As a American Welding Society (AWS) Certified Welding Inspector (CWI) and a member of the American Society of Nondestructive Testing (ASNT), I receive periodicals from both professional organizations. Sometimes the information isn’t interesting to me; I mean, I’m not too concerned with what is taking place at the various charters around the country and indeed the world. Every now and then, though, there are nuggets of information I get to read I find interesting and while not applicable in it’s everyday application, Neutron Radiography just so happens to be one of those articles.

In the field of Non Destructive Testing (NDT), I only get to dabble with Visual as a CWI. More than anything I certify welders but since my CWI class I have taken more interest in the various means and methods of NDT. Indeed, I have been familiar with many of the processes to inspect welds during my 20 years as a welder in the field. I had been around and seen welds inspected with Radiograph testing (RT or X ray as it is commonly called), Ultrasonic Testing (UT), Dye Penetrant Testing (PT), Magnetic Particle Testing (MT) and of course Visual Testing (VT). Earlier in the year I was exposed to Eddy Current Testing (ET) when a tube bundle needed to be overhauled and I thought out of the inspection methods, I had been around most then. Just as there are a host of other welding processes, so too are there other inspection methods.

Neutron Radiography can be very detailed – note the individual grains in the munitions.

Neutron Radiography

Enter Neutron Radiography. I’ll be honest: I had never heard of it until I read an article the other day and this alone demonstrates the benefit of belonging to professional organizations and their associated periodicals. I was surprised to read about another form of radiation used to inspect welds and materials in general. The article, found in Materials Evaluation, an ASNT periodical revealed that, “while similar to RT, NR depends on the unique properties of neutron radiation to provide insights into the composition of materials and detect discontinuities that may be partially or even completely invisible to other forms of NDT.”

I found this interesting because I already knew UT could pick up things RT couldn’t. But to say there are some things both methods could be deficient on sparked my interest considerably.

The article continued to explain that as a NDT method, it remained rather unknown due to the difficulty of it’s creation. The reason for this is that “unlike X-rays, which could be produced by a cathode ray tube, neutron radiation is a product of nuclear fusion and fission, which requires more advanced technology.” Personally, I find that fascinating, not the least because “X-rays and N-rays are two distinct types of radiation that interact with materials in very different ways.”

I’ll quote at length the article now because I think the author does a fantastic job explaining the difference. I like to think I’m capable of writing substance, and indeed I am, but I must give credit where credit is due.

He begins with “X-rays, which are comprised of high-energy photons, readily pass through light elements but have difficulty penetrating dense elements since the high-energy photons, comprising a beam of x-rays, are more easily absorbed by a material based on how many electrons it has. In other words, the denser an atom is (the more electrons it has), the more energy it absorbs and the more opaque to X-rays it is.”

“Neutrons, which are composed of neutral particles, do not interact with the electrons of an atom and can be absorbed only by colliding directly with the nucleus. Because of this, the opacity of a given material to neutron radiation doesn’t depend on the material’s density at all. Instead, it depends on the size of the nucleus in a given atom and how far apart the nuclei are in a given material. The nucleus is already the smallest part of an atom, dwarfed by orders of magnitude by the electron cloud that surrounds it. Denser materials tend to have more space between the nuclei, so neutrons pass through them more easily. On the other hand, light materials have less space between the nuclei, so neutrons struggle to pass through them.”

I find this all very fascinating, and not merely because I am a CWI. I enjoyed chemistry and think better informed individuals make better citizens, better informed welding inspectors make better inspectors, and better informed welders make better welders. Taking the time to explain to a welder the science behind the code book (proper storage of low hydrogen rods, for example) can, at times anyway, provide welders with more background and encourage why – not just the you must – in code enforcement.

The author additionally notes that, “functionally, N-ray imaging produces almost the opposite effect as X-ray imaging, since neutrons can penetrate the dense materials that X-rays cannot.” And this is where I find it fascinating as an applicable NDT method in the future, complimenting the already numerous methods out there. Furthermore, “because composite materials are made of two or more materials layered together, they can be difficult to comprehensively examine using traditional NDT methods alone, but NR can provide a valuable new perspective.”

