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MICHAEL SHORT: Hey guys.

So quick announcement, we're not doing nuclear activation

analysis today, because the valve that

shoots the rabbits into the reactor

broke and needs to be repaired.

So we'll likely just do this next Friday.

And instead, we'll have the whole recitation

for exam review, like I think we'd

originally, originally planned.

I also want to say thanks to whoever said, please write

lecture notes for this class.

It's something I think needs to be done.

And I just now biked back from meeting

with a publisher, or a potential publisher,

to actually get this done.

Because as I think as you guys have seen,

there's no one reading that does this course justice.

There are some that are too easy,

there are some that are too hard.

And there are giant pieces missing,

like most of what we're going to go into today--

sources of background radiation.

And where do cosmic rays come from, and what are they?

So thanks to whoever said that, because it spurred me off

on a let's make this into a book kind of thing.

Want to quickly review what we did last time.

We went over all the different units of dose and radiation

exposure, from the roentgen--

which is pretty much just valid for those measurements in air.

And I realized that yesterday I brought in the civil defense

dosimeter and passed it to one person,

and we didn't continue bringing it.

So I'll bring it to recitation.

It's not in my bag.

There is the unit of the gray, which

is just joules per kilogram.

Which you can calculate from stopping

power or exponential attenuation equations.

This is where you start, because it's

how you get from the world of physics

to the world of biology.

Into units of increased risk, or sieverts,

which is just gray times a bunch of quality factors, which

are either tabulated, or I'd like to see somewhat better

calculated.

They are calculated sometimes, but by empirical relations,

because that's usually good enough.

Biomeasurements tend to be pretty sloppy,

and I'm not that upset that these are empirical relations.

I'm going to skip way ahead past the detector stuff.

We didn't quite finish up the IF2D idea.

I think this is where we ended last time, where

we were talking about how do you detect dose

during cancer treatment.

And I was outlining one proposed way that we're thinking of.

Using this F-center based dosimeter,

which changes color when it gets irradiated,

you can implant it in the tumor.

And as it moves from things like breathing or swallowing,

you could feedback to the proton beam,

and only irradiate when it's in range.

Or play nuclear operation, and try not to hit the sides.

However, whatever you want to do.

And there's multiple implantation options for this.

We've thought about things like implanting a fiber optic

cable into the tumor, and then having a port

on the side of you that could do some in body spectrometry,

which would be pretty cool.

Could also put it all in a chip.

You could have the emitter--

either a broadband or single color LED--

the F-center, and the spectrometer all in a chip

that's implanted.

And with radio frequency power transfer,

so you don't need to put a fiber optic port

and plug it into the side of you.

Or however it goes.

And what we're doing next in this development

is nailing down what color change is given by what dose--

so the physics.

Develop an on-chip version.

Find a bio-compatible casing.

We've done the IP part, which is pretty cool.

As you can see with patent office at least

has taken in the application.

But it's neat that you need to know pretty much everything

you learned at MIT to pull off a project like this.

From the nuclear physics stuff, to the material science,

to the 22.071 electronics, to the medical stuff for biology,

to the financial stuff for econ.

In order to pull off an actual nuclear start

up project like this, you need everything you learn here.

Which is kind of a neat case study.

But now let's get back into what does a sievert really mean

in terms of increased risk?

Usually means the increased risk of some sort

of long-term biological effect, whether it's cancer

or some other genetic effects-- let's say mutations--

anything that would take a while to manifest,

and would manifest by slow but steady cell division.

And if you notice the difference between adults

and whole population, let's see--

yeah, sorry.

You can see right here that these--

not very much dose.

Let's say in the realm of 10 to the minus 2

sieverts can give you some increased risk for cancer

or some other effect.

So we're talking on the realm of 40 millisieverts

or so would give you some additional cancer risk.

That's not a lot of dose.

That's up to about the limit of the occupational dose

that you're allowed.

So when we talk about how much is too much,

I've taken some excerpts from this Committee

on Radiation Protection document.

This is from Turner, but the entire document,

as I mentioned, is up on the learning module site.

So you guys can see the actual verbiage where this is defined.

So your lifetime dose should never

exceed in tens of millisieverts the value of age in years.

Which means in some years if you get a little bit less dose,

you can get a little bit more dose

and still be considered safe or not

have any appreciable increased risk.

