Development of a well-type phoswich detector for low concentration Krypton-85 measurement

Abstract

The measurement of low-level Krypton-85 (⁸⁵Kr) in low-volume influent samples from nuclear power station is required nowadays. We present the development of an original internal gas scintillator detector along with Low-level Automated ⁸⁵Kr Analyzing System (LAKAS) for that purpose. The designed phoswich detector is consist of a thin BC404 cell in the inner side for the detection of ⁸⁵Kr beta rays, and a well-type NaI(Tl) in the outer side for the reduction of background with anti-coincidence method, and a single photomultiplier tube is used for the fluorescence collecting make it very compact. Moreover, a lightweight shield and a high-speed digital pulses analyzer are constructed. The preliminary test with ⁸⁵Kr gas shows that the typical Minimal Detection Activity of the detector is as low as 0.012 Bq, and the Minimal Detection Concentration of LAKAS system is about 5 Bq/m³, which indicate that the detection efficiency to ⁸⁵Kr is comparable with the traditional liquid scintillation spectrometry and internal gas proportional counters.

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.