The Energy Dependence of the Photoelectric Attenuation Coefficient of Substances

The photoelectric attenuation coefficient of substances is known to depend upon the energy E of the photon and the effective atomic number of substances (Zeff) as (Zeffx/Ey). No definitive values about these indices x, y are given in the literature. The index x is said to lie between, 3.0 and 4.0, while for ‘y’ different values have been assigned, between 3.0 and 4.0. We followed a methodology to find both the exponent y explicitly, from a formula which does not contain x. Through this way, the risk of one parameter leading to an estimation error for the other is automatically eliminated. With the value of y being unmistakably established, we determined the exponent x for different elements. It was found from the NIST data that ‘y’ = 3.0669 for most substances with low atomic number but no single value can be assigned for the exponent ‘x’. These results help us to perform model calculations for the attenuation coefficients of different substances. They can also provide important inputs for the diagnostic purposes in the DECT method.