31. Frontiers in Nuclear Medicine, Where One Finds Ionizing Radiation (Background and Other Sources)

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visit MIT OpenCourseWare at ocw.mit.edu. 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. There’s been all sorts of
arguments for or against it. Has anyone heard of this
LNT or Linear No Threshold model of dose versus risk? This is something
we’ll talk a lot about on the last day of class. This is the theory
that the amount of risk versus the
amount of dose is linear. And no threshold means that
every little bit of dose gives you additional risk. This is not supported
very much by science. I’d say it’s not
supported by science. The converse argument is also
not supported by science. We just don’t have the
statistics at super low doses to say what happens. But the official recommendation
is that there is a unit of dose that we define as nothing,
and it’s 0.01 millisieverts– about 100 bananas–
per event, let’s say– yeah, where does it say–
yeah, per source or practice. So eating 100 bananas
in one sitting is considered to give you
zero additional risk according to the official guidelines. So the guidelines
put in place do not follow the linear
no threshold model. But anyone that would claim
that one or the other model is absolutely correct has
either got a huge sample size of people that
we don’t know about, or is probably
extrapolating beyond what the data will tell them. So you’ll see this argument
flaring up quite a bit. For the last day of class we’ll
have you read some arguments for and against the linear
no threshold model that aren’t just blogs
on the internet, but they’re actual
published articles that have passed peer review. And it’s really hard
to tell exactly what’s going on at low doses. But meanwhile, let’s
focus on what you do get that we can measure. The actual contribution from
background levels is about 50% radon. This is a natural decay
product of radium. It’s just everywhere. It’s here right now. It’s all through the atmosphere. This is why you
want your basement to be rather well
ventilated, because it’s a heavy noble gas
that accumulates down in unventilated basements. So you don’t actually
want your house to be sealed up super
tight, because you can have radon accumulation. Especially if you happen to
live near a granite deposit or on granite bedrock– which
everyone in New Hampshire does– radon levels are
a little higher. There’s actually a
story about a guy that used to work at a
nuclear plant somewhere in Pennsylvania. I don’t know where, but he lived
on top of a pretty good granite deposit that was a few parts
per million radium more. So the radon levels in his
house or in his basement were much, much higher
than would normally be allowed for background. And this guy used to
set off the radiation alarms coming into work. Then he would breathe
the nice clean radiation free air in the plant and go out
without setting off the alarm. Eventually something
had to be done. I don’t actually know what
was done, but if any of you can find that original story,
that would be pretty cool. Cosmic rays, another source
that you can’t shield from, is about another 10%. And we’ll talk a lot about
where these things come from. Terrestrial radiation, well
we’ll count that as stuff in the soil, stuff
in the cinder block. Wood happens to be a fairly
radioactive substance pound for pound, but it’s
not very dense compared to things like brick or– well, banana ashes is probably
about the same as wood. Internal, coming from you. You’ll have this on
problems at number eight. Because you all now know your
internal radioactivity thanks to going to environmental
health and safety. Did anyone see anything
disconcerting in your spectra, other than a tiny
little potassium peak? Anyone? I’d say, ah, it’s too bad. But that’s great, especially
for you guys that work at the reactor. Medical X-rays. It’s assumed that everyone
gets a couple of these a year. You all go to the dentist
and look for cavities. You break something, you may get
an X-ray of your hand or foot. And this is let’s say an average
amount of medical X-rays. Then a little bit of stuff
leftover, consumer products. This isn’t counting
things like Fiestaware, those orange glazed
plates and bowls that were painted with a
uranium based neon orange paint. So we also don’t tend to
drink from Revigators anymore. Has anyone ever seen or
heard of a Revigator? AUDIENCE: Yeah, was this
in the ’50s or ’60s? MICHAEL SHORT: Or
even earlier, yeah. So back in the
’20s, people would put radium ore in these
containers and pour water in it and say, natural
radioactivity gets in. It cures croup or the Jimmy
legs or astigmatoid rheumatism, or whatever other quack
diseases there were in the ’20s. You can still find them
