Patent Application
20150060663
The invention is related to the technical field of X-ray fluorescence analysers, and sources of incident radiation used in X-ray fluorescence analysers. In particular the invention is related to the use of an electron beam as the incident radiation in an X-ray fluorescence analyser.
X-ray fluorescence analysers typically use an X-ray tube as the source of incident radiation. The purpose of the incident radiation is to energize the atoms of the sample material sufficiently so that when relaxation from the energized states occurs, the atoms of the sample material emit fluorescent X-rays at their characteristic energies. The energy spectrum of the incident X-rays produced in an X-ray tube is defined by the acceleration voltage, the anode material, and the filtering and attenuating effect of radiation windows and possible other material layers and media through which the radiation must pass. Conventional X-ray fluorescence analysers are well suited for analyzing the concentration of relatively heavy constituents, such as metals, in a sample. However, due to the relatively deep penetration of incident X-rays into the sample material, they are not well suited for measuring lighter elements, such as carbon, nitrogen, oxygen, fluorine, and sodium.
A prior art publication U.S. Pat. No. 6,452,177 introduces the use of an electron beam as the incident radiation in an X-ray spectrometer. Electrons are produced by heating a filament, accelerated, and focused into a beam with an electromagnetic field inside a vacuum tube. A thin, electron-transparent membrane covering one end of the vacuum tube allows directing the produced electron beam towards a sample, in which the impinging electrons excite atoms of the sample materials and cause them to emit X-rays at characteristic wavelengths.
The apparatus described in said prior art publication has been designed for use on space missions, and would be ill suited for portable or benchtop analysers for various reasons. It consumes a relatively large amount of energy, and yet achieves a limited resolution and measurement reliability.
The following presents a simplified summary in order to provide a basic understanding of some aspects of various invention embodiments. The summary is not an extensive overview of the invention. It is neither intended to identify key or critical elements of the invention nor to delineate the scope of the invention. The following summary merely presents some concepts of the invention in a simplified form as a prelude to a more detailed description of exemplifying embodiments of the invention.
According to an aspect of the invention there is provided an electron source that can be used as the source of incident radiation in an X-ray fluorescence analyser, and is energy efficient, lightweight, and robust in construction. According to another aspect of the invention there is provided an X-ray fluorescence analyser that utilizes the advantages of such an electron source. According to a further aspect of the invention there is provided an X-ray fluorescence analyser that is capable of measuring concentrations of light elements in a sample. According to yet another aspect of the invention there is provided a method for performing X-ray fluorescence analysis utilising the advantageous features of the electron source and X-ray fluorescence analyser described above.
Advantageous objectives of the invention are achieved by using a photon source to excite a photoelectric converter and multiplying the electrons emitted by the photoelectric converter. The multiplication of electrons can be a built-in feature of the photoelectric converter, if for example one or more multichannel plates are used. The characteristics of the photon source can be selected so that they match the excitation requirements of the photoelectric converter as optimally as possible. For example an ultraviolet-emitting LED can be used as the photon source. The photon source may emit photons in continuous mode or in some timed fashion, for example in short pulses or with an intensity that is modulated over time.
The multiplied electrons are focused into an electron beam, which is directed towards the exposed surface of a sample. In an X-ray fluorescence analyser the sample is typically placed in a sample holder, and a detector is used to detect and measure an energy spectrum of fluorescent X-rays emitted by the sample as a response to being hit by the electron beam.
The photoelectric converter and the electron multiplier are placed inside a gas-tight casing, inside which vacuum conditions can be set up in order to keep the electrons from being slowed down and attenuated. An electron-transparent membrane covers a first opening in said casing, through which the electron beam is directed towards the sample. The X-ray fluorescence analyser may comprise a dedicated chamber for enclosing the sample and the detector, but since the requirements for avoiding attenuation are less strict for X-rays than electrons, the chamber may have a coarser vacuum (meaning a gaseous atmosphere, the pressure of which is not quite as low) than the inside of the gas-tight casing of the electron source. Instead of vacuum, a gaseous medium like helium that attenuates X-rays only weakly can be used inside the chamber, at least if the distance that the incident electron beam must travel within the gaseous medium is small.
Compared to a glow filament, which is commonly used as an electron source in X-ray tubes, an electron source of the kind explained above has the advantage of significantly lower energy consumption. In a glow filament a very large majority of the initial energy is dissipated in the form of heat, and only a fraction of the initial energy becomes utilized in the form of accelerated electrons. Photon sources such as gallium nitride LEDs are much more effective in converting the applied electric energy into energy of emitted photons, and while the use of photoelectric conversion and electron multiplication add more steps and corresponding power dissipation factors to the method, the overall efficiency still remains much better than in a glow filament. A further advantage of a photon source over glow filaments is the immensely faster response of a photon source to control signals, which allows accurate timing of incident radiation and consequently very accurate time resolution in measurements.
