Spectrometric measurements of 131I and 99mTc activity in thyroid

A modified method for calibration of the thyroid counter with 131I was investigated in order to take into account the depth of the thyroid in human neck. The depth of the 131I source was determined from the ratio of the counting rate in 364 keV peak to the counting rate in the energy band of 110–140 keV. Dependence of detector efficiency on the source depth was determined. The results were used for analysis of the energy spectra recorded for hospital patients. Similar method was investigated also for 99mTc radiation source.