Effective atomic numbers for photon energy absorption of essential amino acids in the energy range 1 keV to 20 MeV

Effective atomic numbers for photon energy-absorption (ZPEAeff) of essential amino acids histidine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan and valine have been calculated by a direct method in the energy region of 1 keV to 20 MeV. The ZPEAeff values have been found to change with energy and composition of the amino acids. The variations of mass energy-absorption coefficient, effective atomic number for photon interaction (ZPIeff) and ZPEAeff with energy are shown graphically. Significant differences exist between ZPIeff and the ZPEAeff in the energy region of 8–100 keV for histidine and threonine; 6–100 keV for leucine, lysine, tryptophan, phenylalanine and valine; 15–400 keV for methionine. The effect of absorption edge on effective atomic numbers and the possibility of defining two set values of these parameters at the K-absorption edge of high-Z element present in the amino acids are discussed. The reasons for using ZPEAeff rather than the commonly used ZPIeff in medical radiation dosimetry for the calculation of absorbed dose in radiation therapy are also discussed.