FACTORS AFFECTING NEUTRON MEASUREMENTS AND CALCULATIONS

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1 Chapter 10 FACTORS AFFECTING NEUTRON MEASUREMENTS AND CALCULATIONS Part A. Neutron Background In Situ Measurements Satoru Endo, Kenichi Tanaka, Masayori Ishikawa, Kazuhisa Komura, Kiyoshi Shizuma, Masaharu Hoshi Introduction Background levels of activation radionuclides in A-bomb exposed samples have been intensively discussed. It has been of concern that background environmental neutrons may be a possible reason for the neutron discrepancy problem. However, no experimental results for the background environmental neutron flux in Hiroshima and Nagasaki were available to resolve this matter. Therefore, we decided to measure the environmental neutron flux in two ways: (1) gas proportional counting, and (2) measurement of naturally induced gold activation. Materials and Methods Gas Proportional Counter Measurements at Sample Storage Locations To determine the contribution of environmental neutrons to the residual activities in A-Bomb exposed samples in Hiroshima, we measured the background thermal neutrons using a proportional counter at several locations. A 3 He-gas proportional counter (Reuter Stokes, Model P ), filled with 3 He gas at 4 atmospheres pressure, was used for the measurements. The counting efficiency for thermal neutrons of the proportional counter was calibrated using a 252 Cffission neutron source (~12 kbq). The proportional counter was surrounded by nylon blocks (thickness 50 mm) to moderate fission neutrons. Three gold foils (1 1 cm 2, thickness 20 µm) in bare form and three other foils covered by cadmium sheets (thickness 0.5 mm) were put on the counter surface to monitor thermal and epithermal neutrons. The gold nuclei were activated by neutrons by the 197 Au(n,γ) 198 Au reaction. The induced 198 Au activity was determined using an ultra-low-background germanium detector (Shizuma et al. 1991). The thermal neutron flux (φ th ) 722

2 can be determined by φ th = ε A th /N Au σ, where N Au is the number of gold nuclei, σ is the cross section of the 197 Au(n,γ) 198 Au reaction for thermal neutrons and ε is a self-shielding factor for 20- µm thick gold foil. The activity of A th induced by only thermal neutrons can be determined using the induced activity of bare gold foil (A) and the cadmium covered gold foil (A cd ) as A th = A F Cd A Cd, where F Cd is a absorption correction factor in the cadmium cover. The factors of ε and F Cd were estimated to be 1.22 and 1.06, respectively, by means Monte Carlo calculations using the MCNP4B computer program (Briesmeister 1997). The bare gold foil activity was determined to be Bq (±3%), and the cadmium-covered gold activity was Bq (±6%). The thermal neutron flux was estimated to be 4.2 ± 0.5 n/(cm 2 s). The peak counting rate of the 3 He-gas proportional counter with the 12-kBq 252 Cf source was 148 cps. Using the surface area of the counter active volume of cm 2, and the thermal neutron incident rate of cm 2 times 4.2 n/(cm 2 sec) through the counter, the efficiency of the counter was estimated to be 0.27 for isotropic thermal neutrons (0.99 for the parallel neutron beam due to cross-sectional area of the counter of 35.5 cm 2 ). We measured the environmental neutrons at various locations in several buildings of Hiroshima University. The environmental neutrons were measured (1) at center points of floors from the basement to the ninth floor in the building of the Graduate School of Engineering, (2) the storage room of A-bomb samples in the same building (6th floor), and (3) the storage room of A-bomb samples in the Research Institute for Radiation Biology and Medicine (RIRBM). For the latter measurement, we arranged for the neutron irradiator with the 252 Cf source to be used at RIRBM. Therefore, measurements at the RIRBM were made at three positions around the irradiator room. One was in the storage room (located at about 20 m distance from the irradiator), and the others were the operation room of the irradiator (at about 6 m) and a room in the next building behind the operation room (at about 7 m). Gas Proportional Counter Measurements at Sample Collection Locations A 3 He gas proportional counter (2.54 φ 25.4 cm, gas pressure 10 atm) was used for the measurement of thermal neutrons outside the buildings. The detection efficiency was estimated using the MCNP4B computer program. Efficiency was estimated to be 0.88 by the calculation for a parallel beam of thermal neutrons incident on the counter in a vertical direction to the shaft of the counter. Measurements were made in four locations: (1) a rooftop of the RIRBM (ground range of 2,630 m), (2) Higashi-Senda Campus of Hiroshima University (ground range of 1,435 m), (3) rooftop of the Takeichi Clinic (ground range of 1,830 m), and (4) rooftop of the NHK Building (ground range of 440 m). Gold Activation Measurements The level of environmental neutrons was measured by the gold activation method. Known amounts of gold target (10-50 g of gold plate, grain, necklace etc.) were placed at the point for measurement. The gold sample was then recovered after more than 3 weeks exposure to environmental neutrons (>8 half-lives of 198 Au: d) and sent back to the Low-Level Radioactivity Laboratory (LLRL) of Kanazawa University covered by a Cd sheet. The 198 Au activity was measured in the Ogoya underground laboratory to obtain the number of 198 Au atoms per g of gold. The 198 Au yield depends on the thickness of gold target, because the neutron cross 723