on eBay, and that’s not accounted for in today’s
consumer products. But this all accumulates
the amount of dose that you tend to
get on your own. You might get a couple of
millisieverts a year of dose just from background, especially
depending on where you are. And the big one– whose spectrum I think you’re
all familiar with by now– is that of radon. Because we saw most of
these peaks in the bananas. Anyone have any idea why you
would find so many radon decay products in bananas? Given that radon’s
everywhere, we did notice elevated levels
of specifically bismuth 214 and actinium I think 228
was the isotope we saw. Where would those come from? The what? AUDIENCE: The soil. MICHAEL SHORT: Absolutely, yeah. Whatever radium’s coming
out from the bedrock, that radon has to come
up through the soil. If that happens to
decay in the soil, it makes lead,
bismuth, polonium, other heavy metals that are
taken by the plant’s tissues. In addition, radon
daughter products can plate out on the
leaves themselves. So this is one of those
reasons that smoking is such a bad thing to do. Aside from the
chemical effects, you have giant fields of
high surface area tobacco that you then
concentrate and dry up into these tiny little sticks. You have an enormous
amount of leaf surface area and dry vegetation that has
taken all these radon daughter products. So most the dose
you get from smoking is lead, bismuth,
polonium, actinium, radium. Alpha emitters. As we saw last
time, to remind you guys of the quality factor
for alpha particles, it’s as big as it gets. Anyone remember why that is? Why are alphas so damaging if
they get into your tissues? AUDIENCE: Because they’re big. MICHAEL SHORT: They are
big, so they have high mass. And? AUDIENCE: Short range. MICHAEL SHORT:
Short range coming from their high relative charge. They have quite
high stopping power. And they will deposit
a lot of energy very close to where
they are emitted. So their range is very small. So that armor piercing
bullet analogy comes to just an armor piercing
bullet that explodes right out of the barrel of the gun. And they do quite a bit
of radiation damage. We jump back to the right slide
without inducing a seizure. I think we’ve looked before
at the radon decay chain. This is a simplified
version, because there are different branches or
different possibilities for decay, but some of them
have extremely low percentages. So this one’s
simplified quite a bit. But it is whenever radon
decays, it gives off a bunch of alpha
and beta emitters that last anywhere from minutes
or seconds to days and so. For every radon atom
that you absorb, you end up getting four
or five alphas and betas, depending on how long it
stays into your system. And then mapping
out radon in the US. You have quite different
amount of radon dose depending on where you are. And I wanted to skip
ahead and overlay a couple of maps of the US. So this is a terrestrial– oh, I’m sorry,
that’s the wrong one. Let’s just look
at this one, yeah. Anyone notice any patterns here? Where do you tend to
get the most radon? What sort of features
would one live near when you get a lot of radon dose? AUDIENCE: Mountains. MICHAEL SHORT: Mountains,
which tend to be made of? AUDIENCE: Rocks. MICHAEL SHORT: Rocks, which
tend to contain a lot of radium. Especially granite
and other such rocks. The Conway granite, named after
up in Conway, New Hampshire, is about 52 parts per
million uranium or radium. So it’s a fairly toasty rock. You can actually tell there
with your Geiger counter, if its count long
enough, that there is a little bit of radioactive
ore in that Conway granite. Not nearly enough
to matter at all, and certainly not enough to stop
you from making fancy kitchen countertops. But I wonder if
folks would buy those if they knew that there was 52
parts per million of something with a half life
of 9 billion years. I somehow think it
would matter to people, but it really doesn’t. AUDIENCE: They put the radiation
symbol on it [INAUDIBLE] MICHAEL SHORT:
Yes, engrave that. If they made the radiation
symbol a little induction stove, that would
be pretty slick. Yeah. And then in terms of
relative radon risk, how much actually matters? I like this graph. Despite being difficult
to read, it actually shows how much the
average indoor level compares to all the different
things you could do. Like getting 2,000 chest X-rays
per year versus 100 times the average indoor level. That’s about what I heard
that fellow in Pennsylvania had is like 100 or 80
times the normal radon levels from living underground
on this giant granite deposit. It’s like getting 2,000
chest X-rays per year, or something like
smoking four packs a day. I know some people that do this. They don’t tend to be that
