P. W. Zitzewitz

Measurement of the vacuum decay rate of orthopositronium formed in an MgO-lined cavity

Abstract Orthopositronium decay rates are measured in MgO-lined cavities with various volumes and entrance apertures. Systematic effects of the entrance aperture, cavity geometry, and collisional pick-off are measured. The vacuum decay rate is determined to be 7.050 ± 0.013 μs −1 .

Beta decay and the origin of biological chirality: New experimental results

The proposed connection between the parity-violating handedness of beta particles in radioactive decay and the sign (L) of biological chirality (the Vester-Ulbricht [V-U] hypothesis) is being investigated by measuring the theoretically predicted asymmetry in the formation of triplet positronium in amino acid enantiomers by low energy positrons under reversal of the helicity of the positrons. We find the asymmetry in leucine to be (0.8±1.0)×10−4, i.e. consistent with the theoretical, prediction of 10−6 to 10−7. The apparatus is now sensitive enough to test the predicted asymmetry in optically active molecules which have heavy atoms at their chiral centers. The connection between these results and asymmetry in radiolysis by beta-decay electrons is made, and the implications of our limits for the V-U hypothesis discussed. Although the above limits are 106 times lower than direct measurements of radiolysis, they are still not small enough to allow us to rule out the V-U hypothesis.

Spin-polarized low-energy positron beams and their applications

The production and use of low-energy (100 eV to 5 keV) high-intensity, spin-polarized positron beams is reviewed. Methods for obtaining beams with high polarization are discussed. Applications include studies of the moderation process, surface and bulk magnetism, optically active molecules, and the production of polarized anti-protons.

FAST POSITRONIUM FORMATION AND DISSOCIATION AT SURFACES

The origin of short‐lived components in the annihilation lifetime spectrum of positronium (Ps) is shown to be due to fast Ps that is collisionally dissociating at the surfaces of the surrounding confinement cavity. The results are consistent with a model of fast (10–100 eV) Ps production by backscattered positrons from the incident beam. It is found that the typical lifetime of dissociating Ps scales with the mean free path of the cavity, and the relative formation intensity depends inversely on the incident positron beam energy. This ubiquitous effect will be present in any Ps formation experiment involving a free surface and can only be eliminated at beam energies less than 10 eV. More practical methods of minimizing the undesirable systematic effects of fast Ps quenching in depth‐profiled positron lifetime spectroscopy and in precision Ps decay rate measurements will be discussed.

Antihydrogen: Production and Applications

The formation of antihydrogen (\(\overline H\)) is of interest for a variety of reasons. Properties of the \(\overline H\) such as the electronic energy levels, fine structure, Lamb shift, and hyperfine structure can be measured and compared to the corresponding quantities in hydrogen as tests of CPT invariance. Novel investigations of the interactions of \(\overline H\) with atoms and with gravitation can be undertaken. Finally, applications such as the production of polarized antiprotons or the storage of macroscopic quantities of \(\overline H\) can also be pursued.

Studies of the formation of slow positrons in MgO‐coated moderators

Studies have been made of the production of slow (∼1 eV) positrons by high‐energy positrons from a radioactive source. Moderators consisting of thin metallic foils coated with MgO smoke were used in the transmission mode. The thinnest foils gave the largest fluxes of slow positrons. A double moderator, consisting of a MgO‐coated grid following the MgO‐coated thin foil, gave a flux almost double that of either the foil or grid alone. The positron beam machine used is described. It has electrostatic optics for the collection, analysis, and transmission of the slow positrons.

Experimental Tests of QED in Positronium: Recent Advances

The current experimental situation regarding tests of fundamental physics using positronium is reviewed. Five measurements are discussed and compared with theoretical predictions: the singlet and triplet annihilation decay rates, the ground state and the n = 2 energy intervals, and the Doppler-free two-photon excitation of the 1S to 2S transition. Previous results, recent progress (where appropriate), and the outlook for future improvements in these measurements are discussed.

Positron accumulation and storage for antihydrogen production

We propose a scheme to stack and accumulate positrons, emitted randomly from a radioactive source. The positrons are moderated and accumulated at low energy.

Limits on asymmetric orthopositronium formation in high Z optically active molecules

AbstractThe proposed connection between the parity violating handedness of beta particles in radioactive decay and the sign (L) of biological chirality (the Vester-Ulbricht [V-U] hypothesis) is experimentally tested. The theoretically predicted asymmetry in triplet positronium formation (APs) is measured in several highZ optically active molecules using low energy positrons with a net helicity. We findAPs<3×10−4 in selenocystine (Z=34) and thyroxine (Z=53), excluding part of the theoretically predicted range of 4×10−3> APs>2×10−6 in these molecules. The connection between these limits and limits on asymmetric radiolysis (AR) is made, with a new limit ofAR>10−9 being placed. This limit onAR, which is thirty times lower than a previous measurement in the amino acid leucine (Z=6), is still not small enough to rule out the V-U hypothesis.

Performance of a spiralled continuous electron multiplier in a magnetic field

The gain and detection efficiency of a spiralled electron multiplier (Spiraltron) have been measured in a magnetic field. Results are presented as a function of field strength and angle between the field and detector. The multiplier is useful in fields as high as 900 G, considerably higher than the limit that has been obtained on a Channeltron curved electron multiplier.