External-beam PIXE

Applications The main application fields of PIXE are earth science, air pollution studies (aerosol analysis), mineralogical studies, forensic science, arts and archaeology. In the external-beam PIXE technique, the proton beam is taken out to ambient air. This mode finds application in the analysis of art objects (paintings, books, etc.). 

Calligaro, T., Colinart, S., Poirot, J. P., and Sudres, C. (2002). Combined external-beam PIXE and mu-Raman characterisation of garnets used in Merovingian jewelry. Nuclear Instruments and Methods in Physics Research B 189 320-327. 

Internal beam and external beam PIXE chambers (a and b, respectively). See text for details... 

Fig. 1.32. A typical external beam PIXE spectrum of 13th century Iranian bowl...

Chlorine in oil is very difficult to analyze by conventional methods. External beam PIXE-PIGE method is adequate for the fast simple analysis of a small amount of oil sample, especially for determining chlorine content. Choi et al. (1998) made analysis of chlorine in industrial oil samples by external beam PIXE and PIGE. It has been found that because of the presence of sulfur in most industrial oils, PIGE has usually better detection limit than PIXE. 

In external beam or nonvacuum PIXE (in air or in a helium or nitrogen atmosphere), poorer detection limits are expected because of the background contribution from interactions in the beam exit foil material and in the air or chamber gas and, for the light elements, also because of the substantial attenuation of their soft X-rays by the same gases. However, practical detection limits in nonvacuum PIXE appear to be comparable to those in vacuum PIXE, at least for analyte elements with atomic number above 25. 

The bombarding particle beam is accelerated and transported in the high vacuum system of the accelerator. The X-ray detector unit is encapsulated in a separate clean vacuum system. The irradiation of the sample may be performed in vacuum or under atmospheric conditions, depending on the size, physical state, and other parameters of the sample. To define the physical conditions mentioned above, special PIXE chambers of internal beam and external beam configurations are used for in vacuo and atmospheric irradiations, respectively. 

In recent years, the external beam technique has been subjected to successive steps of improvements to meet the serious requirements with respect to its application in microbeam setups. Among others, the objectives were solving the problems of increased accuracy of beam monitoring, reducing the scattering of the beam in exit windows, and making possible the simultaneous application of PIXE with other ion beam analytical (IBA) methods. A detailed description of one of the latest external microbeam facility can be found in the work of Salomon et al. (2008). 

On the other hand, a typical PIXE spectrum of 13th century Iranian bowl taken in air using external beam is presented in Fig. 1.32. The two peaks each corresponding to Fe and Cu are their Ka and K(3 peaks, while the three peaks corresponding to Pb are its La, L(3, and Ly peaks. The presence of Argon peak in the spectrum is noteworthy. 

Besides micro-PIXE, macro-PIXE has also been extensively used in art and archaeology, and found to provide very useful information. Actually, the distinction between micro- and macro-PIXE is difficult to make in this field of application, as a lot of work is carried out with (external) probes with beam... 

As NRA has grown from accelerator-based nuclear physics and expanded after the invention of solid-state detectors (the surface barrier Si detector for the detection of particles and Ge(Li) detectors for the detection of y rays), its instrumentation is very much similar to those used in particle and y-ray nuclear spectroscopy. The PIXE method also started to use an existing instrumentation, the Si(Li) X-ray detectors, nearly a decade later. Consequently, this review will refer to the previous O Sects. 33.1 and O 33.2 on PIXE and RBS concerning the acceleration and the formation of energetic ion beams, the internal and external sample chambers, scanning particle microprobe facilities, particle detection, and data acquisition. It will only deal with the characteristic features of the detection of ions and y rays produced in nuclear reactions. Neutrons are also produced in these reactions, but in practice they are rarely used for NRA. Because of space limitations, that technique (Bird and Williams 1989) will not be discussed. 

The same approach has been turned to some polymer systems. It is not trivial to use external ion beams successfully, but using a helium atmosphere to reduce the energy loss and hroadening of the ion heam, the feasibility of this technique has heen demonsttated if not widely adopted. The external heam may he scanned (or the objea may be scanned) to reveal compositional changes over large areas. Again, PIXE is the most widely used technique in conjunction with external ion beams, but there has also been some use of nuclear reactions to study deuterated water vapor transmission in polyester films, where the fast protons ejected by the D( He,p)a reaaion can be detected. 

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