DEVELOPMENT OF HIGH STABLE MONITOR FOR MEASURERING ENVIRONMENTAL RADIATION

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Annabelle Octavia Fisher
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1 DEVELOPMENT OF HIGH STABLE MONITOR FOR MEASURERING ENVIRONMENTAL RADIATION Ken ichiro Moriai.,Hiroshi Kawaguchi,Shohei Matsubara, Naoki Tateishi(ALOKA CO.,LTD.) Masatoshi Egawa,Hideaki Kakihana(THE KANSAI ELECTRIC POWER CO.,INC.) INTRODUCTION Many radiation monitors for measuring environmental dose equivalent rate are provided for radiation protection in outside area of each nuclear power plant in Japan. High sensitive NaI(Tl) scintillation detectors are used for these monitors because of accurate measurement of background level. Generally, their crystal size is 2 inches diameter 2 inches length or 3 inches diameter 3 inches length. By gamma ray spectrum analysis, energy response is flatly compensated. As NaI(Tl) detectors have pulse height change by ambient temperature and long term pulse height change by gain change of photo-multiplier tube (PMT), monitoring systems have climate box for keeping constant temperature and need energy calibration once or twice a year. We developed the automatic energy calibration method utilizing photo peak of potassium-40 gamma rays from natural ground. New type of this monitor with this method is able to measure continuously environmental radiation without influence of temperature or gain change of PMT, by continuous energy calibration without using calibration source.we would like to report the outline of this new system, details of the method and measurement result. SYSTEM CONFIGURATION funct Analog (0-1 V) Pulse Contact Temperature (NaI, IC) Telemeter substation Warming control device To telemeter master station Precipitation detection Various status signals Data collection device (ACE-152) Collected data Dose rate pulse Pulse-height analyzer (ASM-352) Pulse-height distribution data Dose rate, counting rate, SCA Dose rate Low-dose measurement device (ACE-451) High-dose measurement device (ASE-452) NaI detector Dose rate, counting rate, SCA Dose rate IC detector 4-20 ma output for direct transmission 4-20 ma output for direct transmission Fig. 1 Configuration of the environmental radiation monitoring system 1

2 A low-range measurement system incorporates an NaI(Tl) scintillation detector, from which the pulseheight analyzer collects and stores pulse-height distribution data and performs automatic gain calibration. The pulse-height analyzer outputs dose rate pulses passing through an energy compensation within this system. The low-dose measurement device is capable of counting the dose rate pulses from the pulse-height analyzer and displaying the dose rate and counting rate on its liquid crystal display in real time. A high-range measurement system uses a spherical ionization chamber. The high-dose measurement device is capable of amplifying by a preamplifier a signal generated at the ionization chamber, converting into the dose rate, and then displaying it on its liquid crystal display in real time. The data collection device is capable of storing and displaying data from the low-dose and high-dose measurement devices, temperatures within the detector, various status signals, and precipitation data, and transmits this data to the telemeter main station via the telemeter substation. Shown below is the configuration of the pulse-height analyzer (ASM-352) (Fig. 2). ADC unit RS-485 Preamplifier Detected pulse AMP Amplified pulse ADC MCA analog MCA BUS digital RGB NaI detector DATA BUS Display control unit liquid crystal display HV setting Drawing Pulse-height distribution data conversion and analysis processing Dosimetric unit To low-dose measurement device Dose rate pulse Compensation Memory control unit To telemeter substation Data retention processing Fig. 2 Configuration of the pulse-height analyzer (ASM-352) The measuring component of the pulse-height analyzer is comprised of four unit sheets, each providing ions to facilitate maintenance; a liquid crystal display module; a key switch; an FDD; and a power unit, all of which are installed within an enclosure. The liquid crystal display is capable of displaying information such as pulse-height distribution data, the current time, preset time, ROI, and a cursor (See Fig. 3). The display can also store pulse-height distribution data for a period of up to one week (with 10-minute measurements) in memory backed up with a battery. The ADC unit amplifies pulses obtained from the NaI detector, converting them into channel data. The display control unit stores the channel data, converts the data into pulse-height distribution data, and display the data on the liquid crystal display for each second. It also forwards the data to the memory control unit for each preset time period, investigates a photopeak of 40 K, and conducts HV adjustment of the NaI detector preamplifier. The dosimetric unit performs energy compensation for the channel data and outputs the 2

