Presentation on theme: "X – Ray methods of Analysis : X – Absorption spectroscopy. X – Ray Fluorescence Spectroscopy. X – Ray Diffractometry."— Presentation transcript:
X – Ray methods of Analysis : X – Absorption spectroscopy. X – Ray Fluorescence Spectroscopy. X – Ray Diffractometry.
X-rays were discovered in 1895 by Wilhelm Röntgen while investigating the glow produced by cathode rays in a discharge tube. X-rays are short wavelength (typically l » 1Å) electromagnetic radiation The are produced either when accelerated energetic electrons bombard elemental metallic targets or energy is released from K shell transitions by electrons returning to the ground state. The principle of the X-ray tube is shown in figure, the design by Coolidge in The advances in vacuum technology was made possible to design a variety of X-Ray tubes. Less than 1% of the electron energy is converted into photons the rest is dissipated as heat within the target material. If the bombarding electrons have sufficient energy, they can knock an electron out of an inner shell of the target metal atoms. Then electrons from higher states drop down to fill the vacancy, emitting x-ray photons with precise energies determined by the electron energy levels. These x-rays are called characteristic x-rays.characteristic x-rays The short wavelength cut-off of an X-ray beam is determined by the tube voltage Production of X - Rays
Part of the kinetic energy is emitted in a continuous spectrum of X rays covering a broad wavelength range with a broad maximum in intensity and falling off to a definite short- wavelength limit. The emitted X - rays have longer wavelengths than the short-wavelength cutoff value of ‘λ o ’ It is independent of the target element and depends only on the voltage across the X-ray tube. At the cutoff wave length. The relationship between the voltage and ‘λ o ’ in angstrom units (A o ) is given by V is the X-Ray tube voltage in volts, e is the charge of the electron, h is Plank’s constant, and C is the velocity of light. This assumes that an electron looses all its kinetic energy in only one collision whereas in practice an electron will be involved in impacts with many atoms producing lots of low energy photons, this so called ‘Bremsstrahlung’ - braking radiation produces a continuous spectrum. Ionisation of the K shell by electrons with just the right amount of energy results in a characteristic line spectrum, when electrons in the atom return to the ground state with the emission of an x-ray photon.
(now the rotating anode variety is more widely used in medical applications) An X-ray tube is a vacuum tube that produces X-rays. They are used in X-ray machines. X-rays are part of the electromagnetic spectrum, an ionizing radiation with wavelengths shorter than ultraviolet light. X-ray tubes evolved from experimental Crookes tubes with which X-rays were first discovered in the late 1800s, and the availability of this controllable source of X-rays created the field of radiography, the imaging of opaque objects with penetrating radiation. X-ray tubes are also used in CAT scanners, airport luggage scanners, X-ray crystallography, and for industrial inspection.vacuum tubeX-raysX-ray machineselectromagnetic spectrumionizing radiationultravioletCrookes tubesradiographyradiationCAT scannersX-ray crystallography 99% of incident energy is converted to heat) the dissipation of heat at the focal spot is one of the main limitations on the power which can be applied. By sweeping the anode past the focal spot the heat load can be spread over a larger area, greatly increasing the power rating. With the exception of dental X-ray tubes, almost all medical X-ray tubes are of this type. Increasing demand for high-performance CT scanning and angiography systems has driven development of very high performance medical X-ray tubes. Contemporary CT tubes have power ratings of up to 100 kW and anode heat capacity of 6 MJ, yet retain an effective focal spot area of less than 1 mm2.CTangiography X-ray tube is basically a large vacuum tube containing a heated cathode and an anode or target of Copper or Molybdenum. Chromium, Iron, Nickel, & Tungsten are also used for special purposes. Electrons emitted by the cathode are accelerated through a high-voltage field between the target and cathode. Commonly used potentials are between 50 and 60 kV. Tubes with 100 kV rating also available for increased sensitivity. Higher voltage increases the intensity of lines emitted by the X – tube target On impact with the target, the stream of electrons is quickly brought to rest & transfer their kinetic energy to the atoms of the material that makes up the target.
