Development of a well-type phoswich detector for low concentration Krypton-85 measurement
Introduction
Krypton-85 (85Kr) is a radioactive fissiogenic isotope of noble gas with the longest half-life of 10.76 years. Most of the 85Kr is produced in anthropogenic nuclear activities, such as nuclear fuels reprocess, nuclear weapon tests, and nuclear power generate [1], [2], while a very small part is produced in cosmogenic activated reaction [3]. In accordance with an increase use of nuclear energy from 1950s, the world wide 85Kr inventory is observed to increase dramatically. It was reported that the atmospheric 85Kr concentrations has been reach to approximately 1.3 Bq/m3 in 2005 [4] and 1.5 Bq/m3 in 2010 (in the northwest region of Russia) [5]. The baseline level has been increasing steadily at a growth rate of approximately 0.04 Bq/m3 per year [6]. 85Kr is treated as a potential environmental pollutant for this reason [7], monitoring of 85Kr level in gas influent especially around nuclear power station and nuclear fuel reprocessing plant are very necessary. Moreover, the special origination and stable character of 85Kr make it a good tracer for detection of clandestine plutonium production [8], underground nuclear test monitoring [9], groundwater dating [10], and atmospheric dispersion study [11]. In these fields, a reliable and effective 85Kr measurement method is also required.
85Kr decays by emitting beta rays with maximum energy of 687 keV with a major branch ratio of 99.6%, or emitting a 173 keV beta ray associated with a 514 keV gamma ray with a small branch ratio of 0.43%, see Fig. 1. Various methods for 85Kr measurement have been developed. For high level 85Kr measurement, Gamma Spectrometry (GS) is normally used with detectors placed in situ [12]. For low level such as determination of 85Kr concentration in the general environment requires collection and enrichment of krypton gas from the air, followed by measurement of beta rays emitted from 85Kr with Low-Level Counting (LLC) method. In the past decades, Liquid Scintillation Counting (LSC) [13], [14], internal gas proportional counter (IGPC) [15], [16], internal gas CaF 2(Eu) scintillator detector [17] and GM detector [18] have been proposed, in which LSC and IGPC are most popular. Because of the ultra-low concentration of krypton in the air (1.14 ppm), sampling of 85Kr from air is a key point for low level 85Kr measurement, and a lot of krypton separation systems have been constructed for analyzing [19], [20], [21], [22]. In addition, a laser-based Atom Trap Trace Analysis (ATTA) method has been used to measure the single-atom level 85Kr in recent years [23], [24].
Among those methods for 85Kr measurement, GS is widely using as regular monitoring of gas influent from nuclear power station in China, which can only achieve Minimum Detectable Concentration (MDC) of tens of thousands of Bq/m3, 85Kr concentration cannot be determined in most situation. It was supposed that an effective method with MDC lower than 10 Bq/m3 to 3 L sample is a solution to this problem [25], [26], based on this requirements, a prototype named Low-level Automated 85Kr Analyzing System (LAKAS) has been constructed in Institute of Nuclear Physics and Chemistry (INPC) of China Academy of Engineering Physics (CAEP), see Fig. 2. LAKAS consists of three parts, the PLC (Programmable Logic Controller) control and gas pipeline part, the gas chromatography separation column, and the radioactive quantification part. LAKAS is designed to separate and measure 85Kr automatically from a 3 L marlin sampling bottle at normal temperature condition. The analyzed sample was measured with high-purity Germanium (HPGe) spectrometry in advance, and proven to be with low 85Kr concentration. Among the LLC method mentioned above, LSC and IGPC are the most popular but not suitable to this project, because of the insulation of 85Kr separation and measurement process in LSC, and the high demand of impurity removal in IGPC. To fulfill the requirements of LAKAS, a NaI(Tl)/BC404 well-type phoswich detector was designed and assembled in INPC. In this paper, we will describe the design of the phoswich detector and the performance tests by standard source and 85Kr sample gas in the latter sections. Moreover, the MDC of LAKAS system was calculated based on the background measurement and air sampling capability.
Section snippets
Design of the detector
The primary goal of the detector is to meet the requirement of MDC below 10 Bq/m3 [26], and to match with the output of the gas chromatography separation process. The idea of a well-type phoswich detector for 85Kr measurement originates from our past work on development of a 85Kr internal BC404 scintillator detector [27] and a radioxenon phoswich detector named NAPD (NaI-based Phoswich Detector) [28], [29], and other works [30].
The geometry of the 85Kr phoswich detector is shown in Fig. 3. A
Typical pulses of Kr phoswich detector
Typical pulses of 85Kr phoswich detector were acquired with the condition describe above: the input high voltage was set at the middle of plateau area near 750 V; the acquire range and the trigger threshold of high-speed oscilloscope were set at 0.5 V and at 0.008 V, respectively. By switch of acquirement mode among beta, gamma and coincidence; typical BC404, NaI(Tl) and BC404&NaI(Tl) pulses were recorded in particular discrimination algorithms, as shown in Section 2.2. The beta of BC404 pulses
Conclusion
In this work, we proposed a well-type phoswich detector for low concentration 85Kr measurement, and introduced the first generation of LAKAS system development for low-level 85Kr measurement in low-volume gaseous effluent of nuclear power station. The preliminary test with 85Kr gas shows that the typical MDA of the detector is as low as 0.012 Bq, and the MDC of LAKAS system is about 5 Bq/m3, which indicates that the detection efficiency to 85Kr is comparable with the frequently-used LSC and
CRediT authorship contribution statement
Jun Zeng: Project leader, Propose idea, Writing original draft. Yongchun Xiang: Detector construction. Fei Luo: Software. Changfan Zhang: Review & editing, Monte Carlo simulation. Xiaonan Wu: Gas charging. Qingpei Xiang: Krypton-85 production. Fanhua Hao: Supervision, Review & editing. Rende Ze: Electronic circuit.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
This work was supported by the National Natural Science Foundation of China (Grant No. 11905195). The authors would like to thank Dang Yufeng and Tian Jie of INPC for their contributions to this work.
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