Speaker 1: Welcome to Bedtime Astronomy. Explore the wonders of the cosmos
Speaker 1: with our soothing Bedtime Astronomie podcast. Each episode offers a
Speaker 1: gentle journey through the stars, planets, and beyond, perfect for
Speaker 1: unwinding after a long day. Let's travel through the mysteries
Speaker 1: of the universe as you drift off into a peaceful
Speaker 1: slumber under the night sky.
Speaker 2: I want you to just take a second and look
Speaker 2: around you. Think about the device you're listening on, the
Speaker 2: coins in your pocket, maybe the ring on your finger,
Speaker 2: every single atom of the heavy elements in those things,
Speaker 2: you know, copper, nickel, gold, silver, anything heavier than iron. Really, yeah,
Speaker 2: is an ancient relic.
Speaker 3: It is it was forged inside a star, or maybe
Speaker 3: a dying star, or in some cases a star that
Speaker 3: exploded in a just a cataclysmic event.
Speaker 2: We're literally made of stardust, and we're standing on stardust.
Speaker 3: That's not just poetry, it's literal truth. And for physicists,
Speaker 3: you know, figuring out the exact recipes for that start
Speaker 3: us the cosmic mechanisms that build everything from the ground
Speaker 3: beneath our feet to the phone in our hands. That's
Speaker 3: one of the big fundamental quests.
Speaker 2: And for a long long time it felt like we
Speaker 2: had most of that recipe book figured out. We had
Speaker 2: our two main characters, right, the two protagonists of element creation.
Speaker 3: Right, the s process and the process, the slow and
Speaker 3: the rapid exactly.
Speaker 2: We thought we understood the slow, steady way elements were built,
Speaker 2: and we also thought we understood the violent explosive way.
Speaker 3: And they did a remarkable job. I mean, they accounted
Speaker 3: for a stunning amount of what we see out there.
Speaker 3: If you looked at the sort of the chemical inventory
Speaker 3: of our own solar system, those two processes gave you
Speaker 3: this beautiful, almost textbook explanation for how you get from
Speaker 3: iron all the way up to uranium.
Speaker 2: That's where things get interesting because modern science, specifically modern astronomy,
Speaker 2: got a lot more precise, it really did, and these
Speaker 2: new incredibly detailed observation of certain types of stars started
Speaker 2: sending back data that well, it just didn't add up.
Speaker 3: The numbers were just wrong.
Speaker 2: Yeah, the ratios of elements. They were seeing, things like strontium,
Speaker 2: ettrium's or conium, they just couldn't be explained by the
Speaker 2: slow process or the rapid process. It was like finding
Speaker 2: a dish that had ingredients from two completely different recipes
Speaker 2: that shouldn't work together.
Speaker 3: It was a huge signal, a persistent anomaly that told
Speaker 3: us our cosmic recipe book was well incomplete. Astronomers were
Speaker 3: seeing the final products of a process that had to
Speaker 3: be something in the middle, not slow, not rapid, but
Speaker 3: some kind of middle ground our models just weren't ready for.
Speaker 2: And that's exactly what we're going to be diving into today.
Speaker 2: We are unpacking the proposed solution to this cosmic puzzle,
Speaker 2: the intermediate or as it's known, the eye process. We're
Speaker 2: going to get into the physics that makes it so unique,
Speaker 2: explore the really mind bending experimental challenges of trying to
Speaker 2: measure it here on Earth. And this is the amazing part.
Speaker 2: Talk about why solving this mystery have these surprising, profound
Speaker 2: implications for technology we use every day.
Speaker 3: It really does. I mean, we are talking about designing
Speaker 3: the next generation of nuclear reactors, for one.
Speaker 2: It's incredible.
Speaker 3: This whole field requires this amazing convergence of expertise. You've
Speaker 3: got the biggest telescopes in the world, the fastest supercomputers
Speaker 3: running simulations, and the most powerful particle accelerators all working
Speaker 3: on this one question, you know, where did all the
Speaker 3: elements heavier than iron really come from? It's still one
Speaker 3: of the biggest unanswered questions in physics, and this eye
Speaker 3: process might just be the key to unlocking the final chapters.
Speaker 2: Okay, so before we get to the eye process, let's
Speaker 2: just lay the groundwork. Let's talk about the core mechanism.
Speaker 2: Because when we say creating elements heavier than iron, we
Speaker 2: are almost always talking about one thing. Neutron capture.
Speaker 3: Yes, that is the engine absolutely, you know, for everything
Speaker 3: lighter than iron, carbon, oxygen, the stuff of life, that's
Speaker 3: all built through stellar fusion. Just burning hydrogen into helium,
Speaker 3: helium into carbon, and so on.
Speaker 2: That's like a furnace.
Speaker 3: It's a furnace exactly. But once you get to iron,
Speaker 3: that furnace shuts down. Iron is the ultimate ash. Fusing
Speaker 3: iron actually costs you energy instead of releasing it, so
Speaker 3: the star hits a wall. To get any heavier, you
Speaker 3: have to switch to a completely different mechanism.
