Evaluation of absolute measurement using a 4π plastic scintillator for the 4πß-γ coincidence counting method.

Absolute measurement by the 4πß-γ coincidence counting method was conducted by two photomultipliers facing across a plastic scintillator to be focused on ß ray counting efficiency. The detector was held with a through-hole-type NaI(Tl) detector. The results include absolutely determined activity and its uncertainty especially about extrapolation. A comparison between the obtained and known activities showed agreement within their uncertainties.

Homogeneity of wheat flour in 5ml containers for certified reference materials.

We aimed to develop certified reference materials that could be used for well-type HPGe detector. We chose wheat flour as a sample and evaluated the homogeneity of the sample in well-type container (5ml). Results showed that inhomogeneity was sufficiently small for validation checks of well-type HPGe detector (uhom = 0.44%).

Results of an international comparison of activity measurements of 68Ge.

An international key comparison, identifier CCRI(II)-K2.Ge-68, has been performed. The National Institute of Standards and Technology (NIST) served as the pilot laboratory, distributing aliquots of a 68Ge/68Ga solution. Results for the activity concentration, CA, of 68Ge at a reference date of 12h00 UTC 14 November 2014 were submitted by 17 laboratories, encompassing many variants of coincidence methods and liquid-scintillation counting methods. The first use of 4π(Cherenkov)ß-γ coincidence and anticoincidence methods in an international comparison is reported. One participant reported results by secondary methods only. Two results, both utilizing pure liquid-scintillation methods, were identified as outliers. Evaluation using the Power-Moderated Mean method results in a proposed Comparison Reference Value (CRV) of 621.7(11)kBqg-1, based on 14 results. The degrees of equivalence and their associated uncertainties are evaluated for each participant. Several participants submitted 3.6mL ampoules to the BIPM to link the comparison to the International Reference System (SIR) which may lead to the evaluation of a Key Comparison Reference Value and associated degrees of equivalence.

Proficiency testing with uncertainty evaluation for measuring activities per unit mass of 134Cs and 137Cs in brown rice in Japan.

In Japan, we conducted proficiency testing of activity measurement by using high-purity germanium detectors for 134Cs and 137Cs in brown rice grains. Among 176 reported results, 86 % (for 134Cs) and 93 % (for 137Cs) of the results satisfied |En| â‰¦ 1. However, 58 reports for 134Cs and 51 reports for 137Cs had some failures in their evaluations of uncertainties. The proficiency testing was effective to improve the ability of uncertainty evaluation.

Activity measurement of (68)Ge-(68)Ga by use of 4π(ß(+)+γ) integral counting method.

A 4π(ß(+)+γ) integral counting method using 4πß-4πγ detector configuration composed of a large well type NaI(Tl) scintillation detector and stacked plastic scintillators positioned in the center of the well and coupled with a slender PMT was adopted for activity measurement of (68)Ge-(68)Ga. Several source preparation schemes were studied to reduce the activity loss due to volatility. The possible contribution of EC events were rejected with pulse-height discrimination. Owing to the high counting efficiencies in both channels and the multiplicity of photons and ß-particles emitted, the 4π(ß(+)+γ) integral counting system gives a count rate very nearly equal to the positron emission rate. The activity can be determined simply from this value divided by the positron emission branching ratio. The remaining overall inefficiency was evaluated by the EGS5 code.

Simulation technique for extrapolation curves in 4πß-γ coincidence counting method using EGS5 code.

A simulation technique was developed for the extrapolation technique in 4πß-γ coincidence counting method. Simultaneous emissions of ß and γ rays were calculated using EGS5 code to obtain coincidence counting between both ß and γ channels. The simulated extrapolation curves were compared with experimental data obtained with (134)Cs measurements using a plastic scintillator in the ß channel. The variation of the extrapolation curves with γ-gate configuration was investigated by the simulation technique.

Gamma geometry dependency of efficiency functions in the 4πß-γ coincidence measurements of complex decaying nuclides.

In order to clarify the γ-efficiency dependency of 4πß-γ efficiency functions, a series of 4πß-γ efficiency extrapolation measurements of a (134)Cs source were carried out for a wide variety of γ-geometries using a 4πß(PS)-4πγ detector configuration. As the source is situated in the plastic scintillator (PS) ß-detector, the γ-efficiency of the system can be readily changed by extracting the ß-detector from the well-hole in a series of stages. For data acquisition and analyses, a list-mode two-parameter data acquisition system was employed. The forms of the extrapolation curves were monitored with decreasing γ-geometry, eventually exhibiting a similar behavior to those obtained in a usual 4πß-γ coincidence counting system. The experimental results and considerations suggested that the γ-geometry dependency of the efficiency functions were due to summing effects in the γ-channel, and some qualitative remarks on the form of the extrapolation functions are given.

Absorption and backscatter of internal conversion electrons in the measurements of surface contamination of ¹³7Cs.

We measured 4π and 2π counting efficiencies for internal conversion electrons (ICEs), gross ß-particles and also ß-rays alone with various source conditions regarding absorber and backing foil thickness using e-X coincidence technique. Dominant differences regarding the penetration, attenuation and backscattering properties among ICEs and ß-rays were revealed. Although the abundance of internal conversion electrons of (137)Cs-(137)Ba is only 9.35%, 60% of gross counts may be attributed to ICEs in worse source conditions. This information will be useful for radionuclide metrology and for surface contamination monitoring.

Monte Carlo simulation of a beta particle detector for food samples.

