COMPARISON OF GAMMA–RAYS ATTENUATION MEASURED BY DIRECT AND COINCIDENCE SPECTRA

Abstract

Measurements of gamma –rays attenuation in lead and iron sheets were carried out using direct spectrum and gamma- gamma fast slow coincidence spectrometer composed of two 3"×3"NaI(Tl) detectors and  Na-22 source. Measurements of mass attenuation coefficient at 511keV were carried out and its values (in cm²/g units) for lead from direct spectrum for the cases of without and with using a collimator (1cm diameter and 5 cm length) were 0.12029 and 0.13591and  from coincidence  spectrum were 0.10721 and 0.10602 respectively. For iron sheets the mass attenuation coefficient values from direct spectrum for the cases of without and with using a collimator were 0.0780 and 0.0766 andfrom coincidence spectrum were 0.0593 and 0.0616 respectively. The present results registered by direct measurement are little lower than the standard values due to making only photoelectric absorption needed  for comparison with coincidence results where the latter showed lower values.

The build-up factor were also measured as function of thickness using lead and iron sheets and found fluctuating  around unity for lead and iron coincidence  spectra and iron direct spectrum, whereas its  increases gradually for lead direct spectrum for the measured thickness range. The coincidence spectrometer greatly reduces scatteringevents in the measured material, and thus its results somewhat resemble that of direct measurements with using the collimator.