In X-ray fluorescence (XRF), an electron can be ejected from its atomic orbital by the absorption of a light wave (photon) of sufficient energy. The energy of the photon (h?) must be greater than the energy with which the electron is bound to the nucleus of the atom. When an inner orbital electron is ejected from an atom (middle image), an electron from a higher energy level orbital will be transferred to the lower energy level orbital. During this transition a photon maybe emitted from the atom (bottom image). This fluorescent light is called the characteristic X-ray of the element. The energy of the emitted photon will be equal to the difference in energies between the two orbitals occupied by the electron making the transition. Because the energy difference between two specific orbital shells, in a given element, is always the same (i.e. characteristic of a particular element), the photon emitted when an electron moves between these two levels, will always have the same energy. Therefore, by determining the energy (wavelength) of the X-ray light (photon) emitted by a particular element, it is possible to determine the identity of that element.
For a particular energy (wavelength) of fluorescent light emitted by an element, the number of photons per unit time (generally referred to as peak intensity or count rate) is related to the amount of that analyte in the sample. The counting rates for all detectable elements within a sample are usually calculated by counting, for a set amount of time, the number of photons that are detected for the various analytes' characteristic X-ray energy lines. It is important to note that these fluorescent lines are actually observed as peaks with a semi-Gaussian distribution because of the imperfect resolution of modern detector technology. Therefore, by determining the energy of the X-ray peaks in a sample's spectrum, and by calculating the count rate of the various elemental peaks, it is possible to qualitatively establish the elemental composition of the samples and to quantitatively measure the concentration of these elements.
X-ray tube excitation
Like the formerly common vacuum tubes, X-ray tubes are comprised of a cathode - which emits electrons into the vacuum - and an anode to collect the electrons, thus establishing a flow of electrical current through the tube. A high voltage power source, for example 4 to 150 kilovolts (kV), is connected across cathode and anode to accelerate the electrons to impact the anode. The X-ray spectral output of an X-ray tube, which includes both characteristic lines from the anode material and Bremsstrahlung radiation, depends on the anode material and the accelerating voltage.
A common service or repair item, X-ray tubes must be replaced periodically for a variety of reasons, including: broken filament, pitted anode (spectral purity), current leakage, and vaccum loss.
Radioisotope source excitation
Made of encapululated artificially created isotopes of various elements, radioisotope sources have the advantage that they do not need a supply of electrical power to function and are semi-monochromatic, but they also have the enormous disadvantage that they can not be switched off. Because of the obvious safety issues associated with any intrinsically radioactive material, and the threat posed by the potential misappropriation and weaponization of radioactive sources - such as Iron-55 (55Fe), Cadmium-109 (109Cd), Cobalt-57 (57Co), Americium-241 (241Am), and Curium-244 (244Cm), use of radioisotopes has fallen out of favor for use in EDXRF, XRF, and XRT spectrometers and related gauges.
A common service or repair item, radioisotope (RI) sources - as used in ASOMA or SPECTRO gauges - must be replaced if the activity falls to the point where the analytical results are no longer satisfactory due to poor counting statistics. Leakage from, or breach of, source capsule also requires source replacement and/or source disposal.
Proportional counter EDXRF detectors
A proportional counter (PC) is a type of gaseous ionization detector - it works on the same principle as the Geiger-Müller counter, but uses a lower operating voltage. An inert gas is used to fill the tube, with a quench gas added as a stabilizer. A common proportional gas mixture is 90% argon, 10% methane, known as P-10. An incoming photon liberates electrons from the atomic orbitals of the gas atoms, leaving an electron and positively charged atom, commonly known as an ion pair. As the charged particle travels through the chamber, it leaves a trail of ion pairs along its trajectory. The electrons created in this process drift toward a readout electrode, known as the anode, under the influence of an applied electric field. At the same time, the positive ions drift towards the cathode, at much lower speed.
A proportional counter differs from an ionization chamber in that the operating voltage high enough that the drifting electrons gain enough energy over a mean free path to create further ion pairs as they collide with other neutral atoms of the gas. Thus, a cascade of ion pairs (Townsend avalanche) is created. If the operating voltage is chosen correctly, each avalanche occurs independently of others from the same initial event. Therefore, even though the total number of electrons liberated can increase exponentially with distance, the total amount of charge created remains proportional to the amount of charge liberated in the original event. By measuring the total charge (time integral of the electric current) between the electrodes, the initial photon's energy can then be measured.
Another common service or repair item for ASOMA and SPECTRO benchtop EDXRF analyzers, sealed proportion counter (PC) detectors suffer performance degradition after a finite number of events have been recorded. In addition to simply wearing out, beryllium window breakage, loss of resolution, or a compromised enclosure require replacement.
Lithium drifted silicon detectors, for EDXRF spectrometric analysis, essentially consist of a specially treated and manufactured 3-5 mm thick silicon cylinder (junction type p-i-n diode) with a bias of about -1000 V across the two flats. The lithium-drifted center forms the non-conducting intrinsic layer. When an x-ray photon passes through, it causes a specific number (depending on the photon energy) of electron-hole pairs to form, which causes a voltage pulse. To obtain sufficiently low conductivity, the detector must be maintained at low temperature, with liquid nitrogen (LN2) being employed for best resolution. With some loss of resolution, the much more convenient Peltier cooling can be employed.
