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visit MIT OpenCourseWare at ocw.mit.edu. MICHAEL SHORT: So, before we
begin today's bit on dose, dosimetry, and
background radiation, I promised you guys
a story about how to use 22.01 to get out of
Apartheid, South Africa. So, I got told this story when
my cousin was about to get married because he needed a
diamond and he was going to go buy one and his dad said, don't. So back in the 70s, when
my uncle and his family were living in
South Africa, anyone who wasn't Dutch white,
so that would be blacks, Jews, including
us, anyone else was considered second-class citizens
by the Apartheid Government. And you were allowed to leave,
because they didn't want you there, but you had to
surrender all of your funds to the government
in order to leave, which gives me everything
you have and then you can leave the
country penniless is not a winning proposition. So, my uncle and his brother
devised a pretty brilliant idea to get their funds out of
South Africa unnoticed. They were both dentists
or radiologists or some sort of medical doctor
that requires x-ray reading. So, one of them gave the
other one all of his money, then reported bank statements
to the Apartheid Government and said, I'm pretty
much penniless. My practice went broke. I want to leave the country. And they said, OK,
get out of here. So he went to the
US, established a dental or radiological or
some other sort of practice, I forget which one, and
started requesting his brother, back in South Africa, to
send him x-rays to read to help boost his business. Because that was their actual
business, it was pretty legit. So then my uncle would
send him packets of x-rays to interpret and send back. Except that there would
be 10 x-rays on the front and 10 on the bottom,
and the middle 80 would be hollowed out. AUDIENCE: Oh! MICHAEL SHORT: Now that
would have ordinarily have tripped off alarms because
any change in density would trigger a change
in x-ray contrast. Because these packages were
being inspected by x-ray, and if it looked like
these x-rays were hollow and you were smuggling
something out, they'd be caught
and confiscated. So what sort of materials
are valuable that you find in South Africa,
that are pretty similar in x-ray contrast to
other light media like film? AUDIENCE: Diamonds. MICHAEL SHORT: Diamonds. So the remaining brother
went and converted all of his life savings into
diamonds, which is something that you can do in South
Africa because this is where diamonds come from. He then slowly, over a
period of months or years, sent packets of hollow
x-rays full of diamonds to his brother in the
states, knowing full well that the mass attenuation
coefficients of soft tissue and carbon are pretty similar
and so are their densities. So their total
attenuation coefficients are pretty similar, to. Let's pull those up
so we can check it. Carbon graphite. I'm going to add a new tab
and bring up soft tissue and we can compare them. Let's see, what's
the most similar-- so long as you can see
it-- thing to film? What should we call film here? X-ray film? AUDIENCE: I don't even know
what X-ray film [INAUDIBLE] MICHAEL SHORT:
Photographic emulsion. How about that? AUDIENCE: Kodak. MICHAEL SHORT: Is
there something Kodak? OK. Kodak, standard nuclear. I don't think that sounds right. Let's go with
polyethylene and plastic. Carbon, plastic. Carbon, plastic. AUDIENCE: [LAUGHTER] MICHAEL SHORT: Carbon, plastic. Basically identical. So this is a way they were
able to smuggle wealth out of the country without
x-ray contrast tripping off the guards. And once all of
the slave savings had been converted
into diamonds, he then went to the government
and said, I'm penniless. My practice went broke. I want to get out of here. And they said, good riddance. AUDIENCE: [INAUDIBLE] MICHAEL SHORT: Yeah. So they were able to
restart their life in the states with
all of the money that they had had
in South Africa. And when my cousin wanted
to get married and said, I'm thinking of buying a
diamond, my uncle just said oh, don't. He's like, what,
don't get married? He said, no, don't
buy a diamond. Here, let me take you
to the diamond drawer. AUDIENCE: [LAUGHTER] MICHAEL SHORT: Yeah. Because there was some leftover. So he said, pick out
an extra diamond. And that's the story of my
cousin's engagement ring, as well as why part of my
family's here in the States. AUDIENCE: [INAUDIBLE] That's
like a very cool diamond story. MICHAEL SHORT: Yeah, very,
very nuclear diamond story. Diamonds aren't forever, as
the finish have shown us, but they can get you out
of repressive regimes. OK, back to the dose stuff. So today, and for the
rest of the course, we get into biological and
chemical effects of radiation. As soon as this pops up. And so in the first
slide is everything you need to know
about dose and units. Don't worry, it's
not up there yet. I know that different
units of dose are a common point of confusion. So I wanted to put
everything on one slide so you can refer back to
it like a cheat sheet. So, you've probably heard
of the roentgen before. You've definitely seen
the roentgen as a unit because you've all looked in
those pen or pocket dosimeter that you actually had
at the nuclear reactor, when you guys took a tour of
it and controlled the control rods. The roentgen is not really a
unit that we use much anymore for very careful
calculations because you have to do some tissue
equivalency stuff in order to go from
ionizations in air, which is what it actually
measures, it's the amount of charge
dissipated or built up in air, to some damage to soft tissue. And the way you actually
calculate roentgens from first principles, linking
the physics and the biology part, is remember
this equation here? Stopping power, which
is some energy transfer, divided by the energy
required to make an ion. Each one of those will give
one electron unit of charge towards [INAUDIBLE] coulombs. And so this is the direct
link between the physics and the chemistry/biology
in this course. It's not something that's
done that carefully in any of the readings, which is why
I'm going to harp on it here in lecture. These two parts of the course
are often taught differently. And they're actually
