Creation of a monoenergetic pulsed positron beam

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Abstract
In this letter we describe a new and versatile source of high-intensity, cold, pulsed positrons extracted from a Pen- ning trap. This source unites many of the attractive features not available previously using any single technique. There are numerous potential applications for bright sources of slow and cold positrons. Examples include material surface characterization, such as defect depth profiling, positron and positronium gas scattering, and annihilation studies. 1 Fur- thermore, many applications of positron beams, such as time-of-flight measurements, positron lifetime experiments, and time tagging, require pulses of positrons. One advantage of pulsed beams, as compared with steady state beams, lies in the potential for greatly enhanced signal-to-noise ratios. Various techniques to create pulsed positron beams have been reported.2-4 However, many of these techniques have disadvantages, for example the degrading of the perpendicu- lar and/or parallel energy spread in order to achieve pulse compression. There are several possible approaches to generate slow positron beams.1,5 The positrons originate from a radioactive source or from a particle accelerator, but in either case they must be slowed from initial energies of several hundred keV to energies in the electron Volt range before beam formation and handling becomes practicable. At present this is accom- plished most effectively using a solid-state moderating material.1,6-8 In general, positrons emerge from the modera- tor with an energy of several electron Volts and an energy spread in the range 0.3-2 eV, although methods have been described to reduce this energy spread by as much as an order of magnitude in some instances.9,10

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