Development Of Neutron Leakage Spectrometers With Emphasis On The Kev Range

The objectives of this study were (a) to develop techniques for measurement of delayed neutron spectra with resolutions approaching 8.0 keV at the thermal peak and (b) to explore techniques for measuring gamma rays and neutrons in coincidence. During the contract period the following have been accomplished: (a) the electronic, acoustical and inherent noise characteristics of the 3 He neutron ionization chambers purchased from SEFORAD, LTD have been characterized. Through construction of an acoustical shield and replacement of several components at the input stage of the preamplifiers associated with the detectors, the resolutions of the spectrometers have been improved by about 10 to 15 percent and the environment has been controlled to permit maintenance of the improved resolution under normal laboratory conditions. The resolution of the best spectrometer is now 9.3 keV; (b) it has been shown that the amplitude and resolution for neutrons from a thermalized Pu-Be source are dependent upon pulse risetime. For the risetime range 0.2--1.2 .mu.s, the amplitudes of pulses were found to vary by 0.7 to 0.8 keV and the longer risetime pulses led to a full energy peak narrower than that from the short risetime pulses by about 0.2 keV. (c) based on the information available in the literature on the Fano Factors of gas mixtures approximating that existing in the spectrometers, and the measurements made on one of the detectors, it was concluded that external factors are still contributing significantly to the resolution and that it should be possible to obtain a resolution of near 8.0 keV with no change in the gas mixture; (d) the principal limitation in the use of the 3He spectrometers for neutron--gamma ray coincidence measurements lies in the long times required for drift of electrons from the site of interaction in the device to the grid surrounding the anode.