The proton recoil spectrum of thermal neutrons decaying in vacuum

Abstract

The decay of neutrons in an evacuated tangential reactor beam tube is considered. Due to the thermal motion of the decaying neutrons, the proton recoil spectrum in the laboratory system (z-axis parallel to the beam tube) differs from that in the c.m. system. This difference is calculated for a point at the beam tube exit, and expressed in terms of the average¯q _z and of the variance of thez-component of neutron momentum. An activation method for measuring the neutron current densityJ _z (which is closely related to¯q _z) is suggested. The distortion of the spectrum by the finite resolution of an electrostatic spectrometer is also given. The results are to be used in connection with a planned measurement of the proton recoil spectrum in neutron decay.