Partial fragmentation of CaF3- at low MeV energies and its potential use for 41Ca measurement

Abstract The fragmentation yield distribution of CaF 3 - and KF 2 , 3 - in the tandem terminal can be significantly different at low Ar stripper gas pressures. A partial fragmentation of CaF 3 - into CaF + has been found to have about 10% yield at terminal voltage from 0.6 to 1.6 MV, while only 0.1% was found with beams of KF 2 - changing into KF + . Although the presence of KF 3 - as the main source of 41 K background in 41 Ca AMS is now known, the accompanying current of KF 3 - is too low for its yield of KF + to be reliably determined using Faraday cups. However, it is likely to be present, also at the 0.1% level, as indicated by indirect evidence. KF 3 - was found to fragment most readily into the superhalogen anion KF 2 - upon its first collision with residual gases in vacuum at tens of keV, as well as at eV energies in a RFQ gas-cell. When CaF 3 - is used for 41 Ca measurement with a small AMS system directly, a sensitivity of 41 Ca/Ca around 10 −11 can be expected if low levels of impurities in both Cs + beams and samples can be achieved and good noise suppression in the final energy spectrum is available. It is conceivable that even at low tandem energies, the employment of a second gas stripper, to eliminate 60 Ni +2 and other [Mass-60] +2 interferences, could bring the sensitivity to 10 −13 or below, a level well suited for medical applications, without the need for a RFQ gas-cell.