totally related and everything's all the
same, which is kind of nice. You don't have to just relearn
a whole new lingo and field. Then there is the
SI units of dose. The ones that, when you do
calculations in the homework and the rest of your
life, I recommend that you use whenever
possible because they're in units that we're familiar
with, in standard units. The one where you start
with all along is the gray. A gray is the simple measure
of absorbed energy in joules per kilogram of whatever. And so calculating it is
fairly straightforward, too. For example, if you want to know
what sort of dose you would get in gray from
absorbing gamma rays, you can use this old
equation from the first third of the course. And if you integrate
this from, let's say, over the range of
whatever object or person you happen to be
irradiating, you'll get some fractional
difference in intensity. That, multiplied by the original
intensity of gammas, which could be given in maybe
gamma rays per centimeter squared per second,
maybe times time to get total number of gammas
per centimeter squared. All that multiplied by
the energy of each gamma, divided by the mass of whatever
is doing the absorbing, equals your dose in gray. So you can use all the old
stuff from the previous parts of the course directly to
calculate dose in gray. And this is the starting
point for any calculation. If you don't know what
tissue was exposed, what type of radiation provides
what biological effectiveness, it doesn't matter. You just start here. Then the other unit
you may have seen is a unit, not of
energy absorption, but of increased risk
for something going wrong in the biological sense. It's called the sievert. For simple things like whole
body dose from gamma rays, sieverts equals
gray, because sievert is multiplied by this quality
factor, or this effectiveness factor. That Q factor is actually
a couple of factors. There's a Q for
the type of tissue and there's a Q for
the type of radiation. And the total quality
factor for whatever you're trying to calculate
is just the multiplication of these two. And so it's fairly
easy, if you're ever dealing with whole body gamma
dose, gray equals sieverts. If you're dealing with pretty
much anything different, gray does not equal sieverts. There'll be just some
factor to add in, which can be looked
up from a lookup table but, as I've told you before,
I don't like that explanation, look it up on a table. We're going to explore why the
lookup tables are constructed the way they are. And then there's the
CGS units, the ones that are based in
centimeter-gram-second, instead of kilogram-meter-second. The rad is a simple measure. It's, let's say, 100
rad is just 1 gray. Where the rad
actually comes from is, it's defined as 100 ergs
absorbed energy per gram, where an erg is 10
to the minus 7 joules and a gram is 10 to
the minus 3 kilograms. And so, you can do
the mental math there to make sure these all work out. And then a REM, a
roentgen equivalent man, is just a hundredth
of a sievert. So there's historical
basis for this. Back in the day, more
folks used CGS units. There's been a push
to SI units, which I happen to like because
everything works out and you don't have to
remember things like 10 to the minus 7 joules
or whatever like that. So I'd say, when in doubt,
for equal comparisons always use the SI
units and always start with the
gray because that's something where you can take a
physical calculation of energy per kilogram and go into
some increased cancer risk by using the dose
quality factors. So let's take a look
at how these appear. First of all, gammas,
x-rays, electrons, positrons of any LET. LET, as I mentioned before,
is linear energy transfer. And to put this
mathematically, you've actually seen this before, which
would be some change in energy over some change in distance. It's the stopping power. It's not like the stopping
power, it's the stopping power. So with the formulas you got in
the second half of this class, you can calculate
linear energy transfer. Now, why are these things given
in, let's say, discrete tables, or now what's currently
recommended is these functions? Does anyone have any idea? How many of your
Core Seven friends know the formula for stopping
power, or could parse it even, or understand it? You don't have to,
not everybody has to. So, for the rest of us, there
are simpler empirical relations or relations that get the
numbers right that aren't necessarily based on physics. So a simple lookup
table, for those who don't have time to take
22.01 or something beyond, is the easiest thing to do. And in most cases it works. It's not exact, but it's
probably close enough. Given uncertainties in
the amount of radiation that one could absorb or the
weight of a certain organ or the energy of
some x-ray tube, I think these empirical things
are pretty much good enough. And what this tells
you is that there is some effectiveness of
different types of radiation and different
energies of radiation at imparting energy to the
parts of cells and organs that cause damage. To say that in a little
smaller sentence, different energies of radiation
can have different effects biologically, and these tissue
factors account for that. It's also a table. I want you to keep in
mind because you're going to have to do a
calculation about it. The principle elements in
soft tissue in unit density, otherwise known
as number density, which you've seen before. If you want to calculate the
dose using a stopping power formula to a human, you have to
know what this human is made of and this is a pretty
good assumption. And this is something you'll
be doing on homework number 8, is finding the dose
that you're giving to each other in a
particular situation. If anyone's seen the
particular situation, check OCW for last
year's course and you'll see what that situation will be. Has everyone gotten your whole
body counts at the EHS Office? If anyone hasn't, do
them in the next week because you'll need that
data for the homework. You'll also need this table. Because if you think about
it, if you want to say, well, what's the damage by
electrons to soft tissue? And you want to calculate
this from scratch. You have these four
number densities, so we'll keep those in account. And let's put up the
