Munehiko Kowatari

Measurement of air kerma rates for 6- to 7-MeV high-energy gamma-ray field by ionisation chamber and build-up plate

The 6- to 7-MeV high-energy gamma-ray calibration field by the (19)F(p, αγ)(16)O reaction is to be served at the Japan Atomic Energy Agency. For the determination of air kerma rates using an ionisation chamber in the 6- to 7-MeV high-energy gamma-ray field, the establishment of the charged particle equilibrium must be achieved during measurement. In addition to measurement of air kerma rates by the ionisation chamber with a thick build-up cap, measurement using the ionisation chamber and a build-up plate (BUP) was attempted, in order to directly determine air kerma rates under the condition of regular calibration for ordinary survey meters and personal dosemeters. Before measurements, Monte Carlo calculations were made to find the optimum arrangement of BUP in front of the ionisation chamber so that the charged particle equilibrium could be well established. Measured results imply that air kerma rates for the 6- to 7-MeV high-energy gamma-ray field could be directly determined under the appropriate condition using an ionisation chamber coupled with build-up materials.

INFLUENCE OF DIFFERENT TYPES OF PHANTOMS ON THE CALIBRATION OF DOSEMETERS FOR EYE LENS DOSIMETRY

Both a cylinder and a slab phantom have been recommended to be used as calibration phantoms for eye lens dosimetry in the International Atomic Energy Agency TECDOC. This study describes investigations on the influence of the type of phantom on the calibration of dosemeters. In order to fulfil the purpose, backscatter radiation from practically used water-filled phantoms was evaluated by calculations and experiments. For photons, the calculations showed that the cylinder phantom had 10 % lower backscattered effect at maximum than a slab phantom, and simulated well the backscattered effect of the human head or neck to within ±10 %. The irradiation results of non-filtered optically stimulated luminescence and radio-photoluminescence glass dosemeters indicated that the differences of the calibration factors between the two types of phantoms were up to 20 and 10 %, respectively, reflecting the response to backscattered photons. For electrons, no difference was found between the two types of phantoms.

Application of a CZT detector to in situ environmental radioactivity measurement in the Fukushima area

Instead of conventional Ge semiconductor detectors and NaI(Tl) scintillation spectrometers, an application of a CdZnTe semiconductor (CZT) whose crystal has the dimension of 1 cm cubic to the in situ environmental radioactivity measurement was attempted in deeply affected areas in Fukushima region. Results of deposition density on soil for (134)Cs/(137)Cs obtained seemed consistent, comparing obtained results with those measured by the Japanese government.

In Situ Environmental Radioactivity Measurement in High–Dose Rate Areas Using a CdZnTe Semiconductor Detector

For the purpose of determining a surface deposition density on soil for radio-cesiums, a CdZnTe (CZT) semiconductor detector whose crystal has dimensions of 1 cm cubic was applied to the in situ environmental radioactivity measurement in deeply contaminated areas in Fukushima region. Even in high–dose rate areas where pulse height spectra weren’t able to be properly obtained by the conventional high-purity Ge (Hp-Ge) semiconductor detector, proper pulse height spectra were obtained by the CZT detector with certain accuracy. Results of deposition density on soil for 134Cs and 137Cs derived from net peak areas by the CZT detector seemed consistent, comparing with those measured by the Japanese government. Air kerma rates were estimated by the same pulse height spectra for determining surface deposition density on soil for radio-cesiums. Estimated results showed almost the same values as obtained by the NaI(Tl) scintillation survey meter. The results indicate that the CZT detector can be applied to rapid and simple in situ gamma ray radioactivity measurement in higher–dose rate areas whose dose rates exceed several tenth μSv h−1. The study also strongly supports that the CZT detector is one promising candidate for the detector to be used for checking the effect of decontamination works and for long-term monitoring in heavily contaminated areas.

An attempt to decrease anisotropic emissions of neutrons from a cylindrical 241Am-Be-encapsulation source

An attempt to decrease the anisotropic emissions of neutrons from a cylindrical (241)Am-Be-encapsulated X3 source was conducted with Monte Carlo calculations and experiments. The influence of metal materials and shapes of the external casing to the anisotropy factor were focussed on. Results obtained by calculations using MCNP4C implied that a light and spherical-shaped external casing decreases the anisotropic emission of neutrons. Experimental results using the spherical-shaped aluminium protection case also revealed that the anisotropy factor was close to 1.0 with wide zenith angle ranges.

CHARACTERIZATION OF AN IN-HOUSE DEVELOPED MULTI-CYLINDRICAL MODERATOR NEUTRON SPECTROMETER

This article describes the characterization of an in-house developed multi-cylindrical moderator neutron spectrometer, which consists of a cylindrical 3He proportional counter and cylindrical moderator shells of different sizes. The response matrix of the spectrometer was calculated by Monte Carlo simulations for neutron energies from 1 × 10-8 to 10 MeV and verified with measurements in 0.144 MeV, 1.2 MeV and 241AmBe neutron standard fields. Influence of scattered neutrons was properly eliminated from the measured response using the shadow cone technique. The calculated and measured responses were in good agreement in most cases. Differences were <10% for all moderated counter configurations, while larger deviations were observed for the bare counter configuration. The performance of the neutron spectrometer in terms of spectrum unfolding was verified in the 241AmBe neutron standard field, showing reliable neutron spectrum and fluence rate in the energy range up to 10 MeV as investigated in this work.

CORRECTION FACTORS FOR ATTENUATION AND SCATTERING OF THE WALL OF A CYLINDRICAL IONIZATION CHAMBER IN THE 6–7 MeV HIGH-ENERGY PHOTON REFERENCE FIELD

In high-energy photon reference fields the value of the air kerma rate is determined by using ionization chambers (ICs). From the charge collected inside the IC the dose can be calculated using a set of calibration and correction factors according to ISO 4037-2. A crucial parameter is the correction for the attenuation and scattering of the primary radiation due to the chamber wall. This parameter can be determined using Monte Carlo calculations. The evaluation of the factor was performed for a commercially available IC of the type Victoreen 550-3 under different build-up conditions. The results were verified by measurements in the R-F high-energy photon fields according to ISO 4037-1 at the Physikalisch-Technische Bundesanstalt (PTB) and the Japan Atomic Energy Agency (JAEA).

Comparison of the High-Energy Photon Reference Fields of PTB and JAEA

A comparison has been conducted between the 6 MeV and 7 MeV (R-F) high-energy photon reference fields of the Physikalisch-Technische Bundesanstalt (PTB) and the Japan Atomic Energy Agency (JAEA). Both fields are set up according to ISO 4037. The results of both participants are in agreement within the combined uncertainties, thus the equivalence of the measurements is demonstrated.

An Inter-comparison of the Neutron Calibration Fields by D2O Moderated 252Cf Source at JAEA and KAERI

An inter-comparison of the neutron calibration field of the D2O moderated 252Cf source was carried out at JAEA and KAERI. Measuring the neutron spectra at KAERI by means of the method employed at JAEA, we attempted to evaluate the basic parameters for the D2O moderated 252Cf source and validate our method for determination of these parameters. We obtained the differences in the spectrum at each facility and theoretically clarified the effect of the geometries of the source assembly and the dimensions of the irradiation room on the neutron standard field.