FISSILE MASS FLOW MONITOR GAMMA RAY DETECTOR SYSTEM DESIGNED FOR LARGE-SIZE PROCESS PIPES

In this paper, a detailed design description and the performance characteristics are presented for the gamma ray detector system developed for the fissile mass flow monitor (FMFM) to be used on 8-in.-diam process pipes. The FMFM continuously measures the 235U fissile mass flow rate of a UF6 gas stream. It uses moderated and modulated 252Cf neutron sources for fission activation of the UF6 gas. Four pairs of bismuth germinate (BGO) scintillation detectors are placed around the process pipe (on the top, bottom, front, and back) downstream of the neutron sources so that a high detection efficiency can be achieved for measuring the delayed gammas (> 0.3 MeV) emitted from the fission fragments in the stream for the flow measurements. The BGO crystal was selected because it is a novel scintillation material (a rugged, nonhygroscopic, neutroninsensitive, high-density, and high-Z material) with high absorption (stopping) power and because it has high peak photoefficiency for high-energy gammas. Each 4-in.-diam, 2-in.-thick BGO scintillation crystal is coupled to a 3-in.-diam photomultiplier tube (PMT). Both are shielded with lead to reduce the background signal. Each detector pair is housed in a metal enclosure that also contains an electronics board for signal shaping and counting. The front-end electronics boards independently amplify and shape the detector signals before they are summed. The combined signal is then fed into two single-channel analyzers (SCAs), which separate the pulses into low- and high-energy bands and subsequently determine the count rate in each band. A local area network node for providing secure data transmission to the FMFM computer and a local temperature sensor are also parts of each detector electronics board. The temperature response of the detector signal resulting from the intrinsic temperature coefficient of each BGOPMT assembly is used for real-time corrections of the system by monitoring the 186-keV spectrum line of 235U obtained with the lower-energy SCA. The lower-energy SCA is also scanned during manual detector energy calibration to capture 186- or 96-keV spectral lines from the UF6, depending upon its enrichment. The higher-energy SCA output provides for the measurement of the 0.3- to 2-MeV delayed gamma ray signal from the fission fragments in the UF6 stream.