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MICHAEL SHORT: So I'd like to do a quick two or three

minute review of the stuff we did last time

to get you back into where we were.

We were talking about different types

of technologies that use the stuff you'll

be learning in 22.01.

Everything ranging from nuclear reactors for producing power,

and the Cherenkov radiation that tells you-- well,

that the beta particles are moving faster

than the speed of light in water.

It's a neat thing, too.

I've actually been to this reactor

at Idaho, to the spent fuel pool.

Even the spent fuel, once it comes out of the reactor,

is still giving off betas and still

giving off Cherenkov radiation.

And you can tell how long it's been out of the reactor

by how dim the glow gets, which is pretty cool.

So you can tell how old a fuel assembly is by the blue glow.

So remember, I told you guys, if someone says,

oh, you're nuclear, do you glow green?

You can be like, no, it's blue.

That's the right way of things.

We talked about fusion energy and got

into some of the nuclear reactions involved

in fusion energy.

And I'll be teaching you more about these today.

Why fission and fusion work, it all

has to do with the stability and binding

energy of the nuclei involved.

And that'll be the main topic for today,

is excess mass, binding energy, nuclear stability.

We looked at medical uses of radiation,

from implanting radioactive seeds called

brachytherapy seeds in certain places to destroy tumors,

to imaging, to X-ray therapy, to proton therapy using

accelerators, or cyclotrons, to accelerate protons and send

them into people.

If you remember, last time we ran the SRIM code

of the stopping range of ions and matter,

and actually showed that protons all

stop at a certain distance in tissue,

depending on their energy and what you're sending them into.

Let's see-- we talked about brachytherapy.

We talked about radiotracers, and this

is going to be one of the other main topics for today, is

these decay diagrams, and figuring out not only

what products are made, but what energy levels do

the nuclei have, and how do you calculate

the energy of the radioactive decay

products and the recoil nuclei, which do take away

some of the energy.

We talked about one way to get rich.

If you guys can figure out one of the ways

to solve this moly-99 shortage.

Right now, it's mostly made in reactors.

The future has got to be accelerators

or some sort of switchable device

where you don't need to construct a reactor

to make these medical isotopes for imaging, and tracers,

and such.

And finally, we got all the way up

to space applications, shielding, crazy, different

types of shielding, like electromagnetic shielding,

to protect from high energy protons, all the way

to radiothermal generators, which use alpha decay

to produce a constant amount of energy

on the order of one to 200 watts for like 100 years.

And finally, to a different configuration

of nuclear reactors, where you can

design them to produce thrust, not necessarily electricity.

And that's where we stopped on Friday.

So let's move on to one of the things I'd alluded

to earlier, which is semiconductor processing.

This is actually a diagram from the MIT reactor,

because we have this beam port here.

Has anyone got to see the silicon beam

port at the MIT reactor?

Oh, seven, eight-- OK, about half of you.

For those who haven't, who here has not

had a nuclear reactor tour?

Oh man.

OK.

Well, you'll get one when you get to control

the thing in early October.

So you actually get to go down to the control room

and see the rest of what's going on.

So make sure to ask them, show us the beam port

for silicon ingots.

And I think I already told you the story about the poor UROP

who held the ingots up to their chest,

getting about 10 months of dose, which is not dangerous,

but it meant that for 10 months, they

could have no radiation exposure,

and they had to answer the phone.

So that's how we ensure safety around here.

There's other ones that--

applications that you're probably

carrying around in your pocket.

You can use the fact that charged

particles have very finite ranges in matter

to separate little bits of that matter from other things.

So this is actually how single-crystal sapphires

can be separated in little slivers for protective phone

covers, because sapphire is one of the most--

the hardest, or the most scratch-resistant materials

there is.

Single-crystal sapphire is exceptionally strong,

and optically transparent, and expensive.

I know that because on one of our experiments,

we use a single-crystal sapphire window to see into reactor

conditions at like 150 atmospheres, and 350

degrees Celsius, and pretty corrosive chemistry.

So you want to use as little as possible.

So you can use a big, expensive accelerator

to limit the amount of sapphire that you use.

And this is actually done here in Boston.

There's a facility not far from here that uses an accelerator.

And this is their super detailed diagram

of what the radiation looks like--

yeah, whatever.

