Cryogenic helium as stopping medium for high-energy ions

We have investigated the survival and transport efficiency of Ra-219 ions emitted by a Ra-223 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 160 V/cm and for three temperature and density combinations: 77 K, 0.18 mg/cm(3), 10 K, 0.18 mg/cm(3) and 10 K, 0.54 mg/cm(3). 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 160 V/cm, the efficiency stays above 10% up to an ionisation rate density of 10(12) ion-electron pairs/cm(3)/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. (c) 2008 Elsevier B.V. All rights reserved.