Most users of X-ray tubes intuitively understand that X-ray tube lifetime is finite, and that X-ray tubes are consumable items due to the lightbulb-like filament that is generally used as an electron source. One of the first questions when dealing with any consumable is, how long will it last? This question informs total cost of machine ownership, preventative maintenance schedules, and a number of other key decisions needed to accurately compare one source against another.

In today’s post, we’ll look at the common failure modes of X-ray tubes, what can be done to manage them, how long your tube might be expected to last in the field, and why it’s a surprisingly hard question to answer.

A Brief Review

X-ray tubes are vacuum devices, meaning they require a high vacuum to operate. This high vacuum is needed for high voltage insulation, but it is also useful in allowing the electrons to travel from the cathode side of the X-ray tube to the anode, in order to generate X-rays. With a lower quality vacuum, the journey from cathode to anode becomes more challenging for the electrons, and therefore the filament must be run harder. For more information on this phenomenon, please see our previous articles on how X-rays are generated.

All of Micro X-Ray’s X-ray tubes are sealed type X-ray tubes. Sealed tubes are (as the name suggests) completely vacuum sealed at manufacturing, and contain no user-serviceable parts. This makes an extremely compact, robust, and durable package compared to their open type counterparts.

Filament Failure

The first failure mode to explore is failure of the filament. Modern X-ray power supplies control the beam current by tightly controlling the current through the X-ray filament. As the filament current increases, more electrons are released from the filament itself, and as more electrons are available inside the tube, more of them cross the high voltage gap to the target material, increasing the beam current. Over time, this electron emission wears down the filament, leading to an eventual end of life failure where the filament breaks completely – just like an Edison lightbulb. The lower the filament current, the longer the X-ray tube will last. The effect is a classic exponential curve – as an example, the standard 1.7A MXR filament has an estimated life of 100,000,000 hours at 1.34A, dropping to just 10,000 hours when run at the filament current limit of 1.7A. If the filament is run over the limit, say at 2.2A, it would break in a matter of seconds.

Other electron sources, for example dispenser cathodes like those used in the Microbox, have an ever longer service life than a filament-based X-ray tube.

Vacuum Degradation

Vacuum degradation is the other main cause of X-ray tube end of life. The metal components inside the X-ray tube outgas over time; this is an unavoidable law of physics, and while MXR’s X-ray tubes are built and processed with an eye towards minimizing this outgassing and improving the quality of the vacuum before sealing, the nature of sealed tubes means that not only can no gasses enter the tube, but none can escape either. As the tube heats up, the rate of outgassing internally increases.

As the vacuum degrades (meaning the pressure increases inside the X-ray tube), several things start to happen concurrently. First, the high voltage stresses inside the tube increase, which can lead to increasing leakage current, instability, and eventually arcing.

In addition to leakage increasing, the electrons from the filament have a higher change of hitting gas molecules as they traverse the high voltage gap between the anode and cathode. If an electron hits a gas molecule, it is knocked off course and loses energy, so it may not hit the target and produce an X-ray. As the vacuum degrades, therefore, the filament will need to be run hotter to compensate for those lost electrons in order to maintain a constant beam current. Over time, this process runs away, and the filament will eventually exceed its limit and open.

Race to the Death

What we have is a race to the death. As the vacuum degrades, the filament needs to work harder to keep up. As the filament works harder, the tube heats up more which causes the vacuum to degrade faster. What will happen first, will the filament run so hot it burns itself out? Or will the tube vacuum deteriorate so much that the high voltage arcs to ground, causing internal damage. There’s no one right answer, it could be either. If you’re running low kV/high current, it’s likely that the filament will burn out first. If you’re running high kV/low current, it’s likely that arcing due to vacuum degradation will occur first.

So…How Long Will my X-ray Tube Last?

10,000 hours is a good starting point for a lifetime estimate. Unless you run it hot, in which case it can be significantly less. Or, if you keep it cool, it can last longer. And of course you can’t run the filament (or cathode) too hard. If you run the filament conservatively, and keep the tube cool, it could easily last for 10x the 10,000 hour rule of thumb or more.

In practice, MXR has X-ray tubes in the field that are over 10 years old. These are usually in lab environments with a controlled atmosphere, minimal duty cycles, and adequate heating while operating. If you treat your X-ray tube well, it can last a long, long time. On the other hand, if you run 24/7 in a non-climate controlled factory in a hot part of the world, your tube just isn’t going to last as long as in the laboratory use case.

What can I do?

