Fig. Principle of particle-induced X-ray emission

Fig. Schematic picture of PIXE set-up

Particle-Induced X-ray Emission Analysis (PIXE)

Principle
A 3 MeV proton beam produced with the Åbo Akademi MGC-20 cyclotron is used to generate characteristic X-rays in a solid sample. The proton creates a vacancy in the inner electronic shell of the atom. The vacancy is filled with an electron from the outer shells. The energy can be released during the de-excitation by the emission of an X-ray or an Auger electron. The X-rays originating from the electronic transition from the L- shell to the K-shell are usually referred to as K_α and the transition from the M-shell is called K_β according to the Siegbahn notation. The emitted X-rays are detected with an energy dispersive Intrinsic Germanium Planar (IGP) detector with a 25 µm thick beryllium window. The concentrations are obtained by analyzing the X-ray spectrum. The energy of the emitted X-rays is characteristic for the elements in the sample. The intensity of the X- rays representing the element Z gives the elemental concentration.

Fig. Set-up for PIXE at Åbo Akademi University. The IGP detector is to left and the beam line to left. The vertical aluminum pipe is a collimator fot light into the PM-tube used for beam current monitoring.

Sample preparation
A special sample holder has been constructed in order to obtain a fixed measuring geometry. The sample is then mounted on a computer controlled X-Y stage. The stage used in this job is a commercial stepper driven X-Y stage (XYMR-8080, Danaher Precision Systems). The travel distances are 15 cm in both directions and the lead screw accuracy is 30 µm.With this scanning device profiles of elements are determined even at ppm level. The irradiations are monitored using an USB PC-camera (ToUcam, Philips).

Fig. User interface for operating XY-stage to right and a view of sample to the left. The sample is a piece of wood.

For geological samples 0.1 mm polished uncovered sections of minerals for polarizing microscopy (frame 27 x 48 mm) are especially suitable. Another possibility is to mount the sample in a 5 x 5 cm slide projector frame.
Biological samples can be preconcentrated by dry ashing at the Laboratory of Analytical Chemistry in order to enhance the sensitivity of the method. The ashes are pressed to pellets. Only a few mg of ash is needed for the analysis.

Calibration
In order to obtain quantitative results the peak areas in the X-ray spectra have to be normalized with respect to the integrated charge on the sample (the total amount of protons incident on the sample) during the irradiation. The integrated charge is determined utilizing light emission from N₂ in air excited by the proton beam. The light intensity, measured by a photo-multiplier tube is proportional to the proton-beam intensity. The method has been developed at the Åbo Akademi Accelerator Laboratory. The X-rays spectra are analyzed by using the GUPIX software. The calibration of GUPIX is evaluated by utilizing international standard reference materials.

Special features

• All elements with atomic number higher than 14 (Si) can be analyzed. For instance in biological samples the concentrations of about 15 elements are normally determined simultaneously.
• The method is fast. A normal irradiation lasts about 10 min. This enables analyses of large numbers of samples to collect the statistics needed in many scientific studies.
• PIXE is a sensitive method. Concentrations of elements down to a few 100 ppb can be measured.
• The spatial resolution of the beam is 1 mm. The penetration depth in a solid sample is about 100 µm. These features enable determinations of distributions or profiles of elemental concentrations in the surface region, e.g. analyses of tree rings or zonation of elements in minerals.
• The accuracy is high for the method. Normally the relative errors are between 1-10 % depending on element and matrix (sample composition).
• Small-size samples down to a few mg can be analyzed.
• The PIXE-technique is nondestructive. Most materials are not affected by the particle beam, and the samples are still intact after the analysis. This enables analyses of precious objects, e.g. ancient coins or paintings.