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MIKE SHORT: OK, guys.
Welcome to the first filmed
and hands-on installation
of 22.01, Introduction
to Ionizing Radiation.
I'm Mike Short.
I'm the department's
undergrad coordinator.
I'm also your 22.01 instructor.
But I also want to introduce
you to Amelia Trainer
in the back, who one of the
three TAs for the course.
She took it last year.
Everything is still very
fresh in your head, I bet.
AUDIENCE: More or less.
MIKE SHORT: Cool.
So she'll be-- she and
Ka-Yen Yau and Caitlin Fisher
will be with us all
throughout the term.
So if there's something that you
don't like my explanation for,
you've got three people
who just took the course,
and struggled through
my own explanations,
and can say it in
a different way.
So let's start off by taking
your knowledge of physics
from the roughly 1800s education
of the GIRs, the a General
Institute Requirements,
up till 1932
when the neutron was discovered.
And I would argue that
this particle is what
makes us nuclear engineers.
It's the basis behind reactors.
It's what differentiates us
from the high energy physics
folks and everything,
because we've studied these
and use them quite a lot.
And so we want to
retrace Chadwick's steps
in discovering the neutron.
And this is the only
time you're ever
going to see me have a
bunch of words on a slide.
It's not a presentation
technique I like,
but this paper is awesome in
the clarity and expressiveness
of him saying I
ran this experiment
and found something unknown.
I'll use basic conservation
of energy and things
you learned in 8.01
and 8.02 to prove
that it has to be a neutron,
that a neutron must exist.
It's elegant and
brilliant, and I want
to walk you guys through it.
Did any of you get a chance to
read the Chadwick article yet?
OK.
I'll show you where that is,
because hopefully by now you're
all aware that we have
a learning module site.
It's where I'm going
to post everything.
It's where you're going
to submit everything
for the class.
But I'll save to the
end of this class
to go through the
actual syllabus
because I want to
get into the physics.
So let's bring your knowledge
from classical mechanics
and E&M up till about 1895 when
Wilhelm Roentgen used X-rays
and used them to, well, image
something for the first time
ever.
Showing the contrast
between bone and tissue,
he was able to illuminate
the bones in a hand.
And then about a year later, the
X-rays got a whole lot better.
So by then, it was known that
there were high-energy photons
that had differential contrast
between different types
of material.
A year later, after
the nicer X-ray,
J.J. Thompson
conclusively proved
that there is an electron
by taking these cathode
rays, as they were
called at one point,
and sending them through
two charged plates.
And he was able to show
a slight deflection.
So these cathode rays, as they
pass through an electric field,
change direction a little bit.
And from the change
in direction,
you may not know the
mass or the charge,
but you can get the
mass to charge ratio.
Because if you guys remember
from 8.02, from electricity
and magnetism, as
a charged particle
passes through an electric
field, it's deflected.
And the amount of that
deflection, or the curvature,
is based on the mass
to charge ratio.
So Chadwick knew that
electrons existed.
This was a known thing, as well
as alpha, beta, and gamma rays.
So the electrons that
came out of the nucleus
were later renamed beta rays.
And at around the same time,
Ernest Rutherford and Paul
Villard, working in
Canada and France,
discovered that there are some
heavy charge particles that
have very little penetrating
power, while Paul
Villard discovered that there
are some other radiations--
I think he called it produced
by disintegration of nuclei--
that have very high
penetrating power.
And they named them
alpha, beta, gamma
in order of their penetrating
power or their range.
And so it was later figured
out that these were also
high-energy photons.
So this is something to note is
that gamma rays, x-rays, light,
whatever, it's all photons.
However, once this pops
back up, gamma rays
emanate from the nucleus.
So when we refer
to a gamma ray, we
mean a photon that came out
of a nuclear interaction
or a nuclear disintegration,
not an electron transition.
So this is one-- this is what
makes a gamma ray a gamma
ray, is where it comes from.
Otherwise it's a photon.
It behaves just
like any photons.
So what did Chadwick
see in 1932?
This is the first
one-page article
that he sent out to Nature to
say, I found something weird.
So he found out that when
you take alpha particles
from polonium-- so let's say
we had a source of polonium
sending off alpha particles,
which I haven't told you
what they are yet.
It emits a radiation of
great penetrating power
when it hits a
foil of beryllium.
And it was not known
what these things were.
So in goes the
alphas to beryllium.
Something happens,
and something comes
streaming out that couldn't be
explained by current theories.
It was also noticed
that when hydrogen
was placed in front of it,
when a piece of hydrogen
in the form of wax, which
contains a lot of hydrogen,
was put in front of it,
the amount of ionization
increased, as measured by
what's called an ionization
chamber and an
oscillograph, nothing
more than an
almost-sealed chamber,
a piston with some charge on
it that would then deflect.
As it were to pick up
positive or negative charges,
it would move
inwards or outwards
and send an electrical signal to
something like an oscilloscope.
So this was a way that
you could figure out
how many ions were
created by this highly
penetrating radiation
interacting in the ionization
chamber.
And they estimated that
with the old theories,
if this highly penetrating thing
were a photon or a gamma ray,
it would have to have
an energy of 50 times 10
to the 6 electron
volts, or 50 MeV.
He said, OK.
Well, if that's to be basically
the experimental observation,
say, a 50 MeV photon must be
responsible for the ionizations
that we saw.
