Stopping Power Measurements Research Articles

High energy heavy ions in materials characterization at NSC Pelletron

High energy heavy ions available at 15 MV Pelletron at Nuclear Science Centre (NSC), Delhi, are being extensively used for research in materials science. The energetic heavy ions play an important role in materials characterization and in materials modification. High energy ion backscattering and elastic recoil detection (ERD) provide identification as well as depth profiling of high Z and low Z elements, respectively. Elastic recoils produced by the impingement of energetic ions have vital role in characterization of materials by the technique of ERD. A variety of high energy ions have been used at the NSC Pelletron for the analysis of different types of materials. H depth profiling in diamond-like carbon (DLC) and diamond films provide valuable information on the growth of these films and their dependence on deposition conditions. The developments for ion beam analysis at NSC are briefly outlined. Ion beam techniques provide an excellent opportunity for in situ monitoring of the modification of materials in some specific cases. Elastic recoils produced by heavy ions have also been used for stopping power measurements.

Bethe-Bloch stopping-power parameters for GaAs and ZnSe

If one is to calculate the stopping power for a specified projectile–target combination within the energy interval of applicability of modified Bethe–Bloch theory, values of several parameters appearing in the formulation must be ascertained. In the past, the author has established such values for numerous target materials through fits of stopping-power measurements with modified Bethe–Bloch theory. However, the semiconductor materials ZnSe and GaAs have not yet been thus characterized. A set of very recent measurements of the stopping powers of each compound for low-energy protons and alpha particles, reported by members of the Helsinki group, has been analyzed in order to remedy this dearth of parameter values. Moreover, some corresponding measurements for 7Li ions traversing ZnSe have been analyzed for the purpose of obtaining the value of a single effective charge parameter. Results of these studies are reasonably consistent with expectations, and values are recommended for the mean excitation energy and the Barkas-effect parameter for each compound. © 1998 John Wiley & Sons, Inc. Int J Quant Chem 70: 919–924, 1998

MeV heavy ion stopping power measurements using NSC Pelletron

Surface modification and characterization are being undertaken increasingly using the MeV heavy ion beams. Interpretation of such data requires reliable and precise values of stopping powers involved. Sufficient data exists above 2.5 MeV/n but less information is available at lower energies. The present series of experiments report the results of stopping power measurements for various ion species between Z = 8 and 29, covering the energy range ∼ 0.2–1.0 MeV/n in carbon. The elastic recoil ion detection system provided a unique method to generate variety of variable energy ion species utilising a fixed energy heavy ( Z = 47, 53, 79) projectile beam from the NSC Delhi 15 MV Pelletron facility. The data has been compared with available theoretical and semiempirical fitting treatments. Relative merits and demerits of these formulations have been highlighted. Future projections to our line of investigation have been indicated in the light of some recent innovative theoretical predictions.

Stopping power determinations by the transmission technique

Details of the experimental procedures related to the transmission technique are described. Parameters affecting the stopping power measurements by this technique are pointed out and discussed. For example, the importance of an accurate foil areal density determination is emphasized, and methods for the determination are introduced. The accuracy and possible error sources of the stopping power values are reviewed. Advantages as well as limitations of the method are discussed. As an example the stopping powers of 600–3200 keV 4He ions in polyimide (Kapton) have been determined. The experimental results are compared with the predictions of widely used semiempirical models.

Stopping power measurements of 1H, 4He and 14N in Si in the energy range of 0.02–1 MeV/amu

In order to obtain precise stopping power data for ion beam analysis of surface layers we have used high-quality target films produced by advanced molecular beam epitaxy. Thus, multi-δ-layers of Sb or Ge in an amorphous Si matrix have been tailored to define a set of targets for energy-loss measurements in backscattering geometry. The stopping powers of ions of 1H, 4He and 14N in the energy range of 0.02–1 MeV/amu have been obtained with a precision as high as about 1% with an additional systematic error of similar magnitude. Discrepancies of up to 10% with recent experimental literature data will be discussed. Scaling laws for electronic stopping have been checked by comparing theoretical predictions with our measurements. Very close agreement with the experimental data is obtained, if the projectile charge is described within the formalism given by Betz et al.

Electronic stopping power of 〈100〉 axial-channelled He ions in Si crystals

Measurements of the electronic stopping power of He 2+ ions along the 〈100〉 direction in Si crystal with energies ranging between 200 keV and 4.5 MeV are presented. The Rutherford backscattering technique has been used with SIMOX samples consisting of a Si single-crystal layer on top of a buried layer of 500 nm SiO 2 built into a Si 〈100〉 wafer.