Speaker 2: And that mechanism for over ninety nine percent of the
Speaker 2: heavy elements is neutron capture. Could you just walk us
Speaker 2: through the basic physics of that. How does it work?
Speaker 3: Of course, so you start with what we call a
Speaker 3: seed nucleus. Think of it as an iron nucleus, which
Speaker 3: is very stable. This seed is sitting deep inside a
Speaker 3: star in an environment where there's a flow of free
Speaker 3: neutrons floating around. The iron nucleus will every so often
Speaker 3: grab one of those neutrons. It captures it. Now it's
Speaker 3: a little heavier. Maybe a bit later it grabs another one.
Speaker 3: So its mass is going up step by step.
Speaker 2: But it's still iron, right, because the number of protons
Speaker 2: hasn't changed.
Speaker 3: That is the absolute key. It's just a heavier isotope
Speaker 3: of iron. But if you keep packing more and more
Speaker 3: neutrons into that nucleus, eventually you reach a tipping point
Speaker 3: where it becomes unstable. It's radioactive. It has too much tension.
Speaker 2: And it needs to relieve that tension.
Speaker 3: Somehow exactly, and it does that through a process called
Speaker 3: beta decay.
Speaker 2: And this is where the magic happens. This is the
Speaker 2: transmutation what happens in beta decay.
Speaker 3: So in beta decay, one of those extra neutrons inside
Speaker 3: the nucleus spontaneously transforms. It turns into a proton, and
Speaker 3: it spits out an electron to keep the charge balanced.
Speaker 2: So you've just added a proton.
Speaker 3: You've added a proton, and when you change the number
Speaker 3: of protons, you change the fundamental identity of the element.
Speaker 3: You've climbed one rung up the periodic table. Iron becomes cobalt,
Speaker 3: Cobalt becomes nickel, and so on. It's this beautiful step
Speaker 3: by step ladder, all powered by the cycle of capturing
Speaker 3: neutrons and then undergoing radioactive decay.
Speaker 2: And you mentioned earlier that the speed of this process
Speaker 2: is what really matters. That's what separates the two classical
Speaker 2: ways of doing this.
Speaker 3: Precisely, it all comes down to the neutron density and
Speaker 3: the timescale. They dictate which path you take up that ladder.
Speaker 2: So's talk about the first path, the slow one, the
Speaker 2: S process.
Speaker 3: Right, the S process or slow neutron capture. This is
Speaker 3: happening inside certain types of evolved stars, specifically in what
Speaker 3: we call asymptotic giant branch stars AGB stars.
Speaker 2: So what are the conditions like in there? When we
Speaker 2: say slow, what does that actually mean in cosmic terms?
Speaker 3: We're talking about a process that unfolds over thousands of years.
Speaker 3: It's a very leisurely paced The environment has a neutron
Speaker 3: density that's well. It sounds high to us, but it's
Speaker 3: relatively low for a star. We're talking maybe tens of
Speaker 3: millions up to a few hundred billion neutrons per cubic centimeter.
Speaker 2: Which on a nuclear timescale is an eternity. It means
Speaker 2: the nucleus has plenty of time between one neutron capture
Speaker 2: and the next.
Speaker 3: That's the crucial part. Because the captures are so infrequent.
Speaker 3: If a nucleus becomes unstable, it has all the time
Speaker 3: in the world to undergo that beta decay and stabilize
Speaker 3: itself before the next neutron comes along.
Speaker 2: So it's always staying close to the most stable configurations exactly.
Speaker 3: It walks a very predictable path right along we call
Speaker 3: the valley of stability on the chart of all nuclei,
Speaker 3: and this clean, steady process is responsible for creating a
Speaker 3: lot of the stable heavy elements we know, all the
Speaker 3: way up to bismuth, which is element eighty three.
Speaker 2: But bismuth isn't the end of the line. If we
Speaker 2: want the really good stuff, the gold, the platinum, not
Speaker 2: to mention radioactive elements like uranium, the s process just
Speaker 2: can't get us there. For that, you need do I
Speaker 2: need violence.
Speaker 3: You need the R process, the rapid neutron capture process.
Speaker 3: This is the complete opposite extreme. It requires environments with
Speaker 3: just I mean apocalyptic neutron densities.
Speaker 2: We're talking about a huge jump here.
Speaker 3: I'm mind boggling jump. For the S process, we were
Speaker 3: talking about billions of neutrons per cubic centimeter. For the
Speaker 3: R process, we're talking about densities well over ten to
Speaker 3: the power of twenty one, a trillion trillion neutrons per
Speaker 3: cubic centimeter.
Speaker 2: That is just an impossible number to comprehend. What kind
Speaker 2: of cosmic event could possibly create that level of neutron saturation.
Speaker 3: Only the most catastrophic events we know of the leading
Speaker 3: candidates for a long time, where the core collapse supernovae
Speaker 3: of massive stars. But more recently the evidence is pointing
Speaker 3: very strongly toward the merger of two neutron stars.
Speaker 2: When two of the densest objects in the universe.
