Micro PIXE

Neutron Activation Analysis X-Ray Fluorescence Particle-Induced X-Ray Emission Particle-Induced Nuclear Reaction Analysis Rutherford Backscattering Spectrometry Spark Source Mass Spectrometry Glow Discharge Mass Spectrometry Electron Microprobe Analysis Laser Microprobe Analysis Secondary Ion Mass Analysis Micro-PIXE... 

The sample environment was filled with He gas to prevent the argon X-ray emission from air. Beam scanning, data acquisition, evaluation and the generation of elemental maps were controlled by a computer. Micro-PIXE measurements were performed with a scanning 2.5MeVH+ microbeam accelerated by the 3 MV single-end accelerator. The beam diameter was 1-2 pm, so that individual particles could be analysed. The beam current was < 100 pA and the irradiation time was about 3(M0 min. 

Figure 4.20. Schematic diagram of the micro-PIXE beam scanning and data acquisition...

Figure 4.21. Micro-PIXE spectrum of a coarse particle (> 1.17 pm) collected during an Asian dust storm. Beam 2.5MeVH+, current 70pA, irradiation time 30min. (Kasahara et al. 2001.)...

Faiz et al. (1996) have applied micro-PIXE analysis to study solute distributions in a single crystal sample of YiBa2Cu307 5 high temperature superconductor (YBCO) of dimensions 1.3 mm x 1.5 mm x 75 pm. It contained a small secondary crystal overgrowth of dimensions 340 x 340 x 100 pm3. The interface region between the smaller crystal and the base crystal was covered with a material which appeared to be residual flux. The instrument employed a 2.5 MeV focused proton beam of about 4 pm resolution, which could scan an area of 500 x 500 pm2 on the sample surface. The microbeam current was kept low (typically about 30 pA) to avoid any damage to the sample. 

Figure 4.24 shows the spatial distribution maps obtained from Y, Ba, Cu and Au by micro-PIXE. 

PIGE is a rapid, non-destructive technique that is employed in the analysis of light elements such as lithium (10-100 ppm limit of detection), boron (500-1000 ppm limit of detection), and fluorine (1-10 ppm limit of detection), which are often difficult to determine by other analytical means. Because the technique is based upon specific nuclear reactions, the sensitivity of PIGE varies greatly from isotope to isotope, and this non-uniformity of sensitivity has limited its widespread use as a complementary technique to micro-PIXE. 

PIXE Several tends of microns N/A In cross-sections Yes Yes Yes 1-10 pm in micro-PIXE... 

Kramer U, Grime GW, Smith JAC, Hawes CR, Baker AJM. Micro-PIXE as a technique for studying nickel localization in leaves of the hyperaccumulator plant Alyssum lesbiacum. Nucl Instr Methods B 1997 130 346-350. 

Earlier studies of PGE distribution in magmatic sulfides have used microprobe, SIMS and micro-PIXE (e.g. Cabri et al. 

Cabri, L.J., Sylvester, P., Tubrett, M., Peregoedova, a., Tubrett, M., LaFlamme, J.H.G. 2003. Comparison of LAM-ICP-MS and micro-PIXE results for palladium and rhodium in selected samples of Noril sk and Talnakh sulfides. The Canadian Mineralogist, 41, 321-329. 

Figure 11 Schematic diagram of the in-air micro-PIXE system.

In 2006, Lobinski et al.1 reported on the imaging and speciation analysis of trace elements to study the element distribution, oxidation state, metal site and metal structure in biological environments using mass spectrometric techniques (LA-ICP-MS, SIMS, MALDI-MS) and non-mass-spectrometric techniques such as micro-PIXE (proton induced X-ray emission), XANES (X-ray absorption near edge structure) and EXAFS (extended X-ray absorption fine structure) -the latter two techniques are very sensitive due the use of a more intense synchrotron beam.1... 

An important variant on PIXE is micro-PIXE. By using a proton beam whose spatial dimension is 0.5 pm (rather than the usual 10 mm), one can determine the trace-element content of a small portion of the sample, giving one a trace-element microscope. This application is important in probing samples of medical interest. A related technique is used in the electron microprobe where the ionization is caused by electron impact. 

