Gross Neutron Counting

Gross neutron counting for safeguards purposes is applied in searching for undeclared nuclear material and activities, process monitoring, measuring holdup in glove boxes, and in the assay of spent fuel. 

The handheld neutron monitor (HHNM) is a portable ( 4 kg) neutron detection device with three He proportional neutron counters, a GM counter and integrated electronics, which provide a means of searching for and localizing neutron radiation sources. A measurement sequence consists of background and verification measurements. When a predetermined threshold is exceeded, the detector triggers an alarm and records the relevant information. 

The spent fuel plutonium canister counter (SPCC) is designed to operate underwater and to count neutrons emitted from natural uranium spent fuel rods stored in dry stainless steel canisters (Menlove et al. 2002). The counter uses three He detectors located inside watertight and Pb-shielded pipes spaced 120° apart and surrounding a cylindrical cavity. SPCC determines Pu mass from total neutron counts corrected for background, spontaneous fission and multiplication. 

The portable neutron uranium holdup (PNUH) monitor system is a neutron counting system to determine the quantity of uranium holdup within the cascade halls of an enrichment plant (Beddingfield and Menlove 2002). PNUH is custom built and the detector head is a polyethylene-moderated assembly with 25 He neutron proportional tubes. PNUH measures the total neutron signals at various prescribed locations. Measurement data are acquired with standard INCC software and evaluated with specialized PNUH software using distributed source term analysis (DSTA). 

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In early work gross counting of delayed neutrons was used to determine the abundance of a single fissionable nuclide known to be in the sample. Brownlee 101> has reported techniques by which two or more fissionable species may be determined at the submicrogram level in a single irradiated sample. Nuclides fissionable only with fast neutrons may also be determined by this technique. One of the more interesting applications of the method is in the non-destructive determination of uranium and thorium at trace levels in minerals, rocks, and stony meteorites 102,108). 

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