And so again, this is
what the experiment
looks like where you've got
a polonium source naturally
emitting alpha rays.
They hit a foil, a beryllium.
They produce what he did not
know at the time was neutrons.
We actually do know that
beryllium produces neutrons
pretty well.
Beryllium is an interesting
neutron multiplier.
It undergoes what's called
an n 2n reaction where
one neutron comes in, two
neutrons can come out,
and it transmutes
into something else.
And we'll go over what
this notation means, what
these nuclear reactions mean.
If you don't understand
it, don't worry.
The whole point of today
is to open up questions
that we'll spend the rest of the
semester closing and answering.
So again, if you're
lost, don't worry.
It's the first day of class,
and it's your first day
of Modern Physics.
So not to worry.
And this is an actual
picture of what
it looked like in the
paper, a simple polonium
source on a disk that was made
by the natural decomposition
of radium into polonium, a piece
of beryllium, a vacuum chamber.
Because it was already known
that the alpha particles
coming from polonium have
an extremely short range.
We're going to figure out
why as part of this class.
But without that vacuum
there, the alpha particles
wouldn't make it
to the beryllium.
So that much was known.
What wasn't known was why are
we getting so many ionizations.
They attributed it to
what they called a process
similar to the Compton effect.
To tell you what
that is, in 1923,
Arthur Compton figured
out, among other things,
Compton scattering, where a
photon can strike an electron.
The photon changes energy.
The electron picks
up some energy.
They exit at very
well-known angles,
and they transfer very
well-known amounts of energy.
So this is how they knew how
much energy the photon, if it
were to exist, should have.
And they said the process
was analogous to Compton
scattering because
they said in this case,
a proton would be ejected.
It would take a lot of energy to
eject a proton using a photon.
And Chadwick saw
this and said, well,
if we ascribe this phenomenon
to a Compton recoil,
we should see about 10,000 ions.
We actually saw about 30,000.
So there was more
ionization going
on than can be explained
by what's going on.
In addition, those
protons should
have a range in air of
about 1.3 millimeters,
and they saw much more.
So this is something simple--
theory and experiment
don't match.
There's got to be a different
theoretical explanation
if the experiment was correct.
And so finally, what I love--
the last sentence in this--
the quantum
hypothesis-- a quantum
was the way they
referred to a photon.
It was called a
quantum back then,
a little packet of energy.
Can only be upheld if we
forget about conservation
of energy and momentum.
Now, I'll ask you guys
from 8.01 to 8.02.
So Sean, when can you throw
out energy and momentum
conservation?
AUDIENCE: [INAUDIBLE]
MIKE SHORT: That's
pretty much right.
You can't.
A situation probably
wasn't given to you
where you can just
throw away conservation
of momentum and energy.
In fact, nature gives
us three quantities
that we can measure
and conserve--
mass, momentum, and energy.
And throughout this course,
if something is not conserved,
you've probably got the
math or the physics wrong.
So this is something to
remember throughout the course
and our derivations and
in your problem sets,
is conserve mass, conserve
momentum, conserve energy, just
like what was taught
in 8.01 and 8.02.
So I'll call your
answer correct.
You don't remember a situation
because, well, it didn't exist.
And that's what Chadwick noted.
He said theory and
experiment don't
work unless we throw
out conservation
of energy and momentum.
Whether this was a kind of
passive-aggressive thing
to say-- well,
this clearly can't
exist-- or he was suggesting
maybe it doesn't work,
I don't know.
I wasn't there.
But later on,
about a year later,
he published a follow-on
paper confirming the existence
of a neutron by reconciling
these differences
in theory and experiment.
So he restated
what he saw before.
This was the first
paragraph of it.
And again, it said that
radiation excited in beryllium.
Whatever happened after the
alpha particle came out.
It had a highly penetrating
radiation, distinctly greater
than that of any
gamma radiation found
from radioactive elements.
Something is different.
And I want us to take
a sec to digest this.
This is the part I actually
want you guys to read,
so take a minute and read
through some of this stuff.
And then we'll begin
explaining his argument.
Let me know when you
guys are done reading.
OK.
I see some folks
starting to look down.
So let's take this
apart and figure out
what was Chadwick saying.
He was saying that if a
quantum was responsible
for this energy,
a photon, then we
can write a nuclear reaction.
I'll write it in the
notation that we use now,
which would be beryllium-9,
the only naturally occurring
isotope of beryllium,
plus an alpha particle
would lead to carbon-13
plus a gamma ray.
And that gamma ray
would take away
the energy from this reaction.
So now we can start to figure
out, is energy conserved?
Could this gamma
ray actually exist?
And if it does, does it
account for the ionizations
that Chadwick saw?
So for each of
these isotopes, we
know a few different quantities.
We know what's called its rest
mass energy, which is this.
It's rest mass times
speed of light squared.
This should look
familiar to everyone.
I've seen it on t-shirts
all over campus.
And it may take
two or three weeks
to really wrap your head around
what Einstein's equation really
means.
It is that mass and
energy are equivalent.
You can express mass in terms
of energy, and vise versa.
And you will be doing so
to conserve energy and mass
in nuclear reactions, one of
which is written right here.
So if each of these things
has a given rest mass energy,
let's say a rest mass
energy of beryllium
and a rest mass energy
of an alpha particle,
and this alpha particle maybe
had some kinetic energy--
it was moving pretty
fast, so we'll
give that the symbol
t for kinetic energy,
because that's what you're
going to see in your notes
and in the reading
and everywhere.
