Development of TOF-spectrometry of laser-accelerated proton pulses using silicon microdosimeters

Summary form only given. With the continuous advances in the development of high-intensity lasers, laser-driven ion acceleration may become a compact alternative to produce ions with energies suitable for medical applications [1]. However, the unique properties of laser-accelerated ions, such as the mixed radiation field, a broad energy spectrum of up to 100% energy spread and ion fluxes exceeding 10
⁷
protons/cm
²
/ns are a challenge for electronic detectors. Due to relatively large shot-to-shot fluctuations in the energy spectrum, a non-destructive characterization of each ion bunch is necessary for reliable biomedical applications. Our approach to obtain the spectra exploits the time-of-flight (TOF) difference of ions with different kinetic energies, which requires fast and sensitive detectors. We used a novel silicon microdosimeter (“bridge” microdosimeter), developed at the Centre for Medical Radiation Physics at the University of Wollongong for our experiments. The detector consists of an array of 3D sensitive volumes (30 x 30 μm2), connected in parallel via thin silicon bridges [2]. The sensitive thickness of each pixel is 10 μm. In order to test the setup under reproducible conditions, first TOF-measurements were performed at the Maier-Leibnitz-Laboratorium Tandem van-de-Graaf accelerator. By using a chopper and buncher system, 20 MeV proton pulses with a bunch duration of less than 2 ns and a high flux could be obtained. Two dedicated energy-degrader filters were designed to obtain poly-energetic spectra from the mono-energetic proton beam. Moreover, we measured the TOF-spectrum of laser-accelerated ion bunches, where a 35 fs laser pulse of the ATLAS-300, a 300 TW Ti:Sa laser system, was focussed onto thin plastic and gold foils. In both cases, the TOF-signal was acquired using a 4 GHz oscilloscope. The proton energy spectrum was deconvolved from the measured TOF-signal using an iterative deconvolution algorithm.