afraid of the radiation that they’re taking
in from smoking. But yeah, it’s pretty insane
how much radiation you get. People are afraid to get one
chest X-ray, which is not even on this map. One chest X-ray
worth of radiation gives you much, much less than
living for a year in a house, which we all tend to do. Then if you live in a brick
or cinder block house, you actually get a fair
bit more radiation, because these are fairly
radioactive building materials. And I’ll show you those
activity levels in just a sec. As far as exposure
sources, again, to look at the relative amount
of terrestrial gamma ray exposure, you can
correlate that pretty well with not green regions
on a topological map. So look at the really
low levels down here, correlates with low lying
vegetative areas around here. Colors are a little more
extreme on my screen, but you can see this is
all low lying right here. From Louisiana, Florida,
up the east coast, until you get to the
Appalachian Mountains and such. So pretty striking
correlations there. And it all comes from what we
call the primordial nuclides. These are unstable nuclides
that have such long half lives that they still
exist, despite the universe being 15 billion years
old, or whatever supernova formed our solar system being
five plus billion years old. Things that we’ve already
studied almost to death, like potassium-40. And you can see that’s about
0.011% of all potassium is radioactive potassium-40. About 10% of which
gives off gammas in– what is it, 90/10? I forget the split. But can give off either
betas and then a gamma, or positrons and then a gamma. Then things like rubidium,
in the same column as sodium, potassium, and
cesium, so it behaves kind of like a salt-like element. There should be some for– are
there any for chlorine too? Some of the other important
ones to note, oh, platinum. Does anyone has any
platinum jewelry? Does anyone have any
platinum jewelry? Good answer, yeah. I don’t either. I teach at a college. Also, I don’t like
wearing jewelry. But there’ll be all sorts
of these primordial nuclides that you can’t really
do anything about. They’re just there. Note that the half lives
are really, really long. And as you know
now, the half life is going to be inversely
proportional to the activity. Despite almost all indium– look at that, 95% of the indium
that you’ll find emits betas. Doesn’t stop people from using
it as these awesome glass to metal seals for
vacuum components. Because once in a while
it might emit a beta ray, like a whole gasket
might emit a beta once every millennium or so. But these half lives
actually are measurable. And that begs the question,
are there elements with longer half lives that
are just too long to measure? You think what does it mean
to have a half life of 10 to the 15 years, given that the
universe is on the order of 10 to the 10 years old? Is it possible that
all nuclei will decay till the end of time? I’ve seen some
documentary such things and don’t call this a science
that say, oh 10 to the 40 years from now, the last
protons decay, or the last whatever
elements decay into all protons and neutrons. Don’t know if
that’s true, but it does make me wonder, are some
of the other so-called stable elements just have
ridiculously long half lives? It’s something to think about. So let’s take a look at
the building materials, and see what’s in the typical
things around us right now. You can see how much
granite, radioactive thorium, and potassium are in
these building materials. And how many usually is
measured in picocuries per gram. A picocurie is already less
than a becquerel per gram. Because a curie is what, 3.7
times 10 to the 10 becquerels. And a pico is a 10
to the minus 12. So things on the order of
a few picocuries per gram, a gram of that
material might emit one disintegration per second. Not a lot of radiation. But take a look at how much
potassium there is in granite. Nanocuries per gram, that’s
getting into the integers worth of becquerels. If you look at wood,
check that out. Anyone heard of potash before? It’s one of the ways
we get potassium. So if you take wood based things
and you burn them in a fire, you drive off all the
carbon in the water, you’re left with these kind of
whitened salt and pepper ashes. That’s potash. That’s the ashes
left over in the pot after you burn stuff in a fire. They’re quite potassium rich. So I think what
we’ll do next year, instead of burning
a bunch of food, is just burn a bunch of wood. We’ll have a nice bonfire,
collect the ashes. And we can see how much
potassium there is in wood. Which pound for pound,
if you see on this list, is the most radioactive
building material there is. Just that wood happens
to be pretty inexpensive, and consists mostly of water,