And then this
carbon-13 nucleus has
got to have a rest mass
and a kinetic energy,
and then this gamma
ray, it's going
to have some e gamma energy.
Now, the question is,
is the mass and energy
conserved in this equation?
What we're actually
starting to write
is what's called the q
equation, or the universal mass
and energy balance for any
kind of nuclear reaction.
So let's say we have a
large initial nucleus
i and some small
particle i moving
at it with some great speed.
And after some
reaction occurs, you
have a small, final
particle leaving
and a different, large
final particle leaving.
They don't necessarily
have to be the same.
Let's give these particles
designations 1, 2, 3, and 4.
In the end, we should be
able to write the difference
in either total
energy or total mass
of the system as this value q.
q is, let's say,
the amount of energy
that turns into
mass, or vise versa.
So let's say energy transfer.
And so if we start writing some
mass conservation equation,
we can say that the
mass of nucleus 1
plus the mass of
nucleus 2 should
equal the mass of nucleus 3
plus the mass of nucleus 4
plus however much energy
from nuclei 1 and 2 turned
into energy into 3 and 4.
We could also write
the same thing
for their kinetic energies.
In this case, the
finals are on the end.
So I'm sorry.
I should use t for
kinetic energy.
So what this is saying is
that if some mass has turned
into energy at the
end, that energy
had to come from somewhere.
It had to come from the initial
kinetic energy or conversion
of mass to energy
from this reaction.
And so notice that
now, you can actually
express the masses of the
nuclei in terms of their energy,
of their initial and
final kinetic energies.
And this right
here is what we're
going to be spending the first
two or three weeks deriving,
using, and exploring in order
to balance nuclear reactions
and explain why they
are the way they are.
So let's make sure-- we'll keep
this nuclear reaction up here,
because Chadwick proposed
a different one to explain
what he saw.
And some of the
evidence for this
was that he put
some aluminum foil
in between the beryllium where
things were being liberated
and the ionization
chamber and oscilloscope,
or oscillograph, as
he liked to call it.
And that way, by putting
more and more pieces of foil
in there, you can
deduce what's called
the range, or the distance
that the radiation will travel
before it stops by losing
energy through a whole host
of different processes that
we'll be working through
together.
If this were to be
ascribed to a proton,
then it should have had
a certain range in air
by this curve b right here.
Instead, he found this curve a
where things moved about three
times farther than
could have been
explained if that
were a proton to be
liberated by all this stuff.
So he's saying, OK, something
has got more penetrating power.
We know now that part
of the reason for this
is if there's a neutron,
and there's no charge on it,
then it's not going to interact
with the electrons in matter.
It won't even see them.
Whereas protons or
any other charged
particles will see the
electrons in matter
and will interact with the
electrons and the nuclei.
So a little flash
forward to say,
we can explain this pretty
simply with what we generally
know.
But this was the
first time somebody
had to come up with
[INAUDIBLE] explanation,
and it was quite hard.
And so moving on,
he can say, well, I
know what protons should
be injected from paraffin.
I know a formula to describe
what quantum or photon
energy had to create them.
And then instead, he says-- this
is where his major hypothesis
is--
either we relinquish
conservation
of energy or neutron or
adopt another hypothesis.
And this was already put
forth by Rutherford back
in the '20s that there
may be a neutron,
but there wasn't any proof.
And this is what
provided the proof.
He gave an alternate
nuclear reaction
if there were to be
a neutron which had
roughly the mass of a proton.
Then let's write a
second one down here.
I'm going to erase
these extra notation,
and we'll write the competing
nuclear action below.
And he said that-- let's say
we start with beryllium-9
plus an alpha
particle could instead
become carbon-12 and a neutron.
So I'd like to ask you guys
right now to work this out.
Are both of these reactions
balanced in terms of mass?
Are there the same number
of protons, neutrons,
and electrons at either side?
And just to let you
know, an alpha particle
is better known as
a helium nucleus.
So that means that
there's two protons.
There's four protons
plus neutrons,
and beryllium-9 has four protons
and nine protons plus neutrons.
And carbon-12 has six protons.
A neutron has zero protons.
So in each of these-- and I'll
fill in the other ones here.
So that's a 4, 4, 2, and 6.
Do we have the same number
of protons and neutrons
on both sides of both equations?
I see a number of heads and
one person saying yes, we do.
So both of these reactions
are balanced in terms of mass.
The next thing to do is balance
them in terms of energy.
Now, they can both be
balanced in terms of energy
because you could attribute the
change in the amount of mass
from here to there
and attribute that
to the energy of the photon.
That's when you'd have to have a
photon of energy around 50 MeV.
But if a proton--
I'm sorry-- a photon
of energy around 50 MeV
can't explain what we saw.
Instead, if there is something
like a neutron which also
has its own rest mass and
its own kinetic energy,
and that neutron were
highly penetrating,
it could explain
what Chadwick saw.
And so the masses and
things of these nuclei
were fairly well known
back then to, well,
six significant digits
based on some very careful
experimentation.
And all he did is
say, all right.
Let's take all of the
energies in this reaction.
Remember how I
told you over here,
you can write any
nuclear reaction
in terms of its
kinetic energies,
and the difference will
give you the q value,
which you can attribute to the
conversion of mass to energy?
That's what Chadwick
did right here.
