Measurement of Chernobyl Fallout by Onboard Gamma-ray Spectrometer in Train

Transcription

1 Measurement of Chernobyl Fallout by Onboard Gamma-ray Spectrometer in Train Masaharu Okano, Hidenori Yonehara and Kenzo Fujimoto National Institute of Radiological Sciences ABSTRACT Several kinds of mobile gamma-ray spectrometer have been used for environmental radiation survey. Airborne, carborne and manborne systems are often used for this purpose. Our manborne system was utilized for the onboard measurement in a train. The system is composed of a spherical Nal(T1) scintillator of 3 inch diameter with lightweight portable MCA. Measurements were carried out in a train between Stockholm and Gävle, Sweden, Moscow and Kiev and Gomel and Mogilyov in 1990, Russia. The measurement point was defined by the elapsed time from a station. The Chernobyl fallout of Cs-137 was separated from natural potassium and U, Th-series in the energy spectrum obtained by onboard gamma-ray spectrometer in a train. The maximum air dose rate change with time due to radiocesium deposited on the ground was observed with our system in Sweden. The detected maximum dose rates due to cesium deposition by the Chernobyl accident near Gävle were 280nGy/h,240nGy/h,130nGy/h,110nGy/h,70nGy/h and 33nGy/h on June 28, 1986, Sep.3,1986, Sep. 17, 1987, Mar. 4, 1988,Sep. 27, 1991 and Oct.21, 1999 respectively. These values were lower than the exposure rate calculated with the assumption of surface distribution considering only physical decay of cesium since the cesium concentration shows some depth profile in the soil. The relaxation factor which ICRU report 53 were 0.4g/cm 2, 1.3g/cm 2,2.5g/cm 2 and 8g/cm 2 after 1.37y, 2y, 5.4y and 13.5y, respectively. In the train measurements from Moscow to Kiev, maximum dose rate was observed to be 180nGy/hv near Bryansk in May 1990, and also observed to be 170nGy/h near Mogilyov. INTRODUCTION Several kinds of mobile gamma-ray spectrometer have been used for environmental radiation survey. Airborne, carborne and manborne systems are often used for this purpose. The mobile gamma-ray spectrometer using onboard train have several advantage. The measuring point can be easily defined by time tables on schedule and/or real mileage of train. And also measured data is always reproducible. A portable scintillation spectrometer with a spherical Nal(Tl) scintillator of 7.5 cm diameter and its weight of 10 kg was used to study not only natural environmental radiations but also the effects of the Chernobyl fallout of Cs-137 The spectrometer somewhat improved since 30 year ago has been used for the measurements in various situations of the human environment. Measurements were carried out several times in a train between Stockholm and Gävle, Sweden for understanding the change of depth profile and migration of Cs-137. In 1990 onboard train measurement results from Moscow to Kiev, and Gomel and Mogilyov in Russia were compared with the assumption of surface distribution of radioactive cesium,. The incident photon energy spectrum was obtained from the measured spectrum by applying the peeling-off method, and Cs-137 component was separated from natural K-U-Th contribution. 1

2 PORTABLE SCINTILLATION SPECTROMETER FOR FIELD USE The Nal(Tl) scintillation spectrometer is known to be a useful instrument. Its use in the field, however, is handicapped by its high power consumption as well as its weight. These difficulties have been overcome by the portable scintillation spectrometer, called "spectro-survey-recorder" (SSR). The syatem was presented at the Symposium NRE III on April 1978 at Houston 1) and used in the field intercomparison scheduled in association with the Symposium 2), as well as the survey by M.Okano(1983) in Kerala after NRE Sp II on January ). These measuring could provide natural standard gamma-ray field spectrum of Uranium-series and Thoriumseries. Figure 1 and Figure 2 are the block diagrams of the SSR and configuration of spherical NaI(Tl) detector. The weight of the whole system including the scintillation head and a commercial stereo cassette recorder was about 10kg. 2

