Premium
The dependence of mass energy absorption coefficient ratios on beam size and depth in a phantom
Author(s) -
Cunningham J. R.,
Woo M.,
Rogers D. W. O.,
Bielajew A. F.
Publication year - 1986
Publication title -
medical physics
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.473
H-Index - 180
eISSN - 2473-4209
pISSN - 0094-2405
DOI - 10.1118/1.595946
Subject(s) - imaging phantom , dosimetry , bremsstrahlung , monte carlo method , absorption (acoustics) , physics , absorbed dose , spectral line , beam (structure) , materials science , attenuation coefficient , radiation , range (aeronautics) , photon , atomic physics , computational physics , optics , nuclear medicine , mathematics , medicine , statistics , astronomy , composite material
The Monte Carlo computer code “electron gamma shower” (EGS) has been used to determine photon spectra in a water phantom. Spectra used by Johns and Cunningham and for the AAPM dosimetry protocol have been used as input data and ratios of average mass energy absorption coefficients have been calculated for a number of depths and field sizes. The results show that there is a slight dependence on both of these parameters. For example, (μ̄ en/ρ) graphite waterfor cobalt‐60 varies from a value of 1.111 for the primary spectrum in air, to 1.135 at a depth of 20 cm in a phantom for a beam approximately 1 m 2 in area. This variation of over 2% is relevant for dosimetry. The variation is less than this for high‐energy radiation beams and in most cases can be ignored. The effect is greater for high atomic materials such as bone, where the range of variation of (μ̄ en/ρ) water bone , again for cobalt radiation, may be as great as 15%. This too is less for high‐energy bremsstrahlung spectra.