X-ray tube : Crookes tube is the first device designed to generate X-Rays.Crookes tube As with any vacuum tube, there is a cathode, which emits electrons into the vacuum and an anode to collect the electrons, thus establishing a flow of electrical current,cathodeanode A high voltage power source, for example 30 to 150 kilovolts (kV), is connected across cathode and anode to accelerate the electrons.voltagekilovolts The X-ray spectrum depends on the anode material and the accelerating voltage.X-ray Electrons from the cathode collide with the anode material. Usually tungsten, molybdenum or copper, are the anode materials.tungstenmolybdenumcopper About 1% of the energy generated is emitted/radiated, usually perpendicular to the path of the electron beam, as X-rays. The rest of the energy is released as heat. Over time, the anode metal will be deposited from the target onto the interior surface of the tube, including the glass surface. This will slowly darken the tube and was thought to degrade the quality of the X-ray beam, The range of photonic energies emitted by the system can be adjusted by changing the applied voltage, and installing aluminum filters of varying thicknesses. Aluminum filters are installed in the path of the X-ray beam to remove "soft" (non-penetrating) radiation. The Crookes tube was improved by William Coolidge in The Coolidge tube, also called hot cathode tube, is the most widely used. It works with a very good quality vacuum (about 10-4 Pa, or 10-6 Torr).Crookes tubeWilliam Coolidge Crookes X-ray tube from early 1900s. The cathode is on the right, the anode is in the center with attached heat sink at left. The electrode at the 10 o'clock position is the anticathode. The device at top is a 'softener' used to regulate the gas pressure.
Coolidge tube In the Coolidge tube, the electrons are produced by thermionic effect from a tungsten filament heated by an electric current.thermionic effecttungstenfilament The filament is the cathode of the tube. The high voltage potential is between the cathode and the anode, the electrons are thusaccelerated, and then hit the anode.accelerated There are two designs: end-window tubes and side- window tubes. In the end-window tubes, the filament is around the anode, the electrons have a curved path. In the side-window tube, An Electrostatic Lens to focus the beam onto a very small spot on the anodeElectrostatic Lens The anode is specially designed to dissipate the heat and wear resulting from this intense focused barrage of electrons the area heated is cooled by circulating coolant. The anode is precisely angled at 1-20 degrees off perpendicular to the electron current so as to allow escape of some of the X-ray photons which are emitted essentially perpendicular to the direction of the electron current. The anode is usually made out of tungsten or molybdenum. The tube has a window designed for escape of the generated X-ray photons. The power of a Coolidge tube usually ranges from 1 to 4 kW.kW Coolidge side-window tube (scheme) K: filament A: anode Win and Wout: water inlet and outlet of the cooling device (C)
Rotating anode tube The rotating anode tube is an improvement of the Coolidge tube in X-Ray production. X-ray production using this method is very efficient The anode consists of a disc with an annular target close to the edge. The anode disc is supported on a long stem which is supported by bearings within the tube. The anode can then be rotated by electromagnetic induction from a series of stator windings outside the evacuated tube. Because the entire anode assembly has to be contained within the evacuated tube, heat removal is a serious problem, further exacerbated by the higher power rating available. Direct cooling by conduction or convection, as in the Coolidge tube, is difficult.conductionconvection In most tubes, the anode is suspended on ball bearings with silver powder lubrication which provide almost negligible cooling by conduction. A recent development has been liquid gallium lubricated fluid dynamic bearings which can withstand very high temperatures without contaminating the tube vacuum.galliumfluid dynamic bearings The large bearing contact surface and metal lubricant provide an effective method for conduction of heat from the anode. The anode must be constructed of high temperature materials. The focal spot temperature can reach 2500°C during an exposure, and the anode assembly can reach 1000°C following a series of large exposures. Typical materials are a tungsten-rhenium target on a molybdenum core, backed with graphite.molybdenum The rhenium makes the tungsten more ductile and resistant to wear from impact of the electron beams.rheniumtungsten The molybdenum conducts heat from the target.molybdenum The graphite provides thermal storage for the anode, and minimizes the rotating mass of the anode.graphite Simplified rotating anode tube schematic A: Anode C: cathode T: Anode target W: X-ray window
