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PROFESSOR: Okay. We can get started. Is that volume okay. Maybe. Maybe not.
Push it up a little bit and try not to get too much feedback.
So this is always a kind of fun time, because it's Nobel Prize season. And Berkeley
actually did very well this week. You probably heard that the Nobel Prize in medicine was
shared among three people. One of whom is a U.C. Berkeley faculty member. More often
than not it's been in chemistry or physics. Although the Nobel Prize in physics went this
year to people who weren't part of Berkeley faculty, the reason they got it is because
of
experiments done over at the large hadron collider over in Switzerland. And people from
from the Lawrence Berkeley National Laboratories that had to do with discovering something
called the Higgs-Boson. This is Mr. Higgs. Together with this gentleman, Francois
Englert who I never heard of before. We try to understand why particles have the masses
they do. In order to explain the particle masses, they predict there had to be another
particle which Mr. Higgs was lucky enough to have named after him. The problems they
were
trying to address is shown here. We've talked about a number of different particles in
this class. Primarily electrons, protons, neutrons, I alluded to the fact that the
neutrons and protons isn't simple. They have quarks inside them. We talked a little bit
about muons., a tau. And there are things called neutrinos. They all have distinct
properties. One of properties we like to determine for all these particles are what the
masses are. This cartoon is meant to illustrate the range of masses that people who have
observed these various particles to have. So, for example, photons, the particles which
we use to see one another, which is actually the particle responsible for the electric
magnetic interaction is exactly massless. Really zero mass. Although we don't know this
for sure there's a theory that says the particle that really carries the strong nuclear
force which is a gluon is also massless. There's another particle which isn't shown on
here which you've talked about here, the graviton, the particle we believe responsible for
gravity is also massless. On the other hand these other particles all have masses but
the
masses seem to be pretty arbitrary. The neutrinos we used to think are massless, turns
out they're not. At least one has little tiny mass and the other are going to be heavier
than that. The electron, that you know has a rest mass energy of something like half
an
MeV or in these units GeV. Point zero zero five GeV. The quarks aren't massless. They
have strange masses. Turns out there are six different quarks and they all have different
masses for reasons we don't totally understand. Here's the electron, here's its cousin
the muon. A hundred times heavier. The particles that are responsible for the weak
interaction the w and z have masses comparable to the mass of a Germanium.
STUDENT: Contributed to the pion.
PROFESSOR: That's how I talked about it. That's the historical way people described
it. These days don't really talk about it that way. So what I showed you in drawings
you
can imagine you had a neutron and a proton. The thing holding them together was, this
is
shooting pions back and and forth, maybe pi zero or pluses or minuses. And, that was
before people really understood about quarks. And furthermore, understood about what held
the quarks together which were muons. Now the way it's being pictured, I think you're
familiar with atomic physics, with the van der Waal's interaction. If you have a neutral
atom of some substance of some neutral atom of some other substance, when I say neutral
that means there's the same number of positive and negative charge in here. To first
order, these are chargeless objects. There should be no electromagnetic interaction
between them. In fact, the charges are not necessarily uniform deliberately in here.
You
could have excess of charge here and excess of the opposite sign over here. Then you
might have excess here like that. These distributions of charges would interact. And
that gives rise to this van der Waal's interaction. And that can lead to net attraction
between the electrically neutral objects. The basic idea is the same here. We're not
talking about electric charge any more. We're talking about the charges being carried by
the quarks. There are quarks inside each one of these guys. Up, up, down and in this
case it would be down, down, and up. The charge I'm talking is this thing called color.
They come in red, green and blue. And the way the mathematics works out, the neutron
is
colorless, it has equal amounts of all three colors. The proton is colorless, the quarks
are exchanging gluons between each other in here. These are meant to be gluons. And so
this is a colorless object just like the atom is an electrically neutral object. The fact
you have colors inside means you can imagine being polarized in such a way that there's
a
residual coupling between the quark in one nucleon and the quarks in another. This is
what we call nuclear force. The real strong interaction the particle theorists talk about
is quarks within a given nucleon. This is the strong interaction. This leakage out is
the van der Waal interaction that forces neutrons and protons to be attractive to one
another. Does that help.
STUDENT: So then theoretically since the gluon is massless would that give us an
infinite range.
