Standard Test Method for Measuring Fast-Neutron Reaction Rates by Radioactivation of Iron

Abstract

5.1 Refer to Guide E844 for guidance on the selection, irradiation, and quality control of neutron dosimeters. 5.2 Refer to Practice E261 for a general discussion of the determination of fast-neutron fluence rate with threshold detectors. 5.3 Pure iron in the form of foil or wire is readily available and easily handled. 5.4 Fig. 1 shows a plot of cross section as a function of neutron energy for the fast-neutron reaction 54Fe(n,p)54Mn (1).3 This figure is for illustrative purposes only to indicate the range of response of the 54Fe(n,p)54Mn reaction. Refer to Guide E1018 for recommended tabulated dosimetry cross sections. 5.5 54Mn has a half-life of 312.19 (3) days4 (2) and emits a gamma ray with an energy of 834.855 (3) keV (2). 5.6 Interfering activities generated by neutron activation arising from thermal or fast neutron interactions are 2.57878 (46) h 56Mn, 44.494 (12) days 59Fe, and 5.2711 (8) years 60Co (2, 3). (Consult the latest version of Ref (2) for more precise values currently accepted for the half-lives.) Interference from 56Mn can be eliminated by waiting 48 h before counting. Although chemical separation of 54Mn from the irradiated iron is the most effective method for eliminating 59Fe and 60Co, direct counting of iron for 54Mn is possible using high-resolution detector systems or unfolding or stripping techniques, especially if the dosimeter was covered with cadmium or boron during irradiation. Altering the isotopic composition of the iron dosimeter is another useful technique for eliminating interference from extraneous activities when direct sample counting is to be employed. 5.7 The vapor pressures of manganese and iron are such that manganese diffusion losses from iron can become significant at temperatures above about 700 °C. Therefore, precautions must be taken to avoid the diffusion loss of 54Mn from iron dosimeters at high temperature. Encapsulating the iron dosimeter in quartz or vanadium will contain the manganese at temperatures up to about 900 °C. 5.8 Sections 6, 7, and 8 that follow were specifically written to describe the method of chemical separation and subsequent counting of the 54Mn activity. When one elects to count the iron dosimeters directly, those portions of Sections 6, 7, and 8 that pertain to radiochemical separation should be disregarded. Note 1: The following portions of this test method apply also to direct sample counting methods: 6.1 – 6.3, 7.4, 7.9, 7.10, 8.1 – 8.5, 8.18, 8.19, and Sections 9 – 12.