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Monte Carlo study of correction factors for Spencer–Attix cavity theory at photon energies at or above 100 keV
Author(s) -
Borg Jette,
Kawrakow Iwan,
Rogers D. W. O.,
Seuntjens Jan P.
Publication year - 2000
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 - 10.1118/1.1287054
Subject(s) - monte carlo method , photon , physics , atomic physics , electron , ion , photon energy , monte carlo method for photon transport , computational physics , nuclear physics , optics , quantum mechanics , dynamic monte carlo method , statistics , mathematics , direct simulation monte carlo
To develop a primary standard for192 Ir sources, the basic science on which this standard is based, i.e., Spencer–Attix cavity theory, must be established. In the present study Monte Carlo techniques are used to investigate the accuracy of this cavity theory for photons in the energy range from 20 to 1300 keV, since it is usually not applied at energies below that of137 Cs . Ma and Nahum [Phys. Med. Biol. 36 , 413–428 (1991)] found that in low‐energy photon beams the contribution from electrons caused by photons interacting in the cavity is substantial. For the average energy of the192 Ir spectrum they found a departure from Bragg–Gray conditions of up to 3% caused by photon interactions in the cavity. When Monte Carlo is used to calculate the response of a graphite ion chamber to an encapsulated192 Ir source it is found that it differs by less than 0.3% from the value predicted by Spencer–Attix cavity theory. Based on these Monte Carlo calculations, for cavities in graphite it is concluded that the Spencer–Attix cavity theory with Δ = 10 keV is applicable within 0.5% for photon energies at 300 keV or above despite the breakdown of the assumption that there is no interaction of photons within the cavity. This means that it is possible to use a graphite ion chamber and Spencer–Attix cavity theory to calibrate an192 Ir source. It is also found that the use of Δ related to the mean chord length instead of Δ = 10 keV improves the agreement with Spencer–Attix cavity theory at60 Co from 0.2% to within 0.1% of unity. This is at the level of accuracy of which the Monte Carlo code EGSnrc calculates ion chamber responses. In addition, it is shown that the effects of other materials, e.g., insulators and holders, have a substantial effect on the ion chamber response and should be included in the correction factors for a primary standard of air kerma.