Measurement of fission-rate distribution on a standard uranium shell

Makrofol KG solid-state track recorders (SSTRs) are used to measure the absolute fission-rate distribution in the NISUS (Neutron Intermediate Standard Uranium Source) natural-uranium shell in the three orthogonal planes. The results show that the fission-rate ratios of the outer surface to the inner surface of the shell at different positions are not constant, contrary to the prediction of the ANISN one-dimensional transport code. The minimum and maximum fission-rate ratios in the NISUS uranium shell are 7.64 +- 0.24 and 11.58 +- 0.27 in the horizontal midplane at theta = 300 and theta = 1800C, respectively (theta = 00 is the out-of-pile direction). These ratios are compared with that of the ANISN calculation which gives the value of 10.86. The discrepancy is because of the non-uniformity of thermal neutron source distribution in the graphite cavity block. (author)