated(single direction), but after passing through a thickness of Δx in a random medium their direction of light propagation certainly differs from normal direction.\nAs a result, both angular and lateral deviations from the initial direction can occur.\nThese two parameters are discussed below.\nHence, path length will be increased than expected causing fluctuations in ion beam.\nThis process is called multiple scattering, and it is statistical in nature due to the large number of collisions.\n# [Multiple scattering scheme where the ion beam is directed in the x direction. Lateral displacement perpendicular to the beam direction is ρ(y,z), and α is the total angular deviation after the penetrated depth x | upload.wikimedia.org/wikipedia/commons/b/b6/ERDA6.png]\n# [Propagation of multiple scattering angular distribution through matter. The half-width of the angular distribution is α1/2. There is a considerable difference between the shape of Gaussian peak (ideal condition) and angularly deviated peak. | upload.wikimedia.org/wikipedia/commons/3/35/Angular///deviation///MS.png]\n\n## Theory and experiment of multiple scattering phenomena\nIn the study of multiple scattering phenomenon angular distribution of a beam is important quantity for consideration.\nThe lateral distribution is closely related to the angular one but secondary to it, since lateral displacement is a consequence of angular divergence.\nLateral distribution represents the beam profile in the matter.\nboth lateral and angular Multiple scattering distributions are interdependent.\nThe analysis of multiple scattering was started by [Walther Bothe](https://bliptext.com/articles/walther-bothe) and [Gregor Wentzel](https://bliptext.com/articles/gregor-wentzel) in the early 1920s using well-known approximation of small angles.\nThe physics of energy straggling and multiple scattering was developed by Williams in 1929–1945.\nWilliams devised a theory, which consists of fitting the multiple scattering distribution as a Gaussian-like portion due to small scattering angles and the single collision tail due to the large angles.\nWilliam, E.J., studied beta particle straggling, Multiple scattering of fast electrons and alpha particles, and cloud curvature tracks due to scattering to explain Multiple scattering in different scenario and he proposed a mean projection deflection occurrence due to scattering.\nHis theory later extended to multiple scattering of alpha particles.\nGoudsmit and Saunderson provided a more complete treatment of multiple scattering, including large angles.\nFor large angles Goudsmit considered series of Legendre polynomials which are numerically evaluated for distribution of scattering.\nThe angular distribution from Coulomb scattering has been studied in by Molière in the 1940s and then by Marion and coworkers, who tabulated energy loss of charged particles in matter, multiple scattering of charged particles, range straggling of protons, deuterons and alpha particles, and equilibrium charge states of ions in solids and energies of elastically scattered particles.\nScott presents a complete review of basic theory, mathematical methods, as well as results and applications.\nA comparative development of multiple scattering at small angles was presented by Meyer, based on a classical calculation of single cross section.\nSigmund and Winterbon extended the Meyer's calculation to a more general case.\nMarwick and Sigmund carried out development on lateral spreading by multiple scattering, which resulted in a simple scaling relation with the angular distribution.\n\n# Applications\nERDA has applications in the areas of polymer science, semiconductor materials, electronics, and thin film characterization.\nERDA is widely used in polymer science.\nThis is because polymers are hydrogen-rich materials which can be easily studied by LI-ERDA.\nOne can examine surface properties of polymers, polymer blends and evolution of polymer composition induced by irradiation.\nHI-ERDA can also be used in the field of new materials processed for microelectronics and opto-electronic applications.\nMoreover, elemental analysis and depth profiling in thin film can also be performed using ERDA.\nERDA is also used to characterize hydrogen transport near interfaces induced by corrosion and wear.\nCharacterizing how polymer molecules behave at interfaces between incompatible polymers and at interfaces with inorganic solid substances is crucial to our fundamental understanding and for improving the performance of polymers in applications.\nFor example, the adhesion of two polymers strongly depends on the interactions occurring at the interface between polymer segments.\nLI-ERDA is one of the most attractive methods for investigating these aspects of polymer science quantitatively.\nElectronic devices are usually composed of sequential thin layers made up of oxides, nitrides, silicides, metals, polymers, or doped semiconductor–based media coated on a single-crystalline substrate (Si, Ge or GaAs).\nThese structures can be studied by HI-ERDA.\nThis technique has one major advantage over other methods.\nThe profile of impurities can be found in a one-shot measurement at a constant incident energy.\nMoreover, this technique offers an opportunity to study the density profiles of hydrogen, carbon and oxygen in various materials, as well as the absolute hydrogen, carbon and oxygen content.","current_revision":null,"created_by":"kACnuQmH5k9Nr5VNVEpVeg7XpvUmMctD","created_at":"2025-01-25T21:54:06.037Z","updated_at":"2025-01-25T21:54:06.037Z","search_vector":"'detect':3 'elast':1 'recoil':2","search_content":" \n \n\n# \n**Elastic [recoil](https://bliptext.com/articles/recoil) detection analysis** (**ERDA**), also referred to as forward [recoil](https://bliptext.com/articles/recoil) scattering or [spectrometry](https://bliptext.com/articles/spectrometer), is an [ion beam](https://bliptext.com/articles/ion-beam) analysis technique, in [materials science](https://bliptext.com/articles/materials-science), to obtain elemental [concentration](https://bliptext.com/articles/concentration) depth profiles in [thin films](https://bliptext.com/articles/thin-films).\nThis technique can be achieved using many processes.\nIn the technique of ERDA, an [energetic](https://bliptext.com/articles/energy) ion beam is directed at a sample to be characterized and (as in [Rutherford backscattering](https://bliptext.com/articles/rutherford-backscattering-spectrometry)) there is an [elastic nuclear interaction](https://bliptext.com/articles/nuclear-interaction) between the [ions](https://bliptext.com/articles/ions) of the beam and the [atoms](https://bliptext.com/articles/atoms) of the target sample.\nSuch interactions are commonly of [Coulomb](https://bliptext.com/articles/coulomb-s-law) nature.