Accelerator mass spectrometry of 236U with He stripping at the Centro Nacional de Aceleradores

In January 2015, the 1 MV AMS facility at the Centro Nacional de Aceleradores (CNA, Seville, Spain) was upgraded with He stripping. In this work, we present a comprehensive study of the uranium transmission through the accelerator together with the background sources that limit the current sensitivity of our system to measure the U-236/U-238 atom ratio. In addition to the well-known background caused by scattered U-238(3+) molecular fragments, we identify an additional component, not observed with Ar gas, that we relate to scattered U-238(3+) molecular fragments. Stripper gas pressures below the one corresponding to the equilibrium charge state distribution are used to minimize the resulting overall background, which is in the (0.8-1.3) x 10(-10) range. This is at least a factor of 3 higher than the background obtained with Ar gas. On the other hand, new information on the stripping process of U on He gas at the 400-1000 keV stripping energy range is presented. A maximum transmission of 45% for U3+ is achieved at about 650 keV, which correspond to a stripping yield of about 50%. This is the maximum yield that can be actually obtained working with He gas at low energies according to previous results.