Z.E. Switkowski

Ion‐beam‐induced atomic mixing

Calculations based on the diffusion model are presented of atomic mixing by ion bombardment. This mixing is assumed to have its basis, as does sputtering, in the collision cascades generated by the primary beam. Sharp interfaces within a target are seen to be smoothed by ion bombardment. Mixing may place fundamental limits on the resolution of ion microprobes.

On the sputtering of binary compounds

A simple physical model is presented to describe some aspects of the sputtering of compound targets. In particular, expressions are developed for the partial sputtering yields for binary systems in terms of the elemental sputtering rates, the stoichiometric concentrations, and surface binding energy. The partial yields depend nonlinearly on the bulk target concentrations. Comparison of the theoretical predictions with the data on sputtering of PtSi, NiSi, and Cu3Au indicates that the general features are well described.

Gamma-ray yields from 12C + 13C reactions near and below the coulomb barrier☆

Abstract The γ-ray yields from low-lying transitions in heavy residual nuclei produced in the 12 C+ 13 C reaction have been measured from E c.m. = 3.1 to 11.9 MeV using a Ge(Li) detector. Total cross sections for compound nucleus formation were deduced from the experimental data with the aid of the Hauser-Feshbach model. Several independent checks on this procedure are described. These tests verify the assumptions made in the analyses of this reaction and suggest that the deduced cross sections have an absolute uncertainty of ±30 %. The present experimental results for the 12 C+ 13 C reaction are qualitatively very different from those for the 12 C+ 12 C reaction and do not provide any striking evidence for either broad singleparticle resonances in the total reaction cross section or for narrow non-statistical (quasimolecular) resonances in summed cross sections for proton and for α-particle emission to bound states of 24 Na and 21 Ne, respectively. The predictions of several optical models employing attractive nuclear potentials are compared to the data. None is successful in reproducing the measured cross sections over the entire range of bombarding energy. The predictions at low energies depend sensitively on the shape of the potential a few fm inside the region of the nuclear surface. A narrow, rapidly varying energy dependence of the γ-ray yields is observed, with a peak-to-valley ratio of typically 1.1. However, a statistical analysis shows that these fluctuations, and those observed in recent charged particle measurements of α-particle yields, are reasonably consistent with those expected from the formation and decay of strongly overlapping levels in the compound nucleus. Finally, several observations are made on the validity of certain approximations often made in statistical analyses of heavy-ion reactions.

IWB analysis of scattering and fusion cross sections for the 12C+12C, 13C+16O and 16O+16O reactions for energies near and below the Coulomb barrier

Abstract Calculations are presented of the elastic scattering and fusion cross sections for the astrophysically interesting reactions 12 C+ 12 C, 12 C+ 16 O and 16 O+ 16 O. The calculations are performed using the incoming wave boundary condition (IWB) and a real ion-ion interaction potential. The results are compared with the available experimental data for the energy region near and below the Coulomb barrier. With values of two adjustable potential parameters (the radial position of the l = 0 barrier and the diffuseness) determined by fitting elastic scattering data, good agreement is obtained for the average energy dependence of the 12 C+ 12 C and 12 C+ 16 O fusion cross sections. In the case of 16 O+ 16 O, both the calculated absolute magnitude and the energy dependence of the fusion cross section are inconsistent with the data and this discrepancy is discussed.

Measurement of fusion cross sections for 14N + 14N and 12C + 16O at low energles

Abstract Total fusion cross sections for 14 N + 14 N and 12 C + 16 O have been measured for bombarding energies extending from the Coulomb barrier down to E c.m. ≈ 4.6 MeV. Absolute cross sections were deduced from the yields of individual γ-ray transitions observed with a Ge(Li) detector and a statistical-model calculation of the decay modes of the compound nucleus. This technique was checked by a detailed comparison of the present results for 12 C + 16 O with the earlier results of Patterson et al. for charged particle decay, with the study of Spinka and Winkler of activation following neutron emission, and with the measurements of Cujec and Barnes on total γ-ray emission. The present method for determination of the total fusion cross section was found to compare very favourably with other methods. The gross energy dependence of the fusion cross section for 14 N + 14 N was observed to be similar to that of 12 C + 16 O. In contrast to 12 C + 16 O, however, the cross section does not exhibit any resonant-like structure. A simple barrier-penetration model employing the incoming-wave boundary condition is able to reproduce the observed fusion cross section for 14 N + 14 N. An analysis of the transfer reaction 14 N( 14 N, 13 N) 15 N at sub-barrier energies is also presented and a comparison is made between the direct and compound reactions.

