High resolution, portable gamma spectrometer not requiring liquid nitrogen cooling for field use for nuclear security and safeguards applications

Abstract

In-situ characterization of samples by experts, using high resolution gamma spectrometers, is required if low resolution NaI detector-based hand-held radionuclide identification devices are unable to provide definite spectral identifications. In some remote locations, the use of liquid N-2 cooled HPGe detectors is not possible. Electrically cooled HPGe detectors are not always a feasible alternative, as they are expensive, bulky and heavy. Therefore we have developed a portable gamma spectrometer with a unique, nearly room temperature semiconductor (CdTe) detector to close this gap. The technology of CdTe crystal growing developed at ACRORAD (Japan) and the technology of p-i-n structures with high breakdown voltage developed at PNPI (Russia) allowed creation of high performance detectors with dimensions of 13.9mm x 13.8mm x 2.23 mm, i.e. with a sensitive volume of more than 400 mm3. The spectrometer system uses a charge loss correction circuit to improve energy resolution, and detector temperature stabilization by Peltier cooler to ensure consistent operation of the detector during field measurements over a wide range of ambient temperatures. The system can operate continuously for up to 12 hours on rechargeable batteries. Using the combination of the hardware with dedicated software, the device can determine the isotope vector of Pu and U samples without calibration and characterize unknown, shielded or masked radionuclides. Experimentally, we have recently been able to perform the first Uranium and Plutonium isotopic analysis with a device other than a cryogenically-cooled germanium spectrometer. The CdTe spectrometer is capable of measuring small plutonium reference samples and uranium samples with the mass of a few grams in about 20 minutes, with an energy resolution ≤ 2.0 keV for the gamma line of 186 keV (U-235) and ≤ 0.5 % FWHM for the gamma line 662 keV (Cs-137). Thus, the efficiency of γ-radiation registration for such a detector in the energy range from 200 keV to 1500 keV is equivalent to a HPGe detector with sensitive volume about 3 cm3. This extends significantly the express measurement capability for nuclear materials examination for highly portable instruments. The development of this device has been supported under Research Contracts of the International Atomic Energy Agency and the US DOE Research Grant.