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

D. J. La Russa, M. R. McEwenand D. W. O. RogersCarleton Laboratory for Radiotherapy Physics.

Physics Dept, Carleton University

Ottawa and

Ionizing Radiation Standards, NRC

http://www.physics.carleton.ca/~drogersLNHB Workshop, Paris, May 9-11, 2007

NRC-CNRCNRC-CNRC

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Acknowledgements

Dan La Russa: the doctoral student doing the work Malcolm McEwen: the NRC cellist

supervising the measurements

Support from NSERC, the Canada Research Chairs program, CFI/OIT and the WestGrid computing facility.

Thanks to Hong Shen of NRC for technical assistance and Elsayed Ali for help with calculations of x-ray spectra

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PTP: the pressure-temperature correction for ion chambers

te-

ion chamber

independent of ρ

PTP is constructed so

So Edep(ρ) is proportional to the density ρ.

Edep(ρο) is independent of the density ρ.

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PTP: (cont)

e-

ion chamber

independent of ρ

Edep(ρ) is no longer proportional to the density ρ.

Hence the standard PTP correction factor may no longer work.

What happens if the electron does not cross the cavity?

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Pressure vs. altitude

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NE2571 A12

“A4”

NRC x-ray monitor

• EGSnrc Monte Carlo code• cross-sections for DRY air

of different densities• Calculate Dcav (dose to air)

• Standard PTP correction inherent in results

• take (W/e)air =constant• Dcav ∝ M’(ρ) x PTP

• PTB catalogued spectra

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Thimble chamber calculations

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Spherical chambers

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Conclusions of paper I

• there is a significant breakdown of the standard PTPcorrection 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).

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Experiments to demonstrate the effect

complete BEAMnrc model to give x-ray spectrum

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A variety of chambers studied

Kawrakow’s egs_view

A12A2 NE2571

NE2505 A19

C552 aluminium

C552 graphite dural, C552

Calculations with cavity.cpp, using Kawrakow’s C++ geometry package and interface to EGSnrc

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measurement uncertainties

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Farmer-like chambers: 60 kV

Closed symbols:PTP corrected measured responses

open symbols:

calculated responses

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Farmer-like chambers: 150 kV

Closed symbols:PTP corrected measured responses

open symbols:

calculated responses

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Exradin A2 & modified: 60 kV

Closed symbols:PTP corrected measured responses

open symbols:

calculated responses

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Exradin A2 and modified: 150 kV

Closed symbols:PTP corrected measured responses

open symbols:

calculated responses

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Exradin A12: effect of impurities

Measured for C552 walled A12 was not flat as expected from calculations:

C552 has impurities: 600 ppmmatches NKcurve 1.1% impurities in graphite had no effect on

NE2505 response vs pressure

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Exradin A12: quantifying impurities

Thinning the wall 15%makes 22keV beam agree within 1.2%.

Had no effect on the response vs pressure

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NK curves without impurities

Impurities would have a big effect.

Agreement => there are negligibleimpurities

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Effects of geometry details

CAVRZnrcuses a cylindrical model

cavity.cppincludes the conical end.

These geometry differences have no effect in a Co-60 beam

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Conclusions

• measurements confirm the calculated breakdownof the PTP correction factor for low-energy x-ray beams

• EGSnrc is capable of reproducing air-kermacalibration coefficients well within 1% – impurities are important at low photon energies– NK 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

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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.

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Thank you for your attention