\nDepending on the [kinetics](https://bliptext.com/articles/kinetics-physics) of the ions, [cross section](https://bliptext.com/articles/cross-section-physics) area, and the loss of energy of the ions in the matter, ERDA helps determine the [quantification](https://bliptext.com/articles/quantification-science) of the [elemental analysis](https://bliptext.com/articles/elemental-analysis).\nIt also provides information about the depth profile of the sample.\nThe energy of incident energetic ions can vary from 2 [MeV](https://bliptext.com/articles/electronvolt) to 200 [MeV](https://bliptext.com/articles/electronvolt), depending on the studied sample.\nThe energy of the beam should be enough to kick out (“recoil”) the atoms of the sample.\nThus, ERDA usually employs appropriate source and detectors to detect [recoiled atoms](https://bliptext.com/articles/atomic-recoil).\nERDA setup is large, expensive and difficult to operate.\nTherefore, although it is commercially available, it is relatively uncommon in [materials characterization](https://bliptext.com/articles/characterization-materials-science).\nThe [angle of incidence](https://bliptext.com/articles/angle-of-incidence-optics) that an ion beam makes with the sample must also be taken into account for correct analysis of the sample.\nThis is because, depending on this angle, the recoiled atoms will be collected.\nERDA has been used since 1974.\nIt has similar theory to Rutherford backscattering spectrometry (RBS), but there are minor differences in the set-up of the experiment.\nIn case of RBS, the [detector](https://bliptext.com/articles/sensor) is placed in the back of the sample whereas in ERDA, the [detector](https://bliptext.com/articles/sensor) is placed in the front.\n\n# Characteristics of ERDA\nThe main characteristics of ERDA are listed below.\n\n# History\nERDA was first demonstrated by L’Ecuyer et al. in 1976.\nThey used 25–40 MeV [35Cl](https://bliptext.com/articles/isotopes-of-chlorine) ions to detect the recoils in the sample.\nLater, ERDA has been divided into two main groups.\nFirst is the light incident ion ERDA (LI-ERDA) and the second is the heavy incident ion ERDA (HI-ERDA).\nThese techniques provide similar information and differ only in the type of ion beam used as a source.\nLI-ERDA uses low voltage [single-ended accelerators](https://bliptext.com/articles/electrostatic-particle-accelerator), whereas the HI-ERDA uses [large tandem accelerators](https://bliptext.com/articles/electrostatic-particle-accelerator).\nThese techniques were mainly developed after heavy [ion accelerators](https://bliptext.com/articles/particle-accelerator) were introduced in the materials research.\nLI-ERDA is also often performed using a relatively low energy (2 MeV) [helium](https://bliptext.com/articles/helium) beam for measuring the depth profile of [hydrogen](https://bliptext.com/articles/hydrogen).\nIn this technique, multiple detectors are used: [backscattering detector](https://bliptext.com/articles/electron-backscatter-diffraction) for heavier elements and forward (recoil) detector to simultaneously detect the recoiled hydrogen.\nThe recoil detector for LI-ERDA typically has a “range foil”.\nIt is usually a [Mylar](https://bliptext.com/articles/mylar) foil placed in front of the detector, which blocks scattered incident ions, but allows lighter recoiling target atoms to pass through to the detector.\nUsually a 10 [μm](https://bliptext.com/articles/m) thick Mylar foil completely stops 2.6 MeV helium ions but allows the recoiled [protons](https://bliptext.com/articles/proton) to go through with a low energy loss.\nHI-ERDA is more widely used compared to LI-ERDA because it can probe more elements.\nIt is used to detect recoiled target atoms and scattered beam ions using several detectors, such as [silicon](https://bliptext.com/articles/silicon) [diode](https://bliptext.com/articles/diode) detector, [time-of-flight detector](https://bliptext.com/articles/time-of-flight-detector), Gaseous ionization detector etc. The main advantage of HI-ERDA is its ability to obtain quantitative depth profiling information of all the sample elements in one measurement.\nDepth resolution less than 1[nm](https://bliptext.com/articles/nanometre) can be obtained with good quantitative accuracy thus giving these techniques significant advantages over other surface analysis methods.\nAdditionally, a depth of 300 nm can be accessed using this technique.\nA wide range of ion beams including [35Cl](https://bliptext.com/articles/isotopes-of-chlorine), [63Cu](https://bliptext.com/articles/isotopes-of-copper), [127I](https://bliptext.com/articles/isotopes-of-iodine), and [197Au](https://bliptext.com/articles/isotopes-of-gold), with different [energies](https://bliptext.com/articles/energies) can be used in this technique.\nThe setup and the experimental conditions affect the performances of both of these techniques.\nFactors such as [multiple scattering](https://bliptext.com/articles/scattering) and ion beam induced damage must be taken into account before obtaining the data because these processes can affect the data interpretation, quantification, and accuracy of the study.\nAdditionally, the [incident angle](https://bliptext.com/articles/angle-of-incidence-optics) and the [scattered angle](https://bliptext.com/articles/scattering) help determine the sample surface topography.\n\n# Prominent features of ERDA\nERDA is very similar to [RBS](https://bliptext.com/articles/rutherford-backscattering-spectrometry), but instead of detecting the [projectile](https://bliptext.com/articles/projectile) at the back angle, the recoils are detected in the forward direction.\nDoyle and Peercey in 1979 established the use of this technique for hydrogen depth profiling.\nSome of the prominent features of ERDA with high energy heavy ions are:\nERDA can overcome some of the limitations of [RBS](https://bliptext.com/articles/rutherford-backscattering-spectrometry).\nERDA has enabled depth profiling of elements from lightest elements like [hydrogen](https://bliptext.com/articles/hydrogen) up to heavy elements with high resolution in the light mass region as discussed above.\nAlso, this technique has been highly sensitive because of the use of large area position sensitive telescope detectors.\nSuch detectors are used especially when the elements in the sample have similar masses.\n\n# Principles of ERDA\nThe calculations that model this process are relatively simple, assuming projectile energy is in the range corresponding to Rutherford scattering.\nProjectile energy range for light incident ions is in 0.5–3.0 [MeV](https://bliptext.com/articles/electronvolt) range.\nFor heavier projectile ions such as [127I](https://bliptext.com/articles/isotopes-of-iodine) the energy range is usually between 60 and 120 MeV; and for medium heavy ion beams,[36Cl](https://bliptext.com/articles/chlorine-36) is a common ion beam used with an energy of approximately 30 MeV.