Efficient formation of negative ions for plutonium AMS

In an Accelerator Mass Spectrometry (AMS) instrument, the detection efficiency for rare radioisotopes is determined by a combination of ionisation efficiency, charge state yield and beam transmission. Of these, ionisation efficiency remains as the principal limitation, with few instances where efficiency greater than 1 % has been reported. Using the Vega AMS system at ANSTO, we have achieved reproducible ionisation efficiency, for formation of PuO anions, of 3–4 %. However, the achievement of high overall efficiency has come at the cost of operational efficiency, as it can take at least 10 h to consume each sample.

We have performed a series of tests to understand what determines ionisation efficiency for plutonium AMS. In the standard method used at ANSTO, plutonium is dispersed in iron oxide and mixed with niobium as ‘binder’. The overall efficiency for samples run to exhaustion is found to be linearly proportional to the total mass of loaded mixture. We have performed a series of tests investigating the effects of: (i) recess depth of material in the cathode; (ii) use of layered samples; (iii) binder / iron oxide mix; (iv) cathode materials; (v) different binder; (vi) sample surface area. We have also determined the molecular composition of the Pu anions: PuO_x⁻ of extracted beams for x  = 0 to 3. The results are compared to a sputtering model that has been developed to account for the observed variation in count rates versus time as the sample is consumed. Ionisation efficiency up to 6.5 % has been observed for PuO anions.