Le Petit G, Cagniant A, Morelle M, Gross P, Achim P, Douysset G, Taffary T, Moulin C
CEA, DAM, DIF, 91297 Arpajon, France.
Canberra Semiconductor NV, Olen, Belgium.
J Radioanal Nucl Chem. 2013;298(2):1159-1169. doi: 10.1007/s10967-013-2525-8. Epub 2013 May 17.
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, Xe, Xe, Xe and Xe, 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 Xe and Xe with a gain in minimum detectable activity (about 2 × 10 Bq) relative to current CTBT SPALAX™ system (air sampling frequency normalized to 8 h) of about 70 and 30 respectively.
《全面禁止核试验条约》（CTBT）的核查机制基于三种不同波形技术的网络，同时对气溶胶和惰性气体进行全球监测，以便探测、定位和识别当量低至1千吨TNT的核武器爆炸。对于低强度的地下或水下核爆炸，似乎只有放射性气体，特别是难以遏制的惰性气体，才能识别低当量核试验。四种放射性氙同位素，Xe、Xe、Xe和Xe，在裂变反应中大量产生，具有合适的半衰期和辐射发射，能够在远离释放地点的低水平大气中被探测到。为了高灵敏度地采样和测量它们，已经开发了四种不同的CTBT监测系统，即ARIX、ARSA、SAUNA和SPALAX™。法国原子能委员会（CEA）最新开发的系统，配备新型转换电子（CE）/X射线符合光谱仪后，通过更高的采样率，其探测灵敏度（特别是对于亚稳同位素）可能会大幅提高。这种新型光谱仪基于两个组合探测器，二者均具有极低的放射性本底：一个用于光子探测的井型碘化钠（铊）探测器环绕着一个气室，气室配备了两个用于电子探测的大型钝化注入平面硅芯片。其特点是电子能量阈值低，CE的能量分辨率比现有CTBT设备通常测量的要好得多。此外，光谱仪的紧凑结构为与四种相关放射性氙的衰变模式相关的X射线和CE提供了高效率。本文重点介绍这种新型光谱仪的设计，并根据放射性氙标准和气样测量的最新结果，展示了一个原型的光谱性能。已经实现并量化了探测灵敏度的重大提高，特别是对于亚稳放射性同位素Xe和Xe，相对于当前CTBT的SPALAX™系统（空气采样频率归一化为8小时），其最低可探测活度（约2×10 Bq）分别提高了约70倍和30倍。