In an X-ray diffraction measurement, a crystal is mounted on a goniometer and gradually rotated while being bombarded with X-rays, producing a diffraction pattern of regularly spaced spots known as reflections. The two-dimensional images taken at different rotations are converted into a three-dimensional model of the density of electrons within the crystal using the mathematical method of Fourier transforms, combined with chemical data known for the sample. Poor resolution (fuzziness) or even errors may result if the crystals are too small, or not uniform enough in their internal makeup.crystalgoniometerFourier transforms
X-rays primarily interact with electrons in atoms. When x-ray photons collide with electrons, some photons from the incident beam will be deflected away from the direction where they originally travel, much like billiard balls bouncing off one anther. If the wavelength of these scattered x-rays did not change (meaning that x-ray photons did not lose any energy), the process is called elastic scattering (Thompson Scattering) in that only momentum has been transferred in the scattering process. These are the x-rays that we measure in diffraction experiments, as the scattered x-rays carry information about the electron distribution in materials. On the other hand, in the inelastic scattering process (Compton Scattering), x-rays transfer some of their energy to the electrons and the scattered x-rays will have different wavelength than the incident x-rays. Diffracted waves from different atoms can interfere with each other and the resultant intensity distribution is strongly modulated by this interaction. If the atoms are arranged in a periodic fashion, as in crystals, the diffracted waves will consist of sharp interference maxima (peaks) with the same symmetry as in the distribution of atoms. Measuring the diffraction pattern therefore allows us to deduce the distribution of atoms in a material. The peaks in a x-ray diffraction pattern are directly related to the atomic distances. Let us consider an incident x-ray beam interacting with the atoms arranged in a periodic manner as shown in 2 dimensions in the above illustrations. The atoms, represented as green spheres in the graph, can be viewed as forming different sets of planes in the crystal (colored lines in graph on left). For a given set of lattice planes with an inter-plane distance of d, the condition for a diffraction (peak) to occur can be simply written as
Scheme of X-ray spectrometer ( Block diagram ) 1 high voltage unit with stabilizer, 2 current stabilizer, 3 X-ray tube, 4 slit, 5 collimator, 6 specimen, 7 detector, 8 amplifier, 9 scaler, 10 rate meter 11 printer 12 recorder The scheme of a goniometer (S source, C l & C 2 coIlimtros, D detector) Collimator: It is a device that filters a stream of rays so that only those traveling parallel to a specified direction are allowed through. Collimators are used in neutron, X-ray, and gamma-ray optics because it is not yet possible to focus radiation with such short wavelengths into an image through the use of lenses as is routine with electromagnetic radiation at optical or near-optical wavelengths. Source of radiation Un-collimated radiation Collimated radiation Stack of parallel slits or bunch of fine tubes acts as collimator
Various elemental effects Results Using a Mo X-Ray generator: ZirconiumZirconium - Absorbs Bremsstrahlung & K-Beta. Iron - Absorbs the entire spectra. Molybdenum - Absorbs Bremsstrahlung - Leaving K-Beta & K-Alpha. Aluminium - 'Pinches' Bremsstrahlung* & Removes 3rd Generation peaks. Silver - Same as Aluminium, But to greater extent. Indium - Same as Iron, But to lesser extent. Copper - Same as Aluminium, Leaving only 1st Generation Peaks.BremsstrahlungK-Beta Iron MolybdenumK-BetaK-Alpha Aluminium Silver Indium Copper - Bremmstrahlung pinching is due to the atomic mass. The denser the atom, the higher the X-Ray Absorption. Only the higher energy X-Rays pass through the filter, appearing as if the bremmstrahlung continuum had been pinched. - In this case, Mo appears to leave K-Alpha and K-Beta alone while absorbing the Bremsstrahlung. This is due to Mo absorbing all of the spectra's energy, but in doing so produces the same characteristic peaks as generated by the target.
X-Ray Diffraction Tubes with copper targets, which produce their strongest characteristic radiation (K 1) at a wavelength of about 1.5 angstroms, are commonly used for geological applications. If an incident X-ray beam encounters a crystal lattice, general scattering occurs. Although most scattering interferes with itself and is eliminated (destructive interference), Diffraction occurs when scattering in a certain direction is in phase with scattered rays from other atomic planes. Under this condition the reflections combine to form new enhanced wave fronts that mutually reinforce each other (constructive interference). The relation by which diffraction occurs is known as the Bragg law or equation. Because each crystalline material has a characteristic atomic structure, it will diffract X-rays in a unique characteristic pattern.
