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

The fragmentation yield distribution of CaF3- and KF2,3- in the tandem terminal can be significantly different at low Ar stripper gas pressures. A partial fragmentation of CaF3- 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 KF2- changing into KF+. Although the presence of KF3- as the main source of 41K background in 41Ca AMS is now known, the accompanying current of KF3- 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. KF3- was found to fragment most readily into the superhalogen anion KF2- 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 CaF3- is used for 41Ca measurement with a small AMS system directly, a sensitivity of 41Ca/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 60Ni+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.