PROFESSOR: No it's actually very bizarre. The further apart the quarks get the
strong the force becomes. The closer, the weaker. It's very, asymptotic, for example,
take a particle physics class and you'll learn about this. As the quark get close
together the force get weak and as you pull them apart it gets stronger. The result is
as
you start pulling the quarks apart inside a given nucleon, I have an up and down held
together by some gluon. And I try stretching them apart. What happens is this thing ends
up breaking. They end up getting an object like that. And ends up getting an object like
this. I get two pions coming out. As I put energy in to separate them I break the gluon
apart and quarks and anti-quarks here at the end and I end up making pions.
STUDENT: Just, I just clarification, you say colors, I always learned it as white
and black.
PROFESSOR: But there are three colors.
STUDENT: The three combined together are supposed to give white and the anti
particle is supposed to give black.
PROFESSOR: No I think they're all supposed to be white essentially. The like the
way you add all the colors of rainbow together you get white. There are anti-colors.
Whether in fact they add to black or not, that's not something I had thought B maybe.
The point is you have different particles with different masses. Why do have the masses
they do? Why aren't they all in one number or a simple pattern to understand. This is
where Mr. Higgs and Englert came in, they said everywhere in the universe there's a
field. And particles are traveling through this field. Depending upon how they interact
with that field they apply a different mass. And you might think this is very strange,
if
you've ever taken a sold state physics class, if you talk about how a electron travels
through material, in matter an electron has a different mass than it does in free space.
It has an effective mass of that has to do with the interaction of electron with the
material it's traveling through. That's the same idea with the Higgs field. This Higgs
field was created in the Big ***. It's ever where. As particles travel through it they
interact. Each has different strange during the interaction, that's the basic idea of
Higgs model. I basically told you everything I know about the Higgs model right now. One
of consequences of it is it there has textbook a particle associated with that field.
That's what's known as the Higgs-Boson. This idea has out there for 50 years. People
STUDENT: That seems cellular to try to explain givens differences in mass saying
it's because of differences in strengths of interaction, why now the strength of
interaction is different.
PROFESSOR: You should talk to a real particle theorist. I read what I can and
understand and that's what I get out of T a particle theorist would give more detail
and
maybe more convincing argument. I'm not sure. Why is it that neutrinos only interact
weakly? Why don't they participate in strong interaction or other interactions? I don't
know. There's some intrinsic property that forces them to only have weak interactions.
What is it about the quark that make them have strong interactions? I don't know. There
are a lot of thing that seem simple S the more you think about it the more mysterious
they
were one of thing I come back to all the time is electric charge. What is electric charge
really? I have in idea. I know put a charged object in a electric field it will
accelerate. But what is charge in I don't know. I think I have a better feeling for
what
mass is because I can push on something and I know it was resistance and I know that's
mass. But at a deep level what is mass. That's interesting which you talk about quarks.
Because these objects say they have masses, but turns out in the theory that predict their
existence it's also a theorem that you can never isolate a free quark. You can't is a
freak quark and put on a came and figure out what the mass S so what does that number
mean? I don't know. It's very strange.
So getting back to the Higgs business just it finish up. It was predicted there
should be a particle called the Higgs DOE son. Nobody now thousand produces T what it's
mass would be. They start at low energy and does experiments with they collided thing
together and watched for a particle that had certain properties come out. One of the
properties was it had to have spin zero and even parity. It could decay only into
particles that were lighter in particles than it. There were predictions about which
particles would be preferentially produced as a result of those decays and so on. In
the
last year there was \experiments\experimented done at the large hadron collider in suiter
lands where the world's high energy protons can be accelerated. And it's actually a
colliding beam machine. You have protons coming in from one direction and protons coming
in from the other. And they collide. When they do hell breaks lose basically. Every
particle that has an energy or rest mass energy less than (ã) the combines energy of two
beams can be created. This is a cartoon I am straighting a real event which was seen
a
little over a year ago at CERN in one of their large detectors where the two protons
collide. Each one of these tracks represented a particle that emerged. The ones shown in
red are muons, the heavy can you say inns of electron. Turns out in this particular
event, there was four of them that came out. With very, very high energies. And if you
assume they came from the decay of one object, the mass of that object, the mass of the
four left ton is what ethenes works out to be 124 GeV they think that event was the decay
of Higgs-Boson that was created in it experiment. Since them a number of either event
like this has been scene. It's now believed the mass of a Higgs is about 125 GeV. It has
the decay properties that was predicted for this to really be the hissing Boson. This
is
why Higgs got the prize this yeah. Because it has by validated. Because there was a
particle associated with all the properties of Higgs-Boson.
