AMS of 41Ca and 36Cl at a 3-MV tandem with a ΔTOF detector

Thursday, 8 September 2005

This presentation is part of: Poster Session II

AMS of ⁴¹Ca and ³⁶Cl at a 3-MV tandem with a ΔTOF detector

Peter Steier¹, Robin Golser¹, Walter Kutschera¹, Alfred Priller¹, Katharina Vorderwinkler¹, Anton Wallner¹, and Christof Vockenhuber². (1) Vienna Environmental Research Accelerator (VERA), Institut für Isotopenforschung und Kernphysik, Universität Wien, Währinger Str. 17, Wien, A-1090, Austria, (2) TRIUMF, 4004 Wesbrook Mall, Vancouver, BC V6T 2A3, Canada

ΔTOF is a new method for isobar separation at energies below 1 MeV/amu [1]. The residual energy of initially mono-energetic ions is measured precisely with a Time-of-Flight detector. Differences in the specific energy loss lead to a clear separation even at low energies. In this respect, ΔTOF is superior to other methods, since angular straggling and charge state fluctuations do not reduce the measurement precision. Additionally, the energy resolution can be made arbitrarily high by increasing the flight path. This allows studying the physical limitations due to energy straggling independently from detector properties, but reduces the efficiency. Silicon nitride foils [2] used both in the start timing detector and as degraders have proven to be extremely homogenous, and the peaks in the residual energy spectra are almost free of tails.

Systematic studies have led to an optimized ΔTOF setup, which achieves a detector efficiency of ~8% for ⁴¹Ca⁴⁺ (14.6 MeV) while suppressing ⁴¹K⁴⁺ by a factor of ~10³(we use CaF₃^- for injection). A first succesful application of ⁴¹Ca at VERA for a cross section measurement in nuclear astrophysics is described in a separate presentation [3]. For ³⁶Cl, where no suppression of the isobar ³⁶S is possible in the ion source, a similar suppression of ³⁶S of 10³ was achieved, but will require improved chemical sulfur suppression before competitive measurements are possible. The ΔTOF method should be useful not only for AMS, but may also find applications in other fields where isobar separation is required.

[1] C. Vockenhuber, R. Golser, W. Kutschera, A. Priller, P. Steier, K. Vorderwinkler, A. Wallner. The ΔTOF detector for isobar separation at ion energies below 1 MeV/amu. Nucl. Instr. and Meth. in Phys. Res. B (2005), in press.

[2] M. Doebeli, C. Kottler, M. Stocker, S. Weinmann, H.-A. Synal, M. Grajcar, M. Suter. Gas ionization chambers with silicon nitride windows for the detection and identification of low energy ions Nucl. Instr. and Meth. in Phys. Res. B 219-220 (2004) 415.

[3] Wallner et al., contribution to AMS-10.

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