The N2920-2550 Passive Neutron Crate Counter is used for the determination of the 240Pu mass in transuranic (TRU) waste transport containers. It can be configured to the customer’s requirements to measure a variety of waste boxes including B25 Crate or the Standard Waste Box (SWB). The instrument performs passive neutron counting of the correlated neutrons arising from the spontaneous fission of even plutonium nuclides, principally 240Pu.
The neutron detection system consists of polyethylene modules. Each of these modules contains eight 3He tubes connected to a junction box. The junction boxes are hermetically sealed and contain not only the high voltage distribution and charge collection circuitry, but also Amptek model A-111 charge sensitive amplifiers. The detector modules are housed inside a 270 mm (10.63 in) thick polyethylene outer shield.
A cadmium liner on the internal face of the modules can be removed to change the drum monitor to a higher efficiency unfiltered configuration. The N2920-2550 has a typical detection efficiency of ~20% with the cadmium liners deployed and ~30% with the cadmium liners removed.
The N2920-2550 is coupled with a Multiplicity Shift Register that uses an optimised version of the Los Alamos National
Laboratory (LANL) INCC code based on super-HENC technology for cosmic-ray reduction. This involves multiplicity distribution truncation and veto counting of adjacent detectors to reduce the cosmic-ray generated coincidence background.
The N2920-2550 is available with an optional ‘Add-a-Source’ 252Cf based matrix correction capability. This is a well established technique for matrix correction and is appropriate when low count rates and small quantities of material are present in the measurement chamber. ‘Add-a-Source’ complements matrix correction by neutron multiplicity counting. The system typically consists of an 80 μCi 252Cf source deployed on a ‘Teleflex’ cable with associated shielding, motor drive and control.
The N2920-2550 can be operated in conventional shift register coincidence counting (reals) mode with a calibration function, the absolute multiplicity counting mode (histogram function), or totals counting mode. Plant measured isotopic ratios can be used by the software to convert 240Pueffective mass to total plutonium mass.