All right, we're talking about alpha decay. Which at some very deep level it's
connected to the Higgs-Boson S.
Just to remind you, the alpha decay was the first radioactive decay that
Mr. Rutherford found. I showed you it's least penetrating of all of them. Meaning it
takes the left material to stop an after pardon me. There are a lot of heavy nuclei for
which alpha decay happens. If you go back to the chart of nuclides, the upper right
hand
corner of that chart has boxes colored with the signature alpha decay. I mentioned that
Rutherford didn't know what the particles were any, will I. Again he was a very clever
guy. One of thing I did was check them in a jar essentially. Letting them symptom of
recombine with electrons to form knew really atom and it did a electronicing spectroscopy
and showed it was a helium atom. That's what showed these are really particle of --
what's going on in alpha decay, you have some parent nucleus. Let's say atom. Parent
atom of z protons in the nucleus, enough neutrons so the sum of neutrons and protons is
a. And enough electrons bonds to make this electrically neutral. Decays into something
that has too fewer proton and two fewer neutrons and the helium four atom. Again using
atoms here to indicate that the electrons take care of themselves. There was a question
the other day I think about what really happens in alpha decay. And if you think about
it, I have a neutral atom to start with. Or let's assume I have a new traditional atom
to
start with. It spits out an object which is two protons and two neutrons bounds together.
That's an alpha particle. It doesn't spit out a heel four atom. I come out with two
positive charges. This atom initially had z electrons orbiting it. And right after the
alpha is emitted, the electrons don't necessarily know anything's happened. So this
object is going to have two more electrons that it needs to be electrically neutral.
This
thing is going to be negatively charged two units initially. Eventually these electrons
are rearrange and it will lose a neutralize itself. But initially this has two units of
negative charge. This carries away two unit the positive charge the but it sent
districted initially. Eventually thing stop the. Strong recombine and everything
neutralizes.
Okay. Again just to motivate alpha decay, the neutrons and protons are held together
by strong nuclear force which grows with A. But the coulomb repulsion which is due to
all the protons electric magnetically interacting with one another grows like z≤.
Eventually that becomes a limiting factor. That limit how many protons you can cram into
a nucleus. And you can reduce the total energy in the system by reducing the charge. One
way to do that is emit positively charged particles. You can mother emitting protons
or
deuterons, tritons, alphas, carbon nuclei. The question is why alphas. To look in detail
and try to answer a little more quantitatively if you want to reduce the energy of a
system by removing charged particles, here's a list of particles you might think of
emitting.
Turns out if you go back to the binding energy formula and look at the masses of
nuclei we're discussing, it isn't energetically possible to emit a single proton. Turns
out there's still a proton binding energy, so the next heaviest thing emitting is a
deuteron. What I'm doing is looking at the binding energy per nucleon of light ions I
could emit in the course of radioactive decay. What I'm going to argue is one that has
the highest binding energy per nucleon gives me the most decay possible and therefore is
most likely to occur. For the deuteron you know the binding energy is two MeV. So the
binding energy per nucleon is 1.1 MeV. For tritium and helium three it's a little hire.
Lithium six it's higher distill. But for the light nuclei is helium four.
STUDENT: Do we ever see the other ones come up. I feel it should be the highest
probability. The least some probability.
PROFESSOR: Very good. In fact in certain nuclei some of these other particles have
been observed. In Krane's discussion in chapter eight on alpha decay, he also talks about
still heavier nuclei being emitted. For example, carbon 14 has been observed to be
emitted by certain very heavy nuclei. Very rarely compared it alpha decay it's 10 to
the
minus 12 or 10 to the minus 14 in intensity. But energy levels it has been seen much some
of these have also been seen but rarely. Part of reason for that we'll get to in a little
is the potential as a function of distance. And this is where the potential is. And this
is the nucleus. And this is the coulomb potential. So if radioactive decay is
energetically possible, what that means is the particle has some energy above zero. This
is the energy of the decay let's say. And so I can draw this in like this. And that
energy is calculated by calculating the q value between the initial and finite states.