3 dose rate pulses for the low-dose measurement device. The memory control unit retains pulse-height distribution data as well as control of serial data communications with the telemeter substation. METHOD In the gain calibration against 40 K, when a photopeak channel of 40 K calculated from a spectrum collected during the preset time period deviates from a theoretical value by more than ±1.0 ch, the HV of the detector is altered by 0.1 V/ch to effect a gain calibration. However, if the ROI counting rate of 40 K is below or above a specified value by its N% as a result of irradiation effects from a radiation source ( 60 Co), and if the preset time is equal to or less than measurement time T minutes previously measured in an installation environment and verified to be sufficient to obtain a count sufficient for calculating the photopeak channel of 40 K, gain is not calibrated. The criterion value is set to the counting value of 40 K due to the experimentally confirmed fact that the ROI of 40 K remains almost entirely unaffected by radon and thoron, depending upon rain or snow fall (approximately within ±5%). Where, the ROI of 40 K described here indicates an energy window in the range 1.39 to 1.54 MeV. 1. Condition setting (Fig. 4) (1) Allowance determination A determination method for avoiding gain calibration when the variation in the ROI counting rate of 40 K exceeds the allowable range is selected. The allowable range is specified with respect to the value of criterion, which may be selected from among the three following alternatives: varies. Measured Value: An ROI average counting rate of 40 K obtained in automatic measurement is used. Fixed Value: A predetermined value for an ROI average counting rate is used as a fixed value. No Determination: Gain is calibrated unconditionally, even when the ROI counting rate of 40 K (2) Allowable range An allowable range of variation (in %) for the ROI counting rate of 40 K is set. (3) Offset channel A difference channel between the photopeak channel of 40 K obtained after energy calibration for 137 Cs is conducted (full scale: 5 MeV/1000 ch) and a theoretical photopeak channel ch is set. (4) Fixed value An ROI average counting rate value (in cps) to be used for allowance determination is entered. (5) Measurement time A measurement time (in minutes) in automatic measurement is set. (6) The number of measurements The number of measurements in automatic measurement (number of times) is set. 2. Automatic measurement (Fig. 6) Measurement is performed for measurement time and number of measurements, both of which are determined by the condition setting, an ROI average counting rate of 40 K is obtained, and the criterion value is calculated for determining whether automatic gain calibration is to be performed. 3. Measurement of success time (Fig. 5) A measurement time enabling calculation of the photopeak channel of 40 K at an installation environment is measured. An offset channel is set based on the photopeak channel obtained in this step. 3

Fig. 3 ASM-352 Fig. 4 Condition setting Fig. 5 Measurement of success time Fig.

channel of 40 K are calculated using results obtained through measurements of pulse-height

installed near the Takahama nuclear power station of the Kansai Electric Power Co., Inc.

4 Fig. 3 ASM-352 Fig. 4 Condition setting Fig. 5 Measurement of success time Fig. 6 Automatic measurement RESULTS AND CONSIDERATIONS Temporal variations in the photopeak channel of 40 K are calculated using results obtained through measurements of pulse-height distribution data from the NaI(Tl) detector with this system at the monitoring post, installed near the Takahama nuclear power station of the Kansai Electric Power Co., Inc., from June 1, 1999 to August 15, 1999, as indicated in Fig. 7. Fig. 7 Temporal variation in the photopeak channel of 40 K (detector: φ2 2inches, cylindrical type; with 10-minute measurement) at the monitoring post (MS), installed near the Takahama nuclear power station of the Kansai Electric PowerCo., Inc. 4

From the results shown in Fig. 7, it was found that the measurement accuracy of the photopeak channel of 40 K was approximately ±3 ch.

photopeak channel calculated by the computer (See Fig. 9-10).

pulse-height distribution data measured using a 137 Cs radiation source on the roof of a main office building belonging to ALOKA Co., Ltd. (located in Mitaka-city, Tokyo), as shown in Fig.

5 From the results shown in Fig. 7, it was found that the measurement accuracy of the photopeak channel of 40 K was approximately ±3 ch. This accuracy is attributable to the fact that background counts of 40 K in the photopeak region inevitably suffer variations due to statistical errors in counting; this also affects the value of the photopeak channel calculated by the computer (See Fig. 9-10). To evaluate the photopeak channel calculated by the computer and an actual gain variation in NaI(Tl), we obtained temporal variations in the photopeak channels for both 40 K and 137 Cs from pulse-height distribution data measured using a 137 Cs radiation source on the roof of a main office building belonging to ALOKA Co., Ltd. (located in Mitaka-city, Tokyo), as shown in Fig. 8. Fig. 8 Temporal variation in photopeak channels of 40 K and 137 Cs (detector: φ2 2 inches, cylindrical type; with 10-minute measurement) on the roof of a main office building belonging to ALOKA Co., Ltd. (located in Mitaka-city, Tokyo) These results indicate that gain variations of NaI(Tl) evaluated using a variation in the photopeak channel of 137 Cs was controlled to within ±1 ch, and that variation in the photopeak channel of 40 K in Fig. 7 (Takahama Nuclear Power Station) was found to be comparable to that of 40 K in Fig. 8 (Mitaka Head Office). Therefore, it can be said that measurements of data of Fig. 7 has been improved to the point that they can be performed with a stable gain of NaI(Tl). Fig. 9 Spectrum data Fig. 10 Spectrum data (in the vicinity of the photopeak of 40 K) These results suggest that changes in gain of a PMT with the passage of time, formerly handled by performing regular energy calibrations using a radiation source, can be calibrated without generating missing data by performing automatic gain calibrations of the NaI(Tl) detector using the background photopeak of 40 K. A method is still required for correcting the effects of the statistical error in counting of 40 K in its photopeak region while performing arithmetic operations for the photopeak channel. 5

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