Understanding the nucleus means understanding just two things: the
Proton and the Neutron. The proton is a electrically positive particle of
matter. Electrical charges attract and repel. Like charges repel,
opposites attract. Therefore protons generally repel each other even over
large distances. Neutrons are nearly identical to protons, but have no
electrical charge.
The other thing to know about neutrons and protons in that they really
stick together. When they get close enough to stick, they do so and
release a large amount of energy. They can only be unstuck by applying
an identical amount of energy to release them.
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Our world consists of a bewildering variety of objects, materials,
substances, liquids, solids, gasses, naturally-occurring, manmade, and some synthesized by other living things. Humans are
pre-disposed to trying to make sense and order of this variety, a
curiosity about what is inside matter, and a need to learn to
manipulate our material surroundings. We became aware over
thousands of years that certain useful materials could be produced
by combining certain others. We also became aware that certain
substances were somehow more basic or “elemental” and could
not be manufactured by any means.
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Take gold, for example. People attempted for hundreds of years to
manufacture gold out of more plentiful materials, but failed in every
attempt. They did, however, succeed in synthesizing a new science:
Chemistry.
Yet gold remained aloof as one of a growing list of substances which
could not be created out of other substances. We called them Elements.
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By arranging squares representing these substances in order of mass and
grouped by chemical properties, this interesting and useful pattern
emerges for which we had no rational explanation. Why do elements
combine in only certain ratios? What makes one substance different
from another? Why can we combine sodium, a poisonous white metal,
with chlorine, a poisonous gas, into a delicious condiment and essential
dietary mineral called salt?
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One idea that seemed to help our understanding was that perhaps the
elements were composed of tiny, indivis ible units. These units could
combine in ratios of 1:1 or 2:1 etc, but never any in-between ratios. This
would account for the exact ratios of elements in compounds.
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How big would these atoms be? As early as 1865, the year Abraham
Lincoln was shot, it was understood that atoms would have to be very
small. So small, that 1 gram of hydrogen gas would contain about a
million billion billion atoms. A single grain of sand would contain more
atoms than there are grains of sand on the entire beach, about 10 billion
billion. If the atoms in this paperclip were the size of this steel ball, then
the end of the paperclip would be about 50 km in diameter, larger than
Perth.
Could there ever be any way of proving that something so small even
exists?
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One bit of evidence is the existence of something called Brownian
Motion. When small objects are viewed under a microscope, they appear
to be jostled around. It was only in 1905 that this evidence could be
considered proof of the existence of atoms, when Albert Einstein gave
the first correct mathematical description of how miniscule atoms could
produce this effect.
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Now it is possible to make direct images of atoms. In 1987, researchers
in Zurich, Switzerland invented a way of “seeing” individual atoms by
scanning over a surface in roughly the way a blind person reads braille.
So the existence of atoms is no longer a matter of debate, but is an
integral part of our everyday lives and the technology we use. But what
are atoms? What makes the atoms of one element distinct and different
to those of another element? Long before the existence of atoms was
proven, people were trying to find out.
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In 1897, J. J. Thompson discovered that electricity could cause a ray of
matter to emit from a metal electrode. He called them “cathode rays”
and found out that they were individual particles of mass and negative
electric charge. He hypothesized that they come out of atoms, and that
the atom must resemble this picture: a massless haze of positive electric
charge studded with hard negative particles that came to be called
Electrons.
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In an attempt to verify this, a kiwi gentleman named Ernst Rutherford
conducted experiments involving a stream of charged particles passing
through a few layers of atoms. The expected result of the particles
passing through with minor deflections was not found.
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Instead, he found that about 1 out of every 8000 particles would ricochet
off something inside the atom. The idea of the Nucleus was born.
Something very heavy, positively charged, and very, very small lay at
the heart of the atom.
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How small? Imagine that Subiaco Oval is an atom. How big would the
nucleus of that atom be? As big as a building? A car? A football?
