Investigation of the standard temperature- pressure correction factor at low x-ray energies

Transcription

1 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 LNHB Workshop, Paris, May 9-11, 2007 NRC-CNRC 1 NRC-CNRC

2 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

3 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 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.

5 Pressure vs. altitude 5/23

6 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

7 Thimble chamber calculations 7/23

8 Spherical chambers 8/23

9 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

10 Experiments to demonstrate the effect complete BEAMnrc model to give x-ray spectrum 10/23

11 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

12 measurement uncertainties 12/23

13 13/23 Farmer-like chambers: 60 kv Closed symbols: P TP corrected measured responses open symbols: calculated responses

14 14/23 Farmer-like chambers: 150 kv Closed symbols: P TP corrected measured responses open symbols: calculated responses

15 15/23 Exradin A2 & modified: 60 kv Closed symbols: P TP corrected measured responses open symbols: calculated responses

16 16/23 Exradin A2 and modified: 150 kv Closed symbols: P TP corrected measured responses open symbols: calculated responses

17 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 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 19/23 N K curves without impurities Impurities would have a big effect. Agreement => there are negligible impurities

20 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

21 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

22 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

23 Thank you for your attention 23/23