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Radiation Protection Dosimetry Advance Access originally published online on April 8, 2008
Radiation Protection Dosimetry 2008 130(4):410-418; doi:10.1093/rpd/ncn093
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© The Author 2008. Published by Oxford University Press. All rights reserved. For Permissions, please email: journals.permissions@oxfordjournals.org

Corrections to air kerma and exposure measured with free air ionisation chambers for charge of photoelectrons, Compton electrons and Auger electrons

N. Takata1,* and A. Begum2

1 Ionizing Radiation Section, NMIJ, AIST, Tsukuba, Ibaraki 305-8568, Japan
2 Department of Physics, BUET, Dhaka-1000, Bangladesh

* Corresponding author: n.takata{at}aist.go.jp

Received November 26, 2007, amended February 26, 2008, accepted February 28, 2008

The signal charge from a free air ionisation chamber for the measurement of air kerma and exposure consists of not only the charge of ion pairs produced by secondary electrons (i.e. photoelectrons, Compton electrons and Auger electrons), but also the charge of the secondary electrons and single and multiple charged ions formed by the release of the secondary electrons. In the present work, correction factors for air kerma and exposure for the charge of the secondary electrons and ions were calculated for photons with energies in the range from 1 to 400 keV. The effects of an increase in the W value of air for low-energy electrons were also taken into consideration. It was found that the correction factors for air kerma and exposure have a maximum value near a photon energy of 30 keV; in the lower energy region, the correction factor for exposure monotonically decreases with a decrease in photon energy except for a small dip due to K-edge absorption by argon atoms in air. The values of the correction factors were found to be 0.9951 and 0.9892, respectively, for a spectrum with a mean energy of 7.5 keV, the reference X-ray spectrum with the lowest mean energy in ISO 4037-1. The air kerma correction is smaller than that for exposure, because for air kerma the signal due to the charge of secondary electrons and ions is partly compensated by the decrease in the number of ion pairs produced by the secondary electrons due to the increase of the W value of air for lower energy electrons.


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