J.A. Davies

How accurate are absolute rutherford backscattering yields

High precision backscattering assemblies for absolute yield measurements have been built and calibrated to ±1% on three different MeV ion accelerators. The Bi-content in a set of Bi-implanted silicon standards (from the recent Karlsruhe ‘round-robin’ project) has been accurately measured in each of these assemblies: the resulting value is 4.77(±0.07) × 1015 Bi atoms/cm2. Unexpectedly large deviations from the Rutherford scattering law were observed, viz. approximately 4% for 1.0 MeV He on Bi. The magnitude of this screening correction has been measured over a wide range of Z1/E0 and is found to be in good agreement with theoretical estimates. Several thin gold targets, calibrated by Matteson et al.6) to ±1% by comparison with an NBS standard, have also been measured. Our thickness values are (4±1)% lower than the Matteson values, but no explanation has yet been found for this discrepancy.

The pt(100) (5 × 20) ⇋ (1 × 1) phase transition: A study by Rutherford backscattering, nuclear microanalysis, LEED and thermal desorption spectroscopy

Abstract The reconstruction exhibited by clean Pt(100) surfaces [(5 × 20) LEED pattern] is removed by the adsorption of CO. Rutherford backscattering (RBS) indicates that 1.65 ± 0.05 × 10 15 Pt atoms cm −2 move into registry with the bulk upon adsorption of 6.4 ± 0.4 × 10 14 CO molecules cm −2 ( θ = 0.50 ± 0.03 monolayers). The data indicate that some atoms in the second and perhaps even subsequent layers must be displaced by ≳0.01 nm in the reconstructed surface. By contrast, only 1.3 ± 0.1 × 10 15 Pt atoms cm −2 move back into registry upon adsorption of H 2 or D 2 , and the LEED pattern also indicates that residual reconstruction remains. The stability of the CO-covered, H-covered and “almost clean” (1 × 1) surfaces (the latter prepared by NO and H 2 treatments with a residual H-coverage of ∼1 × 10 14 H atoms cm −2 ) was investigated by RBS. The CO-covered surface starts to reconstruct only when the CO coverage drops below 0.5 monolayers ( T ≳ 450 K) while the H-covered surface (produced by adsorption on the (5 × 20) surface) reconstructs rapidly at T ≳ 350 K, by which temperature the adsorbed hydrogen coverage drops below ∼0.2 monolayers. The “almost clean” surface reconstructs at T ≳ 390 K and the data indicate that the process exhibits an activation energy of 88 ± 17 kJ mol −1 . The absolute coverages of CO and D were determined by nuclear microanalysis (NMA) and excellent agreement was achieved between the LEED and NMA data. The saturation CO coverage was found to be 0.77 ± 0.03 monolayers, consistent with the observed c(4 × 2) LEED pattern. Deuterium (and hence hydrogen) coverages of 1.54 ± 0.1 × 10 15 D (H) atoms cm −2 ( θ = 1.20 ± 0.08) were found at saturation at ∼150 K and the hydrogen adsorbed on the (1 × 1) surface was more strongly bound than that resulting from adsorption on the (5 × 20) surface.

Absolute coverage and isostemc heat of adsorption of deuterium on Pt(111) studied by nuclear microanalysis

Abstract The absolute coverage (θ) of deuterium adsorbed on Pt(111) in the ranges 180 T −6 P −2 Pa D 2 has been determined by nuclear microanalysis using the D( 3 He, p) 4 He reaction. From these data, the isosteric heat of adsorption ( E a ) has been determined to be 67 ± 7 kJ mol −1 at θ ≲ 0.3. This heat of adsorption yields values of the pre-exponential for desorption (10 −5 to 10 −2 cm 2 atom −1 s −1 ) that lie much closer to the normal range for a second order process than those determined from previous isosteric heat measurements. The E a versus θ relationship indicates that the adsorbed D atoms are mobile and that there is a repulsive interaction of 6–8 kJ mol −1 at nearest neighbour distances. At 300 K the coverage decreases to ≲ 0.05 monolayer (≲ 8 × 10 13 D atoms cm −2 ) as P → 0, apparently invalidating a recent model of site exchange in the adsorbed layer.

