Development of ∅2×2 inch NaI(Tl) Scintillator Gamma Spectrometer Coupled with Compact Photo Sensor

Thallium doped sodium iodide (NaI(Tl)) scintillator with photomultiplier tube (PMT) has generally been used for a gamma spectrometer. However, a silicon photomultiplier (SiPM) has recently been considered as an alternative to... [ view full abstract ]

Thallium doped sodium iodide (NaI(Tl)) scintillator with photomultiplier tube (PMT) has generally been used for a gamma spectrometer. However, a silicon photomultiplier (SiPM) has recently been considered as an alternative to PMT due to compact size, insensitive to magnetic field, and low power consumption. In fact, several studies of SiPM in scintillator gamma detector were reported, which covered experiments of NaI(Tl) scintillator detector read out with SiPM and light guide. In this study, we focused on the implementation of array SiPM into inch NaI(Tl) scintillator gamma spectrometer as simple as possible. Three systems of gamma spectrometer were tested, as shown in Fig.1. One is composed of scinitllator (Epic Crystal) and PMT (Hamamatsu, H7195), another system is scintillator, SiPM (SensL, ArrayJ-60035-4P-EVB) with light guide (PMMA, 10mm and 30mm) and the other is scintillator directly coupled with SiPM. The parts where components were contacted each other were coated with optical grease to minimize loss of photon due to difference of reflective index. Surface of light guide or SiPM was covered with teflon as a diffuse reflector to transmit photons generated in scintillator to photo sensor. Fig. 2 shows an energy resolution as a function of energy of gamma ray. Energy resolution for 662 keV gamma ray achieved by scintillator directly coupled array SiPM spectrometer was 9.76 %. It was degraded to only about 28 % of the PMT. To generate photons at various position in scintillator, 662 keV gamma source was positioned around the center of scintillator. As shown in Fig. 3, energy resolution relative to that of 0° was increased as the angle was ascended, but light guide let the variation of energy resolution reduced. To verify the behavior of photons in scintillator and light guide, GATE based on Monte Carlo method was used to optical simulation. Optical photons were generated from 800 points in scintillator. The ratio of detected photons to generated photons, called light collection efficiency (LCE), was degraded when using light guide as shown in Fig. 4. Considering the result of angle experiment and simulation, light guide reduced the variation of LCE at each points, but get considerable photons disappeared without being delivered to sensor. Hence, SiPM scintillator detector without light guide is more appropriate as alternative to traditional detector. In the last August, we will plan to test the multi array SiPM scintillator detector and its performance could be comparable to that of PMT system.