Assessment Of The Effectiveness Of Collimation Of Cs–137 Panoramic Beam On Tld Calibration Using A Constructed Lead Block Collimator And An ICRU Slab Phantom At SSDL In Ghana

The objective of calibrating TLD badges and radiation survey meters demand that accurate dose gets to the TLDs and radiation survey meters. A lead block mould and an ICRU slab phantom had been designed and constructed to collimate the panoramic Cs-137 source at the secondary standards dosimetry laboratory (SSDL) by experts at the National Nuclear Research Institute (NNRI) workshop of the Ghana Atomic Energy Commission according to required specifications. This is to concentrate the panoramic isotropic emission into narrow beam geometry to enhance the calibration of personnel dosimeters and radiation survey meters and further reduces scattered radiations due to backscatter and transmission through the biological shield. The designed block was tested by a series of dose measurement of 1mSv using thermoluminescent dosimeters (TLDs) placed on a standardized ICRU slab phantom to cater for backscatter conditions of the human body at specified distances of 1, 2 and 3 meters with and without the collimation. The transmission dose rates measurements through the biological shield were taken by a survey meter. The results show percentage distribution of an effective reduction of transmitted dose rate to the laboratories and offices. However, the lead door recorded very high transmitted dose rates. It was finally concluded that further collimation will be necessary to completely eliminate scatter radiations introduced by the panoramic bench and transmission through the lead door.