Overview
At BNC-WIGNER, proton induced X-ray emission (PIXE) provides simultaneous elemental analysis from sodium to uranium in conventional vacuum and from aluminum to uranium external-beam in air arrangement. The sample is bombarded by protons of 1-4 MeV energy, inner shell ionization occurs followed by a prompt electron re-arrangement processes which is associated by emission of characteristic X-ray photons. The energies of the X-ray photons are determined by the atomic number of the emitting element, while the relative intensities are related to its concentration. Because of the intensive energy loss of the penetrating protons and absorption of the emitted X-rays in the sample itself the PIXE inherently sensitive to the near surface region up to some tens of micrometer depth. The elemental sensitivity of the PIXE analysis is determined by several factors, in favorable conditions it is in the ppm range. The external-beam PIXE technique is expansively applied for the non-invasive analysis of matrix and trace elements in unique archaeological artifacts and fine art creations.
At MTA Atomki the PIXE technique is available in vacuum, as well as in an external beam arrangement. In-vacuum measurements are preferred when the sample is small (a few cm) and not especially sensitive. In this case, even the carbon signal can be detected, although usually not quantified. The quantification is straightforward for oxygen and upwards in atomic number. The lateral resolution is excellent in vacuum, down to the micron range. For large objects and objects which are made of sensitive materials, the external arrangement is applied. In this case the lateral resolution is a few tens of microns and the elements can be detected from magnesium or silicon, depending on the more specific details of the set-up and the sample. Elemental maps can be recorded in both cases to display the distribution of elements.
Technical details
BNC-WIGNER: The external milli-beam PIXE setup at the 5 MV Van de Graaff accelerator of Wigner Research Centre for Physics is dedicated to cultural heritage applications. The properly collimated proton beam of 2–3.5 MeV energy is extracted from the evacuated beam pipe to air through a 7.5 micrometer thick Kapton foil. The target-window distance is 10 mm, at which distance the beam diameter was is about 0.8 mm. The extracted proton beam current can be set within the 1–10 nA range. The objects for analyses are fixed on a three-dimensional positioning stage. Manual positioning available for large objects up to 50 kg, and a computer controlled fine positioning for the smaller ones up to 5 kg The characteristic x-rays are detected and analyzed by an AMPTEK X-123 detector system. The SDD detector is positioned at 135° backward angle with variable absorbers to reduce the low energy x-ray counts and to stop the backscattered protons.
MTA Atomki: The PIXE technique is applied at the Scanning Nuclear Microprobe (Oxford Microbeams) which was built at the 0°beam line of our 5 MV single ended vertical Van de Graaff accelerator. Two X-ray detectors placed at 135° geometry to the incidence beam collect the emitted characteristic X-rays. An SDD detector with ultra-thin polymer window is used to measure low and medium energy X-rays. A permanent magnet protects the detector from the scattered protons. A more traditional, Be-windowed X-ray detector equipped with an appropriate additional filter detects the medium and high energy X-rays. The beam dose is measured with a beam chopper. When we are working outside the chamber, the beam is extracted through a thin polymer foil, using an add-on system also by Oxford Microbeams. The quantification for the concentration of elements is carried out with the GUPIX software.
Further readings
- M. Mödlinger, P. Picardo et al. Archaeometallurgical characterization of the earliest European metal helmets, Materials Characterization 79:22-36,·February 2013
- B. Constantinescu, Daniela Cristea-Stan at al. Provenance studies of Central European Neolithic obsidians using external beam milli-PIXE spectroscopy, Nuclear Instruments and Methods in Physics Research Section B Beam Interactions with Materials and Atoms 318:145-148, January 2014
- Zs. Török, R. Huszánk et al. Development of a new in-air micro-PIXE set-up with in-vacuum charge measurements in Atomki. Nuclear Instruments and Methods in Physics Research B 362 (2015) 167–171.
- M. Sánchez de la Torre, A. Angyal et al. Trace element mapping of two Pyrenean chert deposits (SW Europe) by PIXE. Nuclear Instruments and Methods in Physics Research B 400 (2017) 58–64.
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