Development of a multichannel parabolic guide for thermal neutron beam focusing

Abstract

We have developed a multichannel parabolic guide for thermal neutron beam focusing. The guide was designed with multichannel structure, seven channels in the present case, to focus a neutron beam with wide cross section effectively. The multichannel structure is made with parabolically bent NiC/Ti supermirrors with critical reflection angles m = 3.0 and 3.6 times as much as that of nickel and thickness of 0.55 mm. The guide consists of two focusing devices with each length of 900 mm, which are separated by a gap of 150 mm for neutron beta decay experiments. The focal length of the guide is 995 mm away from the exit of the guide. The cross section of the guide is 90 × 20 mm 2 at the entrance and 52.3 × 12.1 mm 2 at the exit. The guide was installed at a thermal neutron beam line for neutron-induced prompt gamma-ray analysis (PGA) of JRR-3M in JAEA with straight supermirror guides with m = 3.0 . Focusing properties of the parabolic guide were measured at the focal point with a CCD imaging system and we obtained a gain of 9.6 and 4.1 in peak flux compared with those in cases of using the vacuum tube and the straight guide, respectively. The parabolic guide has been successfully used to enhance the efficacy of PGA and have also applied to neutron beta decay experiments.