Identifying the sources of beam transmission loss of carbon isotopes

The continuous decrease of 12C beam transmission through the accelerator at high beam currents is well known. Depending on the machine design, this effect can be strong as is the case for the 250 kV SSAMS unit at CAIS. In such a case, the post acceleration 13C+/12C+ ratio of a sample is not constant, which is detrimental for online δ13C determination. In this study we attempted to identify the sources of transmission loss. We found that at high currents, 14C+/12C- ratio is not constant for a given sample but decreases linearly with increasing 12C- beam. This result is surprising because 14C being a rare isotope, its transmission is not expected to be affected during acceleration. It implies that the observed steady decline in transmission is not due to scattering effects at the stripping column but due to beam emittance growth as a result of Coulomb repulsion of the charged particles. Beam loss due to scattering by argon stripper gas is found to be independent of the beam intensity.