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

摘要

Effective atomic numbers for photon energy-absorption (Z_(PEAeff)) of essential amino acids histidine, leucine, lysine, methionine, phen-ylalanine, threonine, tryptophan and valine have been calculated by a direct method in the energy region of 1 keV to 20 MeV. The Z_(PEAeff) 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 (Z_(PIeff)) and Z_(PEAeff) with energy are shown graphically. Significant differences exist between Z_(PIeff) and the Z_(PEAeff) 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 Z_(PEAeff) rather than the commonly used Z_(PIeff) in medical radiation dosimetry for the calculation of absorbed dose in radiation therapy are also discussed.