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ABSTRACTPassive γ-ray spectroscopy is a useful technique for surveying the radioactive wastes and spent nuclear fuels under nuclear decommissioning. However, this method depends on material properties such as the activity, density, element, scale, and (especially) for low-energy γ rays from 235U and 239Pu. The γ-decay lines of 134Cs, 137Cs, 60Co, and 154Eu occur at greater energies (than those of 235U and 239Pu), and these nuclides provide significant information on spent nuclear fuel and radioactive wastes. A CeBr3spectrometer with a small-volume crystal has been previously developed for use in intense radiation measurements. We exposed the spectrometer to radiation dose rates of 0.025, 0.151, 0.342, 0.700, and 0.954 Sv/h under a standard 137Cs radiation field. A 6.38 MBq 60Co calibration source was placed in front of the detector surface. Identification of the full energy peak at 1173 keV was impossible at dose rates higher than 0.700 Sv/h. However, subtraction of the 137Cs radiation spectra from the γ-ray spectra enabled the identification of the full energy peaks at 1173 and 1333 keV at dose rates of up to 0.954 Sv/h; the relative energy resolution at 1173 and 1333 keV was only slightly degraded at this dose rate.

After the accident of the Fukushima Daiichi Nuclear Power Plant, radionuclide were spread out over large area. It has been past almost two years since the nuclear accident. Therefore, Cs-137 and Cs-134 nuclei are the main sources of gamma rays. The field gamma rays, however, are not mono-energy due to photons scattered from the ground, the air, etc. The effective dose for external exposure depends on the energy of radiations, thus photon energy spectra are important for the evaluation of effective dose for the people who live in Fukushima. In the present study, the photon energy spectra have been measured at several points in Fukushima to evaluate reference energy spectra after the nuclear accident. The energy spectra in Fukushima area were measured using a cadmium zinc telluride (CdZnTe) detector, and an unfolding method was applied to evaluate photon energy spectra. The fraction of low-energy photon is increased by decontamination around the measurement point. This can be used to estimate the efficiency of decontamination effect by removing the radionuclides on the target surface. The photon energy spectra measured by the present study would be useful for the evaluation of the effective dose for the people who live in Fukushima.

A new calibration technique, referred to as e-trace, has been developed by the National Institute of Advanced Industrial Science and Technology (AIST). The e-trace technique enables rapid remote calibration of measurement equipment and requires minimal resources. We calibrated radioisotope calibrators of the Japan Radioisotope Association (JRIA) and the Nishina Memorial Cyclotron Center (NMCC) remotely and confirmed that remote calibration provided results that are consistent with the results obtained by existing methods within the limits of uncertainty. Accordingly, e-trace has been approved as the standard calibration method at AIST. We intend to apply remote calibration to radioisotope calibrators in hospitals and isotope facilities.

Parallel-plate free-air ionization chambers are used for X-ray air-kerma rate standards at the National Metrology Institute of Japan (NMIJ), AIST. The electron-loss and photon-scattering correction factors are needed for the evaluation of air-kerma rate from measured current. The electron-loss correction factor (Ke) is a correction of the charge loss by giving energy to the electrode part without the high-speed electron stopping in the air area where the charges are collected. The scattering correction factor (Ksc) is for a correction of extra charges produced by scattered photons generated after an incidence photon is interactive. The electron-loss and photon-scattering correction factors for 3 different size parallel-plate free-air chambers are estimated by the EGS4 code. One chamber is used to get primary standards for absolute measurements of air kerma in beams of medium-energy X-rays and the other two are used for the same purpose in beams of low-energy X-rays. These correction factors are calculated for mono-energetic photons. It is found that electron-loss and photon-scattering correction factors depend on the chamber size, and the latter especially changes greatly depending on the size. The Keand Kscvalue for medium-and low-energy X-ray fields at AIST are estimated by averaging the energy deposition contributions over the X-ray spectrum.

We measured angular energy spectra of secondary neutrons from large Cu and Pb targets bombarded by 400 MeV/nucleon Fe ions to obtain the benchmark data for the newly-developed heavy ion transport Monte Carlo code HETC-CYRIC. The HETC-CYRIC code is made by incorporating a heavy ion reaction calculation routine, which consists of the HIC code, the SPAR code, and the Shen’s formula, into the hadron transport Monte Carlo code HETC- 3STEP. The results calculated with the HETC-CYRIC were compared with the measured data and the HETC-CYRIC gave good agreement with the experiment.

