[en] A computer program to calculate the response of a 20 cm dia phoswich (3mm thick NaI(Tl) primary detector) to a source of low-energy photons distributed in the lungs of a heterogeneous (MIRD) phantom, approximating ICRP Reference Man, has been developed. Monte Carlo techniques are employed to generate photons and trace their fates in the thorax of MIRD phantom. The acceptable points of photon interactions in skeletal, lung and ordinary tissue are determined by Coleman technique. The photon interactions considered are photoelectric and Compton. The calculations yield the exit photon energy spectrum which is smeared with experimentally determined Gaussian resolution function to convert into pulse-height spectrum observable with the detector. The computer program has provisions for incorporating the effects of iodine K x-ray escape as well as variable intrinsic efficiency of the detector. Computed calibration factors (cpm/μCi integrated over the full spectrum) are given for the phoswich located centrally over and in contact with the chest for several low-energy photon sources distributed uniformly or as points in the lungs of the phantom. The radionuclides considered are ²³⁸Pu, ²³⁹Pu, ²⁴¹Am, ²⁴⁴Cm, ²⁴⁶Cm, ²⁵⁰Cf and ¹⁰³Pd. Examples of generated exit photon and the corresponding pulse-height spectra are included. The spectral changes observed in these generated spectra, which are also discerned in experimental pulse-height spectra, are discussed in detail. Thus, photopeak energies of 18.4 and 55.5 KeV for Usub(L) x-rays and ²⁴¹Am gamma-rays respectively have been observed. It is shown that consideration of the total (i.e. both uncollided and those escaping after collision instead of the uncollided alone) flux of escaping photons improves the calibration factors by about 50% for ²³⁹Pu, 70% for ¹⁰³Pd and as much as 340% for ²⁴¹Am gamma-rays. In addition, calibration factors are calculated for point ²³⁹Pu sources located at different sites in the phantom lungs and data on the escape efficiencies are presented. The suitability of the MIRD phantom as a calibration device for low-energy photon in-vivo spectrometry is examined in the light of these results. (auth.)