LaBr3γ-ray spectrometer for detecting 10B in debris of melted nuclear fuel

Neutron resonance densitometry has been proposed as a nondestructive analytical method for quantifying special nuclear material (SNM) in the rock- and particle-like debris that is to be removed from the Fukushima Daiichi nuclear power plant. The method is based on neutron resonance transmission analysis (NRTA) and neutron resonance capture analysis combined with prompt-γ-ray analysis (NRCA/PGA). Although quantification of SNM will predominantly rely on NRTA, this will be hampered by the presence of strong neutron-absorbing matrix materials, in particular 10B. Results obtained with NRCA/PGA are used to improve the interpretation of NRTA data. Prompt γ rays originating from the 10B(n, αγ) reaction are used to assess the amount of 10B. The 478 keV γ rays from 10B, however, need to be measured under a high-radiation environment, especially because of 137Cs. To meet this requirement, we developed a well-shaped γ-ray spectrometer consisting of one cylindrical and four rectangular-cuboid LaBr3 scintillators combined with a fast data-acquisition system. Furthermore, to improve the gain stability of the main detector, a special high-voltage divider was developed. Because of the reduction in gain shift, a 3.8% resolution at 662 keV was obtained for long-term measurements. By using the data-acquisition system, which consists of eight 250 MHz digitizers, input signals of over 500 kHz per channel were recorded. The work reported herein demonstrates that, with such a spectrometer, the impact of the Compton edge of 662 keV γ rays from 137Cs is significantly reduced, which allows the 10B amount to be determined with greater sensitivity.