Investigation of the standard temperaturepressure correction factor at low x-ray energies D. J. La Russa, M. R. McEwen and D. W. O. Rogers Carleton Laboratory for Radiotherapy Physics. Physics Dept, Carleton University Ottawa and Ionizing Radiation Standards, NRC http://www.physics.carleton.ca/~drogers LNHB Workshop, Paris, May 9-11, 2007 NRC-CNRC 1 NRC-CNRC
Acknowledgements Dan La Russa: the doctoral student doing the work Malcolm McEwen: the NRC cellist supervising the measurements Thanks to Hong Shen of NRC for technical assistance and Elsayed Ali for help with calculations of x-ray spectra Support from NSERC, the Canada Research Chairs program, CFI/OIT and the WestGrid computing facility. 2/23
P TP : the pressure-temperature correction for ion chambers 3/23 PTP is constructed so t ion chamber e- independent of ρ So E dep (ρ) is proportional to the density ρ. E dep (ρ ο ) is independent of the density ρ.
4/23 P TP : (cont) e- What happens if the electron does not cross the cavity? ion chamber independent of ρ E dep (ρ) is no longer proportional to the density ρ. Hence the standard P TP correction factor may no longer work.
Pressure vs. altitude 5/23
A4 EGSnrc Monte Carlo code cross-sections for DRY air of different densities Calculate D cav (dose to air) NE2571 A12 NRC x- ray monitor Standard P TP correction inherent in results take (W/e) air =constant D cav M (ρ) x P TP PTB catalogued spectra 6/23
Thimble chamber calculations 7/23
Spherical chambers 8/23
Conclusions of paper I there is a significant breakdown of the standard P TP correction for low energy photon beams basic cause: e- stopping in the cavity, not crossing magnitude of the effect depends on: mismatch of wall to air cross sections fraction of dose due to photon interactions in the cavity air a similar effect was reported in 2005 by the UW ADCL for well ion chambers for I-125 Burns and Pritchard reported measurements of this breakdown for NE2561 in 1977 in an NPL report (but spoke of non-air equivalence of the chamber). 9/23
Experiments to demonstrate the effect complete BEAMnrc model to give x-ray spectrum 10/23
A variety of chambers studied A2 C552 aluminium A12 C552 NE2571 NE2505 A19 graphite dural, C552 Kawrakow s egs_view Calculations with cavity.cpp, using Kawrakow s C++ geometry package and interface to EGSnrc 11/23
measurement uncertainties 12/23
13/23 Farmer-like chambers: 60 kv Closed symbols: P TP corrected measured responses open symbols: calculated responses
14/23 Farmer-like chambers: 150 kv Closed symbols: P TP corrected measured responses open symbols: calculated responses
15/23 Exradin A2 & modified: 60 kv Closed symbols: P TP corrected measured responses open symbols: calculated responses
16/23 Exradin A2 and modified: 150 kv Closed symbols: P TP corrected measured responses open symbols: calculated responses
Exradin A12: effect of impurities Measured for C552 walled A12 was not flat as expected from calculations: C552 has impurities: 600 ppm matches N K curve 1.1% impurities in graphite had no effect on NE2505 response vs pressure 17/23
18/23 Exradin A12: quantifying impurities Thinning the wall 15% makes 22keV beam agree within 1.2%. Had no effect on the response vs pressure
19/23 N K curves without impurities Impurities would have a big effect. Agreement => there are negligible impurities
Effects of geometry details CAVRZnrc uses a cylindrical model cavity.cpp includes the conical end. These geometry differences have no effect in a Co-60 beam 20/23
Conclusions measurements confirm the calculated breakdown of the P TP correction factor for low-energy x-ray beams EGSnrc is capable of reproducing air-kerma calibration coefficients well within 1% impurities are important at low photon energies N K vs beam quality curves allow quantification of the size of impurity effects geometry details appear to have some effects at these low energies although not at Co-60 energies 21/23
A challenge Can these effects be seen in results of any key comparisons? Look at the ratios of responses for chambers with air-equivalent vs graphite walls. 22/23
Thank you for your attention 23/23