lignin, and other carboniferous materials. They don’t have
carbon-14 on this list, but let’s take a look at
some of the other ones. So sandstone cement has a
pretty toasty signature to it. Gypsum drywall. What is it, 13 parts per million
uranium in all the drywall you tend to find. Anyone scared yet? Because you shouldn’t be. It’s like I mentioned
before, this is the slide I want
to show to most folks in the general public. There is such a dose as nothing. And that’s pretty much
what you get from, let’s say, a day’s worth of
being around these building materials. It’s just about nothing. It’s after the
building materials. There we go. Seawater is another great
source of radioactivity. Enough so that
people have actually proposed harvesting
uranium from seawater. So the total amount of
activity in the ocean, there’s something
like 11 exabecquerels of radiation contained in
the uranium in the seawater. Which means you should be able
to have a gigantic trawler flying out around in the
ocean, and just floating through the seawater,
picking up atoms of uranium here and there. Because technically,
there’s enough to power the world for like 2,000 years. The problem is the ocean is big. It doesn’t stop people from
actually working on it. So it’s neat to
think that there’s a whole lot of
carbon-14 and tritium and uranium in seawater. 300 picocuries per liter. Anyone have any idea why there’s
so much more potassium activity than anything else in seawater? It’s because it’s salty, yeah. And potassium’s
just like sodium. So there’ll be a
fair bit of potassium in the seawater, 0.0117%
of which is potassium-40. And so last year, people asked,
what, uranium from seawater? How is that even possible? So this is the part
in the course where I’m going to pull out a
lot more recent papers to show you some of the
cooler innovations going on. So you could use this adsorbent. And adsorbent. Does anyone know what adsorption
means, not absorption? Adsorption is when
something sticks to the surface of something. Absorption with a B is
when it actually gets incorporated into the bulk. So folks are thinking about
making high surface area materials that can adsorb
selectively atoms of uranium. So you send out this huge
braided net, or a huge stack of adsorbing material with
a D, and just go around in the ocean. Attach them to tankers
or cargo ships, and just have them
pull in product as they go from coast to coast. And by actually changing around
the chemistry and the geometry of this, you can enhance
things specifically for uranium by about a factor of three. And this yellowcake right
here next to an actual ruler– that’s 50 millimeters
right there– that was obtained
directly from seawater. So you can actually pull
yellowcake out of the ocean. Just not very much of it. And the way these work is there
are interesting compounds that selectively absorb uranium
into their structure, not by direct chemical bonding,
but by getting something close. For example, my wife tends
to study metals bonding to proteins. And it’s not necessarily
always a full chemical bond like you might think,
but the protein can kind of wrap around a
metal ion, transport it places. Similar thing going here. I’m not going to
pontificate anymore on it because the title
has the word organic in it, and I am definitely
not an organic chemist. Don’t want to tell
you anything wrong, but I do want to tell you you
can actually see this paper to find out what
sort of compounds selectively grab onto uranium. And if there’s a seawater
floating through, and uranium happens to flow
nearby, it can bond to it. You can then somehow squeeze
out or burn off that adsorbent, and there you get uranium. Now let’s talk about
what’s in the body and interpreting the spectra
that you got from EHS, your full body spectra. If you take a look
at how much uranium there is in the body,
wow, there’s some. Every one of you is got
about a becquerel of uranium in the body. But if you look at the relative
amounts, rate it was at. The only things that
really matter a ton, potassium-40, about 4.4
kilobecquerels per human. So each of you is giving
off 4,400 potassium gammas per second. Most of them just flow right
through all the other people. And then carbon-14,
about 3.7 kilobecquerels. Pretty interesting to know
just how much radioactive stuff you have in your own body. You will need this table
for homework number eight when I ask you a
pretty fun question. Then there’s all the
various medical procedures. The ones that aren’t counted in
your annual dose because they tend to save people. But now let’s look at
the dose in millirem So if you want to figure out
what this is in sieverts, just divide by 100. So a regular old
chest X-ray, 0.1– let’s see, how does it go? 100 rem in a sievert. Yeah, so about 0.1
