Background reduction for heavy element accelerator mass spectrometry

The energy distribution of negative ions generated by low energy (keV) Cs⁺ bombardment of solids has been studied over 11 decades of negative ion intensity. The higher energy component of this ion spectrum is characterized approximately by an E^?2 distribution, terminated by the kinematic limit for two-body collisions. In the case of 27 keV Cs bombardment on carbon an excess energy of 6 keV beyond the mean emission energy was observed. A more complex, low energy ion spectrum was attributed to molecular fragmentation processes within the ion extraction field and subsequent beam transport system. The elimination of these tails and other ions prior to tandem acceleration, and hence the removal of potential sources of background in accelerator mass spectrometry (AMS) are discussed in this paper, with a view to extending the detection limits in AMS for elements other than ¹⁴C and especially for heavy elements.