Charge-state distributions of chlorine ions interacting with cold gas and with fully ionized plasma

Charge transfer of 4.3 MeV/u chlorine ions passing through a discharge plasma target is used as a probe to determine the plasma density and the ratio of impurities inside the plasma column. Charge-state distributions of 2 MeV/u chlorine ions passing through the plasma are then measured and compared to corresponding measurements in the cold gas. Stopping power measurements are also performed in both cases.

Mean excitation energies of Al 2O 3 and SiO 2 extracted from alpha particle stopping power measurements

Previously reported measurements of the stopping powers of Al 2O 3 and SiO 2 for 0.2–2 MeV and 5.48 MeV alpha particles have been analyzed for extraction of values of parameters from modified Bethe-Bloch theory. Two-parameter fits were conducted for values of mean excitation energy and one of two available Barkas-effect parameters. Resulting mean excitation energies lay below additivity-based predictions, whereas Barkas-effect parameters essentially coincided with values prescribed by the formalism.

Electronic stopping power measurements using secondary ion beams

Stopping powers for O and Ti ions were measured in a mylar foil 2 μm thick. The choice of the foil was made because of its wide application in elastic recoil detection analysis experiments and thin windows for gaseous detectors. As a new approach, recoil ions produced in heavy ion scattering were utilized for (d E /d x ) measurements. The energy loss of the recoil ions was measured by keeping two surface barrier detectors, with and without the mylar foil, at the same recoil angle, with their gains matched with pulsers before the experiment. Different energies of the secondary beam could be selected by changing the detection angle. Also, various species of secondary ions could be obtained by merely changing the target. We demonstrate here our new technique and some of the results obtained.

Measurement of the stopping powers of He and H ions near their maximum values by backscattering from a multilayer target

Rutherford backscattering at two different angles on a multilayer target is used to measure the stopping powers of light ions in matter. This method has the advantage that no spectrum width determination is needed so that any error related to this procedure is avoided. Moreover it is almost insensitive to most of the sources of errors suffered by transmission stopping power measurements. This new method has been used to measure the stopping powers of magnesium for helium ions ( 3He and 4He) from 100 to 350 keV with an accuracy better than 10%.

Charge state analysis of heavy ions after penetration of uncleaned and sputter cleaned conducting surfaces

The evolution of the charge state distribution of fast ions inside a solid is of basic interest in various research fields as stopping power measurements etc. The existing models for the charge state evolution differ in the treatment of the projectile-exit-surface interaction, which has a strong influence on the final charge state distributions. We measured the charge state distributions for C +, N +, and O + (30 ≤ E/ M ≤ 130 keV u ) impact on thin C, Cu, and Au foils, where the surface properties were modified by sputter cleaning. The measurements show a pronounced change of the mean projectile charge state to lower values in the case of sputter cleaned surfaces. This result underlines the importance of the projectile-surface interaction for the generation of the outcoming charge state distribution.

RBS and ERD analysis using lithium ions

The advantages of using ions heavier than α-particles for RBS analysis with a noticeable improvement of mass resolution, sensitivity and depth resolution in RBS are well known since the 1970s. The applications are still very limited because of the uncertainty in stopping power, complicated detection techniques and the lesser accessibility of heavy ion beams. In this paper, we emphasize the applications of lithium ions for RBS and channeling analysis, based on the analysis of recent stopping power measurements and its systematic, the development of a silicon surface barrier detector for heavy ions as well as the accessibility of lithium ions at small accelerators. Examples of RBS analysis of high temperature superconductors and other thin films will be presented, as well as ERD analysis of hydrogen in CVD samples using lithium ions.

The stopping power of 1–3 MeV hydrogen and helium in polyimide

The stopping power for hydrogen (1H)and helium (4He) in thermally imidized pyromellitic dianhydride-oxydianaline polyimide was measured using the thin-film backscattering method. As the incident energy increased from 1 to 3 MeV, the hydrogen stopping power decreased from 4.2 to 1.8 eV/(1015 atoms/cm2), whereas the helium stopping power decreased from 33.6 to 18.6 eV/(1015 atoms/cm2). These results were in excellent agreement with values predicted by either Bragg’s rule or the cores-and-bonds model. This work demonstrates the advantages of using spun-cast thin (<3 μm) polymer films supported on silicon wafers for stopping power measurements. Prior measurements of the stopping power relied on transmitting the ion beam through relatively thick (∼10 μm) free-standing films where the uncertainty in the film thickness often introduced significant systematic errors. With polymer films supported on silicon, ellipsometry can be used to accurately determine film thickness.

