Evaluation of the probability distribution of radioactivity estimated by inverse problem solution using Monte Carlo Method.

In this study, we derived the posterior probability distribution of the total activity estimated by inverse problem solution. The posterior probability distribution was derived by applying the Monte Carlo approach of the GUM Supplement 1 to the model of evaluation. The decision threshold, the detection limit, and the limits of the coverage interval for the results, all of which are defined in ISO 11929-2 as characteristic limits, were also derived.

α-particle discrimination in the measurement of α/ß decaying chains by the use of ultra-thin plastic scintillation sheets.

We found that problems of α/ß pulse-height overlapping can be solved by use of ultra-thin plastic scintillator sheets. In order to determine individual α-counting efficiencies for nuclides in the 223Ra decay-chain, the 4πα-γ anti-coincidence spectrometry technique was employed with a 4πα-γ detector configuration composed of a source sandwiched between ultra-thin plastic scintillator sheets and a Ge detector. This technique was applied to the activity measurements of 223Ra. This type of sandwich source is useful to prevent possible leakage of 219Rn gas.

UNCERTAINTY OF MEASUREMENT IN THE RESPONSE TEST OF A THYROID MONITOR.

A test device consisting of an I-131 source, a phantom on a 3D moving table, and a NaI(Tl) scintillation detector on a fixed flat table was developed to carry out response tests of a thyroid monitor. A measurement uncertainty due to variation of configuration of the source and the detector, as well as other factors, was estimated. The estimated relative standard uncertainty of measurement at a source-detector distance of 100 mm, which was deduced to be the optimum, was 1.9%. This is sufficiently small for calibrating a thyroid monitor used for radiation protection.

Calibration of (55)Fe activity with a lithium-drifted silicon detector.

NMIJ participated in the CCRI (II) key comparison of (55)Fe activity measurement. A planar-type Si(Li) detector and a liquid-scintillation counter were used to calibrate the activity. The calibration result obtained by the Si(Li) detector agreed with that obtained by the liquid-scintillation counter using the CIEMAT/NIST method within uncertainty. The result was reported to the BIPM in 2006. The activity of (55)Fe was also measured by using a 2pi pressurized proportional counter. The result obtained by the 2pi pressurized proportional counter agreed with the above results within uncertainty. The measurement by the Si(Li) detector was very simple and its procedure was a combination of conventional methods. The measurement by the liquid-scintillation counter is most popular to calibrate the activity of (55)Fe. However the Si(Li) detector is also useful.

Measurement of the response-ratio of an ionization chamber filled with dry air to that filled with P-10 gas in the calibration of gas monitors.

We developed a new method to determine the response ratio of an ionization chamber (Ohkura ionization chamber: type I-4096 01/06) filled with dry air to that filled with P-10 gas. The response-ratio was obtained by comparing the output currents of the ionization chamber after introduction of radioactive gasses of identical activities contained in two small vessels. The obtained ratio was 0.690 ± 0.003, which agreed with the results obtained by other methods.

Development of a flexible γ-ray detector using a liquid scintillation light guide (LSLG).

A flexible γ detector using a liquid scintillation light guide (LSLG) was developed. The analyzed pulse height (PHA) spectrum depended on the diameter, length and scintillator concentration of the LSLG, and the distance of a γ ray irradiation point from the head of photomultiplier tube (PMT). From the analysis of PHA spectrum, it was found that the count ratio of two divided channel regions linearly decreases as the distance from the PMT head increases. It was further found that the radiation dose rate can be estimated by setting the flexible LSLG tube to a circular shape since the count rate is proportional to the dose rate measured by a conventional NaI (Tl) scintillation detector. Therefore, a flexible and long LSLG detector using a single PMT is useful for determination of the dose rate and has a potential to detect local contaminations in a certain narrow space.

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.

Response calculation for standard ionization chambers in the APMP using EGS4 Monte Carlo code.

