Positron extraction and transport in a nuclear-reactor-based positron beam

This paper describes the design of a positron beam which is primarily based on positron generation by pair formation near the core of a nuclear reacto r. Several configurations of the positron source have been tested. All rely on large emitting surface areas: ranging from 1000 to 2000 cm(2). For effi cient extraction of the emitted positrons the positron emitting foils are a rranged into 3-4 disks which are composed of cylinders with a 6-9 mm diamet er and lengths from 10 to 40 mm, The electrical potential between adjacent disks could be varied up to 50 V so that a field gradient was present to ca rry the positrons through the cylinders. All source tests unexpectedly indi cated a high reflectivity of moderated positrons at the tungsten surfaces o f the moderation disks. A substantial number of positrons is emitted even w ithout electrical field. A model including the effects of positron reflecti on and electrical field extraction explains the experimental results reason ably well. At 2 MW reactor power positrons were observed with an intensity of 0.7 x 10(8) e(+) s(-1) in a 10 mm beam spot. (C) 1999 Published by Elsev ier Science B.V. All rights reserved.