Radial ionisation distribution of heavy ions: Experimental results with OPAC and comparison with TRAX

The difference in the relative biological efficiency (RBE) of ions of same linear energy transfer (LET) but different atomic number (Z) can be attributed to the difference in the radial ionisation distribution. We have performed systematic measurements of the spatial ionisation pattern of heavy ions of various Z but similar LET at GSI. The results are compared with track structure data obtained by the Monte Carlo code, TRAX [1]. The measurements were performed with a time projection chamber with optical readout (Optical Avalanche Chamber, OPAC) [2] which is able to quantitatively capture the spatial ionisation pattern of an ion traversing the chamber [3]. In the previous reports we have described the first results of measurements with C ions [3] and the modifications implemented in the TRAX code, in order to account for the specific properties of the OPAC interaction chamber [2]. In the meantime the optical system of OPAC was further improved and we are now able to obtain well focussed images in the whole field of view. Also a reduction of the stray light background by a factor of 13 as compared to the earlier measurements was obtained