MEASUREMENT OF THE ATTENUATION COEFFICIENT FOR LIVERMORE THORACIC PHANTOM LUNGS FABRICATED USING CONTEMPORARY MATERIALS

The University of Cincinnati has reproduced the original formulation f or the Livermore Thoracic Phantom lungs using contemporary materials a nd has adopted the linear attenuation coefficient as the primary quali ty assurance parameter for evaluating the performance capabilities of these new lung phantoms. The Livermore Thoracic Phantom was originally fabricated in 1978 to intercalibrate detector systems used to measure plutonium and other low-energy, photon emitting radionuclides deposit ed in the respiratory tract. The linear attenuation coefficient is a c ritical performance indicator for these phantom lungs since the presen ce of any material with a high effective atomic number (where Z greate r-than-or-equal-to 20) will make a significant change in the photoelec tric cross section, the predominant mode of interaction for plutonium x rays. A set of test lungs was fabricated with KCl to introduce a kno wn quantity of K-40 in the phantom and to determine, by measurement an d calculations, what change would be made to the attenuation coefficie nt at photon energies below 100 keV as a result of the modified formul ation. The KCl increased the linear attenuation coefficient below 60 k eV by more than a factor of two, which would produce a substantial sys tematic error in any subsequent calibration measurements performed wit h these modified phantom lungs. These results support use of the atten uation coefficient as an important performance indicator for the Liver more Thoracic Phantom lungs and also suggest that KCl not be added to the lung tissue substitute formulation as a means to incorporate K-40 in the phantom for low energy calibrations.