Allen P. Mills

Surface Analysis and Atomic Physics with Slow Positron Beams

Recent advances in slow positron beam techniques are making it possible to study the interactions of low-energy positrons with gas molecules and solid surfaces and to measure the properties of free positronium atoms. New surface related results include the observation of surfaces with negative positron affinity and the thermionic emission of slow positronium atoms, low-energy positron diffraction measurements, and the sensitive detection of near-surface crystalline imperfections. Two recent successful experiments in atomic physics are the formation of the positronium negative ion and the optical excitation of positronium for high precisin spectroscopy. Prospects for a positron microscope and the study of exotic antimatter systems such as the two-component Fermi gas are based on the imminent possibility of enormous increases in the brightness and instantaneous intensity of positron beams.

Thermionic emission of hydrogen isotopes (Mu, H, D and T) from W and interpretation of a role of the hot W as an atomic hydrogen source

Abstract Resonant ionization was applied to learn about the production mechanism of thermal neutral hydrogen isotopes including muonium (Mu; μ + e − ) from a hot tungsten surface as reaction products with pulsed 500xa0MeV protons. The activation energy E a obtained in the temperature dependence of production of neutral muonium (1.72 (5)xa0eV) as well as that of T atom (1.89 (1)xa0eV), is consistent with the emission of the atoms directly from bulk sites in the solid rather than from the surface for which one should expect E a ≈3xa0eV. Since hydrogen molecules are easily dissociated into hydrogen atoms interacting with a surface of the hot tungsten, even residual H 2 in the ultra high vacuum chamber of 3×10 −10 xa0mbar at 1200xa0K was found to be easily detected and extracted by this method. From the results obtained through the temperature and pressure dependence measurements for D 2 and H 2 , we propose a view of the role of hot W as a catalyzer to dissociate H 2 molecules and producing hydrogen atoms. After dissociation on the surface, the hydrogen atoms are dissolved into the bulk solid, and are thermionically emitted from the bulk to the vacuum at high temperatures.

A High Intensity Positron Beam at the Brookhaven Reactor

We describe a high intensity, low energy positron beam utilizing high specific activity 64Cu sources (870 Ci/g) produced in a reactor with high thermal neutron flux. Fast-to-slow moderation can be performed in a self moderation mode or with a transmission moderator. Slow positron rates up to 1.6 × 108 e+/s with a half life of 12.8 h are calculated. Up to 1.0 × 108 e+/s have been observed. New developments including a Ne moderator and an on-line isotope separation process are discussed.

A positronium beam and positronium reflection from LiF

At the Brookhaven National Laboratory we have constructed a positronium (Ps) beam by transmitting monoenergetic, low energy positrons through a gas cell containing either Ar or He which provide an electron to form positronium. A description of the positron beam and of the Ps formation mechanisms are found in these Proceedings (see M. Weber, et al. and B. L. Brown). The positrons were obtained by magnetically deflecting positrons in the straight section of the positron beamline (see Fig. 1) into a beamline which contained the gas cell and a Ps detection chamber. By having two beamlines we are able to switch from an experiment which uses positrons (a study of the angular correlation of annihilating radiation--ACAR) to one which uses Ps atoms without breaking vacuum, nor moving equipment. This, however, put a constraint on the placement of the Ps beamline because it could not interrupt the annihilation gamma ray in its long flight from the target chamber to a gamma ray position imaging detector (Anger camera). At present this constraint has resulted in a degradation of the positron beam intensity and energy resolution in the Ps beamline. Efforts are presently underway to eliminate this problem.

Excitation of the 1S-2S Transition in Muonium

Muonium, the bound state of a positive muon and an electron, is one of the simplest systems for testing QED. Spectroscopic measurements in muonium are particularly attractive since the atom does not possess the proton structure effects present in hydrogen. Positronium is free of nuclear structure offsets, but the relativistic two-body problem presents formidable calculational difficulties. The ultimate precision of a measurement of the muonium energy levels will be limited by the 2.2 μsec lifetime (72kHz line width) of the muon.

Radiation overload warning system for Geiger-Müller detectors

Abstract We call attention to the characteristic that common Geiger-Muller detectors can give false safe readings when actually under dangerous radiation overload. We describe inexpensive circuitry which reliably warns the operator of this condition.

Review of Precision Positronium Experiments

In the years since its discovery by Martin Deutsch, [1] positronium has been studied by many workers not only because of the intrinsic interest generated by this exotic atom containing antimatter, but also because in some ways positronium being the lightest of all atoms is also in principle the simplest to interpret theoretically. The measurement of the structure of simple atoms provides one of the best opportunities for testing the predictions of quantum electrodynamics, the theory which most accurately accounts for the interactions of electrically charged particles and electromagnetic radiation. For example, atomic hydrogen is the simplest nuclear atom, and its ground state hyperfine splitting is one of the most accurately measured quantities [2]. It is unfortunate, however, that theoretical uncertainties stemming from the finite size, mass, and polarizability of the proton have so far precluded a meaningful comparison with the measurement to better than a few parts per million. In an attempt to be free of such nuclear effects, experimenters have persued the study of the non-nuclear atom positronium which is describable to a very good approximation using only the electron, positron, and photon fields.