The following example is a illustration of how photon and charged particles techniques like the micro-PIXE and micro-Raman spectrometry can be combined to solve an important archaeological issue, namely to unveil the intriguing provenance of the red gemstones mounted on barbarian jewels from the early Middle-Ages [11]. 

The application of PIXE (Proton Induced X-Ray Emission) and micro-PIXE to quantitative analysis of impurities in polyethylene are described. The equipment, sources of background which affect the sensitivity and the precision of PIXE measurements are discussed for both thick and thin targets. A number of applications of micro-PIXE to the identification and location of trace elements in the "Trees" found in the polymer insulation of HV cables are presented. 

A brief discussion of the physical principles of PIXE as they relate to the background, sensitivity, and precision of the method will be presented. The design and performance of a typical experimental system will be illustrated by a description of both the conventional PIXE and micro-PIXE setups at the Laboratoire de Physique Nucl6aire of the Universite de Montreal. 

Electrical trees are essentially breakdown channels whose size, typically 50 to 200 microns, together with the large variations in impurity concentrations in the surrounding polyethylene, makes the identification of the impurities associated with both kinds of trees very difficult by traditional techniques. The use of micro-PIXE for the location and analysis of trace elements in electrical and water trees found in the polyethylene insulation of high voltage cables will be described. 

Data Analysis. For both the macro- and micro-PIXE systems an on line display of the X-ray and RBS spectra is provided by an LSI 11/23, while detailed analysis of the RBS and PIXE spectra are performed using the programs RUMP (1L) and MENUGF (UL) The latter analyzes the 1024... 

Figure 4. The electrostatic quadrupole triplet lens and target chamber used for micro PIXE showing the 30 sq mm Si(Li) X-ray detector and the absotber ladder at 135 the RBS detectors which are at 158 above and below the lens the Ge(Li) PIGE detector and collimator the electron flood gun and the movable zoom microscope used for focusing and alignment of the beam.

To illustrate the use of PIXE and micro-PIXE in the study of breakdown phenomena in polyethylene high voltage cable insulation and other related topics we will describe a few typical measurements, first the study by standard PIXE of impurities in the organic semiconductor H2PC and in the carbon black semicon used in high voltage cables. Examples of the use of the microbeam to study some electrical and water trees as well as the diffusion of impurities from the semicon into polyethylene under typical electric field and humidity conditions will be given. 

Unfortunately PIXE does not have any significant depth resolution and thus does not provide any information on the impurity distribution in this dimension. Micro-PIXE data from this tree taken at Ep = 2.5 MeV confirmed the presence of K, Ca, Fe and Cu. This data also showed the presence of Zn and Pb, as well as 20 ppm of Ti at a single point near the center of the tree. 

Figure 10. Micro-PIXE spectra of polyethylene at a depth of 50 0 lm from the interface with a layer of semicon containing commercial carbon black. A) after two weeks in dry air at 55 °C, B) after two weeks immersed in water at 55 °C and C) the virgin polyethylene.

Micro-PIXE with a 20 micron beam spot has proved to be a versatile tool for investigations of water and electrical trees, as well as a variety of other samples. This system is simple to operate and does not require complex target preparation, so that multi-element surveys of a variety of samples can be rapidly performed. 

More work remains to be done to better understand the role of impurities on water tree growth and we feel that micro-PIXE is a very powerful technique for such measurements since it has the required sensitivity and spatial resolution to provide detailed contour maps of the impurity concentrations, which can then be correlated with the visual tree. Our present micro-PIXE equipment with its 20 micron diameter beamspot is ideally suited for such measurements as it is very easy to use and provides online data. However, the use of only a few point measurements could miss essential components of the tree, and raster scans of the whole tree area would provide more complete information, and at the same time reduce beam induced damage. 

Figure 8. Micro-PIXE spectra from the bow tie tree (a) summed over seven points along the axis on the side B of the bow tie tree (b) summed over side A of the sample (c) at the center of the tree side on side B and (d) at the center of the tree on side A.

Kachenko, A.G., Bhatia, N.P., Singh, B., and Siegele, R. 2007. Arsenic hyperaccumulation and localization in the pinnule and stipe tissues of the gold-dust fern (Pityrogramma calo-melanos) (L.) Link var. austroamericana (Domin) Farw. using quantitative micro-PIXE spectroscopy. Plant and Soil, 300 207-19. 

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