For: occupational workers in fuel fabrication and reprocessing plants handling actinides (Plutonium Uranium Isotopes and ²⁴¹Am), intake monitoring requires assessment of internal contamination of lung/skeleton/liver. Assessment of lung/organ burden of actinides is carried out inside the totally shielded steel room whole body counter using 1.2 em thick Phoswich detector. It is not unusual to come across cases of low level internal contamination due to ¹³⁷Cs in the workers of fuel reprocessmg plants. Presence of trace levels of this radionuclide in the body can give false indication of Pu/Am lung contammation, In this paper, response of the 1.2 em thick phoswich detector is studied by uniformly distributing ¹³⁷Cs standard sources m the BOMAB phantom to evaluate its contribution in low energy regions. The contribution factors thus denved will be helpful in evaluating the net lung burden due to actinides in radiation workers.