E-mail:
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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.