Characteristics of the Scintillators for a Whole Body Counter

Characteristics of sodium iodide crystal and plastic scintillators are investigated for use in whole-body counters. The backgrounds of a 5 in. diam by 4 in. NaI (T1) crystal are 28809 cpm for an energy band from 60 to 1640 kev when the crystal is unshielded and 363 cpm when the crystal is shielded by 20 cm of steel and 3 cm of lead. The measured total efficiency is found to be consistent with the calculated total efficiency within experimental error. For the photo- peak efficiency the measured figure is much larger than the calculated figure, which is considered to be due to total energy absorption by multiple Compton scattering. The peak to total ratio of the crystal is measured for a source at different depths in a water phantom, and from this experiment the depth of radioisotopes in the human body may be estimated within an error of a few cm. The minimum detectable amount of radioisotopes is around 1 m mu c. The results with plastic scintillators are as follows: The counting efficiency becomes larger as the size of the scintillator increases. The pulse height becomes smaller as the size increases. The background counting rate is nearly proportional to the surface area. Based on the results, 115 mm diam by 100 ram plastic scintillators are used for whole-body counting and locating radioisotopes in the human body. The measured countirg efficiency of the plastic scintillators is consistent with the calculated figure. (N.W.R.)