millisieverts, or 1,000 banana equivalent dose. Not terrible. Which is why if we go
back to that chart of all those relative
risks, if you look at the average indoor level,
it’s like getting– well, I would have to guess maybe
40 chest X-rays per year based on this
rather crude scale. And you’re allowed to
get around a millisievert or a few millisieverts
of radiation per year. That sounds like it
checks out mathematically. Let’s look at some of
the other crazier ones. Dental bitewing. Yeah. So anyone ever bite
down on something where you have to get an X-ray
through the side of your mouth? Quite negligible amount
of dose, yet they still put the lead apron on you. I’m guessing that’s mostly
for show, because that’s very, very little dose. It’s well beyond the 0.1
microsieverts of something that counts as nothing. But there’s not a lot of
dose going into these things. Let’s see what really does
give you a whole lot of dose. 10 millisieverts for a CT scan,
or a whole body CT screening. That’s a pretty
hefty amount of dose. Right there with
one procedure you may get more than your normal
annual background dose. But if you’re going
in to look for stuff, chances are you need to find
whatever you’re looking for. So we don’t count it. And it may give you a
slightly higher chance of developing cancer
much later down the line when that cell that
gets mutated divides. But it’s probably going to save
your life in the next hour, or the next day. So definitely worth it. Let’s see, the worst– what’s the worst
procedure we can find? Noncardiac embolization. I don’t know enough
biology words know exactly what that means. AUDIENCE: I know what
noncardiac means. MICHAEL SHORT: Yeah, I think we
all know what noncardiac means. Good. This isn’t 7012. Nonmedical procedures– or more
medical procedures, I’m sorry. Where’s the techneitum scan? Yeah, notice how
many of these things have technetium imaging where
you’ll inject technetium into a certain organ or a
certain vascular or lymph or whatever system. Some of these things can
give you a fair bit of dose. Like again, maybe a
heart stress rest test can give you double or triple
your normal background dose. This is why you have
to declare to airports if you’ve just had a
medical imaging procedure, because this is well more
than enough to pick up on any sort of airport
radiation monitor. So again, if you ever
get a medical procedure with any sort of
radiation, anything, do declare it, because
it’s quite measurable. Then there’s radiation
from altitude. I may have mentioned already
that the reason that pilots can’t fly for a certain number
of hours is not fatigue, but it’s radiation exposure. When you start to
look at how many microsieverts you get per
hour on the ground, 0.03. Right about down at that–
hanging around for an hour is a negligible dose. You go up to
international air travel, that goes up by a factor
of little more than 100. And so you get a fair bit
of radiation exposure. If you take your annual
allowable occupational limit of 50 millisieverts,
divide by 3.7 microsieverts, you’re getting close to 86– what is it? How many hours in a year? Let’s see. There’s three times 10 of
the seven seconds in a year. So divide that by 3,600. That’s getting on
the realm of 10,000. That’s about the
conversion factor. So you can’t spend
your life in the air, because you’d get too
much radiation according to the occupational risks. And so actually in addition to
some interesting measurements that have been
published in papers, we’ve actually had
students go out and build radiation altimeters
based on the MIT Geiger counter– removing the speaker,
of course, because you don’t want to clicking
Geiger counter on a plane. That’d be kind of a
stressful situation. Let me show you one
example of these. Are also published from a paper,
but we have pretty similar data from– if you want, to
go talk to Max Carlson, one of my graduate students who
hooked up his Geiger counter to an alarm clock– the case that had
[INAUDIBLE] in it. I think this was a poor
choice of case for a plane because it looks kind
of like a time bomb. But luckily nobody found it,
and he was able to get the data. But you can see just how
much more data you get. And you can correlate the height
that you’re at with the dose– in this case, microroentgens per
hour or microrad depending on– what is it– ambiguous unit
definition right there. But it’s quite noticeable. So for those of you who
have built Geiger counters and have cell phones, and don’t
want to have a fake time bomb, you can actually hook
in your Geiger counter to the microphone port
of your cell phone. And with a few available
radiation apps, you can actually monitor
your dose in microsieverts. Assuming that it all comes
from gamma rays, which is most of what a lot of
