Active-Passive Neutron Crate Counter

The Neutron detection system consists of cadmium shielded ³He proportional counters, typically in 4π geometry. The detectors are positioned vertically in each of the four side walls and detectors in the top and bottom of the instrument are positioned horizontally. The detectors in each of the four walls are connected to a high voltage junction box that contains an Amptek charge sensitive amplifier and discriminator circuit and connections for high voltage, low voltage and signal cables.

Polyethylene back shielding is provided to reduce the effects of ambient background. The outer surface of the counter is covered in stainless steel cladding and the graphite liner is clad in aluminium. Additional detector packages are mounted on the interior walls of the chamber cavity to acquire information on the degree of interrogating neutron self-shielding in fissile material. Flux monitors are installed to correct waste matrix bias effects.

The D-T neutron generator (Thermo Fisher Zetatron accelerator) is normally mounted in the roof of the chamber to provide a uniform interrogation of the crate during the assay. In conventional active totals mode pulses of neutrons are used to interrogate a waste drum. A neutron generator is pulsed at 100 Hz producing 14-MeV neutrons, which are slowed down in the graphite layer providing a source of thermal neutrons that cause induced fission of the ²³⁵U and ²³⁹Pu in the waste. Cadmium shielded detector packages are used to measure the prompt fission neutrons (from induced fission) in a time period (~500 µs) when neutrons from the initial generator burst have been cleared from the fast detector packages. Additional ³He flux monitors are placed in the measurement chamber and provide a measure of the interrogating neutron intensity and its die away characteristics. A later time period is used to determine the background due to delayed neutron production from fission products and from any spontaneous fission neutrons present. The fissile mass is proportional to the net fissile signal normalised to the interrogating flux. The system must be calibrated for uranium and plutonium mass using representative matrices.

The passive mode operation uses conventional neutron counting of the correlated neutrons arising from spontaneous fission of the even plutonium nuclides, principally ²⁴⁰Pu. Plant measured isotopic ratios (from high resolution gamma spectrometry) can be used by the software in order to convert ²⁴⁰Pu effective mass to total plutonium mass.