Speaker 3: Collide exactly in that kind of environment. The neutron capture
Speaker 3: is so unbelievably fast that the nucleus has absolutely no
Speaker 3: time to decay and stabilize. It can't keep up, not
Speaker 3: even close. The whole thing happens in less than a second.
Speaker 3: A nucleus just gets flooded, swallowing dozens of neutrons one
Speaker 3: after another, pushing it way way out into the most exotic,
Speaker 3: unstable neutron rich territory on the nuclear chart. These are
Speaker 3: isotopes that might only exist for milliseconds. And then what
Speaker 3: And then after the explosion is over and the neutron
Speaker 3: flood subsides, this whole chain of incredibly unstable nuclei starts
Speaker 3: to decay back towards stability. It's this rapid accumulation followed
Speaker 3: by a cascade of decays that forges the heaviest possible elements,
Speaker 3: including the actinides like uranium and plutonium.
Speaker 2: So for a very long time that was the complete story.
Speaker 2: You had the slow, steady S process giving us elements
Speaker 2: up to bismuth, and the fast violent R process giving
Speaker 2: us the really heavy, rare stuff. It seemed to cover
Speaker 2: all the bases.
Speaker 3: It did. It was a really elegant picture. But then
Speaker 3: as astronomers started using these new high resolution spectrographs on
Speaker 3: very old, very pristine stars, they started seeing cracks in
Speaker 3: that picture. Ani the anomalies they were finding elemental signatures.
Speaker 3: These abundance patterns that were clearly made by neutron capture,
Speaker 3: but the ratios were all wrong. They didn't fit the
Speaker 3: slow path and they didn't fit the rapid path. It
Speaker 3: was too fast for one, but not extreme enough for
Speaker 3: the other. It was a clear sign that there had
Speaker 3: to be a third way, a mechanism that fell squarely
Speaker 3: in the middle.
Speaker 2: Which is the perfect setup for what was really a
Speaker 2: rediscovery of this missing link. The eye process.
Speaker 3: Yes, and structurally it's very simple to define. The eye
Speaker 3: stands were intermediate and it sits, you know, right in
Speaker 3: between the sn R processes, both in terms of how
Speaker 3: many neutrons are available and how fast it all happens.
Speaker 2: So we're talking about neutron densities that are way higher
Speaker 2: than what you'd find in a typical one of those
Speaker 2: AGB stars, high enough that the captures happened pretty quickly,
Speaker 2: but still what many many orders of magnitude less than
Speaker 2: the insane conditions of a neutron.
Speaker 3: Star Merger exactly. The density is sort of in this
Speaker 3: sweet spot, roughly ten to the power of fourteen, maybe
Speaker 3: up to ten to the sixteen neutrons per cubic centimeter.
Speaker 2: Okay, so what does that density mean for the timing.
Speaker 3: It means it's fast enough to build up some really
Speaker 3: neutron rich nuclei, pushing the reaction path a good way
Speaker 3: off that value of stability we talked about. But it's
Speaker 3: slow enough that the whole process takes say hours or
Speaker 3: maybe even a few days instead of being over in
Speaker 3: a single second.
Speaker 2: Now, the history of this idea is fascinating to me.
Speaker 2: This isn't some brand new concept that someone dreamed up
Speaker 2: last year. The idea was first proposed what back in
Speaker 2: nineteen seventy seven, that's right.
Speaker 3: It was a compelling theoretical idea even then, but the
Speaker 3: problem was there was no solid observational evidence to back
Speaker 3: it up, so it kind of well was almost forgotten about.
Speaker 3: It remained this niche theory because we just didn't have
Speaker 3: the tools to go out and find its signature in
Speaker 3: the cosmos.
Speaker 2: You couldn't drive the research because it was nothing.
Speaker 3: To measure precisely. You can have a great theory, but
Speaker 3: if you can't test it, it's hard for it to
Speaker 3: gain traction in the wider community.
Speaker 2: So what change What brought the eye process back from
Speaker 2: the dead in the last say ten or fifteen years.
Speaker 3: In a word technology, specifically, huge advancements in our telescopes
Speaker 3: and our detectors. We're talking about new generations of both
Speaker 3: space based observatories like Hubble and massive ground based telescopes
Speaker 3: that can now analyze starlight with a level of precision
Speaker 3: that was unimaginable in the seventies.
Speaker 2: And the key technique here is absorption spectroscopy, right.
Speaker 3: Yes, that's the one. You look at the rainbow of
Speaker 3: light coming from a star and you see these dark
Speaker 3: lines like a barcode. Each dark line core responds to
Speaker 3: a specific element in the star's atmosphere, absorbing that particular
Speaker 3: color of light, and.
Speaker 2: The darker the line, the more of that element there.
Speaker 3: Is, exactly. And with these new instruments, we can measure
Speaker 3: that barcode with incredible detail. And that's when we started
Speaker 3: finding these really hard anomalies in very specific types of stars.
Speaker 3: Stars that were perfect laboratories for this kind of thing.
Speaker 2: The research you're referencing specifically calls out carbon enhanced, metal
Speaker 2: poor stars. Now that's a mouthful. Can you break that
Speaker 2: down for us? Why is that type of star so important?
