First platinum moderated positron beam based on neutron capture

A positron beam based on absorption of high energy prompt gamma-rays from thermal neutron capture in Cd-113 was installed at a neutron guide of the high flux reactor at the ILL in Grenoble. Measurements were performed for various source geometries, dependent on converter mass, moderator surface and extraction voltages. The results lead to an optimised design of the in-pile positron source which will be implemented at the Munich research reactor FRM-II. The positron source consists of platinum foils acting as gamma - e(+)e(-)-converter and positron moderator. Due to the negative positron work function moderation in heated platinum leads to emission of monoenergetic positrons. The positron work function of polycrystalline platinum was determined to 1.95(5) eV. After acceleration to several keV by four electrical lenses the beam was magnetically guided in a solenoid field of 7.5 mT leading to a NaI-detector in order to detect the 511 keV gamma-radiation of the annihilating positrons. The positron beam with a diameter of less than 20 mm yielded an intensity of 3.1 x 10(4) moderated positrons per second. The total moderation efficiency of the positron source was about epsilon = 1.06(16) x 10(-4). Within the first 20 h of operation a degradation of the moderation efficiency of 30% was observed. An annealing procedure at 873 K in air recovers the platinum moderator. (C) 2002 Elsevier Science B.V. All rights reserved.