The accident at the Fukushima Daiichi Nuclear Power Plant in March 2011 released radionuclides into the environment. There is concern that (90)Sr will be concentrated in seafood. To measure the activities of (90)Sr in a short time without chemical processes, we have designed a new detector for measuring activity that obtains count rates using 10 layers of proportional counters that are separated by walls that absorb beta particles. Monte Carlo simulations were performed to confirm that its design is appropriate.

Efficiency fitting for TDCR measurement data using polynomial approximation and the Newton-Raphson method.

A tritium radioactivity source was measured by triple-to-double coincidence ratio (TDCR) equipment of the National Metrology Institute of Japan (NMIJ), and measured data were fitted using polynomial approximation and the Newton-Raphson method, a technique whereby equations are solved numerically by successive approximations. The method used to obtain the activity minimizes the difference between statistically calculated data and experimental data. In the fitting, since calculated statistical efficiency and TDCR values are discrete, the calculated efficiencies are approximated by quadratic functions around experimental values and the Newton-Raphson method is used for convergence at the minimal difference between experimental data and calculated data. In this way, the activity of tritium was successfully obtained.

A simple and versatile data acquisition system for software coincidence and pulse-height discrimination in 4πß-γ coincidence experiments.

A simple but versatile data acquisition system for software coincidence experiments is described, in which any time stamping and live time controller are not provided. Signals from ß- and γ-channels are fed to separately two fast ADCs (16 bits, 25 MHz clock maximum) via variable delay circuits and pulse-height stretchers, and also to pulse-height discriminators. The discriminating level was set to just above the electronic noise. Two ADCs were controlled with a common clock signal, and triggered simultaneously by the logic OR pulses from both discriminators. Paired digital signals for each sampling were sent to buffer memories connected to main PC with a FIFO (First-In, First-Out) pipe via USB. After data acquisition in list mode, various processing including pulse-height analyses was performed using MS-Excel (version 2007 and later). The usefulness of this system was demonstrated for 4πß(PS)-4πγ coincidence measurements of (60)Co, (134)Cs and (152)Eu. Possibilities of other extended applications will be touched upon.

Standardization of 99mTc and 99Mo by the use of a 4pibeta-4pigamma counter.

Technetium-99m is widely used for many diagnostic investigations in nuclear medicine, thus the standardization of this nuclide is important. In classical 4pibeta-gamma coincidence counting of (99m)Tc, the steep slope of the efficiency function and the large extrapolation range may cause a large measurement uncertainty. In order to overcome these difficulties, we apply the 4pibeta+4pigamma sum counting technique for the standardization. Furthermore, we adopt both the 4pibeta-4pigamma coincidence counting method and the 4pibeta+4pigamma sum counting technique for the standardization of its parent nuclide (99)Mo. High efficiencies for both nuclides were easily achieved by the use of our 4pibeta-4pigamma counter. All the results were consistent with the reference value obtained by the conventional method obtained by NMIJ.

Monte Carlo simulation of the standardization of 22Na using scintillation detector arrays.

In order to calibrate PET devices by a sealed point source, we contrived an absolute activity measurement method for the sealed point source using scintillation detector arrays. This new method was verified by EGS5 Monte Carlo simulation.

Standardization of (18)F using the 4pi(beta+gamma) integral counting technique.

Alpha 4pi(beta+gamma) integral counting technique using a 4pibeta-4pigamma detector configuration was adopted for the standardization of (18)F. In this technique, the beta-detector is composed of two thin plastic scintillators sandwiching the source, coupled with a slender photomultiplier tube. The beta-detector part with the source was inserted into a large well-type NaI(Tl) scintillation detector for gamma-ray detection, making a 4pibeta-4pigamma coincidence counting system. In this work, positron particles were detected with high efficiency in the beta-channel and annihilation quanta were also detected with high efficiency in the 4pigamma channel. The very small inefficiency of the 4pi(beta+gamma) integral counter for the beta-plus branch has been confirmed by EGS5 Monte Carlo simulation. The result using this technique agreed within the uncertainties with the result obtained by the conventional 4pibeta-gamma coincidence counting with the efficiency extrapolation technique using the same detector configuration and a conventional 4pibeta-gamma coincidence counter.

Observation of beta-ray spectra after penetrating absorbing materials.

beta-Ray spectra after penetrating absorbing materials of various thicknesses were observed by the use of a scintillation-type beta-ray spectrometer equipped with a flat NE-102 plastic scintillator of 5mm thickness for sources of (60)Co, (90)Sr-(90)Y, (137)Cs, (147)Pm and (204)Tl. Although the spectra changed rapidly with increasing absorber thickness, the average beta-ray energy was kept nearly constant for a wider range. These results are consistent in that the beta-ray absorption curve becomes quasi-linear in a semi-logarithmic plot. Spectra including scattered beta-rays from several materials placed behind the source were also measured for (137)Cs and (204)Tl. It may be concluded that mean energy measurements by the use of beta-ray spectrometer of this kind is useful for the identification of nuclides in radiation protection purposes even in worse source-conditions.

The efficiency variation method for 4pibeta-gamma coincidence counting by ink-jet printing.

In order to vary the counting efficiencies in the 4pibeta-gamma coincidence extrapolation technique, a radioactive source was coated directly with varying amounts of an electrical conducting pigment using an ink-jet printer. This method can be used to efficiently prepare the multiple sources needed to generate efficiency extrapolation curves, and was successfully applied to the standardization of a (54)Mn source.