The history of X-rays goes back to 1895, when Wilhelm Conrad Roentgen discovered and identified X-rays. Then, in 1909, Charles Glover Barkla discovered a connection between X-rays radiating from a sample and the atomic weight of the sample. Four years later, in 1913, Henry Gwyn Jeffreys Moseley discovered that there was a relationship between the atomic number of an element and the reciprocal of the wavelength for the spectral series of emission lines for each element. This provided the foundation for refining the periodic table by atomic number rather than by atomic weight and for making an X-ray spectrometer. However, it was not until 1948 when Herbert Friedman and Laverne Stanfield Birks, Jr. built the first X-ray fluorescence (XRF) spectrometer that opened the way for commercial use of XRF technology.
XRF is a phenomenon where a material emits photons as the result of being bombarded with enough high-energy X-rays. The types of atoms in the material and how they absorb the external X-ray energy affects the radiation energy (analogous to color for optical light) they emit. Each type of atom emits a different energy or color. An XRF spectrometer relies on this phenomenon to be able to identify what types and how much of each type of atom exist in a sampled material. By analyzing the energies emitted by a material, it is possible to determine which elements are present. By analyzing the relative intensities of the energies emitted by a material, it is possible to determine how much of each element is present.
XRF spectrometry is a nondestructive, non-intrusive, fast analytical technique that can be used to determine the chemical composition of different materials.
These features make XRF spectrometry practical and advantageous for many uses, including positive material identification (PMI) of metal alloys, hazardous material detection, and certification verification for applications such as salvage, recycling, machining, and forensic science.
Among the most important advancements that are expanding the use of XRF are those that shrink the cost and size of the XRF instrumentation. The commercial release of the first handheld XRF (HHXRF) technology nearly two decades ago enabled an important transition from stationary benchtop XRF instrumentation to mobile devices.
68 years later, a new X-Ray fluorescence technology
The first users that will benefit from this advancement are QA/QC in manufacturing, machining, metal processing, and scrap recycling—basically anyone concerned about the quality of their metals. It is also likely that applications that have never considered using XRF technology before because it was too expensive may start using it. These include aerospace, automotive, and medical instrumentation applications.
This article provides a brief overview of older XRF technology and the new triboelectric technology for XRF. It explores how the technology works to enable you to more confidently decide how, where, and when your business can benefit from using this technology.
Anatomy of conventional XRF spectrometry
There are a number of ways to implement XRF spectrometry, so to avoid confusion we focus on the specifics of XRF spectrometry using an energy dispersive XRF (EDXRF) approach. XRF spectrometers consist of four major sub-systems that form a signal chain: (1) X-Ray tube, (2) X-ray detector, (3) multi-channel analyzer, and (4) computer.
The X-ray tube is where the X-rays are generated and directed at the target sample. The X-ray tube will house one of two types of X-ray sources: radioactive or high-voltage. Radioactive sources are simple, small, and inexpensive. However, they cannot be turned off and pose enough environmental risks to the user and the community that there are registration requirements, restrictions on transportation and disposal, and periodic testing required to use radioactive sources. On the other hand, because high-voltage X-ray sources do not contain radioactive sources and they can be “turned-off,” they do not suffer from the same limitations. However, they do require a source of high-voltage electricity to be able to generate and emit the desired X-rays.
The X-ray tube contains a vacuum housing with a wire filament and a target anode inside the tube. An electric current applied to the filament heats it up to about 1,000 degrees Celsius so that it emits electrons. Once the filament is emitting electrons, the high-voltage is applied across the filament and the target anode, which accelerates the electrons from the filament towards the target anode. The interaction between the accelerated electrons and the target anode causes the emission of X-rays. The type of element in the target anode determines the energy of the emitted X-rays.
The X-ray detector is used to measure the fluorescent X-rays emitted from the target sample. There are different types of detectors available for XRF applications. EDXRF systems typically use solid-state detectors, such as a Si-PIN detector or a Silicon Drift Detector (SDD). Each type of sensor has advantages in different applications; neither is always best. The resolution and sensitivity are two important properties of these detectors. A higher resolution means the detector can detect the difference between more energy levels. A higher sensitivity means that a higher percentage of incoming photons are detected.
The computer manages the user interface and communications, as well as data storage, retrieval, and display. XRF spectrometers were able to become mobile, handheld devices in part because the computer functionality was able to reside on smaller embedded application processors that supported the small form factor.
A new way in XRF powered by Triboluminescence
Triboluminescence is the phenomenon of creating light through mechanical action such as pulling apart, ripping, scratching, crushing, or rubbing different materials. For example, this phenomenon is observable when breaking sugar crystals and peeling adhesive tapes. This phenomenon has been known since ancient civilizations. Following the 1980s discovery of mechanical action producing luminescence in the X-ray energy range in a vacuum tube, in 2008, a team of physicists at Tribogenics and UCLA backed by DARPA funding expanded on this discovery and confirmed that they could use triboluminescence to generate X-rays in a useful and repeatable way.
It was found that using triboluminescence to generate X-rays can have a profound impact on lowering the complexity and cost of X-ray generation. It is now possible to rely on the triboelectric effect caused by mechanically pushing materials together and pulling them apart to discharge enough electrons at the target anode to generate the necessary amount of X-rays to successfully perform XRF spectrometry. In short, a mechanical system replaces and eliminates the need for a high-voltage supply to generate X-rays. This is the primary innovation that is driving down the cost of entry for XRF spectrometers by almost half and creating many new uses for HHXRF.
The new technologies being deployed are listed here.
Anatomy of breakthrough XRF spectrometry
The high-voltage power supply and supporting components are no longer needed when using triboluminescence. An electric motor, battery, switch, microcontroller, and a low-voltage connector replace the need for inverters, transformers, and control system needed for the high-voltage supply. There is no thermal cycling because there is no need to heat up a filament. It is no longer necessary to provide a cable or mono-block connection between the high-voltage supply and the X-ray source. All of the other sub-systems in the XRF spectrometer remain the same.
The computer interface and value-added analytical functions it supports represents an important area of differentiation with the new technologies.
One of the new developments is the use of standard Android-based smartphones to provide the computing capabilities. The use of Nexus 5 smartphone technology, and its familiar user interface, has created a new, more powerful tool to analyze the data and provide the user with detailed information. Color touch screens allow the user to interact with and display information as needed. Another important factor is the smartphone’s capability to communicate with the outside world through WiFi, allowing for on-demand access of past data.
Using new smartphone technologies creates an environment where communication options continue to mature, enabling users to safely and securely store and retrieve thousands of results directly from cloud storage applications over wired and wireless interfaces.
Because XRF is a non-invasive, nondestructive process, the new, lower cost XRF devices more easily permit multiple measurements throughout the lifecycle of a material or component to perform live verification of the material you are working with. Low-cost, mobile XRF spectrometers powered by patented triboelectric technology will complement rather than replace site spectrometers and lab testing. Personnel will be able to test critical components before and after they are put into service.
For quality managers, metallurgical labs, metal processors, machine shops, fabricators, recyclers, welders, and anyone that is concerned about the quality of their metals, XRF with new triboelectric technology offers QC/QA departments a very cost effective insurance policy. For their aerospace, automotive, medical, and manufacturing customers, conformance certificates are no longer enough of a validation. XRF with triboelectric technology is the verification technology that they have long been asking for.