James R. Macdonald

Efficient, high‐spectral‐resolution Bragg crystal x‐ray polarimeter

Utilizing Bragg scattering near 90 degrees , an x-ray polarimeter is described that has high spectral resolution and reasonable efficiency. The latter is achieved by using a position-sensitive proportional counter to discriminate spatially resolved x rays dispersed by the crystal. This instrument has been used to measure the linear polarization of lines from one- and two-electron oxygen ions. For this study, aligned initial states have been populated in oxygen beams by various excitation processes in foil, gas, and thick solid targets. In spite of the small probability for the production and radiative decay of few-electron excited states in the presence of intense background radiation, the instrument resolution and efficiency are sufficient to permit polarization measurements.

Experimental differential cross section for electron capture from helium by 293 keV protons

The differential cross section has been measured for scattering angles less than 0.07 degrees with an angular resolution of 0.003 degrees FWHM. The differential cross section was found to decrease monotonically with scattering angle. In particular the node in the angular distribution, which had been predicted by some theoretical calculations, was not observed.

Lifetimes of metastable X-ray emitters in helium- and lithium-like chlorine☆

Abstract We have used coincidence techniques to identify the charge states of metastable K X-ray emitters produced in foil excitation of energetic chlorine beams. At 55 MeV the emitting states are found to belong primarily to helium- and lithium-like chlorine ions. Charge-state separated decay curves for these ionic species are characterized by lifetimes of 2.2±0.3 ns and 0.95±0.2 ns for two- and three-electron systems respectively. The corresponding emitting states are thought to be the (1s2p) 3 P 2 0 and ( s 2 s 2 p ) 4 P 5 2 0 levels in helium- and lithium-like chlorine respectively, both of which may radiatively decay by M2 photon emission.

Impact-parameter dependence of K-vacancy production in Cuq+-Kr collisions

The authors have measured the impact-parameter (b) dependence of the probability (P) for production of K vacancies by 55 MeV Cuq+ ions on a Kr target. The absolute probability scale is found to increase in proportion to the number of L vacancies brought in by the projectile, but the shape of P(b) is nearly independent of projectile charge state. This shape is not in good agreement with that expected for the 2p pi -2p sigma rotational-coupling mechanism.

Polarization fractions of K X-rays emitted by fast fluorine ions incident in different charge states on helium

Abstract Polarization fractions have been measured for K X-rays emitted by energetic fluorine ions passing through helium gas. A strong monotonic charge state dependence is found, increasing to an observed polarization of ∼ 30% for highly charged ions.