The dissociation of 13 CH and 12 CH 2 molecules in He and N 2 at beam energies of 80–250 keV and possible implications for radiocarbon mass spectrometry T. Schulze-König ⇑ , M. Seiler, M. Suter, L. Wacker, H.-A. Synal Laboratory of Ion Beam Physics, ETH Zurich, Switzerland article info Article history: Received 23 June 2010 Received in revised form 27 August 2010 Available online 16 October 2010 Keywords: Molecule dissociation Stripper gas Radiocarbon AMS He stripping Dissociation cross section abstract Isotopic ratios of 14 C at natural levels can be efficiently measured with accelerator mass spectrometry (AMS). In compact AMS systems, 13 CH and 12 CH 2 molecular interferences are destroyed in collisions with the stripper gas, a process which can be described by dissociation cross sections. These dissociation cross sections determine the gas areal density required for sufficient attenuation of the interfering molecular beams, and are therefore key parameters in the effort to further reduce the terminal voltage and thus the size of the AMS system. We measured the dissociation cross sections of 13 CH and 12 CH 2 in N 2 and He in the energy range of 80–250 keV. In N 2 , cross sections were constant for energies above 100 keV with average values per molecule of (8.1 ± 0.4)  10 16 cm 2 for 13 CH and (9.5 ± 0.5)  10 16 cm 2 for 12 CH 2 . In He, cross sections were constant over the full measured range of 80–150 keV with average values of (4.2 ± 0.3)  10  16 cm 2 and (4.8 ± 0.4)  10 16 cm 2 , respectively. A considerable reduction of the termi- nal voltage from the currently used 200 kV while using N 2 for 13 CH and 12 CH 2 molecule dissociation is not possible: the required N 2 areal densities of 1.4 lg/cm 2 , consequential angular straggling and a decreasing 1+ charge state fraction would reduce the ion beam transmission too much. This is not the case for He: sufficient molecule dissociation can be obtained with gas densities of 0.4 lg/cm 2 , for which angular straggling is relatively small. In addition, the 1+ charge state fraction still increases at lower strip- ping energies. Thus, the usage of He for stripping and molecule dissociation might allow the development of even smaller 14 C-AMS systems than available today. Ó 2010 Elsevier B.V. All rights reserved. 1. Introduction Shortly after the invention of accelerator mass spectrometry (AMS) [1,2], dedicated 14 C-AMS systems came up analyzing C ions stripped to 3+ charge state [3], where a complete dissociation of molecular interferences is achieved [3,4]. More recently, another significant step forward in the reduction of size and complexity of AMS systems has been achieved. It was made possible by an alternative method to suppress molecular interferences for the iso- tope of interest. Lee et al. [5] showed that molecular beams in charge state 1+ or 2+ are exponentially attenuated when passing through a section of rarefied gas. Early 14 C measurements at termi- nal voltages of approx. 1 MV utilizing 1+ and 2+ charge states re- vealed the feasibility to reach the 14 C detection limits required for 14 C dating applications [6]. A dedicated 14 C-AMS system which utilizes the breakup process of molecular ions in charge state 1+ was developed at ETH Zurich [7] demonstrating the capability of high performance 14 C dating [8]. Equilibrium charge state fractions in the gas are a function of beam energy and thus analysis of charge state 1+ went along with a reduction of the terminal voltage U T . Today, AMS systems for 14 C analysis are available with terminal voltages below 300 kV and 12 C beam transmissions of more than 40% [9,10]. A further reduction of the terminal voltage and consequently of the size of the AMS system depends on the stripper gas properties, i.e. its molecule dissociation capability and its stripping and scat- tering properties. The exponential attenuation of molecule intensi- ties in a gas of areal density a can be described by a dissociation cross section r: NðaÞ¼ N 0  e ra : ð1Þ At very low densities, the strict exponential law may not hold be- cause molecular dissociation may depend on a gradual excitation of molecular bonds or on the projectile charge state. For the same reason, r is likely an average value of charge or excited state depen- dent dissociation cross sections. Under the assumption, that r does not depend on the gas areal density, a measured molecule suppres- sion factor N(a 0 )/N 0 will define a corresponding gas areal density a 0 which leads to a phase space expansion due to scattering events. 0168-583X/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2010.09.015 ⇑ Corresponding author. Tel.: +41 44 633 2041; fax: +41 44 633 1067. E-mail address: schulze@phys.ethz.ch (T. Schulze-König). Nuclear Instruments and Methods in Physics Research B 269 (2011) 34–39 Contents lists available at ScienceDirect Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb