RDMIS Radiation Detection and Medical Image Sensor lab. Composition Optimization of Polystyrene Based Plastic Scintillator Yewon Kim, Hyunjun Yoo, Chankyu Kim, Kyungjin Park, Eunjoong Lee and Gyuseong Cho* Department of Nuclear and Quantum Engineering, KAIST, Republic of Korea *Corresponding author: I.Introduction II. Material and Methods III. Experimental Results IV. Acknowledgment Posters III - Physics & Security Applications and Signal Processing Wednesday (6/11) 3:00 - 4:00 pm Ballroom Poster Index : 26 Plastic scintillator has advantages such as fast decay time, easy fabrication, low production cost and so on. Due to the scintillation property depends on the composition of the scintillator, it is important to find optimum additives ratio that plastic scintillator has maximum light yield. The main substance of the fabricated plastic scintillator was styrene solution and small amount of wavelength shifters (PPO (2,5-Diphenyloxazole) and POPOP (1,4-Bis(5-phnyl- 2oxidazolyl)benzene) were added. The wavelength shifters plays a roll converting ultraviolet (~300nm) emitted from polystyrene to visible light (330~500nm) with relatively longer wavelength by Stoke’s shift. - Fabrication of polystyrene based scintillator In order to optimize the plastic scintillator composition ratio, we developed polystyrene based cylindrical plastic scintillators. The scintillators include various ratio of PPO and POPOP in the 100g of styrene solution. Styrene solution and additives were weighted precisely by an electronic balance measured to five places of decimals. Mixed solutions were blended by an electronic stirrer with a magnetic bar for the uniform concentration of the mixture. Then each sample was separated into the cylindrical vials to make cylindrical scintillators and put into an electronic vacuum oven. In order to get rid of the air bubbles in the mixture formed during blending, made the oven vacuum at atmospheric temperature during 30 minutes. After the vacuum treatment, samples were heat in the oven. For the sake of sufficient and stable polymerization, the oven temperature was not only increased from 25°C to 120 °C by 20 °C per hour at atmospheric pressure but also holed 120°C of the oven temperature and cooled down to room temperature. - Experimental Setup Disc type Cs-137 gamma source was located far from scintillators. PMT was coupled with scintillators by optical grease. Photons emitted from scintillator go into the PMT through the grease applied to avoid light loss by scattered reflection. About 1g of the optical grease was applied to between PMT and scintillators. PMT was connected to a shaping amp and the measured signal was amplified by the shaping amp and the signal transferred to a multi channel analyzer (MCA). High-voltage, gain, low- level discriminator (LLD) and integration time were 1700V, 10, 20 and 300sec respectively. Cs-137 Source Plastic Scintillator Optical Grease PMT Shaping AMP MCA - PPO Energy Spectrum Following graph shows Compton spectrum of Cs-137 source coupled PPO only mixed polystyrene based plastic scintillator. As the mass of the PPO increase, the spectrum was shifted lower channel of MCA. Polymerized plastic scintillators, samples, were taken after removing of the vial glass. Each samples were cut to have same size by a diamond cutting machine. After the cutting process, the surface of the scintillator was polished one after another using sandpapers and the polishing machine. The polished samples were wrapped with PTFE tape as a reflector and this reflector material. - PPO Energy Spectrum As the graph indicate, Compton spectrum of Cs- 137 source was shifted higher channel of the MCA according to the increase of POPOP mass in the plastic scintillator. Fabricated plastic scintillators and the fluorescence - Plastic Scintillator Energy Spectrum Following graph shows Compton spectrum of Cs-137 source coupled PPO only mixed polystyrene based plastic scintillator. As the mass of the PPO increase, the spectrum was shifted lower channel of MCA. -Result According to the increase of the POPOP mass in the scintillator, the number of emitted photon was increased whereas it was decreased as the mass of PPO increase. However, the saturated concentration of POPOP was 0.2 g in the 100 g of styrene solution and appropriate concentration of the PPO in this mixture should be found. Emission spectrum of scintillators shows that UV(300~400 nm) emitted from PPO was absorbed to POPOP and reemitted longer wavelength, lower energy, with higher intensity by Stoke’s shift. “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 ) Schematic of energy spectrum measurement Fabricated plastic scintillators and the fluorescence 2014 Symposium on Radiation Measurements and Application (SORMA XV) June , 2014, University of Michigan (Ann Arbor)