Standardization of cesium-137 gamma-ray sources in terms of exposure units (Roentgens)

Abstract

3'CS y~ray so urces, whi c h have bee n proposed by th e Pub li c Health Se rvi ce as a s ubs titute for radium in th e treatm e nt of ma lignant di seases, req uire s uitab le calibrati on if co ns is te ncy is to be ac hi e ve d in radiation treatm e nt.Sever al '"'Cs so urces we re ca libra te d usin g a graphite cavi t y ionizat ion cha mb e r in an ope n·ai r geo me try. Th e co ntributi on of scatter from th e room s urfaces was calculated and the so urce meas ureme nts correc ted [or thi s e ffe c t.A large-volum e aluminum-wa lled ioni za tion c hambe r was des igned a nd fabricated [or use in routin e ca libra tions. This c hamber will be used as a mea ns [or in te rco mpariso n of the so urce to be calibrated with one o[ th e sta nd ards.

Calibration of gamma-ray-emitting brachytherapy sources

“…The working standard sources were measured in 1967 [7] using a graphite cavity ionization chamber now identified as chamber 1 [9].…”

“…For detailed information about the graphite ionization chamber measurements of the three working standards, reference can be made to [7], but since the "500" source is used as the reference standard, only that source is used in the following review to show the effect of updating the standards.…”

“…The correction factors for the number 1 chamber, identified in [7] as VIII, are given in table 3-6. The ratio of the product of the correction factors in the 1974 column to that in the 1967 column is 0.9935, which is the factor used to bring the 1967 brachytherapy source calibrations into agreement with the 1974 exposure standard.…”

“…Correction factors 1967 [7] 1974 [9] Collision mass stopping power ratio (C to air) [7] (identified as VIII) is 1.130 cm3.…”

“…A test of the consistency of the standard chamber measurements for these two sources using the replacement method showed a difference between the ratios of the open-air measurements for ICN 100 and ICN 500, and the replacement measurements, of about 0.1% (table [3][4][5][6][7][8][9][10][11][12][13].…”