1. Characteristics of Miniaturized CsI(Tl) Scintillator for Mobile Radiation Detector Hyunjun Yoo 1, Chankyu Kim 1, Yewon Kim 1, Eunjoong Lee 1, Segyeong Joo 2, Gyuseong Cho* 1 1 : KAIST, 291 Daehak-ro, Yuseong-gu, Daejeon 305-701, Republic of Korea 2 : ASAN MEDICAL CENTER, 88, Olympic-ro 43-gil, Songpa-gu, Seoul 138-736, Republic of Korea E-mail : cmos@kaist.ac.kr III. Results and Conclusions Fukushima disaster made people worry about radiation exposure from daily living environment. Existing radiation detectors using Geiger Muller tube or photo multiplier tube are not suitable for being integrated into small or mobile devices like smart phones due to its bulky volume. To build a truly miniaturized radiation detector, characteristics of a scintillator coupled with a silicon detector should be confirmed, especially when a scintillator is miniaturized. There are two issues in designing the geometry of the scintillator to be integrated in a mobile radiation detector. One is the performance in detecting background radiations, which depends on the volume of the scintillators. When the volume is too small, the scintillator cannot detect enough amounts of background radiations for calculating dose from background radiation. The other is shaping the scintillators to minimize the photon loss due to the absorption at outer wall (Teflon taping), which are arose by the tapered structure with small output area of the scintillators to reduce the dark current of silicon detector (3×3mm2 of CsI(Tl) and 3.16×3.16mm2 of SiPM). Therefore, in this paper, we suggested various geometries of scintillators and tested the performance of the background detection and the amount of photon loss. “This work was supported by the Center for Integrated Smart Sensors funded by the Ministry of Science, ICT & Future Planning as Global Frontier Project” (CISS-2011-0031870) II. Materials & Simulation ◆ CsI(Tl) Crystal The CsI(Tl) crystal used in these experiments had three shapes. The first was cube shape had three different size of area as 3×3, 6×6, 9×9 mm2 and six different thickness as 2, 3, 4, 6, 8, 10 mm for checking the performance in background detection with increasing the volume of CsI(Tl) scintillators. The second was cylinder shape with different tapered structures for the experiment for measuring the reduction of the output signal due to the photon loss from absorption at outer walls of scintillators with various shapes. Each CsI(Tl) crystals were manufactured in Sinoma(China). The detail information of shapes and size was in bottom. ◆ Silicon Photo Multiplier(SiPM) SiPMs are considered better alternatives than photo multiplier tubes; which are also more expensive and bulkier. Another kind of silicon detector, the PIN diode, has a very low gain (about ‘1’) and thus needs very low feedback capacitor, also, high feedback resistor in the preamplifier (more than 10 MΩ) for enough gain to amplify the charge signal. The SiPM used in these experiments had 3 different area as 3.16×3.16, 6.32×6.32, 9.48×9.48mm 2 and it was developed in SensL(Ireland ) ◆ Light Tools Simulation ◆ Background Detection First experiment checked the performance in background detection with increasing the volume of CsI(Tl) crystals. The experiments were performed in a concrete room with area of 20m 2 and height of 2.5m. A silicon detector, SiPM(SensL), and a photo multiplier tube (PMT, Hamamatsu, H6410) were used. Experimental results are shown in bottom figure. Background counts of the SiPM(red circles)s showed increasing with crystal volume, but, slight differences depending on the its area while the PMT(black squares) showed linear increase of background counts. The background counts of the SiPM of 9.48×9.48mm2 area(blue ellipse) was distinctly higher than that of the SiPM of 6.32×6.32mm2 area(green ellipse) since dark count of a SiPM proportionately increases according to the area of the SiPM. Therefore, the area of a silicon detector like SiPM should be minimized to decrease the dark noise. ◆ Photon loss Second experiment was done for measuring the reduction of the output signal due to the photon loss from absorption at outer walls of crystals with various shapes and Teflon taped. The results showed that the output signal reduction in type 1 shapes was smaller than that of type 2 shapes depends on its thicknesses, which is similar to the simulation results. Type1 crystal with thickness 20mm and type2 crystal with thickness 30mm had similar volume, but, the photon loss of type1 was smaller than type2, it means the scattering probability in type 2 crystals were higher than type1 because of its narrow inside width. In conclusion, to determine the geometry of CsI(Tl) scintillator for mobile radiation detectors, the enough large volume for performances of background detection and the optimized shape with length for decreasing the photon loss should be considered. Specification of CsI(Tl) for Experiement1 Specification of CsI(Tl) for Experiment2 We simulated the experiment 2 concerning two types of CsI(Tl) crystals had different shapes of tapered structures by LightTools 5.1.0. Powers of output signal on receivers were compared for each structure when a gamma ray(662keV) incident in crystal. The reflectance of Teflon tape of outer wall was 99% with Lambertian scattering. The decrease rate of power of output signal of type 2 crystal with increasing the volume was higher than Type 1, moreover, the power loss, same meaning with photon loss, of type 2 was higher than type1 in same volume (365mm 2 ). AbstractAbstract Active region 3x3 mm 2 Active region 6x6 mm 2 Active region 9x9 mm 2 To build a truly miniaturized radiation detector for integrating in mobile device as smart phone, characteristics of a crystal coupled with a silicon detector should be confirmed, especially when a scintillator is miniaturized. There are two issues in designing the geometry of the crystal, One is the performance in detecting background radiations, which depends on the volume of the scintillators. The other is shaping the crystal to minimize the photon loss due to the absorption at outer wall Teflon taped, which are arose by the tapered structure with small output area of the scintillators to reduce the dark current of silicon detector. Background counts of the SiPM showed increasing with crystal volume, but, slight differences depending on the area of SiPM while the PMT showed linear increase of background counts. The background counts of the SiPM of 9.48×9.48mm 2 area was distinctly higher than that of the SiPM of 6.32×6.32mm 2 area since dark count of a SiPM proportionately increases according to the area of the SiPM. Therefore, the area of a silicon detector like SiPM should be minimized to decrease the dark noise. In second experiment, Type1 crystal with thickness 20mm and type2 crystal with thickness 30mm had similar volume, but, the photon loss of type1 was smaller than type2, it means the scattering probability in type 2 crystals were higher than type1 because of its narrow inside width. In conclusion, to determine the geometry of CsI(Tl) scintillator for mobile radiation detectors, the enough large volume for performances of background detection and the optimized shape with length for decreasing the photon loss should be considered. Dark count increases 60% Decreases In similar volume 76% Decreases In similar volume