Dresden 2000 – scientific programme

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ST: Strahlenphysik und Strahlenwirkung

ST 4: Radiometrie

ST 4.6: Talk

Tuesday, March 21, 2000, 17:45–18:00, PHY C213

Measurement of the average energy required to produce an ion pair in air by 7.6 MeV/u C-12 ions — •Jose Rodriguez-Cossio¹, Caterina Brusasco¹, Dieter Schardt¹, Bernd Voss¹, Uli Weber¹, Frank Flesch², and Wolfgang Heinrich² — ¹Gesellschaft für Schwerionenforschung, Planckstr. 1, 64291 Darmstadt — ²Univ.-GH Siegen, Fachbereich Physik, 57068 Siegen

In radiation therapy an accurate measurement of the absorbed dose is essential to verify that the target volume receives the prescribed dose and that the dose distribution is in accordance with the treatment plan. As in the interactions of heavy ions with tissue the dose is deposited by ions of different energies and secondary fragments of different atomic numbers and energies, the dosimetry faces the problem of the dependence of the detector efficiency on the particle’s atomic number and energy. Using ionization chambers filled with air or nitrogen gas these dependences can be reduced to a few percent.

An important physical quantity,however, ist the W-value (the average energy required to produce an ion pair in a gas) which has to be known accurately in order to obtain the absorbed dose from the chamber output. While many measurements have been carried out at low energies for various ions and gases, only a few data are available in the energy range of interest for heavy-ion therapy. We present our experiments performed at the UNILAC facility at GSI, where a preliminary value of 34.1 ± 0.4 eV for 7.6 MeV/u C-12 ions in air was found.