Simulation of a well-type HPGe detector for samples both in the hole and on top of the endcap.

A well-type HPGe detector model with the existence of a particular dead area in the HPGe crystal was developed for use in Monte Carlo simulation codes to better model gamma-ray efficiency characteristics. Dissimilarities of counting efficiencies between simulation and calibration were successfully minimized for the case of point and volume sources. Simulations using the resulting model are applicable to the activity measurement of samples both in the hole and on top of the endcap of the well-type HPGe detector.

A practical evaluation of measurement uncertainty in gamma-ray efficiency curve determination through an analytical approach.

An example of uncertainty evaluation of an efficiency curve is reported. In the applied method, the efficiency curve is determined through the weighted least square method based on measured efficiencies employing a mixed-radionuclide standard source. The variance-covariance matrix of the efficiencies is determined by evaluating the uncertainties of the counting, the calibrated radioactivities, the correction factors for true coincidence summing, and the inadequacy of the fitting function. The proposed method can be a basis to develop a more general procedure.

Characterization of brown rice as a certified reference material for Fukushima accident-related radioactivity measurements.

We developed a certified reference material of brown rice to measure radioactivity from the Fukushima Daiichi Nuclear Power Plant accident. The rice was planted in the spring of 2011, just after the Fukushima accident occurred, and it was harvested in the autumn of 2011. The certified value of radioactivity concentration in the rice was 33.6 Bq kg(-1) of Cs-134 and 51.8 Bq kg(-1) of Cs-137 on August 1, 2012. The reference material is being widely distributed by the National Metrology Institute of Japan. To determine the radioactivity and its uncertainties in the brown rice, we employed gamma-ray spectrometry with a high-purity germanium detector and Monte Carlo simulation.

Estimation of immediate fallout after the accident at Fukushima Daiichi Nuclear Power Plant by using HPGe detector and EGS5 code.

After the accident at the Fukushima Daiichi nuclear power plant, we managed to carry out emergency measurements of the radioactive fallout. The included nuclides were identified via gamma-ray spectrometry using an HPGe detector. Quantifications of each radionuclide in the fallout were determined based on the efficiency calibrations and relevant corrections. The collected samples had a variety of shapes, densities, and compositions. EGS5 Monte Carlo code was used for the flexible estimation of these parameters. The measurement results show the temporal changes in the fallout quantity about a month after the accident.

Measurement of anisotropic angular distributions of photon energy spectra for I-125 brachytherapy sources.

The angular distribution of photon energy spectra emitted from an I-125 brachytherapy source was measured using a specially designed jig in the range of ±70° in the plane of the long axis of the source. It is important to investigate the angular dependence of photon emissions from these sources for the calibration of the air kerma rate. The results show that the influence of the distributions between 0° and ±8° is small enough to allow a calibration using current primary instruments which have a large entrance window.

Effects of anisotropic fluences and angular depended spectra of beta-particles in the use of large area reference sources.

Calibrations of instrument efficiency of surface contamination meters are usually made with extended reference sources which are standardized in terms of 2π surface ß-particle emission rates from the source surface including backscattered particles. Extended sources supplied from various metrology institutes or calibration laboratories, but the source-types such as structure, preparation method, backing and covering materials vary between manufacturers. In this work first we show how the calibration results are dependent on the source type. Second, in order to clarify the possible reason of such discrepancy, we examined the isotropy of ß-particle fluences by the use of a proportional counter and also observed the angular dependence of ß-particle spectra by the use of small plastic scintillation spectrometer, where the source mount can rotate relative to the detector window at various obliquities. The discrepancy in the instrument-calibration of surface contamination meters, which are mainly used under the conditions of large source-to-detector geometry, can be explained.

Activity measurement of 85Kr diluted by a large volume balloon technique.

The National Metrology Institute of Japan (NMIJ) participated in the CCRI(II) Key-comparison of (85)Kr activity measurement. The absolute internal gas-counting method was used, with a set of proportional counters of different length. The original (85)Kr gas was diluted by a factor of 2x10(5) in order to achieve the suitable activity concentration for the proportional counters. A new dilution method was introduced, based on a large-volume balloon and gravimetric determination of the dilution volume with small uncertainties.