H. Hayashi

H. Miyake

K. Nishimura

M. Isobe

M. Shimo

M." Tanaka

S. "Yokoyama

Y. Yamamoto

National Institute for Fusion Science (NIFS-Repository)

In an actual monitoring system containing a proportional gas counter (PG), a quantity of sample air is mixed with counting gas followed by counting of the mixed gas. However, air contains oxygen with a high electron attachment coefficient and removes electrons from electron-ion pairs generated through ionization by the radiation in the counting gas. This electron trapping depletes the number of electrons, which reduces radiation counts. In the present study, a decompression procedure method was proposed to reduce the influence of air and its effectiveness was investigated. When applying the decompression method, the pressure of counting gas inside the PG counter is reduced below one atmospheric pressure (atm) before the radiation count is obtained. To determine the effect of decompression, energy spectra were measured simultaneously under 1 and 0.5 atm using methane-air mixtures in addition to pure methane as the counting gas. For these measurements, a cylindrical proportional gas counter with a length of 400 mm and an inside diameter of 60 mm was manufactured out of stainless steel pipe. A gold-coated tungsten wire 0.02 mm in diameter was stretched along the centerline of the counter. The active volume of the PG counter was approximately 1000 cm3. An unsealed 55Fe radiation source was used to simulate beta rays emitted from tritium and mixtures of methane and air (5.0, 10, and 15%) were prepared as counting gas containing air. For these measurements, the voltage applied to the PG counter was fixed at 2300 V. Results are shown in Figs. 1(A) and (B). Figure 1(A) shows four energy spectra obtained at a pressure of 1 atm. Only the spectrum obtained using 0% air contained one complete peak, and the other three peaks were indistinct because they moved to the zero channel side and their peak height decreased in proportion to the amount of added air. Thus, peaks produced in energy spectra shrank significantly due to the presence of air in the counting gas when compared to spectra produced with no air.By contrast, definite peaks occurred in the spectra for all four counting gases, as shown in Fig. 1(B). The width of the peaks narrowed upon shifting to the lower energy channel side as the proportion of air increased, and peaks were well formed even when 15% air was allowed in the counting gas under 0.5 atm..    To quantitatively evaluate the effectiveness of the decompression procedure for reducing the adverse influence of air, peak areas were evaluated for the energy spectra in Figs. 1(A) and (B). Results, shown in Fig. 2, indicate that peak areas in energy spectra measured at 1 atm decreased dramatically as percentage of air increased. The peak areas obtained at 5, 10, and 15% air were less than 3% of the peak areas in the spectrum produced with no air in the counting gas (pure methane). However, in spectra obtained at 0.5 atm, peak areas were not affected by the amount of air in the counting gas. In these spectra, peak areas for 5, 10, and 15% air were 94% larger than those in the spectrum obtained with no air in the counting gas. These results demonstrate that air in the counting gas influences the energy spectra, but that the decompression procedure appears to effectively eliminate or reduce the adverse influence of air on peaks in energy spectra. This new method enables measurement of tritium concentration while eliminating the influence of up to 15% air in the counting gas. 1101001000100000 100 200 300 400㻯㼛㼡㼚㼠㼟Enegry(arbitrary)䕿䠖㻭㼕㼞㻌㻜㻌㻑䕧䠖㻭㼕㼞㻌㻦㻌㻡㻌㻑䕻䠖㻭㼕㼞㻌㻝㻜㻌㻑䕕䠖 㻭㼕㼞㻌㻝㻡㻌㻑(A)1atm1101001000100000 200 400 600 800 1000CountsEnergy(arbitrary) 㻔㻮㻕㻌㻜㻚㻡㻌㼍㼠㼙䕿䠖㻭㼕㼞㻌㻜㻌㻑䕧䠖㻭㼕㼞㻌㻦㻌㻡㻌㻑䕻䠖㻭㼕㼞㻌㻝㻜㻌㻑䕕䠖 㻭㼕㼞㻌㻝㻡㻌㻑Fig. 1. Reduction in the effect of air on energy spectra by lowering counting gas pressure with an applied fixed voltage of 2300 V to a proportional gas counter at (A) 1 atm and (B) 0.5 atm.  11010010001000010000010000000 5 10 15 20 Peak area (counts)Contained air  (%)䕻䠖㻱㼚㼑㼞㼓㼥㻌㼜㼑㼍㼗㻌㻔㻝㻌㼍㼠㼙㻕䕿䠖㻱㼚㼑㼞㼓㼥㻌㼜㼑㼍㼗 㻔㻜㻚㻡㻌㼍㼠㼙)   Fig. 2. Dependence of peak area on amount of air contained in counting gas in both energy and rise-time spectra.   In the atmosphere and soil, there are natural radionuclides such as U-238, U-235, and their progeny; K-40, which is a component of the soil; and H-3, N-13, and Be-7, which are caused by a spallation reaction between cosmic rays and components in the air. In addition, radioactive fallout, such as Cs-137 (T1/2 = 30 y) and Sr-90 (T1/2 =29 y), was released into the atmosphere as a result of nuclear tests carried out in the 1950s and 1960s. Furthermore, Cs-134 (T1/2= 2 y) was released into the atmosphere and deposited on soil after the Fukushima Daiichi nuclear power plant (1FNPP) accident, and they may still be present. To ensure the safety of the general public near the National Institute of Fusion Science (NIFS), which houses a high-performance Large Helical Device (LHD), it is important to investigate variations in environmental radiation via long-term monitoring, and to analyze the critical factors. The aim of this study is to show the difference in both the total amount and the depth profile between the radioactive fallout, Cs-137, of past nuclear tests and the 1FNPP accident, and natural radionuclides in soil.  