D T Burns and C Kessler 2009 Phys. Med. Biol. 54 2737 doi:10.1088/0031-9155/54/9/009
D T Burns and C Kessler
Show affiliationsAt present, only a correction factor for photon transmission, kl, is systematically applied for the entrance diaphragm of free-air chamber standards for air kerma. In the present work, the Monte Carlo code PENELOPE is used to re-evaluate kl for the BIPM standards and new correction factors are calculated for photon scatter and for fluorescence production in the diaphragm. An additional effect arising from electrons emitted from the diaphragm is shown to be significant at the highest photon energies. The results for the radiation qualities used for international comparisons give a combined diaphragm correction factor kdia = 0.9980(3) for the BIPM medium-energy standard at 250 kV. This is significantly different from the factor kl = 0.9996(1) in use at present and it might be concluded that differences are likely to exist for all free-air chamber standards. The effect of using a conical taper at the downstream edge of the diaphragm is shown to be negligible for these radiation qualities.
Issue 9 (7 May 2009)
Received 22 December 2008, in final form 9 March 2009
Published 8 April 2009
A Corrigendum for this article has been published in 2009 Phys. Med. Biol. 54 4343
D T Burns and C Kessler 2009 Phys. Med. Biol. 54 2737
Licia Verde and Sabino Matarrese 2009 ApJ 706 L91
P W Martin 2003 Metrologia 40
Janice C. Lee et al. 2009 ApJ 706 599
Stijn Wuyts et al. 2009 ApJ 706 885
Sigmund Heller and Walter T Strunz 2009 J. Phys. B: At. Mol. Opt. Phys. 42 081001
T Moiseev et al 2009 J. Phys. D: Appl. Phys. 42 072003
Syud A Ahmed et al 2009 Environ. Res. Lett. 4 034004
Anh-Thu Le et al 2008 J. Phys. B: At. Mol. Opt. Phys. 41 081002
G I Fann et al 2005 J. Phys.: Conf. Ser. 16 461