1. 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, 220-710, Republic of Korea E-mail: radkim@jwu.ac.kr II. Materials and Methods IV. Conclusion & Further Study III. Results & Analysis  There is more energy absorption in the region of a scintillator which is close to a radiation source and more light transmission efficiency in the region of a scintillator which is close to the detector. Thus, as shown in the fig.1, to obtain the optimum thickness of a scintillator(1cm x 1cm) of each layer of staked layer structure, a bulk of the scintillator is segmented by sections measuring 0.5cm and analyzed with respect to absorbed energy, light generation, and the LTE. This yields the final scintillation count on the detector of that position and is then summated as a whole. This can be deducted by the equation 1 below. I. Introduction  FNGR (Fast Neutron and Gamma Radiography) consists of detectors and scintillators for two different types of radiation; neutron and gamma-ray. It can be employed as a detection system of containers in airport facilities and as a security system for analyzing the compositions of objects by detecting specific gamma-rays occurring through interaction during penetration. For the detections of neutron and gamma-ray, usually a plastic scintillator coupled photo-sensor and a CsI crystal scintillator coupled photo-sensor are used respectively.[1]  Owing to the high energy of radiations, the thickness of the scintillator cannot be sufficiently thickened. The energy cannot be completely absorbed by the scintillator. Although it is optimized with respect to the fact that there is dissipation and penetration as much as absorption.  This study reconfigures the structure with the 2 nd detector module stacked behind the 1 st detector module. The FNGR detector module as a stacked layer structure requires the photon count be simultaneous in order to enhance the performance of the existing FNGR system. The verification and analysis of the possible optimization of the scitillator performance are conducted and simulated in the study.  Through the simulation study of neutron and gamma-ray scintillators, the optimum thicknesses were deducted as 5.5cm for the BC430 plastic scintillator (1cm x 1cm) and 4.5cm for the CsI(Tl) crystal scintillator (1cm x 1cm).  In spite of the reduced thickness of the existing structure, the performances of BC430 and CsI(Tl) is both efficiently and economically enhanced by 40% and 58%. Also the optimum conditions of the scintillators can be enhanced by the surfaces treatments of the scintillators with polishing and metal coating.  The study demonstrates the possibility of applications for other high- energy radiography systems exists.  The actual experiments are planned and in process. The staked layer structure boards are built in order to combine the scintillators coupled PIN photodiodes and Charge Sensitive Preamplifiers. The results of the experiments will be discussed in future studies into the various aspects of spectroscopic energy resolutions. 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.  We have deducted the optimum thicknesses of both types of scintillators which are a CsI(Tl) crystal scintillator for the gamma-ray (Co-60: 1.17 & 1.33MeV) radiography and a plastic scintillator (BC-430) for the fast neutron (14 MeV) radiography.  The study employed the Monte Carlo simulation. To obtain absorbed energies and transmitted light of the scintillators, the MCNPX code and the Detect-97 code were used respectively.  For the comparison of the stacked layer structure with the existing system, we have set the geometry of the scintillators of the Brisbane Air Port FNGR system in Australia. As a standard, it’s dimensions are 2cm x 2cm x 7.5cm of Saint Gobain’s red emitting plastic scintillator BC430 for fast neutron radiography, and 1cm x 1cm x 5cm of SICCAS’s CsI(Tl) for gamma-ray radiography. In the selection of photodiodes, the simulated geometry was 1cm x 1cm x 7.5cm due to the active area of the photodiode selected for the later experiment. The surface of each scintillator was treated as polished and paint coated.  The scintillators surface treatment is an important parameter to be optimized due to the fact the generated light within the scintillator can escape from the scintillator. In this study, the optimum surface-treatments of the scintillators are analyzed by simulation with the DETECT 97 code as well. 1st2ndStackedExistingEnhancement(%) CsI(Tl)4.5 cm14225.834.5 cm5431.19619657.035 cm14011.7440.29 BC4305.5 cm8103.2695.5 cm4196.88812300.167.5 cm7774.74358.2 Table 1. Photons counted on the detectors (1) Table 2  The optimum thicknesses of the both scintillators are deducted by the equation (1) as 1cm x 1cm x 4.5cm for CsI(Tl) and 1cm x 1cm x 5.5cm for BC430. The table 1 shows the final counts of each condition and comparison between existing structure and the stacked layer structure. Fig. 2 the surface-treatments of CsI(Tl) and BC430  As shown in Fig. 2 and Table 2, the ground condition of the scintillator shows poor condition. For both scintillators, the polished surfaces perform notably higher in efficiency.  Because the polished surface with metal coated scintillators perform better by 9.7% for CsI(Tl) and 5.9% for BC430 than the polished surface with paint coated surfaces, the simulation results which were conducted with paint coating can be enhanced more. The final photo counts with metal surface treatment for CsI(Tl) can be 21566.22 and for BC430 can be 13031.81, then both are optimized. 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 0.5 cm intervals Fig. 1 Simulation structure of stacked layer detecting system 1st layer 2nd layer Source