Speaker 3: Okay, so that name is basically a description of a
Speaker 3: cosmic fossil metal poor is astronomer speak for a very
Speaker 3: very old star.
Speaker 2: Right, because in astronomy, metal is anything heavier than hydrogen
Speaker 2: and helium exactly.
Speaker 3: So a metal poor star is one that formed early
Speaker 3: in the universe's history, when it was still mostly just
Speaker 3: hydrogen and helium. It's a pristine sample. It hasn't been
Speaker 3: polluted by generations of other stars exploding and seeding the
Speaker 3: galaxy with heavy elements.
Speaker 2: So it's chemical make is a much cleaner record of
Speaker 2: whatever nucleosynthesis happened inside of it, or to it correct.
Speaker 3: And the carbon enhanced part is the other key. It
Speaker 3: means that at some point these old stars got a
Speaker 3: big dump of carbon onto their surfaces, most likely from
Speaker 3: a companion star that evolved and puffed away its outer layers.
Speaker 3: And when astronomers pointed their new powerful spectrographs at these
Speaker 3: specific old pristine stars.
Speaker 2: They found the smoking gun.
Speaker 3: They found the smoking gun the ratios of certain heavy
Speaker 3: elements barium, lanthanum, europium. They just could not be made
Speaker 3: by any combination of the S and R processes. It
Speaker 3: was the definitive evidence that a third mechanism, an intermediate one,
Speaker 3: had to be at work in these environments.
Speaker 2: So let's talk about the physics of why. How does
Speaker 2: that intermediate timing hours or days create a different set
Speaker 2: of elements than the other two.
Speaker 3: This is where it gets really cool. It's all about
Speaker 3: hitting what we call branching points on the nuclear chart.
Speaker 3: These are specific unstable nuclei where there's a competition between
Speaker 3: capturing another neutron and undergoing beta decay. The fork row
Speaker 3: a fork in the road exactly. In the S process,
Speaker 3: it's so slow you almost always take the decay path.
Speaker 3: The nucleus stabilizes before another neutron.
Speaker 2: Arrives, And in the R process, it's so fast you
Speaker 2: just blow right past the fork. You capture another ten
Speaker 2: neutrons before decay is even an option precisely.
Speaker 3: But the EYE process, because of its intermediate speed, it
Speaker 3: hits some of these forks where the half life of
Speaker 3: the unstable nucleus is on the order of hours or days, the.
Speaker 2: Same time scale as the process itself.
Speaker 3: Yes, so the nucleus gets to that fork and it
Speaker 3: sits there. There's a real competition does it decay or
Speaker 3: does it capture another neutron, And that specific timing allows
Speaker 3: the reaction path to zigzag in a way that's totally unique.
Speaker 3: It can bypass some elements that the cess process makes
Speaker 3: a lot of and create other elements in ratios that
Speaker 3: neither of the other two can explain.
Speaker 2: So it's this unique timing that produces that specific anomalous
Speaker 2: fingerprint that astronomers were seeing.
Speaker 3: That's it. It's the convergence of the app sstronomical observations
Speaker 3: with the nuclear physics theory that told us, yes, the
Speaker 3: eye process isn't just a possibility, it's a necessity. It's
Speaker 3: a required piece of the cosmic puzzle.
Speaker 2: Okay, so it seems like there's a strong consensus that
Speaker 2: the eye process is real and necessary, which brings us
Speaker 2: to the next, much more complicated part. How do you
Speaker 2: actually solve it. Let's talk about this research ecosystem, because
Speaker 2: it's not one person in a lab. It sounds like
Speaker 2: a massive, coordinated effort across completely different fields of science.
Speaker 3: It's a fundamentally iterative process. It's complex, it's expensive, and
Speaker 3: it absolutely relies on constant, detailed communication between these different groups.
Speaker 3: I like to think of it as a three leg stool.
Speaker 3: You have observation, theory, and experiment, and.
Speaker 2: If any one of those legs is weak, the whole
Speaker 2: thing falls over it does.
Speaker 3: It all starts with the observers, the astronomers. They're the
Speaker 3: ones at the telescopes pointing at these stars and collecting
Speaker 3: the light. They provide the ground truth.
Speaker 2: They're providing the what and the where. They're saying, look
Speaker 2: at this carbon enhanced metal poor star. It has one
Speaker 2: hundred times more europium relative to iron than our sun.
Speaker 2: Does explain that exactly.
Speaker 3: They provide the target. They give us the final abundance
Speaker 3: pattern and say to the other groups, your models, your experiments,
Speaker 3: they must reproduce this specific fingerprint.
Speaker 2: So once they have that target, the baton gets passed
Speaker 2: to the theoretical physicists and the modelers, and in.
Speaker 3: Some ways they have the hardest job. Their task is
Speaker 3: to build a virtual star inside a supercomputer. They have
Speaker 3: to model everything, the star's temperature, its pressure, convection, these
Speaker 3: things called thermal pulses and AGB stars, and simulate its
Speaker 3: evolution and its nucleosynthesis over millions or billions of years.
