PIXE experimental set-up in Cologne, 2003
Particle Induced X-ray Emission (PIXE) analysis is a nuclear method used for multielemental analysis of various samples and surfaces (up to tens of micrometer). Recently, the particles used for PIXE are mostly protons, PIXE can be also understood as Proton Induced X-ray Emission. PIXE is a non-destructive technique used for elemental composition analysis with high importance for geologists, archaeologists, art conservators and many others (see below PIXE applications). Lots of other fields are opened for PIXE usage (e.g. biology, medicine…). When PIXE installation is well constructed, the limit of detection and quantification range lies in units of Parts Per Million (ppm) (see e.g. link http://www.chem.vt.edu/chem-dept/long/6164/files/LOD-98.pdf).
First of all to remember is that PIXE analysis has nothing to do with atomic nuclei of the explored elements and samples. Proton beam accelerated to energies usually of 2 – 3 MeV bombards the sample (target) and protons mainly interacts with electron shells of the atoms. Exactly this interaction is demanded for PIXE analysis. Interaction of protons with atomic nucleus is undesirable (e.g. higher background in the spectra and nuclear reactions causing higher amount of particles (p,n) entering and harming the detector).
From the theory can be understood, that proton beam interacts preferably with inner electron shells of atom [?]. The most inner electron shell is called K-shell, second inner is called L-shell, then comes M, N, O…-shell. When e.g. K-shell electron is kicked out by the proton, there appears a vacancy in the K-shell. To keep the energy of the atom the lowest, some electron from more outer shell then K (L, M, …) drops down to replace this K-shell vacancy and emits its redundant energy in a form of X-rays. As the energy gap between K and L (or M, N,…) shell is well defined (according to quantum theory-orbiting electrons of an atom must occupy discrete energy levels), the emitted X-ray photon will have some characteristic energy. When L-shell electron is kicked out, most preferably electron from M-shell will take the free position and will emit the energy difference between L and M shell in form of X-rays.
K, L, M-lines in spectra:
Spectra of X-rays emitted by electrons filling the vacancies in K-shell (they jump from L-, M-, N-…shells) of atoms are called K-lines. The picture explains better this designation [scan of picture]. L-lines are that ones in spectra caused by L-shell electron to be kicked out and replaced by electron from M-, N-…shells. Every element has different energy gap between electron shells and therefore K-lines (L-, M-…lines) of one element will differ from K-lines (L-, M-…lines) of other element. That is why X-rays originated from this process (proton interaction with electron shells) are called characteristic X-rays.
Instead of emitting the redundant energy in form of X-rays, this energy can be accepted by one of outer shell electron and if getting more than binding energy, this one is ejected and leaves the atomic orbit. Such an electron is called Auger electron and this process is competitive to X-ray emission.
Protons are accelerated usually by electrostatic accelerator (in our case by
Tandem = double Van de Graaff) to energy of few MeV (between 2MeV – 3MeV)
of intensity 1nA to 100nA [link to tandem]. This proton beam is directed at
the sample. Induced characteristic X-rays of many elements present in the sample
are detected by SiLi detector and analysed. These X-rays come mainly from K
and L electron shells of the deexciting atoms. The low energy detectors like
SiLi, planar HPGe or X-flash have a detecting range between 1keV up to 30 –
40keV. This corresponds to K- and L-shells of elements with proton number between
11 (Na) and 92 (U). [link x-ray chart and site: http://www.amptek.com/pdf/xraychrt.pdf]
Depending on the sample composition, mass, proton current, detector-sample distance and geometry, … the acquisition time can vary from few minutes to about 1 hour. Pixe samples are preferably solid.
The Pixe chamber is designed for automatic analysis of a batch of twenty five samples. They can be attached to a wheel with 25 sample holders. This wheel is driven by stepping-motor [link to some nice site e.g. http://www.cs.uiowa.edu/~jones/step/] with a 1600 steps per revolution. The correct position of the sample is watched by a small camera.
At present, we are using X-flash [link to http://www.rontec.com/en/3/4/1/2]detector for the spectra acquisition. The greatest advantage along with the very good resolution (~144eV at 5.90keV) is the working temperature at –15°C. This means no nitrogen cooling and therefore a compact form of the detector body.
Our Pixe experimental beam-line can be seen on this picture and the drawing:
One of the main goal of the Pixe-group in Cologne is to use a sweeper for fast
and precise ‘macroscanning’ of the sample surface. The expected
scan dimension is (20x20)mm² with scan-spot about 0.4 mm² but this
can be varied according to the requests.
At present, the differences between analog versus digital acquisition system are tested and compared.
Application of PIXE:
- Atmospheric aerosols
This application has been very popular since the beginning of the technique. The air-pollution (mainly the heavy metals) is very easily analysed this way. Aerosols are collected on a thin film or filter made of light
elements (e.g. Nuclepore polycarbonate filter, Mylar films, Kapton). These filters are afterwards analysed by PIXE. Analysis of e.g. S, V, Ni, Cu, Zn, As, Pb can be done in 5 minutes. Air pollution analysis is requested indoors (e.g. in working environments) and outdoor near specific pollution sources or urban pollution of large cities.
- Art and Archaeology
(e.g. for authenticity, provenance,
deterioration and conservation questions)
PIXE is very convenient for art and archaeology samples analysis because of its non-destructivity, when possible. By samples bigger than space at the target position of the PIXE chamber or fragile samples (e.g. analysis of ink in the Gutenberg Bible), very small piece of the object is extracted and analysed. The questions of authenticity, provenance, deterioration and conservation are possibly solved this way (e.g. provenance of pottery). . Large samples need to be analysed in external beams (proton beam is focused out of the guide-line to the target position on air. If matrixes of the samples are of high Z elements (bronze, copper, gold,...), then filters and corrections
- Medicine and Biology
There is an interest in ultratrace analysis of K, Ca, Mn, Fe, Cu, Zn, Se,... in organic material. Due to the biodiversity of organic samples, high number of samples is required to make statistical analysis.
Samples are either solid and then no preceding preparation is required:
- bones, finger nails, teeth, hairs (comparison of healthy vs. sick person)
- wood (tree cores - acidic rains change Ca content and Ca/Mn ratio)
In case of soft tissues and liquids, preceding preparation before analysis is required. This means mainly freeze-drying or wet-ashing with addition of internal standards:
- brain (Alzheimer), liver, ...
- single cells (with microbeam)
- blood, urine
- Earth Sciences
(e.g. aquatic systems, mineral prospecting)
PIXE is used in aquatic systems science for sea water analysis. Ice cores drilled from glaciers (variation with depth gives information on climatic changes) are analysed by PIXE too. Another applications are e.g. mineral prospecting (filtering geogas or analysing drill cores) and study of liquid inclusion in minerals by micro-pixe.
- Materials Analysis
PIXE is used to study corrosion and erosion in Solid-State Physics, to study impurities in single crystals by “channelling PIXE” vs. “common PIXE” analysis. Another possibility is to study high-temperature semi-conductors based on ceramics with oxides of rare elements.
Other related techniques of IBA (Ion Beam Analysis)
- PIGE (Proton Induced Gamma-ray Emission) for analysis of low-Z elements (Li, Be, B, N, O, F, Na, Mg, Al, Si, P, S, Cl, K)
- NRA (Nuclear Reaction analysis) e.g. Nitrogen depth profile for biomedicine using the reaction 14N(d,p)15N
- RBS (Rutherford Backscattering) mostly for depth profiling using alpha particles