The use of fast neutron threshold detectors for measuring the fast neutron (neutrons with energies greater than 1 Mev) flux in nuclear reactors takes advantage of the (n, p) and (n,α) reactions which various nuclides undergo when exposed to high-energy neutrons. In order to qualify as a good dosimeter system the reactant nucleus must have a convenient and well-known reaction cross-section and the decay characteristics of the product nucleus must be well known. Other desirable characteristics include a chemical state which facilitates handling of the dosimeter and nuclear properties which allow simple radioassay procedures. The chief consideration for a dosimeter to be used for monitoring long irradiations is the halflife of the product nuclide. It should be long enough so that a significant fraction of that activity produced early in the irradiation is present at the conclusion of irradiation. This usually leads to selection of a system in which the product nuclide's half-life is at least twice the length of irradiation, although it is possible to use the system when the irradiation time is nearly equal to the half-life provided the calendar time over which the irradiation is made and the decay time from end of irradiation to analysis are not excessive.