Gamma dose characterization of the neutron field in the ΣΣ irradiation facility

We present a method to estimate the gamma dose in a neutron field using a parametrization of the doses measured with thermoluminescent detectors. The source for the high neutron fluences was the ΣΣ facility, a uranium multishell sphere placed in the thermal column of the VVRS reactor of the INPE Bucharest, which has a high precision neutron fluence calibration (4%) and a very well defined neutron spectrum. The photon spectrum inside the ΣΣ, degraded by absorption and Compton scattering was simulated with a Monte Carlo method. The doses, up to 150 Gy, were measured with thermoluminiscent detectors (TLDG MgF2(Mn) and CaF2) calibrated up to 8 MeV with radioactive sources and in the bremsstrahlung radiation of the IPTRD-Bucharest linear electron accelerator. A rough model for separating the contribution of prompt and delayed gammas is used to calculate the doses for different time characteristics of irradiation. For the ΣΣ facility, the ratio between the prompt gamma dose and the neutron fluence is (1.73±0.09) pGy cm2/neutron; the contribution of the delayed gamma depends on the time characteristics of the irradiation. The method can be used to evaluate the gamma dose in any fission environment if the gamma and neutron spectra are approximately known.