But what they do is they take-- they accelerate protons.

They send them through bending magnets

to steer that beam path.

And then they send them into a large piece

of single-crystal sapphire, which is exceptionally

expensive to make.

And they can actually lift off a thin sliver

with micron precision.

The reason for that is the same reason

that we showed with that SRIM code.

If you have this exact energy of protons

going into well-known matter, you

know what its range is going to be with an uncertainty

or so of about a micron.

So you can have things that come out thin, uniformly

thick, and smooth, right away.

There's some other really wacky products--

like has anyone heard of these betavoltaic batteries?

No?

They rely on beta decay or the direct capture and electricity

generation from a radiation source

like radioactive tritium.

So in this little chip is about 2 curies worth of tritium.

You guys will learn in about a week,

how to go from activity in curies to mass,

or something like that.

And so this chip actually contains

a lot of radioactive tritium that

directly creates electricity.

So you can hook into that chip and produce

nanowatts for years.

So it's one of these batteries that lasts-- well,

as long as a couple half-lives of the isotope that's inside.

Now there's a trade-off here.

The shorter the half-life, the more active

a given isotope will be for the same number of atoms,

but the shorter it will last.

So you can have higher power for lower time, or lower power,

higher time.

It's the classic energy trade-off--

works the same way with irradiation.

And so now I wanted to get into some

of the more technical stuff, where

we'll be talking about nuclear mass and stability.

And this is where the nuclear stuff really begins in 22.01.

First, I want to make sure that we all agree on notation.

So I'll be writing isotopes in this sort of fashion,

where we refer to A as the atomic mass, or just

the total number of nucleons.

This is not the exact mass of a nucleus.

It just refers to the sum of the protons and neutrons

in the nucleus itself.

And a lot of what we'll be talking about today

is the difference between this nice integer mass number,

and the actual mass of the nucleus,

and that difference is given by the binding energy,

or the excess mass, which are directly related.

Z is just referred to as the atomic number,

or the number of protons.

It's what makes the element what it,

which makes the name kind of redundant.

But it's-- humans learned by association.

It's easier to remember element names or symbols

than which element is which just by the number of protons.

So a lot of times we'll use the name, or at least the symbol

just so we know what's going on.

And anything up here is some sort of a charge.

I do want to warn you guys of the dreaded multiple symbol

use or multiple use of symbols.

I'll try to stick for a lower case q--

will be charged.

And uppercase Q is going to refer

to the Q value or the energy consumed or released

by a nuclear reaction.

So they're both Q's but we're going to keep

one upper and lower case.

And like we mentioned before, let's

say we were to write a typical nuclear reaction,

like the capture of neutrons by boron

to produce lithium-7, helium-4, better

known as an alpha particle, and some amount of energy.

There's two places where we actually use this reaction.

One of them is as control rods.

A lot of reactors use boron for carbide,

or this compound B4C, which is conveniently solid, fairly

dense, and contains a whole lot of boron in one place.

Specifically, enriched in boron-10,

because boron-10 has a high cross section, or probability,

for neutron capture.

And the other one is in what's called

boron neutron capture therapy.

Have I discussed this with you guys already, BNCT?

Good, because that's what we'll be

talking about for a few slides.

And to write this whole reaction is the same thing

as writing this shorthand nuclear reaction.

So this is often how you see them in the reading,

and in papers, because it's shorter to do that.

But it's the exact same thing.

So I have a couple of questions for you guys then.

I have this extra Q here.

Where does that Q actually go?

So let's say boron and neutron absorb,

it produces two nuclei with different binding energies.

What happens to the excess energy

created from the conversion of mass to energy?

Yeah, Alex?

AUDIENCE: That could be heat.

MICHAEL SHORT: Yep.

And heat, and more specifically?

AUDIENCE: Kinetic energy.

MICHAEL SHORT: Kinetic energy of the radiation released.

And so that kinetic energy is actually

used to our benefit in BNCT, or Boron Neutron Capture Therapy.

The way this works--

once I hit play--

is you can either-- you can use any sort of source of neutrons,

either a reactor or an accelerator,

through a lower complex chain of events, like this.

In this case, an accelerator-- so you don't need a whole

reactor--

fires a beam of high-energy protons