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 reactor. Several configurations of the positron source have been tested. All rely on large emitting surface areas: ranging from 1000 to 2000 cm2. For efficient extraction of the emitted positrons the positron emitting foils are arranged into 3–4 disks which are composed of cylinders with a 6–9 mm diameter 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 carry the positrons through the cylinders. All source tests unexpectedly indicated a high reflectivity of moderated positrons at the tungsten surfaces of the moderation disks. A substantial number of positrons is emitted even without electrical field. A model including the effects of positron reflection and electrical field extraction explains the experimental results reasonably well. At 2 MW reactor power positrons were observed with an intensity of 0.7×108 e+ s−1 in a 10 mm beam spot.