Speaker 2: I can't even imagine the complexity of those simulations.
Speaker 3: They are monstrously complicated, and to run they rely on
Speaker 3: a huge library of pre existing nuclear data. Things like
Speaker 3: reaction rates, decay, half lives for thousands of different isotopes.
Speaker 2: But this is where the problem starts, and where the
Speaker 2: feedback loop begins.
Speaker 3: This is it. When they run these dudably complex models,
Speaker 3: they'll find that the final outcome, the amount of barium
Speaker 3: or lanthanum produced, is extremely sensitive to a handful of
Speaker 3: specific nuclear reactions along the eye process path. Okay, And
Speaker 3: often when they look up the data for those critical
Speaker 3: reactions in the nuclear databases, they find that the numbers
Speaker 3: have what we call large uncertainties. The value might be known,
Speaker 3: but only to within say, a factor of five or ten.
Speaker 2: So the model basically says, okay, the final abundance of
Speaker 2: barium should be x, but because we're not sure about
Speaker 2: this one key reaction, X could be anything between ten.
Speaker 3: And one thousand precisely, And that kind of uncertainty is
Speaker 3: completely useless if you're trying to match the very precise
Speaker 3: data coming from the telescopes. It's at that point that
Speaker 3: the modelers pick up the phone figuratively and call the
Speaker 3: experimental nuclear physicists.
Speaker 2: Like Mathiswheediking, the physicists whose work we're drawing on today.
Speaker 3: People exactly like him. The modelers go to the experimentalists
Speaker 3: and they say, listen, our models are completely stuck. The
Speaker 3: outcome is dominated by the uncertainty in this one specific
Speaker 3: neutron capture reaction on this one specific unstable nucleus. We
Speaker 3: need you to go to your lab and measure it
Speaker 3: for us. We need better data.
Speaker 2: So this massive cosmic question gets boiled down to a
Speaker 2: very specific, actionable challenge for a team at a particle
Speaker 2: accelerator here on.
Speaker 3: Her Yes, their job is to design an experiment that
Speaker 3: can reduce that uncertainty, to shrink those error bars. And
Speaker 3: it's this constant back and forth. The experimentalists provide new,
Speaker 3: more precise data, the modelers plug it into their simulations.
Speaker 3: The simulations get better, which then often highlights another reaction
Speaker 3: that needs to be measured even more precisely. It's a
Speaker 3: collaborative dance that's pushing our knowledge forward, one reaction at
Speaker 3: a time.
Speaker 2: So let's zoom in on that experimental work, because this
Speaker 2: is where the difficulty just goes off the charts. What
Speaker 2: is that single most fundamental piece of data that the
Speaker 2: modelers are always asking.
Speaker 3: For, the absolute number one most critical ingredient for any
Speaker 3: neutron capture model, whether it's sr or I is something
Speaker 3: called the neutron capture cross section.
Speaker 2: Okay, that sounds pretty technical. Is there a more intuitive
Speaker 2: way to think about what a cross section is?
Speaker 3: Absolutely the easiest way to think about it is as
Speaker 3: the size of the nucleus from the neutron's point of view.
Speaker 3: It's the probability that if you shoot a neutron at
Speaker 3: a nucleus it will actually get captured, that it will stick.
Speaker 2: So a big cross section means it's a big, easy target.
Speaker 2: A small cross section means it's a tiny target that
Speaker 2: the neutron will probably just miss.
Speaker 3: That's a perfect analogy. And if you don't know that
Speaker 3: cross section, you have no idea how often the capture
Speaker 3: will happen. And if you don't know that, you can't
Speaker 3: predict how fast you climb the periodic table, your whole
Speaker 3: model falls apart.
Speaker 2: Okay, so we need to measure these cross sections. But
Speaker 2: this is where the huge problem comes in, especially for
Speaker 2: the eye process.
Speaker 3: This is the massive challenge. Measuring a cross section is
Speaker 3: I won't say easy, but it's relatively straightforward. If you're
Speaker 3: working with a stable isotope, you can make a physical
Speaker 3: target out of the material like a thin foil, stick
Speaker 3: it in a beam of neutrons and just count how
Speaker 3: many captures happen direct measurement.
Speaker 2: But the eye process, by its very nature, doesn't happen
Speaker 2: with stable.
Speaker 3: Stuff, not at all. Because of that intermediate speed and
Speaker 3: neutron density, its reaction path wanders far away from the
Speaker 3: valley of stability. The nuclei that are crucial for the
Speaker 3: eye process are almost all unstable.
Speaker 2: And when you say unstable, what kind of time scales
Speaker 2: are we talking about.
Speaker 3: We're talking about half lives that range for maybe a
Speaker 3: few days down to fractions of a.
Speaker 2: Second, which means making a physical target out of them
Speaker 2: is completely impossible. You can't make a foil out of
Speaker 2: something that disappears a second after you create it, You
Speaker 2: absolutely cannot.