And while you're working, you should never

get more than about 50 millisieverts in a given year.

How was it for radiation workers?

So for you guys, what dose are you

allowed per year working at the reactor?

AUDIENCE: About five rem per year.

MICHAEL SHORT: Five rem per year,

which is 50 millisieverts.

Awesome, so it's--

AUDIENCE: [INAUDIBLE] to your eyes [INAUDIBLE]..

MICHAEL SHORT: Oh sure, yeah.

To jump back to that, if you're saying that there's actually

tabulated differences for different organs,

that's where they come from.

Let's say you can take less radiation to the same organ

and get the same dose in sieverts

measured in equivalent risk.

So I wouldn't be surprised if these are the organs

that you're not allowed to irradiate as much.

AUDIENCE: [INAUDIBLE].

MICHAEL SHORT: I'm surprised the eye isn't here.

Does it say retina anywhere?

Oh, OK.

Well that's just one table.

It's not necessarily the complete answer.

Yeah, so 50 millisieverts isn't that much.

Although if you think of it in the old xkcd units of banana

equivalent dose, eating a banana gives you

about 0.1 microsieverts.

So you would have to eat 50,000 bananas in a year--

no, I'm sorry, 500,000 bananas in order to incur that.

Well yeah, I was talking to someone at dinner

last night about the banana burning

experiment where we measured the activity in becquerels.

And then we can calculate how much dose in sieverts

it would take.

And he said, how many bananas would it

take to get some increased cancer risk?

About double this.

It would take about a million bananas

to give you about 100 millisieverts.

And I said, you know what else it would cost?

And he goes, 100 millisieverts.

No shit.

And I said, that's right.

Yeah.

Yeah, he totally didn't plan that,

but it just worked out that way.

Yeah.

And then how much is too much?

Let's say for the general public for a background for excluding

things like natural background and medical exposures,

you're not supposed to get about more than one millisievert just

walking about outside.

And you don't tend to get that much more.

Why are medical exposures not included, despite them being

pretty radioactive procedures?

Yeah?

AUDIENCE: Because they're very targeted.

MICHAEL SHORT: They are targeted,

and so they could give a lot of dose to certain organs.

But the amount of dose isn't necessarily

why we don't count medical procedures.

Anyone have any idea?

Yeah.

AUDIENCE: Was it because usually you wear a lead vest

if you're getting an X-ray?

MICHAEL SHORT: In some cases, like if you go to the dentist,

you'll get a lead X-ray.

But let's say you get a chest X-ray.

Why don't we care that a chest X-ray is way more than you get?

Because these things tend to save lives.

So you're absolutely willing to get extra radiation exposure

that may have a delayed effect if the immediate effect is

to save your life.

You had a question?

Or no.

OK.

Yeah, so we don't count medical things

because chances are you're doing them to improve or save

your life.

So what's a little bit of radiation

compared to let's say finding the blood clot or the aneurysm

or whatever it would take?

And then how much is enough?

AUDIENCE: Yeah, that's the table.

MICHAEL SHORT: This is the table that you're familiar with?

Yeah.

They actually talk about the lens of the eye.

And that's a heftier dose.

But also, the lens of the eye is not a very massive organ.

So this would mean do not stick remaining eye in neutron beam,

right?

Or you've seen that sticker, do not

stare into a laser with remaining eye.

Yeah, the same goes for the neutron beam ports coming out

of the reactor.

But the lens of the eye can take a fair bit more dose per unit

mass than the whole body.

The lens of the eye is not a particularly fast developing

tissue.

It can cloud up with an insane amount of radiation exposure.

That would take a lot more than 150 millisieverts to do,

though.

And then things like 500 millisieverts

for skin, hands, feet.

Pretty much just groups of muscle, bone, and dead skin

that not much is going on biologically.

Blood's flowing through it, but that's about it.

And notice that the regulations do differ a little bit,

but on the whole, they're fairly similar.

Same for the eye, same for the feet, same for the year.

Cumulative is a little different.

This says 10 millisieverts times age.

This allows you a little bit more.

Whichever recommendations you follow,

they're all pretty similar.

And our knowledge of how much dose

leads to how much risk hasn't changed

a ton in the last decade or so.