Comparison of counting efficiencies of Shadow Shield Whole Body Monitor for Paediatric BOMAB and reference computational phantoms

The Shadow Shield Whole Body Monitor (SSWBM) is a partially shielded system used for the assessment of internal dose to occupational workers due to high energy photon (Eγ > 200 keV) emitters. The SSWBM installed at Health Physics Laboratory, BARC, Tarapur consists of a 12.7 cm diameter and 7.6 cm thick NaI(Tl) detector surrounded by 20.5 cm thick mild steel (MS) plates. This system has been calibrated using BOMAB and voxel phantoms, both having BARC reference dimensions. Recently, ICRP has published reference paediatric computational (voxel) phantoms for male and female representing various age groups of members of public. In the present study, 1-year (1Y) and 5-year (5Y) old voxel phantoms have been used to estimate the counting efficiency (CE) of SSWBM for ¹³⁷Cs and ⁶⁰Co and the results are compared with their BOMAB counterparts. The results show that, for any age group CEs for BOMAB phantom are greater than voxel phantom. It is because of the localized axial source distribution in BOMAB phantoms, which imposes a relatively greater solid angle at detector face. Also, CEs for ¹³⁷Cs are lower than ⁶⁰Co because both peaks (1.17 & 1.33 MeV) are considered for the latter