formula for ionization stopping power again. Comes out with 4
pi, k not squared, little z squared, big
Z, number density, e to the fourth, over
MeV squared, log. Let's see, what
goes on top again? Oh, yeah, 2MeV squared, over
the mean ionization energy. The nonrelativistic form. Which of these
terms vary depending on the atom that an electron
or whatever would strike? Let's circle them,
there's a couple. AUDIENCE: Big Z. MICHAEL SHORT: Big Z, what
the electron's hitting. N, the number density. AUDIENCE: Ionization potential. MICHAEL SHORT: Yes, the
mean ionization potential. So these three terms
are the only things that change when
you're doing a stopping power to dose calculation. So if you want to get the
total dose in gray to a human, you have to sum over these
four different isotopes. Actually, wait. Let me put all the other junk
in front to make it quicker. So the 4 pi k0 squared
little z squared e to the fourth
over MeV squared is just a constant, times the
sum over all your isotopes of zi, the number
density of isotope i, which inside there has the
element fraction of that, times the log of 2 MeV squared
over the mean ionization potential of isotope i. And in this case, notice
there's no isotopes given. They're just given as elements. Why do we do that? Why don't we care for humans? How many isotopes of hydrogen
tend to exist in you? One, pretty much one, to
about five significant digits. You all have a little deuterium. Something like 1 in every 20,000
atoms of hydrogen is deuterium. But it's not a lot. I think it's even
less than that. Carbon is just carbon-12,
except for the tiny amount of carbon-14 you use
for radio carbon dating. Oxygen, it's oxygen--
think what, 16? Nitrogen is nitrogen-14. So as long as you know
what isotopes to use, you'll know what
z's, what i bars. And the number
density is given here. So this right here is
how you calculate dose in gray to a human
over some distance. Then all you'd have to do is
integrate that over your-- let's say thickness
of the human, whatever that happens to be. And you get the total amount
of dose imparted to them. So a lot of questions came
up in last year's class. How do we actually do
these calculations? Well, this is how right here. First, separate out
everything that's a constant. Because you only have
to calculate it once, as I hope problem set 7
and 6 taught you guys, is separate out
whatever you can first, and don't repeat yourself. Then sum over all
the things that are unique to each isotope. And inside this number
density is the fraction of that isotope in every human. So it's all built in. So these calculations
aren't that bad. Since you know how
to do stopping power, you can take out
2/3 of the terms. I don't know why
I still have this. It's not terrible. Can anyone not see
through how to do this? Or yeah, have a question? AUDIENCE: Where do we
find the ionization term? MICHAEL SHORT: The mean
ionization potential can be usually approximated
as about 10 electron volts times z. Except for the very light
isotopes like hydrogen, it's somewhere
between 10 and 19 eV. I would say one, you
can just look them up. Or two, you can use
this empirical relation to get a good approximation. And empiricism definitely enters
into the biological world, because uncertainties abound. And it's not always worth
being ultra crazy exact, although it can't hurt. Any other questions on
how to actually carry out a dose calculation
using stopping power? Hopefully it's pretty
straightforward. Guess we'll find
out on the homework. The other quality
factors to mention-- there are some different ideas
about these quality factors. Notice the scales
are fairly coarse. So again, there's a lot
of uncertainty or slop in these values. But notice that for, let's
say, X-rays, gammas, betas of all energies and charges,
the quality factor is 1. Why do you think that is? Let's go for the case
of X-rays or gammas. What tends to be the
attenuation coefficient of any photon in soft tissue
of considerable energy? Pretty low. And the amount of energy
that can be transferred by those photons is
variable, anywhere from pretty low to pretty high. And so the resulting
electron cascade isn't going to be that damaging. And it might not even
be that localized. So let's say if you really
want to know how much damage is it going to go do to
the DNA of a cell, where it could mutate and cause
cancer, not that much. Most of the gammas
pass through you, and the ones that
do get absorbed can have rather long
energy deposition tracks. Neutrons, however, interact
nuclear stopping power. Or let's say they just interact
with the nuclei of you, or whatever they're irradiating. And so when you
knock out a nucleus, it can then slam
into other atoms, causing a huge and dense
cascade of ionization. If that cascade happens to be
near the nucleus of a cell, you better believe it's going
to cause a lot of damage. And that's why these
more energetic neutrons have a much higher quality
factor, because they're much better at causing
damage until you reach some sort of threshold. Why do you think it goes
down at higher energies? Yeah. AUDIENCE: Well, because
there's less of a probability that it would actually
interact with [INAUDIBLE].. MICHAEL SHORT: Absolutely. Right around 2 MeV or 1
MeV, these cross sections tend to go down. If we look at the cross-section
for neutrons in anything-- let's look at
hydrogen. [INAUDIBLE] still got to do the screen
cloning thing again. Bear with me. Let's look at any old cross
section for, let's say, neutron scattering in,
I don't know, oxygen. We've looked at
hydrogen enough already. Cool. You can see that. Oxygen-16. We have incident neutrons,
elastic scattering, the bouncing off. Let's look at the form
of this cross-section. Right around 1 MeV,
things start to dip. And so yeah, the probability
of any sort of interaction is going to go down. In addition, the
atom that struck has a much higher energy,
and therefore a higher range. So chances are,
even if the neutron strikes an atom
near the nucleus, it will have a higher range
and can travel farther before that secondary cascade ends up
depositing most of its damage at a lower energy. So if you remember for the
stopping power for ionization or nuclear stopping power,
