Fast neutron dosimetry by the reaction 93Nb(n,n'). Counting technique for 93Nb activity

Nb-Fe-Cu activation detectors have been used, for some time, for fast neutron dosimetry measurements in a number of Swiss power reactors and the use of such detectors will continue. The use of Fe and Cu detectors leads to no particular problems but, in the case of Nb, there are difficulties related to the preparation of the detectors, the measurement of the intensity of the emitted radiation and the evaluation of the results. The main problem is that the niobium, unavoidably, contains a few ppM of tantalum. The intense Bremsstrahlung of 182Ta excites the 16.6 keV x-rays of Nb which are emitted in addition to the /sup 93m/Nb radiation which is of interest. The production rate of the externally produced Nb x-rays as a function of the sample thickness and 182Ta specific activity was determined. It was found that to avoid this effect it would be necessary to prepare counting samples with a thickness not greater than 0.1 to 0.5 mg/cm2