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Physica Scripta

Positron Traps: Towards High Resolution Positron Beams

Jonathan Peter Merrison

Published Jan 1, 2003• 0 citations
Publisher

Abstract

The low intensity and poor resolution of conventional slow positron beams is limiting the study of atomic and molecular collisions and prohibits more ambitious fundamental studies. The application of trapping promises huge enhancement in instantaneous beam intensity; ideal for studying short lived positron bound states. Cooling of trapped positrons gives vast improvement in energy and spatial resolution and gives access to a new low energy regime. This new technology is proving to be both challenging and exciting; the relative merits of several different trapping schemes will be discussed and recent progress presented.

DOI
10.1238/physica.regular.068ac0054

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•May 8, 1994•0 citations
The low intensity and poor resolution of conventional slow positron beams is limiting the study of atomic and molecular collisions and prohibits more ambitious fundamental studies. The application of trapping promises huge enhancement in instantaneous beam intensity; ideal for studying short lived positron bound states. Cooling of trapped positrons gives vast improvement in energy and spatial resolution and gives access to a new low energy regime. This new technology is proving to be both challenging and exciting; the relative merits of several different trapping schemes will be discussed and recent progress presented.
An overview is presented of recent studies of the interaction of low-energy positrons with atoms and molecules using a trap-based positron beam. The beam is tunable from ~100 meV to many electron volts, with a typical energy resolution of 25 meV (FWHM). A variety of scattering processes are investigated using the properties of positron orbits in a spatially varying magnetic field. Recent scattering measurements are reviewed, and new data for electronic excitation, ionization, and positronium formation in noble gases are presented. In a separate set of experiments, positron annihilation below the threshold for positronium formation is studied in molecules as a function of positron energy. The data show large Feshbach resonances in the range of energies of the molecular vibrations. Within the framework of this model, these results provide the first experimental evidence that positrons bind to hydrocarbon molecules.
The field of experimental positronium physics has advanced significantly in the last few decades, with new areas of research driven by the development of techniques for trapping and manipulating positrons using Surko-type buffer gas traps. Large numbers of positrons (typically ≥106) accumulated in such a device may be ejected all at once, so as to generate an intense pulse. Standard bunching techniques can produce pulses with ns (mm) temporal (spatial) beam profiles. These pulses can be converted into a dilute Ps gas in vacuum with densities on the order of 107 cm−3 which can be probed by standard ns pulsed laser systems. This allows for the efficient production of excited Ps states, including long-lived Rydberg states, which in turn facilitates numerous experimental programs, such as precision optical and microwave spectroscopy of Ps, the application of Stark deceleration methods to guide, decelerate and focus Rydberg Ps beams, and studies of the interactions of such beams with other atomic and molecular species. These methods are also applicable to antihydrogen production and spectroscopic studies of energy levels and resonances in positronium ions and molecules. A summary of recent progress in this area will be given, with the objective of providing an overview of the field as it currently exists, and a brief discussion of some future directions.
The development of buffer-gas trapping and magnetized beam formation has provided positron beams with significantly improved energy resolution $(\ensuremath{\Delta}E<~0.025\mathrm{eV})$ compared to those available previously. Analysis techniques have been developed to take advantage of the fact that the beam is in a magnetic field of $\ensuremath{\sim}0.1\mathrm{T}.$ This has enabled scattering experiments at lower energies and with significantly improved state selectivity for excitation experiments than had been possible previously. A detailed description of these techniques is presented. Data are presented for a variety of cross sections for scattering of positrons from atomic and molecular targets.
This is a personal account of the development of our buffer-gas positron trap and the new generation of cold beams that these traps enabled. Dick Drachman provided much appreciated advice to us from the time we started the project. The physics underlying trap operation is related to resonances (or apparent resonances) in positron-molecule interactions. Amusingly, experiments enabled by the trap allowed us to understand these processes. The positron-resonance "box score" to date is one resounding "yes," namely vibrational Feshbach resonances in positron annihilation on hydrocarbons; a "probably" for positron-impact electronic excitation of CO and NZ;an d a "maybe" for vibrational excitation of selected molecules. Two of these processes enabled the efficient operation of the trap, and one almost killed it in infancy. We conclude with a brief overview of further applications of the trapping technology discussed here, such as "massive" positron storage and beams with meV energy resolution.
We numerically study laser deceleration, cooling and trapping of host fast ortho-positronium (Ps) bunches as produced in silica microchannel targets using positron bunches from a Surko trap. While Doppler laser cooling routinely allows to cool neutral atom species below milli-Kelvin kinetic temperatures, the fast ground state annihilation and the high recoil shift to line-width ratio in Ps pose new challenging limitations on timing and the required laser power. Earlier theoretical work and first experiments already exhibit viable prospects for radiation pressure cooling to create unprecedented cold and dense Ps ensembles. Our extended numerical studies confirm very good prospects to simultaneously decelerate, cool and optically trap Ps atoms in a standard Doppler cooling geometry on the 13S1^3\mathrm{S}13S--23P2^3\mathrm{P}23P transition. Including the full Zeeman state manifolds and adding additional transverse lasers on the 23P2^3\mathrm{P}23P--33D3^3\mathrm{D}33D transition improves timing and final temperature to allow for optical trapping at the end. The required laser powers and geometry parameters to implement effective slowing and trapping within a time frame, where the majority of atoms is not annihilated, is in reach of current technology of mJ\mathrm{mJ}mJ energy pulses of approximately 100 ns100\ \mathrm{ns}100 ns duration. Future extensions to collectively enhance cooling and trapping in optical cavities or hollow-core fibres should finally allow fast preparation of ultra-cold Ps systems as future basis of superradiant lasing and Bose--Einstein condensates.
With the ultracold temperatures (T 1012 cm-3) of atoms that can be produced with laser-cooling and trapping techniques, it is now possible to carry out a new kind of high-resolution spectroscopy.1 This photoassociation is based on optical absorption resonances between the initially free states of colliding atoms and specific, bound excited molecular states. High resolution can be obtained because of the narrow energy distribution of the ultracold gas.1 Because the initial states are formed by collisions, it becomes possible to probe unique, excited molecular states with very large interatomic separations that are impossible to probe with conventional molecular spectroscopy. The feasibility of this new spectroscopy has been demonstrated in two recent experiments.2,3