Optimization of 10Be and 26Ai detection with low-energy accelerator mass spectrometry

Resumen

The Spanish Accelerator for Radionuclides Analysis (SARA) is the 1 MV accelerator mass spectrometry (AMS) facility hosted at the Centro Nacional de Aceleradores (CNA, Seville, Spain). As a multi-elemental system, SARA is capable to perform measurements over a wide mass range of the nuclear chart. In order to progressively improve its overall performance, several upgrades have been carried out since it started being operative in 2006. In 2014, SARA was improved with two features: (i) the stripper gas was changed from Ar to He and (ii) a high resolution gas ionization chamber was installed for the detection of the rare istopes. The first upgrade has opened to a variety of pioneering studies concerning the stripping process at terminal voltages of 1 MV and below and the behaviour of different radionuclides with the new stripper conditions. The second one has improved the ions detection, and has allowed the implementation of the so-called passive absorber technique, for which an optimal energy resolution is essential. The different processes and parameters of merit playing a role during the measurement of light ions as 10Be and 26Al have been investigated. The optimization work has included: (i) the analysis of the overall efficiency and the causes of beam losses, and (ii) the identification and minimization of background events. 10Be AMS measurements are affected by the intense interference of its isobar 10B, for which dedicated suppression techniques have to be applied. The degrader and the passive absorber methods have been tested and analyzed at the SARA system. The degrader technique consisted in placing a thin silicon nitride foil in the beam path before the electrostatic deflector placed in the high-energy side of the facility. As 10Be (Z=4) and 10B (Z=5) lose different energies in the foil according to their atomic numbers, they can be separated by the subsequent cinematic filter, that works as an energy dispersive analyzing device. The passive absorber technique consisted in positioning an absorber cell in front of the rare isotope gas ionization chamber with a proper mass thickness to stop 10B and allow 10Be to enter the chamber. The degrader technique is the most conventional and effective way to measure 10Be through AMS, but is characterized by severe beam losses that reduce the measurement efficiency. Passive absorbers, on the other hand, are in principle suitable at the energy range that can be reached at the CNA facility, but just with a proper absorber design and detecting system, for which further studies are required. In both cases, the overall efficiency has been quantitatively estimated and the background sources identified. The difficulty of 26Al AMS measurement is due to the low Al- currents which can be extracted from the samples. A detailed study has been performed in order to identify the optimal conditions to measure it at the upgraded SARA facility. The passive absorber technique has been successfully applied also to remove the disturbing 14C1+ molecular fragments during 26Al2+ detection. Even if the absorber design still needs to be improved, this technique, combined with the high transmission of the 2+ charge state through the accelerator, provides an improvement for the 26Al measurement efficiency.