Before I begin, let me clarify that I’m not claiming you will be a nuclear physicist in
10 minutes. Most of you probably don’t want to do that anyway. It’s
not for everyone. But if you want to understand what I believe are the most important and
essential concepts in nuclear physics, then stick around because I’m about to make you dangerously
interesting at your next dinner party. And that’s about to happen right about now…
Before we start, I want to acknowledge the inspiration for this video,
which comes courtesy of today’s sponsor MagellanTV.
It’s where I saw a documentary called, “North Korea vs USA:
A Nuclear Chicken Game“ – It’s a fascinating story of how the Kim
dynasty of North Korea has been working on becoming a nuclear power for over 60 years,
during which time the US has gone through 13 presidents with different policies, and how this
proved to be a big advantage for North Korea. It got me thinking about nuclear physics in general.
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Atoms are made of a positively charged nucleus surrounded by negatively charged electrons.
The nuclei of atoms are made up of protons and neutrons. These are called nucleons.
Because all protons have the same charge, and like charges repel,
force can be calculated using the classical coulombs law equation here:
You can see that the force between charged particles in inversely proportion to the
distance between them “r.” Because positively charged protons are in such close proximity to
each other in a nucleus, the electrostatic repulsive force between them is enormous.
Two protons typically separated by the width of a neutron is on the order of 60 Newtons, or 12 lbs.
But what keeps them together against this tremendous repulsion, is an even stronger
force called the strong nuclear force, which is about 100 times stronger than
electromagnetism. This force comes from one of the 4 fundamental forces in the universe,
the Strong Force. But this force, unlike gravity and electromagnetism,
operates at very small distances, only about the width of a proton. And it only operates within
and between protons and neutrons. Electrons, photons, and neutrinos are not affected by it.
You can think of the strong force like Velcro. When two objects carrying the strong force are
very close, they stick to each other like glue. But if they are far apart, they are unaffected.
This means that a nucleus can only grow so large. The reason is that
the two opposing forces electromagnetism and the strong force operate differently.
The strong force only acts at short distances, so a given proton or neutron’s attractive effect
diminishes rapidly as the nucleus grows larger. But since electromagnetism has an infinite range,
its force accumulates as more and more protons are added together. So all the
protons in a nucleus participate in repelling each other. This repulsion quickly overwhelms
the attractive effect of the strong force as more protons are added, disrupting
the stability of the nucleus. The heaviest stable element is lead, which has 82 protons.
The number of protons in an element is important because it is solely responsible
for its atomic properties. This proton number corresponds to the position of the element
on the periodic table. A given element or atom can have the same number of protons,
but different numbers of neutrons. These are called isotopes of the element.
They have exactly the same chemical properties and differ only in mass.
Stable nuclei consist of roughly the same number of protons and neutrons. The neutrons serve to
provide additional strong force needed to keep the nucleus stable. Without neutrons,
not even two protons can be held together against their repulsion.
Free protons are stable, but free neutrons are not stable.
Lone neutrons decay into a proton, an electron, and an antineutrino within about 15 minutes.
But inside a nucleus they remain stable because it is energetically unfavorable
for them to decay. In other words, the energy gained from a neutron decaying to a proton is
lower than the increased energy required to keep an additional proton in the nucleus,
if there are plenty of other protons around. But this type of neutron decay in the nucleus
is not impossible and can happen in very neutron-rich isotopes of a given element.
But a nucleus cannot have too few neutrons, or the nucleus will not be stable.
It also cannot have too many neutrons, because eventually the strong force can
no longer keep them from decaying into a proton, and forming a different element.
Protons and neutrons, just like all quantum objects are subject to the laws of quantum
mechanics. One consequence of this is that quantum particles have a very small but
non-zero probability of appearing anywhere in the universe. The reason I mention this
is because it is tied to a certain kind of natural radioactivity called alpha-decay.
If a nucleus is very large, or has an excess number of protons or neutrons, then the strong
force can barely hold it together. Quantum objects have a higher probability of appearing on the
other side of an energy barrier, if the barrier is not very high. In the case of very large nuclei,
such as uranium, the energy needed to liberate protons or neutrons is small enough that,
sooner or later, an alpha particle, which is essentially a helium nucleus consisting of 2
protons and two neutrons, can quantum-mechanically tunnel through the strong-force barrier and appear
outside the nucleus. Then, they speed away very fast. This is what we call radioactivity.
