Increased AMS Ion Source Efficiency and Ion Currents by Modifying SNICS Cathode Material and Geometry

A series of tests have been conducted at the Wood Hole Oceanographic Institution's National Ocean Sciences Accelerator Mass Spectrometry facility (NOSAMS) to investigate the effect of sample well geometry and cathode material on C− extraction efficiency and beam currents. Ion current production tests were performed on aluminum cathodes that were prepared by drilling sample wells with various diameters (Ø), ranging from 0.50 mm to 1.50 mm, and depths ranging from 1.3 mm to 4.3 mm. Cathodes with sample well diameters of 1 mm and 0.75 mm had marginally better C− current, while current for the larger sample wells was lower but more consistent. Depth tests showed an obvious difference in ion beam currents, with shallow wells outperforming the deeper wells. Efficiency tests were first conducted on Al cathodes to find an optimum diameter. Cathodes with Ø of 0.50 mm, 0.75 mm, and 1.00 mm were drilled to a depth of 2.3 mm, hand pressed with approximately 250 μg of Alfa Aesar graphite, and then run to exhaustion. The best performers were cathodes with Ø of 0.75 mm, measuring as much as 16.5% efficiency compared to 13% from the 0.50 mm and 15% from the 1.00 mm cathodes. Cathodes with Zn inserts were then prepared in the same manner, with a 0.75 mm diameter, and showed further improvement, increasing the ion source efficiency to as much as 27%.