Interaction of O2 with Pt(100). I: Equilibrium measurements

Abstract Two newly discovered phases on the Pt(100) surface produced by the adsorption of oxygen have been investigated using Rutherford baekscattering (RBS), nuclear microanalysis (NMA), work function changes (Δφ) and LEED. One phase is associated with the oxygensaturated surface (0.63 ± 0.03 monolayers0.81 × 10 15 O atoms cm −2 ), where a very complex LEED pattern is observed; the other is observed at an average coverage of 0.44 ± 0.05 monolayers and gives rise to a (3 × 1) LEED pattern (when observed at room temperature). For both surfaces, RBS measurements indicate large (⩾ 0.025 nm) Pt atom displacements. Also discussed is a new method for preparing the “clean” (1 × 1)-Pt(100) surface without the need for NO adsorption/decomposition.

Evidence for a thermal spike mechanism in the erosion of frozen xenon

Abstract Recent measurements of the sputtering of frozen gases by MeV ions exhibit yields that are several orders of magnitude larger than predicted by collision cascade theory. Of the models considered, only a “thermal spike” model is consistent with our experimental observations. The various experimental parameters studied include the energy (0.3–2.0 MeV), atomic number (H+. He+. N+ and Ar+). angle of incidence and current density of the incident ion beam. the thermal conductivity of the substrate, and the temperature and thickness of the frozen film. Calculations for 1.0 MeV He + in Xe. based on a thermal spike mechanism, indicate that a localized cylindrical region around each incident ion track is raised to a temperature sufficiently high to evaporate a significant quantity of the frozen gas film.

Absolute coverages and hysteresis phenomena associated with the CO‐induced Pt(100) hex⇄(1×1) phase transition

Temperature hysteresis in the hex⇄(1×1) phase transition in the Pt(100)–CO system has been investigated using Rutherford backscattering spectroscopy, nuclear microanalysis, work function measurements, LEED, and thermal desorption spectroscopy. The onset of the hex→(1×1) transition occurs at an average surface coverage of 0.08±0.05 monolayers while the onset of the (1×1)→hex transition occurs at 0.25±0.05 monolayers. The results are consistent with a recently proposed model for the phase transition.

A cryogenic system for low-temperature and clean-environment channeling measurements

Abstract An experimental apparatus is described for studying channeling in a clean environment at temperatures as low as 25 K. The target is mounted on a standard 2-axis goniometer and is almost completely surrounded by a large cryoshield at 20 K. This cryoshield reduces the build-up rate of carbon and oxygen contamination on the surface to less than 5 × 1014 molecules/cm2. For low-temperature measurements, the target is connected by a length of copper braid to the refrigerator system. Some preliminary measurements at 25 K and at 300 K are presented.

Intercomparison of absolute standards for RBS studies

Abstract A Harwell Series I Bi-implanted standard calibrated in Chalk River has been compared with a vacuum-deposited thin Ta standard calibrated in Paris. The standards agree to within 1–2%; the “best” value for the Bi standard is now (4.83±0.05)×1015 Bi/cm2 .

A study of the phase transition and relaxation of a Pt(100) surface by MeV ion backscattering and channeling

Abstract MeV He + ion backscattering has been used to show that the phase transition on a Pt(100) surface involves the lateral displacement of an entire plane of atoms. It is further shown that the outward relaxation of the unreconstructed surface plane is negligible, i.e. 0.5 ± 0.5% of thebulk interplanar spacing.

On the electromagnetic separation method of preparing radioactive sources for precision β-spectroscopy

Abstract With the recently developed techniques of high-resolution β-spectroscopy, the precision that can be realized is limited by source quality rather than the performance of the β-spectrometer. It is therefore of interest to consider in detail how source quality might be improved. Throughout the present work an inert gas has been used so as to study the problems of source preparation under the most difficult and general conditions. First, the general effect of source quality is examined experimentally by studying the shapes of the K 54.96 and the K 188.4 conversion lines from a wide variety of Xe 125 sources using momentum-resolution settings of ∽ 0.05% and ∽ 0.1%. Techniques for decelerating ions and thus minimizing source thickness are then described, followed by experimental measurements of the ranges of Na 24 , K 42 , Rb 86 , Cs 137 , and Xe 133 ions in Al and of Xe 133 ions in Au. Experiments relating to the diffusion and to the long-term retention of inert-gas atoms in a variety of metal targets are described. Finally, a number of effects which tend to limit source strength are considered, including beam current, beam scattering and neutralization, target collection, source thickness, and sticking factors.