Investigation of the Air Kerma Response of Spherical Ionization Chambers for Unfolded Pulse Height Distributions of 60Co and 137Cs using the EGS4 Monte Carlo Code

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  • CHUN Kook Jin
    Health Metrology Group, Division of Metrology for Quality Life, Korea Research Institute of Standards and Science
  • YOO Gwang Ho
    Division of General Education, Daebul University

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Data required for the determination of the absolute air kerma rate for 60Co and 137Cs gamma-rays using spherical cavity chambers were calculated using the EGS4 Monte Carlo system. Mass energy-absorption coefficient ratio and the stopping power ratio were calculated for a 10 cm3 primary standard graphite-walled ionization chamber from the unfolded energy pulse height distributions of 60Co and 137Cs sources. Wall correction factors and non-uniformity correction factors for two graphite and one air equivalent plastic walled ionization chambers were also calculated with EGS4 code. The wall correction factors were compared with those determined by an experimental extrapolation method.<br> To check the accuracy of the calculations the results were compared with those obtained from other primary standard laboratories such as NIST and NRCC. For a 10 cm3 graphite ionization chamber, the mass energy-absorption coefficient ratios were 0.99917 for 60Co and 1.0004 for 137Cs. The values differed by 0.02~0.05 % for 60Co and 0.11 % for 137Cs from those of two laboratories. The stopping power ratios were 0.99984 for 60Co and 1.0087 for 137Cs. Comparison with NIST values showed differences of 0.06 % for 60Co and 0.04 % for 137Cs.<br> The wall correction factors were obtained and they were different by 0.6~1.1 % for 60Co and 137Cs compared to the experimental linear extrapolation method. These values were compared with Monte Carlo derived values from other laboratories.<br> The non-uniformity correction factors were also calculated and they differed from unity, the traditional value used in most standard national metrology laboratories.<br>

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