Particle-Induced X-ray Emission

Particle-Induced X-ray Emission or Proton Induced X-ray Emission (PIXE) is a technique used in the determining of the elemental make-up of a material or sample. When a material is exposed to an ion beam, atomic interactions occur that give off EM radiation of wavelengths in the x-ray part of the electromagnetic spectrum specific to an element.

Particle-induced X-ray emission (PIXE), is a powerful yet non-destructive elemental analysis technique now used routinely by geologists, archaeologists, art conservators and others to help answer questions of provenience, dating and authenticity.

Recent extensions of PIXE using tightly focused beams (down to 1 μm) gives the additional capability of microscopic analysis. This technique, called microPIXE, can be used to determine the distribution of trace elements in a wide range of samples.

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Theory

Three spectra can be collected from a PIXE experiment;

1. X-ray emission spectrum.
2. Rutherford backscattering spectrum.
3. Proton transmission spectrum.

X-ray Emission

Quantum theory states that orbiting electrons of an atom must occupy discrete energy levels in order to be stable. Bombardment with ions of sufficient energy (usually MeV protons) produced by an ion accelerator, will cause inner shell ionization of atoms in a specimen. Outer shell electrons drop down to replace inner shell vacancies, however only certain transitions are allowed. X-rays of a characteristic energy of the element are emitted. An energy dispersive detector is used to record and measure these X-rays.

Only elements heavier than Fluorine can be detected. The lower detection limit for a PIXE beam is given by the ability of the X-rays to pass through the window between the chamber and the X-ray detector. The upper limit is given by the ionisation cross section, the probability of the K electron shell ionisation, this is maximal when the velocity of the proton matches the velocity of the electron (10% of the speed of light), therefore 3 MeV proton beams are optimal.

Proton Backscatter

Protons can also interact with the nucleus of the atoms in the sample through elastic collisions, Rutherford backscattering, often repelling the proton at angles close to 180 degrees. The backscatter give information on the sample thickness and composition. The bulk sample properties allow for the correction of X-ray photon loss within the sample.

Proton Transmission

The transmission of protons through a sample can also be used to get information about the sample.

Protein Analysis

Protein analysis using microPIXE allow for the determination of the elemental composition of liquid and crystalline proteins. microPIXE can quantify the metal content of protein molecules with a relative accuracy of between 10% and 20%.^[1]

The advantage of microPIXE is that given a protein of known sequence, The X-ray emission from sulfur can be used as an internal standard to calculate the number of metal atom per protein monomer. Because only relative concentrations are calculated there are only minimal systematic errors, and the results are totally internally consistent.

The relative concentrations of DNA to protein (and metals) can also be measured using the phosphate groups of the bases as an internal calibration.

Analysis of Data

Analysis of the data collected can be performed by the programs Dan32,^[2] the front end to gupix^[3][4]

Limitations

In order to get a meaningful sulfur signal from the analysis, the buffer should NOT CONTAIN SULFUR, (i.e. no; BES, DDT, HEPES, MES, MOPSO or PIPES). Excessive amounts of chlorine in the buffer should also be avoided, this will overlap with the sulfur peak, KBr and NaBr are suitable alternatives.

Advantages

There are many advantages to using a proton beam over an electron beam, there is less crystal charging from Bremsstrahlung radiation, although there is some from the emission of Auger electrons, there is significantly less than if the primary beam was itself an electron beam.

Due to the higher mass of protons relative to electrons, there is less lateral deflection of the beam, this is important for proton beam writing applications.

Scanning

2 dimensional maps of elemental compositions can be generated by scanning the microPIXE beam across the target.

Cell / Tissue analysis

Whole cell and tissue analysis is possible using a microPIXE beam, this method is also referred to as Nuclear microscopy^{[citation needed]}.

Artifact Analysis

microPIXE is a powerful technique for the non-destructive analysis of paintings and antiques. The exact chemical properties of the material can be determined which can be useful for dating and preserving artifacts, and the exact recreation of old paints and glosses for restoration. ^{[citation needed]}

Proton Beam Writing

Proton beams can be used for writing through either the hardening of a polymer (by proton induced cross-linking), or through the degradation of a proton sensitive material. This may have important effects in the field of nanotechnology.

Notes and references

1. ^ Garman, E.F. and Grime, G.W. Elemental analysis of proteins by microPIXE. Progress in Biophysics and Molecular Biology, vol. 89/2, October 2005, 173-205.
2. ^ Geoffrey W Grime Dan32: recent developments in the windows interface to gupix. Tenth International Conference on Particle Induced X-ray Emission, Portoroz, Slovenia, 2004
3. ^ J.L. Campbell, J.A. Maxwell, W.J. Teesdale, The guelph-pixe software package-II. Nucl. Instrum. Methods B, 95 407-421, 2005.
4. ^ J.A. Maxwell, Z. Nejedly, J.L. Campbell, T.L. Hopman. The guelph pixe software package III: alternative proton database. Nucl. Instrum. Methods B, 170 193--204, 2000.