The novel features which are considered as characteristic of the invention are set forth in particular in the appended claims. The invention itself, however, both as to its construction and its method of operation, together with additional objects and advantages thereof, will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.
The exemplary embodiments of the invention presented in this patent application are not to be interpreted to pose limitations to the applicability of the appended claims. The verb “to comprise” is used in this patent application as an open limitation that does not exclude the existence of also unrecited features. The features recited in depending claims are mutually freely combinable unless otherwise explicitly stated.
The multiplied electrons are focused into an electron beam using a focusing element 104. The electron beam is directed towards a sample. Since the generation, multiplication, and focusing of electrons is advantageous to implement in a relatively good vacuum, the directing of the electron beam towards the sample typically takes place through an electron-transparent membrane 105, which seals the evacuated space at the output location of the electron beam.
The method for performing X-ray fluorescence analysis additionally comprises detecting the energy spectrum of fluorescent X-rays that the sample emits as a response to being hit by the electron beam. Aspects of this step of the method will be described in more detail later in association with the description of an X-ray fluorescence analyser according to an embodiment of the invention. The electric current carried by the electron beam may be for example between 1 and 10 microamperes, and the method may comprise scanning the electron beam across an exposed surface of the sample.
In the embodiment of
The primary emission of photoelectrons, i.e. the actual photoelectric conversion, can be enhanced by treating at least the entry side of the first microchannel plate with a substance that efficiently emits electrons when hit by photons of the wavelength of the photon source to be used. As an alternative, a separate photocathode can be placed on the entry side of the first microchannel plate. Some microchannel plates are suitable for use in photoelectric conversion even without any specific treatment, because the activation substance used to plate the pores of these microchannel plates is responsive to both photons and electrons.
Especially when microchannel plates are used for photoelectric conversion and electron multiplication, these tasks are spatially spread over an area that may be larger than the diameter of the desired output beam. The focusing element that is configured to focus electrons coming from the electron multiplier appears as schematically illustrated coils 206 and 207, and the associated conductors 208, in
Electrons are relatively easily attenuated in any elongated passages through a medium. In order to keep the created, multiplied, and focused electrons from attenuating it is advantageous to perform these tasks in a relatively good vacuum. For this purpose at least the photoelectric converter and the electron multiplier 203 and 204 are enclosed in a gastight casing 209. In the embodiment of
Photons experience relatively little attenuation in air or other gaseous media, at least on the wavelengths that are useful for creating photoelectrons. This means that the generation of, and directing towards the photoelectric converter, of photons does not necessarily need to take place in vacuum conditions. In the embodiment of
If the electron-transparent membrane 213 and/or a frame to which it is attached exhibit sufficient electric conductivity, one or both of them may be used as an additional acceleration electrode for the electron beam by coupling it or them to a suitable electric potential. Structures related to the electron-transparent window 213, such as the window frame or holder for example, may also have some function related to the focusing of the electron beam, if their shape and electric potential are such that a suitable electromagnetic field is formed.
In some cases it may be advantageous if the electron beam can be controllably deflected, in addition to being focused. Such a case occurs for example if a significantly larger area of the surface of a sample needs to be examined than what the focal spot of the electron beam covers. Scanning the electron beam across the surface of the sample could naturally be implemented by physically moving the electron source, the sample, or both. Another possibility is to make the electron source comprise an electron beam deflector configured to controllably deflect electrons focused by the focusing element. Electron beams were routinely deflected with electromagnetic fields in e.g. CRT (cathode-ray tube) displays, so the technology and a multitude of technical implementations of electron beam deflectors is widely known. In the electron source that is schematically shown in
If the photon source itself is capable of producing light on a visible wavelength, a visual indication may be provided with a configuration simpler than that of
The visual indication provided by a laser beam or a beam of light may be useful if the irradiated spot on the sample surface must be selected very carefully. Depending on the structure and operation of the focusing element it may be possible to irradiate even a very small portion of the sample surface with the electron beam, in which case the visual indication may provide a useful aiming aid. A tightly focused electron beam may allow e.g. in analyzing the short-range differences in the constitution of a somewhat heterogeneous sample. On other hand, not focusing the electron beam too tightly may be advantageous if the granularity or other heterogeneity of the sample surface is not an issue, and/or if the aim is to produce more fluorescent X-rays by irradiating a larger portion of the sample surface with the electron beam. The irradiated area on the sample surface may be for example a couple of square millimetres.
