Single Atom Counting with Accelerators

europhysics news BULLETIN OF THE EUROPEAN PHYSICAL SOCIETY J. A. Volume 15 February 1984 Number 2 Single Atom Counting with Accelerators W. W ölfli, Zürich (E T H ) Anew method of measuring small concentrations of long lived radioisotopes is beginning to have a major impact in many areas of science, where they are used for dating or tracing. Direct detection of radioisotopes with conventional mass spectrometers Is possible when the potential background atoms, In particular stable isotopes of the same mass (Isobars) or molecules of similar mass are present In sufficiently low concentrations. Most of the long liv ed radioisotopes of interest for dating purposes however, occur In such small concentrations that their peak in the mass spectrum Isobscured by the stable Isobar and molecule distributions. Table 1 gives the half-lives, the approximate ranges of natural concentrations In ter restrial and meteoritic materials, and the Interfering stable isobars of some of these radioisotopes. Up to now, none listed in the table has been directly observed by classical mass spectrome try or by the more modern separationidentification technique using lasers. The key Idea of the new AMS techni que which allows us to measure directly such small concentrations Is the accele ration of the sample atoms to MeV ener gies and to use various filter processes and particle Identification techniques developed for nuclear physics research to eliminate the isobaric and molecular Interferences. The detection methods used for each radioisotope depend on the dominant background atoms and these in turn depend on the specific ac celerator used. Physicists In many places have been developing these techniques since the idea was first mooted about six years ago in Berkeley and Rochester. Most In terestingly it was found that the sen sitivity required to detect such isotopes can be achieved quite easily with virtual ly any existing particle accelerator com bined with a reasonable mass filter and particle detection system. Flowever, It was soon realized that there is an enor mous difference between detecting a radioisotope at its natural level and mea suring the relevant Isotope ratios with the high precision required for dating. For Instance, to compete with the classi cal radiocarbon dating method, introdu ced by Libby in 1946, the 14C/12C ratio has to be measured with a precision of at least 1%. An error of 1% In the ratio measurement corresponds to an uncer tainty of 80 years in dating, Independent of the sample age. The problems encountered in trans forming an existing particle accelerator Into a high precision dating tool are con siderable and have been solved only recently for one type of accelerator, no tably the tandem van de Graaff. For this reason the following description of the AMS method and some of its applica tions will be restricted to this type of ac celerator only, taking the dating facility of the ETH Zurich as an example. AMS with TandemAccelerators The schematic layout of the facility Is shown in Fig. 1in the configuration used for radiocarbon dating. It consists of an electrostatic EN tandem accelerator de livering voltages up to 6 MV, an ion source attached to a 90° double focuss ing Inflection magnet at the low energy side and an electrostatic energy selector followed by another double focussing 90° analyzing magnet and a particle detector system at the high energy end. The currents of the abundant stable Iso topes are measured directly with con ventional Faraday cups, whereas the rare isotopes are identified and counted Individually by means of a AE/E gas counter telescope. With such an arran gement both the atomic and the mass Fig. 1— Schematic layout of the ETHaccelerator dating facility. Contents Single Atom Counting with Accelerators 1 Three-Nucleon Forces 5 8 New Members of the EPS Memories from the Childhood of Particle Accelerators 9 What Are Cosmic Rays Made Of? 12 Third World Academy of Sciences 15 European Geophysical Society 15 H.-P. Europhysics Prize 16 Publication of the EPS Computational Physics Group 16 Europhysics News is published monthly by the European Physical Society. © 1984. Reproduction rights reserved. ISSN0531-7479 1 Table 1— Long-lived Radioisotopes Isotope 10Be 14C 26AI 32Si 36CI 41Ca Half-life (a) 1.6 x 106 5.7 x 103 7.4 x 105 1.3 x 102 3.1 x IO5 1.3 x 105 Approx. Range of Terrestrial Cone. 1) 10 8 - 10-14 10-12 . 10-16 ~ 10-14 1015- 10-1 7 IO-12. 10-14 10-14- 10-15 Production Process Spallation 14N (n,p) 14C Spallation Spallation Spallation 40Ca (n, )41Ca Interfering Isobars 10B 14N 2) 26Mg 32S 36S, 36A 2) 41K 1) Compared to the stable isotope of the same element. 2) These elements do not form stable negative ions. number of each registered particle can now, no triply charged stable or meta stable molecules have been observed to be identified. Tandem accelerators require negative exist. Therefore any beam emerging ions, which are accelerated to high velo from a tandem accelerator will be con cities in the first section through the taminated only with fragments of mole large potential difference. These ions are cules, if charge states of 3+ or higher converted into multiply charged positive are selected out. A normal mass filter ions and further accelerated through the can then be used to separate these same potential difference. The energy of fragments from the wanted rare iso the ions emerging from the accelerator topes. Fig. 2 shows a three dimensional is given by E = eU (1 + q) where eU is given in eV and q is the charge state. representation of a particle spectrum Typically 3-6 MV are required to produce measured with the counter telescope. 3+ charge states with optimum efficien The sample was modern carbon with a cy. In our case, optimum beam intensity 14C/12C ratio of 1.2 x 1012. As can be for C3+ is obtained with 4.5 MV giving a seen, the 14C peak is well resolved from the few background peaks. These are final beam energy of 18 MeV. The negative ions are produced by a produced by molecular fragments which Cs sputter source. The sample atoms, somehow outwitted the high energy e.g. carbon, are sputtered and negatively mass filter. Similar tests with dead car charged, using an intense (0.5 - 1 mA) bon have indicated a dating limit of and well focussed 40 keV Cs beam. The about 60000 years. In order to determine the isotopic negative ions are extracted, accelerated to the same energy and focussed with ratios, required for a 14C dating with an electrostatic einzel lens onto the ob high precision, not only the rare but also ject slits of the inflection magnet. For the abundant stable isotopes should be graphite-like samples, almost 10% of all accelerated and measured at the high atoms are converted into negative ions energy side of the accelerator. Most of and more than 40 µA of carbon beam the existing accelerators, however, in can be obtained. Up to 25 samples can particular electrostatic tandem accelebe loaded simultaneously whic (...truncated)