Subnanosecond bunching of a positron beam

Abstract

A simple scheme for bunching spatially distributed charged particles is described. In the present application, sub-nanosecond width pulses are obtained from a milliliter volume containing slow «2 e V) positrons. The utility of this technique is demonstrated by a qualitative observation of the temperature dependance of the surface state lifetime for positrons on aluminum.

Plasma and trap-based techniques for science with positrons

“…If the bunching cavity is short, a good approximation to a quadratic potential can be conveniently provided by a cylindrical cavity with a characteristic aspect ratio. As described in Crane and Mills (1985), this technique was used to bunch positrons from a dc beam producing 470 ps duration pulses (including detector response) at a 50 kHz repetition rate.…”

Creation of a monoenergetic pulsed positron beam

“…Various techniques to create pulsed positron beams have been reported. [2][3][4] However, many of these techniques have disadvantages, for example the degrading of the perpendicular and/or parallel energy spread in order to achieve pulse compression.…”

Development and applications of time-bunched and velocity-selected positron beams

“…Hulett et al, 27 who used a device similar to the buncher described here as an ionic mass spectrometer, have found from both a theoretical approach and direct measurement that the pulse rise time had very little influence upon the time focusing properties of their device. This is probably because their voltage distribution was still quadratic and therefore time focusing, 7,13,27 despite the fact that the amplitude of the voltage pulse changed with time. However, the situation here, with a variable rise time, results in a more complex behavior and further discussion of this can be found in Sec.…”

“…Similar expressions can be found in the work of Hulett et al 27 and Crane and Mills. 13 Inserting values for our case, and with (V b /V e )ϭ200, we find that the maximum flight time is 79.2 ns ͑corresponding to x 0 /lϭ1) while the minimum value is 70.8 ns for x 0 /lϭ0.3, the last portion of the buncher which is pulsed on. Thus, the expected time width ͑8.4 ns͒ is much broader than that measured.…”

“…4,5 Usually they are based around a radioactive ␤ ϩ source such that the ensuing beam is continuous in nature ͑i.e., the positrons are randomly spaced in time͒. ͑Notable exceptions include beams based around pulsed machines, such as microtron electron accelerators and linear accelerators; see for example Ley 6 for a summary and Mills et al 7 and Merrison et al 8 for discussions of other relevant instruments.͒ However, over the years there have been many successful attempts both to time [9][10][11][12] and bunch ͑or pulse͒ [13][14][15][16] radioactive source-based beams. The scientific motivation for doing so has been varied and includes the following; the measurement of positron lifetimes at surfaces 11 and in the bulk 16 of materials, atomic cross section measurements 9 and the production of positronium beams, 12 positronium spectroscopy, 17,18 and the prospect of studying systems containing more than one positron.…”