Y. Kawada

Standardization of 18F using the 4π(β+γ) 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.

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

In order to clarify the γ-efficiency dependency of 4πβ-γ efficiency functions, a series of 4πβ-γ efficiency extrapolation measurements of a (134)Cs source were carried out for a wide variety of γ-geometries using a 4πβ(PS)-4πγ detector configuration. As the source is situated in the plastic scintillator (PS) β-detector, the γ-efficiency of the system can be readily changed by extracting the β-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 4πβ-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.

A simple and versatile data acquisition system for software coincidence and pulse-height discrimination in 4πβ–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 β- 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 4πβ(PS)-4πγ coincidence measurements of (60)Co, (134)Cs and (152)Eu. Possibilities of other extended applications will be touched upon.

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.

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.

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 β-particles and also β-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 β-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.

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.

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

In the standardization of (134)Cs by the 4πβ-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.

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.