Advancing 36Cl measurement capabilities: Performance limits of the 1 MV AMS system at Centro Nacional de Aceleradores (CNA, Seville, Spain)

The detection limit for 36 Cl (T 1/2 = 0.301 My) using mass spectrometry techniques is primarily constrained by the system’s ability to discriminate between 36 Cl and its stable isobar 36 S. Conventional accelerator mass spectrometry (AMS) systems typically operating at terminal voltages of 3 MV and above, achieve background levels for the isotopic ratio of 36 Cl/ 35 Cl on the order of 10⁻ 15 using techniques based on the stopping power difference. However, compact AMS systems are limited in their ability to discriminate heavy isobars due to their lower operating terminal voltages (i.e., 1 MV and below). Driven by the need to study 36 Cl in low- and intermediate-level nuclear waste (LILW), for which 36 Cl/ 35 Cl atom ratios on the order of 10⁻ 9 represent the detection limit of radiometric techniques, the performance of the 1 MV AMS system for the analysis of 36 Cl at the Centro Nacional de Aceleradores (CNA, Seville, Spain) was investigated in this work. Two strategies were adopted: (i) optimisation of radiochemistry and control of the 36 S signal using 33 S as a proxy, and (ii) exploitation of the partial overlap between the 36 Cl and 36 S signals in a ΔE–Eres gas detector. These approaches yielded background levels of approximately 10⁻ 9 and 10⁻ 10 , respectively. To enable these experiments, the stripping process of chlorine in helium gas at terminal voltages below 1 MV was investigated. Results from standard samples and blanks are presented in this paper.