J. W. Cooper

Observation of the (3)He(n, tp) Reaction by Detection of Far-Ultraviolet Radiation.

We have detected Lyman alpha radiation, 121.6 nm light produced from the n = 2 to n = 1 transition in atomic hydrogen, as a product of the 3He(n, tp) nuclear reaction occurring in a cell of 3He gas. The predominant source of this radiation appears to be decay of the 2p state of tritium produced by charge transfer and excitation collisions with the background 3He gas. Under the experimental conditions reported here we find yields of tens of Lyman alpha photons for every neutron reaction. These results suggest a method of cold neutron detection that is complementary to existing technologies that use proportional counters. In particular, this approach may provide single neutron sensitivity with wide dynamic range capability, and a class of neutron detectors that are compact and operate at relatively low voltages.

Total photon attenuation cross sections of helium at energies below 10 keV. An assessment of the accuracy of measured and calculated cross sections

Recent measurements and calculations of helium photoionization, photon attenuation and scattering cross sections for energies below 10 keV are critically evaluated and compared with older measurements, calculations and tabulations. Oscillator strength sum rules are used to assess the validity of various results.

Double ionization of argon by electron impact: new theoretical results

Recent experiments (Jia et al 2003 J. Phys. B: At. Mol. Opt. Phys. 36 L17 and 35 1103) have measured the relative double ionization cross sections of argon at an incident energy of 561.4 eV and the measurements do not agree with first-order plane-wave Born approximation (PWBA) calculations. A new approach to this problem has been developed which uses distorted waves (DWBA) in a first-order approach rather than the plane waves inherent in first-order PWBA. The calculations based on this approach provide better agreement with the experiments.

Knockout Reactions to Study Atomic and Molecular Electronic Structure: The Future

The momenta of the scattered and ejected electrons in electron impact ionization of an atom or molecule are related to the precollisional momenta of the target electrons. Electron knockout, or (e,2e), reactions have thus been employed to measure the single-electron momentum density function for atomic and molecular electrons. Future electron knockout experiments promise the possibility of (i) observing subtle chemical effects in molecular electron momentum densities, (ii) measuring the momentum density in a specified direction relative to a molecular axis, and (iii) determining the relative momentum density for pairs of atomic or molecular electrons in order to assess the effect of electron correlation. Interpreting these experiments will pose significant new challenges for theorists.

Double electron knockout to study electron correlation

Electron-impact double ionization probes the role of multi-electron interactions in the dynamics of electron scattering, as well as the nature of electron correlation in a target atom. Electron impact double ionization of magnesium is being studied with the ultimate goal of measuring the pairwise joint momentum distribution of atomic electrons. A multi-detector apparatus permits 120 triple-coincidence measurements to be performed simultaneously. (e,3e) and (e,(3-1)e) experiments have been carried out at incident-electron energies of 400 to 3500 eV with ejected-electron energies between 35 and 100 eV. Resonant (Auger) and nonresonant (direct) double ionization have been studied. Analysis of the data in terms of the net momentum of the ejected electrons and the momentum of the residual doubly-charged magnesium ion has revealed the existence of underlying symmetries during the course of the collision and unexpectedly large residual ion momentum. Current models of double ionization fail to account for many of...

An investigation of basic symmetries in electron-impact ionization of magnesium

The symmetry of electron ejection about the momentum-transfer direction in electron-impact ionization has been investigated in (e, 2e) measurements and distorted wave calculations of the cross sections for ionization from the 2s, 2p and 3s subshells of magnesium. Measurements were carried out for a scattering angle of 18° with incident energies of 400, 750 and 1500 eV. The energy loss was chosen so that the momentum transfer direction was oriented at 45° to the incident electron direction. Ejected electrons were observed both in and out of the scattering plane defined by the incident- and scattered-electron momentum vectors. Both measurements and calculations demonstrate a breakdown of the symmetry about the direction of momentum transfer.

(e, 3e) collisions on Mg in the impulsive regime studied by the second Born approximation

Five-fold differential cross sections for electron-impact double ionization of the 3s electrons of magnesium have been calculated in the second Born approximation in the impulsive regime. Comparing these results with calculations carried out in the first Born approximation demonstrates the dominant contribution of the second Born term. The second Born calculation shows that the contribution of the two-step 2 (TS2) process becomes large under the condition where sequential binary collisions on the Bethe ridge can occur. The effect of electron correlation in the initial target state is also examined by using a configuration interaction wavefunction.

Distinguishing between target structure and ionization mechanisms in (e,3e) experiments

Electron impact double ionization with full determination of the kinematics of the collision, (e,3e), can, in principle, provide direct information on the correlated motion of the ejected electrons at the instant of ejection, provided that the mechanism of ejection is known. The symmetries of the observed double ionization cross sections in a variety of kinematic regimes can be used to experimentally investigate ejection mechanisms. The sensitivities of various experimental geometries and kinematic regimes to different ionization mechanisms are discussed in the context of the double ionization of the 3s electrons of magnesium. The practical implications for extracting information about electron correlation are examined.

Partitioning of Momentum in Electron-Impact Double Ionization

A complete description of electron-impact double ionization requires the determination of the momenta of the scattered projectile and two ejected electrons. The experiment is referred to as (e,3e) implying a collision in which an incident electron gives rise to three electrons that are detected in coincidence. The cross section for a given incident-electron energy and a specified final state of the doubly-charge ion is eightfold differential and is typically reported as a function of the solid angle of acceptance of the detectors of the three outgoing electrons and the energy bandpass of two of these detectors (the energy of the third electron being fixed by conservation of energy).

New directions in the study of atomic autoionizing states

Abstract The Auger process, discovered by Auger in 1923, was for many years viewed primarily as the end process of a nuclear reaction. However, in the early 1960s, it became clear that basically there was no difference between the Auger process and autoionization, and that much could be learned by a detailed study of these processes both experimentally and theoretically as atomic processes. Since that time such studies have played an important role in the evolution of our understanding of atomic processes. This article will give a brief summary of these developments and will attempt to outline some promising directions for future work.