the energy dependence of the photoelectric attenuation coefficient of substances

introduction: 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 (zeff x /e y ).  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. methodology: 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. results: 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’. conclusions: 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.