Monte-Carlo simulation studies have been undertaken to study the transport of uranium L X-rays and the 241Am 59.5-keV gamma photon in a heterogeneous mathematical phantom. The phantom transmission function is found to be expressible in the form f =alfa1 ealfa2(micros xs+microL xL) for transport through soft tissue and lung, respectively. For Pu counting in the front position, alfa1 = 0.21 ± 0.03 and alfa2 = 1.42 ± 0.09 while for Am counting, alfa1 = 0.48 ± 0.06 and alfa2 = 2.36 ± 0.16. For rear counting of Pu, a, = 0.24 ± 0.03 and alfa2 = 1.55 ± 0.06, while for rear counting of Am, alfa1= 0.40 ± 0.05 and alfa2 = 2.10 ± 0.16. The Monte-Carlo method predicts an effective soft tissue thickness (ESTT) of 5.2±0.9cm for the mathematical phantom, in excellent agreement with an average ESTT of 5.0 ± 1.0 cm calculated from previous 51Cr-103Pd experiments using human volunteers. These results are also utilized to predict phoswich calibration factors and to predict the L X-ray line shape for the phoswich detector.