they both look the same. So I'm not going
to label which one. It's fairly low
at high energies, peaks at a rather low
energy, and then comes down. So it's at the end of the
range of whatever particle, whether it be the neutron
or the atoms that it struck, that it does the most damage. So you can kind of think
like all forms of radiation-- except for photons-- as armor-piercing bullets. They don't do damage
right where they enter. They do damage right where
they stop and explode. In the case of
armor-piercing bullets, it's a literal explosion. In the case of neutrons,
electrons, protons, heavy ions, it's at the end of their range
they have the most stopping power. And that's where the dense
cascade's going to be. So chances are,
again, if a neutron happens to interact with
the nucleus of a cell-- that's the simplest
cell I can draw-- if a neutron comes in
and strikes another atom, it may move far away before
its armor-piercing explosion. Let's see the other ones. For protons, depending
on which book you go to, you get a different
effectiveness. It's higher than that of gammas
and X-rays and electrons, because you get a big
cascade at the end. And alpha particles,
really, really, damaging. And energy is like
even a few MeV, they have a very short range. They tend to deposit
a ton of energy. And that's why they have
a huge effectiveness. This is also why
alphas are the-- that's the one
cookie you don't want to eat, if you guys remember
this, the four cookies problem. Never eat the alpha. It's not going to get
in through your skin, but if it gets
into your body and incorporated into the
material directly surrounding your nucleus, that's how
things go really bad. That's why smoking's so bad. Did anyone end up
getting into a smoke shop to do that measurement? I did check the one by
my house, and they just put up a sign that
said no one under 21. So you guys are
right about that. Weird law. Whatever. Now let's look at the
tissue weighting factors. We've talked about the
factors for different types of radiation. It also matters what
tissue it enters. So for things like
skin or bone surface, you think of these tissues
as not that critical. If you get a scratch
and you lose some skin, it's not that bad for the body. Same thing if a little
bone chip flakes off. You might get shin splints,
but it's not so bad. Not for the same reason. These tissues aren't
dividing very fast. The surface of your
bone, the hard part, is basically standing still. It's just calcified minerals
with some osteocytes trapped in there. Not much happens. What's happening in these
tissues constantly-- gonads, bone marrow,
colon, lung, stomach? AUDIENCE: [INAUDIBLE] cells. MICHAEL SHORT: Yep. This is where stem cells and
fat-- rapidly dividing cells-- tend to be found. So there's been some theories
and some papers saying whatever cancer you're going to
get, you probably already got it in the first
few years of life, when you're just a big
rolling sack of stem cells. This is part of why
occupational hazards for infants and pregnant women are much,
much lower, because they are giant sacks of
stationary stem cells. And you don't want to
irradiate something that's dividing really, really fast. And so the older
you are, the more OK it is to get more and
more radiation dose, because a lot of these effects
take a long time to manifest. Because they all start
with a single cell. And it takes a long time
for that single cell to exponentially grow and
divide over a longtime scale into a mass that be
considered a tumor. So it's a little
worrying to think, OK, probably most of the cancer
I'll get I got when I was five. But then again, it means,
don't worry about it. What can you do? There's what? AUDIENCE: It's free. MICHAEL SHORT: Yeah, it's free. It's done. Not all of it. You can still limit your
dose, because well, one, any acute radiation exposure
will have short term health effects. And two, your cells
are still dividing. As we had a seminar speaker
say, a biological organism at static equilibrium
is not very interesting. It's dead. It's not dividing anymore. So your cells are
still dividing. It still means you should
minimize radiation exposure. But a lot of what
happened already happened. And if you can see
here, the pattern is the more rapidly
the cells are dividing, the more it has this
tissue quality factor. Because the more
quickly these effects would manifest themselves,
and the more quickly the cells may divide with that mutation
before these cellular repair mechanisms fix that
mutation before a division. We'll get into a
lot of that when we talk about biological
effects, probably next week. So now, how do you do
a calculation of dose in Sieverts? First you can take the dose in
gray, like we have over here, multiply by these radiation
weighting factors, and you get a total
amount of dose to that tissue,
where that tissue may be a certain organ, a part
of your muscle, whole body if it was a broad blast of
radiation from a bomb far away or something like that. And you get these
single doses to tissue, where each of these
radiation equivalent factors can be equated to this
average quality factor, where you integrate the quality
factor as a function of length times the dose at that length. This is where the stopping
power formula comes in, is you'll have a stopping
power as the energy decreases, as the particle moves
through the material. And its stopping
power will change. So then you have to integrate
that stopping power times some constants and stuff over
that distance times the quality at that distance to get the
simple weighting factor. You then take these
weighting factors, plug them into your
dose to tissue, and sum up all the exposures
to different tissues. So each tissue will have
its own dose in Sieverts. It'll have its own
tissue weighting factor. You sum up all the
tissues exposed, and you get the total effective
dose to the whole body. This means that some
organs incur dose faster than others for the
same radiation exposure. So the whole body dose
should sum up to 1. And I believe-- I remember
doing this calculation, but it may be worth your
while to just try it. These numbers
should sum up to 1, because all of these
individual organs plus the remainder
of your body should constitute the whole body. And so this is,
