Estimating the γ-ray contribution to the signal of fission chambers with Monte Carlo simulations

The fission chambers that provide in-core measurements of the neutron flux in nuclear reactors are also sensitive to the gamma-ray flux. According to experimental data, this contribution can be dominant in current mode. We present a calculation route, relying on a description of the physical phenomena, that compute the gamma-ray signal in a fission chamber, coming directly from the gamma-ray flux, and indirectly from the neutron flux and activated structures. We use it in a working example for which we show that the main contribution of the gamma-ray flux to the signal comes from the interaction of the gamma-rays with the electrodes. The relative contribution of gammas created by neutron interactions in the detector is small. The comparison of this example with experimental results yields a good qualitative agreement: the gamma-ray signal may dominate in current mode, but is effectively suppressed in fluctuation mode. This calculation route will be useful for the design of fission chambers to be operated under high neutron and gamma-ray flux. (C) 2011 Elsevier B.V. All rights reserved.