Si(Li) detectors, as used in ASOMA and SPECTRO spectrometers, are a high maintenance item because most such detectors employ liquid nitrogen cooling. Forgetting to refill the Dewar in a timely manner, to the point where the detector warms up to room temperature, almost always results in some loss of resolution. The resulting resolution degradation may impact analytical performance to the point where the Si(Li) detector requires either refurbishment or replacement. Other factors necessitating service or repair include: loss of vacuum, microphonics, loss of resolution, window contamination and window failure.
PIN-diode EDXRF detectors
A PIN-diode is a diode with a wide, lightly doped 'near' intrinsic semiconductor region between a p-type semiconductor and an n-type semiconductor regions. Peltier cooled silicon PIN photodiodes are commonly employed as high resolution energy dispersive detectors for X-ray fluorescence (XRF)spectrometry. The detection efficiency is a function of the thickness of the silicon wafer; for example, a wafer thickness of 300 microns provides nearly 100% detection efficiency at 10 KeV but only about 1% efficiency at 150 KeV.
While robust in nature, ASOMA and SPECTRO sourced PIN-diode EDXRF detectors do require service or repair whenever the vacuum can is compromised, the Peltier stack stops cooling correctly, the X-ray window is damaged or contaminated, or the PIN-diode degrades due to radiation damage.
A new category of Peltier cooled X-ray detectors, silicon drift detectors (SDD), are chiefly used in X-ray spectrometry (EDXRF and MDXRF) as well as electron microscopy (EDX). This technology has become very popular because their characteristics, compared with other x-ray detectors, include very high count rates and comparatively high energy resolution. Like other solid state x-ray detectors, silicon drift detectors measure the energy of an incoming photon by the amount of ionization it produces in the detector material. The major distinguishing feature of an SDD is a transversal field generated by a series of ring electrodes that forces charge carriers to 'drift' to a small collection electrode. This 'drift' concept of the SDD allows for throughput up to 1,000,000 counts per second (CPS).
Current generation ASOMA and SPECTRO SDD EDXRF detectors, with the field effect transistor (FET) moved out of the radiation path, are far more reliable than the first generation devices. Nevertheless, service or repair is required whenever the vacuum can is compromised, the Peltier stack stops cooling correctly, the X-ray window is damaged or contaminated, or the SDD degrades due to radiation damage.
Pulse processor and multi channel analyzer
Pulses generated by high resolution X-ray detector are processed by pulse-shaping amplifiers (pulse processor). As it takes time for the amplifier to shape the pulse for optimum resolution, there is necessarily a trade-off between resolution and count-rate. Long processing times deliver better resolution but can result in "pulse pile-up" in which the pulses from successive photons overlap. Current state-of-the-art digital pulse processing techniques rely on linear filtering methods which attempt to reduce the pulse length to improve detector performance. However the inability to resolve closely
spaced pulses means pulse pile-up remains a problem. This results in limited detector throughput, decreased spectral accuracy and energy resolution, increased spectral noise, and detector dead time.
In EDXRF, the multichannel analyzer (MCA) is the component used to store information from the pulse processor. Each channel corresponds to a small energy increment and each pulse from the detector is stored in the appropriate channel according to the amplitude of the pulse (that is, the photon energy).
EDXRF spectrometers are the elemental analysis tool of choice, for many applications, in that they are smaller, simpler in design and cost less to operate than other technologies like inductively coupled plasma optical emission spectroscopy (ICP-OES) and atomic absorption (AA) or atomic fluorescence (AF) spectroscopy. Examples of some common EDXRF applications are: Cement and raw meal: sulfur, iron, calcium, silicon, aluminum, magnesium, etc; Kaolin clay: titanium, iron, aluminum, silicon, etc; Granular catalysts: palladium, platinum, rhodium, ruthenium, etc; Ores: copper, tin, gold, silver, etc; Cement and mortar fillers: sulfur in ash; Gasoline, diesel and RFG: sulfur, manganese, lead, etc; Residual gas oils: sulfur, chlorine, vanadium, nickel, etc; Secondary oil: chlorine, etc; Kerosine, naphtha: sulfur, etc; Crude oil and bunker fuels: sulfur, vanadium, nickel, etc; Plating, pickling & pre-treatment baths: gold, copper, rhodium, platinum, nickel, sulfates, phosphates, chlorides, etc; Acetic acid: magnesium, cobalt and bromine; Terephthalic acid (TPA): cobalt, manganese, iron, etc; Dimethyl terephthalate (DMT): heavy metals; PVC copolymer solutions: chlorine; Photographic emulsion: silver; Clay: metals and non-metals; Waste and effluent streams: RCRA metals, chlorides, phosphates, etc; Food, pet food and other animal feed: potassium, phosphorus and chlorine; Cosmetics: zinc, titanium, calcium, manganese, iron, silicon, phosphorus, sulfur, aluminum, and sodium; Wood treatment: CCA, Penta, ACQ, ACZA, phosphorus-based fire retardants, copper naphthanate, zinc napthanate, TBTO, IPBC and combinations of these; Antacids: calcium; and Toothpaste: phosphorus and tin.
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