Rapid measurement of 131I in the thyroid gland using a portable Ge system

Rapid yet accurate measurement of the 131I activity in the thyroid gland as well as in the air, water and vegetation may have an important role in obtaining quantitative information on internal doses for the people living in the vicinity of nuclear facilities shortly after an accidental release of radionuclides. Whole body counting technique is still the standard method for measuring radionuclides in the body while necessity for in situ measurement techniques has considerably increased especially after the Three Mile Island and Chernobyl accidents. For measurement of 131I in the thyroid gland in emergency situations, NaI (Tl) detectors, as in a scintillation survey meter as in the simplest case, are most often used while measurement of urinary excretions for members of the public may also effective. The scintillation survey meter method, being easily implemented, may not have enough selectivity for radioiodine and even be liable to an elevated background radiation spectrum. This would possibly lead to higher detection limits and lower accuracy. A use of a laboratory Ge (Li) detector system in the thyroidal radioiodine measurement was suggested to overcome the problem. A real measurement with a similar instrument was reported for the residents in U.K. after the Chernobyl accident. A use of a scintillation spectro-survey meter with a NaI (Tl) probe with lead collimation to thyroidal radioiodine measurement was also reported to give satisfactorily accurate evaluation of the thyroidal 131I burden. In this paper, a movable Ge system was developed for the above purpose and preliminarily evaluated particularly for counting efficiency. It is consisted of a portable high-purity Ge detector and a battery-operated MCA. It employs a laboratory made thin Pb shield with a collimation window and an elevator for the detector platform. The elevator was designed to adjust the height of the thyroid radioiodine probe in relation to the height and position of the neck of a subject sitting in a chair. Gamma spectra were analyzed and stored by the portable MCA. The acquired spectral information, sent to a notebook computer on line, was processed for determination of the 131I activity. In the preliminary measurement, the system was checked for its counting efficiency for 131I radioactivity by using neck-thyroid phantoms for the adult and some different ages that were previously prepared. The absolute counting efficiency was observed to be satisfactory, i.e. 0.20 to 0.28%. However, it was suggested that certain modification in the shield part of the probe might improve counting efficiency. (author)