Yield of the Fricke Dosimeter to 14.6 Mev Neutrons

Abstract

Cheap, relatively copious supplies of fast neutrons are now available from small laboratory accelerators that utilize the H3(d, n)He4 reaction. Such sources make possible a variety of experiments, including studies of radiation chemistry and radiation damage, in an essentially 7-free environment. Whereas the dosimetry of fast neutrons from nuclear reactors is difficult and inexact, the dosimetry of fast, monoenergetic neutrons is amenable to straightforward attack. A previous paper (1) reported a study of the radiation chemistry of fast neutrons impinging on mixtures of liquid N2 and 02. For dosimetry the neutron flux was measured with a counter-a boron trifluoride proportional counter with a thick paraffin shield that provides a uniform response to 14-Mev neutrons and to neutrons from a calibrating plutonium-beryllium source. The flux measurement was then combined with cross-section and nuclear reaction data, as well as a geometrical analysis, to give the dose. The method required constant monitoring of the neutron flux during irradiations and frequent instrumental checks and calibrations. Because such studies often involve long irradiations during which the neutron flux varies appreciably, it would be most convenient to have a reliable chemical dosimeter. The Fricke dosimeter (an air-saturated dilute solution of ferrous ion in 0.8 N H2S04 with a small amount of NaCl added) has gained universal acceptance for work in radiation chemistry that involves X-rays, y-rays, or energetic electrons. G(Fe3+), the number of ferric ions produced per 100 ev deposited in the dosimeter, is a well-studied and monotonically decreasing function of ILET (initial linear energy transfer). G(Fe3+) decreases from 15.6 for lightly ionizing radiation, such as the y-rays from Co60, to an asymptotic value of about 4 for densely ioning particles such as the recoil a's and lithium atoms from the B10(n, a)Li7 reaction. Allen has summarized the work on the Fricke dosimeter through 1959 (2). Other, more sensi-