S. J. Gilbert

Creation of a monoenergetic pulsed positron beam

We have developed a versatile, pulsed source of cold (ΔE=0.018 eV), low-energy positrons (E≈0–9 eV). Multiple pulses of 105 positrons, each 10 μs in duration, are extracted from a thermalized, room temperature positron plasma stored in a Penning trap. The frequency, duration, and amplitude of the pulses can be varied over a wide range.

Search for resonances in the scattering of low-energy positrons from atoms and molecules

In a search for quasi-bound states, or resonances, high-resolution measurements (?E~25?meV) of the total positron scattering cross sections have been carried out in the energy region of the first electronically excited states of H2, N2, CO and Ar. In the case of H2, a recent calculation by Varella et al predicts such a resonance in the total elastic cross section near the excitation threshold for the B 1?u+ state. We find no experimental evidence for the existence of this resonance and also find similar negative results for the other atomic and molecular targets.

Low-energy positron scattering from atoms and molecules using positron accumulation techniques

Positron scattering from atoms and molecules is studied at low values of incident positron energy. The experiments use a cold magnetized positron beam formed in a positron accumulator. A discussion of positron scattering in a magnetic field is presented along with diAerential cross-sections (DCS) for positron‐atom collisions and positron‐ molecule total vibrational excitation cross-sections. Absolute values of the DCS for elastic scattering from argon and krypton are measured at energies ranging from 0.4 to 2.0 eV. The first low-energy positron‐molecule vibrational excitation cross-sections have been measured (i.e., for carbon tetrafluoride at energies ranging from 0.2 to 1 eV). Using information gained from these experiments a second generation scattering apparatus is described, which was designed and built specifically for scattering experiments using a magnetized positron beam. This apparatus has a number of improvements, including an order of magnitude higher throughput, better energy separation between elastic and inelastic scattering events, and improved measurement of the absolute pressure of the test-gases. Analysis techniques for the data taken in these experiments using a retarding potential energy analyzer and possible extensions of these experiments are also discussed. ” 2000 Elsevier Science B.V. All rights reserved.

Atomic and molecular physics using positron accumulation techniques – Summary and a look to the future

An overview is presented of current techniques to accumulate and cool large numbers of positrons from a radioactive 22 Na source and neon moderator, and the first operation of a new generation of positron accumulator is described. Experiments are discussed that use these techniques to study the interaction of positrons with atoms and molecules at low energies (i.e., below the threshold for positronium formation), including systematic studies of the dependence of positron annihilation on chemical composition. By measuring the Doppler-broadening of gamma-ray annihilation radiation, the quantum state of the annihilating electrons in atoms and molecules was identified. These experiments indicate that positrons annihilate with approximately equal probability on any valence electron. Annihilation with inner shell electrons is infrequent, but is measurable at the level of a few percent in heavier atoms. Measurements of annihilation rates in molecules as a function of positron temperature revealed a number of interesting trends that are briefly discussed. We have developed a new technique to make a cold, bright positron beam. This technique is now being used for a new generation of scattering experiments in the range of energies 61 eV. Other possible experiments to study aspects of atomic and molecular physics using positron accumulation techniques and this cold positron beam are briefly discussed. ” 2000 Elsevier Science B.V. All rights reserved.

Progress in creating low-energy positron plasmas and beams

A summary is presented of recent research to create positron plasmas in new regimes of density, temperature, and particle number. The operation of a new, compact positron accumulator is discussed. It has a number of improvements including enhanced vacuum capabilities and an easily modified electrode structure. Using a 90 mCi 22Na source and neon moderator, a plasma of 3×108 positrons, with a diameter of 6 mm (FWHM) and a density of 2×107 cm−3, has been accumulated in 8 minutes. This is a factor of 50% more positrons and an order of magnitude increase in plasma density over the performance of the previous accumulator. Plans for a separate, high magnetic field (i.e., 5 Tesla), low-temperature (<10 Kelvin) trap are described. This trap is expected to permit the creation and long-term storage of cryogenic plasmas with more than an order of magnitude larger particle number and more than two orders of magnitude in plasma density. A method is described that uses positron accumulation techniques to create a cold,...