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[SQUEAKING] [RUSTLING]
[CLICKING] MARKUS KLUTE:

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Welcome back to 8.701.

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We're starting a new
chapter now, Chapter 9,

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on nuclear physics.

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And this video is the
first introduction

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into the topic,
where I'm explaining

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some of the terminology
and some of the concepts.

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We dive in much more
detail as we go on.

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So given an atom, you can
specify the number of neutrons,

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the number of protons,
and the number

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of electrons, which is equal
to the number of protons,

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for neutral atoms.

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Atoms of the same element, they
have the same atomic number,

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Z, but they're not all the same.

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Isotopes of the same element
have different numbers

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of neutrons.

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So we can have uranium with
a number of neutrons varying.

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You typically write an
isotope by specifying

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the mass, the number of protons,
and the number of neutrons.

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But that information
is redundant.

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So typically, we simplify
this by just writing things

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like 238 uranium,
and that specifies

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a specific isotope of uranium.

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When talking about
different nuclei,

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we sometimes refer
to them as nuclide,

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atom/nucleus with a
specific number of neutrons

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and a specific
number of protons.

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Isobars are nuclides
with the same mass,

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with the same number, same
sum of protons and neutrons,

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but with varying individual
number of protons and neutrons.

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An isotine is a nuclide with
the same number of neutrons,

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but with varying
number of protons,

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and an isomer is
the same nuclide

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but different
[? eigen ?] states,

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which means that the
energy states are--

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an energy state.

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So we can excite nuclides at
their component particles.

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The nuclear radius
typically can be extracted

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from the mass of the nuclide.

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And it's simply we add
little balls to the sum,

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and it scales with A to the
1/3 the mass of a number

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of elements in this nuclide.

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There's many isotopes.

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There's many nuclides.

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And so you typically can look
at all of them, if you want,

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or a subset of them
in nuclear charts

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that's given here, where we
plot here the number of protons,

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and here the number of neutrons.

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And we look at many more
of those charts later.

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Here's another representation
of the very same thing.

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You see a nuclear chart again.

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And here, what's spotted in
red are the stable nuclei.

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We will see that
nuclei can decay,

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and we'll understand why
they decay and in what form

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they decay.

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It's a core part
of this chapter,

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understanding how
nuclei can decay,

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and what we can learn about
them by studying their decays.

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One way to look at
it, for example,

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we see that here I
plot Z over A, so

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the number of protons over the
sum of the number of protons, Z

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plus N. And you see that
most of the stable nuclei,

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with the exception of the one
with very small mass number,

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have less protons than neutrons.

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So there's an
excess of neutrons.

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This can also be seen here.

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The stable nuclei are
typically on or below this axis

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where Z is equal to A.

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Radioactive decays can be
characterized, typically,

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as a parent nuclide, and
then a daughter nuclide.

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And so radioactive
decay is a process

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in which an unstable nucleus
spontaneously loses energy

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by emitting
particles, ionization

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particles and radiation.

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The decay and the loss
of energy results, then,

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in an atom of one type, the
parent particle or parent

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nuclide, transforming into
another type of an atom,

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or the daughter nuclide.

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We have already
looked at decay rate

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in the concept of particle
physics interactions,

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but we can define this very
similar here, the decay rate,

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or sometimes it is
called decay constant.

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And then as we
did before, we can

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define the mean lifetime or the
half life of a parent nuclide.

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So this is it for
the introduction.

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And in the next lecture, we'll
start looking in the energy

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which is used to bind
the nuclei, the protons

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and neutrons together, and
how we can understand this

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from an empirical model.