Volatile Elements Production Rates in a 1.4-GeV Proton-Irradiated Molten Lead-Bismuth Target

The IS419 experiment at the ISOLDE facility at CERN dedicated to the measurement of production and release rates of volatile elements from an irradiated Pb/Bi target by a proton beam of 1/1.4GeV has been completed. The release of He, Ne, Ar, Br, Kr, Cd, I, Xe, Hg, Po and At isotopes was investigated at different target temperatures, ranging from 250 ◦ C to 600 ◦ C. Three experimental methods were used for the mass-separated, ionized beams: i) implantation of short- and medium-lived isotopes in a tape and on-line detection with a HPGe γ detector; ii) implantation of longer-lived isotopes in Al foils and off-line detection with a HPGe detector; iii) a Faraday Cup used mainly for stable nuclides. The results were compared with predictions from the FLUKA and MCNPX codes using different options for the intra-nuclear cascades and evaporation/fission models. Results show good agreement with calculations for Hg and for noble gases. For other elements such as iodine it is apparent that only a fraction of the produced isotopes is released. The results from FLUKA and MCNPX with the INCL4/ABLA models are in general more satisfactory than those obtained using MCNPX with the standard Bertini/Dresner model combination. Interestingly also significant yields of 204−210 At isotopes were observed. At isotopes are produced either by (p, π − xn) charge exchange reactions on 209 Bi or by secondary reactions involving 3 He and 4 He. Despite the non-release of polonium from Pb/Bi targets at typical operation temperatures, a smaller amount of highly radiotoxic Po isotopes can actually be liberated indirectly as decay daughters of the released astatine.