He took the full
reaction, saying here's
the mass of beryllium, the
mass of the alpha particle,
the kinetic energy of
the alpha particle.
Note that he assumed that the
kinetic energy of beryllium
was zero.
It was just sitting
at room temperature.
Does anyone know the approximate
kinetic energy of atoms
at room temperature?
Order of magnitude, even?
It's around 1/100 to 1/1,000
of an EV, or an electron volt.
So when we're talking
about beryllium,
whose kinetic energy, we'll
say, is around 0.01 EV,
and the alpha particle whose
kinetic energy was around 4
times 10 to the 6th EV, you
can see why it's neglected.
And you can do that too.
You do not have to account
for the initial kinetic energy
of a nucleus at rest.
This is the first
approximation that we
tend to make to the q equation
to just have fewer variables.
And don't worry if you
don't remember this now,
because we have a whole
lecture on the q equation.
And so finally, he
said, all right.
We'll subtract all the masses.
We're left with the kinetic
energies and a little bit
of excess rest mass.
That's got to be--
this has got to exist.
And so this inequality has to be
satisfied, which indeed it was.
Using this inequality,
he said that the velocity
of the neutron has to be
less than its kinetic energy
if it had all of that
energy, 3.9 times 10
to the 9 centimeter per second.
Indeed it was lower--
not by that much, but it still
satisfied this criterion.
So things are checking out.
That's pretty cool.
He looked at another
nuclear reaction
that was known at the time.
If you were to bombard
boron-11 with helium,
you end up with
nitrogen-14 and either--
either end up with
nitrogen-15 and a photon
or nitrogen-14 and a
neutron, explaining
another reaction that
wasn't as well known before.
So I'd like to write
this nuclear reaction,
because I want you all to get
very familiar with writing
nuclear reactions.
Let's say boron-11 plus
an alpha particle--
we'll say it has a mass of 4--
becomes nitrogen
14 and a neutron.
We also have a
shorthand of writing
this nuclear reaction
which I'll use
on the board for speed's sake.
Usually if you put
the initial nucleus
and the initial incoming
radiation, comma,
the exiting radiation,
and the final nucleus,
these two right
here are equivalent.
This is just a shorthand
for nuclear reactions.
This is what you'll
tend to see because it's
a lot easier to write this
shorthand and parse it visually
than it is to parse a
whole nuclear reaction.
So I just want you to know if
you don't know what these are,
just remember to
stick the arrow here,
stick plus signs in
for the parentheses,
and you've got the same thing.
And using these, you should be
able to very quickly determine
is this reaction balanced.
What's actually going on?
And there will be tabulated
values of q values or energy
amounts for these
sorts of reactions
in all sorts of tables
I'll be showing you.
And so finally, he
figured out what
the energy or the mass defect
of the neutrons should be.
Does anyone know what
a mass defect is?
This is another core concept.
Let's say you were to want
to make an atom of helium.
So you would have to take
two protons whose masses are
very well known,
and two neutrons,
and bring them together.
So if you were to
have-- let's say
the initial mass
would be 2 times
the mass of a neutron plus 2
times the mass of a proton.
And the final mass is just
the mass of a helium nucleus.
You'll actually find that
the initial mass does not
equal the final mass.
In bringing nuclei
or nucleons together,
they actually release what's
called their binding energy.
It's what keeps the
nucleus bound together.
There's a little bit of
mass turned into energy.
And so you know how we like
say the whole is usually
more than the sum of its parts?
In nuclear
engineering, the whole
is a little less than
the sum of its parts.
It definitely is not equal.
And Chadwick was proposing
that a neutron should actually
be made up of a
proton and an electron
in very close proximity.
And since the masses of
the proton and the electron
were known, he said, well,
if we bring the proton
and electron very
close together to have
an overall neutral
neutron particle,
it should have roughly that
mass defect or that difference
between the energy of
its constituent nucleons
and the energy of the
assembled nucleus.
And you'll hear the
words "mass defect"
and "binding energy" used.
Mass defect is in terms
of mass in either--
you can give it in kilograms
or in atomic mass units,
or AMU, or in, let's
say, MeV c squared.
And you'll also hear of the
binding energy just given
in things like MeV.
I want to show you where you
can find these things now.
I'll give you the single
most useful website
that you'll be referring to.
And I've posted it up on
the Learning Module site,
so now is a good time for me to
show you that the site exists.
And let me just clone
my screen real quick.
It's a wireless HDMI thing, so
it takes a sec to pop back up.
Great.
Has anyone not been
to the site yet?
It's OK.
You don't have to
be embarrassed.
OK.
About half of you.
I recommend tonight that you
start looking through the site.
One, make sure that
you can log in,
because you'll need to log in
to see some of the copyrighted
materials that I've posted,
and two, because this is where
you'll be posting
all your homeworks,
getting the assignments,
checking due dates.
Especially if I
postpone a problem set,
I'll put out an announcement
and post it here.
So this is the place
to look for everything.
And in addition, I've posted
a lot of useful materials
for you guys.
They're all at the
bottom, and the top one
is the [INAUDIBLE]
table of nuclides.
Anyone seen this
kind of thing before?
We have posters of it down on
all the first-floor classrooms
in Building 24.
This is our go-to chart.
When you want to find out
all of the nuclear half-life,
radioactive decay
and decay of energy,
probability of certain
direction, whatever,
this is where you go.