\nFor the instrumentation section, the focus will be on heavy ion bombardment.\nThe E2 transferred by projectile ions of mass m1 and energy E1 to sample atoms of mass m2 recoiling at an angle ϕ, with respect to the incidence direction is given by the following equation.\nEq.\n1 models the energy transfer from the incident ions striking the sample atoms and the recoiling effect of the target atoms with an angle of ϕ.\nFor heavier ions in elastic recoil detection analysis, if m2/m1 <<1, all recoiling ions have similar velocities.\nIt can be deduced from the previous equation the maximum scattering angle, θ’max, as Eq.\n2 describes:\nUsing these parameters, absorber foils do not need to be incorporated into the instrument design.\nWhen using heavy ion beams and the parameters above, the geometry can be estimated as to allow for incident particle collision and scattering at an angle deflected away from the detector.\nThis will prevent degradation of the detector from the more intense beam energies.\nThe differential elastic recoil cross-section σERD is given by:\nwhere Z1 and Z2 are the atomic numbers of projectile and sample atoms, respectively.\nFor m2/m1 <<1 and with approximation m=2Z; Z being the atomic number of Z1 and Z2.\nIn Eq.\n3 two essential consequences can be seen, first the sensitivity is roughly the same for all elements and second it has a Z14 dependence on the projector of the ion.\nThis allows for the use of low energy beam currents in HI-ERDA preventing sample degradation and excessive heating of the specimen.\nWhen using heavy ion beams, care must be taken for beam-induced damage in sample such as [sputtering](https://bliptext.com/articles/sputtering) or [amorphization](https://bliptext.com/articles/metamictisation).\nIf only [nuclear interaction](https://bliptext.com/articles/nuclear-interaction) is taken into account, it has been shown that the ratio of recoiling to displaced atoms is independent of Z1 and only weakly dependent on the projectile mass of the incident ion.\nWith heavy ion bombardment, it has been shown that the sputter yield by the ion beam on the sample increases for [nonmetallic](https://bliptext.com/articles/nonmetallic-material) samples and enhanced radiation damage in [superconductors](https://bliptext.com/articles/superconductivity).\nIn any case, the acceptance angle of the detector system should be as large as possible to minimize the radiation damage.\nHowever, it may reduce the depth profiling and elemental analysis due to the ion beam not being able to penetrate the sample.\nThis demand of a large acceptance angle, however, is in conflict with the requirement of optimum depth resolution dependency on the detection geometry.\nIn the surface approximation and assuming constant energy loss the depth resolution δx can be written:\nwhere Srel is the relative energy loss factor defined by:\nhere, α and β are the [incidence angles](https://bliptext.com/articles/angle-of-incidence-optics) of the beam and exit angle of the recoiling ion respectively, connected to the scattering angle ϕ by ϕ=α+β.\nIt should be noticed here that the depth resolution depends on the relative energy resolution only, as well as the relative stopping power of incoming and outgoing ions.\nThe detector resolution and energy broadening associated with the measuring geometry contribute to the energy spread, δE.\nThe detector acceptance angle and the finite beam spot size define a scattering angle range δϕ causing a kinematic energy spread δEkin according to Eq.\n6:\nA detailed analysis of the different contributions to depth resolution shows that this kinematic effect is the predominant term near the surface, severely limiting the permitted detector acceptance angle, whereas energy straggling dominates the resolution at larger depth.\nFor example, if one estimates δϕ for a scattering angle of 37.5° causing a [kinematic](https://bliptext.com/articles/kinematics) [energy](https://bliptext.com/articles/energy) shift comparable to typical detector [energy](https://bliptext.com/articles/energy) resolutions of 1%, the angular spread δψ must be less than 0.4°.\nThe angular spread can be maintained within this range by contributions from the beam spot size; however, the solid angle geometry of the detector is only 0.04 msr.\nTherefore, a detector system with large solid angle as well as high depth resolution may enable corrections for the kinematic energy shift.\nIn an [elastic scattering](https://bliptext.com/articles/elastic-scattering) event, the kinematics requires that the target atom is recoiled with significant energy.\nEq.\n7 models the recoil kinematical factor that occurs during the ion bombardment.\nEq.\n7 gives a [mathematical model](https://bliptext.com/articles/mathematical-model) of the collision event when the heavier ions in the beam strike the specimen.\nKs is termed the kinematical factor for the scattered particle (Eq. 8) with a scattering angle of θ, and the recoiled particle (Eq. 9) with a recoil angle of Φ.\nThe variable r is the ratio of mass of the incident nuclei to that of the mass of the target nuclei, (Eq. 10).\nTo achieve this recoil of particles, the specimen needs to be very thin and the geometries need to be precisely optimized to obtain accurate recoil detection.\nSince ERD beam intensity can damage the specimen and there has been growing interest to invest in the development of low energy beams for reducing the damage of the specimen.\nThe [[cathode](https://bliptext.com/articles/cathode)](https://bliptext.com/articles/[cathode](https://bliptext.com/articles/cathode)) is divided into two insulated halves, where particle entrance position is derived from charges induced on the left, l, and right, r, halves of the [[cathode](https://bliptext.com/articles/cathode)](https://bliptext.com/articles/[cathode](https://bliptext.com/articles/cathode)).\nUsing the following equation, x-coordinates of particle positions, as they enter the detector, can be calculated from charges l and r:\nFurthermore, the y-coordinate is calculated from the following equation due to the position independence of the [anode](https://bliptext.com/articles/anode) pulses:\nFor transformation of the (x, y) information into scattering angle ϕ a removable calibration mask in front of the entrance window is used.\nThis mask allows correction for x and y distortions too.\nFor notation detail, the cathode has an ion drift time on the order of a few milliseconds.\nTo prevent ion saturation of the detector, a limit of 1 kHz must be applied to the number of particles entering the detector.