ENHANCED RADIOACTIVE CONTENT OF 'BALANCE' BRACELETS.

During a routine whole body counting measurement of a worker at the Nuclear Research Center Negev, abnormal activities of (232)Th and (238)U were measured. After a thorough investigation, it was found that the radioactivity was due to a rubber bracelet ('balance bracelet') worn by the worker during the measurement. The bracelet was counted directly by an high pure germanium gamma spectrometry system, and the specific activities determined were 10.80 ± 1.37 Bq g(-1) for (232)Th and 5.68 ± 0.88 Bq g(-1) for natural uranium. These values are obviously high compared with normally occurring radioactive material (NORM) average values. The dose rate to the wrist surface was estimated to be ∼3.9 µGy h(-1) and ∼34 mGy for a whole year. The dose rate at the centre of the wrist was estimated to be ∼2.4 µGy h(-1) and ∼21 mGy for a whole year. The present findings stresses a more general issue, as synthetic rubber and silicone products are common and widely used, but their radioactivity content is mostly uncontrolled, thus causing unjustified exposure due to enhanced NORM radioactivity levels.

Fast determination of 9°Sr/9°Y activity in milk by Cherenkov counting.

Cherenkov counting of the 9°Sr/9°Y pure beta emitters is an attractive method for 9°Sr activity determination, but the color quenching effect may be significant, especially for strongly colored or semi-opaque media. A quench correction method based on the external source of some liquid scintillation systems (named ESAR - external source area ratio) was proposed and checked for aqueous solutions and was proved to be effective also for urine samples. In the present work, the application of the ESAR method for fast determination of 9°Sr/9°Y activity in milk samples is described.

Determination of 90Sr-90Y activity in urine samples by using Cherenkov counting.

Cherenkov counting of the (90)Sr-(90)Y pure beta emitters in aqueous samples is an attractive method; but color quenching correction is needed, this being especially significant for urine which is characterized by a strong coloration. A quench correction method based on the external source of some liquid scintillation systems (named ESAR-External Source Area Ratio) was proposed for aqueous solutions. In the present work, the application of the ESAR method for determination of (90)Sr-(90)Y in human urine samples is described.

A comparative study of color quenching correction methods for Cerenkov counting.

The color quenching correction for Cerenkov counting is normally based on indexes defined for liquid scintillation spectra. Because of the differences in the spectrum shapes, these indexes are not optimal, and a different method was proposed based on the integral counting of the external (152)Eu source spectra of a 1200 Quantulus system. We have shown that this method, named ESAR (external source area ratio), is superior to all the other correction methods, offering the highest sensitivity and the widest range of application.

Color quench correction for low level Cherenkov counting.

The Cherenkov counting efficiency varies strongly with color quenching, thus correction curves must be used to obtain correct results. The external (152)Eu source of a Quantulus 1220 liquid scintillation counting (LSC) system was used to obtain a quench indicative parameter based on spectra area ratio. A color quench correction curve for aqueous samples containing (90)Sr/(90)Y was prepared. The main advantage of this method over the common spectra indicators is its usefulness also for low level Cherenkov counting.

Determination of radioiodine activity in charcoal cassettes.

Sampling of radioiodine in air is accomplished by passing an air sample through an activated charcoal cassette. A mathematical model was developed, which assumes that radioiodine distribution along the cassette axis can be expressed by an exponential function. The model was validated experimentally for cassettes used for air sampling of radioiodine production boxes. There is a good agreement between the measurements and model predictions. Furthermore, when breakthrough occurs, the model can be used to estimate the activity that passed through the cassette unabsorbed.

Improved localization of a 'hot spot' in the lungs for an array of four HPGe detectors--the isotopic ratio effect.

Considerable errors in the determination of radioactivity in lungs are caused by the usual assumption of homogeneous distribution, while actually the radioactive contamination can be distributed, and even in the form of a 'hot spot'. Modern lung counter systems use several HPGe detectors, and the ratio of the different count rates of the detectors can be used to locate a 'hot spot' and apply correction algorithms for activity calculation. In the present work it is shown that the variability of the isotopic composition of the uranium source has a major effect on the process of localization of the 'hot spot' in the lungs. A way to eliminate this effect is proposed by separate normalization of the count rates of each energy peak.

Locating a "hot spot" in the lungs when using an array of four HPGe detectors.

Considerable errors in the activity determination in lungs can be induced for the case of a "hot spot". Modern lung counter systems use several HPGe detectors, and the count rate ratios of the detectors can be used to locate the "hot spot" and apply correction algorithms. Some criteria for location determination of a point source in the lungs were investigated, and it is shown that an average error of up to about 10% can be achieved.