Speaker 3: So direct measurement techniques are off the table, and this
Speaker 3: is what forces scientist into this incredibly clever and complex
Speaker 3: field of indirect techniques.
Speaker 2: Okay, this is what I really want to understand. How
Speaker 2: on earth do you do this? How do you measure
Speaker 2: the probability of a neutron capture without ever actually you know,
Speaker 2: capturing a neutron on the nucleus you care about.
Speaker 3: It's a beautiful piece of experimental physics. It's a kind
Speaker 3: of substitution. So first you need a very advanced particle
Speaker 3: accelerator facility, a place that can produce beams of these
Speaker 3: rare unstable isotopes.
Speaker 2: We're talking about major international.
Speaker 3: Facility, yeah, places like the eighty eight inch cyclotron at
Speaker 3: Berkeley Lab, or the New Facility for Rare Isotope Beams
Speaker 3: FRIB at Michigan State or Argone National Lab. These are massive,
Speaker 3: cutting edge machines.
Speaker 2: So step one is you use one of these machines
Speaker 2: to create a beam of the specific unstable nucleus the
Speaker 2: astrophysicists asked for. That alone sounds incredibly difficult.
Speaker 3: It's a huge challenge. Once you have that beam, you
Speaker 3: can't hit it with neutrons, so instead you hit it
Speaker 3: with a different particle that acts as a sort of surrogate,
Speaker 3: a stand in for the neutron. A common method is
Speaker 3: to use something called a deuteron.
Speaker 2: A deuteron is the nucleus of heavy hydrogen, right, one
Speaker 2: proton and one neutron stuck together exactly.
Speaker 3: So you take your beam of unstable nuclei and you
Speaker 3: shoot it through a target made of deuterons. When one
Speaker 3: of your unstable nuclei hits a deutero on, the deuteron
Speaker 3: might break apart. The unstable nucleus grabs the neutron from
Speaker 3: the deuteron, and the proton goes flying off.
Speaker 2: So you've successfully added a neutron, just not a free one.
Speaker 3: Correct. We call it a DPRP reaction for deuteron in
Speaker 3: proton out Now, this is not the same as a
Speaker 3: pure neutron capture. But and here's the clever part. By
Speaker 3: surrounding the collision point with incredibly sensitive detectors, we can
Speaker 3: measure the properties of that outgoing proton with extreme precision.
Speaker 3: It's energy, the angle it flies off at.
Speaker 2: You're reconstructing the crime scene.
Speaker 3: Basically, that's a great way to put it. We're looking
Speaker 3: at the debris field. By applying the laws of conservation
Speaker 3: of energy and momentum and some sophisticated nuclear theory, we
Speaker 3: can use the information from that proton to work backwards
Speaker 3: and infer the properties of the nucleus we created.
Speaker 2: And from those properties you can calculate what the cross
Speaker 2: section for a direct neutron capture would have been.
Speaker 3: That's the final step. It's a very complex chain of
Speaker 3: logic and measurement. You have to account for every single
Speaker 3: bit of energy in the reaction. You need gamma ray detectors,
Speaker 3: particle detectors, all working in concert. But at the end
Speaker 3: of this enormous experimental effort, you can provide the astrophysicists
Speaker 3: with the number they needed. The neutron capture cross section
Speaker 3: for a nucleus that only exists for a fraction of
Speaker 3: a second. It's an incredible bridge between the lab and
Speaker 3: the stars.
Speaker 2: Okay, we've been deep in the weeds of neutron densities
Speaker 2: and unstable nuclei and particle accelerators. Let's pull back up
Speaker 2: for a minute and connect this all back to life
Speaker 2: here on Earth. Because I think for a lot of
Speaker 2: people listening, it might be hard to see the connection.
Speaker 2: Why should we as a society care about the neutron
Speaker 2: capture cross section of some obscure, short lived isotope? What
Speaker 2: is the tangible so what?
Speaker 3: And that connection is often the most surprising part of
Speaker 3: this whole story. The fundamental nuclear data that is being
Speaker 3: painstakingly gathered to answer a question about stellar chemistry has immediate,
Speaker 3: and I mean immediate relevance to some of our most
Speaker 3: critical technological challenges. Right here.
Speaker 2: It's a classic case of pure curiosity driven research leading
Speaker 2: to massive practical payoffs. So let's break them down. Application
Speaker 2: number one nuclear energy.
Speaker 3: Yes, the data from these indirect measurements, specifically getting more
Speaker 3: accurate cross sections for these unstable isotopes, is absolutely crucial
Speaker 3: for designing the next generation of nuclear reactors.
Speaker 2: Why is that precision so important for a reactor. I
Speaker 2: think most people imagine it's just about keeping a chain
Speaker 2: reaction going safely.
Speaker 3: For older reactor designs, that's more or less true. But
Speaker 3: for advanced reactors what we call generation four V designs,
Speaker 3: things like fast reactors or molten salt reactors, the physics
Speaker 3: is much more complex. They operate with different neutron energies
Speaker 3: and different fuel cycles, and to model how these reactors
Speaker 3: will behave safely and efficiently over a lifespan of sixty
Speaker 3: or eighty years, engineers need incredibly precise data. They need
Speaker 3: to predict exactly how the fuel will change or burn
Speaker 3: up over time, and that involves a whole chain of
Speaker 3: neutron captures and decays, creating all sorts of exotic unstable
Speaker 3: isotopes right there in the reactor core.