\n# [Schematic of ERDA. ϕ, α and β are recoil angle, incident angle, and exit angle, respectively. x is the probing depth. | upload.wikimedia.org/wikipedia/commons/c/c1/ERDA///schematic.png]\n\n# Instrumentation\nElastic recoil detection analysis was originally developed for [hydrogen](https://bliptext.com/articles/hydrogen) detection or a light element ([H](https://bliptext.com/articles/hydrogen), [H](https://bliptext.com/articles/hydrogen)e, [Li](https://bliptext.com/articles/lithium), [C](https://bliptext.com/articles/carbon), [O](https://bliptext.com/articles/oxygen), [Mg](https://bliptext.com/articles/magnesium), [K](https://bliptext.com/articles/potassium)) profiling with an absorber foil in front of the energy detector for beam suppression.\nUsing an absorber foil prevents the higher energy ion beam from striking the detector and causing degradation.\nAbsorber foils increase the lifetime of the detector.\nMore advanced techniques have been implemented to negate the use of absorber foils and the associated difficulties that arise through the use of it.\nIn most cases, medium heavy ion beams, typically 36Cl ions, have been used for ERDA so far with energies around 30 MeV.\nDepth resolution and element profiling of thin films has been greatly advanced using elastic recoil detection analysis.\n\n## Ion source and interactions\n[Particle accelerators](https://bliptext.com/articles/particle-accelerator), such as a [magnetron](https://bliptext.com/articles/cavity-magnetron) or [cyclotron](https://bliptext.com/articles/cyclotron), implement [electromagnetic fields](https://bliptext.com/articles/electromagnetic-field) to achieve acceleration of elements.\nAtoms must be electrically charged (ionized) before they can be accelerated.\n[Ionization](https://bliptext.com/articles/ionization) involves the removal of electrons from the target atoms.\nA magnetron can be used to produce hydrogen ions.\n[Van de Graaff generators](https://bliptext.com/articles/van-de-graaff-generator) have also been integrated with particle accelerators for light ion beam generation.\nFor heavier ion production, for example, an [electron cyclotron resonance](https://bliptext.com/articles/electron-cyclotron-resonance) (ECR) source can be used.\nAt the [National Superconducting Cyclotron Laboratory](https://bliptext.com/articles/national-superconducting-cyclotron-laboratory), neutral atoms have their electrons removed using an ECR ion source.\nECR works by ionizing the vapor of a desired element such as chlorine and iodine.\nFurther, utilizing this technique, metals ([Au](https://bliptext.com/articles/gold), [Ag](https://bliptext.com/articles/silver), etc.) can also be ionized using a small oven to achieve a vapor phase.\nThe vapor is maintained within a [magnetic field](https://bliptext.com/articles/magnetic-field) long enough for the atoms to be ionized by collisions with [electrons](https://bliptext.com/articles/electron).\nMicrowaves are applied to the chamber as to keep the electrons in motion.\nThe vapor is introduced via injection directly into the “magnetic bottle” or the magnetic field.\nCircular coils provide the shape for the magnetic bottle.\nThe coils are found at the top and bottom of the chamber with a [hexapole](https://bliptext.com/articles/hexapole) magnet around the sides.\nA hexapole magnet consists of permanent magnets or superconducting coils.\nThe plasma is contained within the magnetic trap that is formed from electric current flowing in solenoids located on the sides of the chamber.\nA radial magnetic field, exerted by the hexapole magnetic, is applied to the system that also confines the plasma.\n[Acceleration](https://bliptext.com/articles/acceleration) of the [electrons](https://bliptext.com/articles/electron) is achieved using [resonance](https://bliptext.com/articles/resonance).\nFor this to occur, the electrons must pass through a resonance zone.\nIn this zone, their gyrofrequency or [cyclotron frequency](https://bliptext.com/articles/cyclotron-frequency) is equal to the frequency of the microwave injected into the plasma chamber.\n[Cyclotron frequency](https://bliptext.com/articles/cyclotron-frequency) is defined as the frequency of a charged particle moving perpendicular to the direction of a uniform magnetic field B.\nSince the motion is always circular, cyclotron frequency-ω in radians/second-can be described by the following equation:\nwhere m is the mass of the particle, its charge is q, and the velocity is v.\n[Ionization](https://bliptext.com/articles/ionization) is a step-by-step process from collisions of the accelerated electrons with the desired vapor atoms.\nThe [gyrofrequency](https://bliptext.com/articles/gyrofrequency) of an electron is calculated to be 1.76x107 [Brad](https://bliptext.com/articles/binary-radian)/second.\nNow that the vapor of the desired has been ionized, they must be removed from the magnetic bottle.\nTo do this, a high voltage is between the hexapoles applied to pull out the ions from the [magnetic field](https://bliptext.com/articles/magnetic-field).\nThe extraction of the ions, from the chamber, is carried out using an electrode system through a hole in a positively biased plasma chamber.\nOnce the ions have been extracted from the chamber, they are then sent to the cyclotron for acceleration.\nIt is very important that the ion source used is optimal for the experiment being carried out.\nTo perform an experiment in a practical amount of time, the ions provided from the accelerator complex should have the correct desired energy.\nThe quality and stability of the ion beam needs to be considered carefully, due to the fact that only the ions with the correct flight trajectory can be injected in the cyclotron and accelerated to the desired energy.\nDuring ERDA, the idea is to place an ion beam source at a grazing angle to the sample.\nIn this set up, the angle is calculated as to allow the incident ions to scatter off of the sample so that there is no contact made with the detector.\nThe physical basis that has given the method its name stems from the elastic scattering of incident ions on a sample surface and detecting the recoiling sample atoms while the incident ions [backscatter](https://bliptext.com/articles/backscatter) at such an angle, that they do not reach the detector; this is typically in reflection geometry.\nAnother method for preventing incident ions from making contact with the detector is to use an absorber foil.\nDuring analysis of the elastically recoiled particles, an absorber foil with selected specific thickness can be used to \"stop\" the heavy recoil and beam ions from reaching the detector; reducing the background noise.\nIncorporating an absorber into the experimental set up can be the most difficult to achieve.\nThe stopping of the beam using either direct or scattered methods can only be accomplished without also stopping the light impurity atoms, if it is heavier (beam ions) than the impurity atoms being analyzed.\nThere are advantages when using absorber films:\nThe major criterion for absorber foils used in ERDA is whether a recoiling impurity [atom](https://bliptext.com/articles/atom) can be transmitted through the absorber, preferably a commercially available [metal foil](https://bliptext.com/articles/metal-foil), while stopping heavy particles.