in a nutshell, how you do these dose calculations. So let's take an example
from the actual reading. Let's say a worker gets 14
milligray of uniform whole body dose-- this would just
be from background radiation, cosmic
rays, food, whatever-- plus a targeted
dose of 8 milligray to the lung from alphas,
plus 180 millgray from betas in the thyroid. Anyone know why they chose
alphas in the lung and betas in the thyroid? What's the most likely
sources of those? AUDIENCE: Iodine and nicotine. MICHAEL SHORT: Iodine
and let's say smoking. Yeah. Yeah, exactly. As we saw, a lot of the
radon daughter products tend to be alpha
emitters, and you tend to inhale those
through the lungs. And iodine is a beta decay with
about an eight day half life, and that gets preferentially
absorbed in the thyroid. So this is a pretty
realistic scenario. And we'd say, how much effective
dose did this person get? To do this calculation,
you first-- well, let's go through those steps. You look at the dose to each
tissue times the effectiveness for that type of radiation. So the dose got 8
milligray of alphas. So you multiply by 20
for that quality factor. And the lung gets
160 millisieverts. You do the multiplication for
the thyroid, the multiplication for the whole body. And then you do a summation
of these single tissue doses here, here, and here, times
the tissue weighting factors, which you then look
up from that table or calculate from the cell
division rate here, here, and here. And you get a total dose
of 42 millisieverts. So I want to point
something out to you guys. His thyroid got
180 millisieverts. He got 42 millisieverts. Interesting. Sounds a little
counterintuitive. But in this case, these
doses for each tissue are calculated for that tissue. So if you have a probability
of organ failure or mutation, you can now then know
how much increased risk he may have
of thyroid cancer specifically, or sum it up to
an equivalent whole body risk. And it just so
happens you're only allowed about 50
millisieverts of exposure. So he'd only be
allowed another eight. But we'll get into
limits and what this ICRU whatever
whatever means. I'll actually show
you the document. It's also posted online. This International
Committee on Radiation U. I forget what the U is. But this is the
whole document that has the basis for these
recommendations, the numbers, and the reasons. And that's all online for
you guys to check through. So anyone have any questions
on a example dose calculation? Cool. Let's look at some of the
ways that you'd actually measure dose. One of them we looked at
the first day of class, the old Chadwick experiment. Send radiation through
a fixed amount of area so you know the flux. If you know the total
amount of gammas is produced by this X-ray
tube, you know the area, then you know the solid angle. You know the flux. Then you have just a free air
chamber with a high voltage to suck up those ions
before they recombine. And that's how you can calculate
things like dose in roentgens. You also have pocket
versions of these things, sealed tubes with
two electrically insulated electrodes. And that cannot discharge unless
ions in the gas allow them to. And this is the basis behind
these civil defense air wall chambers, one of which I
happen to have right here. So I want to pass
this around and let you guys take a look at it. Like the ones you
saw in the reactor, you can look through
and see a little needle that will tell you the-- not dose, but the
amount of ionization this thing has
received in roentgens, which can then be equated
with some calculations to dose in soft tissue. And also there's the base for
the civil defense dosimeter. I want to tell you
guys how it works. It's a battery. That's it. If you open that
thing up, there's a couple of big T cells in
there and a voltage divider. And you just turn the knob-- which sounds complicated,
all you're doing is turning a potentiometer-- to decide at what voltage this
wire end inside of the tube will be. And that voltage deflects
the wire a little bit by just coulombic
attraction or repulsion, bends it, and
gives you the dose. And it stays there. Nothing can get in or out
of that sealed chamber, except penetrating radiation. When gammas or
neutrons get through, they can cause
ionizations in the gas. Those gas ions can move
to these electrodes, partially neutralizing them,
making the needle tilt a little farther and a little farther. So the interesting thing is when
that needle read 0 roentgens, it's fully charged. And as the detector discharges
from radiation interactions, the needle moves higher
on the roentgens scale. Because you can
paint whatever scale you want on it, as long
as the physics works out. So it's kind of neat to think
that the charge is highest when the dose is lowest. Because all the
charge is doing is deflecting that little needle. Quite simple design,
and quite robust. These things were
designed by civil defense in case the Cold War
became a hot war, and they'd have to last
for a very long time. So having had this one
for about 10 years, I can tell you I've not
yet filled the meter, which is probably a good thing. And I took that on flights
across the world and hiking in Nepal, where the background
dose was considerably higher. And the needle moved like half
a roentgen for the entire three weeks up at elevation
and plane rides. And I got more dose on the plane
ride than I did on the hike. Interesting fun fact. That's pretty much exactly
what you should see. And so Alex tells
me these things are starting to get kind of rare
on eBay, and that's a shame. Because this is the best
possible teaching dosimeter there is. If you understand 8.02 and
a little bit of radiation, you know how these work and
can predict what the dose is actually going to be. Now, how do you do things
like detect neutrons? If you want to detect neutrons,
you want a good moderator. Because well moderated neutrons
deposit all their energy in the detector instead of
bouncing off, transferring a little energy, and leaving. If you want to know how many and
what energy neutrons you have, you can fill a similar chamber
that's got a high voltage. It's got a wire on the inside. But instead of air or
