DEVELOPMENT OF LARGE AREA PLASTIC SCINTILLATION DETECTOR FOR RADIOACTIVE CONTAMINATION MONITOR

Transcription

1 DEVELOPMENT OF LARGE AREA PLASTIC SCINTILLATION DETECTOR FOR RADIOACTIVE CONTAMINATION MONITOR Katsuhito Ito, Ryohei Yanagishima, Daisuke Inui*, Tomoya Minagawa Fuji Electric Systems Co., Ltd. Fuji-machi 1, Hino, Tokyo , Japan Abstract. We developed a large area plastic scintillation detector which fits to the shapes of the measured objects. Effective area of the detector is from about 300 to about 3000 cm 2, and the outer thickness of the detector unit is less than 10 cm for β ray measurements. The detection efficiency is about 25% in 2π geometry with the 60 Co 318 kev β ray source of 100mm x 100mm size set 1cm away from the detector center. The uniformity of detection efficiency over the whole sensitive area is acceptably good within about 20%. KEYWORDS: Plastic scintillation detector, Contamination monitor 1. Introduction The radioactive contamination monitor, such as body surface contamination monitor (portal monitor) and laundry monitor, requires a detector with large sensitive area to monitor the whole objects with various shapes to be measured. The gas flow proportional detector has ever been used usually as the detector for the contamination monitor, however it requires a gas supply system, which has the problem to bring high pressure gas cylinder into a radiation controlled area. We therefore developed a large area plastic scintillation detector without gas supply which fits to the shapes of the measured objects. Effective area of the detector is from about 300 to about 3000 cm 2, and the outer thickness of the detector unit is less than 10 cm for β ray measurements. Table 1 lists the comparison of physical characteristics between gas flow proportional detector and plastic scintillation detector. 2. Detector structure and detection principle We developed a thin (about 0.5 mm thick) and large area (about 300 cm 2 to 3000 cm 2 ) plastic scintillation detector which has a uniform sensitivity for using as body surface contamination monitor (portable monitor), laundry monitor etc. A thin plastic scintillator was used to depress the sensitivity to gamma rays. Figure 1 shows the inner structure of the detector, and Figure 2 shows an example of the outlook of the detector. As shown in Fig.1, the plastic scintillator (Saint-Gobain Industrial Ceramics Inc.) is coupled with air light guide made of aluminum reflector and the scintillation light from β rays is reflected at the light guide like a mirror, and enters into two photomultiplier tubes (ET Enterprises Co. Ltd.) set in parallel. We could realize a thin detector system by using the light guide structure and two photomultipliers setup in parallel to the detector as shown in Fig. 1. In the detector system, the amplifier circuit, high voltage power supply, coincidence circuit and pulse-shaping circuit for photomultipliernoise cut-off are built in, and the pulses of constant pulse heights are outputted without depending on the detector size. Figure 3 shows the block diagram of the detector system. We developed various sizes of plastic scintillators for different purposes. * Presenting author, inui-daisuke@fesys.co.jp 1

2 Table1: Comparison of physical characteristics between gas flow proportional detector and plastic scintillation detector. 1.Background count rate 2.Detection efficiency 3.Measures for the breakage of light protecting film 4.Maintenance 5.Peripheral equipment Gas flow proportional detector Low background counts due to low sensitivity to gamma rays. High efficiency to β rays, but may decrease due to gas leakage through light-protecting film. On the breakage of light protecting film in gas flow counter, the broken film should be replaced by the users as temporary repair. The gas flashing is needed for a few hours after the replacement. Supply of gas is needed. Gas flow is checked in everyday. Gas supply system is required and high voltage power supply is required externally. Plastic scintillation detector Low background counts are realized by taking the coincidence of two output pulses from two photomultipliers. High efficiency to β rays keeps constant for long-term use. On the breakage of light protecting film in plastic scintillator, the broken film should be replaced by the users as temporary repair. It can be used after 30 minutes of the replacement. The routine running cost is not necessary. No gas supply system is needed and high voltage circuit is built in the detector system. Figure 1 : Inner structure of the thin and large area plastic scintillation detector β ray Plastic scintillator Air light guide Aluminum reflector Scintillation light Photomultiplier tube 2 2

3 Figure 2: Outlook of the detector Signal output Light protecting film Low Voltage input Figure 3 : Block diagram of the detector Photomultiplier tube Amplifier Discriminator Printed circuit Light shielding film Plastic scintillator Light guide High-voltage power supply Coincidence Photocoupler Connector Photomultiplier tube Amplifier Discriminator 3. Physical characteristics of the detector system To investigate the physical characteristics of the detector system the detection efficiency and its uniformity over the whole detector surface were measured by using β rays of three different energies, 156 kev of 14 C, 318 kev of 60 Co and 709 kev of 36 Cl at Fuji Electric Systems Co. Ltd. These sources are 100 mm x100 mm in size and were set at the center of the large-area detector 3cm away from the detector sensitive surface. For uniformity investigation, a 36 Cl β ray source was collimated into 25-mm diameter with aluminum plate and set 1cm away from the detector surface. The 60 points at each 5-cm interval in the central detector area of 300 mm by 500 mm were selected and the surface distribution of detector counts was measured. As shown in Fig.4, the selected area of 500 x 300mm was divided into 10 (A to J) segments in 500-mm width and into 6 (1 to 6) segments in 300-mm length. A source collimated into 25mm diameter was set at the center of each divided area and the surface distribution of β- ray detection efficiency was measured. 3