\nSince the lighter [atoms](https://bliptext.com/articles/atom) leave the absorber with smaller energies, the kinematic calculations do not provide much help.\nFavorable results have been obtained by using heavier ion beams of approximately 1 MeV/ nucleon.\nThe best overall candidate is the [35Cl](https://bliptext.com/articles/isotopes-of-chlorine) ion beam; although, [79Br](https://bliptext.com/articles/isotopes-of-bromine) would give better sensitivity by one order of magnitude compared to the [35Cl](https://bliptext.com/articles/isotopes-of-chlorine) ion beam.\nThe mass resolution, of the detector at θ= 0°, of thin samples is ΔM/Δx ~ 0.3 amu/1000 Angstroms of the profile width.\nWith thick samples, the mass resolution is feasible at θ≤30°.\nIn thicker samples there is some degradation of [mass resolution](https://bliptext.com/articles/mass-resolution) and slight loss of sensitivity.\nThe detector solid angle has to be closed, but the thick sample can take more current without heating, which decreases sample degradation.\n# [Van de Graaff generator coupled with a particle accelerator | upload.wikimedia.org/wikipedia/commons/b/b2/Image///of///a///Van///de///Graaff///generator///coupled///with///a///particle///accelerator.JPG]\n\n## Detectors\nOnce the ion beam has ionized target sample atoms, the sample ions are recoiled toward the detector.\nThe beam ions are scattered at an angle that does not permit them to reach the detector.\nThe sample ions pass through an entrance window of the detector, and depending on the type of detector used, the signal is converted into a spectrum.\n\n### Silicon diode detector\nIn elastic recoil detection analysis, a silicon [diode](https://bliptext.com/articles/diode) is the most common detector.\nThis type of detector is commonly used, however, there are some major disadvantages when using this type of detector.\nFor example, the energy resolution decreases significantly with a Si detector when detecting heavy recoiled ions.\nThere is also a possibility of damage to the detector by radiation exposure.\nThese detectors have a short functional lifetime (5–10 years) when doing heavy ion analysis.\nOne of the main advantages of silicon detectors is their simplicity.\nHowever, they have to be used with a so-called “range foil” to range out the forward scattered heavy beam ions.\nTherefore, the simple range foil ERD has two major disadvantages: first, the loss of energy resolution due to the energy straggle and second, thickness inhomogeneity of the range foil, and the intrinsic indistinguishability of the signals for the various different recoiled target elements.\nAside from the listed disadvantages, ERDA range foils with silicon detectors is still a powerful method and is relatively simple to work with.\n\n### Time of flight detector\nAnother method of detection for ERDA is time of flight (TOF)-ERD.\nThis method does not present the same issues, as those for the silicon detector.\nHowever, the throughput of TOF detectors is limited; the detection is performed in a serial fashion (one ion in the detector at a time).\nThe longer the TOF for ions, the better the time resolution (equivalent to energy resolution) will be.\nTOF spectrometers that have an incorporated solid state detector must be confined to small solid angles.\nWhen performing HI-ERDA, TOF detectors are often used and/or ∆E/E detectors-such as [ionization chambers](https://bliptext.com/articles/ionization-chamber).\nThese types of detectors usually implement small solid angles for higher depth resolution.\nHeavier ions have a longer flight time than the lighter ions.\nDetectors in modern time-of-flight instruments have improved sensitivity, temporal and spatial resolution, and lifetimes.\nHi mass bipolar (high mass ion detection), Gen 2 Ultra Fast (twice as fast as traditional detectors), and High temperature (operated up to 150 °C) TOF are just a few of the commercially available detectors integrated with time-of-flight instruments.\nLinear and reflectron-TOF are the more common instruments used.\n\n### Ionization detector\nA third type of detector is the gas [ionization](https://bliptext.com/articles/ionization) detector.\nThe gas ionization detectors have some advantages over silicon detectors, for example, they are completely impervious to beam damage, since the gas can be replenished continuously.\nNuclear experiments with large area ionization chambers increase the particle and position resolution have been used for many years and can easily be assimilated to any specific geometry.\nThe limiting factor on energy resolution using this type of detector is the entrance window, which needs to be strong enough to withstand the atmospheric pressure of the gas, 20–90 [mbar](https://bliptext.com/articles/bar-unit).\nUltra-thin [silicon nitride](https://bliptext.com/articles/silicon-nitride) windows have been introduced, together with dramatic simplifications in the design, which have been demonstrated to be nearly as good as more complex designs for low energy ERD.\nThese detectors have also been implemented in heavy-ion rutherford backscattering spectrometry.\nThe energy resolution obtained from this detector is better than a silicon detector when using ion beams heavier than helium ions.\nThere are various designs of ionization detectors but a general schematic of the detector consists of a transversal field ionization chamber with a [Frisch grid](https://bliptext.com/articles/frisch-grid) positioned between anode and cathode electrodes.\nThe anode is subdivided into two plates separated by a specific distance.\nFrom the anode, signals ∆E(energy lost), Erest(residual energy after loss), and Etot (the total energy Etot= ΔΕ+Erest) as well as the atomic number Z can be deduced.\nFor this specific design, the gas used was [isobutane](https://bliptext.com/articles/isobutane) at pressures of 20–90 mbar with a flow rate that was electronically controlled.\nA [polypropylene](https://bliptext.com/articles/polypropylene) foil was used as the entrance window.\nIt has to be noted that the foil thickness [homogeneity](https://bliptext.com/articles/homogeneity-physics) is of more importance for the detector energy resolution than the absolute thickness.\nIf heavy ions are used and detected, the effect of energy loss straggling will be easily surpassed by the energy loss variation, which is a direct consequence of different foil thicknesses.\nThe [cathode](https://bliptext.com/articles/cathode) electrode is divided in two insulated halves, thus information of particle entrance position is derived from charges induced at the right and left halves.