EMSL, Poland scientists enhance radiation damage models

How exactly do materials and devices degrade when exposed to radioactivity? The more detailed answers scientists uncover for this question, the better equipped engineers will be to improve the quality and safety of, for example, key structures and parts within nuclear power plants. But these answers are far from simple—they depend on a wide variety of complex factors at the atomic scale, and they must be reliably predictive. Thus, to improve confidence in such answers, scientists must deeply integrate experimental evidence with computational simulations and models.

Among EMSL's visiting users in fiscal year 2011 are two scientists who traveled across the globe to do just that. Jacek Jagielski and Iwona Jozwik-Biala from the Institute for Electronic Materials Technology in Warsaw, Poland are part of a team that develops advanced simulation code for predicting radiation damage. In October 2010 and May 2011, they performed experiments at EMSL in collaboration with onsite scientists, yielding new data they needed to refine the code.

During their second visit, they joined EMSL scientists Bruce Arey and Libor Kovarik to answer a few questions about their experiences at EMSL.

Describe the scientific problem you are addressing in your work here at EMSL?

Jagielski: Our focus is on radiation damage in materials. In particular, there are two main driving forces. The first is the nuclear energy industry, especially the materials in nuclear reactors. The second is ion implantation in semiconductors [a materials engineering process used in semiconductor device fabrication and other applications], which can also lead to radiation damage. As we study these phenomena, our two main goals are to quantify the amount of damage created by a given radiation and to identify the types of damage that result from different kinds of radiation. So, these are both quantitative and qualitative assessments.

Kovarik: The qualitative and quantitative assessment will help to predict how these materials that are developed for nuclear applications will respond to radiation damage over a prolonged period of time. We need to have confidence that these materials will be reliable after decades of use, and the best way to predict this is to tie experimental analysis with simulation techniques.

Tell the story of how you learned about EMSL, became users, and began collaborating with the scientists here.

Jagielski: Our story overlaps with the reason we came here. Around three years ago, we met an EMSL scientist named Yanwen Zhang at a conference [Dr. Zhang is now an associate professor at the University of Tennessee Knoxville]. We began a discussion about radiation damage, and it came out that EMSL has tremendous experimental capabilities. And she was very interested in the simulation code we had developed, which reproduces certain types of damage. So we thought it would be interesting to join the code with the capabilities—to allow us to quantify extended defects such as dislocation in the material. We can collect various spectra at different arrangements and identify the kind of defects by high-resolution TEM. This gives us the input we need to refine our program further.

Arey: So they submitted a user proposal, which was accepted, and their first visit was in October 2010.

Jagielski: Our most important aim when we arrived at EMSL was to get enough data to validate the simulation procedure. But sometimes when you make experiments, there are unexpected byproducts. During this second visit [May 2011], we made an observation just three days ago that has already been accepted for a conference presentation in August—and we think it will be a hot topic.

What was this result?

Jagielski: Well, when these projectiles enter a material, they leave a long "ion track" or "damage track." And for the first time, we were able to observe the complete evolution of a damage track—from the surface all the way to the track’s end—from the side. This has never been done before; traditionally scientists have only been able to see a cross-section.

Kovarik: To get the results, it requires a use of several instruments that are not available together in many places, but we have them here at EMSL. These include high-resolution aberration-corrected TEM to see the tracks at the atomic scale, and the focused ion beam sample preparation techniques.

Jozwik-Biala: Yes, and the sample preparation is crucial. If you consider the depth of the ion entering the material, it’s on the scale of a few microns. To prepare a TEM-ready sample for such a large area using traditional mechanical techniques is very difficult; in fact, it has never been done. Only FIB [focused ion beam] allows you to create such a large area of transparency.

Arey: It starts with sample preparation and understanding what you’re working with. If it isn’t prepared right, the TEM does you no good. If it is, then the analytical part becomes very straightforward and very interesting—because now you’re seeing structures you wouldn’t have seen otherwise. That’s what happened in this case. We were able to prepare and orient the sample in such a way that the long damage track became more easily observable.

Kovarik: This kind of work takes great equipment, but also expertise— plus there is definitely some intuition required when it comes to sample preparation for TEM.

Jagielski: It is worth mentioning that the track is only roughly 100 atoms wide, so you really need atomic-level resolution to be able to see it in detail. I’m very impressed by your TEM capabilities here!

Do you have anything further to say about your overall experience at EMSL?

Jagielski: Everything has gone so smoothly during both visits, we almost forget how complex the coordination can be. We arrived here, got our keys to our rooms in the Guest House, and we were able to begin our experiments the same day. I have to say, the User Support Office has made the details run very smoothly.

Arey: Behind the scenes, when you're coordinating multiple instruments and scientists, communication becomes critical, to make sure each sample, each instrument, and each scientist is available and ready at the right time.

Kovarik: Yes, and we work hard to make sure everything is ready to go—it would be a shame to come all the way from Europe and find the instruments unavailable for some reason.

As you leave EMSL and return to Poland, what are your future plans for your research?

Jagielski: It depends on the time scale you’re referring to. In the very near term, we want to continue to push forward to enhance our simulation code even more and validate damage models with these experimental data. Longer term, we are developing various exciting partnerships with industry related to how our simulation codes can provide the knowledge needed for new nuclear power plants. Beyond that, we don’t know what the future holds.

Jacek Jagielski, PhD, is a research scientist and professor of physics at the Institute for Electronic Materials Technology in Warsaw, Poland and the Andrzej Sultan Institute for Nuclear Studies in Swierk, Poland.

Iwona Jozwik-Biala, PhD, is a research scientist at the Institute for Electronic Materials Technology in Warsaw, Poland.

Bruce Arey is a technologist in EMSL's microscopy capability group. Libor Kovarik, PhD, is a senior research scientist in EMSL's microscopy capability group.

Provided by Environmental Molecular Sciences Laboratory