14N fusion with 13C and 16O at sub-barrier energies

Abstract Total fusion cross sections for the 14N+12C and 14N+16O reactions have been measured in the c.m. energy ranges 3.6–9.2 MeV and 5.6–12.6 MeV, respectively. Cross sections are also reported for important individual exit channels which were studied by observing discrete γ-ray transitions in the evaporation residues with a Ge(Li) detector. Excitation functions reveal no evidence of intermediate structure in these reactions. The fusion cross sections for 14N induced reactions on 12C, 14N and 16O are compared with an IWB calculation using recently published semi-empirical parameters for the real ion-ion interaction potential. Such a comparison supports the view that low-energy fusion studies may be sensitive probes of the nuclear potential in the interior of the interaction barrier.

9Be + 12C, 9Be + 16O,9Be + 19F reactions at energies below the barrier

The ^(16)O + ^9Be reactions have been studied from E_(c.m.) = 2.0 MeV to 5.1 MeV, an energy near the top of the Coulomb barrier. The cross section for the neutron transfer reaction ^9Be(^(16)O,^(17)O^∗ (0.87 MeV))^8Be has been measured over this range by detecting the prompt 0.87 MeV γ-rays. The total fusion cross section has been determined from E_(c.m.) = 2.8 to 5.1 MeV by observing individual γ-ray transitions in the evaporation residues with a Ge(Li) detector, and then summing the separate yields. Direct processes are found to dominate the reaction yield below E_(c.m.) = 4 MeV. A comparison of the energy dependence of the fusion cross section for this reaction and the ^(12)C + ^(13)C reaction, which proceeds via the formation of the same compound nucleus, ^(25)Mg, reveals differences at sub-barrier energies. Optical model and incoming-wave boundary condition calculations are presented. Data have also been obtained for the near optimum Q-value neutron-transfer reactions ^9Be(^(12)C, ^(13)C^∗)^8Be and ^9Be(^(19)F, ^(20)F)^8Be, and these are discussed in terms of a simple model of sub-barrier direct reactions.

Neutron branching in the reaction 12C + 12C

Abstract The cross section for the reaction 12 C( 12 C, n) 23 Mg has been measured in the energy range E c . m . = 3.54−4.94 MeV by counting the delayed γ-rays from 23 Mg decays (half-life = 11.57 sec), and a theoretical model has been employed to extrapolate the results to threshold ( E c . m = 2.60 MeV). By combining these results with previous measurements of the reactions 12 C( 12 C, p) 23 Na and 12 C( 12 C, α) 20 Ne, the neutron branching ratio in the energy interval from threshold to 8 MeV is deduced, and a thermal average is computed that should be valid for use in astrophysical environments characterized by temperatures in the range (0.5–5) × 10 9 °K. The neutron branching at temperatures appropriate to hydrostatic carbon burning in stars ( T ≈ 109 °K) is found to be much smaller than previously estimated.

A new technique for the measurement of sputtering yields

Abstract We have measured the yields of 90 keV 40Ar+ and V4He+ sputtering of Mo and V samples by the use of a new radio-tracer technique. This technique involves activating the samples by high-energy charged-particle irradiation before sputtering, and using conventional γ-ray counting methods to analyze the material subsequently sputtered onto collector foils. We have also measured angular distributions of the sputtered material, and compared these results and our total sputtering yields with the predictions of Sigmunds sputtering theory. Further comparisons between our radiotracer results and those obtained for 40Ar+ sputtering of unactivated Mo and V samples, determined from elastic backscattering measurements using 12 MeV 16O ions, show that the techniques give consistent results.

14C production by the reaction 11B(α, p)14C☆

To estimate the production rate of long-lived radio-nuclides during the controlled thermonuclear burning of exotic fuels, cross sections for the ^(11)B(α, p)^(14)C reaction have been measured for α-particle energies in the range 1.43 to 2.94 MeV. Angular distributions have been measured at 20 keV intervals within this energy range. Five resonances have been identified corresponding to states in ^(15)N at excitation energies from 12 to 13 MeV. A discussion of possible spin, parity assignments is presented.