\n# [Gas ionization chamber, in which positive charges are migrating toward the cathode and the negatively charged ions are migrating toward the sub divided anode through a Frisch Grid. | upload.wikimedia.org/wikipedia/commons/c/c4/Gas///ionization///chamber///%281%29.png]\n\n## ERDA and energy detection of recoiled sample atoms\nERDA in transmission geometry, where only the energy of the recoiling sample atoms is measured, was extensively used for contamination analysis of target foils for nuclear physics experiments.\nThis technique is excellent for discerning different contaminants of foils used in sensitive experiments, such as carbon contamination.\nUsing 127I ion beam, a profile of various elements can be obtained and the amount of contamination can be determined.\nHigh levels of carbon contamination could be associated with beam excursions on the support, such as a [graphite](https://bliptext.com/articles/graphite) support.\nThis could be corrected by using a different support material.\nUsing a Mo support, the carbon content could be reduced from 20 to 100 at.% to 1–2 at.% level of the oxygen contamination probably originating from residual gas components.\nFor nuclear experiments, high carbon contamination would result in extremely high background and the experimental results would be skewed or less differentiable with the background.\nWith ERDA and heavy ion projectiles, valuable information can be obtained on the light element content of thin foils even if only the energy of the recoils is measured.\n\n## ERDA and particle identification\nGenerally, the energy spectra of different recoil elements overlap due to finite sample thickness, therefore particle identification is necessary to separate the contributions of different elements.\nCommon examples of analysis are thin films of TiNxOy-Cu and BaBiKO.\nTiNxOy-Cu films were developed at the [University of Munich](https://bliptext.com/articles/ludwig-maximilian-university-of-munich) and are used as tandem solar absorbers.\nThe copper coating and the glass substrate was also identified.\nNot only is ERDA is also coupled to [Rutherford backscattering spectrometry](https://bliptext.com/articles/rutherford-backscattering-spectrometry), which is a similar process to ERDA.\nUsing a solid angle of 7.5 mrs, recoils can be detected for this specific analysis of TiNxOy-Cu.\nIt is important when designing an experiment to always consider the geometry of the system as to achieve recoil detection.\nIn this geometry and with Cu being the heaviest component of the sample, according to eq.\n2, scattered projectiles could not reach the detector.\nTo prevent pileup of signals from these recoiled ions, a limit of 500 Hz needed to be set on the count rate of ΔΕ pulses.\nThis corresponded to beam currents of lass than 20 particle pA.\nAnother example of thin film analysis is of BaBiKO.\nThis type of film showed superconductivity at one of the highest-temperatures for oxide [superconductors](https://bliptext.com/articles/superconductivity).\nElemental analysis of this film was carried out using heavy ion-ERDA.\nThese elemental constituents of the polymer film ([Bi](https://bliptext.com/articles/bismuth), [K](https://bliptext.com/articles/potassium), [Mg](https://bliptext.com/articles/magnesium), [O](https://bliptext.com/articles/oxygen), along with [carbon](https://bliptext.com/articles/carbon) contamination) were detected using an [ionization chamber](https://bliptext.com/articles/ionization-chamber).\nOther than potassium, the lighter elements are clearly separated in the matrix.\nFrom the matrix, there is evidence of a strong carbon contamination within the film.\nSome films showed a 1:1 ratio of K to carbon contamination.\nFor this specific film analysis, the source for contamination was traced to an oil [diffusion pump](https://bliptext.com/articles/diffusion-pump) and replaced with an oil-free pumping system.\n\n## ERDA and position resolution\nIn the above examples, the main focus was identification of constituent [particles](https://bliptext.com/articles/particle) found in thin films and depth resolution was of less significance.\nDepth resolution is of great importance in applications when a profile of a sample's elemental composition, in different sample layers, has to be measured.\nThis is a powerful tool for materials characterization.\nBeing able to quantify elemental concentration in sub-surface layers can provide a great deal of information pertaining to chemical properties.\nHigh sensitivity, i.e. large detector solid angle, can be combined with high depth resolution only if the related kinematic energy shift is compensated.\n\n# Physical processes of ERDA\nThe basic chemistry of forward recoil scattering process is considered to be charged particle interaction with matters.\nTo understand forward recoil spectrometry, it is instructive to review the physics involved in elastic and inelastic collisions.\nIn elastic collision, only kinetic energy is conserved in the scattering process, and there is no role of particle internal energy.\nMeanwhile, in case of inelastic collision, both kinetic energy and internal energy are participated in the scattering process.\nPhysical concepts of two-body elastic scattering are the basis of several nuclear methods for elemental material characterization.\n\n## Fundamentals of recoil (backscattering) spectrometry\nThe Fundamental aspects in dealing with recoil spectroscopy involves electron back scattering process of matter such as thin films and solid materials.\nEnergy loss of particles in target materials is evaluated by assuming that the target sample is laterally uniform and constituted by a mono isotopic element.\nThis allows a simple relationship between that of penetration depth profile and elastic scattering yield.\n\n## Main assumptions in physical concepts of Back scattering spectrometry\nPhysical concepts that are highly important in interpretation of forward recoil spectrum are depth profile, energy straggling, and multiple scattering.\nThese concepts are described in detail in the following sections:\n\n## Depth profile and resolution analysis\nA key parameter that characterizes recoil spectrometry is the depth resolution.\nThis parameter is defined as the ability of an analytical technique to measure a variation in atomic distribution as a function of depth in a sample layer.\nIn terms of low energy forward recoil spectrometry, hydrogen and deuterium depth profiling can be expressed in the following mathematical notation:\nwhere δEdet defines as the energy width of a channel in a multichannel analyzer, and dEdet/dx is the effective stopping power of the recoiled particles.\nConsider an Incoming and outgoing ion beams that are calculated as a function of collisional depth, by considering two trajectories are in a plane perpendicular to target surface, and incoming and outgoing paths are the shortest possible ones for a given collision depth and given scattering and recoil angles.