3 Fig. 2 Block diagrams of spectro-survey-recorder (SSR) (1) INSTRUMENT During field measurement, a cassette tape runs till its end without stopping and locations of the measurement are booked or recorded by video as a function of time since starting the tape, The scintillation output stored in the cassette tape is reproduced later in the laboratory usually at an interval of one minute by PHA via an I/O interface, and transferred to computer data processing. (2) DATA PROCESSING Pulse height spectra from the detector are divided into two energy regions to be analyzed, a gammaray region (up to 3 MeV) and a cosmic-ray region (above 3 MeV). The former region deduces the intrinsic photon energy spectrum and exposure rate due to environmental gamma rays by applying the peeling off method. The Cosmic-ray region in the scintillation spectrum is mainly a contribution of penetrating electrons and charged mesons. As a result of measurements of altitude dependence of dose rate determined by ion chambers and counting rate above 3 MeV in flight experiments using airplane (DC-8), a conversion factor for the counting rate to exposure rate has been determined to be µr/hr per cps (for a 3 inch spherical Nal(TI) scintillator). For data processing and presentation, two computer programs, "Result" and "Search" were employed. A function of the "Result" program is as follows: 1. To adjust energy per channel of the pulse height spectra stored in the standard file media by use of polynomial fitting to 10 kev/channel, and, in the cosmic-ray region, to obtain the counting rate above the energy of 3 MeV. 2. From the gamma-ray region, the background of the detector and the small contribution of cosmic rays estimated from the spectrum obtained at 10,000 meters are subtracted. Then, the photon energy spectrum and exposure rate and/or objective dose are calculated. Figure 3 is a standard representation of results. Scintillation pulse height spectrum, photon energy spectrum, gamma-ray exposure (G=) at 10 kev energy interval, exposure integrated from high energy side, and also cosmic-ray exposure (C=) calculated from counting rate above 3 MeV. In the figure an example of output format is also shown including detector count, photon flux, exposure, and it integral from high energy part. For determination of natural radioactive contribution, using three energy regions of MeV, MeV,and MeV are used for potassium, Uranium series(u) and Thorium-series(Th), respectively. Then the exposure rate due to each component are obtained by inverting the equation using measured spectrum of standard U and Th field. The "Search" program digitally and graphically represents some of the following quantities calculated by the "Result" program: file number, output time, collect time (live-time), cosmic-ray and gamma-ray doses, and dose contributions of K U Th and these sum. When a detailed local variation is required for examination, gross count for each six-second interval is 3

4 reproduced by a multi-scaler and the result is graphically represented. TRAINBORNE SURVEY RESULT AND CONCLUSION The measurement of Chernobyl fallout effects was carried out with gamma-ray SSR instrument on rack in train. The MCA pulse height is converted into digital signal and automatically recorded on a two-track tape recorder. The recorded signal was reproduced as a fixed interval pulse height distribution in the laboratory. Figure 4 illustrated several spectrum observed in train. They represents radio-cesium and natural radioactivity, and the spectrum obtained shortly after the accident reveals the peaks of Ba-La-140. Figure 5 and 6 shows time profile of air dose due to not only natural but also radiocesium contribution for the data obtained from Moscow to Kiev and from Gomel to Mogilyov, and Table 1 shows each contribution at several points. The dose rate due to radiocesium contribution just coincides with the distribution of surface ground contamination by Cs-137, as shown in Figure 7. Figure 8 and 9 shows air doses at different time near Gävle. Table 2 shows time variations of dose due to Cs-137 at maximum point near Gävle, and the calculated doses by physical decay with the distribution of surface deposition. The table also shows the relaxation factor presented by ICRU report 53 4). The onboard measurement using portable spectro-survey-recorder in train, is a powerful method for measurement selected environmental radiation survey not only natural radiation but also artificial radioactive sources. 4

5 Counting Rate (Arbitrary) Energy (MeV) Counting Rate (Arbitrary) Energy (MeV) Peak point near Bryansk in May 1990 Gävle city in June 1986 Counting Rate (Arbitrary) Energy (MeV) Counting Rate (Arbitrary) Energy (MeV) 1999 Peak point near Gävle in June 1986 Peak point near Gävle in October Fig 4. Scintillation spectrum observed at several points 5