So let's take a look
at, well, helium-4
since we've been
talking about it,
better known as
an alpha particle.
And you'll notice a few
different quantities visible.
The atomic mass, 4.0026032 AMU.
And this is another tip I want
to give you guys right now.
Don't round these numbers.
That's one of the
major trip up points.
If you say that's
approximately 4 or 4.003,
you probably won't get
the p-set questions right,
because 1/1,000 of an AMU can
still represent almost an MeV
of lost energy.
So let's say you have
a nuclear reaction that
liberates a 1 mega electron
volt or one MeV gamma ray,
and you get the
fourth digit wrong
in one of your
mass calculations.
It's like that gamma
ray didn't exist,
and you won't get
the answer right.
So again, word to the
wise-- do not round.
You'll also see what's known as
the excess mass or the binding
energy.
So this binding
energy right here,
if you were to take two
protons and two neutrons
and bring them together and
look at the difference in masses
from, let's say, the same
old formula as before,
you would get a difference of
28,295 keV, or about 28.295673
MeV.
Again, don't round.
Let's figure this
out right here.
So we have 28.295673 MeV.
And there is a conversion factor
that you should either memorize
or write down.
Either way, it's good.
It's about 931.49 MeV
per atomic mass unit.
This is your mass
energy equivalence
that you'll be using over
and over and over again.
And again, don't round.
Those last two
digits are important.
So by taking this energy and
dividing by this conversion
factor, you can figure out
how many atomic mass units are
lost in terms of
actual mass when
you assemble an alpha particle
from its constituent pieces.
And the rest of the stuff
we will get into later.
It's not really relevant
to today's discussion,
but it's definitely
relevant to today's course.
Cool.
OK.
And then on to--
one of the last things
that he mentioned
is some predictions to say, OK,
let's say this neutron exists.
It doesn't have charge.
Most matter interacts
with other matter
by virtue of Coulombic
or charge interactions.
If the neutron has no
charge, it shouldn't really
see matter except for nuclei.
This is exactly what he
said, is an electrical field
of a neutron will be
extremely small except
at small distances, because
he proposed that a neutron is
a proton plus an electron.
So once you get to around
the radius of the neutron,
you might start to see some
charge, but not before.
And so most other
matter, unless you
have a head-on collision with a
nucleus, neutrons won't see it.
And that helps explain
why the neutrons
had such high penetrating
power or high range--
because they just went streaming
through most materials,
invisible to the electrons.
So very forward thinking, and
turned out to be very correct.
And then finally, as a kind
of mic drop conclusion,
came up with the final
concluding statements.
OK, we know there's a neutron.
We know its mass.
The actual mass of the
neutron is about 1.0087 AMU so
within 0.1% of Chadwick's
calculations and predictions
based on 1930s equipment,
which is strikingly awesome.
And there you have it.
That's the discovery
of the neutron using
most of the concepts that we're
going to be teaching you here
in 22.01.
So right now, I'd say
your scientific knowledge,
if we don't count what
you read on the news,
is roughly around
1850 when all the E&M
stuff was being figured out.
We are going to bring you
screaming into the 1930s.
And by about month 1,
we'll hit the present day
when we can start to talk
about the super heavy elements
like the ones that were
discovered last year.
I think there was
even some this year.
But we'll look at the Physics
Today article from last year
to get to the
point of explaining
why super heavy elements
might be stable.
Why are we even
looking for them?
Where do cosmic rays come from?
How do we know that they're
cosmic rays and not something
else?
How can you tell a reactor
turns on anywhere in the world
by measuring different
bits of radiation, which
is an active defense
project that folks
are pursuing right now?
Lots of really fun questions.
And speaking of
questions, do you guys
have any questions about
what we've explained here,
how we've retraced Chadwick's
discovery of the neutron
from basic nuclear
science principles?
So who here has seen these
nuclear reactions before?
Cool.
This is something
that I hope folks
would cover in high school.
But with a general
trend of watering down
science education, I didn't
want to make any assumptions.
I'm glad to hear
this was covered.
Was this coveted at MIT?
Are you guys relying on
high school knowledge?
OK, good.
Not good.
Good I know where you are.
Not good that MIT doesn't
teach anything nuclear
until year two.
That's OK.
You guys, along with
the Physics Department,
will get at least a 20th-century
knowledge of physics and 21st
by the end of month 1.
So I want to come back
to the Stellar site,
and specifically the syllabus.
I've taken a lot of care to
write a very detailed syllabus
of what we're going to do,
what I expect of you guys, what
you can expect of
me, and what we'll
be doing every single day.
So if you want to
know what we're
going to be doing, if you have
a class that you miss, and you
want to know what notes
you're going to miss,
it's all written up here.
I want to get right
into assignments,
because everyone wants to know
what am I responsible for.
Well, not too much--
nine problems sets,
three quizzes.
The final exam is just a quiz.
It's only worth 24% of
your grade instead of 20
to get the math to work
out, because I eliminated
one problem set to avoid running
afoul of MIT regulations,
but not assigning things
at the last week of class.
But there are three quizzes,
so the final exam is just
another quiz.
It's not a super
high-stress, crazy thing,
because I don't see a
point in doing that.
You can make your
assignments however you want.
I don't care as long
as I can read them.
But I do ask that in the
end, you submit a PDF
file on the Stellar site.
And the reason for
this is my first course
that I ever taught at
MIT as a professor were
the graduate modules, 22.13,
Intro to Nuclear Systems,
and 22.14, Intro to
Nuclear Materials.
