Xenon Detectors

There is a need for reliable and sensitive nuclear radiation systems suitable for detecting accidental or intentional transport of special nuclear materials through portals such as border crossings, exits from nuclear storage facilities, gates of nuclear plants, etc. The detectors for these systems must be rugged, affordable, and insensitive to environmental temperature extremes. They must have good efficiency and energy resolution, but not require frequent servicing and maintenance; for example, they cannot be reliant on liquid cryogenic systems.

Practically all nuclear material emits gamma-rays with a characteristic signature. Therefore, these gamma-rays can be used for detection and identification of the nuclear material. To date, two general categories of gamma-ray spectrometers have been used for these purposes:

• Semiconductor detectors (germanium has the highest resolution) have excellent energy resolution (~0.1% at 1 MeV). However, semiconductor detectors are very expensive and are comparatively sensitive to hostile environments.

• Scintillation detectors (NaI(Tl) is the most popular scintillator) have the advantages of ambient-temperature operation, relatively low cost per unit of mass, and are available in large crystals. However, scintillation detectors suffer from low energy resolution, are fragile, and are sensitive to temperature changes.

Recent progress has been made in developing high-pressure xenon ionization chambers that are:

• Relatively insensitive to temperature changes

• Rugged

• Have demonstrated a good energy resolution with high efficiency and without cryocooling

Constellation has developed a family of high-pressure xenon ionization chambers (HPXe) with a mass of working medium between 100 g and 2 kg. These detectors demonstrate good energy resolution (below 3% FWHM at 662 keV for 0.8 kg HPXe mass) that is close to the resolution of room temperature semiconductor detectors with only a few grams working mass. They fill the niche in energy resolution between germanium and scintillation detectors. The detectors employ a relatively simple design, robust construction, and are available in a variety of pipe-like configurations. The sensitive area of HPXe may cover a few square feet and detect gamma rays with an efficiency close to solid state detectors. Constellation HPXe experimental detectors have been constructed in portable and mobile systems available for testing in the conditions typical of nuclear storage facilities or plants.

The HPXe-1001 detection system has the detector mounted on a shock-absorbing frame inside a transportable case. The detector is integrated with a charge-sensitive preamplifier (CSPA) and high-voltage power supply (HVPS). A low-voltage power supply is installed in the case. The system requires standard 120V AC power. The data-acquisition system is not included in the system. Optionally, a number of MCA devices can be provided for data analysis. The HPXe-1001 specifications are:

A schematic drawing of the detector design is shown below. All internal parts are made of stainless steel and ceramic. The cylindrical vessel has the following dimensions: 119 mm outside diameter, 770 mm length, and 3.1 mm wall thickness. The vessel has been hydraulically test at 100 bar internal pressure. The shielding grid is a 40 mm diameter cylinder made of 0.25 mm thick stainless steel mesh with 5 x 3 mm holes spaced 0.25 mm apart. The screening efficiency of the grid is 97%. The grid is secured to the vessel of the chamber with annular ceramic insulators. In addition, these insulators suppress microphonics and protect the chamber from external vibrations and acoustical effects. Two high voltage (HV) ceramic feedthroughs are mounted on the flange. A gas input valve, a charge-sensitive amplifier, and a compact HV power supply with filter are mounted to the flange as shown below. One HV feedthrough is used for signal output and the other one is used for the high voltage supply (connected to the grid). The vessel and Teflon^® cylindrical insulator are covered by several Teflon^®-aluminum-polyolephyne (shrink tube) layers serving as electrostatic and HV shielding. The sensitive volume of the detector is 2 liters. The end-on sensitive area is 100 cm². The operational gas pressure is 40-50 atmospheres (atm). A voltage of 20.5 kV is applied to the cathode, and 12.5 kV to the shielding grid.

The HPXe-1001 detector module is mounted on a shock-absorbing frame inside the transportable case. The case dimensions: 50" x 20" x 13.5."

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