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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,

  • all over the place.

  • So they can react with each other, which

  • is something you'd want to encourage so that they

  • don't go and find something else, causing

  • biological damage.

  • There's a question on last year's OCW problem set,