快猫短视频

FAR, FAR away, on a towering peak at the centre of the Enchanted Island, sits
the Magic Element. No one has ever come near it, and perhaps no one ever will.
The Magic Element presides over a mysterious landscape long spoken of by the
sages of nuclear physics. They foretold that the Enchanted Island, also known as
the Island of Stability, would be home to a group of giant atomic nuclei with
extraordinarily long lives, and that the Magic Element would be the
longest-lived of them all.

Many of these giants are said to have strange shapes that no other nuclei
possess. But perhaps chief among the Island鈥檚 many riches is a seductive promise
of immortality: adventurers that reach its shores might give their name to a
previously undiscovered element.

The Island, however, is a difficult place to get to. It lies off the edge of
the known world of nuclei, far across the treacherous Sea of Instability. But
this year, after some thirty years of questing, human voices finally shattered
the island鈥檚 calm. First to arrive was a band of scientists from the Joint
Institute for Nuclear Research in Dubna, Russia, and the Lawrence Livermore
National Laboratory in California. Together, they created a nucleus containing
175 neutrons and 114 protons.

It was the heaviest nucleus ever seen. And it existed for a whole thirty
seconds鈥攁n age in nuclear physics鈥攂efore decaying into an isotope of
element 112. There was no mistaking where the researchers had landed: a
half-minute lifetime placed them firmly on the Island of Stability. But in June,
before the Dubna-Livermore team had finished their party, another ship sailed
over the horizon. Physicists at the Lawrence Berkeley National Laboratory in
California found elements 116 and 118.

Now that the two research teams have reached the shores of the island, they
can lift their gaze and contemplate the seemingly impossible climb towards the
Magic Element.

The Sea of Instability exists because there is a limit to the size of a
nucleus. In a very heavy element, the enormous electrical repulsion of all the
positively charged protons tends to make the nucleus unstable. It decays
rapidly, usually by emitting an alpha particle, made of two protons and two
neutrons.

Up to a certain size, stability can be regained by adding extra neutrons.
They have the same nuclear attraction as protons, but without the electrical
repulsion. That is why heavy nuclei are packed with neutrons
(see Diagram).
Eventually, though, the repulsion becomes too much. It seems to be
impossible to have much more than a hundred protons in a nucleus.

Magic shells

But a bit of magic can make the seemingly impossible possible. The
nucleons鈥攑rotons and neutrons鈥攖hat make up a nucleus are arranged in
鈥渟hells鈥�: the nucleons in each shell have similar quantum properties, and
similar energies. But quantum theory allows only a limited number of neutrons or
protons in each shell. Adding more means you have to start a new shell, which
means adding a lot more energy. So a nucleus with a full or nearly full shell
has a low energy compared with neighbouring nuclei, making it stable. The magic
numbers are simply the numbers of nucleons that complete a shell: 2, 8, 20, 28,
50, 82 and so on.

With a magic number of protons or a magic number of neutrons, some heavy
nuclei are surprisingly long-lived. And a 鈥渄oubly magic鈥� nucleus, with magic
numbers of both neutrons and protons, is even more stable. Lead-208, for
example, with 82 protons and 126 neutrons, effectively lasts forever.

The Island of Stability is anchored on the next such doubly magic nucleus. It
isn鈥檛 easy to work out exactly where this should be鈥攑redicting the shell
structure involves solving the Schr枚dinger equation for the motions of
neutrons and protons within the nucleus. But according to calculations first
made by Polish physicist Adam Sobiczewski in the mid-1960s, the doubly magic
nucleus should have 184 neutrons and 114 protons, and the majority of nuclear
physicists believe that these are the right coordinates.

Creating this Magic Element is the ultimate goal. But there are also many
other undiscovered elements to find en route to the centre of the Island, and
each of them might have properties that have never been seen before.

鈥淎n exotic structure has been predicted for some superheavy nuclei,鈥�
says Sobiczewski. Some of them should be hollow: their protons and neutrons
almost absent from the centre of the nucleus, sticking instead to an outer wall.
Other giant nuclei may be hideously deformed, squashed into ellipsoids or
bearing eccentric lumps and bumps.

