Ultra-High Sensitivity Techniques for the Determination of 3He/4He Abundances in Helium by Accelerator Mass Spectrometry

We report the development of an Accelerator Mass Spectrometry technique to measure the 3He/4He isotopic ratio using a radio frequency (RF) discharge source and the ATLAS facility at Argonne National Laboratory.~ Control over 3He/4He ratio in helium several orders of magnitude lower than natural abundance is critical for neutron lifetime and source experiments using liquid helium.~ Due to low ultimate beam currents, the ATLAS accelerator and beam line were tuned using a succession of species of the same M/q.~ A unique RF source was developed for the experiment due to large natural 3He backgrounds.~ Analog H\textunderscore 3\textasciicircum $+$ and DH\textasciicircum $+$ molecular ions are eliminated by dissociation via a gold stripper foil near the detector. The stripped ions were dispersed in a magnetic spectrograph and 3He\textasciicircum 2$+$ ions counted in the focal plane detector. This technique is sensitive to 3He/4He ratios in the regime of 10\textasciicircum -12 with backgrounds that appear to be below 10\textasciicircum \textunderscore 14.~ The techniques used to reduce the backgrounds and remaining outstanding problems will be presented along with results from measurements on high purity 4He samples.