cosmic rays will produce. So it’s a pretty
safe approximation that your dose in gray
flying on the plane is also your dose in
sieverts, because it’s whole body, and its gammas. So I’d say try
this at home, kids. This is one of those things
I recommend you try out. Speaking of these cosmic rays,
where did they come from? Well, this is a question
that’s been under debate, and was more completely
answered just a few years ago. They come from very high energy
particles from somewhere. It had been argued, do they
come mostly from solar flares, or did they come from
elsewhere in the galaxy? Mostly we’re talking about
things like high energy protons or other charged particles. We’re also talking neutrinos. Anyone know about
how many neutrinos are theorized to pass
through you every second? Trillions. Yeah, something like that. But they basically don’t
interact with matter. As I showed you guys
near the beginning, it takes a gigantic salt mine
full of water and exploding photomultiplier tubes in order
to catch two or three neutrinos a day if you’re lucky. So let’s say those
don’t really matter in terms of background dose. But when those high
energy particles interact with the oxygen and nitrogen
up here in the atmosphere, they produce a
shower and cascade of additional ionization
and high energy particles. So it’s been said that solar
flares and such will accelerate charged particles
from the plasma in the sun towards Earth. They’re deflected somewhat
by the magnetic field of the Earth, but they
tend to enter right here at the– what is it? At the poles. I’m sorry, that’s the simple
word I was looking for. Until recently in 2001, they
were looking specifically at the evidence for or against
the idea that coronal mass ejections– which means
large ejections of mass from the outer layers, these
sparcer layers of the sun– was responsible for most charged
particles and cosmic rays. Skipping the stuff
that’s not in bold, it appears to be that
the CME bow shock scenario has been overvalued. So for a while,
folks were saying most cosmic rays come from the
sun– that’s our nearest star. By making really, really careful
measurements of the energy and lifetime of
these cosmic rays, this has actually been
somewhat disproven that this is the major
source of cosmic rays, which is pretty cool. But let’s talk about where
they actually come from. Reactions that you can
probably understand. So extremely high protons
enter the atmosphere. They all start as
high energy protons. And when protons are
high enough energy– and like I do probably
in every class ever, I’m going to pull up Janis
to show you something. They can undergo what’s
called spallation. It’s the same principle
that the Spallation Neutron Source at Oak Ridge
National Lab works on is shoot in extremely
high energy protons, out come neutrons. So as usual, it didn’t
clone the screen right. So just bear with me for a sec. I’d like to, for probably the
first time in this course, switch databases to the
incident proton data. Is that actually working? OK, good. So we’ll leave the
incident neutron data, we’ll go to the
incident proton data. We’ll stick with
the same library. Let’s see how much they have. Not much, but enough to
matter, because there’s a lot of nitrogen-14
up in the air. Let’s see what happens when
protons hit nitrogen-14 all the way at high energy. So don’t quite know what a
negative cross-section refers to. But at high energies, this is
definitely a possible event. And let’s see, there’s not
a lot of cross-sections to look at here. Let’s try oxygen-16. Not much. We’ll stick to the slides then. So when a high energy
proton strikes a nucleus, it can eject neutrons. And those neutrons can then
cause activation reactions, and then emit things like
proton or tritium, leading to– that’s where your
carbon-14 comes from. Comes from nitrogen-14. So in comes a high energy
proton, releases a neutron. That neutron hits nitrogen-14,
releases a proton, out comes carbon-14. So this is why it’s being
constantly generated in the atmosphere. It’s not like there’s
a certain amount that was there at the beginning
of the universe and decays, because its half life
is only 5,700 years. So this is part of why
radiocarbon dating works, because we have cosmic rays. It’s kind of a neat
connection to make. If there weren’t cosmic
rays, all of the carbon-14 would decay pretty quickly
in the universe time scale. And we wouldn’t have this
form of radiocarbon dating. And then same thing for tritium
production in the atmosphere. This is where some of that
tritium naturally comes from, is it makes carbon-12– which
is the normal form of carbon– but out come tritium,
which there is going to be some constant isotopic
fraction of tritium in all the world’s hydrogen.
