Sequential monitoring of cosmic-ray neutrons and ionizing components in Japan

A fixed-point measurement of cosmic-ray intensity has being continued at Chiba, near Tokyo in Japan to obtain the time-sequential data. High efficiency neutron rem counter and Multi-sphere neutron Spectrometer (Bonner Sphere: BS) were used for the measurement of cosmic-ray neutron components and for the evaluation of the cosmic-ray ionizing components, NaI(Tl) scintillation spectrometer was employed. A series of measurement of the cosmic-ray intensity (neutron and ionizing components) were compared with the available experimental data obtained by the other cosmic-ray monitoring group. In this study we clarify that cosmic-ray neutrons and ionizing components measured at ground level vary according to an exponential attenuation law with the atmospheric pressure and that the variation due to the solar activity (called Forbush decrease) are observed from the monitoring data of pressure corrected cosmic-ray neutron dose rate and cosmic-ray ionizing component. It is confirmed that the magnitude of the variation of cosmic-ray neutron is rather larger than that of ionizing components. 1.Introduction Exposure to natural source ionizing radiation is continuous and inescapable. Although the assessment of the exposure to cosmic-ray induced neutron is getting more important [1], the experimental studies on the neutron in the environment in low geographical latitude area are quite few [2]. The authors, therefore, have begun the series of investigation on the cosmic-ray neutron energy spectrum and the neutron dose rate in Japan to assess the natural neutron background at sea level in detail in low geographical latitude area. Cosmic-ray intensity at ground level is affected by environmental factors such as, altitude, geomagnetic latitude and so on, and also by solar activity. More detailed information about the variation of cosmic-ray intensity in particular neutron component in the environment, is therefore indispensable to assess the natural background dose rate in Japan with good accuracy. In this study, a fixed-point measurement of cosmic-ray intensity has being continued at Chiba, near Tokyo in Japan to obtain the time-sequential data from May 2002 to May 2003. High efficiency neutron rem counter and Multi-sphere neutron Spectrometer (Bonner Sphere: BS) were used for the measurement of cosmic ray neutron components and NaI(Tl) scintillation spectrometer was employed for the evaluation of the cosmic-ray ionizing components. A series of measurement of the cosmic-ray intensity (neutron and ionizing components) were compared with the available experimental data obtained by the other cosmic-ray monitoring group. 2.Experimental Setup and Measurement In this study, a fixed-point measurement of cosmic-ray intensity has being continued at Chiba, near Tokyo in Japan to obtain the time-sequential data. The geomagnetic latitude in Chiba is about 26.6°. We carried out cosmic-ray measurements by using three kinds of apparatus: High efficiency neutron rem counter equipped with 12.7cm diameter spherical He-3 proportional counter [3], Multi-sphere neutron Spectrometer (BS) with 5.07-cm diameter spherical He-3 proportional counter, and 7.62-cm diameter spherical NaI(Tl) scintillation spectrometer. The list of apparatus is shown in Table I. All the our results of the cosmic-ray intensity (neutron dose rate, counting rate by NaI(Tl) spectrometer and neutron counting rate by each He-3 counter of the BS) were averaged from the accumulation of counts measured for 24 hours to decrease the counting error. Duration of the measurement is from May 2002 to May 2003 and now the sequential monitoring of the cosmic-ray has been also continued. All the measurement apparatus are placed in a well air-conditioned room in one-story concrete building with galvanized iron roofing. Before the beginning of the sequential monitoring of cosmic-ray, the comparison of the cosmic-ray intensity between inside and outside of the building was carried out using neutron rem counter and NaI(Tl) scintillation spectrometer. As the result of this experimental, it was confirmed that there was no influence of shielding by the building materials on the measurement result of cosmic-ray intensity. In parallel with the sequential monitoring of cosmic-ray intensity, ambient temperature and atmospheric pressure were also measured. In order to clarify the relationship between the cosmic-ray intensities and the atmospheric pressure, a series of the result of atmospheric pressure was used. A series of measurement of the cosmic-ray intensity (neutron and ionizing components) were compared with the available experimental data obtained by