²⁶Al measurements below 500 kV in charge state 2+

The use of helium as stripper gas improved the measurement efficiency of compact AMS systems for many radionuclides significantly because of a higher mean charge state and reduced scattering losses compared with other conventional gases. Recent tests at the ETH 500 kV AMS facility (Tandy) with aluminum have demonstrated that a transmission of more than 50% is achievable in the charge state 2+ at terminal voltages between 300 and 500 kV. On the other hand the m/q interference of ¹³C¹⁺ entering the detector at very high intensity has to be suppressed. Based on first positive results with a very simple absorber cell a more elaborate absorber detector configuration was designed and built in order to eliminate the carbon interference. The suppression of carbon with the new detector–absorber design has been studied extensively at 300 kV (950 keV) and 500 kV (1550 keV) and the results are compared with simulated data. With the new configuration an overall transmission for ²⁶Al of more than 42% at 500 kV and about 30% at 300 kV terminal voltage is achieved, while ²⁶Al/²⁷Al blank ratios of aluminum targets in the range of 5–14·10⁻¹⁵ are measured.