D. B. Cassidy

Accumulator for the production of intense positron pulses

An intense pulsed positron source has been developed using a buffer gas trap to accumulate large numbers of positrons and create a dense plasma, which may then be bunched and spatially focused. Areal densities of more than 3×1010e+cm−2 have been achieved in a subnanosecond pulse producing an instantaneous positron current of more than 10mA. We describe various aspects of the device including a detection technique specifically developed for use with intense positron pulses. Two applications are also described as well as future experiments such as the formation of positronium molecules and the positronium Bose-Einstein condensate.

Single shot positron annihilation lifetime spectroscopy

Recent developments in positron trapping technology have made possible experimentation with dense interacting positronium gases. Along with these capabilities comes a need for suitable measurement techniques, and accordingly we have developed a method to measure positronium lifetimes from a single intense burst of positrons. Our method is based on recording the anode signal from a photomultiplier with a fast oscilloscope following a short-time positron burst which allows us to measure transitory effects as well as high density positronium interactions.

Prospects for Making a Bose-Einstein-Condensed Positronium Annihilation Gamma Ray Laser

An annihilation gamma ray laser could be made by a cylinder of high density cold singlet Ps annihilating into a coherent gamma ray burst directed along the axis of the cylinder. Such a laser would have many important uses and prospects seem fair for making a 1J model in the immediate future. Higher intensity lasers that would be useful for controled fusion are envisioned, but involve so many orders of magnitude increase in our ability to produce and control antimatter that no reasonable statement about the possibilities can be made at this time. This paper describes our vision and we briefly report the present status of the experiments.

Prospects for Studies of the Free Fall and Gravitational Quantum States of Antimatter

Different experiments are ongoing to measure the effect of gravity on cold neutral antimatter atoms such as positronium, muonium, and antihydrogen. Among those, the project GBAR at CERN aims to measure precisely the gravitational fall of ultracold antihydrogen atoms. In the ultracold regime, the interaction of antihydrogen atoms with a surface is governed by the phenomenon of quantum reflection which results in bouncing of antihydrogen atoms on matter surfaces. This allows the application of a filtering scheme to increase the precision of the free fall measurement. In the ultimate limit of smallest vertical velocities, antihydrogen atoms are settled in gravitational quantum states in close analogy to ultracold neutrons (UCNs). Positronium is another neutral system involving antimatter for which free fall under gravity is currently being investigated at UCL. Building on the experimental techniques under development for the free fall measurement, gravitational quantum states could also be observed in positronium. In this contribution, we report on the status of the ongoing experiments and discuss the prospects of observing gravitational quantum states of antimatter and their implications.

Antihydrogen from positronium impact with cold antiprotons: a Monte Carlo simulation

A Monte Carlo simulation of the reaction to form antihydrogen by positronium impact upon antiprotons has been undertaken. Total and differential cross sections have been utilized as inputs to the simulation which models the conditions foreseen in planned antihydrogen formation experiments using positrons and antiprotons held in Penning traps. Thus, predictions of antihydrogen production rates, angular distributions and the variation of the mean antihydrogen temperature as a function of incident positronium kinetic energy have been produced.

Positronium emission and cooling in reflection and transmission from thin meso-structured silica films

Measurements of the positronium (Ps) energy and formation fraction in reflection and transmission from a thin meso-structured silica target have been conducted using single-shot positron annihilation lifetime spectroscopy and Doppler spectroscopy. The silica sample is made using glancing angle deposition of vaporized SiO2 on a suspended thin carbon foil. Optical access through the silica sample facilitates measurement of the longitudinal Ps energy, and the Ps energy in the reflection geometry is found to decrease with positron energy as expected, with a minimum achievable Ps energy of 0.203(12) and 0.26(3) eV for the transverse and longitudinal directions, respectively. In the transmission geometry cooling of Ps becomes evident at the minimum positron impact energy required for the positrons to penetrate the carbon foil and enter the meso-structured silica. The minimum energies for this geometry are 0.210(12) and 0.287(14) eV in the transverse and longitudinal directions, respectively, and the minimum achievable Ps energy is found to be limited by the thickness of the structured silica target, since the same energy was found in both geometries.

Evidence for positronium molecule formation at a metal surface

We have observed a reduction in the amount of positronium emitted from an atomically clean Al(111) surface that depends on the incident positron beam density. We interpret this as evidence for the formation of molecular positronium, created following interactions between two pseudopositronium atoms trapped in a surface state. We find that this process is highly sensitive to the condition of the surface and is easily suppressed by changes thereupon. The implications of our data for planned spectroscopic studies of molecular positronium are discussed, as well as improvements to the experimental procedure that will allow more detailed measurements of the thermodynamics of the formation of this molecule from metal surfaces.

Formation of positron-atom bound states in collisions between Rydberg Ps and neutral atoms

Predicted 20 years ago, positron binding to neutral atoms has not yet been observed experimentally. A scheme is proposed to detect positron-atom bound states by colliding Rydberg positronium (Ps) with neutral atoms. Estimates of the charge-transfer reaction cross section are obtained using the first Born approximation for a selection of neutral atom targets and a wide range of incident Ps energies and principal quantum numbers. We also estimate the corresponding Ps ionization cross section. The accuracy of the calculations is tested by comparison with earlier predictions for charge transfer in Ps collisions with hydrogen and antihydrogen. We describe an existing Rydberg Ps beam suitable for producing positron-atom bound states and estimate signal rates based on the calculated cross sections and realistic experimental parameters. We conclude that the proposed methodology is capable of producing such states and of testing theoretical predictions of their binding energies.

A CF4 based positron trap

All buffer-gas positron traps in use today rely on N2 as the primary trapping gas due to its conveniently placed

A trap-based pulsed positron beam optimised for positronium laser spectroscopy.

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