Minimising carbon contamination during ion beam analysis

The highly sensitivity ¹²C(d,p ₀) ¹³C reaction makes Nuclear Reaction Analysis (NRA) an excellent tool with which to quantify trace amounts of carbon. However it is often the build up of carbon on the sample during analysis which limits the sensitivity of this technique in practice. Carbon deposition rates on silicon have been monitored for a range of experimental conditions in order to quantify the problem and minimise contamination during analysis. Under common experimental conditions used at the RMCS facility, 1 × 10 ¹⁴atoms cm ^-2 of carbon were found to be deposited on silicon for each microcoulomb of an incident 1.2 MeV deuteron beam. The total number of carbon atoms deposited over a fixed dose could be reduced by minimising the hydrocarbon content of the residual gas, heating the sample, minimising the analysis area and placing a range foil in the path of the analysing beam.