Preliminary measurements on the new TOF system installed at the AMS beamline of INFN-LABEC L. Palla a,⇑ , L. Castelli b , C. Czelusniak b,c , M.E. Fedi b , L. Giuntini b,c , L. Liccioli b,d , P.A. Mandò b,c , M. Martini e , A. Mazzinghi b,c , C. Ruberto b,d , L. Schiavulli f , E. Sibilia e , F. Taccetti b a Dipartimento di Fisica, Università di Pisa, e INFN Sezione di Pisa, Italy b INFN Sezione di Firenze, Italy c Dipartimento di Fisica e Astronomia, Università di Firenze, Italy d Dipartimento di Chimica Ugo Schiff, Università di Firenze, Italy e Dipartimento di Scienza dei Materiali, Università di Milano Bicocca, e INFN Sezione di Milano Bicocca, Milano, Italy f Dipartimento di Fisica, Università di Bari, e INFN Sezione di Bari, Italy article info Article history: Received 4 December 2014 Received in revised form 9 April 2015 Accepted 10 April 2015 Available online xxxx Keywords: Time of flight Accelerator Mass Spectrometry Background suppression 14 C abstract A high resolution time of flight (TOF) system has been developed at LABEC, the 3 MV Tandem accelerator laboratory in Florence, in order to improve the sensitivity of AMS measurements on carbon samples with ultra-low concentration and also to measure other isotopes, such as 129 I. The system can be employed to detect and identify residual interfering particles originated from the break-up of molecular isobars. The set-up has been specifically designed for low energy heavy ions: it consists of two identical time pick-off stations, each made up of a thin conductive foil and a Micro-Channel Plate (MCP) multiplier. The beam- line is also equipped with a silicon detector, installed downstream the stop TOF station. In this paper the design of the new system and the implemented readout electronics are presented. The tests performed on the single time pick-off station are reported: they show that the maximum contribu- tion to the timing resolution given by both the intrinsic MCP resolution and the electronics is 6500 ps (FWHM). For these tests, single particle pulsed beams of 2–5 MeV protons and 10 MeV 12 C 3+ ions, to sim- ulate typical AMS conditions, were used. The preliminary TOF and TOF-E (TOF-energy) measurements performed with carbon beams after the installation of the new system on the AMS beam line are also discussed. These measurements were per- formed using the foil–MCP as the start stage and a silicon detector as the stop stage. The spectra acquired with carbon ions suggest the presence of a small residual background from neighboring masses reaching the end of the beamline with the same energy as the rare isotope. Ó 2015 Elsevier B.V. All rights reserved. 1. Introduction Accelerator Mass Spectrometry (AMS) measurements are routinely performed at the Tandem accelerator of LABEC (Laboratorio di tecniche nucleari per l’Ambiente e i Beni Culturali), INFN-Firenze [1]. As it is well known, Tandem accelera- tors allow for the suppression of molecular isobars thanks to their fragmentation during the charge-exchange stripping process at the high voltage terminal, while magnetic and electrostatic analyzers, operating on both the low and the high energy sides of the beam- line, are used to discriminate among beam characteristics (energy, charge and mass) and to remove unwanted ions from the beam. In routine radiocarbon measurements, a concentration down to one atom in 10 15 is detectable. In general, the typical sensitivity for the measurement of a rare isotope is set by various kinds of back- ground. These include interferences due to isotopes (originated from the breakup of molecular isobars) and to atomic isobars. For example, in the case of 14 C, since its atomic isobar 14 N does not form stable negative ions, the most important interfering ions derive from the molecular isobars 12 CH 2 and 13 CH. Ions fragments originated from the breakup of these molecular isobars may occasionally show particular energy–charge state–tra- jectory configurations such as to elude the magnetic and electro- static selections in the high energy side of the beam line. For instance, residual charge exchange processes in the high energy accelerator tube may increase the charge state of ions that will be thus accelerated to a higher energy. Furthermore, low-angle http://dx.doi.org/10.1016/j.nimb.2015.04.020 0168-583X/Ó 2015 Elsevier B.V. All rights reserved. ⇑ Corresponding author. E-mail address: palla@fi.infn.it (L. Palla). Nuclear Instruments and Methods in Physics Research B xxx (2015) xxx–xxx Contents lists available at ScienceDirect Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb Please cite this article in press as: L. Palla et al., Preliminary measurements on the new TOF system installed at the AMS beamline of INFN-LABEC, Nucl. Instr. Meth. B (2015), http://dx.doi.org/10.1016/j.nimb.2015.04.020