3 section of 197 Au is high (98.8 b for thermal neutrons). The measured points were Nisseki Hospital, Honkawa Primary School, Radiation Effects Research Foundation, and Graduate School of Engineering of Hiroshima University in Hiroshima, and Nagasaki University in Nagasaki. For comparison control points, locations at LLRL, Tatsunokuchi, and the Metrological Search Institute at Tsukuba were selected and measured. Results Gas Proportional Counter Measurements at the Graduate School of Engineering The environmental neutron flux at the center position of each floor in the building of the Graduate School of Engineering, Hiroshima University depends on the floor number. This is due to the shielding effect of the floors, with the neutron flux in the basement being the lowest and the flux increasing at higher floors. The thermal neutron flux ranged from to n/(cm 2 s). The A-bomb exposed samples of granite cores and steel are stored at the east side on the 6th floor. The thermal neutron flux at this position was determined to be n/(cm 2 s). The specific activity of 60 Co induced by environmental neutrons in the steel by the 59 Co(n,γ) 60 Co reaction was estimated to be 0.27 mbq/g Co at saturation 60 Co production. Similarly, activity of 152 Eu induced in the granite cores according to the 151 Eu(n,γ) 152 Eu reaction was estimated to be 7.9 mbq/g Eu (Table 1). Meanwhile, the specific activities of 60 Co and 152 Eu at 1,200 m from the hypocenter in Hiroshima based on the DS02 calculation are mbq/g Co and mbq/g Eu, respectively, at the time of the bombing. Taking into consideration the decay during the time after the bombing, these values decrease to 5.5 mbq/g Co and mbq/g Eu in 2001 (Table 1). The contributions of the background activity by environmental neutrons, therefore, can be evaluated to be 1/20 for 60 Co and 1/670 for 152 Eu of the activities estimated by the DS02 calculation, respectively, at 1,200 m from the hypocenter. Gas Proportional Counter Measurements at RIRBM In the RIRBM measurement, the neutron flux when the fission neutron irradiator was not in operation was the same as the level on the 6th floor in the building of the Graduate School of Engineering. In the storage room of the A-bomb samples, the neutron flux slightly increased during the working time of the fission neutron irradiator, by 22% above the environmental background neutron level [ n/(cm 2 s)]. Considering that the irradiator is not in continuous operation, the excess of activations induced by neutrons from the irradiator is rather less than 22%. Gas Proportional Counter Measurements Outside Locations Results of environmental neutron measurements outside buildings are shown in Table 2. The thermal neutron flux is estimated to range from to n/(cm 2 s). Using these values, the specific activity of 152 Eu from natural Eu nuclei activated to unstable 152 Eu is estimated to range from 6.77 to 11.0 mbq/g Eu. These values are in agreement with the measurement in the building of the Graduate School of Engineering (Table 1). 724

4 Gold Activation Measurements Results of neutron measurements by gold activation in Hiroshima and Nagasaki are given in Table 3. The measured 198 Au yields are well within the range of typical background values, which indicates that the neutron flux in Hiroshima and Nagasaki is also within the normal range of typical values in the other areas. Typical 198 Au yields for a 20-g gold grain (1 to 1.5 mm in diameter) were measured to be 50 ± 10 atoms 198 Au/(g Au), and the thermal neutron equivalent flux was then estimated to be (3.5 ± 0.4) 10-4 n/(cm 2 s). The self-shielding factor in 0.1 mmthick gold sheet for the thermal neutron field was determined by Hoshi et. al. (Chapter 8, Part H) to be 0.4. Assuming the self-shielding factor of 0.4, the thermal neutron flux is estimated to be (8.8 ± 1.0) 10-4 n/(cm 2 s). This value agrees well with the above-mentioned estimates of thermal neutron flux using the 3 He counters. 725

5 Conclusions Naturally induced activity of 60 Co by the environmental neutrons in Hiroshima is evaluated to be 0.27 mbq/g Co. This value is 1/20 of the activity estimated by DS02 for a location at 1,200 m from the hypocenter. The induced activity of 152 Eu by environmental neutrons is negligibly smaller, 1/670 of the activity estimated by DS02 for a location at 1,200 m. The contribution of the naturally induced activity of 152 Eu can be neglected in comparison with the activity produced by the atomic-bomb neutrons in samples within 1,200 m of the hypocenter. References Briesmeister, J. F.; ed. MCNP A General Monte Carlo N-Particle Transport Code, Version 4B. Los Alamos, New Mexico: Los Alamos National Laboratory; LA-12625; March Shizuma, K.; Fukami, K.; Iwatani, K.; Hasai, H. Low-Background of Ge Detectors for the Measurement of Residual 152 Eu Radioactivity Induced by Neutrons from the Hiroshima Atomic Bomb. Nucl. Inst. Method. B66: ; Next Page