Quantum Dot X-ray Image Sensors
How Quantum Dot Scintillators create a more efficient, cost-effective answer to X-ray inspection
X-ray imaging has contributed to major developments in medicine, but the benefits are not limited to one industry. As technology advances, the application possibilities for X-ray scanning continues to widen.
Since its discovery more than a century ago, X-ray scanning has predominantly been used as a diagnosis tool in the medical field. According to the World Health Organisation as many as 3.6 billion diagnostic medical examinations are performed across the globe each year. There have also been technological advances and more widespread use of X-ray scanners in areas such as security, to minimise the risk of threat and identify concealed items.
However, X-ray scanning is expanding into new industries, predominantly through its ability to perform non-destructive quality inspection and control. Here, the power of seeing inside an object, without damaging it, can be harnessed to ensure quality standards are met and safety is guaranteed, introducing a new, efficient, reactive assessment process.
Examples of X-ray scanning in non-destructive testing and inspection include:
- Food manufacturing – Through the use of X-ray scanners, foods can be inspected, contaminants can be detected, and defects can be identified without any damage to the product or its packaging.
- Automotive industry – X-ray scanners are able to check various parts of the vehicle including brakes, tyres, wheels, batteries, electromechanical components and so on. This can be undertaken throughout the manufacturing process.
- Electronics – X-ray scanners are now being used to ensure electronic products meet standards of reliability and safety, (testing bonding areas, wires, solder joints and components).
- Welding – X-ray scanners can be used for quality assurance for welding inspection, to ensure weld connections are of a suitable standard.
- Aerospace industry – X-Ray scanners can also be used in the detection of cracks, corrosions, pores, or welding defects in aircraft manufacturing. The utilization of X-ray imaging for outer space exploration is another big thing. Today, tens of X-ray satellites and telescopes are observing the space, for example to probe matter as it falls into a black hole, as well as probe the nature of dark matter and dark energy by observing the formation of clusters of galaxies.
How X-Ray scanners work
Typically, an X-ray scanner comprises of two elements, an X-ray Source– which is the device that emits the X-ray beams, and the X-Ray Detector, which is the component that measures the properties of X-rays and converts them into a visual form.
There are two types of X-ray detector, called direct and indirect. The first has a higher level of sensitivity, and is the type often used in medical applications. These detectors apply a direct conversion of X-ray photons into an electrical signal and are the more expensive of the two.
The second utilises an indirect conversion of photons, achieved by using scintillators, (materials which convert X-rays into visible light). These scintillators convert ionising radiation into visible photons that later can be detected by a silicon photodiode. Whilst these detectors are less sensitive, they are also a lot less expensive.
There are certain advantages and disadvantages to using both detectors, depending on the application required. For the purposes of quality control and inspection, indirect conversion using scintillators, is most suitable because it is cost effective and can be easily scaled-up for large area scanning.
Scintillator materials are very useful for non-destructive quality inspection and detection of product failures and inconsistencies, due to their high light output and impressive resolution.
There are many scintillator materials available, but the most commonly used are thallium-doped Cesium Iodide – CsI(Tl) scintillators, and terbium-doped gadolinium oxysulfide (GADOX(Tb)) scintillators.
The commonly used CsI(Tl) scintillators are usually the columnar type, grown in small columns (10 μm in thickness between two substrates). This kind of scintillator offers very high resolution with up to 10 – 20 lp/mm and excellent brightness (of around 10 ph/keV).
However, one big disadvantage of these CsI(Tl) scintillators is that they are extremely expensive, costing between $30,000-$40,000 per m2. Another disadvantage is their low radiation hardness, meaning they degrade very fast at high X-ray doses and have high afterglow (the scintillation light that exists once the X-ray has ceased). A more affordable, and significantly brighter scintillator is GADOX, which costs around $1,000 -$4,000 per m2. This scintillator is most frequently used in non-destructive testing. Whilst this scintillator has high brightness (20 ph/keV), its disadvantage is that it provides a lower resolution of image (reaching up 10 lp/mm max).
The benefits of Quantum Dot Scintillators
Scintillators using ‘Perovskite’ Quantum Dots (QDs) combine the advantages of both CsI (Tl) scintillators, and GADOX scintillators. They offer an extremely high resolution and brightness of image, (equivalent to those that the CsI(Tl) scintillators), but they are as cost-effective as GADOX scintillators). Their ability to provide higher resolution images allows them to show the smaller structures in objects, making them ideal for machine vision.
An additional performance advantage is that perovskite scintillators are more robust, durable and work more efficiently when exposed to hard X-rays and gamma rays.
However, possibly the most important advantage of Perovskite QD scintillators is their super-fast response time, allowing them to cease scintillation when the X-ray is switched off. The quantum confinement of Perovskite QDs means it has a response time measured in ‘ns’ as opposed to the standard response time for conventional scintillators, which is in within µs and ms ranges. This unique property allows for ultrafast X-ray scanning that is perfect when examining objects that are moving (again, ideal for non-destructive testing and inspection).
Lutfan Sinatra, the VP of Product Development of Quantum Solutions noted: “QDot™ Perovskite Scintillators demonstrate the unprecedented advantages over CsI(Tl) and GADOX, offering a new type of highly performing scintillators”.
As a result of these impressive features, perovskite materials are quickly gaining a lot of attention. Marat Lutfullin, the CEO of Quantum Solutions confirmed this stating: “In the last year, we have seen an enormous response from the X-ray industry as the qualities and advantages of QDot™ scintillators have become more widely recognised. Industries are getting convinced about the benefits in terms of overall efficiency and cost. Quantum Solutions has focused on meeting the needs of our clients working in this particular field.”
As X-ray technology continues to advance, it is predicted that evaluation cameras with novel perovskite X-ray scintillators will be available from as early as 2023.
Furthermore, there are many other exciting explorations and developments around perovskite materials that are taking place in the field of medical X-ray.
Since the invention of X-rays, the biggest challenge has always been developing technology that reduces exposure to radiation, without the X-ray image being compromised. Achieving an X-ray with sufficient resolution and depth has naturally led to administering a higher radiation dose, which, as we know, is detrimental to a patient’s health, (and the reason why only a limited number of X-rays per year are recommended). Reducing the dose without compromising resolution relies on the development of a higher level of sensitivity in the sensor.
Perovskite semiconductors have already demonstrated great potential in direct X-ray detection due to their exceptional properties (large X-ray attenuation, ultrafast, extremely high sensitivity and cost-efficiency).
Quantum Solutions is currently exploring and investigating the possibilities in terms of ways in which Perovskite and Quantum Dot technology can play an important role in this process, and we have already seen some very positive results.
Stay tuned for further updates …