some other gas in there, you can fill it with things
like ethylene or propylene or some sort of very hydrogenous
hydrocarbon gas, something full of hydrogen to act
as a good moderator. That hydrogen ion
then becomes a proton, which then damages
a lot of other ions, causing an ionization
cascade, and leading to some current pulse, just
like any other detector. There's going to be some
movement of current, which is picked up as damage. You might then ask also, why
is that alpha source there? Anyone have any idea? That's your calibration source. So as the gas in this chamber
is attacked by neutrons, you will be blasting
some of the hydrogen atoms out of the
ethylene or propylene or whatever gas you have. The gas will change in
effectiveness over time. The energy of the alpha
particles will not. So that is your absolutely
fixed energy calibration source. So you know exactly-- if, let's say, 3.72 MeV
alphas make a current pulse of a certain height,
then you can equate that to a 3.72 MeV neutron that
would strike a hydrogen atom. And because the gas
degrades over time, you have to recalibrate
with that built-in alpha. And any sort of shutter-- like a little piece of
foil-- will block the alpha so that you can use it
as a neutron detector. Quite clever design,
in my opinion. It accounts for the degradation
of the gas and the detector. Anyone seen one of
these things before? Then there's the Geiger counter,
which is an ionization counter run in avalanche mode. You can use these free
air ionization chambers or other counters as an energy
proportional counter, where a higher energy particle will
impart a bigger ionization cascade. And you can then use that
to get energy resolution. Or you crank up the
voltage like crazy, so that any count of any energy
causes an intense ionization cascade and a huge
current pulse. And this is the basis behind
cheap Geiger counters, like the ones we build
in our department. Those old Soviet
SPM 20 tubes that have actually been survived
being stepped on and crushed. As long as the electrodes don't
short out, they still work. So there's a few folks
in the department that have clearly bent Geiger tubes. One of them looks like
they've been chewed, but they still work. Because any sort of anything
interacting with a gas will cause an ionization
cascade, which is immediately sucked into the electrodes
by a high voltage and collected as a current pulse And that's just
a cutaway of what it looks like on the inside. And this is the circuit
for a Geiger counter. There is a voltage, a resistor,
a capacitor, and a tube. That's all you need. Everything else in
the MIT Geiger counter is there to make
lights and sound. It's just for fun. But the actual Geiger counter
itself can be made incredibly . Compact the bigger the
tube, the more radiation it will catch, just
because of its size but otherwise, as long as you
get the voltage high enough to cause this ionization
cascade, it works. It's really, really robust. So I want to skip ahead
for some of that stuff, and then talk about how do
you measure dose in humans. Who here has seen one of these
TLDs, or Thermoluminescent Dosimeters before? Our reactor trainees have. And you've worked on
nuclear stuff too, right? AUDIENCE: [INAUDIBLE] MICHAEL SHORT: You've got one
in the vault. Wait, Kristen, what about you,
since you've worked-- AUDIENCE: We had one with
a little crystal in it. MICHAEL SHORT: That's
exactly what this is. AUDIENCE: [INAUDIBLE] MICHAEL SHORT: Yeah. And you can shake it. It rattles, right? AUDIENCE: Yep. [INAUDIBLE] MICHAEL SHORT: Exactly. So this is how these work. This stands for a
Thermoluminescent Dosimeter. And converting from
Latin to English, that means if you heat it, it
produces light proportional to the dose that it gets. So these little
crystals, aluminum oxide or some sort of assault
or whatever have you, something that creates
permanent ionized defects, they will relax when you
heat them, giving off light. And then you use a light
counter to tell how much dose this crystal has received. And by putting different filters
in the way for different pieces and looking at the light
coming from different parts, you can tell different
types of dose. So one works for
betas, because betas can get through the little
hole in the detector, whereas this may help you figure
out gamma rays or neutrons of different energies. And who here has seen
one of these ring badges? Yeah, so again, the
reactor trainees. Great. If you shake them, you
hear a little rattling. Try it next time. They're usually pretty loose. It's a cheap plastic
crappy casting with a thermal luminescent
crystal in the inside. So again, shake it. It should rattle. If it doesn't, the
crystal might be missing, and you should
probably get a new one. So when you read a TLD,
you use this fancy machine, all it does is heat
it very carefully, and allow the electrons that
are trapped at a higher energy to jump back down,
emitting visible light. That's it. We've seen this process before. What happens after the
photoelectric effect? Electrons fall down
an energy and release light in the form of
X-rays or visible light. Same thing. And then let's talk
about, how can you use dosimeters for dosimetry
in medical applications? And let's take the
example of proton beam therapy, the new and
more upcoming replacement to X-ray therapy. It relies on the fact that
the stopping power of protons is extremely low, until they
reach the end of their range, like we talked about here. So again, we use protons
as armor-piercing bullets to get through the
person, drilling a little hole in the
process, and exploding once they reach the tumor. It's a nice quirk of physics. It's really an elegant
use of the stuff in 22.01 to do damage where
you want it to. There's all sorts of other
methods of cancer treatment. Let's say you're
simple, and you can just go with excision, which
means cutting it out. Chemotherapy, which is
a pretty nasty process, and is usually used as a
backup for radiation therapy to catch whatever else
might be floating. X-ray therapy, which is still
used a lot, but hopefully will get phased out a bit. Brachytherapy, where you
implant a little seed of a beta emitter into an area
near where the tumor is. Let's say you have an easy
route of entry, like that way. Then you can implant
