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Abstract: In order to solve the problem of searching a radioactive source in a wide area, we developed a mobile CsI detector. This paper presented the performance of the detector during the spectra collection investigation. The 1 s spectrum collected by the detector was low-count spectrum and it is hard to distinguish whether it contains radioactive source signals. A rapid detection method of radioactive source based on low-count gamma spectra was proposed. Principal component analysis (PCA) was the key technology of the method. According to the PCA, the source information was efficiently extracted. With the method, the detect sensitivity and accuracy of the detector were optimized.
Keywords: low-count spectrum, gamma ray, detection method, principal component analysis
Introduction
With the development of nuclear science and technology, the problem of human energy shortage was relieved. However, it also brings threat to nuclear safety. The threats from nuclear weapons are increasing. After the 9.11 incident, “dirty bombs” received more and more attention. This nuclear weapon composed of radionuclides will pose a serious threat to people’s health. Besides, the leak of nuclear power plants and large nuclear facilities, the loss of radionuclides also threaten public safety.
Although, radioactive materials are generally subject to very strict management. However, it is difficult to control these risks in the event of an accident or terrorists bring the above-mentioned nuclear weapons to unknown areas through illegal ways. A new method to solve the above problems is to employ multiple detectors to search for radioactive substances in the suspect area. Through the system’s centralized scheduling and search path, the location of the radioactive sources can be quickly obtained in the suspect area. Such mobile radiation detectors are usually required to be small and easy to carry. The Kromek Group has developed such series radioisotope identification device named D3S, which was able to detect a Cs-137 source with activity of 16.1mCi when the detector was 10 m away from the radioactive source. Therefore, employing mobile detectors for joint detection of radioactive sources is a development trend at present. However, mobile detector means the sensor of the detector should be small. The counts of the spectra collected by such detectors are low in common. Under the influence of background count and statistic fluctuation, it is hard to detect the information of the radioactive source.
Prateek Tandon illustrate his research with application to the nuclear threat detection domain, which is related to our application[1]. The detector is consist of double 4 × 16 inch NaI planar spectrometer installed on a van driving in an urban area. Miltiadis Alamaniotis proposed several algorithms for analysis of the radiation detection based on low-count gamma spectra[2-5]. Kirkpatrick proposed poisson statistical methods for the analysis of low-count gamma spectra[6]. There are other groups carried out the similar study [7-10]. However, the algorithms proposed in the above researches are complex. And they are not suitable for ultra-low count spectra.
This paper presented a CsI detector we developed. The specification of the detector was similar with the D3S. During the performance test investigation, we found that the low-count of spectrum make it hard to determine whether the spectra contain radioactive source signal or not. This paper also proposed a rapid detection method of Cs-137 with the low-count spectra, which has optimized the sensitive and accuracy of the detector.
Specification of the CsI detector and performance investigation
Specification of the CsI detector
The dimensions of the CsI crystal of the detector is 5cm×2.5cm×1.25cm. The fluorescence signal of CsI crystal is collected by 8 Silicon photomultiplier (SiPM) array with dimensions of 6mm×6mm. The energy resolution was approximately 6.8% (@662keV). The energy range of the detector was 30keV-3MeV. The endurance time was less than 12 hours.
In this study, the gamma-ray spectra collected by the detectors were calibrated by lutetium 177 powder. With the calibration work, the relationships between the spectra channels and energies were united. The number of channels of each set of spectra was 1024.
Background spectra investigation
In this paper, in order to deeply test the performances of the CsI detectors we developed, the background spectra investigation was carried out. In the investigation, 1-s-measured spectra were employed. A set of background spectrum measured by the detector was showed in figure 1.