| Zugriffsnummer | 11533 |
| Dokumenttyp | Konferenzartikel |
| Sprache | Englisch |
| Titel | Determination of absorbed dose to water for medium-energy x-rays, based on a graphite extrapolation chamber |
| Autor(in); Institution |
Schneider, Ulf; 6.22, Dosimetrie der Gammastrahlung, PTB-Braunschweig
|
| Quelle/Jahr | Kilovoltage x-ray beam dosimetry for radiotherapy and radiobiology:(1999), 113 - 122 |
| Schriftenreihe | American Association of Physicists in Medicine symposium proceedings: 11 |
| Herausgeber(in) |
Ma, Chang-Ming Charlie
|
| ISBN | 1-888340-16-9 |
| Verlag | Madison, Wis.: Medical Physics Publishing |
| Konferenzangaben | Workshop on Kilovoltage X-Ray Beam Dosimetry, Stanford, April, 1997, USA |
| Freie Schlagworte | absorbed dose to water ; medium-energy X-rays ; graphite extrapolation chamber |
| Zusammenfassung | More than twenty years ago, it was developped in the PTB a standard measuring equipment for the determination of absorbed dose to water in the region of medium-energy X-rays, based on a graphite extrapolation chamber within a graphite phantom. With this extrapolation chamber it was possible to determine absolutely the graphite kerma at a reference depth within the graphite phantom, and therefore also to calibrate ionization chambers (called transfer chambers) in terms of graphite kerma. At that time there was no possibility to determine the absorbed dose to water in a water phantom directly, since, for instance, the concept of water calorimetry was not yet developped. The main reason to start with the graphite kerma in a graphite phantom, instead of air kerma in free air, was the consideration that within a liquid or solid phantom material the distribution of the photons with respect to energy and direction is completely different from that in a free air calibration. This is a consequence of the considerable amount of photon scattering within a phantom, whereas in a free air calibration the photons are coming solely from one direction perpendicular to the ionization axis. Graphite was the only practicable material to determine a dose quantity absolutely by means of an air filled extrapolation chamber. This was shown in a model consideration, where the kerma components of the photoeffect and the Compton effect interactions are treated separately. The main reason, why this model works, is the fact that graphite is air equivalent to a high degree with respect to the Compton component for medium energy X-rays. The advantages and disadvantages of the method based on the graphite extrapolation chamber in a graphite phantom are discussed. |