You pick a particular particle here or any other particle you want. Tell me what parent
atom was. We look up the masses and calculate a q value. That determined where this
dashed line should be drawn in energy. It will almost always turn out to be or probable
the always below the height of this column barrier. This distance I call the coulomb
barrier. You can calculate that I by assuming you have two spheres that are touching.
This is the daughter after decay has happened and this is whatever particle you choose to
emit. So what I'm getting at is the quantum mechanical tunnel process which has to be
going on for these charged particle tour emitted from a heavy nucleus. And the higher,
sorry, the higher this energy, the more likely it is for the tunneling to actually occur.
Depending on what q values are which is directly related to binding energy you'll be
further up or down this curve in terms of become close or far away from the coulomb
barrier. So the, it's easier fore this particle it get out than for that particle to get
out of because these values are all smaller than this turns out probability for these
decays are going to be a lot lower for the probability of alpha decay of but you're right
in Rin they can happen.
First we'll talk about kinematics. In general, we think of having a nucleus or an
atom that's initially at rest. And then undergoing alpha decay. So I've got some heavy
nucleus, with all its electrons there and I assume it's sitting there. the q value is
calculated as we've always said. Take the NAS mass of a neutral atom, this substance
and
subtract the mass of the neutral atom of daughter. And multiply by c≤, that's the q
value these masses you can look up in the back of Krane's book. The question value is
the
total energy which gets released by having this rearrangement of neutrons and protons
happen. And the essentially it's e equals mc≤. The mass of the parent is a little
bit bigger than the sum of the masses of two products. That energy manifest itself as the
kinetic energy of alpha particle and also the daughter atom. So what I'm trying to
indicate in it drawing is if the daughter, if the parent atom is initially at rest, and
the alpha goes off in this direction the daughter has to go off in the exact opposite
direction in order to conserve moment because the momentum initially is zero. Moment is
absolutely conserved therefore the momentum vectors have to point in the opposite
directions. And the markets of these two momenta have to be equal to one another. That
means the momentums are the same. The kinetic energy are not the same. As we'll see in a
minute. We'll calculate what the relative kinetic energies are for both the alpha
particle and the heavy nucleus left hand. Energy is also conserved. And so the sum of
these two kinetic energies adds up to the q value for the decay.
So to conserve momentum, the momentum of the alpha has it equal the momentum.
Daughter. If I look at typical alpha decay energy for the book or from looking at masses,
for most nuclei that we're going to discuss they're on the order of five MeV. Five
million electrons. That's relatively a lot of energy in nuclear terms. If I think about
the energy of the kinetic energy of alpha compared to its rest mass energy, what is
roughly the rest mass energy of an alpha particle?
STUDENT: Almost 4000.
PROFESSOR: Four GeV. Roughly speaking, neutrons and protons are receipt mass energy
of about a thousand MeV or one GeV. So the alpha is going to have about 4000 MeV. And
five MeV is very small compared to that. The reason I bring it up is to make my live in
terms of doing calculations simpler if I can get away with doing nonrelativistic
kinematics. If your kinetic energies are small compared to rest mass energies. In case
I've justified in doing. That. I can say the kinetic energy of Beth the alpha part and
daughter are the -- now I can rewrite at that equation for the q evaluate. The q
evaluate is the kinetic energy the after carries away plus the kinetic energy of daughter.
Rewrite that, that's the kinetic energy, alpha again. Now write the kinetic energy of
daughter is p≤ twice the mass of daughter of the atom. I know it's the same for both
the alpha and heavy daughter. Then rewrite this and saying that this Q value is the
kinetic energy of alpha plus that same kinetic energy times this quantity. Mass of alpha
time mass of daughter. And then I can rewrite it and say the kinetic energy of the alpha
is the q value which I calculate from the masses, divided by one plus the mass of the
alpha divided by the mass of the daughter. I haven't made any approximations here yet.
Now I'm going to say for almost all the alpha decays we're going to carry about,
talking about heavy nuke, maximizes 200 on up. Mass alpha is -- to a good approximation
if you like I can expand this thing (refer to Powerpoint 8), and say that the kinetic
energy of alpha is q value time one minus four, the mass of the alpha, roughly, divided
by A, the mass of the daughter. And for our purposes in this class that's good enough.
