Optimization of the proton chopper for 7Li(p,n) neutron spectrometry using a 3He ionization chamber

Thick target 7Li(p,n) neutron spectra were measured with a 3He ion chamber in the proton energy region between 1.95 and 2.3 MeV using the McMaster pulsed accelerator neutron source. The pulsed neutron beam was produced by an electrostatic proton chopper to reject the slow neutron detection events, which seriously limit the fast neutron counting rate of the 3He ion chamber. To collect both arrival time and energy information of 3He detection events, a custom two-dimensional time-energy analyzer was built using a time scaler and a successive approximation peak-sensing ADC. At each proton energy, the optimum chopper operation was determined by taking into account the two competing requirements: high fast-to-slow neutron ratio and reasonable fast neutron counting rate. The proton pulse widths used were 10 μs for 1.95 and 2.1 MeV proton energies, whereas a shorter, 5 μs proton pulse was used for 2.3 MeV acquisition. The raw data were analyzed using three spectral unfolding methods: a simple division by detection efficiency, an iterative algorithm, and a regularized constrained inversion method. The three methods gave consistent neutron fluence spectra within 20% above 30 keV. Thanks to the enhanced fast-to-slow neutron ratio of the pulsed beam, the full detector response function could be employed in unfolding, which led to an extension of the dynamic energy range as well as a better stability of unfolding process in the low energy region.