First results of carbon ion track structure imaging in low pressure gas

The relative biological effectiveness (RBE) is an important parameter in heavy ion treatment planning. It is well known that the RBE does not only depend on the LET of a particle, but also on the ion sort.. This can be explained by the difference in the local ionisation distribution for different kind of ions, expressed by LET and radial ionisation distribution. Our goal is to measure these parameters with high spatial resolution and compare the results with track structure data obtained by Monte Carlo simulations of Krämer [1]. The experimental method is based on a time projection chamber with optical readout (Optical Avalanche Chamber, OPAC). OPAC is able to measure the spatial ionisation pattern of a particle crossing the interaction volume. The chamber is operated with triethylamine (TEA) vapour at a pressure of typically 10 hPa. A description of the experimental setup can be found in [2,3]. During the last year the detection system was modified for the experiments with heavy ions at the GSIfacilities. The data acquisition system had to be upgraded and the intensity dynamic of the recorded image was improved to 11 bit (instead of previously 8 bit). A new gating system of the image intensifier allows for selective capture of images from small slices of the 3-dimensional ionisation track in the interaction volume. In summer 2002 measurements were performed at SIS at GSI with C-ions. Starting with an initial ion energy of 400 MeV/u tracks of carbon ions traversing the chamber parallel to the projection plane were recorded. By means of an adjustable water column the beam energy was stepwise reduced to 300, 200, 100, 50, 30 MeV/u and finally close to Bragg peak energies. More than 1000 tracks for each energy were recorded and stored for later offline analysis. This large number was required to obtain sufficient statistical information on the ionisation density, especially far outside the track core, where only high energy delta electrons deposit energy. For example Fig.1 shows a track of a 400 MeV/u C ion (a) and one with energy close to the Bragg maximum of 0,7 MeV/u (b).