JCC-41: Flat-Squared Neutron Coincidence Counter

Features

• Well-type neutron coincidence counter for in-plant applications.
• Special design features give uniform spatial response axially over the sample cavity.
• Relatively insensitive to sample matrix effects.
• Relatively flat neutron energy response.
• Twenty-four ³He Detectors.
• Fast Amptek electronics.
• Large (61 x 24 cm) sample cavity.
• Exterior neutron shielding.
• Unit's high efficiency (>24%) provides high counting rates and good precision.
• Response is better than 28 counts/sec per gram ²⁴⁰Pu (a 1000 second counting time gives a precision of 2.4% for 0.06 gram ²⁴⁰Pu)¹ .

Description

The JCC-41, which is based on Monte Carlo design calculations performed at Los Alamos National Laboratory¹, measures the effective ²⁴⁰Pu mass in a sample by detecting coincidence neutrons from the spontaneous fission of the even numbered isotopes of plutonium. The JCC-41 is designed for in-plant measurements of large plutonium samples (up to several kg Pu), but can be used for plutonium waste samples with milligram quantities of plutonium. The JCC-41 has a higher efficiency, larger sample size and flatter axial response than the transportable JCC-31. The system consists of a JCC-41 counter head and sample hoist mechanism. A JSR-12 Neutron Coincidence Analyzer, a computer and analysis software are required for coincidence counting but are not included with the JCC-41.

The JCC-41 has a cylindrical-shaped sample cavity. It is intended to assay plutonium samples including PuO₂, mixed oxides (PuO₂-UO₂), metal carbides, fuel rods, fast critical assemblies, solution, scrap, and waste. A cadmium sleeve surrounds the sample cavity to prevent the reentry of thermalized neutrons into the sample, which could induce fission in the sample and adversely affect the results.

Outside the cadmium sleeve is a polyethylene/cadmium liner to flatten the axial response. Twenty-four ³He tubes are embedded in the high-density polyethylene. The tubes are arranged in a single ring around the sample with optimum spacing between the tubes for maximum counter efficiency. The tubes are divided into six groups of four with each group wired together and connected to one of the six JAB-01 Amplifier/Discriminator circuit boards which are mounted inside a high voltage junction box. LED indicator lights are placed externally on the junction box to indicate proper operation of each JAB-01 channel. Electrical connections between the JCC-41 and the JSR-12 include +5 V, HV, and a single "ORed" output signal.

A cadmium sleeve wrapped around the outside of the JCC-41 provides radiation protection for personnel as well as background reduction.

The external polyethylene shielding and special design (graphite end plugs and polyethylene/cadmium liner) give uniform response axially over the sample cavity, making the counter relatively insensitive to matrix effects.