A search for doubly-charged negative ions via accelerator mass spectrometry

The Argonne FN tandem accelerator in conjunction with an Enge split-pole magnetic spectrograph has been used as a highly sensitive mass spectrometer to search for doubly-charged negative ions of 11B, 12C and 16O. No evidence for the formation of these ions in an inverted sputter source and the subsequent acceleration in the tandem has been found. The following limits for the ratio of doubly-charged singly-charged ions were measured: 11B2−/11B− < 1 × 10−15, 12C2−/12C− < 2 × 10−15, 16O2−/16O− < 2 × 10−14. A relatively abundant formation of the short lived, metastable He− ion in the sputter source has been observed.