All around us are elements forged in stars, from the nickel and copper in coins to the gold and silver in jewelry. Scientists have a good understanding of how these elements form: In many cases, a nucleus heavier than iron captures neutrons until one decays, turning it into a heavier element. There's a slow version of this neutron capture, the s-process, and a rapid version, the r-process.
That would be the end of the story, but certain stars don't seem to play by the rules. When astronomers analyze their light, they see unexpected ratios of heavy elements that can't be easily explained by the two processes. The anomalies point to a third way: the "intermediate" i-process.
Mathis Wiedeking, an experimental physicist at the Department of Energy's Lawrence Berkeley National Laboratory (Berkeley Lab), gathers data on nuclear reactions that can improve models of how the elements form. He's also the lead author of a new Nature Reviews Physics article on the current state of i-process research, where experiments, theory, and astrophysical observations converge. In this Q&A, he shares how the i-process fits into the bigger picture of element formation, what it takes to study it, and why it matters - both for understanding the cosmos and advancing technologies here on Earth.
Q: Why are you interested in how elements are formed?
Mathis Wiedeking: Where all the elements around us came from has been and remains one of the big unanswered questions in physics. We want to know how the elements, especially elements heavier than iron, are formed in the cosmos. They're formed in extreme environments, which can be a star, a dying star, or an exploding star, or other scenarios. It's a fascinating topic.
Q: What are the processes we do know behind element formation?
MW: Over 99 percent of the elements heavier than iron are produced in what we call "neutron capture" processes. These start where you have an initial "seed" nucleus that is stable. When it's exposed to a neutron-rich environment, it can capture more and more neutrons. Eventually, it reaches a limit, when the nucleus becomes unstable and decays, and one of the neutrons converts into a proton, forming a heavier element.
For the s-process, or the slow neutron capture process, the neutron density is relatively low. There are tens of millions to hundreds of billion neutrons per cubic centimeter, and the process takes thousands of years, and you can make elements as heavy as bismuth. During the rapid process, or r-process, you have densities well exceeding 10^21 [a 1 followed by 21 zeros] neutrons per cubic centimeters. Because of this huge density, within a second, you can make elements in the actinides, in the uranium and plutonium region. And the intermediate process, or i-process, has neutron densities and a time scale in between those two other processes.
Q: How long have we known about the i-process?
MW: The mechanism was first proposed in 1977, but then it was almost forgotten about until the last decade or so. We have new telescopes that are much better at observing and analyzing light from distant stars, and these new astronomical observations found anomalies in the ratios of elements in certain stars, such as carbon-enhanced metal-poor stars.
There's this indication that something else is going on besides the s- and r-processes. So it's started to become an active research field for many people. It's a relatively new topic, but with a lot of activity in theory, nuclear physics modeling, experimental physics, and astrophysics.
Q: How do these different fields come together to figure out how the i-process works?
MW: There are space-based and ground-based telescopes that capture starlight and analyze it through absorption spectroscopy to determine what elements exist in the stars. But then we have the theoretical physicists and nuclear physics modelers who combine what we know about the i-process, s-process, and r-process, and try to model - based on the nuclear data that are available - and reproduce the abundances that are observed.
These models are very complicated, and there are many different reactions that are all interconnected. Many reactions will have large uncertainties and impact, so they'll come back to experimental nuclear physicists like me and ask us to measure them so we have better nuclear data and can constrain the processes. It's a constant back and forth of requesting measurements, providing measurements, and figuring out what needs to be improved.
Q: What are the challenges in making some of these measurements?
MW: One of the most fundamental and important quantities we need to measure are "neutron capture cross-sections," the likelihood for a neutron to get absorbed by a nucleus. That's fundamental in all the neutron capture processes. The problem is that neutron capture cross-sections can mostly be measured only when you have a stable material. But for the i-process nuclei, where the i-process proceeds across the nuclear chart, almost all of them are unstable nuclei. It's not easy to directly measure them with the direct experimental techniques that exist, so instead we have to employ indirect techniques.
Q: How do you make these measurements?
MW: We need accelerator facilities and, typically, gamma-ray and particle detectors. We have done measurements here at the 88-Inch Cyclotron going back at least 15 years, and we've also done measurements at FRIB [the Facility for Rare Isotope Beams], Argonne National Laboratory, and around the world. Our needs for certain measurements bring us to different facilities that have the detection equipment or the beams that we need. We can use very light to heavy accelerated particles for our measurements. Whenever you can create a nuclear reaction and you can reconstruct all the energies, and don't lose energy that goes undetected, we can use it to extract these properties to feed into the astrophysical models.
Q: What are the open questions about the i-process?
MW: We want to know if it really explains some of these anomalies of ratios we have seen in particular kinds of stars. Another interesting open question is whether the i-process terminates similarly to the s-process in the bismuth region, or if it can go beyond. There's a model that predicts that the i-process could go all the way to the actinides [elements 89-103]. This i-process research has really only taken off in the last decade, so there are many open questions. We're starting to answer some of them, but sometimes when we solve one question, another comes up.
Q: What are the potential applications for this research into the i-process?
MW: Neutron capture reactions needs are plentiful. Many applications need to understand capture reactions on unstable nuclei, so the indirect measurements we're making provide valuable data on the neutron capture reactions we can't measure directly. This can feed into modeling for the next generation of nuclear reactors. It can also help decide whether it's worthwhile to pursue a certain new medical isotope. For example, could it be produced sufficiently in the lab? The indirect techniques also reduce nuclear data uncertainties, which can help engineers when they need to design something. There are also national security and non-proliferation applications.
Q: How do you see the field evolving over the next 5 to 10 years? What do you hope researchers will accomplish in that time?
MW: I think in that time we can really nail down the i-process. Across the world there are tens of data sets currently being analyzed from beam time already received. And there are many future experiments in preparation that will give us more data. So I think we'll put the i-process on very solid footing, similar to the s-process, and bring down the experimental uncertainties to the point where we have a lot of confidence in what the i-process can or cannot do. So maybe it explains the anomalies we see, or maybe not - and then it's time for the theorists to think again.
