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Accelerator Mass Spectrometry at Vera ACCELERATOR MASS SPECTROMETRY AT VERA Walter Kutschera Institut für Isotopenforschung und Kernphysik der Universität Wien, Währinger Strasse 17, A-1090 Wien, Austria Abstract one to measure long-lived radioisotopes by counting The Vienna Environmental Research Accelerator atoms rather than decays, which increases the detection (VERA) is a dedicated facility for accelerator mass sensitivity by many orders of magnitude. This makes it spectrometry (AMS), operated at the University of Vienna possible now to utilize a variety of cosmogenic since 1996. The principle of AMS with VERA and the radioisotopes which were virtually not detectable by advantage of measuring long-lived radioisotopes by atom decay counting. counting (AMS) rather than decay counting (ß decay) is described. A brief overview on AMS applications is also 2 ATOM COUNTING VERSUS DECAY presented. COUNTING Long-lived radioisotopes can be detected either through 1 INTRODUCTION their radioactivity (decay counting) or by mass Mass Spectrometry (MS) is a well-known method to spectrometry (atom countig). In the latter case the determine the abundance of stable isotopes. Typically, an radioisotopes are detected before they decay, and for ion beam with a well-defined energy in the keV range is measuring times much shorter than the half-life this produced from some sample material, and is sent through method is much more sensitive. In Table 1 the conditions a magnet where the ions are separated according to their for measuring 14C by decay counting and by AMS at mass. MS is widely used in research and industry, and VERA are compared. For this comparison it is assumed isotope ratios of stable isotopes can be measured with that the efficiency of detecting ß-rays from the decay of high precision. This method, however, has not enough 14C is 100% (upper limit), whereas the 14C atom counting selectivity to separate the extremely low abundances of numbers are taken from actual measurements at VERA. long-lived radioisotopes in nature (10-12 to 10-16) from an As a practical consequence of the enormous gain in overwhelming background. Such a selectivity can be sensitivity, one can perform 14C measurements with AMS reached by using an entire accelerator system common in requiring 1000 times less sample material (1 mg instead nuclear physics as a mass spectromter, thus extending MS of 1 g), and 100 times less counting time (0.5 h instead of to AMS (accelerator mass spectrometry). AMS allows 2 d). Table 1: 14C detection: beta counting versus atom counting Isotopic abundance of 12C 89.9 % Isotopic abundance of 13C 1.1 % 14 -12 14 12 Isotopic abundance of C in modern (pre-bomb) carbon 1.2x10 C/ C 14 7 14 C content of 1 mg of modern carbon 6x10 C atoms Half-life of 14C 5730 ± 40 y Activity of 1 mg of modern carbon 0.8 14C decays/h Activity of 1 mg of 50,000-year old carbon 17 14C decays/y − Negative ion 12C output from 1 mg of modern carbon a) 30 µA 14 − 1.9x10 12C ions/s − − Negative ion 14C output from 1 mg of modern carbon 230 14C ions/s Counting rate of 14C in the final AMS detector 115 14C3+ ions/s 5 14 3+ 4.1x10 C ions/h Counting rate of 14C from 1 mg of 50,000-year old carbon 970 14C3+ ions/h 14 5 Gain in C detection sensitivity with AMS 5.1x10 AMS/ß-counting a) 1 mg of carbon lasts typically about one hour in a cesium beam negative ion sputter source. Proceedings of EPAC 2000, Vienna, Austria 245 13 − 12 − 2 THE VIENNA ENVIRONMENTAL CH and CH2 molecules very effectively when RESEARCH ACCELERATOR sufficient electrons are stripped off. With the high energy analysing magnet set to select 14C3+ ions, one observes the Figure 1 shows the schematic layout of VERA, a energy spectrum shown in Figure 2b. Although the dedicated AMS facility based on a 3-MV Pelletron 12 13 residual C and C peaks are greatly reduced in intensity, tandem accelerator [1]. As an example we describe the there is still a large number of background peaks which isotopic analysis of carbon, which starts with the 14 3+ happen to have the same magnetic rigidity as C . production of negative carbon ions in a cesium beam 14 3+ However, Figure 2c shows that the C ions can be sputter ion source. cleanly selected by sending the mix of ions in figure 2b Whereas it is relatively easy to measure the isotope through a Wien filter (see fig. 1) set to the velocity of 13 12 -2 14 3+ 14 12 ratio of the two stable carbon isotopes ( C/ C = 1.1x10 ) C . In order to determine C/ C ratios with high − − − with classical mass spectrometry at energies in the keV precision (~0.5%), 12C , 13C , and 14C are injected in fast range, it is virtually impossible to measure the extremely sequence (10 times per second) into the tandem. The low abundance