one of these seeds right where the tumor is. But if you can't have
an easy route of entry, that's where proton
therapy comes in. And as we've talked
about X-ray therapy, if you want to damage the tumor
more than the rest of the brain or the rest of the
person, you have to come in from
different angles so that the sum of all
the dose to this target is less than anywhere else. And X-ray therapy tends
to do a lot of damage. Well, you hear of X-ray therapy
causing hair loss in people. Well, yeah. You're going through the head. It's not good for your
hair follicles, either. And then there's proton therapy. We've talked about
it before, but not in the strict medical sense. And actually I'm going
to reveal to you guys an invention that we've got out
of our department that might help this go a little better. The way it works is you start
with a cyclotron, which I've already explained,
something that accelerates charged particles
to about 250 MeV. And we have one of these across
the river at Mass General Hospital. Send them through
bending magnets, and bend them up so that
they hit the patient. Then you move the patient
on this gurney or table so that-- and the entryf
can rotate anywhere, to come in from any
entry point, minimize the dose to the
rest of the patient, while frying the tumor. AUDIENCE: Is that
a scale diagram? MICHAEL SHORT: Yep, quite big. Yeah. It's pretty to scale, yeah. So time on this instrument
runs in the thousands per hour. If you go through the back
door and know the folks that run the thing and
no one is not being used for cancer treatments. Proton therapy can
run in the hundreds of thousands of dollars. This is one of the
millions of reasons we have medical insurance. It's because when you
need it, you want it. And these cyclotrons
aren't cheap. The way they work
is pretty simple. You inject ionized particles
through these D magnets, and they go faster and faster
and faster every time they cross this electric field. They bend at larger
and larger tracks through these magnets as
their energy increases. Than they exit out the
other side, getting delivered to the patient. And why protons versus X-rays? Well, I made it
quick Desmos graph. To say, let's say
you started off with a equivalent dose
of protons and X-rays, and you're trying to get to
a 40 millimeter deep tumor. This is why. This is the amount of
dose that the X-rays would give compared
to the protons in this highlighted
tumor region. Then you look at the dose
where X-rays and protons give to the rest of the person. It should be
graphically obvious. And you can do some tricks
with proton therapy. If you have a lung tumor, you
can vary the energy and time at each point, such that
you give a uniform dose to the tumor. So you can move that
[? Bragg ?] peak by degrading the proton energy,
by putting filters in line with the beam. You don't tend to like to
change the energy of the beam. So you can put things in
the way of the protons to slow them down. And because when the
stopping power is very low, the proton speed's very
high, and forward scattering is preferable, putting
things in the way of the beam pretty much just
slows them down, but doesn't change
their direction. That assumption breaks down
when the energy gets low. But when the energy gets low,
you better be in the tumor anyway. And you want them to change
direction and explode out and blast everything in sight. The problem with
proton therapy is that humans are not
biological organisms at static equilibrium. In other words, they're alive. They tend to move. Breathing is something I
like to do every few seconds. Swallowing, maybe once
every couple of minutes. And most of your
organs move around and dance without
you controlling them. It's really hard if
you're trying to hit a tumor on a moving target. That is the main problem
with proton therapy. The solution right now, I like
to call it spray and pray. You fire into the person,
hope that things don't move. We know that they do though. Those proton beams
are very narrow. And let's say, for
abdominal patients, let's say you happen to
be digesting something. Your intestines will just
go krschlock like that and get lunch where it's going. This is one of those
reasons that they say don't eat anything
before these procedures. If you're not actively
digesting things, then your abdomen won't
be moving as much. Thoracic patients, better known
as the lungs or your thorax, if you're from France. My wife likes to make-- I like to make fun of my wife. That's right. Because she still refers to
this region as the thorax. And I was like, I am
not a giant beetle. But it is the medical
term for this. You tend to breathe. And if you actually
measure how much you move when you
breathe in and out-- let's say you're trying
to fry a lung tumor. That's a tricky proposition. So how do you keep
the protons on track? Ideally, some dosimeter
would be able to determine absolute dose, and
it would be able to-- where on this list
of things is-- oh, there's even more
things I'd want here. So ideally, if you'd
want a proton dosimeter, you want it to be able to
measure things, provide some data, not be orientation
dependent, and things like not be toxic, be cheap
to build, but also be able to turn on and off if
the tumor moves out of range. And this problem
hasn't been solved yet. Existing dosimetry
methods include making calculations and hoping,
which is what we do now. We do complex Monte
Carlo calculations based on scans of
the patient, try and map out how much
energy is going to be lost and where to go in. There's conventional
port films, which means you put a film on the
entry of the patient, which gives you an idea of
where the beam is, but not necessarily where the organ is. Let me get into some
pictures of these things so you know what they look like. Anyone ever put one of
these in your mouth before? The sort of electronic
dosimeters, the X-ray imagers that you
get at the dentist, you bite down on one
of these, and you get X-rays of your teeth. That's great if you have
a place to put them, but doesn't quite work
for proton therapy. There are tissue
equivalent gels, where you can cast a person in
gel, fire in the proton beam, and see how deep
it goes, and then hope that your tissue
