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Scientists have been slowly extending the periodic table one element at a time, pushing to higher and
higher masses, and have discovered some incredibly useful materials along the way. But the elements
at the current end of the table are so unstable that they decay almost as soon as they’re created
in our particle accelerators. Have we reached the end of the line of discoverable elements? There
are new rows of the periodic table to unlock, and more stable versions of known heavy elements to
synthesize—and while our accelerators are coming up short, astronomers have found a
strange cosmic phenomenon that may populate the periodic table beyond our wildest dreams.
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A little while ago we talked about the Island of Stability—the patch of the periodic table where
nuclear physics tells us we may find heavy elements of unusual stability,
and perhaps with unusual and useful properties. We stopped short of talking about how to actually
make these things. In fact, it turns out it may be next to impossible—at least with current methods
of smashing nuclei together. But astrophysicists of all people may have figured out a way to create
elements in the island of stability. Instead of building a large nucleus from a smaller one,
how about taking two atomic nuclei the size of cities, smash them together, and see what comes
out? Sounds impractical, but the universe does this pretty regularly with neutron star mergers.
Today we’re going to see how this phenomenon may allow us to prove the existence of the Island of
Stability, and how new techniques in the lab are pushing the far end of the periodic table.
Before we can fix the problem, let’s talk about the problem itself—nuclear instability. So a
nucleus is basically a balancing act between the repulsive force between protons trying to
blow the nucleus apart via the electromagnetic force, and both protons and neutrons sticking
together via the strong nuclear force. This is why neutrons are important: they ensure protons
don’t get so close together that their repulsive tendencies overwhelm the attractive strong force.
For small nuclei, the most energetically favorable arrangements tend to involve
roughly equal numbers of protons and neutrons. For many of the most common elements in the universe,
the most stable form is when neutron number equals proton number—like helium-4,
carbon-12, oxygen-16, and magnesium-20. By the way, these numbers are atomic mass number,
and are the total number of nucleons—protons plus neutrons—and they designate the isotope
of the named element. The element is defined by the atomic number—the number or protons,
and each element can have different isotopes—different numbers of neutrons.
For the lighter elements, neutron and protons numbers are often similar.
But the repulsive electrostatic or “Coulomb force” grows quickly as we add protons. The
further we go along the periodic table the more neutrons we need as padding between
protons to dampen that force. For example, iron-56 has 4 more neutrons than protons,
gold-179 has 21 more neutrons than protons, and lead-208 has a ‘neutron excess’ of 44.
Beyond a certain total number of nucleons, it becomes impossible for the short-range
strong nuclear force to hold the nucleus together against the enormous electrostatic
repulsion. The heaviest truly stable element is lead-208, with 82 protons and 126 neutrons.
Part of what makes Lead-208 special is that it is ‘doubly magic.’ Nucleons in a nucleus
are not little balls held together by glue like we see in our textbooks,
they’re fully quantum mechanical objects that occupy 'shells' the same way that electron
orbitals do. And just as with electron orbitals, these nuclear shells are more stable when properly
filled up with “magic” numbers of protons and neutrons—as is the case with lead-208.
Without full nuclear shells, an element is more susceptible to radioactive decay.
But beyond lead-208, even full nuclear shells can’t save these overstuffed nuclei. At some
point, excess repulsive force between the protons causes it to fall apart—in
the most unstable cases splitting into two smaller nuclei in what we call spontaneous
fission. More commonly the decay is via the emission of an alpha particle,
which is the same thing as a helium-4 nucleus with 2 protons and 2 neutrons, or a beta particle,
which is just an electron being spat out to turn a neutron into a proton. All elements beyond lead
are radioactive and eventually decay this way. And the larger the nucleus, the quicker this is
likely to happen. Some of these radioactive elements decay slowly enough that we still
find them in Earth’s crust—albeit diminished in quantity from when they were forged in supernova
explosions and the like. Other naturally occurring radioactive elements are the decay
products of more massive nuclei, or are created when stable nuclei are hit with cosmic rays.