An X-ray diffraction pattern of a crystallized enzyme. The pattern of spots (called reflections) can be used to determine the structure of the enzyme.
The Gemini A Ultra is mounted with a 135 mm diagonal Atlas CCD detector (2:1 demagnification and 2k x 2k Kodak 4-port CCD chip) and co- mounted Enhance (Mo) and Enhance Ultra (Cu) X-ray sources. The Enhance Ultra providing a rotating anode alternative high brilliance copper X-ray source.
The basic geometry of an X-ray diffractometer involves a source of monochromatic radiation and an X-ray detector situated on the circumference of a graduated circle centered on the powder specimen. Divergent slits, located between the X-ray source and the specimen, and divergent slits, located between the specimen and the detector, limit scattered (non-diffracted) radiation, reduce background noise, and collimate the radiation. The detector and specimen holder are mechanically coupled with a goniometer so that a rotation of the detector through 2x degrees occurs in conjunction with the rotation of the specimen through x degrees, a fixed 2:1 ratio. A curved-crystal monochromator containing a graphite crystal is normally used to ensure that the detected radiation is monochromatic. When positioned properly just in front of the detector, only the K radiation is directed into the detector, and the Kß radiation, because it is diffracted at a slightly different angle, is directed away. The signals from the detector are filtered by pulse-height analysis, scaled to measurable proportions, and sent to a linear ratemeter for conversion into a continuous current. Common output devices include strip-chart recorders, printers, and computer monitX-ray diffractometer
X – Ray Diffractometer
Techniques employing the mass absorption coefficient or the critical absorption edges are used for measurement and analysis. It has been known that the mass absorption coefficient, neglecting scattering, is proportional to the 4th power of the atomic number Z, and cubit power of the wavelength. Thus X – Ray absorptiometer : where K is a constant. Hence, if absorption can be measured it is only simple to trace out the constituent by simple relations as in the above equation. The measurement method uses the comparison principle. White or monochromatic X-ray is allowed to pass through two channels alternately, consisting of a reference and the test. The output falls on a fluorescent screen followed by a light funnel and a photo- multiplier or a scintillation detector. By driving in or out a variable thickness wedge attenuator in the reference channel, the photomultiplier output is held constant so that the output of the comparator or better at the servo-output. i.e.. coupled to the wedge movement. The measurement may be made continuously on-line and is most suitable for binary mixtures. A typical example is analysing lead in petrol, i.e., a heavier element in lighter ones.
The general arrangement of a nondispersive X-ray absorptiometer is shown in Figure A tungsten target X-ray tube is operated at 15—45 kV. In the X-ray beam is a synchronous motor-driven chopper that alternately interrupts half of the X-ray beam. A variable-thickness aluminum attenuator (in the shape of a wedge) is placed between the chopper and the reference sample compartment. Duplicate reference and sample cells up to 65 cm long can be accommodated and those for liquids and gases arranged for continuous-flow process stream analyses if desired. Both halves of the X-ray beam fall on a common phosphor-coated photomultiplier tube, which is protected from visible light by a thin metallic filter. In operation, a reference sample is placed in the appropriate cell and the specimen to be analyzed in the sample tube. The attenuator is adjusted until the absorption in the two X-ray beams is brought into balance. The change in thickness of aluminum required for different samples is a function of the difference in composition.
Prior calibration enables a determination in terms of the solute in an unknown. Liquids are simplest to handle. With solids the thickness of solid specimens and the density of samples of powders must be uniform to a precision greater than that expected in the results. Polychromatic absorptiometry can be used to determine chlorine in hydrogen. Sulfur in crude oil can be distinguished from the carbon-hydrogen residuum. Other examples are barium fluoride in carbon brushes, barium or lead in special glasses, and chloride in plastics and hydrocarbons. In fact, the method is applicable to any sample that contains one element that is markedly heavier than the others and when the matrix is essentially invariant in concentration.
