Particle induced X-ray emission (PIXE) (Continued)
| In this lecture we will see various applications of PIXE. Because of direct information of the elements from their characteristics x-rays, PIXE has been used as a tool for wide range of materials analysis. PIXE is also often used in geology, archaeology, biology and environmental pollution. The following are typical outcome of a PIXE analysis: | ||
| 1. Within a short time (3-10min) all elements (Na) can be analyzed simultaneously. 2. For all elements sensitivity is quite high and equal. 3. Extremely small amount of samples (1mg) can be quantitatively analyzed. 4. Some untreated samples. (Hair, Urine, Nail, Blood, Organ etc.) can be analyzed non-destructively. 5. Analysis of liquid samples like an oil, bio-samples staying in alive, large samples. such as an ancient pottery, powdered samples such as soil, ash and aerosol etc. 6. If a micro-beam is used in PIXE an micron scale elemental mapping is possible. | ||
| One way of quantification using PIXE is what is known as internal standard method. The sequence of this analysis is as follows: (i) Weight measurement of the sample and the internal standard, (ii) Addition of internal standard and unknown sample and homogenization. (iii) Preparation of the target on a film (e.g. on mylar). (iv) PIXE analysis of the target. The peaks corresponding to unknown and standard are analyzed. The Atomic fraction of the specimen with respect to standard can be quantified from the following equation: | ||
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| Where, Xu, Yu and σu stand for atomic fraction, peak yield and cross section of unknown and other three quantities are same corresponding to standard. Before going to analyze a PIXE spectrum it is important to perform proper energy calibration. The analog signal generated by a PIXE detector is fed to an analog to digital converter (ADC). The ADC channel number (nch) can be approximated as linearly proportional to the incident X-ray energy Ein as per following relation: | ||
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| Coefficients α and β are energy calibration factors, which decide the position of X-ray peak in the spectrum. The FWHM of the peaks are also calibrated as per the following relation: | ||
/equ5_24.png){kind=small} | ||
| Where, λ and η are the calibration factors to be determined. All these parameters are needed for fitting theoretical spectrum to experimental one. As mentioned earlier that PIXE has an advantage due to its ability to make reliable measurements down to ppm level (μg/g). This is due to the fact that the background signal of PIXE is quite low. | ||
| Let us now see what are the sources of background and how to eliminate them: | ||
| (i) Interaction between high energy protons and low mass elements can lead to nuclear reactions involving gamma rays. The scattering of gamma rays will make a continuous background. If the energy of the proton is in the range of 2-3 MeV, these nuclear reactions can be avoided and the the continuous background of the spectrum can be eliminated. (ii) Electrical charging can be generated to a nearby conductor due to charging of the sample by proton beam and may add to the background signal. This can be eliminated by coating a conductive layer on the sample or spraying it with electrons to neutralize the charges. | ||
| Let us know about data analysis of PIXE: | ||
| (i) The first step is to decompose the overlapped features of the x-ray spectrum, and to estimate background contributions to the spectrum. (ii) Theoretical relative line (Kα, K β, Lα) intensities for each elements are computed using a computer programme. (iii) Line shapes are also estimated. (iv) Using non-linear least square iterations of peak intensities and position parameters, a model spectrum can be generated using a standard software (e.g. GUPIX [x]) and thereby intensities of all peaks are calculated. (v) A chi square test is used after that to determine the degree of correspondence between experimental and theoretical intensities. (vi) Finally, after calculation of intensity of each peak, for each element, the standard theoretical formulation (as given above are used to determine) involving, X-ray fluroscence yield, ionization cross section of protons for each element, detector efficiency, detector transmission and calibration factor. (vii) In practice, a standard (as mentioned before) is measured before and after every set of samples and a proper calibration is made. | ||
| Now I will make a comparison between PIXE and standard X-ray fluroscence (XRF) method: | ||
| Both PIXE and XRF are based on analysis of characteristics X-ray based element quantification. In XRF, X-ray is used to eject inner shell electrons, whereas in PIXE, proton beam is used for this purpose. As X-ray can penetrate more in a solid, XRF is bulk sensitive. However, greater depth analysis requires more corrections for interelement and matrix effect. Proton beams in PIXE in other hand can be used to generate X-rays for entire range of elements present in a sample without adding much background error, thereby decreasing the detection limit. | ||