A low-noise He-3 ionization chamber for measuring the energy spectrum of acold neutron beam

We developed a longitudinally and transversely segmented He-3 ionization ch amber that can simultaneously measure the energy spectrum and total flux of a cold neutron beam (T-n = 30 K, lambda (n) = 5.6 Angstrom) in each of fou r transverse segments. The detector directly measures the neutron range alo ng the beam direction and the energy distribution of the beam is obtained f rom the known energy dependence of the n-He-3 absorption cross-section, (si gma (n) proportional to 1/nu (n)). For operating voltages in the ionization regime, the detector channels registered dark currents of 0.2 pA and typic al signal currents of 30 nA for an integrated neutron flux of 10(7) n/s. Th e measured currents agreed at the 2% level with other independent measureme nts of the neutron flux and energy spectrum: the majority of this error is believed to arise from an inability to accurately characterize the neutron beam. (C) 2001 Elsevier Science B.V. All rights reserved.