I accepted paper submissions.
And by week 3, I had
to microwave them.
Because three or four times,
I definitely saw blood.
And there were also
some weird stains
that I didn't want to explain.
So I added the habit of
unstapling, microwaving,
and re-stapling the
p-sets before grading.
So in the digital
world, it's sterile.
I'm not a germophobe.
I just don't like
blood in my house,
especially if it's not mine.
So I ask that you guys submit
PDFs on the Stellar site.
They're due at 5:00 PM to
make sure that you're done
and you can go home and relax
or work on something else.
I used to some have
things due at midnight,
and I had every
submission was 11:59 PM.
I'm not going to
do that anymore.
Do make sure to submit
15 minutes early.
So if your computer or the
Stellar site has trouble,
send me an email or a
text or whatever saying,
I'm trying to submit,
and it's not working.
Here is a backup, or I'm leaving
something under your door.
And if you want my
cell phone number,
that's also my office number.
It's also my only number.
It's in the MIT directory.
So if there is some emergency
you need to make me aware of,
please do communicate.
I'd rather you tell me than be
worried about not telling me
and then find out later.
So are we all clear on that?
As far as what the
assignments are,
each assignment is going to be
about 50% basic calculations,
working out things like these
to make sure you've mastered
the material, that
you understand
writing nuclear reactions,
you can balance a q equation,
you can tell me about
what your cancer
risk would be from a
certain dose of material.
So this is like when you
go out in the real world,
the sort of
calculations everyone
would expect you as a nuclear
engineer to be able to do.
And then 50% of each
problem set is either
going to be analytical questions
of considerable difficulty.
This is MIT.
We're not just here
to give you the basics
so that you can
regurgitate a textbook
onto the first person who asks.
We're here to make sure
that you can go farther.
Because you guys are
the future of this very
small and diminishing field
at the moment if you look
at the nuclear power in the US.
I would say growing
in terms of the world,
but not in terms of the US.
And you guys are going to be
in charge of leading this field
and determining where
it's going to go.
So you've got to be up
at the cutting edge,
and we're going to take you
to the edge of your abilities.
My favorite kind of
problem is to give
one sentence for the question,
five or 10 pages for the answer
if you don't get the trick.
Now, that's OK.
It's perfectly fine
not to figure out
the answer in the end.
In fact, I'll usually
give you the answer
for the analytical
questions because I
want to see your approach.
I'm not interested in
you nailing the answer.
I'm interested in
seeing how you think.
And copious partial credit will
be given for the way you think.
So if you have a missing
step, and you say,
I don't know the step, I'm
going to assume variable a
and keep going,
you will get credit
for the subsequent steps.
I want to see how you
think from start to finish
and how you cover for holes
that you can't get through.
So everyone clear on that?
Partial credit, yes.
Use it to your fullest ability.
The other half of
the problem sets
will have take-home
laboratory assignments.
It's not just enough
for me to tell you
about nuclear engineering.
You have to see it
for yourself, and you
have to feel it for yourself.
And once in a while, you'll
get a mild electric shock
by yourself if things
go wrong, but that's OK.
It happens to the best of us.
I got zapped by our--
you guys have all made
Geiger counters, right?
Has anyone not made
a Geiger counter yet?
Oh, OK.
It sounds like we need
to run another workshop.
Well, our Geiger counters
rely on a neat little boost
converter power supply
that takes 9 volts
and steps it up to 400 volts
via some switching things.
That means you have 400
volts on a big metal tube.
And if you're working
on your circuit
and you happen to brush against
it, you get zero current,
so it doesn't hurt you in the
medical sense, but it hurts.
I also have a dance I
call the 60 Hertz shuffle.
It's the high speed
shaking that you
do when you're
connected to 60 Hertz
somewhere from the wall outlet.
None of you guys will
be exposed to this,
but I've done it
enough times that I
have a name for the dance.
If you get 400 volts,
you'll just kind of scream.
And I don't care how
manly men you guys are.
Everyone makes the same
pitch scream with 400 volts.
We're all equal in the
eyes of electricity.
For these laboratory
questions, I'm
going to ask you to both
complete an assignment where
you'll, for example, measure
the half-life of uranium,
measure the radioactivity
of one banana, confirm
or refute the
linear no threshold
hypothesis of the dose.
And the experiment itself
won't take that long,
but I want you to write it up in
proper documented format using
these sections.
So I'm going to be
teaching you guys how
to write scientific articles.
So actually, this is kind of
a good time to ask you guys.
How would you define
the word "science?"
Luke, what do you think?
AUDIENCE: It's a process
of getting knowledge
by fitting theories
to empirical evidence.
MIKE SHORT: Gaining
knowledge by fitting theories
to empirical evidence.
OK.
So I hear knowledge
gaining by some sort
of well-justified and
accepted means, right?
Monica, what do you think?
AUDIENCE: Science is the
study of the natural world
through patterns and
mathematics, I suppose.
MIKE SHORT: Cool, yeah.
Let's say the studying,
modeling, and abstraction
of the natural world into
ways we can understand.
Jared, what would you say?
AUDIENCE: Which one?
MIKE SHORT: Oh,
there's two Jareds.
I want to hear both,
and then I'll--
yeah.
AUDIENCE: Science is--
I'd probably go with
it's the same thing
Luke said, gaining knowledge
through experimentation
and trial.
