Results supporting calculated wall correction factors for cavity chambers

Abstract

Thick walled cavity ionization chambers are used by primary standard laboratories as primary air kerma standards in 137Cs and 60Co gamma-rays. Application of the cavity theory requires correction for the effects of photon attenuation and scattering in the chamber walls. For more than a decade there have been intensive discussions about the validity of wall correction factors determined by more traditional extrapolation methods versus those calculated by Monte Carlo methods. For existing primary standards the alternative methods lead to results that differ by up to 50% of the correction itself. This report presents both experimental and theoretical results which strongly support the validity of calculated wall correction factors. Moreover, it is demonstrated that, in selected cases, the application of a linear extrapolation method leads to errors in the determination of the air kerma reaching up to 13%.

A radiation quality correction factor kQ for well‐type ionization chambers for the measurement of the reference air kerma rate of 60Co HDR brachytherapy sources

“…33,34 The HRK-3 is one of PTB's primary standards for air kerma free in air for 60 Co and 192 Ir gamma radiation 35,36 based on the Bragg-Gray cavity theory.…”

“…The calibration coefficient N LS01 Co-60 for 60 Co is derived by traceable calibration of the LS01 secondary standard chamber in a 60 Co reference field to a custom-built graphite walled cavity ionization chamber referred to as HRK-3. 33,34 The HRK-3 is one of PTB's primary standards for air kerma free in air for 60 Co and 192 Ir gamma radiation 35,36 based on the Bragg-Gray cavity theory.…”

Monte Carlo calculated correction factors for primary standards of air kerma

“…14 -16 There is one particularly convincing experiment from the PTB which demonstrates that the linear extrapolation technique for determining K wall is inappropriate. 17 We have undertaken a sensitivity study to investigate the uncertainties associated with these calculated values, and the sensitivity of the calculated values to such things as the Monte Carlo code used, the 60 Co spectrum used, the diameter of the 60 Co source, and the distance from the source. As well as investigating K wall , we have undertaken to investigate the sensitivity of the calculated values of the other factors in Eq.…”

On the accuracy of techniques for obtaining the calibration coefficient NK of Ir192 HDR brachytherapy sources

“…There has been concern about the proper way to determine the values of A wall . 14 This issue has recently been unequivocally resolved in favor of using the Monte Carlo calculated values [19][20][21] over the linear extrapolation technique used by Goetsch et al This has led to a change of about 1% in the NIST air-kerma standards for 137 Cs and 60 Co. 32 Table I presents values of A wall . The first observation is that the values "measured" using linear extrapolation for the graphite-walled thimble chamber are in good agreement with the Monte Carlo calculated values.…”

“…For this reason, we have resorted to the Monte Carlo method, which has been shown to estimate accurately both the dose inside the cavity of an ion chamber and A wall , the correction factor for attenuation and scatter in the chamber's wall. 3, [14][15][16][17][18][19][20][21] In particular, Seuntjens et al 17 demonstrated that EGSnrc calculates ion chamber response with an accuracy of 0.1% relative to its own cross sections down to a mean photon energy of about 100 keV which makes it an ideal tool for this work.…”