Speaker 2: So the same physics happening in the star is happening
Speaker 2: on a small scale inside the reactor.
Speaker 3: It is, and if the cross sections for those isotopes
Speaker 3: are uncertain, the engineer's models are uncertain. They can't accurately
Speaker 3: predict things like neutron poisoning, which is where some of
Speaker 3: these newly created isotopes are really good at absorbing neutrons
Speaker 3: and can actually stall the reaction if you're not careful.
Speaker 2: So better data from astrophysics leads directly to safer, more
Speaker 2: efficient reactor designs.
Speaker 3: Directly, when an experimental physicist manages to reduce the uncertainty
Speaker 3: on a cross section by a factor of ten for
Speaker 3: an astrophysical model, that same factor of ten improvement gets
Speaker 3: plugged into the nuclear engineering codes. It takes this research
Speaker 3: out of the cosmos and puts it right into our
Speaker 3: future energy grid.
Speaker 2: That's a powerful connection. What about application number two you
Speaker 2: mentioned medicine.
Speaker 3: This is another huge one. This fundamental research directly benefits
Speaker 3: the entire field of medical isotope.
Speaker 2: You're talking about the radioactive materials used in things like
Speaker 2: pay scans or cancer therapy.
Speaker 3: Exactly. Many of the most promising new isotopes for say,
Speaker 3: targeted alpha therapy for cancer are very unstable, short lived nuclei.
Speaker 3: And before a pharmaceutical company or a research hospital invests
Speaker 3: billions of dollars to build the infrastructure to produce a
Speaker 3: new medical isotope, they have to know if it's even feasible.
Speaker 2: They need to know if they can make enough of
Speaker 2: it to be useful.
Speaker 3: Right, can it be produced in sufficient quantities and with
Speaker 3: sufficient purity? Answering that question comes down to knowing the
Speaker 3: exact reaction cross sections. The data from the eye process
Speaker 3: research helps them determine the most efficient way to make
Speaker 3: a new isotope, what target material to start with, what
Speaker 3: energy to use for your particle beam, what you're expected
Speaker 3: yield will be. It's a very practical, multi billion dollar
Speaker 3: decision that rests on this very fundamental physics data.
Speaker 2: And finally, application three, which you said extends into engineering
Speaker 2: and even national security.
Speaker 3: Yes, again, it all comes back to reducing those new
Speaker 3: clear data uncertainties. Better data helps in any complex engineering
Speaker 3: field where you have to deal with neutron interactions, designing
Speaker 3: power sources for deep space probes, for example, but it's
Speaker 3: also vital for national security also for things like nuclear
Speaker 3: non proliferation and treaty verification. To be able to detect
Speaker 3: and identify nuclear materials, or to verify that a country
Speaker 3: is adhering to an arms control treaty, you need the
Speaker 3: ability to model and predict nuclear signatures with very high confidence.
Speaker 3: The more accurate our fundamental database of nuclear cross sections is,
Speaker 3: the better our predictive and our verification capabilities become on
Speaker 3: a global scale.
Speaker 2: So the quest to understand a weird chemical signature in
Speaker 2: a star a thousand light years away ends up making
Speaker 2: our power plants safer and our world more secure.
Speaker 3: It's a perfect illustration of how you can never predict
Speaker 3: where fundamental research will lead. You chase the answer to
Speaker 3: a deep academic question about the universe, and you end
Speaker 3: up developing tools and data that have profound benefits for everyone.
Speaker 2: So let's bring it back to the cosmos for the
Speaker 2: last part of our discussion. We've talked about what the
Speaker 2: eye process is and why it's so hard to measure,
Speaker 2: but there's still a huge open question, right. We don't
Speaker 2: really know what its ultimate contribution is.
Speaker 3: That's right. The big unanswered question is about the termination point,
Speaker 3: where does the eye process stop.
Speaker 2: We know this process kind of fizzles out around bismuth
Speaker 2: element eighty three. It just can't build anything.
Speaker 3: Heavier, correct, The nuclear physics just doesn't allow it to
Speaker 3: proceed further. So the big question is does the Eye
Speaker 3: process also hit a wall there or can its unique
Speaker 3: pathway allow it to go beyond bismuth?
Speaker 2: And why is that specific question so critical?
Speaker 3: It's critical because of what lies beyond bismuth the actinides
Speaker 3: elements eighty nine through one hundred and three, things like thorium, uranium, plutonium.
Speaker 2: The heaviest naturally occurring elements.
Speaker 3: Exactly. If the Eye process is confirmed to stop at
Speaker 3: or near bismuth, then it solidifies the idea that the
Speaker 3: R process, those incredibly violent neutron star mergers or supernovae,
Speaker 3: is the sole source for all the uranium and plutonium
Speaker 3: in the universe. It means you absolutely need one of
Speaker 3: those rare catastrophic events to forge them.