MIKE SHORT: Cool.
And other Jared?
AUDIENCE: I think what Luke
said about fitting theories
to empirical evidence and
testing them that way.
MIKE SHORT: OK, cool.
I like these.
And these are the
generally accepted theories
and descriptions I've
heard of science.
And I want to pose a
question to you guys.
If a tree falls in the woods
and nobody is around to hear it,
can it win the Nobel Prize?
It's kind of an expression.
So if somebody
discovered the neutron,
and they wrote up
their findings,
and proved that it exists,
and they put it in their desk,
and the house burned
down, and the person died,
was the neutron discovered?
What does discovered mean?
So to me, science is
equal parts everything you
guys said and communication.
If you discover something
and you don't tell anyone,
the information
technically doesn't exist.
It dies with you.
And you don't want
that to happen.
So I want to make sure that
you guys both understand
the science and understand the
importance of communicating it
effectively to people.
Because that's the
other thing you're
going to be doing as
leaders in this field
is explaining things.
You better believe when
Fukushima happened--
I was a postdoc at the time.
I was not a person, I guess,
in the academic sense.
People here treated
me very well,
but I was also very aware that
I was not one of the greats.
Still I am not old enough yet.
I was getting calls all
day, all night from news
agencies saying, you're at MIT.
I saw your name
on the directory.
Do a radio interview and tell
us all if we're going to die.
And you can only imagine what
the professors on this hall
were dealing with.
So folks were traveling
around, answering things
left and right.
I ended up doing some weird
podcast on a Brazilian news
channel that I don't
think ever got aired
and stopped doing it after that.
You as undergrads even
might be called if somebody
wants to know something.
And so it's best that you
not only know the material,
but you can convey
it effectively,
briefly, and in a way that
your audience can understand.
The audience for these
articles is any undergraduate
in any engineering
program anywhere.
That's your lowest
common denominator--
not to say that
that's a bad thing,
but it is the audience that you
want to aim your writing at.
So what I want you
to be able to do
is say what you did, why you
did it, and what it means.
In communication terms, this
means a less-than-100-word
abstract, a very brief synopsis
of what you did and why
it's important.
That's the teaser.
This is the trailer to make
somebody read what you actually
did and see why they care.
This is the main
method and currency
through which
scientists communicate
is articles of this type.
An introduction and
background which
says why are we
studying this problem.
And the answer is not
because I told you to,
and your grade depends on it.
I want you to think about why
this problem is important,
and put it into
context, and give
any of the scientific
background to understand
what's going to come next,
like the experimental section.
Describe what you did in nitty
gritty scientific detail.
This is usually the easy part.
I put this gamma ray in this
bucket, and it made this color,
and I made this noise, whatever.
A results section
where you show all
of your data and a
discussion section--
notice that these are different.
You want to separate
your actual results
from your interpretation
of your results,
because someone else may have
a very different interpretation
of results--
for example, Chadwick.
Somebody found that
beryllium bombarded
by alpha particles
emitted radiation
of great penetrating power.
That's the result. The
interpretation or discussion
said it's probably a
Compton-like effect
from a photon.
By separating your results
and your discussion,
you allow people to mentally
say, OK, I get your results.
I believe that you
found these numbers.
I have a different explanation.
And you all may have
different explanations
for what you see
in your own labs,
because you're also probably
going to get different results.
And then finally, a conclusion
where you quickly re-summarize
your major contributions.
Your abstract is the teaser.
Your conclusion is like your
re-abstract with the context
that people now believe--
or don't-- what you did.
And think about how
you guys read articles.
So who here has read
scientific articles before?
More than half of you.
Let's see.
Alex, what do you read first?
AUDIENCE: If it's a journal,
probably the abstract.
But given that I'm mostly
interested in the topic,
I tend to go to the
conclusion section.
MIKE SHORT: That's right.
OK.
I'm glad you said that.
That was my next question.
You read the abstract.
The next thing you
read is the conclusion.
The next thing you
usually read is
you skim through the
results and the figures
and see if it's
worth looking at.
Then if you're like, OK,
this is worth my time,
then you slog through
and read everything
to make sure you
understand it all.
So when you're writing
these articles,
think about who's reading
them and how they read them.
Because if you guys don't tend
to read an article from top
to bottom, neither
will your audience.
And that's true.
Most scientists skim things
because we have a lot to read.
So that's OK.
And I am very interested
in you guys completely
documenting your experiment.
Pictures are also
awesome to use.
Accuracy of results
and analysis--
so did you round when
you weren't supposed to?
Did you have a clear numerical
typo that you can't explain?
And the readability
of the report--
I want you to spend time
making this readable.
I expect that this
part of the assignment
will take roughly five hours,
whereas the basic questions
will take roughly three
to four hours depending
on how well you're doing.
And that leaves three
hours of class time
and a couple hours for
whatever else happens in life,
let's call it.
Since you've never
written these before--
wait a minute.
I shouldn't say that.
Who's written these
kinds of things before?
Anyone here wrote a
scientific article?
Two.
OK, three.
Cool.
So most of you
haven't, and that's
where I assumed you'd all be.
We have a whole lab dedicated to
scientific communication called
the Comm Lab run by
someone, who happens
to be my wife, four doors down.
We live and work
next to each other.
It's pretty cool.
And you get an automatic
three-day extension
on the lab assignment if
you go to the Comm Lab.
