Space charge effect on energy resolution for high-energy heavy ions in allene-doped liquid argon

An allene-doped liquid argon ionization chamber has been constructed for the measurement of precise energy of heavy ions at around 100 MeV/n. An energy resolution of 0.6%-0.7 % (FWHM) was achieved for Ca and Ar ions. On the other hand, it was worse than 1% (FWHM) for O and Ne ions. The energy resolution deteriorated with increasing beam intensity. We have studied the reason for this degradation using a simple model which assumes that positive ions are left along the track of the incident high-energy heavy ions and that they modify the electric field and hence the amount of charge collection. The amount of positive space charge increases with increasing beam intensity, and the electric field due to the positive space charge was found to be strong enough to modify the rate of recombination between argon ions and electrons along the track of incident heavy ions. Furthermore, the electric field is not uniform because heavy ions do not enter the chamber uniformly. So, the fraction of collected charge does not become uniform and the nonuniformity in charge collection seems to have caused the degradation of energy resolution.