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Actually, this big. No larger than a marble. It would not even be visible
from outside the atom, even magnified to this extent. This in turn raised
another mystery: how did the electrons manage to stay so far away from
the nucleus? What made the atom so big compared to the nucleus?
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Niels Bohr thought he knew, but Heisenberg expressed a lot of
uncertainty. Einstein did not wish to be bothered with such small
matters.
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It took Erwin Schroedinger and his famous equation to finally explained
everything about the atom, how it works, why it behaves the way it does.
In short, all of chemistry was now understood from the point of view of
the fundamental forces of nature.
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Now we knew almost everything there was to know, and were ready to
tackle the questions of Life itself. Almost. There were still a few details
to be swept up – a few cracks in the wall. But these cracks opened up an
entirely new world that we could not have supposed existed before.
What were these missing pieces?
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Firstly, the Curies and others had reported seeing unexplained rays and
energies emanating from certain elements which could not be accounted
for through chemistry. Also, certain elements appeared to spontaneously
change into something else, violating the age-old rule that the Alchemists
had proven through centuries of persistent failure.
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Also, where was Element 43? And why were Cobalt and Nickel in
reverse order in the periodic table? Chemically, this is how they fit, but
their masses are reversed. To answer these questions and more, it would
be necessary to discover the structure of the nucleus.
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When we finally did, we found a rather simple thing. The nucleus of any
atom is just a cluster of neutrons and protons. The number of protons
gives the nucleus its total charge, determines how many electrons it will
attract, and therefore which element it will be. The neutrons add mass to
the nucleus and give it stability.
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The nucleus comes in many combinations of protons and neutrons, so we
need a way of keeping track of them all. We can create a grid in which
each square is a unique combination of protons and neutrons. The
number of neutrons will be counted from left to right, and nuclei with a
fixed number of protons will be in rows with increasing numbers of
protons towards the top of the grid.
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When we fill in the details of what is known about each unique
combination, it looks like this. Only about 30% of the complete table is
shown here.
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Looking closely, we find that each element occupies a row of squares.
Each square is a unique “Isotope.” The black squares are stable nuclei,
while the coloured squares are combinations which do not form stable,
compact shapes. They may exist temporarily, but will spontaneously
break apart. 50% of all such nuclei break up during each period of the
nuclei’s “half life” shown in the square, in seconds, minutes, days or
years. Many elements have more than one isotope, and the element’s
atomic mass is a weighted average of all stable isotopes.
For stable nuclei, the percentage of each isotope as it occurs naturally is
shown. For unstable nuclei, the most probable means of decay is
indicated in the lower left corner.
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Some nuclei are more stable than others. When a nucleus forms, it
releases energy, but there is usually some left over. This chart roughly
shows the “leftover energy” of the different nuclei. When they are lined
up in order of size from left to right, an interesting pattern emerges.
If smaller nuclei combine to form larger ones, there is a reduction of this
“leftover energy” within the nucleus. Where did it go? It was released
into the environment, eventually becoming heat that we can access.
Also, if larger nuclei on the right are broken up into smaller nuclei, heat
is released as well. Only iron, the most stable nucleus, cannot release
any further energy by either breaking it up or by adding to it.
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How does a nucleus break up and why? A large nucleus is under a lot of
strain from the positively-charged protons pushing against each other
through their positive electrical charges, but being stuck together with a
bunch of sticky neutrons. If the nucleus were given a little kick, it might
be enough to unstick them and two or more pieces will fly apart, repelled
under enormous static electrical force.
This little kick usually comes in the form of a free neutron in whose path
the nucleus happens to lie. Protons usually cannot get close enough to
the nucleus to affect it due to the strong electrical repulsion it would
experience.
This breakup also usually produces a couple more neutrons.
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If those neutrons in turn happen to bump into other nuclei, more
breakups could occur, and the process could repeat. This is a “chain
reaction.” The conditions needed for a chain reaction are that there be a
sufficient quantity of marginally unstable nuclei such that the odds of a
neutron escaping the mass without encountering another nucleus are less
than 50-50. Such an assembly of matter will spontaneously fission.
