XRF and its types

X-RAY FLUORESCENCE (XRF) SPECTROMETRY AND ITS TYPES and limitations.

1. X-RAY FLUORESCENCE (XRF) SPECTROMETRY AND ITS TYPES Name: Hamza Suharwardi Roll No: MM-03 Course: Advanced Material Characterization Technique MM-(505) DEPARTMENT OF MATERIALS ENGINEERING

2. INTRODUCTION:- X-ray fluorescence (XRF) is the emission of characteristic "secondary" (or fluorescent)  X-rays  from a material that has been excited by being bombarded with high-energy X-rays or gamma rays . The phenomenon is widely used for elemental analysis and chemical analysis, particularly in the investigation of  metals,   glass , ceramics and building materials, and for research in geochemistry, forensic science, archaeology and art objects  such as  paintings. Most of the XRF instruments in use today fall into two categories: energy-dispersive (ED) and wavelength-dispersive (WD) spectrometers. Within these two categories is a tremendous variety of differing configurations, X-ray sources and optics, and detector technologies

3. PRINCIPLE:- XRF is based on the principle that individual atoms, when excited by an external energy source, emit X-ray photons of a characteristic energy or wavelength. By counting the number of photons of each energy emitted from a sample, the elements present may be identified and quantities.

4. THEORY:- When an electron beam of high energy strikes a material, one of the results of the interaction is the emission of photons which have a broad continuum of energies. This radiation, called braking radiation, is the result of the declaration of the electron inside the material.

5. Another result of the interaction between the electron beam and the material is the ejection of photoelectrons from the inner shells of the atoms making up the material. These photoelectrons leave with a kinetic energy which is the difference in energy between that of the incident particle and the binding energy of the atomic electron. This ejected electron leaves a "hole" in the electronic structure of the atom, and after a brief period, the atomic electrons rearrange, with an electron from a higher energy shell filling the vacancy. By way of this relaxation the atom undergoes fluorescence, or the emission of an X-ray photon whose energy is equal to the difference in energies of the initial and final states. Detecting this photon and measuring its energy allows us to determine the element and specific electronic transition from which it originated.

6. AUGER ELECTRON:- The Auger effect is a physical phenomenon in which the filling of an inner-shell vacancy of an atom is accompanied by the emission of an electron from the same atom. When a core electron is removed, leaving a vacancy, an electron from a higher energy level may fall into the vacancy, resulting in a release of energy.

7. SCHEMATIC DIAGRAM OF WAVELENGTH OF DISPERSIVE X-RAY FLUORESCENCE

8. WDXRF:- The WD-XRF analyzer uses a x-ray source to excite a sample. X-rays that have wavelengths that are characteristic to the elements within the sample are emitted and they along with scattered source x-rays go in all directions . A crystal or other diffraction device is placed in the way of the x-rays coming off the sample. A x-ray detector is position where it can detector the x-rays that are diffracted and scattered off the crystal. Depending on the spacing between the atoms of the crystal lattice (diffractive device) and its angle in relation to the sample and detector, specific wavelengths directed at the detector can be controlled. The angle can be changed in order to measure elements sequentially, or multiple crystals and detectors may be arrayed around a sample for simultaneous analysis Wavelength-dispersive X-ray spectroscopy is based on known principles of, how the characteristic x-rays are generated by a sample and how the x-rays are measured.

9. X-RAY GENERATIONS:- X-rays are generated when an electron beam of high enough energy dislodges an electron from an inner orbital within an atom or ion, creating a void. This void is filled when an electron from a higher orbital releases energy and drops down to replace the dislodged electron. The energy difference between the two orbitals is characteristic of the electron configuration of the atom or ion and can be used to identify the atom or ion. The lightest elements,  hydrogen, helium, lithium, Beryllium up to atomic number 5, do not have electrons in outer orbitals to replace an electron displaced by the electron beam and thus cannot be detected using this technique.

10. X-RAY MEASUREMENT:- According to Bragg's law, when an X-ray beam of wavelength "λ" strikes the surface of a crystal at an angle and the crystal has atomic lattice planes a distance "d" apart, then constructive interference will result in a beam of diffracted x-rays that will be emitted from the crystal at angle if nλ  = 2d sin Θ, where n is an integer . This means that a crystal with a known lattice size will deflect a beam of x-rays from a specific type of sample at a pre-determined angle . The x-ray beam can be measured by placing a detector (usually a scintillation counter or a proportional counter in the path of the deflected beam and, since each element has a distinctive x-ray wavelength, multiple elements can be determined by having multiple crystals and multiple detectors . To improve accuracy the x-ray beams are usually  collimated by parallel copper blades called a  Soller collimator The single crystal, the specimen, and the detector are mounted precisely on a goniometer with the distance between the specimen and the crystal equal to the distance between the crystal and the detector . It is usually operated under vacuum to reduce the absorption of soft radiation (low-energy photons) by the air and thus increase the sensitivity for the detection and quantification of light elements (between boron and  oxygen. The technique generates a spectrum with peaks corresponding to x-ray lines. This is compared with reference spectra to determine the elemental composition of the sample .

