JCC-15: Glovebox Neutron/Gamma System

Features

• Designed for high accuracy measurements of plutonium, reducing the need for destructive analysis.
• Mounts below the drywell of a glovebox.
• Provides simultaneous neutron coincidence and gamma isotopic analysis.

Description

The Glovebox Neutron/Gamma System is a combination of a passive neutron coincidence counter and a gamma spectroscopy system that is placed around the drywell of the glovebox of the On-Site Laboratory (OSL). The OSL concept was developed by the Euratom Safeguards Directorate to allow high accuracy measurements of plutonium samples, reducing the need for destructive analysis. The system includes the OSL Neutron Counter, a JSR-12 Neutron Coincidence Analyzer, a Low Energy Germanium (LEGe) Detector, the NIM electronics, and a data processor with software.

The neutron counter is a modification of the Los Alamos National Laboratory INVS III, which was designed to provide high efficiency with an axial response that is less sensitive to variations in sample position. The design was modified to place a LEGe detector inside the counter, close to the sample, to simultaneously measure the plutonium isotopics. Both the top and bottom plugs are made from graphite and polyethylene. The bottom plug was altered to fit around the LEGe detector and the cold finger, reflecting neutrons back into the counter and flattening the axial response. The system is mounted on a trolley to facilitate positioning the counter under the glove box.

The neutron counter was further modified by splitting the body into two parts to allow placement around the drywell. Eighteen ³He tubes, arranged in a double ring with nine tubes each, are embedded in the high density polyethylene moderator surrounding the sample cavity. The tubes are divided into six groups of three, with each group wired together and connected to a JAB-01 Amplifier/Discriminator board. These are mounted inside a high voltage junction box that has also been split into two pieces. LED indicators are placed external to the junction box to indicate proper operation of each JAB-01. Connections between the OSL and the JSR-12 include +5 V, HV, and a single ORed output signal.

The plutonium gamma-ray spectrum is very complex, with several multiplet structures. Software like MGA, which was developed at Lawrence Livermore National Laboratory, uses the integrals of these close lying peaks to calculate the isotopic composition of plutonium. To make the measurement, the gamma system must have good energy resolution (FWHM at 122 keV of approximately 550 eV) and a high counting efficiency. CANBERRA's LEGe detectors are used for those measurements because they have demonstrated better resolution and far less slow pulses than other types of detectors with the same active area.

The measured isotopics are used as input to the neutron coincidence software to convert the ²⁴⁰Pu-effective to total plutonium mass.