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Here is the map of matter, the chart of the nuclei, centered on the
uranium isotopes. U235 is spontaneously fissile, but constitutes less than
1% of all uranium. To get it to “burn,” the concentration of U235 must
be increased up to 5% or 10%. Enrichment is the process of separating
some the U238 in order to make a burnable nuclear fuel.
However, if a neutron happened to land on U238 and stick to it, it would
become U239, which decays into Neptunium 239, then into Plutonium
239. Pu239 is a highly fissile, weapons-grade material with one
redeeming quality: it decays into useful U235. Thus, a “breeder reactor”
can produce its own fuel from otherwise-useless U238, of which there is
a vast quantity, 99% of all uranium.
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A nuclear reactor is simply an assembly of enriched fuel rods separated
by “control rods” used to control the reaction. By inserting the rods, the
free exchange of neutrons is stopped, and the reaction dies out. By
removing the rods, the neutrons from other rods set off reactions in each
other and the reaction grows exponentially. The control rods must be
continually shifted to maintain a safe and constant reaction. Surrounding
the rods is a fluid – often water – which is heated by the nuclear reaction.
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The rest of the reactor is exactly like a coal-fired plant. A boiler turns
water into steam which spins a turbine connected to a generator. The
exhaust steam is recirculated through a cooler, which uses an immense
cooling tower to dissipate the waste heat. The tower has nothing to do
with anything nuclear, and can be seen on most modern coal stations,
too.
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Another kind of reactor dispenses with moveable control rods and
instead uses a passive regulation scheme. The pebble-bed reactor uses
enriched fuel packed inside a tennis-ball-sized sphere of carbon and
metal cladding. A number of these are then placed into the reactor vessel
together where they react in the normal way by exchanging neutrons. As
the reaction progresses, the cladding becomes hotter and begins to
restrict the passage of neutrons through a process described as “doppler
scattering.” Thus, the reaction is limited by temperature and is
essentially self-regulating. One must only take care that the cladding
remains intact, or all hell could break loose.
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Another potential nuclear fuel is Thorium, two rows below Uranium on
the Map of Matter. It consists of only one nearly-stable isotope
compared to Uranium’s 3 isotopes. 100% of thorium can be used,
compared to uranium’s 0.72% However, it requires an external source of
neutrons to make it burn. This could be an advantage, as the reaction
would be easy to stop if it got out of hand. Simply turn off the flow of
neutrons into the reactor, and the reaction dies down. It cannot melt
down or explode.
When thorium absorbs a neutron, a chain reaction is started which results
in the production of U233, a fissile nuclear fuel with the advantage that it
cannot be turned into Plutonium or anything else suitable for nuclear
weapons. Therefore, countries which can’t be trusted with uranium
might be trusted with thorium.
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To illustrate what all the fuss over nuclear energy really is, this graph
compares a tonne of coal to a kilogram of enriched uranium.
Unfortunately, one of the bars is grossly exaggerated, or else it would be
too small to be visible in the picture.
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And those unanswered questions that chemistry alone could not answer?
Well, cobalt and nickel really are in the correct order, even though cobalt
is heavier than nickel. It has, on average, slightly more mass due to
having more neutrons. 100% of cobalt has 32 neutrons, while 68% of
nickel has just 30 neutrons, and 26% has the same number, and an
insignificant number have more. Cobalt’s 32 neutrons make up for the
extra proton that nickel has.
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What DID ever happen to element 43? As it happens, no combination of
neutrons and protons in which there are exactly 43 protons happens to
form a stable nucleus. Therefore, Technetium does not occur naturally
and was never seen until it was produced artificially in a nuclear
laboratory.
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The science of the nucleus is simple, really. Just neutrons and protons.
That’s all you need to think about to be a nuclear physicist!
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