\nImpinging ions reach the surface, making an angle θ1, with the inward-pointing normal to the surface.\nAfter collision their velocity makes an angle θ1, with the outward surface normal; and the atom initially at rest recoils, making an angle θ1, with this normal.\nDetection is possible at one of these angles as such that the particle crosses the target surface.\nPaths of particles are related to collisional depth x, measured along a normal to the surface.\nFor the impinging ion, length of the incoming path L1 is given by:\nThe outgoing path length L2 of the scattered projectile is:\nAnd finally the outgoing path L3 of the recoil is:\nIn this simple case a collisional plane is perpendicular to the target surface, the scattering angle of the impinging ion is θ = π-θ1-θ2 & the recoil angle is φ = π-θ1-θ3.\nThe target angle with the collisional plane is taken as α, and path is augmented by a factor of 1/cos α.\nFor the purpose of converting outgoing particle in to collision depth, geometrical factors are chosen.\nFor recoil, R(φ, α) is defined as:\nFor forward scattering of the projectile, R(φ,α) is given by:\nPaths of scattered particles are considered to be L1 for incident beam, L2 is for scattered particle, and L3 is for recoiled atoms.\n# [Planar representation of scattered projectile path of an ion beam. This figure is plane representation of a scattered projectile on the target surface, when both incoming and outgoing paths are perpendicular to target surface. | upload.wikimedia.org/wikipedia/commons/8/84/ERDA1.png]\n# [Planar representation of recoiled path of an ion beam. This figure is plane representation of a recoiled ion path on the target surface, when both incoming and outgoing paths are perpendicular to target surface. | upload.wikimedia.org/wikipedia/commons/0/07/ERDA2.png]\n# [Typical geometrical configuration of recoil spectrometry | upload.wikimedia.org/wikipedia/commons/7/7b/3-///d///picture///final.png]\n\n## Energy depth relationship\nThe energy E0(x) of the incident particle at a depth (x) compared to its initial energy E0 where scattering occurs is given by the following equations.\nSimilarly, energy expression for the scattered particle is:\nand for the recoil atom is:\nThe energy loss per unit path is usually defined as stopping power and it is represented by:\nSpecifically, stopping power S(E) is known as a [function](https://bliptext.com/articles/function-mathematics) of the [energy](https://bliptext.com/articles/energy) E of an ion.\nStarting point for the energy loss calculations is illustrated by the expression:\nBy applying the above equation and energy conservation Illustrates expressions in 3 cases:\nHere, and ; S(E) and S_r(E) are the stopping powers for the projectile and recoil in the target material.\nFinally, the stopping cross-section is defined by, where ε is the stopping cross-section factor.\nTo obtain the energy path scale, we need to evaluate the energy variation δE2 of the outgoing beam of energy E2 from the target surface for an increment δx of collisional depth, while E0 remains fixed.\nThis causes changes in path lengths L1 and L3.\nThe variation of the path around the collision point x is related to the corresponding variation in energy before scattering:\nMoreover, particles with slight energy differences after scattering from a depth x undergo slight energy losses on their outgoing path.\nThen the change δL3 of the path length L3 can be written as:\nδL1 is the path variations due to energy variation just after the collision and δL3 is the path variation because of variation of energy loss along the outward path.\nThe above equations can be solved assuming δx = 0 for the derivative dL_1/dE_2 and :\nIn elastic spectrometry, the term [S] is called as energy loss factor:\nFinally, the stopping cross section is defined by, where N is the atomic density of the target material.\nThe stopping cross-section factor is given by:\n\n## Depth resolution\nAn important parameter that characterizes recoil spectrometer is depth resolution.\nIt is defined as the ability of an analytical technique to detect a variation in atomic distribution as a function of depth.\nThe capability to separate in energy in the recoil system arising from small depth intervals.\nThe expression for depth resolution is given as:\nHere, δET is the total energy resolution of the system, and the expression in the denominator is the sum of the path integrals of initial, scattered and recoil ion beams.\n\n## Practical importance of depth resolution\nThe concept of depth resolution represents the ability of Recoil spectrometry to separate the energies of scattered particles that occurred at slightly different depths δRx is interpreted as an absolute limit for determining the concentration profile.\nFrom this point of view, a concentration profile separated by a depth interval of the order of magnitude of δRx would be undistinguishable in the spectrum, and obviously it is impossible to gain accuracy better than δRx to assign depth profile.\nIn particular the fact that the signals corresponding to features of the concentration profile separated by less than δRx strongly overlap in the spectrum.\nA finite final depth resolution resulting from both theoretical and experimental limitations has deviation from exact result when consider an ideal situation.\nFinal resolution is not coincide with theoretical evaluation such as the classical depth resolution δRx precisely because it results from three terms that escape from theoretical estimations:\n\n## Influence of energy broadening on a recoil spectrum\nStraggling is energy loss of particle in a dense medium is statistical in nature due to a large number of individual collisions between the particle and sample.\nThus the evolution of an initially mono energetic and mono directional beam leads to dispersion of energy and direction.\nThe resulting statistical energy distribution or deviation from initial energy is called energy straggling.\nEnergy straggling data are plotted as a function of depth in the material.\nThe energy distribution of straggling is divided into three domains depending on the ratio of ΔE i.e., ΔE /E where ΔE is the mean energy loss and E is the average energy of the particle along the trajectory.\nThe standard deviation Ω2B of the energy distribution is, where NZ2Δx is the number of electrons per unit area over the path length increment Δx.\nAn expression of energy for straggling was proposed by Symon in the region of 0.2 < ΔE/E ≤ 0.5.\nTschalar et al. derived a straggling function, where σ2(E) represents energy straggling per unit length (or) variance of energy loss distribution per unit length for particles of energy E, and E(x) is the mean energy at depth x. The Tschalar's expression is valid for nearly symmetrical energy loss spectra.