There are three
reasons for this.
One, I want you to
get better grades,
so I want you to learn
how to communicate.
Two, I don't want to spend
time trying to figure out
what you were trying to say.
So better articles means
less grading time for me.
And three-- OK, let's
just say it's two reasons.
That's enough.
And for everything
except for the quizzes,
it's perfectly OK
to work together
as long as you
attribute who did what,
you write your own articles,
don't Xerox anything,
and say who took the data.
So if the whole class wants
to get together and take one
set of data and
work for that, fine.
If you all want to do
the labs yourselves,
which I highly recommend, fine.
But I'm not going
to tell you how
to do the lab assignment
in this, as long as you
say what you did.
And I want all of you,
if you haven't yet,
to head to integrity.mit.edu
to see our official policies
on what is considered
plagiarism, what is considered
working together, what's
considered academic honesty.
I will assume, because
it's on the syllabus
and I'm telling you now,
that you've all read this,
and that there will
be no cheating.
It's just not something that's
part of my job description,
and I don't want
to deal with it,
which means I
won't deal with it,
which means the
consequences will be severe.
So I don't think I'll
have to worry about that.
And then for the late
policy, it's just 10%
of the value of assignment
for each calendar day, not
each business day.
So if you're running really
late and you haven't started
an assignment the day it's due,
better to take the 10% penalty
and do really well than
hand in nothing on time.
So keep in mind how
can you maximize
the points in this course.
I'd rather you hand
in something good late
than terrible on time.
So if you really need that extra
day if MIT gets crazy, take it.
10% of a problem set is
0.4 points on your grade.
It's not that big a deal.
Then as far as the syllabus, I
want to show you very quickly.
We've got when things are due.
I'm going to change these dates
to basically just shift them
all forward by
one day to account
for the new Tuesday,
Thursday classes.
So I've got when the
problem sets are due.
And Friday is
recitation activities.
If there aren't
too many questions
on a particular
Friday, I have a lot
of fun stuff in store for you.
For example,
tomorrow we're going
to be talking about radiation
utilizing technology, including
plasma sputter coders, one of
which we have set up in my lab,
and I'd like to show you.
Because it's a way
that you can coat
materials and other
materials, and you
have to generate this beautiful,
glowing purple plasma in order
to do so.
So you ionize nitrogen. You
induce sputtering, which
is a radiation damage process
which we'll be going over,
to coat things in other things.
There will be once
in a while where
I have to shift a class into
recitation because I'll be
at Westinghouse or in Russia.
And I think that's only
twice during the whole year.
So you won't miss any classes.
We'll just use the
recitation time.
And then other,
times we'll be doing
measuring the radioactivity
of banana hashes.
Or once we talk about
electron interactions,
we're going to go use a
scanning electron microscope.
The carrot at the end of
the stick to make sure
that you guys do well--
the top two people performing
on the quizzes get to pilot
and choose the samples for
the SEM and elemental analysis
and the focused ion
beam demonstration.
So you guys get to pilot
something that's, let's
say, as complicated as a
space shuttle but deals
with things much, much smaller.
So I'll put you in
the driver's seat
in the machines of
our lab, and you
get to bring whatever you
want to analyze and find
the elemental analysis of
and use the world's smallest
machining instrument
that can cut
5-nanometer slices of things
using processes that we're
going to discuss in this class.
So the better you do, the
more you get to use it.
And at the end, we'll
have a nice debate.
I call it arguing
with Greenpeace
when we'll talk
about-- now that you'll
have known all of the nuclear
science and engineering
and can speak
scientifically about topics,
we're going to go after a lot
of societal misconceptions.
Do cell phones cause cancer?
Does living near a nuclear
power plant cause cancer?
Does arguing with
Greenpeace cause cancer,
whatever it's going to be?
So I want to make sure
that you're well-equipped
and confident enough to
go out there and hold
your own in a vigorous debate
with an angry, emotional
environmentalist.
You guys will be calm, peaceful,
and informed environmentalists.
After all, that's why a
lot of us are here, is we
want nuclear energy
to happen because we
care about the environment.
There's other people
that don't want
nuclear energy to
happen because they
care about the environment.
To each their own, I
guess, motivations.
But I want to make sure
you're well equipped
to also tackle things like
is food irradiation bad.
That there's all sorts of
websites with dancing babies
and weird Geocities-like
graphics saying
food irradiation is evil.
You won't find a lot
of scientific articles
if that's the case.
And to see if you'll
put your, let's
say, cancer risk
where your mouth is,
the last day of class, we'll
have an irradiated fruit
party where I'll be buying
only the kinds of fruit that
can be imported into the US
because food irradiation is
done.
Otherwise, the USDA would
not let it into the country.
And this is mostly things like
mangosteens from Thailand,
pineapple from Costa Rica.
And interestingly
enough, Hawaii is
considered a different
country agriculturally.
It is so far away that they have
different agricultural pests.
And without
irradiation, we couldn't
import some of the
produce from Hawaii,
because it could decimate some
of the crops in the Continental
US.
Pretty crazy, huh?
Yeah.
It's the-- what, it's the 49th
state, but agriculturally,
a different country.
So it's about 5 till, so
I'm going to stop here.
And we will start with
radiation-utilizing technology
on Friday, tomorrow,
downstairs in Room 24-121.
And then we'll move over
to my lab at 2 o'clock
to see the plasma sputter coder.