You can keep your tube cool! There’s not much to do about the filament other than ensure it doesn’t exceed the filament current limit, but for most tubes, the filament can live into the millions of hours before being depleted from normal operating conditions. So we’re left to focus on the outgassing, which can be minimized by keeping the tube cool. The cooler the internal components are, the slower the rate of outgassing. With a properly cooled X-ray source running within its design specifications, the lifetime of the tube can reliably exceed 10,000 hours, and often much, much more.

Contact Miro X-Ray Today

As always, let’s talk about it. Our engineers are always happy to have a conversation about YOUR application to help pick the right tube, and provide recommendations for the right cooling system.

The post X-Ray Sources 101: How Long Will My X-ray Tube Last? appeared first on Micro X-Ray.

Much to the chagrin of many in our industry, there’s no legal (or even just widely accepted) definition of a microfocus source. This leads to some understandable confusion among the X-ray source buying population, and to some occasional overly generous prose by the marketers in some X-ray source companies.

In some X-ray market segments, a microfocus is any source with a focused X-ray spot smaller than 1mm (the thinking goes, if it has a µm in front of the spot size, it’s a microfocus). Other manufacturers feel that there’s a cutoff around 100µm in order to be considered a microfocus. Still other X-ray industry pros feel that a microfocus refers to any X-ray source that’s actively focused rather than statically focused (maybe a bit too technical to be useful). There’s no right or wrong answer, it’s not like there’s an ISO definition of microfocus that’s agreed and enforceable.

So what about Micro X-Ray? We specialize in small spots, so by some of those definitions every tube we make is a microfocus tube. But when we say microfocus we generally mean any source that focuses X-rays down to under 10µm like our Microbox integrated source, and we refer to sources with larger spot sizes as Minifocus sources. Our largest spot size sources are 1000-1500µm, and in those cases we don’t generally talk about their spot sizes at all, because in the applications they’re designed for spot size isn’t an important specification point. If you need a refresher on what exactly we mean when we’re talking about focal spot, then check out our post on the topic in the Resources section of our website.

Spot Sizes in Imaging

Imaging applications are generally all about identifying features hidden inside opaque materials. Whether you’re imaging a rock, a pebble, or a grain of sand, you can use X-rays to see inside them all. X-rays have the ability to penetrate objects that visible light cannot, which makes them incredibly valuable in a huge array of application spaces from security to food inspection to electronics manufacturing. With X-ray sources, there are three main knobs you can turn that impact the final image:

1. Excitation voltage: This sets the maximum penetration ability of the X-rays. The higher the voltage, the higher density object you are able to image.
2. Beam current: This sets the absolute number of X-rays from the source. The higher the beam current, the more X-rays. The more X-rays, the faster an image can be acquired, and the faster your product can be moved through the process.
3. Spot size: This sets the minimum feature size able to be imaged in an X-ray system. The smaller the spot size, the smaller the size of the feature that can be resolved on the X-ray image

If we think about these three knobs, you can start to map them on to your application and narrow down your X-ray source options. If you have a general idea of the density of the object to be scanned, the process time you have available to scan it, and the size of the features you’ll need to resolve, you have a great starting point for selecting an X-ray source. Of course, there are always tradeoffs, so you might choose a slightly higher resolution that takes a slightly longer time, or perhaps the ability to have a quick process time might come at the expense of slightly lower contrast on dense materials. Either way, once you have your application properly scoped, at a great point to reach out to our technical team so we can find the right source together.

Spot Sizes in Analysis

Most analysis applications couldn’t care less about spot size. Take X-ray attenuation thickness gauging, for instance. This application works by passing a thin film of some known material between an X-ray source and an X-ray detector. The system is calibrated with the tube and detector a fixed distance apart, and the total flux from the X-ray tube is empirically measured and stored. Because the material is known, the absorption of the material for a given X-ray energy can be calculated. During operation, the X-ray flux is read back continuously from the detector, and the thickness can be calculated with a high degree of precision based solely on the attenuation of the X-ray beam through the film. At no point in this process is the spot size of the X-ray tube impactful, and for this reason the majority of thickness gauging applications tend to use tubes with our largest spot sizes, around 1000um.

Of course, not all X-ray analysis applications are created equally, and for some (I’m looking at you, µXRF) the spot size is critically important. But in general, spot size usually only matters for imaging applications.

Summary

The definition of a microfocus source matters much less than selecting the right spot for you application. If you’re in the market for an X-ray source of any kind, it’s best to understand your application and select a source with the spot size you need, and understand that terms like microfocus, small spot, etc are really all just marketing terms with no standardized definition. The spot size YOU need depends on YOUR use case. We here at Micro X-Ray are always happy to discuss your applications, and get you the right tube for your application, whether you need a 5µm spot, a 1500µm spot, or something in between.

