Innovative concept for a major breakthrough in atmospheric radioactive xenon detection for nuclear explosion monitoring

Abstract

The verification regime of the comprehensive test ban treaty (CTBT) is based on a network of three different waveform technologies together with global monitoring of aerosols and noble gas in order to detect, locate and identify a nuclear weapon explosion down to 1 kt TNT equivalent. In case of a low intensity underground or underwater nuclear explosion, it appears that only radioactive gases, especially the noble gas which are difficult to contain, will allow identification of weak yield nuclear tests. Four radioactive xenon isotopes, 131mXe, 133mXe, 133Xe and 135Xe, are sufficiently produced in fission reactions and exhibit suitable half-lives and radiation emissions to be detected in atmosphere at low level far away from the release site. Four different monitoring CTBT systems, ARIX, ARSA, SAUNA, and SPALAX™ have been developed in order to sample and to measure them with high sensitivity. The latest developed by the French Atomic Energy Commission (CEA) is likely to be drastically improved in detection sensitivity (especially for the metastable isotopes) through a higher sampling rate, when equipped with a new conversion electron (CE)/X-ray coincidence spectrometer. This new spectrometer is based on two combined detectors, both exhibiting very low radioactive background: a well-type NaI(Tl) detector for photon detection surrounding a gas cell equipped with two large passivated implanted planar silicon chips for electron detection. It is characterized by a low electron energy threshold and a much better energy resolution for the CE than those usually measured with the existing CTBT equipments. Furthermore, the compact geometry of the spectrometer provides high efficiency for X-ray and for CE associated to the decay modes of the four relevant radioxenons. The paper focus on the design of this new spectrometer and presents spectroscopic performances of a prototype based on recent results achieved from both radioactive xenon standards and air sample measurements. Major improvements in detection sensitivity have been reached and quantified, especially for metastable radioactive isotopes 131mXe and 133mXe with a gain in minimum detectable activity (about 2 × 10−3 Bq) relative to current CTBT SPALAX™ system (air sampling frequency normalized to 8 h) of about 70 and 30 respectively.

An alpha–gamma coincidence spectrometer based on the photon–electron rejecting alpha liquid scintillation (PERALS®) system

“…This gives detection limits for actinides such as 237 Np and 241 Am of to 2 mBq. As several recent studies have examined the use of solid state scintillators or detector for detection of xenon nuclides [34,[41][42][43][44][45], the behavior of the PERALS® coincidence system was also investigated in beta/gamma operation with a 133 Xe sample. The SCA window was adjusted to cover the region of the beta/gamma pulses in the pulse shape output, and the ancillary electronics were set to generate the gating signal from the beta pulses.…”

Radioxenon net count calculations revisited

“…Additionally, a mechanical structure is required to house the silicon detectors within the gas cell (required for vacuum and structural integrity of the silicon). This outer gas cell causes a reduction of detector efficiency to ~ 50% for the PIPSBox [36] and 67% for the Lares-detector [37]. And finally, the thickness of the silicon attenuates the photons, and further reduces the coincident beta-gamma detection efficiency [37].…”

A Figure-of-Merit for Beta Cell Detector Characterization

“…The beta-gamma simulation tool (BGSim) developed by PNNL 1 was utilized throughout this work in order to study the effect of varying parameters of interest. For these simulations, an 11-hour count time was utilized, with an assumed xenon volume of 1.3 cm 3 . The background count rate was 0.24 events per minute, and the associated live time was taken to be 100% of the count time.…”