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

We developed a longitudinally and transversely segmented 3He ionization chamber that can simultaneously measure the energy spectrum and total flux of a cold neutron beam (Tn=30 K, λn=5.6 Å) in each of four transverse segments. The detector directly measures the neutron range along the beam direction and the energy distribution of the beam is obtained from the known energy dependence of the n-3He absorption cross-section, (σn∝1/vn). For operating voltages in the ionization regime, the detector channels registered dark currents of 0.2 pA and typical signal currents of 30 nA for an integrated neutron flux of 107 n/s. The measured currents agreed at the 2% level with other independent measurements 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.