The post X-ray Sources 101: What’s a Microfocus Source, Anyway? appeared first on Micro X-Ray.

Physics of Bremsstrahlung Radiation

Bremsstrahlung radiation, also known as “braking radiation,” is a type of electromagnetic radiation produced by the deceleration of charged particles, such as electrons, when they pass through the electric field of an atom in an X-ray tube’s target. In X-ray tubes, the charged particles, or electrons, are accelerated by a high voltage, usually in the range of 20 to 150 kV, and then collide with a target material, such as tungsten. These collisions result in the deceleration of the electrons, and the energy lost by the electrons is emitted as Bremsstrahlung radiation.

Briefly, Bremsstrahlung is different than characteristic radiation which is produced when one of the same electrons knocks an electron from an inner shell out of orbit. An electron from a higher shell jumps down to fill the gap, and the difference in energy between the shells is emitted as an X-ray. The energy of that X-ray is equal to the energy difference between the two shells.

The energy of the Bremsstrahlung radiation is directly related to the energy of the electrons, as well as the atomic number of the target material. The higher the energy of the electrons, the higher the energy of the emitted Bremsstrahlung radiation. Similarly, the higher the atomic number of the target material, the higher overall count rate of the emitted Bremsstrahlung radiation.

X-ray Imaging

Bremsstrahlung radiation is significant in X-ray imaging because it is the primary source of X-rays in X-ray tubes. The X-rays produced by Bremsstrahlung radiation are favorable for imaging purposes because they have a broad spectrum of energies, allowing for higher contrast images in materials with varying densities. This broad spectrum of energies is due to the fact that the electrons that produce the X-rays lose different amounts of energy as they pass through the target material, resulting in a wide range of X-ray energies.

The upper limit of the X-ray tube’s energy output is set by the excitation voltage of the source, and the lower limit is set by the inherent filtration of the tube, in the example below we see both the Bremsstrahlung generated at the target face, along with the output of the X-ray tube with the 254μm Be window’s inherent filtration taken into consideration.

The voltage applied to the X-ray tube and the choice of additional filtering materials are carefully chosen to produce X-rays with the desired energy spectrum for a given imaging sample. Often in imaging applications, the characteristic lines of the target material are filtered out to ensure a clean Bremsstrahlung radiation pattern for optimal imaging of the widest possible array of samples.

Summary

Bremsstrahlung radiation is a type of electromagnetic radiation produced by the deceleration of charged particles, such as electrons, when they pass through the electric field of a heavy atom in an X-ray tube. It is the primary source of X-rays in X-ray tubes and plays a crucial role in X-ray imaging. By understanding the generation and properties of Bremsstrahlung radiation, we can better understand the workings of X-ray tubes and their applications in medical and industrial imaging.

If you have any questions about target materials, imaging applications, or X-rays in general, don’t hesitate to reach out to us today!

The post X-ray Sources 101: Bremsstrahlung Radiation and Why it Matters for Imaging appeared first on Micro X-Ray.

• Part 1 of this series
• Our article on voltage, current, and power
• Our article on spot size

Power Loading

Target Material Selection

X-ray spot visualization of an undamaged target (left), and a pitted target (right)

Higher Power Loading Enabling Faster Images

Micro X-Ray Diamond Anode Products

Microbox Operating Range, Maximum Power

 

Microbox Operating Range, Minimum Spot Size

For More Information on Diamond Anodes

The post X-ray Sources 101: Taking Faster Images at Higher Resolution (Part 2) appeared first on Micro X-Ray.

Elements to be Measured

Primary X-ray Spectrum

Spectra from two different Lightbright tubes at 50kV with different target materials

Power Requirements & Target Durability

Conclusion

The post X-ray Sources 101: How to Choose a Target Material for XRF appeared first on Micro X-Ray.

Introduction

Excitation Voltage

Beam Current

Tube Power

X-ray Power Supplies

The post X-ray Sources 101: Voltage, Current, and Power in X-ray Tubes appeared first on Micro X-Ray.

Spot Size and Resolution

Image Quality

• Geometric Unsharpness: The quantitative term for the loss of definition in an image due to focal spot size (and other geometric factors).
• Penumbra: The partially shaded region on the edge of the shadow in an image.

X-ray coverage on a detector shown with a X-ray point source

X-ray coverage on a detector shown with a X-ray spot of some quantifiable size

What does this mean in practice?