\n# [Propagation of the energy straggling distribution through an Al foil for protons of 19.6 MeV with different distribution functions fB:Bohr,fS:Symon,fT:Tschalar | upload.wikimedia.org/wikipedia/commons/2/22/Energy///Straggling///Effect.png]\n\n## Mass resolution\nIn a similar way mass resolution is a parameter that characterizes the capability of recoil spectrometry to separate two signals arising from two neighboring elements in the target.\nThe difference in the energy δE2 of recoil atoms after collision when two types of atoms differ in their masses by a quantity δM2 is:\nMass resolution δMR (≡ δE2/ δM2).\nA main limitation of using low beam energies is the reduced mass resolution.\nThe energy separation of different masses is, in fact, directly proportional to the incident energy.\nThe mass resolution is limited by the relative E and velocity v.\nExpression for mass resolution is: ΔM = √(∂M/∂E.∆E)2 + √(∂M/∂v.∆v)2\nΔM = M(√((∆E)/E)2+√(2.∆v/v)2)\nHere, E is the [energy](https://bliptext.com/articles/energy), M is the [mass](https://bliptext.com/articles/mass), v is the [velocity](https://bliptext.com/articles/velocity) of the [particle beam](https://bliptext.com/articles/particle-beam), and ΔM is the reduced [mass](https://bliptext.com/articles/mass) difference.\n\n## Multiple scattering scheme in forward recoil spectrometry\nWhen an ion beam penetrating in to matter, ions undergo successive scattering events and deviates from original direction.\nThe beam of ions in initial stage are well collimated(single direction), but after passing through a thickness of Δx in a random medium their direction of light propagation certainly differs from normal direction.\nAs a result, both angular and lateral deviations from the initial direction can occur.\nThese two parameters are discussed below.\nHence, path length will be increased than expected causing fluctuations in ion beam.\nThis process is called multiple scattering, and it is statistical in nature due to the large number of collisions.\n# [Multiple scattering scheme where the ion beam is directed in the x direction. Lateral displacement perpendicular to the beam direction is ρ(y,z), and α is the total angular deviation after the penetrated depth x | upload.wikimedia.org/wikipedia/commons/b/b6/ERDA6.png]\n# [Propagation of multiple scattering angular distribution through matter. The half-width of the angular distribution is α1/2. There is a considerable difference between the shape of Gaussian peak (ideal condition) and angularly deviated peak. | upload.wikimedia.org/wikipedia/commons/3/35/Angular///deviation///MS.png]\n\n## Theory and experiment of multiple scattering phenomena\nIn the study of multiple scattering phenomenon angular distribution of a beam is important quantity for consideration.\nThe lateral distribution is closely related to the angular one but secondary to it, since lateral displacement is a consequence of angular divergence.\nLateral distribution represents the beam profile in the matter.\nboth lateral and angular Multiple scattering distributions are interdependent.\nThe analysis of multiple scattering was started by [Walther Bothe](https://bliptext.com/articles/walther-bothe) and [Gregor Wentzel](https://bliptext.com/articles/gregor-wentzel) in the early 1920s using well-known approximation of small angles.\nThe physics of energy straggling and multiple scattering was developed by Williams in 1929–1945.\nWilliams devised a theory, which consists of fitting the multiple scattering distribution as a Gaussian-like portion due to small scattering angles and the single collision tail due to the large angles.\nWilliam, E.J., studied beta particle straggling, Multiple scattering of fast electrons and alpha particles, and cloud curvature tracks due to scattering to explain Multiple scattering in different scenario and he proposed a mean projection deflection occurrence due to scattering.\nHis theory later extended to multiple scattering of alpha particles.\nGoudsmit and Saunderson provided a more complete treatment of multiple scattering, including large angles.\nFor large angles Goudsmit considered series of Legendre polynomials which are numerically evaluated for distribution of scattering.\nThe angular distribution from Coulomb scattering has been studied in by Molière in the 1940s and then by Marion and coworkers, who tabulated energy loss of charged particles in matter, multiple scattering of charged particles, range straggling of protons, deuterons and alpha particles, and equilibrium charge states of ions in solids and energies of elastically scattered particles.\nScott presents a complete review of basic theory, mathematical methods, as well as results and applications.\nA comparative development of multiple scattering at small angles was presented by Meyer, based on a classical calculation of single cross section.\nSigmund and Winterbon extended the Meyer's calculation to a more general case.\nMarwick and Sigmund carried out development on lateral spreading by multiple scattering, which resulted in a simple scaling relation with the angular distribution.\n\n# Applications\nERDA has applications in the areas of polymer science, semiconductor materials, electronics, and thin film characterization.\nERDA is widely used in polymer science.\nThis is because polymers are hydrogen-rich materials which can be easily studied by LI-ERDA.\nOne can examine surface properties of polymers, polymer blends and evolution of polymer composition induced by irradiation.\nHI-ERDA can also be used in the field of new materials processed for microelectronics and opto-electronic applications.\nMoreover, elemental analysis and depth profiling in thin film can also be performed using ERDA.\nERDA is also used to characterize hydrogen transport near interfaces induced by corrosion and wear.\nCharacterizing how polymer molecules behave at interfaces between incompatible polymers and at interfaces with inorganic solid substances is crucial to our fundamental understanding and for improving the performance of polymers in applications.\nFor example, the adhesion of two polymers strongly depends on the interactions occurring at the interface between polymer segments.\nLI-ERDA is one of the most attractive methods for investigating these aspects of polymer science quantitatively.\nElectronic devices are usually composed of sequential thin layers made up of oxides, nitrides, silicides, metals, polymers, or doped semiconductor–based media coated on a single-crystalline substrate (Si, Ge or GaAs).\nThese structures can be studied by HI-ERDA.\nThis technique has one major advantage over other methods.\nThe profile of impurities can be found in a one-shot measurement at a constant incident energy.\nMoreover, this technique offers an opportunity to study the density profiles of hydrogen, carbon and oxygen in various materials, as well as the absolute hydrogen, carbon and oxygen content.","revisionCount":0},"hasMore":false,"nextCursor":null}