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Fast measurements of single Bragg peaks with an active multi-plane phantom

Hadron cancer-therapy [1] with high-energetic heavyions requires the precise knowledge of the energy of the primary beam using an method independent from the accelerator. This may be obtained by measuring the range of the impinging particles. Currently, this task is performed with a system consisting of a water column with remote controlled variable thickness flanked by two ionization chambers (IC). The energy-loss in the IC installed upstream of the setup serves to normalized the signal of the second IC to the beam intensity. Recording this quantity for various thicknesses of the water absorber allows to trace and determine the position of the Bragg-Peak, which coincides with the maximum reading of the second IC. Moreover, the shape of the curve is an additional characteristic feature to be checked. Checking the constancy of the associated material thickness is done during the preparative phase of every beam time. The precision of this setup is limited e.g. by the accuracy of the stepper motor used to steer the thickness of the water absorber. The sum of all contributions is in the order of 100 μm. The measurement of one point of the curve requires one extraction cycle of the SIS of 5 s, which sums up to a total of 10 to 15 minutes per BraggPeak measurement. Thus the total constancy check requires approximately 1.5 hours of beam time for a representative set of energies. It is our aim to minimize this load to only a few seconds per energy. For this purpose, a set of staged ICs has been built which is handy enough to be used during even daily checks.