Cryogenic helium as stopping medium for high-energy ions
Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms(2008)
Abstract
We have investigated the survival and transport efficiency of 219Rn ions emitted by a 223Ra source in high-density cryogenic helium gas, with ionisation of the gas induced by a proton beam. The combined efficiency of ion survival and transport by an applied electric field was measured as a function of ionisation rate density for electric fields up to 160V/cm and for three temperature and density combinations: 77K, 0.18mg/cm3, 10K, 0.18mg/cm3 and 10K, 0.54mg/cm3. At low beam intensity or high electric field, an efficiency of 30 % is obtained, confirming earlier results. A sharp drop in efficiency is observed at a “threshold” ionisation rate density which increases with the square of the applied electric field. At 160V/cm, the efficiency stays above 10% up to an ionisation rate density of 1012ion–electron pairs/cm3/s. The observed behaviour is understood as the result of shielding of the applied field by the weak plasma created by the proton beam: it counteracts the effective transport of ions and electrons, leading to recombination between the two. We conclude that cryogenic helium gas at high-density and high electric field is a promising medium for the transformation of very high-energy ions into low-energy ones.
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Key words
29.25.Rm,29.38.−c,29.90.+r
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