If you want to learn more about quantum tunneling, I have a video right up here.
There are three forms of natural radioactivity, Alpa- Beta- and Gamma-decay. An alpha-particle consists of two
protons and two neutrons. This type of radiation happens via the mechanism I just described.
A beta-particle is a high-energy electron. In some very large nuclei, what are barely bound together,
a second fundamental force, called the weak force, can compete with the strong force and cause a
neutron to decay even though it is in the presence of protons. As I said before, when a neutron
decays to a proton, it gives off an electron and an antineutrino. This electron is the beta-particle.
A gamma-particle is a high-energy photon, also called a gamma-ray. Gamma-rays are usually emitted by excited
nuclei that have been created after either an alpha or beta decay. The reason these processes
release such high energy photons is because they involve nuclear processes, which are due
to the strong force. And since the strong force is so much stronger than, for example the next
strongest force, electromagnetism, whenever there is a shift to lower in energy levels within the
nucleus an enormous amount of energy is released. And this high energy is in the form of gamma rays.
The three forms of radioactivity have very different abilities to
penetrate matter. Alpa-particles can be stopped by a thin piece of paper.
Beta-particles can penetrate your skin, but can be stopped by a thin sheet of metal like aluminum
foil. But Gamma-rays are difficult to stop and can penetrate through even an inch of lead.
Why is this the case? Alpha-particles consisting of two protons and two neutrons are relatively big,
heavy, and thus slow. Because of their large size, they can be easily blocked. Beta-particles travel
much faster and can penetrate matter more easily because of their higher velocity and smaller size.
Gamma-rays have no charge at all and are moving at the speed of light,
so they are very penetrating. These photons pretty much have to hit a nucleus directly to be stopped.
Alpa- and beta-decay change the atomic identity of the decaying nucleus because they change the number of
protons. Alpa-decay subtracts two protons, so the element moves down two notches in the periodic
table. Beta-decay changes a neutron into a proton, or vise versa, so it changes the type of element either up or down.
A radioactive nucleus is characterized by something called its “half-life.” What this
means is that if I have a certain number of radioactive atoms, lets say 16 atoms,
and its half-life is 1 week. Then one week later I will have ½ of this or 8 atoms remaining. And
in 2 weeks, I will have half of the half, or 4, of the original atoms left. And so on.
The half-life is a statistical concept. If you are holding the 16 atoms, you will not
be able to determine whether the atom will decay one week from now or a billion weeks
from now. You can only say that any atom has a 50-50 chance of doing so in the time of one week.
If a large nucleus, like some isotopes of uranium like U235, is hit by a particle,
usually a neutron, then it will split into two smaller nuclei. This is called nuclear fission.
If the total mass of the two smaller nuclei is less than that of the uranium before it was hit,
the missing mass is turned into energy via E = mc2. This is called fission.
In the case of certain isotopes of uranium and plutonium, such as U235, and Plutonium 239,
when these are hit by a neutron. They split into two lower mass nuclei, and three neutrons are also
thrown off. If there is enough fissionable nuclei in high enough concentration, then it is possible
for the three thrown off neutrons to, in turn, fission three more nuclei, which in turn fission 9
more nuclei, which in turn fission 27 more nuclei, creating what’s called a chain reaction. This is
the mechanism behind the atomic bombs dropped on Hiroshima and Nagasaki during World War II.
A kind of opposite of fission is fusion. This happens when two small nuclei such as hydrogen,
which consists of only a single proton, can be brought close enough together they fuse into a
single nucleus. Fusion is very difficult to achieve, because the protons strongly
repel each other. Only gases heated to millions of degrees Celsius have atoms
moving fast enough that they can approach each other close enough to achieve fusion.
Quantum tunneling also plays a role here. A hydrogen bomb achieves very high levels
of energy release, by using the heat of a fission bomb to trigger hydrogen fusion.
I have video on how a hydrogen bomb works if you want to learn about it.
The Sun is powered by hydrogen fusion, but its mechanism is quite different than
that of a hydrogen bomb. It does not need nearly has high temperature because its
mass creates tremendous pressure in its core, pushing hydrogen nuclei together.
The sun has such a huge amount of hydrogen atoms that the low probability of fusion
still amounts to a very high number of nuclei fusing together.
So, now if you have seen this whole video,
I officially crown you a “Dangerously Interesting Nuclear Physics Aficionado" (DINPA)!
Please use your newfound powers for good, and not evil. I will see you in the next video my friend.