of the long-lived radioisotope 14C (14C/12C stable isotopes,12C3+ and 13C3+, are measured via their -12 = 1.2x10 ) in this way. After selecting ions with mass 14 respective ion currents in Faraday cups located at the exit − by the first analysing magnet, the 14C ions are burried of the high-energy analysing magnet, and 14C3+ ions are under an enormous background of molecular ions such as counted in a Si surface barrier detector at the end of the 13 − 12 − 14 − 14 12 CH , CH2 (see fig.2a). Fortunately, N ions are beam line (see fig. 1). For details of C/ C ratio absent in the mass-14 peak, because nitrogen does not measurements and of other radionuclides at VERA the form stable neagative ions and thus gets completely reader is referred to references [3-8]. We now operate suppressed already in the ion source. An important step in VERA fully automated and unattended during nights and the consecutive acceleration in the tandem is therefore the over weekends, with the person in charge monitoring (and stripping process in the terminal, which dissociates the controlling) the performance from home via internet [9]. Cs-BEAM SPUTTER SOURCE FOR NEGATIVE IONS (40 Samples) 75-kV PRE- ACCELERATION VERA INSERTABLE Vienna Environmental Research Accelerator F-CUP 45° SPHERICAL EXTRACTOR ELECTROSTATIC EINZEL LENS ANALYSER (E/q = 92 keV) Y STEERER C - X-Y SLITS BEAM PROFILE MONITOR BEAM PROFILE MONITOR 90° DOUBLE FOCUSING ANALYSING MAGNET +3 MILLION VOLT 2 X-Y SLITS (ME/q = 176 MeV amu) MAGNETIC QUADRUPOLE 12 - PELLETRON TANDEM DOUBLET LENS C 13 - ACCELERATOR C STABLE ISOTOPE F-CUPS 90° DOUBLE FOCUSING INJECTION MAGNET BEAM PROFILE MONITOR WITH FAST SEQUENTIAL 12, 13, 14 n+ ISOTOPE INJECTION C (ME/q2 = 15 MeV amu) GAS + FOIL X-Y SLITS STRIPPER 3+ 12 C FOIL STRIPPER ELECTROSTATIC STABLE ISOTOPE X-Y STEERER QUADRUPOLE 13 3+ F-CUP TRIPLET LENS C 20° SWITCHING INSERTABLE MAGNET F-CUP ELECTROSTATIC (ME/q2 = 176 MeV amu) EINZEL LENS MAGNETIC QUADRUPOLE DOUBLET LENS 20° BEAM LINE FOR FUTURE EXTENSION WIEN FILTER ε x B = 35 kV/cm x 0.4T BEAM PROFILE MONITOR X-Y SLITS 14 3+ C INSERTABLE RARE ISOTOPE DETECTOR F-CUP Figure1: Schematic layout of VERA showing the essential features of the AMS system. 14C measurements are typically performed at a terminal voltage of 2.7 MV. 246 Proceedings of EPAC 2000, Vienna, Austria 3 SMALL IS BEAUTIFUL: THE MINATURIZATION OF AMS FACILITIES For twenty years it was believed that 14C3+ ions (or higher charge states) must be produced in the stripping process to break up the molecules for sure. It is interesting to note that this “dogma” of stripping has now been proven to be obsolete. In a collaborative effort of National Electrostatics Corporation (NEC) and the AMS laboratory of the ETH/PSI Zurich it was demonstrated that high quality 14C AMS measurements can be performed using a 0.5 MV Pelletron tandem and stripping only to 14C ions in charge state 1+ [10, 11]. The most surprising result was the measurement of 14C/12C ratios down to a level that corresponds to a radiocarbon age of 48,000 years. It therefore looks feasible to perform 14C dating measurements with much smaller tandem accelerators than presently in use, approaching essentially the size of table top machines. Such small machines may prove particularly useful for biomedical application with labelled compounds, where generally less precision is required, but ease of operation, sample throughput, and space requirements are important factors. 4 APPLICATIONS OF AMS IN THE ENVIRONMENT AT LARGE Applications of AMS are numerous, and penetrate almost every field of our environment on earth and beyond. The reason is that every piece of matter on Earth and beyond (meteorites, Moon, etc.) contains a certain number of long-lived radionuclides produced mainly by cosmic ray interaction, so-called cosmogenic radioisotopes. Other sources are man-made radioisotopes (anthropogenic) and those which originate from the decay of very long-lived radionuclides such as 238U (radiogenic). AMS makes it possible to measure an increasing number of radioisotopes in the various domains of our environment (Table 2). Among the radioisotopes listed in Table 2, 14C is still by far the most-used one. Approximately 90% of all AMS measurements performed in 50 AMS laboratories worldwide [12] are applications of 14C. This is not surprizing since prior to the invention of 14 14 AMS in 1977 there were already 30 years of C Figure 2: The three steps in the detection of C with AMS applications through the classical ß-counting technique. at VERA. a) In the negative ion mass spectrum after the 14 Most other radioisotopes shown in Table 2 only became ion source the C signal is burried under an enourmous available for applications through AMS.
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