Bingo. I’ll readily admit I know very little experientially in the NDT world, having been exposed to the various methods when welding but not as a CWI. For my part, I would like to actually do more welding inspection than what I am currently involved in, say a couple days a month at a shop to broaden my experience. What’s more, if there was a Non Destructive degree I could pursue close to me (there isn’t) I would be all over it. As it is, I’ll have to settle for the little I get to dabble in NDT with future prospects forthcoming.

The article prompted me to do a little research about Neutron Radiography and while it wasn’t as comprehensive as I could have made it, I discovered that as valuable as it is, it is also rare. I address that point below.

Why is Neutron Radiography so Rare?

The main inhibitor in Neutron Radiography is it’s creation source. That is to say, since X-rays and N-rays are produced differently, one cannot lump them together. It’s fairly obvious X-rays are prevalent because, as noted above, they can be produced by a cathode ray tube. (Click here for a good website discussing x-rays. Additionally, the NDT resource Center states that “to generate X-rays, we must have three things. We need to have a source of electrons, a means of accelerating the electrons at high speeds, and a target material to receive the impact of the electrons and interact with them.”) Neutron radiation, however, is created by nuclear fusion (which is when you join two or more atoms) or fission (where you split atoms). Of course, you also get the radiation from radioactive decay. In any of the cases, though, it often proves difficult to amass enough free neutrons to provide you with a quality image. What are some of those sources providing free neutrons?

Non – Powering – Generating Reactors

Essentially, what you are talking about is a nuclear reactor that isn’t generating power like a power plant; rather, it is used for scientific research. The Nuclear Regulatory Commission, the governing board for all things nuclear, has oversight concerning this and as such, it isn’t a profitable operation in many circumstances (normally people bemoan regulatory constraints; in this case, I’m willing to bet most people are thankful for this – even if they are ignorant of them). That’s not to say they don’t exist; most commercial reactors are operated by colleges and universities. They are costly, however, and with an aging fleet, they aren’t being replaced with like and kind. For the industry, this certainly provides a problem.

Radioactive decay

Here I was going to do a thorough list of those elements but there is no need; for the gist of this post we merely need to understand they are expensive, hard to obtain, and a large quantity is needed to provide enough neutrons for a quality image. Yes, they produce neutrons but no, they are not a serious viable option to produce N-rays commercially.

Spallation Neutron source (SNS)

Oak Ridge National Laboratory, nestled in Tennessee, has what they call an “accelerator-based system that delivers short (microsecond) proton pulses to a steel target filled with liquid mercury through a process called spallation. Those neutrons are then directed toward state-of-the-art instruments that provide a variety of capabilities to researchers across a broad range of disciplines including physics, chemistry, biology, and materials science.”

I was instantly reminded of Jefferson Lab, located in Newport News, Virginia. There, they use a Continuous Electron Beam Accelerator Facility (CEBAF as it’s often referred to) as opposed to “proton pulses” at Oak Ridge. Still, Oak Ridge was a collaborative effort of monumental proportion, not readily available commercially for NDT.

Additional information and articles can be found here, here, here, and here.

Fusion Neutron Generators

This technology uses “a particle accelerator to produce an ion beam that will combine hydrogen isotopes into new elements, resulting in surplus neutrons.” Notably, “a new neutron imaging center located in the United States recently became the first radiographic imaging facility capable of producing Category 1 neutron images using a small, relatively low-cost fusion neutron source as opposed to a reactor source.”

To that end, I link to Phoenix Neutron Imaging Center, because they utilize this technology and this is in fact where the author of the article is employed. With the aging nuclear reactors we have, it is nice to see nuclear energy – even in inspection – finding advocates. With their opening in 2020, it will be interesting to see how far the world of NDT and Neutron Radiography progresses.

Conclusion

While it seems the days of on site testing using Neutron Radiography are far off, at least there are breakthroughs in additional methods of neutron producing technology – which will further the study and practice. For my part, Although I don’t get to work with the various methods of NDT as often as I’d like, I am excited to see this technology and practice emerging just a little bit further for inspection. One day, technological breakthroughs may enable Neutron Radiography to be more commonplace for weld inspection, and as an interested CWI, I like the idea.