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MICHAEL SHORT: So, today we're going

to get into the most politically and emotionally fraught

topic of this course for stuff on chemical

and biological effects of radiation.

Now that you know the units of dose, background dose,

we're going to talk about what ionizing radiation does

in the body, to cells, to other things,

and we're going to get into a lot of the feelings associated

with it.

And by the end of this lecture, or Thursday,

I'm going to teach you guys how to smell bullshit.

Because we're going to go through one of millions

of internet articles about things that cause cancer,

that don't cause cancer.

In this case, it's going to be radiation from cell phones.

So I'm going to try to reserve at least 10 minutes

at the end of this class for us to go through a bunch of quote,

unquote, studies and misinterpretations

of those conclusions.

And I was going to pick my favorite of the 44 studies,

and looking through them all, my favorite are all of them.

AUDIENCE: [LAUGHTER]

MICHAEL SHORT: So we'll see how many we can get through.

But let's get into the science first,

so you can understand a bit about what goes on

with ionizing radiation.

Like radiation damage in materials,

radiation damage and biological systems

is an extremely multi-time-scale process.

Everything from the physical stage, or the ballistic stage,

of radiation damage to biological tissues

acting on femtoseconds, where this

is just the physical knocking about atoms and creation

of free radicals, these ionized species, which in metals you

wouldn't care about, in biological organisms you

do because then they undergo chemical

reactions from the initial movement

and creation of other strange radiolytic species

and the diffusion and reaction of those things,

which starts and finishes in about a

microsecond, before most of these things are neutralized.

And then, later on, the buildup of those oxidative byproducts

of these chemical reactions undergo the biological stages

of radiation damage.

All of the free radicals with biological molecules

have reacted within a millisecond.

So radiation goes in, a millisecond later the damage

is done.

Then you start to affect, let's say, cell division.

It takes, on average, minutes for a rapidly dividing cell

to undergo a division.

That's when the effects would first be

manifest from a DNA mutation.

But then it'd take things like weeks,

or years for these sorts of things

to manifest in a health-related aspect.

So, the division of one cancerous cell into two

won't change the way your body functions,

but the doubling in size of a tumor that blocks other tissue

absolutely would.

And so, it all starts in this sub-femtosecond regime,

when most of you-- well, for this entire year,

we've been approximating humans as water.

We're going to continue to do so for the purposes

of these biological effects.

So, let's say you, a giant sack of water,

gets irradiated by a gamma ray.

And that gamma ray undergoes Compton scattering.

Which, now you know how to tell what the energy of the Compton

electron would be.

We never talked about what happens with the molecule where

it came from.

That molecule remains ionized.

And since you're not especially electrically conductive,

they're not neutralized immediately.

And you can be left over with either a free radical

or an electron in an excited state.

And then what happens next is the whole basis

of radiation damage to biological organisms.

These free radicals can then encounter other ones,

and let's say an H2O+, can very quickly find a neighboring

water molecule, which they're almost touching and form OH

and H3O.

This is better known as H+, and that OH is a kind of unstable

molecule.

And these excited electrons here can also become these

H2O+'s, leading to this cascade of what we call radiolysis

reactions.

There's a few of them listed here,

things like an OH plus an aqueous electron,

which could come from anywhere, like Compton scattering,

like any other biological process that frees an electron,

can make another OH-.

So you can locally change the pH inside the cell

that you happen to be irradiating.

Or, let's say any of these oxidative byproducts

could encounter DNA.

Rip off or add an electron to one of the guanine, thymine,

or other two or three bases in DNA or RNA,

then you've changed the genetic code of the cell.

In the progression of these radiologists byproducts,

like I mentioned, whether you go by excitation or ionization,

then you start to build up these six species-- these five

species tend to be-- or these six ones

tend to be the ending byproducts of a whole host

of radiolysis reactions.

And don't worry, you're never going

to have to memorize all the radiolysis

reactions because the mechanism map is fairly complicated

and there are multiple routes to creating each one.

But the ones that are highlighted here

in these squares, are the ones that

end up building up in your body, things like peroxide.

Has anyone ever put peroxide on a wound before?

What happens?

Yell it out.

AUDIENCE: It bubbles up.

MICHAEL SHORT: Bubbles up.

What happens when you form peroxide in your body

from radiation?

AUDIENCE: It bubbles up.