(refer to Powerpoint 9). If I look at u235 decay, it is an alpha emitter. Look
necessity back of Krane and look at the mass of u235. Mass the daughter, thorium 231
and mass of helium four atom. Calculate the q value is positive 4.6 receive eight MeV.
It close to that five MeV number I was talking about. Now use that formula up there to
calculate what fraction of that energy is carried away by the alpha particle. So the
kinetic energy of the alpha given by that equation is the q value times one minus four
over 235. Mass of the alpha divided by mass of the daughter, it's 4.598 MeV. Almost all
the energy gets carried away by the alpha but if the alpha, if the rest gets carried
away
by thorium daughter because it has to move in the opposite direction. We can calculate
quantitatively what that is. Basically it's energy conservation. Here's the q value.
Here's the kinetic energy we just calculated carried away by the alpha. The difference
must be carried away by thorium. It's .08 MeV. Not very much but it's important in order
to conserve energy and momentum. You might think that's not a big deal. But turns out
to
be a very big deal practically. Because if you have an alpha emitter sitting somewhere
and it decays, the daughter may not stay where you thought it was. So, for example, if
you have some container, like this, and you have some alpha emitting substance, let's
say
it's down here on the floor initially. Suppose it alpha decay and alpha goes that way. I
should think of this as being a box that was a finite thickness. Say it's a steel box.
Suppose the alpha goes that, where is the daughter going to go.
PROFESSOR: Right down. Bury itself if the steel and I'm happy. What happens if the
alpha happens to go the other way? The alpha goes that way. Suppose this thing was
really sitting on the surface initially. Now the daughter has to go that way. Suppose
this box is full of air what's going to happen to that daughter? It's going to clear the
air molecules and stop. The demo on day one we can see with about an inch of air you're
going to stop alpha particles. The daughters will stop in here as oppose this has a
value-creation activity um and I pumped all the air out it's going to go across, hit the
other wall and stick there. The point is if you have alpha emitter the daughter can
travel pretty large distances just from the nuclear recoil associated with the alpha decay
of whenever you're dealing with after \particle or\parlor alpha particle sources you have
to be careful about con Tam of Tam hibernating whatever box you put them in. In our lab
downstairs in nuclear 101 we do experiments that I'll describe. One of thing we have to
be concerned about is we end up contaminating everything because of this. It's
not such an issue for beta decays or gamma decays because the recoil energy there are
much
smaller. For alpha decays it's not negligible.
Okay. One interesting question is where does helium four, turns out helium four was
discovered in the sun because it was discovered on earth. People looked at the spectra
from the sun and can figure out the chemical element helium was there. and people then
found there was sources of helium four on earth. The question is where does it come
from.
The answer is it's all from radioactive decay. When our sun and the solar system formed
four and a half billion years ago, there already was helium present. And that's where the
helium that's in our sun actually came from. we'll see that in a little bit. Presumably
when the earth fir formed there was helium that got trapped gravitation will I. If you
have gone through classical mechanics, you know for any given object there's something
called escape velocity, velocity to he is indicate the gravitational field of that object.
If you think about gases here on earth, they're in thermal equilibrium with their
environmentment in this room the gas molecules are all traveling around with what kind of
energy? Three halves kt. That's the kinetic energy of anything that's in thermal
equilibrium with an environment that temperature t. For room temperature, that energy
turns out to be something like 140th of an electron volt. So that's the mean energy of
particle running around the room now the. If you then translate into a velocity, the
velocity depends on the mass of the particle. And so the lighter the particle, the higher
the velocity will be given you have one, 40th of an electron vole energy. If you compare
that to the escape velocity of earth, which is about 25,000 miles per hour, turns out
hydrogen and helium all have higher velocities that is neated to he is indicate. All the
hydrogen and helium presents on earth have long ago escaped just from, nevertheless
there's still helium here and to call came from the decay of uranium and their yum that
are deep inside the earth. In fact, the federal government subsidized the collection of
helium because what would happen is oil companies and gas exploration companies with drill
into the ground to get the oil and gas out. But they would tap into regions where helium
had accumulated from the decays of isotopes sitting down in the rock. The government
would pay the companies to separate out the helium and chemo keep it for scientific
purposes. They developmentally stopped doing that. you probably read about helium
shortages and that's one of reasons. This is the decay chain of u238 down to lady 206.