But how was anyone to reach the Island? The basic idea was to bang together
lighter nuclei to make heavier ones. Doing this at random would probably leave
physicists running their experiments well into the next millennium. The products
of nearly all such reactions are unstable, breaking up into smaller nuclei or
emitting radioactive particles. So to find reactions that will leave a
superheavy nucleus behind, theorists had to return to their equations.

Knowing the theory is one thing, doing the experiments is quite another: the
probability of getting a head-on collision with just the right amount of energy
is frighteningly low. Which is why it was thirty years before anyone could land
on the Island.

Precious stuff

The Dubna-Livermore team, led by Yuri Oganessian, got there by bombarding a
lump of plutonium for a month with a high-energy beam of calcium ions. They
calculated that this could create a nucleus with 114 protons and 175 neutrons.
No isotope of element 114 had never been made.

Each plutonium atom had 94 protons and 150 neutrons, and the calcium had 20
protons and 28 neutrons鈥攁n extremely rare and expensive isotope. Getting
these two to fuse into a superheavy element was extraordinarily difficult
because the spare energy left over from a collision tends to instantly break
apart anything that heavy. Success relied on tuning the velocity of the particle
beam to give the combined nucleus just the right amount of spare energy. Then
there is a chance that it will only lose a few neutrons rather than fall
apart.

Eventually, as planned, three neutrons fizzed off the ephemeral combination
of calcium and plutonium, leaving a relatively stable nucleus containing 175
neutrons and the magic 114 protons. 鈥淲e bought four cases of champagne,鈥� said
Andrei Popeko, one of the Dubna team. Such celebrations, he says, are rare among
Russian research teams.

The Berkeley team had a similarly arduous voyage. Their cyclotron accelerator
flung 2 trillion krypton ions at the lead target every second, of which 1 in 10
000 ions hit a lead nucleus and 1 in 1000 had a head-on collision. Of those, 1
in 10 000 actually fused. And of those that fused, just 1 in 1 000 000 created
an atom of element 118.

But received wisdom said the Berkeley expedition should have sunk. To create
element 114, the Dubna team had flung light ions at high speed into heavy ones,
whereas the Berkeley experiment fused two middleweight nuclei. 鈥淭he common
knowledge was that this kind of reaction wouldn鈥檛 work,鈥� says Ken Gregorich,
leader of the Berkeley group. As a pair of such nuclei approach, their large
positive electrical charges tend to make them change shape and fly apart.

But Robert Smolanczuk, from the Soltan Institute for Nuclear Studies in
Poland, challenged this prejudice. Arriving at Berkeley as a Fulbright scholar
last October, he approached Gregorich armed with the results of some new
calculations: the optimal energies for the beam of krypton nuclei. At these
energies, they would be more likely to fuse with lead nuclei before they have a
chance to deform and fly off. The result, Smolanczuk assured Gregorich, would be
element 118.

鈥淲e didn鈥檛 expect it to work,鈥� says Gregorich. But the krypton ions were
cheap and easy to obtain, so he decided it was worth a try. Eleven days and
10¹⁸ krypton particles later, Victor Ninov, one of the team鈥檚 chief
researchers, was analysing the data from the detectors. Going through the
figures at home late one night, he found traces of three atoms of element 118.
Within a millisecond of being created, each one had alpha decayed to create
element 116, and then emitted several more alpha particles before ending up as a
more familiar heavy isotope. Ninov鈥檚 shouts of celebration woke his wife.

A millisecond lifetime might not seem very long, but it was long enough to
excite the team. 鈥淲e consider something that lives a microsecond very
long-lived,鈥� says Neil Rowley, the scientific director for nuclear physics at
the Strasbourg Institute for Subatomic Research. A proton or neutron can cross a
nucleus in around 10^-22 seconds, so all the particles from a pair of colliding
nuclei will rattle around inside a new nucleus and settle down on about that
timescale. If the nucleus goes on to live for a whole microsecond, it鈥檚 a real
survivor.

So the achievement of the Dubna group, the thirty-second lifetime of element
114, shows the amazing potential of the Island of Stability. 鈥淣ow you鈥檙e getting
to lifetimes where you can take these things and do all sorts of experiments,鈥�
says Rowley. 鈥淚f you can produce enough of these you can do physics and
chemistry, and examine the nuclear structure.鈥�

There is still great controversy over how long isotopes on the Island might
last. Most say the upper limit will be about a hundred years. But others think
the most stable elements could last hundreds of millions of years. 鈥淲e are very
far from the predicted closed neutron shell,鈥� says Popeko. He points out that
adding six neutrons to element 112 can increase the lifetime by four or five
orders of magnitude.