MICHAEL SHORT: Well, luckily it doesn't quite

bubble up on the macro scale level,

but it is a vigorous oxidizer.

90% H2O2 is used as rocket fuel, as the oxidizing species

in rocket fuel.

You don't make 90% H2O2 from getting irradiated,

but every molecule counts.

Things like O2, you're shifting the amount

of oxygen in the cells.

And then there's things like these superoxide radicals,

or H2O-, H2O+, or all these other things that are available

to rip off or add an electron to something else that normally

wouldn't have it.

And the list of these potential reactions,

as well as their equilibrium constants and activation

energy, is huge.

Here's half of it.

Notice a lot of these equilibrium

constants shift really strongly one way or the other.

So, just because these molecules are made,

doesn't mean that all of them end up

staying and doing damage.

But unless these rate constants are either 0 or infinity,

there's going to be some dynamic equilibrium of these reactions.

So, once in a while, some of these free radicals

will escape the cloud of chemical change and charge

and get to something else.

Here's the other half of the equation set.

And it's under debate just how many of these reactions

there actually are.

Like, how often would O2- radicals combine with water,

which you can see is not quite set in the reaction,

to form [? HO2 - NO2 NH+ ?] Kind of a strange little reaction

right there.

Actually, a lot of them are quite strange.

You don't usually think of them happening

because these are very transient reactions, whose byproducts

do build up.

And that's the chemical basis for radiation damage

to biological tissues.

Now, once those chemical products form,

they have to move or diffuse.

So you can actually calculate or get diffusion coefficients

for some of these oxidizing species,

as well as compute an average radius

that they'll remove before undergoing a reaction.

So this is part of the basis for why

alpha radiation is a lot more damaging than gamma radiation.

Chances are, if you incorporate an alpha emitter into the cell,

it does a whole bunch of damage.

That damage consists of these oxidative chemical species,

that, if they're that far away from neighboring atoms that

happen to be in DNA, they might do some damage.

Whereas, isolated Compton scatters and photoelectric

exhortations from gamma radiation, not so much.

Chances are you hit random water in the cell that

isn't quite close to anything, fragile, and not much happens.

But you can also see this by looking at charged particle

tracks.

These things can actually be experimentally measured.

By firing electrons into gel or film or something like that,

you can actually see tracks of ionization

and watch them as a function of time.

In this case, it's a simulation of a charged particle

track at different timescales.

So, right here, this 10 to the minus 12

for the time in seconds, tells you

where these radiolysis products are.

And the N number, here, tells you how many of those remain.

So after a picosecond, you can pretty much just

trace out the path that the electron took, starts off right

here.

What do you guys notice about the density

of the charged particle track as it moves from the source

to the end?

AUDIENCE: It's much more dense at the end.

MICHAEL SHORT: It's much more dense at the end.

And why do you think that is?

AUDIENCE: Stopping power.

MICHAEL SHORT: OK.

More than just-- yeah.

Stopping power, yes, but fill in the beginning

and end of that sentence.

Chris, do you have your hand up?

AUDIENCE: [? It's all good. ?] So, it's a charged particle,

so it drops off most of it's energy where it has the least

amount of energy, so it does the most damage [INAUDIBLE]..

MICHAEL SHORT: That's right.

So, you're actually visualizing the change

in stopping power as a function of charged particle energy.

It comes in, has a very high energy.

And it might knock a little radiation damage cascade

by hitting another electron, which can have

its own shower of ionization.

And then it moves while doing nothing, in this straight line,

until it hits another one.

And notice right at the end, that's

where the densest amount of damage is done because that's

where the stopping power is the highest.

It's also where the energy is the lowest.

So, this is where the worlds of and physics collide.

You can actually visualize stopping power, like actually

visually in gel or on film or on a computer

by watching these charged particle tracks.

And after 10 to the minus 12 seconds,

all the ballistics are over.

Then you end up with diffusion and reaction.

So, it's going to be a balance between these charged particles

moving away from each other and finding something else,

or finding each other and re-combining.

And that's why, as you go up in timescale,

the particle tracks get more and more diffuse

and the number of these remaining free radicals

goes down until you level out at about a microsecond,

when all of the different particles

are so spread out that there are none

touching each other anymore.

To refresh your memory a bit from a few seconds ago,

take a look at some of the charge

states of these oxidative byproducts.

Some of them plus, some of them minus, sum of them excited,