When selecting a spot size, the amount of geometric unsharpness that you can tolerate depends on the size of the feature you’re measuring and the size of the penumbra.

X-ray image of a line pair resolution gauge, with line pairs from 3 – 30 microns.

The above image is of an X-ray of a line pair gauge, taken with a Microbox with a 5μm focal spot size. The line pairs are a high density material on a low density substrate. Each grouping is a set of three lines of a certain width, spaced the same width apart. This gives us a easy, repeatable way to measure the size of resolvable details using any given X-ray source.

Visualization of 5µm line pairs with the grey value assigned to the Z axis.

If we now zoom in on the 5µm line pair and apply a 3D visualization by assigning the grey value to the Z axis, we can easily resolve the three distinct peaks by eye; one peak for each of the three 5um width lines on the gauge. However, notice the pyramidal shape and how the features tend to blur together at the base of the shape. This is the penumbra from the 5μm spot causing geometric unsharpness that’s blurring the image, and because the lines and spaces are themselves only 5μm, we are nearing the minimum resolvable feature size of this source. Whether or not this resolution is good enough is a choice for the system designer to make.

Visualization of 30µm line pairs with the grey value assigned to the Z axis.

Next, we’ll zoom in on the 30µm pair from the same line pair image, and apply the same 3D visualization. In this case we can see a distinct tabletop shape to the lines, with a clear valley between them. This is because the penumbra from a 5µm spot will only blur the outer 5µm of the lines. Because the lines are 30µm wide and the space between them is 30µm wide, the line pairs are clearly resolvable and can easily be distinguished.

What size spot do you need?

Other factors affecting resolution

The post X-Ray Sources 101: What is Focal Spot Size and Why Does it Matter? appeared first on Micro X-Ray.

Grounded Cathode Topology

Grounded Anode Topology

Bipolar Topology

The post X-ray Sources 101: X-ray Tube Topologies appeared first on Micro X-Ray.

What are the major parts of an X-ray tube?

X-ray Tube Parts: Cathode

X-ray Tube Parts: Anode

X-ray Tube Parts: Window

About Micro X-Ray

The post X-ray Sources 101: Anatomy of an X-ray Tube appeared first on Micro X-Ray.

X-ray Tube Grounding and Instability

When evaluating and using an X-ray tube, it is important to pay attention to the X-ray tube grounding topology. An improper ground path, a loose ground connection, or a mismanaged high voltage return line can result in tube instability that is tricky to diagnose. Recently, we have had a customers call us with an X-ray tube stability issue. They had recently received our X-ray tubes and installed them in their systems to begin testing. We then received notice that there was something wrong with the stability of the tube.

Our tubes are thoroughly tested before they leave our factory to ensure they are plug-and-play ready. Improper grounding is one of the leading causes of instability and can typically be characterized in one of two scenarios:

High Voltage Grounding Connection  

Some customers choose to perform quick tests after receiving the unit as a form of incoming inspection. Even (or especially) when doing a ‘quick test’, care needs to be taken!

Using temporary grounding connections is a sure-fire way of causing instability, or worse, damage! Temporary grounding simply cannot manage the high voltage (HV) it needs to dissipate. Whenever voltages over 600V are being used, proper grounding is a necessity. That means adequate cable sizing (min 12 AWG or larger) with a suitably sized ring or spade connection. Never use an alligator clip to ground any HV circuit!

Earth Ground

This is a ground physically and electrically connected to Earth via a conductive material. Although it seems natural to ground to the nearest bolt or screw, care needs to be taken to make sure that whatever you are connecting to has a suitably low inductance path directly to Earth.

One of the most common causes of tube instability is caused by a floating ground. This occurs when not all grounds are suitably connected to the same earth ground. A floating ground can quickly cause the tube to become unstable and lead to arcing. A ground which seems suitable at a low kV setting may suddenly appear to be failing at higher kV settings. Once the ground path is compromised, arcing in the tube begins to deplete the vacuum, which causes the premature end of filament life.

Test and Verification

It is always a good idea to put a high quality ohmmeter across all ground connections to make sure there is a dead short between them. If these poor grounding conditions exist, then damage to the X-ray tube may occur.

At Micro X-Ray, we put a lot of emphasis on building high-quality and high-performing X-Ray tubes and integrated solutions. The last thing we want is a tube to be damaged from something as simple as an improper ground connection.

Contact us today with your questions about grounding, or to learn more about our X-ray tube options and why we should be your top choice.

The post X-ray Sources 101: X-ray Tube Grounding Considerations appeared first on Micro X-Ray.