Fragmentation and lateral scattering of 120 and 200 MeV/u 4He ions on water targets

Along with an increased popularity of heavy ions in cancer therapy, 4He ions have regained the interest of the medical community as a compromise between protons and 12C ions. Al- though 2054 patients have been treated with 4He beams at Lawrence Berkeley Laboratory (LBL) (Berkeley CA, US) between 1975 and 1992, a comprehensive database of biological and physics measurements in the therapeutic energy range is still missing. One of the first steps necessary for introducing 4He ions in particle therapy, is the develop- ment of a dedicated treatment planning system, for which basic physics information such as the characterization of the beam lateral scattering and fragmentation cross sections describing the loss of primary particles and the build up of secondary fragments are required. Examination of data found in the literature reveals a gap in the therapeutic energy range. These measurements are essential for benchmarking not only the new model developed for the in-house treatment planning code TRiP98 (Treatment Planning for Particles) [1], but also for already existing beam algorithms [2, 3] and for Monte Carlo codes like Geant4 [4] and Fluka [5]. The aim of this work is to provide fragmentation cross sections of 4He ions in the therapeutic energy range. The experimental data presented here were measured at Heidelberg Ion Beam Therapy Cen- ter (HIT) (Heidelberg, Germany) using 120 MeV/u and 200 MeV/u 4He beams. The attenuation of 200 MeV/u 4He beam in water was studied together with the build up of the secondary frag- ments produced by nuclear fragmentation processes. Target thicknesses between 1 and 25 cm H2O were chosen to investigate nuclear fragmentation also beyond the maximum penetration depth of the 4He ions. The mixed radiation field produced by the interaction of 120 and 200 4He ions with wa- ter targets (4.28 and 13.96 cm thick, respectively) has also been investigated in this work by measuring double differential cross sections. A combination of energy deposition and Time of Flight (TOF) acquired with a ∆E-E telescope system provided yields and kinetic energy spectra of all particle species emitted between 0◦ and 23◦ with respect to the primary beam direction. Coupling the angular distributions and the kinetic energy spectra gave an estimate of the dose contribution from all particles types. A direct measurement of the beam dose profile was per- formed independently. For this purpose, a two dimensional (2D) Ionization Chamber (IC) array and radiographic films were used to get information not only on the core of the radial dose distribution but also on its halo. The two datasets have been compared and showed consistent results. As a good parametrization of the beam lateral dose profile is a crucial element in a treatment planning systems, a fit of the measured distribution was performed and compared to the simple Gaussian approach still used by some treatment planning systems. The gap of experimental data in the energy range between 100 and 300 MeV/u proves the significance of this work not only for therapeutic applications but also for any other applications where the benchmark of Monte Carlo codes in simulating 4He fragmentation is required.