If they last that long, some of the most stable isotopes from the Island
could still be around in the Earth鈥檚 crust, having been formed in supernovae
billions of years ago. But Rowley points out that no one has seen them. 鈥淭hese
things were looked for by the Americans for military reasons鈥攊f you can
find this stuff in nature, and separate it, you can make a pretty powerful bomb
with a relatively small amount of material.鈥�

Having failed to find these isotopes in the crust, the only way to be sure
what they are like is to create them in the lab. Now that the celebrations have
died down, however, the teams have realised that further progress will be
difficult. 鈥淚t was a great success, but we understand that it鈥檚 only a
beginning,鈥� says Popeko.

One thing is clear: using stable nuclei won鈥檛 do the trick. 鈥淭hey have too
few neutrons to reach the centre of stability,鈥� says Smolanczuk. Paradoxically,
the solution is to introduce some instability: radioactive, neutron-rich nuclei
would provide enough neutrons to get near the island鈥檚 centre.

Unfortunately, changing the target isn鈥檛 practical. 鈥淚t would be better to do
the experiments with plutonium-246鈥攜ou would get another two neutrons,鈥�
explains Rowley. But its half-life is only 11 days. 鈥淵ou can鈥檛 make a target out
of it, certainly not for a 40-day experiment.鈥�

Instead, researchers are concentrating on making the accelerated beam more
neutron-rich. This, too, is a formidable task: the promising nuclei, such as
krypton-96, have lifetimes of just a few seconds. In that time, they have to be
created by other nuclear collisions, then separated out and accelerated into the
target. Another problem is that it is not yet possible to produce strong enough
radioactive beams: an experiment might take thousands of years to produce any
results.

Inwards and upwards

But Dietrich Habs of Munich University has a plan to get round this. He is
putting together a particle accelerator that will recycle the beam: all the
particles that fail to hit the target properly will be turned through 180
degrees using magnetic deflectors, accelerated again to replace lost energy and
turned through another 180 degrees to rejoin the main beam. The particles can be
recycled perhaps a thousand times before they decay.

The first such experiment, using neutron-rich caesium and krypton particles
obtained from the fission of uranium-235, will probably not take place for five
years. The plan is to fire the krypton into a lead target to make a heavy
isotope of element 118. Gottfried Muenzenberg of the Institute for Heavy Ion
Research in Darmstadt, Germany, is collaborating with the Berkeley team in this
scheme, but he has reservations about the success of such projects. Theorists,
he says, can鈥檛 be sure that neutron-rich unstable beams will actually create
stable superheavy elements. But he believes it鈥檚 worth a try. 鈥淚t may be
difficult, but in principle it should be feasible,鈥� he says.

Even if the radioactive beams do work, Habs warns, they still might not be
enough to take physicists all the way to the centre of the Island. 鈥淵ou鈥檙e much
closer in, but there鈥檚 still a little bit鈥攖wo or three
neutrons鈥攎issing,鈥� he says.

Those vital, frustrating few neutrons could keep the centre of the Enchanted
Island safe from prying eyes for a long time. At the moment, the slopes that
lead up to it are too steep and difficult to climb. Gregorich says he plans to
spend a little more time on the beach, improving their equipment, mapping out
their immediate vicinity, trying different accelerator energies and seeing what
other elements they might create. There鈥檚 another good reason to hang around and
work on these easier elements: each new one needs a name. Our children may yet
see Dubnassium, Oganessium or Gregorichium on their periodic tables.

Eventually, Gregorich does plan to head for the centre, albeit without much
confidence. 鈥淚t鈥檚 going to be a few years. And we probably won鈥檛 get there,鈥� he
adds.

But then, the Island itself once seemed like something from a fairy tale:
even if it did exist, it would be forever beyond our reach. And yet physicists
are now camping on the beach and eyeing its mysterious peak. Perhaps, with a
little ingenuity, they will start to climb it one day.