6 Fig. 5. Route of trainborne and Cs-137 surface concentration 6

7 Gamma Dose Rate from Moscow to Kiev Kiev street Air Dose Rate ngy/h Suchneci Bryansk Shostka Konotop Nezin Kiev station Natural B.G. Cs Fallout :00:27 1:59:27 4:01:40 6:15:07 8:14:27 10:01:58 May 5, 1990 Fig. 6. Gamma dose rate from Moscow to Kiev on May 5, 1990 Gamma Dose from Gomel to Mogilyov Air Dose Rate ngy/h Buda-kosheleva Zlobin Rogachov 21:14:45 21:25:20 21:38:10 21:51:25 22:04:56 22:18:11 22:31:26 22:50:44 22:59:56 23:13:11 23:26:25 23:39:59 23:53:13 0:06:28 0:19:43 0:33:07 0:46:22 0:59:37 May 29-30, 1990 Byhov Mogilyov Natural B.G. Cs Fallout Fig. 7. Gamma dose rate from Gomel to Mogiilyov on May 29-30,

8 Gamma Dose Rate at peak point near Gavle 350 Air Dose Rate ngy/h peak point Gavle station Tatal Gamma Cs Fallout :13:00 9:15:00 9:17:00 9:19:00 9:21:00 9:23:00 9:25:00 9:27:00 9:29:00 9:31:00 9:33:00 9:35:00 9:37:00 9:39:00 9:41:00 9:43:00 9:45:00 9:47:00 June 28, 1986 Fig. 8. Gamma dose rate at pear point near Gävle on June 28, 1986 Gamma Dose Rate from Uppsal to Gavle Air Dose Rate ngy/h Uppsal peak point Gavle station 18:23:01 18:27:01 18:31:01 18:35:01 18:39:01 18:43:01 18:47:01 18:51:01 18:55:01 18:59:01 19:03:01 19:07:01 19:11:01 19:15:01 19:19:01 19:23:01 October 21, 1999 Natural B. G. Cs Fallout Fig. 9. Gamma dose rate from Uppsal to Gävle on October 21,

9 Table 1. Gamma dose rate due to natural and radiocesium contribution at several points from Moscow to Kiev Time/LT Place Gamma Dose rate ( ngy/h) Cosmic Gamma Cs-137 Total K U-ser. Th-ser. 2030/18 Moscow City /16 Moscow Sta /57 Moscow Dept /62 Kaluga / /20 Suchneci /31 Sudimir /16 Navlya / / /10 Shostka /85 Konotop /79 Nezhin nearby / / / / /04 Kiev Station /03 Kiev Street /06 Kiev Street /05 KievHotel RB /09 KievHotel Rm /26 Gomel Dept /20 Buda-koseleva /40 Zlobin /45 Rogachov /17 Byhov /29 to Mogilyov Table 2. Time variations of dose due to Cs-137, calculated dose and relaxation factor DATE Time after the Observed dose Calbrated by physical Relaxation factor assident (y) (ngy/h) decay (ngy/h) (g/cm 2 ) 1986/06/ /09/ /09/ /03/ /09/ /10/ ACKNOWLEDGMENT The authors express appreciation to the staffs of NHK who made measurements just after the Chernobyl accident in Sweden and to Mr. T. Katou and Mr. M. Miyaqgawa of Radiation Safety Division of Institute of Physical and Chemical Research for his help in The authors also express thanks to Dr. Vintersved of FOR in Sweden who accompanied the author for the onboard train measurement in

10 REFERENCES 1. Okano,M. et al. Measurement of environmental radiations with a scintillation spectrometer equipped with a spherical Nal(TI) scintillator, NRE III,Symposium series DOE 51 (CONF ), (1980) 2. Carl V.Gogolak and Thomas F.Gesell Intercomparison Experiment at NRE III,NRE III,Symposium series DOE 51 (CONF ), (1980) 3. Okano,M et al. Environmental radiation measurements in south-western district of India, J.Radiat. Res.24,81 ( 1983) 4. ICRU report 53 Gamma-Ray Spectrometry in the Environment, (1994) 10