equivalent gel is equivalent to the tissue. Usually it's pretty good. There's silicon diodes. You can implant a tiny little
diode or other semiconductor device near or in the
patient, and measure the change in band
gap, or the voltage required to turn on conduction
in the semiconductor. The problem is you can
only use them once. Once you irradiate a piece
of silicon, it's irradiated. And then you would
have to take it out and stick another
one in with a big needle to keep going. There's optically
stimulated luminescence, which means protons hit stuff. Stuff creates light. Light can be measured
in real time. So you could implant a
little crystal like this TLD or Thermoluminescent
Dosimeter, attached to a fiber optic cable. And in that way, you can
measure the amount of light and preamplify it. PM means-- what is it-- Photo Multiplier tube. And use electronics and
software to calculate that light and turn it into dose. Problem is, this scintillation,
it's not very strong. There's a lot that can
go wrong between where the radiation is done and
where it can be collected. Also implanted MOSFETs are
these metal oxide semiconductor field effect transistors. Same problem. You can look at the
change in difference-- I'm sorry, the
change in band gap as you irradiate these things,
or in the MOSFET voltage. But again, you can
only use them once. It's not like you
can reset them. So the problems
with all of these is we don't know what
dose the tumor gets. If we know how much
we need to fry it, but we don't know if we fried
it, the cancer could recur. Or it may not respond
to the radiation. These are liability terms
to say it didn't work, but you can't sue us,
because we don't know why. And you don't know why. Or you may apply too much
dose to the surrounding tissue and induce secondary tumors. This is one of those things
that's not talked about very much, except
in medical circles, and my entire family happens
to be in medical circles. So they confirmed,
yeah, this is true. We don't know how many
times, if you treat a tumor, or do you induce
another one that will pop up five years
later in the same site. All you may think is, OK,
it recurred, despite being dead for five years. It might not have recurred. It might have made a new one. You don't know the
dose rate versus time. The existing in situ methods
haven't worked very well. So we had another
idea, that I'll go in the last
negative 1 minutes, what we call the integrating
F-Center Feedback Dosimeter. We just got the
patent filed on this. Not accepted, but filed
with the US Patent Office. It's pretty simple. You send in calibrated
light into a little crystal of something that creates these
color centers, or F-centers. When it's irradiated, look
at the light coming out. See what's absorbed. You know how much
dose you've received. And so look at
these three parts. A is just an alkali
halide salt, better known as table salt, sodium chloride. B is some biocompatible casing
so your body doesn't reject it. Calibrated white light source,
fiber optic connection cables, and a spectrometer to
read the absorption. These can be little
compact USB spectrometers. And it relies on what's
called F-centers. When you irradiate ionic
materials, they change color. These defects produced by,
let's say, blasting out a chlorine ion or a
sodium or potassium ion are optically active. Because you get differing
regions of electron density. And you absorb certain
wavelengths of light changing their color. If you then send calibrated
light through it, you can tell what happened,
how much dose there was. And F-center creation
versus radiation is extremely well known. We've actually done some
preliminary tests on the Dante accelerator here to show that
the amount of dose that you give in a fractional cancer
treatment on the order of a kilogray or so does
cause the salt to respond very strongly and produce a color. And the best part is,
they relax on their own. After anywhere from 5
seconds to a few hours, the color just disappears. Because the atomic
defects relax. So you don't have to implant,
remove, reimplant, remove. And you can enable
dose rate information by putting multiple
salts in a row that have different response
levels to these protons. So by looking at the amount of
absorption in each wavelength, you can tell not just the
dose, but the dose rate. Calibrate your beam current. And I'm going to skip
all the way to the end to say how this would work. So let's say you had one of
these IF2D dosimeters implanted in your tumor, and
your heart was beating, or your lungs were breathing,
or you were digesting something. You could then feedback
this information to the proton accelerator
to shut the beam off when the tumor
moves out of range and send in tiny little pulses
to say, hey, are you there? Little micro second pulses to
say, microsecond of protons isn't going to do much. But if it senses the
IF2D back in range, it then blasts continuously. And as soon as the
IF2D says no more dose, it starts just putting those
wake up pulses back on. So this would be the
first feedback way to apply proton therapy
without screwing it up. You could also install
IF2D dosimeters near the tumor,
outside the tumor, and play the world's first
game of radioactive proton Operation. Don't hit the sides. If you wonder if your
beam's on target, you then steer it until it
doesn't hit any of the IF2Ds, and you know you're right
through the gates on the tumor. Very important for
certain sensitive tumors, like chordomas,
spinal cord tumors, which tend to happen in
infants and young children. There's various ways
of treating those, other than removal of the neck,
what you don't want to do. They're extremely
difficult to operate on. You don't want to give
radiation therapy. So highly targeted
proton therapy like this, making
sure that you fry the tumor without
the surrounding spinal cord and medulla,
would be probably the way to go for this. So I'm going to stop there,
because it's exactly 10. It's also the perfect
stopping point. And we'll pick up with
background radiation tomorrow. I also want to let you
guys know that we'll be doing our nuclear activation
analysis irradiations Friday at the beginning of recitation,
and we'll finish up recitation by doing the exam review.
