Time-of-flight spectrometry applied to 2 MeV He RBS

Abstract

A time-of-flight (ToF) spectrometer used for heavy ion backscattering spectrometry has been optimized for the detection of slow particles. In order to decrease energy straggling of the particles in the start detector the start signal is produced by the secondary electrons released from a diamond-like carbon (DLC) film of only 0.6 μg/cm 2. The start foil as well as the entrance to the stop detector have been set to ground potential to avoid the influence of the charge state of the particle on energy (`tandem effect'). Significant improvements of the energy resolution of the system have been obtained especially at low velocities where straggling and tandem effect are important contributions. The energy resolution we obtained for 4He, 16O and 28Si ions below 1 MeV agrees well with the calculated energy straggling in the carbon foil. Representative measured values (FWHM) are 1.8 keV for 4He at 250 keV, 7 keV for 16O at 600 keV and 9 keV for 28Si at 600 keV. Without final optimization, the detection efficiency of the start detector was between 50% and 65% in the energy range relevant to He RBS. For 4He the energy resolution of the ToF spectrometer equals the one of a state-of-the-art silicon detector at approximately 1.5 MeV and is substantially better at lower energies. Therefore a ToF detector offers an attractive alternative to solid state detectors for He-RBS.