9. Description of Methods for Stopping-Power Measurements

9. Description of Methods for Stopping-Power Measurements

9. Description of Methods for Stopping-Power Measurements

9. Description of Methods for Stopping-Power Measurements

12C, 15N and 16O stopping powers between 0.5 and 3.4 MeV in solid targets

Stopping powers for 12C, 15N and 16O ions in Al, Cu, Ag and Au targets in the energy range from 0.5 to 3.4 MeV have been measured using transmission and backscattering techniques. The results are compared to available experimental data and to values in Ziegler's compilation. To avoid the limitation due to the magnitude of the kinematic factor, a procedure is described for extracting the stopping power from a backscattering spectrum. The stopping power measurements are compared to LSS predictions and fitted to the power function S e = η( ν ν 0 ) μ .

Binary-collision-cascade simulation of hyperchanneling of high-energy helium ions in silicon.

The binary-collision-cascade code, marlowe, was modified to include an accurate electron-density distribution of silicon and an electronic-energy-loss (EEL) model that is suitable for high-energy studies. The EEL model is based on a theory that relates the energy loss and local valence-electron density via a phenomenological band-structure model. The enhanced marlowe is applied to simulate transmission spectra due to the hyperchanneling of high-energy \ensuremath{\alpha} particles in silicon along the [110] and [111] directions. The transmission spectra due to a ``random'' incident direction was also studied. The channeling peak positions in the energy-transmission spectra agree fairly well with the measurements. The effects of transverse-energy distribution of the incident beam, the effects of electron-multiple scattering and energy-loss straggling, and the effects of nonuniformity of the electron density on the transmission spectra are discussed. The electron-density effects reproduce the stopping-power measurements based on the peak and leading edge of the energy spectrum. The importance of energy-loss straggling, of core-electron contribution to the energy loss at high energies, and of charge-state effects at intermediate energies is also discussed.

Energy loss of 70 MeV protons in organic polymers.

Using Bragg ionization curves, measurements of the average stopping power of organic polymers relative to that of water have been made for 70 MeV protons losing about 30 MeV in the absorbers. By comparing calculated curves to experimental ones, I-values for the polymers were obtained, relative to the I-value for A1. For repeated measurements with A-150 tissue equivalent plastic, the standard deviation of the ratio of the average stopping powers q was +/- 0.06%. It was found that q depends on the year of production of A-150. For other absorbers, the uncertainty of q is estimated to be +/- 0.2%, with a corresponding uncertainty of the I-values of +/- 1.5%. Most measured I-values exceed values calculated with the additivity rule by about 10%. During the course of the measurements, fluctuations of the proton energy from the cyclotron of up to 0.8% were found.

Importance of the high-velocity projectile-z3 term in modified Bethe–Bloch stopping power theory

Modified Bethe–Bloch theory of stopping power provides for inclusion of several parameters in the Bethe–Bloch formula. These parameters include the target mean excitation energy and those associated with target shell corections, with the low-velocity projectile-z3 (Barkas effect) term, and with an effective charge factor. Fits of stopping power measurements permit extraction of one or two (and sometimes three) parameter values while the remaining parameters are assigned fixed values based on other studies. Omission of one or more of the required correction terms or factors induces distortions in the values of the extracted parameters. A recent development has been omission of the (parameter independent) high-velocity Barkas effect term in an analysis where projectile energies were sufficiently high to necessitate inclusion of this term. The importance of incorporating all pertinent terms and factors into the modified Bethe–Bloch formula is strenuously emphasized.

Measurement of collision stopping powers of graphite, aluminium and copper for 10 and 20 MeV electrons

A technique is presented for measuring collision stopping powers of 5 to 50 MeV electrons. Measurements were made at 10 and 20 MeV with slabs of C, Al and Cu ranging in thickness from 0.5 to 1.5 g cm-2. A large, nearly totally-absorbing NaI detector was used to measure the energy distribution in the beam with and without an intervening slab. Detailed Monte Carlo simulations of the full experimental geometry were done. The offset between the simulated and experimental distributions was determined by precise fitting and was used to obtain the stopping power. Reduction of the drift and improved energy linearity should allow stopping power measurements with an accuracy of 1% or better.