Some of it’s being generated in real time. And we do have these
spallation sources on Earth. Like I mentioned, the
Spallation Neutron Source has a gigantic synchrotron. We’ve seen these before, which
just injects in this case protons, which circle around and
around and around, accelerating until some of them are
extracted and fire onto things like a liquid mercury target,
some neutron rich liquid metal. So you want something
that’s very neutron rich. You want something
that’s very dense. You want something that’s fluid
so you can cool it better. And you want something with
high thermal conductivity. That’s where the metal comes in. So a liquid metal you can
keep cool really well. Because when you’re firing lots
of 800 MeV protons into it, you generate a tremendous
amount of heat. And this is what the
actual thing looks like. You can get tours of
this down in Tennessee at the Oak Ridge National Lab. Actually, I’ve driven
up here before. I recognize this from the map. That’s pretty cool. So where the actual
neutron science stuff happens, where all
the scientists sit with their targets, there’s
quite a bit of stuff going on behind it. So there is a gigantic– you can see that’s a
parking lot for scale. There’s a gigantic
linear accelerator shooting into the
synchrotron ring, which then fires the protons
here into the target into one of any number of end
stations, which creates a not quite push button, but
turn on-able pulsed neutron source, which is pretty slick. And again, parking
lot for scale. Takes a lot of magnets to
bend an 800 MeV proton. That’s what it
actually looks like. Has anyone ever seen one
of these synchrotrons? Like at Brookhaven or at
Oak Ridge or at anywhere? They’re quite
interesting things. The closest one
to us is the NSLS, or the National Synchrotron
Light Source version two at Brookhaven National Lab. It’s like a 2 and 1/2 hour
drive down in Long Island. I don’t know if they’re
doing tours yet, but it’s about a
kilometer around. And I was told they did
bike races around it to see who could beat
the protons, which of course everybody loses. But they are pretty
insane collections of magnets, vacuum equipment. And once in a while a
proton will pass through. And then there’s the
spallation source itself. So this is what the
target looks like. There’s going to have to
be liquid metal cooled in. And then out of here come lots
and lots and lots of neutrons. Enough neutrons
that you still need hot cells to deal with things. They’re still quite
radioactive, inactivated. But it’s not a reactor. Other ways of making neutrons. Speaking of, has anybody
seen the pulsed fusion source that we have down
in Northwest 13? No? We have a pulse
neutron source that you can come take a look at. It’s an electrostatic
fusion pulsed machine. There’s a whole bunch
of tritium and deuterium in this palladium sponge,
what happens to hold hydrogen and its isotopes very well. And with a very quick pulse,
you can have a tiny pulse fusion and generate about 10 to the
8th neutrons that actually is a push button neutron source. So if you want to see a
neutron source beyond reactors, go down to the vault
in Northwest 13 and ask to see that. We did a quick
experiment before trying to activate cell phones
to see what was in them. We did not generate
enough neutrons to do so. But this cell phone has
definitely seen a few fusion neutron pulses. And we checked later,
it’s not giving off any residual radioactivity. At least we can measure. That was a fun
failed experiment. And then comes the craziness. Since it’s about five
of, I want to get into things that will
definitely not be on the exam. So just sit back and enjoy
and stop taking notes. Complete insanity can happen
with super high energy electrons. We’ve already talked
about Bremsstrahlung. We have talked about
synchrotron radiation, where you have a charged particle
going along a magnetic field line. It changes direction
and gives off X-rays. We haven’t talked about
inverse Compton scattering. Interesting process here. In comes a low energy
photon, hits an electron, out comes a higher
energy photon. Compton scatterings usually
think of as the other way around, where a high energy
photon comes in, scatters off an electron, loses energy. In this case, a
high energy electron colliding with a
low energy photon can impart energy to the photon. And you can actually
calculate– or in this case, I’ve just taken from a paper– the energy gain from
inverse Compton scattering, as well as whatever
cross-section there is. And even though this is a
very infrequent process– well, the universe
is pretty big, and contains a
lot of things that have magnetic fields
like stars and black– whatever else happens
to have magnetic fields. And you can identify
radio sources by looking for these inverse
Compton scattered X-rays. So the Chandra X-ray map,
I believe a piece of which or a receiver for which is up in
