First positron flux measurements at the Argonne Positron Source (APosS)

Abstract

PACS 23.20.Ra, 41.75.Fr, 78.20.Mb The development of the APosS at the Argonne low-energy linac is discussed. We measured the yields and outline strategies to increase the number of slow positrons. We describe the measurement of the yield of slow positrons as a function of electron-beam energy and compare these results with theoretical calculations.1 Introduction Positron annihilation spectroscopy is an extremely sensitive, non-destructive method to study the nature, concentration and distribution of the defects in materials [1][2][3]. This method has been successfully used for the last forty years, but low positron flux prevents the realization of the full potential of the positron spectroscopy. One of the most promising directions for increase in positron flux is to use an electron linac as a source of fast positrons [4,5]. We are in the process of developing a low-energy positron facility based on the Argonne Chemistry Division High-Current electron Linac [6]. This is an attractive source for the generation of positrons because the linac is capable of producing a relatively high current of electrons (in the present configuration, more than 100 µA average of electrons (6 x 10 14 electrons per second) and with relatively few modifications, greater than 2 milliamp of electrons (1.2 x 10 16 electrons per second) with an energy of 15 MeV. Pulsed accelerators provide a convenient source of positrons. They have the advantage of being able to turn off the source of radiation by simply turning off the accelerator, which is not possible with an isotope source. Because the source of positrons is pulsed, high peak currents of positrons are available, which are well suited for filling positron traps. Traditionally, high-energy electron accelerators (50-100 MeV) have been used as a source of electrons to generate positrons because the yield of positrons per electron will be higher for the higher energy electrons [4,5]. However, when one considers the efficiency of generating positrons for the total beam energy (current times translational beam energy), the increase in efficiency is no longer as clear. Higher energy electron beam produces more positrons per incident electron, but the energy spectrum of the

Simulations of slow positron production using a low-energy electron accelerator

“…This normalized result, γ N , is plotted in figure 12 along with several previously reported measurements 3,5,8 .…”

“…This disagreement is also apparent in the low energy Argonne measurements 8,36 . Much closer agreement is seen with the measurement at the Mitsubishi Electric Corporation LINAC 5 where a 5 mm Ta converter and W moderator were used and an estimated slow positron production efficiency of 1.35 ×10 −7 s −1 at 18 MeV was reported.…”

“…The use of electron accelerators is a well established technique for the production of slow positrons, with numerous facilities based on linear accelerators (LINAC) [1][2][3][4][5][6][7][8][9] or microtrons 10,11 . Positrons are generated via pair creation when energetic electrons are stopped in a high-Z target, the electron-positron converter.…”

Development of the Argonne positron source APosS

“…[4] shows the yield of positrons as a function of electronbeam energy. These results were taken using the microchannel plate detector.…”

Future Sources of Polarised Positrons

“…Positrons produced at other intense sources, such as reactor-based facilities [3], or electron beam facilities [4], [5], [6], are typically unpolarised and are consequently outside the scope of this review.…”