In the APMP radioactivity group (TCRI), a series of intercomparisons were conducted between member laboratories, with the aim to improve the calibration of ionization chambers used as secondary standards for radioactivity measurement. Several sources were sent to the participating institutes and the measurement results of calibration factors have been compared. Good agreement between laboratories was found for radioisotopes where the major contribution to the ionization current stems from high-energy gamma rays. It was expected in this energy region that these ratios would become unity for any type of chamber by normalizing with the same type of reference sources. However, where the dominating contribution to the measured ionization current is due to the presence of low-energy gamma rays, there were some serious discrepancies between participating institutes. In order to understand these differences, Monte Carlo simulation has been utilized by studying the effects of changing the wall thickness of the re-entrant well, the materials of the sample holder and the composition of the ionizing gas.

Improvements of the standardization of (134)Cs by the critical window setting for 605keV photopeak.

In the standardization of (134)Cs by the 4πß-γ coincidence method with a γ-window at 605keV, the satellite components of 563keV and 569keV overlapping the 605keV peak cause a steep slope and non-linearity of the efficiency extrapolation function. By shifting the lower threshold of a γ-window higher, the satellite components are eliminated, and the slope tends to horizontal. Nearly flat efficiency curves were obtained by using a CeBr3 scintillator for detecting γ-photons, as well as a NaI(Tl) scintillator.

Effect of time walk in the use of single channel analyzer/discriminator for saturated pulses in the 4πß-γ coincidence experiments.

Using the TAC technique, the timing properties of a 4πß-γ coincidence counting system were experimentally studied with an emphasis on saturated pulses. Experiments were performed for several discriminators (integral mode of TSCA) each with different kinds of timing techniques. Timing spectra were measured at various applied voltage to the 4π proportional detector covering the entire region of the plateau. Most of timing discriminators show good timing property when the pulses remain the linear region, but suddenly deteriorate after the pulses was saturated, and the timing spectra expands seriously up to a few µs in some types of timing discriminator. To overcome this problem, two techniques were proposed.

Observation of X-ray and Auger electron spectra in a 4π proportional counter for 4π(e, X)-γ coincidence measurements.

For the standardization of electron capture nuclides followed by γ-transitions, the 4π(e, X)-γ coincidence counting method is mainly adopted. To examine how the spectra of the (e, X) channel change with gas nature and pressure, pulse-height spectrum measurements of signals from a 4π pressurized proportional counter were carried out under various conditions for (54)Mn and (88)Y sources together with 4π(e, X)-γ coincidence counting. The spectra were measured for each half of the counter and for the combined 4π signal. The nature of counting gas (P-10 or methane) and its pressure was reflected in the X-ray peak intensity, and the Auger electron spectra were influenced significantly by the absorption in a thin VYNS film and the source conditions. In the measurements, we employed a pill-box type 4π counter in which the anodes were very thin gold-coated tungsten wires (30 µm ∅). In this system, spectra down to 0. 2 keV were clearly seen. The gas pressure can be raised up to 0.6 MPa. Counting electronics used for 4π(e, X)-γ coincidence measurements were conventional modules and a list mode two-parameter data acquisition system.

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.

APMP comparison of measurement of surface emission rate of 36Cl large area source (APMP.RI(II)-K2.Cl-36).

An international comparison of measurement of beta particle surface emission rate from a (36)Cl large area source (APMP.RI(II)-S1.Cl-36) was carried out within the framework of the Asia-Pacific Metrology Program (APMP). Participants from APMP were NMIJ (Japan), KRISS (Korea) and INER (Chinese Taipei). Participants from the other RMOs were NIST (United States), PTB (Germany), NMISA (South Africa) and VNIIM (Russia). All the results of the participants agreed within ±1%. This was the first international comparison of measurement of surface emission rate of beta particle from a large area source.

Performance test and quality control of large area reference sources fabricated by the ink-jet printing technique.

Quantitative performance tests and evaluations were carried out for large area reference sources fabricated by the ink-jet printing technique developed by NMIJ/AIST. The present tests were performed quantitatively with emphasis on uniformity, surface emission rate and long-term stability. Some techniques to minimize the self-absorption and to control the amount of radioactivity were established. The prototype source fabricated in this study safely satisfies the requirements specified in ISO 8769.

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.