11. SPECTRAL LINES BY WDXRF:-

12. DETECTORS:- Detectors used for wavelength dispersive spectrometry need to have high pulse processing speeds in order to cope with the very high photon count rates that can be obtained. In addition, they need sufficient energy resolution to allow filtering-out of background noise and spurious photons from the primary beam or from crystal fluorescence. There are four common types of detector: gas flow proportional counters sealed gas detectors scintillation counters semiconductor detectors

13. SCHEMATIC DIAGRAM OF ENERGY OF DISPERSIVE X-RAY FLUORESCENCE

14. EDXRF :- The ED-XRF analyzer also uses an x-ray source to excite the sample but it may be configured in one of two ways. The first way is direct excitation where the x-ray beam is pointed directly at the sample. Filter made of various elements may be placed between the source and sample to increase the excitation of the element of interest or reduce the background in the region of interest. The second way uses a secondary target, where the source points at the target, the target element is excited and fluoresces, and then the target fluorescence is used to excite the sample . A detector is positioned to measure the fluorescent and scattered x-rays from the sample and a multichannel analyzer and software assigns each detector pulse an energy value thus producing a spectrum The peak positions are predicted by the  moseley's law with accuracy much better than experimental resolution of a typical edx instrument

15. COMPONENTS OF EDX Four primary components of the EDS setup are the excitation source (electron beam or x-ray beam) the X-ray detector the pulse processor the analyzer . X-ray beam excitation is used in X-ray fluorescence  (XRF) spectrometers . A detector is used to convert X-ray energy into  voltage  signals; this information is sent to a pulse processor, which measures the signals and passes them onto an analyzer for data display and analysis .   The most common detector used to be Si(Li) detector cooled to cryogenic temperatures with liquid nitrogen. Now , newer systems are often equipped with silicon drift detectors  (SDD) with  Peltier coolings ystems .

16. SILICON DRIFT DETECTOR:- There is a trend towards a newer EDS detector, called the silicon drift detector (SDD). The SDD consists of a high-resistivity silicon chip where electrons are driven to a small collecting anode. The advantage lies in the extremely low capacitance of this anode, thereby utilizing shorter processing times and allowing very high throughput. Benefits of the SDD include : High count rates and processing, Better resolution than traditional Si(Li) detectors at high count rates, Lower dead time (time spent on processing X-ray event), Faster analytical capabilities and more precise X-ray maps or particle data collected in seconds, Ability to be stored and operated at relatively high temperatures, eliminating the need for liquid nitrogen cooling

17. WAFER DETECTORS:- More recently, high-purity silicon wafers with low conductivity have become routinely available. Cooled by the  Peltier effect, this provides a cheap and convenient detector, although the liquid nitrogen  cooled Si(Li) detector still has the best resolution (i.e. ability to distinguish different photon energies). The pulses generated by the detector are processed by pulse-shaping amplifiers. It takes time for the amplifier to shape the pulse for optimum resolution, and there is therefore a trade-off between resolution and count-rate: long processing time for good resolution results in "pulse pile-up" in which the pulses from successive photons overlap. AMPLIFIER:-

18. COMPARISON BETWEEN EDXF AND WDXF:-

19. APPLICATIONS OF XRF:- It is a method of elemental (metal and Non metal ) analysis with atomic number greater than 12. Quantitative analysis can be carried out by measuring the intensity of fluorescence at the wavelength characteristics of the element being determined, especially applicable to most of the element in the periodic table. In medicine , Direct determination of sulfur in protein. The sulfur content of each of the many different forms in which protein exists in human blood varies considerably. XRF indicates protein distribution and provides a diagnostic link for the medical practitioner. Determination of chloride in blood serum. Determination of strontium in blood serum and bone tissue. Elemental analysis of tissues, bones and body fluids. It is used for detection of pesticides on fruits and herbal drugs

20. Hamza Suharwardi Researcher Slow and steady win the race. Any questions? You can find me at: [email protected] https://www.researchgate.net/profile/Hamza-Suharwardi-2