Techniques of Measuring Neutron Spectra with Threshold Detectors—Tissue Dose Determination

Abstract

A method is described for measuring the spectrum of fast neutrons with a series of foil detectors. The detectors consist of Au and Au plus Cd for determining the thermal flux, Pu²³⁹ shielded with B¹⁰ for determining the total fast flux, Np²³⁷ for determining the total flux above 0.75 Mev, U²³⁸ for determining the total flux above 1.5 Mev, and S³² for determining the total flux above 2.5 Mev. From these data the spectrum of fast neutrons can be constructed reasonably well for spectra similar to that of fission. The amount of induced activity in the detecting elements is determined with scintillation counters, which are calibrated by irradiation of suitable foils with thermal neutrons. Fast neutron tissue dose can be calculated from the measured spectrum. Since the foil method of measuring spectra works well for high intensity bursts of neutrons, dosimetry may be accomplished under conditions where other methods, e.g., the ionization chamber and proportional counter, would lead to questionable data.