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EI2GYB > ASTRO 19.11.25 15:21l 164 Lines 9655 Bytes #56 (0) @ WW
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Subj: Stars forge elements in new, uncharted ways:
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Stars forge elements in new, uncharted ways: Experimental physicist discusses
the 'i-process'
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.
Why are you interested in how elements are formed?
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.
What are the processes we do know behind element formation?
Over 99% 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 1021 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.
How long have we known about the i-process?
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.
How do these different fields come together to figure out how the i-process
works?
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.
What are the challenges in making some of these measurements?
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.
How do you make these measurements?
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.
What are the open questions about the i-process?
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.
What are the potential applications for this research into the i-process?
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.
How do you see the field evolving over the next 5 to 10 years? What do you hope
researchers will accomplish in that time?
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.
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