The chamber 404 allows controlling the atmosphere between at least the sample and the entrance window 405 of the detector 403 so that the fluorescent X-rays generated in the sample would experience as little attenuation as possible before they enter the detector 403. If the electron beam that is used as the incident radiation had to travel through air at room pressure, it would experience heavy attenuation, so it is advantageous if the chamber 404 encloses also the space between the electron-transparent window 213 of the electron source 401 and a sample held by the sample holder 402. It is also possible to use two separate chambers: one on the path of the incident radiation and the other on the path of the fluorescent X-rays.
An atmosphere control subsystem should be available for controlling the atmosphere inside the chamber 404. In its simplest form the atmosphere control subsystem that is comprised by the X-ray fluorescence analyser itself need not be more than a gas flushing joint 406, through which the chamber may be pumped empty and/or flushed with helium or other suitable gaseous medium that causes only little attenuation. It is also possible that the X-ray fluorescence analyser is made independent of all external equipment in this respect by equipping it with a vacuum pump and/or a pressurized purging gas container of its own.
An advantageous detector for use as the detector 403 in an X-ray fluorescence analyser according to an embodiment of the invention is a silicon drift detector (SDD). Its entrance window 405 may comprise a layer of silicon nitride, which has a thickness as small as 40-100 nm, as a continuous window layer. Radiation window foils that have such an extremely thin silicon nitride layer, as well as methods for their manufacturing, are known for example from the patent application PCT/FI2012/050804, which at the time of writing this description is not yet available to the public. A PIN detector could also be used, but an SDD is believed to have better noise characteristics especially at the low characteristic energies of light elements that can be measured with an X-ray fluorescence analyser according to an embodiment of the invention. Using an electron beam as the incident radiation allows exciting, and measuring the characteristic fluorescent X-ray peaks of light elements such as carbon, nitrogen, oxygen, fluorine, and sodium, with relatively high efficiency even with relatively small excitation current values. In an advantageous embodiment of the invention, the electric current carried by the electron beam is between 1 and 10 microamperes.
The X-ray fluorescence analyser of
An analyser control functionality 507 is responsible for the overall operation of the X-ray fluorescence analyser, and comprises typically a computer. It has a data storage 508 at its disposal for storing the measurement data obtained through the detection control functionality 504 from the X-ray detector 503, and also for storing the machine-readable instructions that, when executed by the computer in the analyser control functionality 507, cause the implementation of a method for performing X-ray fluorescence analysis according to the invention. The analyser control functionality 507 is also coupled to a user interface 509, which may comprise various indicator means such as one or more displays and various input means such as keys or touchscreens, for enabling bidirectional communications with a human user.
Other functionalities of the X-ray fluorescence analyser illustrated in
Blocks 502, 504, and 506 to 511 of
The emission of photons from a photon source can be controlled over time in a relatively accurate manner, for example so that the emission of photons takes place in pulses. A LED emits photons as a response to the electric current flowing through it, so if a LED is used as the photon source, the emission of photons can be modulated by modulating the current. Another example of a time-controlled photon source is a pulsed laser, in which Q-switching, mode-locking, or pulsed pumping causes the optical energy to be output in the form of short pulses rather than continuously. Focusing a pulsed laser to a small spot may easily create energy densities large enough to vaporize the material that is hit by the photons, so using a pulsed laser as a photon source in an embodiment of the invention may require using a beam expander or otherwise taking care that the output of the laser becomes distributed over a large enough area of the photoelectric converter.
The operation of an electron source according to an embodiment of the invention (i.e. the photoelectric conversion, the multiplication of electrons, and the forming of an electron beam) takes place with very little additional delay. Thus the intensity of the electron beam hitting the target will follow very accurately any possible modulation over time of the photon emission. Even the creation of X-ray fluorescence in the sample and the detection of the fluorescent X-rays introduce very little additional delay. This means that an X-ray fluorescence analyser according to an embodiment of the invention may be capable of analyzing even very fast dynamic changes in the sample, contrary to prior art X-ray fluorescence analysers where the electron source comprised a glow filament and a high acceleration voltage.
Implementing dynamic measurements in an apparatus of the kind that is schematically illustrated in
Variations and modifications to the features described above are possible. For example, although the description above has mentioned a photon source as the device that is used to initiate photoelectric conversion and electron multiplication, it would be possible to use a piece of a β−-decaying radioactive substance such as 63Ni instead. A β−-decaying radioactive substance produces electrons that are then received by and multiplied in the electron multiplier, making a photoelectric converter unnecessary. However, since the emission levels from a photon source such as an UV-LED are much easier to control, the use of a photon source and a photoelectric converter makes an electron source and an X-ray fluorescence analyser more practical to operate. The use of a β−-decaying radioactive substance could mainly apply to space applications, where the consumption of electric current should be kept at an absolute minimum.
| Number | Date | Country | Kind |
|---|---|---|---|
| 13397529.2 | Sep 2013 | EP | regional |
