Calibration of lung monitoring system using LLNL phantom and comparison with JAERI phantom for 241Am

Due to the low yield and low energies of the photons (E < 100 keV) emitted by actinides and severe attenuation of these low energy photons (LEPs) within body tissues, there is a need of highly sensitive detection systems for in vivo monitoring of actinides. At Bhabha Atomic Research Centre, lung/organ monitoring for actinides is carried out inside the totally shielded steel room whole body counter lined with graded Z lining using Phoswich (20 cm dia and 1.2 cm thick Nal(Tl) primary and 5 cm thick CsI(Tl) secondary detector) and in array of three HPGe detectors (each of 7 cm dia and 2.5 cm thick). At low energies, photon transmission is dependent on the tissue composition and therefore, realistic thorax phantom is required for calibration. Phoswich and HPGe detection systems were calibrated for lung activity estimation using realistic thorax JAERI(Japan Atomic Energy Research Institute) phantom as apart of lAEA inter-comparison exercise. Recently, LLNL (Lawrence Livermore National Laboratory) phantom has been procured under XIth plan. This paper gives the details of the calibration factors (CFs) estimated using LLNL phantom and their comparison with JAERI values for 241Am. On comparing with JAERI phantom measurement data for 241Am, deviation was found to be around 30% in CFs estimated using LLNL phantom at 60 keV. This is due to the difference in shape and size of the lungs in both the phantoms. JAERI phantom is based on the reference Asian man of height 168 cm and weight 63.5 kg whereas LLNL phantom is based on reference US worker of height 177 cm and 76 kg. Large deviation of around 90% is observed in low energy region of 17 keV, where transmission of photons is severely affected by tissue thickness and composition