The commonly used techniques in this type are (i)The wavelength dispersive type, (ii) The energy dispersive type and (iii) The non-dispersive type. Single channel wave-length dispersive type : In single channel type X-ray fluorescence spectrometry. a pair of X-ray sources is used with W- target for shorter wavelength and another with Cr for longer wavelength. Crystals also are changed and a He- atmosphere or vacuum for longer wavelength is maintained. Multi channel types are large systems requiring as many crystals and detectors as the channel numbers. Each detector has its own amplifier, pulse height selector, scaler, counter integrator etc. The best way to tackle the operation of such an instrument is to use a computer/micro-processor which also helps in its control, data processing and display or results. X – Ray Fluorescence Spectrometer : Wave length dispersive type of single channel fluorescence spectrometer
The energy dispersive type does not require collimators and crystal diifractors. They also contain no moving components in the excitation and detection sides. Here source is a polychromatic one and the detector is the semiconductor type. Detector is closely mounted and hence energy reaching the detector is very high. Energy dispersive type Fluorescence diffractometer : The instrument has low resolution at longer wavelengths, longer thaii 1 m. In its multi channel variety all the lines are measured simultaneously and the advantage of FT spectrometry can be taken for improved signal to noise ratio. Wave length detected angle indicating the element and the peak the content of the element. In the non-dispersive type a sample containing a certain element is irradiated with X-ray of suitable wavelength which produces a fluorescent line of the element. This radiation is then passed through a pair of filters with absorption edges just above and below the fluorescent wavelength. The difference in the detected signals in the two cases is in proportion to the amount of element present in the sample. Commercial routine instruments in this category have an accuracy of about 1%.
A schematic set-up for fluorescent spectrometer for liquid samples is shown in the figure. The samples are kept in beakers on a rotating table, the samples are also rotated for uniform exposure. Radiation from an X-ray generator strikes a specimen to produce characteristic fluorescent lines. The fluorescent radiation is analysed by a rotating crystal in an X-ray goniometer. In the goniometer, the primary slit, the analyzing crystal and the secondary slit are on the focal circle for satisfying the Bragg’s conditions. Fluorescent spectrometer for liquid samples : This method can be used for analysis of solid, liquid and gas samples as well but is commonly used for heavier elements and can measure down to a value of 1 ppm. Interference occurs due to absorption, particularly if the fluorescent radiations of the detectants are absorbed by another constituent in the sample. For example, Pb absorbs fluorescent radiation of chlorine as also the main radiation.
Radiation Detectors : Detection actually means measurement of the radiation with its energy content and other related properties. The detection system would, therefore, consist of a detector and a measuring circuit. The detectors are to be chosen according to the requirement. Photographic film is used for general diffraction studies. The GM counter and proportional; counter which are gas ionization types, the scintillation counters and the semiconductor detectors are being increasingly used for detection purposes. The semiconductor type is used for soft or low energy radiation detection. The choice is to a large extent dependent on the X-ray energy to be detected.
The interaction of radiation in different types of detectors is of different nature. There are, two different methods of measurement after detection of the radiation. One is the counter type in which the detector output, in the form of a series of pulses, is counted by a counter over a certain interval of time. Each pulse represents an interaction of radiation with the detector element. The other is mean level detection, where the pulses that occur are so high per unit of time that resolution to individual ones is not possible and an average effect, generally in the form of current output, is measured. Set up of gas-tilled detector chamber Ionizatio chambers including proportional and Geiizer Muller counters employ gas filled chambers with varying applied voltages to the electrodes. A central electrode separated from the chamber by insulator is kept at a potential E. with respect to the chamber wall and this is maintained through the shunt combination of reistsnace R and capacitance C. The RC product is chosen to be much larger compared to the time required by any ion in the chamber to be collected by appropriate electrode. For a charge q across C, the voltage output is When radiation falls on the chamber, let -ion pairs be produced and let them drift towards the oppositely charged electrodes. Then the pulse voltage magnitude, being the measure of the ions collected by the electrode at various supply voltages.
Radiation Detectors : Pulse amplitude as a function of applied voltage for the ionization type of detectors. If the pulse voltages are plotted for various applied voltages, it is as shown in the figure. The curve is divisible in to several regions with respect to applied voltages. I II III IV V