The double-differential cross sections (DDX) of neutron production by high energy heavy ions were measured using the RIKEN Ring Cyclotron of the Institute of Physical and Chemical Research, Japan, and the Heavy Ion Medical Accelerator in Chiba (HIMAC), the National Institute of Radiological Sciences. The neutron energy spectra from 0 to 110 degrees were obtained by using the time-of-flight method. The neutron spectra in the forward direction have a sharp peak which is located at the incident particle energy per nucleon due to the knock-on process, and spreads out to about 3 times the incident particle energy per nucleon. The total neutron production cross sections were obtained by integrating the energy spectra above 20 MeV over a hemisphere from 0 to 90 degrees. The experimental results are compared with calculations using the intranuclear cascade-evaporation Monte Carlo code, HIC, and the quantum molecular dynamics code, QMD. The QMD codes give better agreement with the experimental results than the HIC code.

In order to estimate the accuracy of calculation methods such as empirical formulas and Monte Carlo codes for neutron and gamma ray fluxes through a labyrinth connected with a room having a Cu target irradiated by 68-MeV-proton, experimental analyses are carried out. As results of the analyses, the followings are obtained. (l)The MCNP calculation is applicable to estimation of neutron and gamma ray dose equivalent rates in the labyrinth, although cross sections in the process of neutron thermalization and gamma ray production cross section should be modified for more precise estimation. (2)The empirical formulas are useful for estimation of neutron dose equivalent rate. However, an additional safety factor of 2 should be required to apply to the estimation of total dose equivalent rates, because the contribution of gamma ray dose equivalent rate increases with the distance in the labyrinth and becomes comparable to that of neutron dose equivalent rate at the distance longer than 20 m.

In order to estimate the accuracy of calculation methods such as an empirical formula and Monte Carlo codes for neutron and gamma ray flux in a room with a Cu target irradiated by 68-MeV-proton, experimental analyses are carried out. The analyses reveals that (1) the NMTC/JAERI code is applicable to source neutron calculation, (2) the Patterson’s equation with Ishikawa’s parameter is useful and estimates thermal neutron flux conservatively and (3) the MCNP calculation underestimates the fast neutron scattered back from the wall of the room, consequently the secondary gamma ray from the wall.

We are developing a heavy ion transport Monte Carlo code which can treat the fragments produced by heavy ion reactions. For this purpose, we planned to combine the two codes, the one is the HIC or ISABEL code, to calculate the heavy ion nuclear reaction cross section and the other is the hadron transport Monte Carlo code, HETC-3STEP. First, we investigated the calculational accuracy of the HIC and ISABEL codes by comparing the calculated results with the experimental data. The ISABEL gives better agreement with the experimental data than the HIC. For easier handling we first coupled the HIC code with the HETC-3STEP code for developing the heavy ion transport Monte Carlo code. We also made a simple version of a heavy ion transport Monte Carlo code for limited geometry using the HIC code. The results calculated with this version of the code were compared with the experimental data. We are trying to couple the ISABEL code with the HETC-3STEP code.

We have measured angular and energy distributions of neutrons produced by 100 and 180 MeV/u He, 100, 180, 400 MeV/u C, 100, 180, 400MeV/u Ne, 400MeV/u Ar, 400MeV/u Fe, 400MeV/u Xe and 800MeV/u Si ions stopping in thick carbon, aluminum, copper and lead targets using the HIMAC (Heavy-Ion Medical Accelerator in Chiba) of NIRS (National Institute of Radiological Sciences), Japan. The neutron spectra in the forward direction have broad peaks of about 60 to 70 % of the incident particle energy per nucleon due to the break-up process and spread up to almost the twice of the projectile energy per nucleon. The neutron spectra are similar for the same incident energy for different projectile particles. The experimental results are compared with the calculations using the HIC code, and the calculated results agree with the measured results within a factor of 2. This systematic study on neutron production from thick targets by high-energy heavy ions is the first experimental work performed by NIRS and will be useful for designing the shield of the high-energy heavy-ion accelerator facility.

Following our preceding study on thick target neutron yields by He and C, we measured angular and energy distributions of neutrons produced by 100, 180 and 400 MeV/nucleon Ne ions stopping in thick carbon, aluminum, copper and lead targets using the heavy ion medical accelerator of the National Institute of Radiological Sciences. The neutron spectra in the forward direction have broad peaks which are located at about 60 to 70% of the incident particle energy per nucleon due to break-up process and spread up to almost twice as much as the projectile energy per nucleon. The neutron spectra at all angles consist of two components of cascade neutrons and evaporation neutrons. The phenomenological hybrid analysis of the moving source model for these two components and the Gaussian fitting of break-up process could well represent the measured thick target neutron spectra. The experimental results are also compared with the calculations using the HIC code, and the calculated results generally agree with the measured ones within a factor of 2 margin of accuracy for 180 and 400 MeV/nucleon Ne ions. This systematic study on neutron production from thick targets by high-energy heavy-ions is the first experimental work and will be useful for shielding design of high energy heavy ion accelerator facilities.