Soil samples were collected using a core sampler (5 cmȭ× 5 cm high) and a scraper plate (15 cm wide × 30 cm long, 0–3 cm depth) at five points at the NIFS. Points WB, WM, and WH are located at a wooded area of a hill. Point WC is a narrow ravine [where soil was collected a few meters away from a stream (WC-1) and at the side of the stream (WC-2)]. Point IB, which was created in 1996, is a man-made open section of land where lawns were planted.  The collected soil samples were dried at 104 ºC for 24 h. All dry soil samples were sifted to remove small rocks and plant roots, and particle sizes were classified for the soil collected at points WB and WC.  The dry soil samples were placed in U-8 containers. The radioactivity of the soil samples was measured by Ge (Li) semiconductor detectors (GX3018, Canberra Industries Inc. and Model GMX-20200, ORTEC) for 24–26 h.  Classification of the soil particles revealed that the ratio of clay and silt in the soil from points WB and WC-1 was about 15%, while that at point WC-2 was 4–6%. The reason for this difference is that the clay and silt from the latter sample would have been washed downstream.  Fig. 1 illustrates the radioactivity of Pb-214, Cs-137, and K-40 in soil collected by the core sampler. The radioactivity of Pb-214 in soil did not differ between the sampling points. The radioactivity of K-40 in the soil of points WB, WC, WH, and WM was approximately equal. The concentration of K-40 in the soil of point IB was four times as high as that at the other locations. This indicates that lawns were planted and a fertilizer containing potassium was added to the soil, although the original soil type was the same at all locations. The radioactivity of Cs-137 in the soil was highest at point WB. One of the properties of cesium is that it attaches strongly to clay. The low Cs-137 radioactivity in the soil of point WC-2 may be because that clay, which is a fine particle, and the attached Cs-137 in the surface soils were washed by water from the stream.  Fig. 2 displays the 0–3 cm depth profiles of the radioactivity of Be-7, Cs-134, Cs-137, and K-40 in the soil collected by the scraper plate at point WB. The highest radioactivity of Be-7 was present in the surface soil. This indicates that the origin of Be-7 is radioactive fallout from the atmosphere. The distribution of radioactivity of Cs-134 was also similar to that of Be-7. Thus, it can be concluded that Cs-134 originated from the 1FNPP accident. The radioactivity of Cs-137 in the soil increased with depth, which means that the fallout of radionuclides released by nuclear tests played a large role in the distribution of radioactivity of Cs-137 in the soil. In soil from the area surrounding Fukushima prefecture, the current ratio of radioactivity of Cs-134 and Cs-137 derived from the 1FNPP accident was 0.44. Considering this value, the estimated radioactivity of Cs-137 at depths of 0–1 and 1–2 cm was one-third and one-seventh of the measured radioactivity, respectively. At 0–1 cm depth, the amount of Cs-137 released from the 1FNPP accident may be analyzed by comparing it with a measurement of radioactivity in a non-contaminated area that is due to radionuclides released from the 1FNPP. The activity of K-40 in surface soil was lower than that in the underlayers. Future analysis will be necessary to clarify this finding.         0.95±0.01Bq/g Fig. 1. Radioactivity of Pb-214, Cs-137, and K-40 in the soil at each sampling point. Radioactivity (Bq/g) 00.10.20.3Pb-214 Cs-137 K-40WBWC(୰ᕞ䠅WC(ἑἢ䠅WHWMIBWC C (near the stream) Radioactivity (Bq/g) 00.10.20.3Be- 7 Cs-134 Cs-137 K - 400-1cm1-2cm2-3cmFig. 2. Depth profiles of radioactivity of Be-7, Cs-134, Cs-137, and K-40 in the soil at the WB point. 169§6. Variation Factor for Long-term Background Radiation Monitoring Data at TokiYokoyama, S., Shimo, M. (Fujita Health Univ.),  Nishimura, K., Isobe, M., Miyake, H., Hayashi, H.,  Yamamoto, Y., Tanaka, M.

Variation Factor for Long-term Background Radiation Monitoring Data at Toki

https://nifs-repository.repo.nii.ac.jp/?action=repository_action_common_download&item_id=1255&item_no=1&attribute_id=25&file_no=1

Variation Factor for Long-term Background Radiation Monitoring Data at Toki

Abstract

Similar works

Full text

Available Versions

National Institute for Fusion Science (NIFS-Repository)