Observation of Radiative Capture in Relativistic Heavy-Ion—Atom Collisions

Abstract

X-ray studies of relativistic heavy-ion-atom collisions allow the direct observation of the radiative-electron-capture photons. The angular distribution of these photons is approximately sin 2 0 lab , because of the cancellation of electron retardation effects when the cross sections are subjected to Lorentz transformation into the laboratory frame. Comparison of measured and calculated cross sections reveals the number of equilibrium projectile K vacancies present in the solid targets, which can be compared with charge-state measurements behind the target.

Lifetime measurements of the yrast8+and9+states inAs70Morse1, Iwasaki2, Lemasson3 et al. 2014Phys. Rev. C5040View full textAdd to dashboardCite

“…Finally, the REC process referenced earlier makes the extraction of the lifetime from the γ-ray singles spectrum impractical. As stated in [16,17], the energy of shows the experimental data gated on the 76.1 keV γ-ray from the 9 + state but attempts to fit this data with simulations which do not account for the 9 + lifetime. These simulations fail to reproduce the experimental lineshape and result in a poor χ 2 distribution.…”

“…REC can have a cross-section of tens of barns [16,17] and can happen either before or after the incoming 78 Rb beam reacts to form 70 As, and so the range of REC energies was calculated using velocities which correspond to 78 Rb as it enters the target (β = 0.43) and to 70 As as it exits the target (β = 0.34).…”

“…This discrepancy is due to the detection threshold on the forward ring being increased during the experiment to reduce the rate of photons being detected to a manageable level. The peak in the backward ring can be explained as originating from a combination of the 76.1 keV 9 + → 8 + transition and a contribution from radiative electron capture (REC) [16,17] in the target which will be discussed in more detail later.…”

Experiments on Few-Electron Very High-Z ions

“…Equilibrium thickness-the thickness at which additional material no longer changes the charge state is typi cally a few ten's of mg/cm : for high-Z targets. The processes for electron capture and loss by rela tivistic heavy ions are well understood and cross sections for ionization, for radiative electron cap ture and for nonradiative electron capture can be reliably calculated 3 What is significant about the self energy at Z-92 is it arises almost entirely from terms' 7 which are of very high order in Za (where a is the fine structure constant). Because these terms are large only at very high Z they are not tested in low-Z Lamb shift and fine structure experi ments.…”

“…The 96 keV x ray is easy to detect. The I0"' 4 sec lifetime of the 2 % state has no effect on the measurement of the 6 X 10"" sec 2 3 Po lifetime provided sufficient distance is allowed for the initial 2 3 Heliumlike uranium in the 2 J Po state is prepared by stripping a beam of 220 MeV/amu uranium 39+ which is obtained from the Lawrence Berkeley Laboratory's Bevalac. An aluminum target produces an equilibrium charge state distribution of roughly 5% U ,2+ , 30% U" + , 60% U 90 *, 5% U ,,+ .…”