the other cosmic-ray monitoring group. In this study, our results of the cosmic-ray measurement were compared with the cosmic-ray neutron counting rate at Fort Smith, Canada performed by BARTOL RESEARCH INSTITUTE NEUTRON MONITOR PROGRAMME [4]. Table I. Measurement Apparatus for the sequential monitoring of cosmic-ray intensity Apparatus High Efficiency NaI(Tl) scintillation Bonner Multi-sphere Neutron rem counter Spectrometer Spectrometer Purpose monitoring of neutron monitoring of ionizing monitoring of the shape component of cosmic-ray component of cosmic-ray of neutron component neutron dose equivalent meter Spherical (7.62cm diameter) Multi-sphere Spectrometer Model and using 12.7cm diam. spherical NaI(Tl) scintillation using 5.07cm diam. spherical He-3 Manufacturer He-3 proportional counter Spectrometer proportional counter (Type : 27036) model NDN4NA11 Aloka Co. Ltd. LND,Inc. Fuji Electric Co. Ltd. duration From May-13-2002 From May-01-2002 From May-04-2002 Now under meaurement Now under meaurement Now under meaurement 2.1 Neutron rem counter Neutron rem counter used for the sequential monitoring was the high efficiency neutron rem counter equipped with 12.7cm diameter spherical He-3 proportional counter, developed by Nakamura et al. [3]. From the counting rate obtained by neutron rem counter, the ambient dose equivalent rate (H*(10)) for neutrons was obtained by multiplying the counting rate to ambient dose equivalent rate (H*(10)) coefficient. Counting rate to ambient dose equivalent rate (H*(10)) coefficient was estimated by performing the calibration by Cf-252 source at the Facility of Radiation Standard of the Japan Atomic Energy Research Institute. It was estimated to be about 15.2 (cps/(μSv·h)). 2.2 NaI(Tl) scintillation spectrometer The purpose of the measurement of ionizing components is to compare the magnitude of the variation of ionizing components with that of neutron component. As well known, ionizing components of the cosmic-ray consists of various kind of the secondary cosmic-ray, muons, pions, electrons and gamma-rays. Many reliable investigations on the assessment of public dose due to ionizing components are reported [1,5,6]. As shown in Fig.1, the counting rate (cpm) of the above 3MeV region measured by NaI(Tl) spectrometer agreed with the air equivalent absorbed dose rate (nGy·h) originated to cosmic-ray obtained by pressurized ionization chamber. This result shows that the counting rate (cpm) of the above 3MeV region measured by NaI(Tl) spectrometer can be used to evaluate the cosmic-ray ionizing components. In addition, it is more reliable than ionization chamber due to its higher counting rate. In this study, NaI(Tl) scintillation spectrometer was employed for the evaluation of the cosmic-ray ionizing components. y = 0.3492x R = 0.82 20 22 24 26 28 30 32 34 36 38 40 90 91 92 93 94 95 96 97 98 99 100 Counting rate by NaI spectrometer (>3MeV) (cpm) >3MeV counting rate (by NaI) fitting curve A ir eq ui va le nt a bs or be d do se ra te (n G y· h -1 ) Fig.1 Correlation between counting rate (above 3MeV) by NaI spectrometer and absorbed dose rate of ionizing component 2.3 Neutron spectrometer The spectrum of the cosmic-ray induced neutrons was determined by using Multi-sphere neutron Spectrometer (Bonner sphere: BS). The BS consists of five 5.07-cm diameter spherical He-3 proportional counters filled with 5 atom He-3 gas, surrounded by spherical polyethylene moderators, with different size together with no moderator (bare He-3 counter). The diameters of the polyethylene moderators are 8.0cm, 11cm, 15cm, and 23cm, respectively. As the response of the BS to neutron with energy from thermal region to 400MeV, the set of response functions determined by Uwamino et al. [7] was used except for the response function of bare He-3 counter. The response function of bare He-3 counter determined by Nunomiya [8] was used in this study. Calibration and tests of the measurement of the BS were performed using monoenergetic neutrons from 250keV to 15MeV in the Dynamitron facility at the Tohoku University [9] and irradiation of thermal neutrons and neutrons from Cf-252 source were performed at the Facility of Radiation Standard of the Japan Atomic Energy Research Institute. Throughout the series of calibration the mean errors between the measured and calculated response were within the range of 20%. From the measured data sets of count rates, the cosmic ray neutron energy spectra were obtained by performing unfolding process using the SAND2 code. The analytical conditions are shown in Table II. Table II. Analytical conditions for estimating neutron fluence rate and neutron dose rate (H*(10)) using SAND2 code Energy region 0.01eV 400MeV