Gas ionization chamber for medium-energy, high-resolution ToF-ERD system at KIST

Time-of-flight – energy (ToF-E) spectrometers, consisting of two time pickoff detectors for ToF and an energy detector, are the most common mass discrimination technology for elastic recoil detection (ERD) analysis. While silicon detectors are easy to use, robust and commercially available, the energy resolution for heavy ions is poor due to the pulse height deficit (PHD) which limits mass resolution. We are constructing a new 40° recoil ToF-E spectrometer with a gas ionization chamber (GIC) for improved energy resolution, consequently mass resolution for a 6 MV tandem accelerator beam line using 30 MeV 127 I. Preliminary test of pressure endurance suggests that 30 mbar isobutane is safe from the mechanical burst with a 100 nm Si 3 N 4 entrance window for routine operation. Initial tests on a 2 MV tandem accelerator beamline show the GIC detector gives similar mass and energy resolutions as a PIPS detector for the light elements, but has atomic mass unit resolution for heavier elements, such as 28 Si and 59 Co, achieving 2–5 times better than those by the PIPS detector. Measurements suggest the potential for isotopic discrimination without need of mathematical unfolding of spectra. The design parameters, theoretical considerations and fabrication procedure for the GIC are discussed.