Tandem accelerators

Principles and Characteristics Particle-induced X-ray emission spectrometry (PIXE) is a high-energy ion beam analysis technique, which is often considered as a complement to XRF. PIXE analysis is typically carried out with a proton beam (proton-induced X-ray emission) and requires nuclear physics facilities such as a Van der Graaff accelerator, or otherwise a small electrostatic particle accelerator. As the highest sensitivity is obtained at rather low proton energies (2-4 MeV), recently, small and relatively inexpensive tandem accelerators have been developed for PIXE applications, which are commercially available. Compact cyclotrons are also often used. 

Many RBS installations use a tandem accelerator, producing a 2.25 MeV He++ beam by removing three electrons from He-. 

I.2. The Tandem Accelerator. As indicated in the diagram of Figure 4.13, a tandem accelerator uses a positive terminal located in the centre of the device. Negatively charged He- particles are injected into the accelerator and attracted to the terminal, where a stripper element removes two or more electrons from each... 

Figure 4.13. Schematic diagram of a tandem accelerator source of high energy alpha particles.

The impact of this new technique, which was called Accelerator Mass Spectrometry (AMS), on the radiocarbon and archaeologist communities, was immediate and revolutionary. The introduction of AMS is indeed recognized by some as the third revolution in radiocarbon dating[22,23] and it has provided the opportunity to date very precious finds by collecting very small samples. The interest in developing the technique of AMS was so evident that, just few years after the measurements cited above, a first dedicated AMS system (based on a tandem accelerator) was designed and built [24] then, the first dedicated... 

Figure 16.1 Block diagram of AMS measurements with a tandem accelerator...

Following this overview on the main features of AMS and the properties of the beam analysers, in the following section the experimental apparatus will be described in detail. In particular, an AMS system based on the use of a tandem accelerator will be considered as reference. 

The general scheme of an AMS beam line installed in a tandem accelerator has already been represented in Figure 16.1. As an example of one of these systems, Figure 16.3 shows the layout of the AMS facility at the LABEC laboratory in Florence,[28] where a 3 MV tandem accelerator by High Voltage Engineering Europe (HVEE) is installed. 

Techniques for the Direct Measurement of Natural Beryllium-10 and Carbon-14 with a Tandem Accelerator... 

At the same time, another group of researchers [3] (from the Universities of Rochester and Toronto, and the General Ionex Corporation) working quite independently and unknown to us, also showed that a Tandem accelerator offered many advantages for direct detection radioisotope dating. Since those initial experiments, many groups have undertaken similar work, and the first commercial systems should appear within the next year. 

Our project to test a Tandem accelerator for multi-beam detection was then to modify the accelerator analyzing magnet such that the various beams could be dispersed and detected simultaneously, to design an injection system to appropriately process the different isotopic beams from the ion source for injection into the accelerator, and to construct an ion-source to allow us to routinely handle small samples. Our interests at present lie in radiocarbon and radioberyllium studies, and so we have designed this system to accommodate these isotopes. 

Southon, J. R., Nelson, D. E., Korteling, R., Nowikow, I., Hammaren, E., McKay, J., Burke, D., Techniques for the Direct Measurement of Natural 10Be and 14C with a Tandem Accelerator, Chapter 4 in this book. 

Elmore, D., Anantaraman, N., Fulbright, H. W., Gove, H. E., Hans, H. S., Nishiizumi, K., Murrell, M. T., Honda, M., Half-life of 32Si using tandem accelerator mass spectrometry, Nuclear Structure Research Laboratory, University of Rochester, NY, Publication UR-NSRL-220, 11 p., 1980. 

It is necessary to calibrate the 14C time scale for greater dating accuracy. However, the second-order variations are at least as important as the first-order constancy of atmospheric 14C. For example, they provide a record of prehistoric solar variations, changes in the Earth s dipole moment and an insight into the fate of C02 from fossil fuel combustion. Improved techniques are needed that will enable the precise measurement of small cellulose samples from single tree rings. The tandem accelerator mass spectrometer (TAMS) may fill this need. 

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