the building in Porter Square. If you guys go down three
stops on the red line, you’ll see this little area
full of Japanese noodle shops, Lesley University,
a bunch of galleries. And in a little– I think it’s still there– and
a little sign that says, oh, and there’s the Chandra
X-ray Observatory. Whatever. They may or may not
have moved, but I recommend you check it out. And then what happens
to those electrons? Well, they can actually decay. Pretty interesting things. And so some of this
inverse Compton scattering has gone into the evidence
for or against where cosmic rays come from,
because you should see electrons of
a certain energy after undergoing this process. I think I will skip ahead. Oh, I won’t skip ahead. And so what these
cosmic rays can produce is what’s called positive,
negative, or neutral pions. Other subatomic particles
with masses somewhere between protons and
electrons that themselves can undergo different reactions
or different decays into muons. And don’t worry, muons
and pions and such are not part of the
topic of this course. But they do have
known lifetimes, they do have known
masses and charges, they do have known
stopping powers. We should be able
to measure them. And there is a cosmic muon
detector at Boston University. Or rather, it’s a
pair of detectors that looks for this
coincidence of one muon scattering off one
detector, or interacting, and another particle being
sensed directly beneath. So we can actually sense these
muons to confirm or refute the theories about
where they come from in terms of cosmic rays. And these neutral
pions are what end up creating these gamma rays. I think they were around
the 70 something MeV range. So if these theories
about them are correct, we should be able to
sense these gamma rays, and sense how many of them there
are as a function of energy. That’s not what I wanted to do. Let me recreate
presentation mode, because this is definitely
delving into the kind of stuff that, well, I’m
not an expert in. So a quick detour into
subatomic physics. You guys probably have heard of
that protons and neutrons are not the smallest building
blocks of matter. They themselves can be composed
of quarks and antiquarks with different charges
and different masses. They’re given different flavors. I don’t know who came up
with this terminology, but it’s kind of fun. Things get kind
of whimsical when you get into subatomic physics. And these quarks and
the gluons between them are what composes protons,
neutrons, in their antimatter counterparts. And these sorts of things can
also undergo their own decay and reactions. So when beta decay
occurs, it’s actually one of these up quarks
turning into a down quark and releasing an electron
and an antineutrino. But again, we’re not going to
delve even deeper into this. There are other particles
composed of other arrangements of quarks. So if you have just an up and a
down, you have a positive pion. Which should have a charge– I think one’s 2/3
of one’s minus 1/3. Yep, that is correct. So a plus pion should
have a charge of 1/3 the charge of an electron. So if you know the
mass of some particle because you know the
number of quarks, and you know the
charge of it, you should be able to calculate
its own stopping power. And figure out how
many should get through the atmosphere
and such, or how many get absorbed in your detector. And so when a very high
energy proton collides with an atmospheric molecule,
it creates neutrons, creates a shower of pions. These neutral pions– much like
electron positron interaction– can produce their own gammas. So they can spontaneously
decay from particles that are mass into gamma
rays of pure energy. Which then go on to create
their own shower of electrons and positrons by
pair production. Because as we saw, the higher
in energy you go for photons, the more likely pair
production becomes. And there you have it. This is part of the 22.01 stuff,
but taken to the nth degree. And then the evidence
for pion decay comes from extremely
fine measurements of the number of these pions
as a function of energy. Or in this case, look at that. They’re– what is it– ergs per
centimeter squared per second. What does that unit
physically mean? Not going to get into that. But at any rate, there
are different models for how many of these pions
or their high energy gamma decay products
should be observed. And by looking at
those very carefully, you can tell where
they came from. Should they have come from
coronal mass discharges, so we should know what energy
those protons should be, or some other source But I am going to stop
there, because it is five of. So after that crazy detour, give
you guys 10 minutes to degas and absorb some neutral
pion gamma decay products. We’ll meet upstairs in 10
minutes for an exam review.

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