W. E. Meyerhof

Positron - hydrogen scattering in the vicinity of the positronium formation threshold

An analysis is made of the behaviour of the elastic scattering and positronium formation cross sections for s-, p- and d-wave low-energy positron - hydrogen scattering in the vicinity of the positronium formation threshold. The accurate cross sections, obtained using the Kohn variational method with elaborate trial functions containing many Hylleraas correlation functions, reveal interesting threshold structures which arise from the coupling between the positronium formation and the elastic scattering channels. Wigners R-matrix method is used to provide a qualitative analysis of these structures. One of the most interesting features of the present results is the presence of a `rounded cusp in the s-wave elastic scattering cross section at the threshold itself, as predicted by Wigners threshold theory. Remarkably good fits to the partial-wave cross sections for elastic scattering, positronium formation and Ps - p elastic scattering are obtained using a five-parameter fit for each partial wave.

Threshold and channel-coupling effects in positron elastic scattering from noble gases

We examine angular-distribution measurements of Kauppila et al and others of positrons elastically scattered from noble-gas targets at energies near the positronium (Ps) threshold. We use an R-matrix theory approach in which the R-matrix elements are used as fitting parameters. An attempt is made to find a set of parameters also consistent with the energy dependence of the (angle-integrated) elastic and Ps-formation cross sections. We show that close to threshold, both below and above, small differences between the experimental angular distributions and calculations by McEachran et al of differential cross sections, which neglect the Ps channel, are due to coupling to the Ps channel. In accordance with calculations of McAlinden and Walters, we find evidence for the dominance of channel coupling near the Ps threshold between the (incoming, outgoing) orbital angular momentum channels (1,0) and (2,1), respectively.

PbK-shell ionization probability in high-energy Xe+Pb collisions

Probabilities for PbK-shell ionization in 5.9-, 7.2-, and 8.5-MeV/a.m.u. Xe+Pb collisions for impact parameters between 10 and 60 fm are reported. The measured probabilities are compared with theories of 1sσ-vacancy production.

Impact-parameter dependence of 1 sσ vacancy production in 4.6-MeV/a.m.u. Xe + Pb collisions

The probability of forming 1sσ vacancies in collisions with impact parameters b ≲ 60 fm was measured using the x-ray-scattered-particle-coincidence method. The probability falls off monotonically with b, in qualitative agreement with calculations of Soff et al.

Optical spectroscopy of highly ionized neon

Abstract Optical spectroscopy studies have been performed on slow, highly charged neon recoil ions. Rydberg states were populated by electron capture from sodium. These spectra reveal the influence of core interactions on the alignment of states populated by electron capture processes.

K-Ionization probability across the 460-keV12C(p, p0) resonance

We have remeasured theK-shell vacancy production probabilityPK across the 460-keV12C(p, p0) resonance, previously investigated by Duinker et al. We are unable to confirm the large (∼70 %) variation ofPK across the resonance, found by Duinker et al. for protons elastically scattered at 125°. Our values for the ratioPK(142°)/PK(16°) are independent (within±20%) of the proton bombarding energy over the resonance region, in agreement with theoretical predictions by Blair and Anholt, Feagin and Kocbach, McVoy and Weidenmüller. Our average value for this ratio, equal to 0.88 ±0.06, agrees with the value 0.88 calculated by Amundsen and Aashamar.

Molecular-orbital X-rays and their anisotropies in heavy-ion collisions in solids and gases

Molecular-orbital spectra and anisotropies were measured in Cl+NaCl, Cl+KCl, Cl +Ti, and Cl+Ar collisions at bombarding energies between 0.5 and 2.6MeV/a.m.u. The results suggest that at low bombarding energies MO x-rays in the KCl and NaCl targets are partly produced by a two-collision process. Qualitative differences between the anisotropies of gas- and solid-target induced MO x-ray spectra underline this result. Also, new information on the bombarding-energy dependency of anisotropies at higher projectile velocities has been obtained, confirming previous observations in low-Z systems.

X-ray study of the deep inelastic reactions 1,160-MeV136Xe+Pb and136Xe+232Th

The spectra ofK x-rays emitted by the target-like reaction products in the deep inelastic collisions 1,160-MeV136Xe+Pb and136Xe+232Th were investigated. Target-like x-ray emission probabilities per projectile-like and per target-like product were determined and compared with theoretical predictions of atomic vacancy production in the higher-Z particle (1s }-vacancy production). It is shown that a large fraction of the x-rays is produced by internal conversion ofγ-rays in the higher-Z reaction product. Measurement of target-like x-rays in coincidence with projectile-like and with target-like reaction products allowed the determination of the average survival probability against fission and of theZ-distribution of the highly excited target-like nuclei as a function of theQ value (total kinetic energy loss). The observed average survival probabilities agree qualitatively with theoretical predictions of Wilcke et al. Nuclearreaction-time effects on the 1s σ-vacancy production in these collisions could not be demonstrated with any certainty.

Impact parameter dependence of Ag K-vacancy production by 60 MeV I ions

The probability of forming Ag K vacancies in 60 MeV I collisions with centre-of-mass scattering angles theta cm between 20 and 144 degrees was measured. The probability varies approximately as sin2 theta cm, as predicted for 2p sigma -2p pi rotational coupling but the magnitude of the probability is much higher than expected.

Molecular-orbital X-rays from 1 GeV 208Pb + 208Pb and 209Bi + 209Bi collisions

Abstract We have measured the spectra of continuum X-rays above the characteristic K lines for 4.5 to 4.8 MeV/N 20882Pb → 20882Pb, Pb → Bi, Bi → Pb and Bi → Bi collisions. Above ≈400 keV X-ray energy the spectral shape and intensity agree roughly with calculations of Kirsch et al. for the 1sσ molecular-orbital (MO) X-ray spectrum from Pb-Pb. Deviations from the theory below ≈400 keV suggest transitions to other MOs.