Optimization of Scintillators for Stacked-layer Detectors of FNGR 1,2 Jea Hyung Cho, 1,2 Kwang Hyun Kim *, and 3 Young Hyun Jung 1 Biomedical Engineering, Jungwon University, Republic of Korea 2 Basic Atomic Energy Research Institute (BAERI), Jungwon University, Republic of Korea 3 Department of Radiological Science, College of Health Science, Yonsei University, Wonju, , Republic of Korea

 Because of high energies of the radiations which FNGR(fast neutron and gamma ray) system uses, there are energy losses for both type of radiations due to the penetration through the detectors.  Thus stacked layer structure is suggested in the study in order to acquire the energy penetrated through the first detector.  Since the Scintillation detecting system is utilized in the customized FNGR system in Brisbane Airport, the optimization of scintillators as stacked layer structure is conducted in the study in aspects of dimensions and surface treatment by the Monte Carlo simulation.  The fig.1 shows that a bulk of scintillator is segmented in order to be analyzed in the aspects of it’s generated light by absorbed energy and LTE(light transmission efficiency) to obtain photo counts on detectors.  Optimum thicknesses of scintillatiors(1cm x 1cm) are deducted by the equation 1 below. Source PC is Photon-counted on the scintillation detector, E i is the absorbed energy (MeV) in each part LY is the light Yield (photon/MeV) of the scintillator LTE i is the light transmission efficiency (%) and n is the number of parts segmented. (1) first layer second layer Fig. 1 Simulation structure of stacked layer detecting system

1st2ndStackedExistingEnhancement(%) CsI(Tl)4.5 cm cm cm BC cm cm cm  The optimum thicknesses of the both scintillators are deducted by the equation (1) as 1cm x 1cm x 4.5cm for CsI(Tl) (for gamma ray detection) and 1cm x 1cm x 5.5cm for BC430 (for fast neutron detection). The table 1 shows the final counts of each condition and comparison between existing structure and the stacked layer structure. Table 1. Photons counted on the detectors  As shown in the fig 2. and table 2, the final photo counts with metal surface treatment for CsI(Tl) can be and for BC430 can be , then both are optimized. Fig. 2 Table 2 Fig. 3. Absorbed energy comparison between the existing structure and the optimized scintillator utilized stacked layer structure. CsI(Tl) for gamma-ray Co-60(1.17 MeV & 1.33 MeV), BC430 for 14 MeV fast neutron.  The Fig. 3 shows that the higher performance can be obtained when FNGR is reconfigured as stacked layer structure compared to existing single layer structure.