H. Ishizu

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

A 4π(β(+)+γ) integral counting method using 4πβ-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 β-particles emitted, the 4π(β(+)+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.

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 β-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 β-particle fluences by the use of a proportional counter and also observed the angular dependence of β-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.

A simple method for activity determination of 134Cs and 137Cs in foodstuffs using NaI(Tl) scintillation spectrometer

A simplified peak fitting technique for the analysis of the overlapped pulse-height spectra of (134)Cs and (137)Cs γ-rays obtained with a NaI(Tl) scintillation spectrometer was studied. In this analysis, nearly an upper half of 662 keV (137)Cs peak data was employed for fitting of the Gaussian peak using the least squares algorithm. Consistent results were obtained as compared with the reference value of test samples mixed with (134)Cs and (137)Cs standard solutions.

Extended utilization of a 68Ge/68Ga reference source as a mock 18F source in the ionization chamber calibration

Possible extended utilization of a 68Ge/68Ga source as a surrogate 18F reference source was studied. Some effects and uncertainties might arise from the difference of constructions between chambers that were used in the source calibration and those practically in use. To extend utilization of the surrogate source, response ratio of real 18F to 68Ge/68Ga sources were determined for three models of chambers. Monte-Carlo simulation using EGS5 code was also carried out to evaluate uncertainties expected. The present result shows that such a surrogate source would adapt to calibration even for chambers have with different construction from the reference chamber used as the determination of 18F equivalent activity of the 68Ge/68Ga surrogate source.

Screening test for radioactivity of self-consumption products in Fukushima after the Fukushima Dai-ichi NPP accident in Japan.

A food inspection service system for home grown products or wild plants collected by individual consumers for self-consumption was implemented in Fukushima in Nov. 2012. About 500 NaI(Tl) scintillation spectrometers were distributed to 300 or more temporary testing laboratories which were set up in public halls or meeting places of the municipalities. The screening method for radiocaesium was adapted to the present inspection service system. The performance of the equipment under field conditions was evaluated using sampled data obtained by temporary testing laboratories. From the present results of evaluation the confidence of these inspections was confirmed. Totally about 550,000 food samples have been tested with a pass rate of 90% or more.

Response of Ge photon detectors to beta-rays and possible reduction of photopeak areas due to beta–gamma coincidence summing effects

In the use of photon detectors having sensitivity to β-rays or internal conversion electrons (CEs), β-γ and/or X-CE coincidence summing may arise. In particular, an n-type germanium detector has a thin dead layer and window, causing considerably higher sensitivity to β-rays and/or CEs especially when sources are located at a small source-to-detector distance. In the present research, reduction of the photopeak area due to β-γ or X-CE coincidence summing has been studied for three germanium detectors.

Development of certified matrix reference materials for quality assurance of screening 134Cs and 137Cs in food

A certified reference material using activated alumina powder certified for activity of (134)Cs and (137)Cs was developed. The results of the verification and the certification are described. The certified reference material can be used for quality assurance of screening activity measurements of (134)Cs and (137)Cs in food/foodstuffs. Commercially available equipments were experimentally tested using the CRM and another CRM including (40)K. The results of these tests are also shown.

Practical correction method for impurities on activity measurements using isotope calibrators

Radioactive impurities might cause significant error in the activity determination of a target nuclide using ionization chambers. In the present study, an impurity correction technique for (201)Tl sources was performed by applying two different responses of an IG12A20 and IG11N20 ionization chamber. This technique can be extended to another method in which an attenuation filter made of tin was used to obtain different responses of an argon filled IG12A20. The results obtained with these techniques were very consistent with each other and with the reference value within their uncertainty after making the impurity correction. Examples of (64)Cu activity determination were also shown.

Sealed 22Na Sources for Positron Annihilation Lifetime Spectroscopy

Sealed radioactive sources of 22Na for positron annihilation lifetime spectroscopy (PALS), free from the legal regulations, are commercially available from an American company. However, thick foils are used to seal 22Na in these sources and large fractions of the positrons annihilate in the sealing materials. Further, it is pointed out that a long lifetime component over 1 ns appears in the positron lifetime spectrum acquired with the Kapton sealed source (POSK-22, IPL Inc.). In this research, attempts were made to develop high quality sealed positron sources, potentially free from regulations, for ubiquitous application of PALS. The sources prepared in the present work are of high quality and applicable to our new PALS, which does not require sample cutting and is potentially applicable to truly nondestructive, onsite inspection of various materials