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

Satoru ENDO Kenichi TANAKA Kazuo FUJIKAWA Tetsuo HORIGUCHI Tetsuo ITOH Yoshihiko ONIZUKA Masaharu HOSHI Ayanori MURATAKA Yasuaki KOJIMA Kiyoshi SHIZUMA J. Radiat. Res., Vol. 50, No. 1 (2009); http://jrr.jstage.jst.go.jp - 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 m site sizes were obtained in the lineal energy range from 1 to 1000 keV/m. 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 m 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 Tilikidiss 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. 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 paper.13) 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.47) 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 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 m-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- 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 f (y) = fraw (y) / where fraw(y) stands for raw data values. On the other hand, d(y) and D(y) are defined by the expressions d(y) = yf (y) / D(y) = y 1keV / m For comparison, the microdosimetry single event spectrum was also measured for fission neutrons and gammarays from 252Cf and 137Cs sources which were installed at the Research Institute for Radiation Biology and Medicine, Hiroshima University.9) The 137Cs gamma-ray spectrum is close to that of the fission prompt gamma-ray therefore the former provides a good reference for comparison. The average value of the lineal energy is a useful parameter for quantifying radiation quality. The average dose weighted lineal energy can be calculated from: yD = 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 RBE = RESULTS AND DISCUSSIONS The microdosimetric single event spectra for the site sizes of 0.5, 1, 2, 3 and 5 m measured at UTR-KINKI are shown in Fig. 1. All the spectra have two broad peak structures at the region from 110 keV/m and from 50140 keV/m. The broad peak at the lower y region is due to gamma-rays and while that at the higher y region is due to neutrons. The decreasing slope in the spectra around 10 and 140 keV/m are called electron- and proton-edges, respectively. The edge structures are caused by the corresponding maximum dE/dx for electrons and protons. Each spectrum for the site size of 0.5, 1, 2, 3 and 5 m had similar trend which shifted to the lower y side as a function of the site size. Electrons with an energy of about 100 eV have track ranges of a few 10 nm in water which result (...truncated)