| Zugriffsnummer | 14775 |
| Dokumenttyp | Konferenzartikel |
| Sprache | Englisch |
| Titel | Comparison of calculated and measured air kerma response of graphite cavity chambers for 30 kV - 300 kV X-ray beams |
| Autor(in); Institution |
Büermann, Ludwig; 6.203, Luftkerma-Normalmesseinrichtungen und Dosimetrie in der diagnostischen Radiologie, PTB-Braunschweig
|
| Quelle/Jahr | Recent developments in accurate radiation dosimetry : international workshop:(2002), 53 - 68 |
| Schriftenreihe | Symposium proceedings / American Association of Physicists in Medicine: 13 |
| Herausgeber(in) |
Seuntjens, Jan P.; McGill University, Medical Physics Unit, Montréal, CANADA
|
| ISBN | 1-930524-12-9 |
| Verlag | Madison, Wis.: Medical Physics Publishing |
| Konferenzangaben | Recent developments in accurate radiation dosimetry, Montréal, 11-13, October, 2001, Canada |
| Freie Schlagworte | Cavity ionisation chamber ; Air kerma response ; Monte Carlo simulations ; EGSnrc |
| Zusammenfassung | The essential improvements which have recently been achieved with the EGSnrc Monte Carlo code system in the simulation of the ionisation chamber response has triggered the current investigation into the response of the PTB air kerma standard for 60Co and 137Cs-γ radiation in low- and medium-energy X-ray beams. This standard consists of a set of three graphite cavity ionisation chambers of different shapes and sizes with volumes varying between about 0.55 cm3 and 6.14 cm3. The chambers were calibrated in terms of air kerma at different X-ray beam qualities with tube potentials between 30 kV and 300 kV using the standard free-air chamber of the PTB. Using the measured X-ray spectra the air kerma response of the chambers was calculated with the EGSnrc code system and compared with the measurements. The relative uncertainty of the ratio of the measured and calculated response was estimated to be 0.5 %. The measured response of the chambers varied by about a factor of 2 over the energy range investigated, a result which was on the whole well reproduced by the calculations. Nevertheless some significant differences between the measured and calculated response remain which have a similar trend for all three chambers and X-ray qualities investigated at normal radiation incidence. The values of the ratios were systematically greater than 1, between 1.005 and 1.015, at mean photon energies between 250 keV and 80 keV. Below 80 keV, the ratios increased up to values of about 1.04 at 30 keV and then slightly decreased to about 1.02 at 17 keV, which was the lowest mean energy of the investigations. The discrepancies observed at the low photon energies depend on the special geometry and dimensions of the cavity. This was clearly reflected when the pancake type chamber was irradiated in its normal position with the photon beam incident on the flat side (0° incidence) of the chamber and rotated by 90°, i.e. the photon beam incident on the curved side of the chamber (90° incidence). The ratios of the measured and calculated responses were about 1.04 and 1.015 at 30 keV for 0° and 90° radiation incidence, respectively. The larger discrepancy of about 4% at 0° compared to 1.5% at 90° photon beam incidence was shown to be correlated with different fractions of dose contributions from photon interactions in the air cavity which were essentially higher for the 0° incidence. This result supports the assumption that the discrepancies are due to a failure of the Monte Carlo code to correctly calculate the fraction of cavity dose of the ionisation chamber caused by electrons liberated by photon interactions in the air of the cavity. At low photon energies the photoelectric interactions dominate. The discrepancies could probably come from an imperfect simulation of the angular distribution of the photoelectrons. |
