On-line characterization of heavy-ion beams with semiconductor detectors

Heavy-ion beams used in biomedical studies suffer a substantial amount of nuclear reactions (fragmentation) as they traverse matter. Since it has been demonstrated that dose and linear energy transfer (LET) are not a sufficient description of a beam for the purpose of understanding its biological effects, it is necessary to be able to separate the components of a complex beam so that their individual effects can be analyzed. A simple and small assembly consisting of a thin silicon LET detector, in time coincidence with a thick germanium residual energy detector has been used in measurements of the components of Ne-20 and Si-28 high-energy ion beams. The detector system can be placed at any experimental area without difficulty and it can carry out a beam analysis in a few minutes, making it very appropriate for fast on-line measurements and verification of beam characteristics. LET values measured by the silicon detector agree well with results of the Bethe stopping-power calculations, and the dose measured for the beam components can be used to obtain Bragg curves that are in good agreement with those obtained by ionization chamber measurements on the same beams. The numbers and LET distribution of primaries and fragments at different positions of the Bragg curves, as well as fractional dose contributed by the different components are determined directly from the experimental data. Particle velocity distributions can be obtained for the higher Z fragments. Limitations and advantages of the simple measurement technique are discussed.