Particle induced X-ray emission (PIXE)
| In particle induced X-ray emission or PIXE one measures X-ray arising from the filling of inner shell vacancies produced by the projectiles. The x-ray energies are the characteristics of the elements. Following physical processes are involved in PIXE process: (i) inelastic collisions between incoming charged particle with target atoms, | ||
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Fig. m5.13 Schematic of PIXE, indicating emission of x-rays to charged particle materials interaction from certain depth of a material. | ||
| The total PIXE yield from any element (n) with a concentration profile c(x) and cross section σn | ||
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| where Q is the total amount of incident charge. For a constant concentration profile of nth element c(x) is constant and if the detector efficiency is e, then the X-ray intensity recorded at the detector and the concentration Cn can be written as | ||
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| Where, | ||
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| The Nn value or the calibration factor remains constant if the geometry of the experiment and the sample matrix composition do not change. The measurement precision depends on the following factors: | ||
| (i) particle energy (ii) accurate calculation of Q (iii) geometry (iv) sample matrix composition (v) detection efficiency (vi) Uncertainty in the calculation of I. Let us now try to understand the X-ray production cross section σn. This cross section is related to the ionization cross section σ through the fluroscence yield, ωx; σn = ωx σ. The fluroscence yield is the probability of a radiative transition relative to all possible transitions (radiative and nonradiative). It has been observed that with increase in atomic number the maximum value of cross section decreases. That means it decreases with increase in binding energy. The variation of X-ray production cross section for various elements with incident proton energy is shown in is shown in Fig. m5.14. | ||
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Fig. m5.14 The cross section σK α versus proton energy for various elements. | ||
| PIXE Instrumentation | ||
| Before we go to applications of PIXE let us see the basic instrumentation of PIXE. A typical PIXE instrumentation is shown in Fig. m5.15. | ||
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Fig. m5.15 The schematic of PIXE instrumentation | ||
| The high energy beam produced in the accelerator are collimated by two collimation stages before it strike the sample kept in the high vacuum chamber. Uniform distribution of the beam across the cross section defined by a collimator is achieved generally in two ways: (i) by intentionally diffusing the beam before it enters the collimator, or (ii) by scanning the focused beam across the particular region of the sample. Sometimes the beam is focused down to micron size spots. A series of magnetic lenses are required for that. The x-ray photons generated by incident ion and the material of the sample under study are detected in a high resolution x-ray detector. Generally semiconductor detectors are used for this purpose. The direct measurement of the total charge delivered to a sample can be done by connecting electrically the sample holder (insulated from the chamber) and the Faraday cup to the charge integrator. If the sample is thin enough the beam transmitted through the sample will be stopped in the beam stop (Faraday cup). For the thick specimens, the charge will be collected directly from the sample holder. A secondary electron suppressor around the sample is often used for accurate estimation of charges. | ||