PIXE - particle induced X-rays
By PIXE method we determine elemental concentrations in the sample which is excited by a proton beam with typical 1 - 3 MeV energy. In collisions with the protons the atoms in the sample become ionized and excited. The inner electron holes are subsequently relaxed by the emission of X-rays whose energy is characteristic of a given atom. PIXE method is based mainly on detection of K-shell transitions in lighter and L-shell transitions in the heavier atoms, because the characteristic energies of these transitions are are well enough separated to discern contributions of different atoms in the spectra, recorded by a typical semiconductor X-ray detector.
After measuring the PIXE spectrum photon yield has to be normalized to the total proton dose, received by the sample during the measurement. The number of photons under the specific line in the spectrum is then proportional to the hole creation cross section, of the experimental geometry and, of course, to the concentration of a given element in the sample.
PIXE method is mostly used for the elements heavier than Na, because the characteristic X-rays of elements with lower Z have not enough energy to penetrate into the detector sensitive volume. Besides, there is an alternative, much more probable relaxation route of inner holes which further reduces the photon yield in the light atoms. In the so called Auger decay the energy is taken away by the Auger electron. For example, the argon Ne-hole has approximately 98% probability for the nonradiative relaxation and only 2% for the X-ray emission and the things get even worse when Z is lower. Therefore the sensitivity of the PIXE method varies with Z and amounts to 1 ppm (µg/g) for light elements (from Na to Cl) bellow 0.1 ppm for transition metals and close to 10ppm for heavier elements.
There are many different PIXE experimental setups possible but the most common is to irradiate sample by the "wide" proton beam with several mm2 cross section. The result of such measurement is then the average surface density of a given element in the area excited by the beam. Usually the sample is placed into the vacuum chamber because the range of the proton beam in the air is relatively low. Nevertheless, the samples which can not be put into vacuum can be analyzed by the same technique using the external proton beam.
PIXE method is used on our PIXE/RBS and external beam beamline.
Last updated: 11/23/2007