4 Figure 4: 36 Cl β ray source setting position A B C D E F G H I J mm φ collimated source position: (x,y)=(j,1) Figure 5 shows an example of measured results of detection efficiencies to the above three β-ray energies for the detector of 520 mm wide and 310 mm long sensitive dimension. The detection efficiency increases with increasing β-ray energy from 3.6% for 156 kev to 32.5% for 709 kev, and is almost equal to that of gas proportional counter. Figure 5: Detection efficiencies to three β-ray energies for the detector of 520mm width and 310mm length 40 β -ray efficiency (%) Co (318keV) 36 Cl(709keV) C(156keV) β- ray energy (kev) Table 2 shows the surface distribution of detection efficiency for 709 kev β rays of 36 Cl as an example for the detector of 520 mm wide and 310 mm long sensitive dimension. The detection efficiency averaged over the whole surface is 33.4 %, and the deviation ratio from the average is 0.78 at minimum and 1.13 at maximum. The uniformity of detection efficiency over the whole sensitive area is acceptably good within about 20 %. 4

5 Table 2 : Surface distribution of detection efficiency for 709 kev β rays of 36 Cl for the detector of 520 mm wide and 310 mm length A B C D E F G H I J % 34.6% 28.5% 27.7% 28.8% 26.0% 28.4% 34.7% 34.1% 33.6% % 33.7% 36.2% 36.4% 36.5% 36.6% 36.6% 36.4% 34.9% 33.8% % 35.2% 35.6% 37.6% 36.1% 36.6% 36.4% 35.3% 35.0% 33.5% % 34.5% 36.3% 35.4% 35.2% 35.2% 35.7% 33.2% 34.3% 33.0% % 32.3% 32.6% 32.8% 32.3% 33.5% 33.8% 34.8% 31.8% 33.6% % 32.0% 29.6% 29.4% 29.4% 30.6% 28.8% 30.6% 31.8% 31.7% Table 3 exemplifies the dimensions and detection efficiencies of five main plastic scintillation detectors for specified applications. The detection efficiencies were obtained with the 60 Co β ray source of 100 mm x 100 mm size set 1 cm away from the detector center. Table 3: Dimensions and detection efficiency data of five main plastic scintillation detectors for specified application Sensitive dimension ( mm ) 700(W) x 350(L) For β -ray 520(W) x 310(L) For β -ray 420(W) x 310(L) For β -ray 420(W) x 310(L) For β -ray+γ-ray Simultaneous measurement 310(W) x 220(L) For β-ray Outer thickness ( mm ) Detection efficiency 60 Co :25.5% Laundry monitor 60 Co :26.0% 60 Co :26.5% 60 Co(β) 60 Co(γ) :23.5% :14.5% 60 Co :21.0% Applications Body surface contamination monitor front and back Small article surface contamination monitor Small article surface contamination monitor Body surface contamination monitor Head, hands and feet Transportable small article surface contamination monitor 4. Application examples (1) Application to the contamination monitor Figure 6 shows two examples of application to the body surface contamination monitor. One is the one-through type to measure the surface contamination on the whole body (head, hands, feet, and front, back and lateral sides of the central body) at one time, which is commonly used in Japan. While in U.S.A, Europe etc., the two-step type to measure the front and the back sides of the body at two times is commonly used. The other is the hand-foot-clothes monitor to measure hands, feet, and clothes. 5

6 Figure 6: Application example for body surface contamination monitor One-through type to measure the surface contamination on the whole body Hand-foot-clothes monitor (2) Application of plastic scintillator to a spot contamination monitoring For the purpose to manufacture a contamination monitor to detect the spot contamination, a large area plastic scintillation detector of 31cm length by 22 to 70 cm width was developed by arraying a couple of detectors with effective areas of 31 cm length by 11 cm width as one unit, as shown at the right side in Fig.7. We aimed to detect the surface contamination radioactivity of 80 Bq 60 Co corresponding to 0.8 Bg/cm 2 radioactivity density for 10x10cm surface source. Although one large area detector shown at the left side in Fig. 6 has ever been used for spot contamination monitoring, this array-type detector using smaller size detectors will be able to have the following better characteristics. Figure 7: Conceptual design of detector array to realize a large area detector for spot contamination monitoring 220~ Large area detector in the past 6

7 By dividing the detector, background count rates can be reduced without decreasing the detection efficiency of spot contamination which is less than 10cm x 10cm. (1) By adding up the counts measured with the two adjacent detectors, the efficiency decrease near the border of these two detectors can be prevented. (2) By putting the detector of 11cm x 31cm together, the area size to detect the spot contamination can be freely changed according to the sizes of measured objects. 5. Conclusion We developed a large area plastic scintillation detector which fits to the shapes of the measured objects. Effective area of the detector is from about 300 to about 3000 cm 2, and the outer thickness of the detector unit is less than 10 cm for β ray measurements. These plastic scintillation detectors will be applied to various contamination monitors. 7