Speaker 2: But there's a possibility that's not the case. Some of
Speaker 2: the models based on the data we have so far,
Speaker 2: suggest the Eye process could actually push all the way
Speaker 2: into the actinides.
Speaker 3: That's the tantalizing possibility. If that turns out to be true,
Speaker 3: it would fundamentally change our understanding of cosmic history. It
Speaker 3: would mean that the heaviest elements in the universe don't
Speaker 3: only come from the rarest, most violent explosions.
Speaker 2: They could be made in what slightly less extreme environments.
Speaker 3: Potentially maybe in certain types of supernovae that aren't quite
Speaker 3: energetic enough for a full R process, or maybe in
Speaker 3: the evolution of stars that are a bit more massive
Speaker 3: than the ones that host the size process. It would
Speaker 3: mean the cosmic pathways to creating uranium here on Earth
Speaker 3: might be more diverse than we currently think. It would
Speaker 3: give us a crucial new actor in the story of
Speaker 3: element formation.
Speaker 2: This field is moving so fast. It's really only been
Speaker 2: a major active air research for about a decade. So
Speaker 2: looking ahead, what does the next five to ten years
Speaker 2: look like?
Speaker 3: It's going to be a period of really intense data
Speaker 3: analysis and experiment preparation. Right now, as we speak, there
Speaker 3: are teams all over the world analyzing data from experiments
Speaker 3: that have already been run. There are literally, as Mathiswede
Speaker 3: King puts it, tens of data sets in the pipeline.
Speaker 2: So we're on the cusp of some major new results,
Speaker 2: I think.
Speaker 3: So the goal for the next decade is really to
Speaker 3: nail down the eye process to put it on the
Speaker 3: same solid, confident footing that we have for this process.
Speaker 2: And what does that mean in practice?
Speaker 3: It means designing and running a new generation of experiments
Speaker 3: to bring those experimental uncertainties way way down, to get
Speaker 3: the cross sections so precise that the astrophysical models no
Speaker 3: longer have huge wiggle room. They will have to make
Speaker 3: a firm prediction, and at that point we'll know either
Speaker 3: the eye process, with these new precise inputs, perfectly explains
Speaker 3: the anomalies we see in those old stars, or it doesn't.
Speaker 3: Or it doesn't, and if it doesn't, that's just as
Speaker 3: exciting because it means our understanding is still fundamentally wrong,
Speaker 3: and it forces the theorist to go back to the
Speaker 3: drawing board and come up with something entirely new. Either way,
Speaker 3: the science moves forward.
Speaker 2: It's just such an incredible story. You start with the
Speaker 2: metals in your phone, and you trace their origin back
Speaker 2: through billions of years to the fiery heart of a
Speaker 2: star we'll never see, and the quest to understand that
Speaker 2: star ends up inside a particle accelerator here on Earth.
Speaker 3: It really is a profound link. The qualtion of how
Speaker 3: elements are formed has been central to physics for what
Speaker 3: almost a century now, and the solution isn't coming from
Speaker 3: one field. It requires this massive international effort linking our
Speaker 3: biggest telescopes, our most powerful supercomputers, and these incredible high
Speaker 3: energy accelerators.
Speaker 2: It just underscores that to answer the biggest questions about
Speaker 2: the universe, you need the most complex collaborative solutions right
Speaker 2: here at home. Every chemical element that makes up our
Speaker 2: world was determined by the simple act of a neutron
Speaker 2: being captured by a nucleus billions of years ago, and.
Speaker 3: We're finally getting the tools to read that history.
Speaker 2: Which brings us to a final provocative thought for you
Speaker 2: to take away. Let's imagine that in the next few years,
Speaker 2: all this new precise data comes in from the global
Speaker 2: accelerator experiments and it confirms, yes, the eye process is real,
Speaker 2: but it also proves definitively that it stops cold well
Speaker 2: before the actinides. Let's say it terminates at element eighty
Speaker 2: five and can go no further. Okay, what does that
Speaker 2: constraint that new wall in our knowledge actually tell us.
Speaker 3: If the eye process truly cannot bridge that gap, Well,
Speaker 3: it intensifies the necessity of violence and the cosmos.
Speaker 2: What do you mean by that?
Speaker 3: It tells us something fundamental about the sheer rarity, the
Speaker 3: extreme physics, the absolute violence required to forge the heaviest elements.
Speaker 3: It means that to get the uranium and thorium that
Speaker 3: are in the Earth's crust right now, the elements that
Speaker 3: help powerplate tectonics and geothermal energy, you had to have
Speaker 3: had an event as catastrophic as two neutron stars colliding nearby.
Speaker 3: In our galaxy's history, there was no easier intermediate way.
Speaker 2: So it reminds us that every single element, whether it's
Speaker 2: in our jewelry or our technology, tells this incredibly complex,
Speaker 2: fiery and as we've learned today, is still very incomplete
Speaker 2: cosmic story, a story whose final chapters are still being
Speaker 2: written right now in labs around the world.
Speaker 3: US