But some nuclei are so short-lived that we never find them in nature. These we have to
make in labs by smacking more stable nuclei with alpha particles or neutrons to build up to higher
atomic numbers. That has allowed us to crack the 100 mark in number of protons—to what we call the
super-heavy elements. But to get all the way up to the current heaviest known elements—currently
Oganesson, with atomic number 118—we can’t build them up slowly. That’s because nuclei this large
decay quicker than we can add neutrons or alpha particles. Instead, these are made by smacking
two smaller nuclei together—for example Oganesson was first made by smashing calcium nuclei into
californium atoms—quite a lot of times to get even a barely detectable amount of Oganesson.
By the way, calcium-48 has been a go-to for this sort of element building because
it also has doubly-magic filled nuclear shells. Very recent work by scientists at Berkeley Lab
has used titanium-50 instead, which greatly increases the yield of super-heavy elements,
and the researchers have their eyes set on the next row of the periodic table,
in particular the as-yet-unnamed elements 120.
But these new elements are going to be extremely unstable, just as the current
super-heavy’s are. For one thing, the versions of these
elements that we create with current methods don’t have enough neutrons.
Remember that the ideal ratio of neutrons to protons increases with atomic number. That
means smashing two lighter elements together will always give you an isotope of the heavy
element with similar neutron-to-proton ratio as the lighter elements that went in. I.e. not
enough neutrons. We could try bombarding the new element with a beam of neutrons to supplement,
but it’s very difficult to do that before the unstable isotope decays.
The extra impetus to find that neutron-to-proton sweet spot is to hit a new doubly-magic region of
the periodic table, similar to lead-208 and calcium-48. That’s this theoretical island
of stability, which is thought to exist at around atomic number 110-114—elements
from darmstadtium to Flerovium, but for isotopes with around 180 neutrons. The versions we’ve
created are at least 10-neutrons short of this and have half-lives between milliseconds and minutes,
while the stable isotopes may have half-lives of at least hours, and perhaps up to a few years.
It’s not yet known whether we can make island of stability elements with particle accelerators. But
there may be another way to at least prove the existence of the island. In the past,
when our accelerators haven’t been up to the job we’ve turned to the universe. For
example the most energetic cosmic rays have vastly more punch than anything the large
hadron collider could produce. Well, it turns out there’s one event, phenomenon,
that can potentially create island of stability elements—neutron star mergers.
Neutron stars are basically city-sized atomic nuclei with the mass of stars.
They are the end state of the core of a massive star after its explosive death,
when all that central matter collapses under gravity and is converted to almost
pure neutrons at nuclear densities. When two neutron stars find each other—perhaps
because their progenitor stars were binary partners—then they’ll eventually
spiral together and merge. From Earth we see these first as a wash of gravitational waves,
immediately followed by a brief gamma ray burst, and then a longer-lasting kilonova explosion.
Perhaps the most important outcome of neutron star mergers is that we now
believe that they are one of the key sources of heavy elements in the universe. When they impact,
huge quantities of neutron matter material is sprayed into surrounding space. Freed from the
extreme pressure of the neutron star, many neutrons decay into protons and electrons,
forming light atomic nuclei. But these nuclei find themselves swimming in a thick soup of
fast-moving neutrons which can quickly build the nuclei up via the r-process,
or rapid neutron capture process. Calculations show that this produces elements all the way
to the end of the naturally-occuring periodic table. We now believe that neutron star collisions
are essential to making many of the heavy elements we see on earth like gold and uranium.
As the collided neutron star settles down into either a much larger neutron star,
or more likely a black hole, the explosion of new heavy elements spreads outward s. Lots
of ridiculously radioactive new elements will be in that cloud,
and these will quickly alpha-decay, beta-decay or fission their way to more stable nuclei.
