Microdosimetry on a Mini-Reactor UTR-KINKI for Educational Uses and Biological Researches

Short Communication J. Radiat. Res., 50, 83–87 (2009) Microdosimetry on a Mini-Reactor UTR-KINKI for Educational Uses and Biological Researches Satoru ENDO1*, Kenichi TANAKA2, Kazuo FUJIKAWA3, Tetsuo HORIGUCHI4, Tetsuo ITOH4, Yoshihiko ONIZUKA5, Masaharu HOSHI6, Ayanori MURATAKA1, Yasuaki KOJIMA1 and Kiyoshi SHIZUMA1 Neutron/Microdosimetry/UTR-KINKI/RBE. Microdosimetry study has been carried out at the education and research mini-reactor of Kinki University (UTR-KINKI) using a tissue equivalent gas proportional counter (TEPC). The microdosimetric single event spectra for 0.5, 1, 2, 3 and 5 μm site sizes were obtained in the lineal energy range from 1 to 1000 keV/μm. Neutron and gamma-ray fractional doses were estimated from the single event spectra. The neutron dose fraction was varied from 35 to 55% for 0.5 to 5 μm site size. The averaged lineal energy, yD , for each site size was likewise estimated and found to be dependent on the site size. The averaged lineal energy for neutron was slightly larger than that of the fission neutrons from 252Cf, and the averaged lineal energy for gamma-ray had similar site-size-dependence of 25 keV gamma-rays and 250 kV X-rays. Relative biological effectiveness was found to be 4.1 ± 0.13 for UTR-KINKI using Tilikidis’s 2 Gy-response function. The estimated RBE for UTR-KINKI neutrons is quite close to the previous biological experimental value of 4.3 ± 0.6 for micronucleated cells in gill cell of Medaka and 4.6 ± 0.5 for induction of lymphocyte apoptosis in the thymus of ICR mice. INTRODUCTION The Kinki University nuclear reactor, UTR-KINKI, has an ample space for the irradiation of biological specimen in the central portion of its core, where both neutron and gamma dose-rate of about 20 cGy/hr are available during operation at a nominal output of 1 W. Due to its simple feature, UTRKINKI is used for reactor physics educational and biological studies. The dose rate of neutron and gamma-ray mixed field and the neutron energy spectrum at UTR-KINKI have been evaluated for biological studies by the same authors of this *Corresponding author: Phone: +81-82-424-7612, Fax: +81-82-424-2453, E-mail: 1 Quantum Energy Applications, Graduated School of Engineering, Hiroshima University, 1-4-1, Kagamiyama, Higashi-Hiroshima 739-8527, Japan; 2Department of Physics, School of Medicine, Sapporo Medical University, 17, Minami 1 Jo, Chuo-ku, Sapporo 060-8556, Japan; 3 Department of Life Science, Faculty of Science and Technology, Kinki University, 3-4-1 Kowakae, Higashi-Osaka 577-8502, Japan; 4Atomic Energy Research Institute, Kinki University, 3-4-1 Kowakae, HigashiOsaka 577-8502, Japan; 5Department of Health Science, Kyushu University, Maidashi, Higashi-ku. Fukuoka 812-8582, Japan; 6Research Institute for Radiation Biology and Medicine, Hiroshima University, 1-2-3 Kasumi, Minami-ku, Hiroshima 734-8553, Japan. doi:10.1269/jrr.08072 paper.1–3) The neutron energy spectra at the irradiation port were evaluated using a multi-foil activation analysis with an artificial neural network.1) The neutron mean energy which is needed in radiobiological studies was estimated and a separate dosimetry of neutron and gamma ray was carried out using a pair of tissue equivalent ionization chambers.2,3) The UTR-KINKI is useful for radiobiological studies of fission neutrons because of its stable output. Fission neutron irradiation is useful for radiobiological and physical studies of nuclear disasters, such as atomic bomb and criticality accident dosimetry. Radiobiological experiments on cultured cells, plant seeds, mice and other samples have also been carried out using this facility.4–7) Reported in this paper is the microdosimetric study that has been carried out to investigate the radiation quality of the mini-reactor, UTR-KINKI. MATERIALS AND METHODS Measurement UTR-KINKI has two fuel tanks separated by 46 cm internal graphite; each tank contains 235U enriched uranium fuels immersed in a small quantity of light water.1) At the center of the internal graphite, a graphite stringer with cross-sectional area of 9.6 × 9.6 cm2 and length of 122 cm can be withdrawn to provide a cavity for sample irradiation. The J. Radiat. Res., Vol. 50, No. 1 (2009); http://jrr.jstage.jst.go.jp 84 S. Endo et al. center of the core height in the cavity is used as irradiation field for biological samples. The geometrical view of UTRKINKI has been shown in reference.1,2) In the present study, the UTR-KINKI was operated at 1 W, and a tissue equivalent gas proportional counter (TEPC, Far West Technology ltd., Model LET-1/2”), which has an active volume with a diameter of 12.7 mm, was inserted about 30 cm into the irradiation cavity of UTR-KINKI (about 100 cm from the center) in order to accommodate the detector which has a dimension larger than the stringer. The TEPC was filled with methane-based tissue equivalent gas: TE-gas (64.4% CH4, 32.5% CO2 and 3.1% N2). To simulate 0.5, 1, 2, 3 and 5 μm-site size in tissue, the TPEC was filled with TE-gas at pressures of 32.3, 74.5, 149, 224 and 373 hPa, respectively. The output signals from the TEPC were amplified by a preamplifier (HOUSHIN, Preamplifier), and divided into two gain-main-amplifiers (ORTEC 672) referred to as “low-gain” and “high-gain” to accumulate wide lineal energy range. The data were taken by two multi-channel analyzers (MCA) for each low- and high-gain signal. The energy calibration was performed using a 244Cm-α source contained in the TEPC wall and the linearity was checked by a pulse generator module (ORTEC 419). Microdosimetric quantities Microdosimetric quantities are characterized by the lineal energy, y, which is the deposited energy in an active volume having a simulated diameter (r) divided by a mean chord length given by l = 2/3 r. The event and dose frequency spectra (f(y)), the accumulated dose distribution (D(y)) and yd(y) are used for the analysis.8) Here, f(y) is defined as 1000 keV / μ m f ( y) = fraw ( y) / ∫ (1), fraw ( y)dy 1keV / μ m weighted lineal energy can be calculated from: 1000 keV / μ m yD = ∫ yd ( y)dy (4). 1keV / μ m We used yD to compare the UTR-KINKI neutron and gamma-ray mixed field with 252Cf-fission neutron field and 137Cs gamma-ray field. RBE estimation Relative biological effectiveness (RBE), defined by the biological response to gamma rays from 60Co, is an important parameter to evaluate the radiation quality. The RBE for UTR-KINKI neutron-gamma mixed fields is estimated from microdosimetric spectrum using a 2 Gy biological response of fractional cell survivals (r(y)). This can be derived from Tilikidis et al. (1996)10) equation for RBE given by 1000 keV / μ m RBE = ∫ r ( y) y f ( y) dy . (5) 1keV / μ m RESULTS AND DISCUSSIONS The microdosimetric single event spectra for the site sizes of 0.5, 1, 2, 3 and 5 μm measured at UTR-KINKI are shown in Fig. 1. All the spectra have two broad peak structures at the region from 1–10 keV/μm and from 50–140 keV/μm. The broad p (...truncated)