Because of the extreme abundance of neutrons, there’s no reason that elements in the island
of stability or even heavier shouldn’t be produced. In fact, if heavier elements
are made we’d expect them to undergo a chain of rapid decays until they hit the island, where the
decay chain would stall for up to years in that semi-stable region and that might be detectable.
OK, this is all pretty speculative. Let’s get to some actual observations and evidence.
The first neutron star merger was observed in 2017, simultaneously spotted as a gravitational
wave signal and as a gamma ray burst. GW170817 was then observed as a kilonova—a long-lasting glow of
radiation as all the radioactive byproducts of the merger decayed. That kilonova gave
us evidence that neutron star mergers do at least produce some heavy elements, but there’s
only so much you can determine with a single event. I’ll come back to what these kilonovae
can potentially can tell us as we see more of them—but first some more real evidence. Evidence
that elements heavier than uranium—so-called transuranics—are produced in neutron star mergers.
Stars that formed early in the universe are expected to have relatively few heavy
elements because there hadn’t been many prior generations of stars to make those
elements. But such an otherwise unpolluted star that formed shortly after a neutron star
merger should bear the particular elemental signature of that explosion’s debris. These
stars give us our current best picture of what exact elements are produced in a neutron star
merger. Many of these stars are actually still around, orbiting in the Milky Way,
and we discover them through the unusual combination of elements seen in their spectra.
Now we can’t directly detect any superheavy elements in these stars—remember, these are
stars formed near the beginning of the universe, and so those unstable elements have long-since
decayed. However we can look for an excess of the stable decay products of these elements—ruthenium,
rhodium, palladium, and silver. And we’ve found cases of the high abundances of
these decay products, suggesting that neutron star mergers forged large amounts of elements
like californium—a transuranic that’s a few ticks up the periodic table from uranium.
We haven’t yet pinned down similar evidence for island of stability elements, but perhaps
by finding more of these ancient stars stained by neutron star merger we’ll be able to do so.
There’s also hope of observing island isotopes a little more directly. I mentioned that after
a neutron star merger, there’s something called a kilonova,
which is a fading afterglow of light from the decay of radioactive nuclei. Well,
if extremely heavy nuclei decay through the island of stability,
we might expect that to be reflected in the way the kilonova fades. The way a kilonova changes in
brightness definitely reflects the half-lives of lighter radioactive isotopes—for example,
there’s a brightness bump peaking at around 100 days that’s driven by the decay of isotopes in the
actinide series that have half-lives in that range. The island of stability elements have
half-lives ranging from hours to years, and it’s speculated that the quicker-decaying elements in
this range might leave a detectable bump in the first several hours after the kilanova begins.
In order to detect this, we’d have to catch the kilonova as soon as possible—really right after
we spot the gravitational waves from the merger. We’ve only observed one highly resolved kilonova,
again from GW170817, and it took astronomers 11 hours to find the
electromagnetic source after LIGO caught the gravitational wave signal. By then any
very massive nuclei would have decayed already, so we missed any transition
in the rate of fading caused by the island of stability—if that transition happened at all.
And it’s not clear that these magic island elements can even be produced this way. Our
nuclear physics tells us that the heaviest elements produced in the r-process after
the neutrons run out should have proton numbers of around 100 and neutron numbers
of around 200. If a dozen-ish of those neutrons then beta-decay into protons,
then it’s possible these could wind up as island of stability elements.
But if we’re lucky, and the nucleus doesn’t just fall apart during that chain of beta decay,
then neutron star mergers could produce enormous numbers of nuclei in the island
of stability. We just need to catch a kilonova quickly enough.
Usually it’s our labs here on earth that help us understand what we’re seeing out
there in the universe. Perhaps now we astrophysicists can give back a
little by proving that the island of stability exists—then our labs can
get on with building them. Perhaps we’ll do it with the very next neutron star merger,
and the following cascade of radioactive decay shining at us from distant parts of spacetime.
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