All these diagonal lines are alpha decays. Start from u238 and alpha decay down to
thorium 234. That alpha if it's happening inside a rock stops at some short distance,
neutralizes itself and become a neutral helium atom and similarly all the way down the
chain. We can get a large number of alpha and a fairly large amount of helium produces
over billions of years from the decays of these nuclei.
And similarly, in the thorium decay chain, that I recall alpha decays which get you
down to lead 208 and again produce helium. These are interesting decay chains and turns
out there are interesting experiments you can do with them. And a lot of them evolve
the
fact in both thorium and European yum decay chains you pass through a noble gas, raid
on,
I think I talked about that a little bit before. In the case of thorium you end up going
through radon 222, these are noble gases which moons they basically are inert chemically.
If you have decays happening inside an object which is not too tight in the sense that
it's not a metal, let's say it's in a rock which is somewhat porous or some other object,
once you decay into the radon there's a fair chance that radon gas will escape. For
example, the building materials here, all have small amounts of European yum and thorium
in them. As a result of their *** radon is being produced. Some of these is emanating
from a the walls and floor and it's part of air we're breathing now. We can study that
in
the lab. That's one of thing we do back if Me 102, this is a little bit of an advertise.
For that class. What what we do is make use of these guys, sources of radon. These are
radium painted watches back old days, in order to make a watch glow in the dark so you
could read the dial they patented numbers and hands with a compound that contained radium.
When the alpha particles were given off they could strike the other material in the
numbers or the hands. It would glow. You can see it in the dark. It would glow. these
are not made anymore because of health hazards associated with them. They're collect
item. If you go on eBay you can find people selling them. Which is where I got these.
People will say they're radium watch dials. You have to be Carol if you're considered
in
buying them because this no longer glow in the dark. All the alpha decays over the years
have so damaged the phosphorus that they don't flow in the dark. Some are still
radioactive, some aren't. Buyer beware is what I'm saying. I bought three. Two turns
out to be radium, the other wasn't. We remove the covers and thousand you've got radium
contained in here. And there's nothing really to stop the radon to come out. We make use
of that in the experiments. Another source of radon that comes from the thorium decay
chain are lantern mantles. This is the thing which goes over the gas flame. And it gets
heated up to the point where it glows. Turns out if you put a little bit of thorium in
this cloth is it glows more brightly. Not because of radioactive decay but because of
chemical properties of thorium. We don't make them in this country any more. I think
they're still be made in India. My colleagues and I have purchase a fair supply of these
that have the radium, thorium in them. So we use them now as sources of radon from the
thorium decay chain.
We do simple experiments using these. So this is a mayonnaise jar I stole from my
wife. We put the radium containing watch down here with its cover removed. This is a
plastic cup and we have a piece of copper. The idea is over a period of time, the radium
decays in here, some of the radon get out and fills the jar. When the radon decays it
produces daughter nuclei which can be identified from that decay chain. From random walk
some of them dost themselves on the copper surface. After a few days row motive the
copper and stick in front of a detector which allows you to see the decay in whatever has
collected there. If go back, the radon fills the jar, when the radon decays you get
polonium 218. Down to lead 214. Lead 214 beta decays with half-life, and another with a
between minute half-life. And, if you collect anything, you look with alpha detector
which you expect to see the alpha decay of Monday yum 218. You end up seeing polonium
214
but not with this half-life. With a much longer half-life.
This is the device we use, this is a little vacuum chamber. When you open the door
there's a silicon detector, here's the piece of copper that we allow to be exposed to the
radon, it's sitting on a tray. Close the door and evacuate this to allow the alpha
particles it travel from the copper night sill congress. You measure the amount of energy
deposited in the silicon. Because each alpha decay has a particular q value you can
measure the energy of alpha and figure out which decay is going on. So this is piece
of
copper that was exposed for three days. To the radon. You quickly take the copper out
of
the jar, get it in front of your detector and start counting. There are two peaks, this
is energy going this way, this is number of events going that way. This is one minute's
worth of counting. Within two minutes of opening the jar, this peak corresponds to the
alpha you expect from polonium 218 decay, that first decay after the radon. This is
the
polonium 214 that has the decay after the two beta decays. You can measure the energy
and
figure out what they are. And then the next part of experiment is to measure the
half-lifes of these by collecting data in different time intervals. In this case 10
measurements of one minute each. To follow how these lines decay with time. If you look
at the first alpha, the one associated with polonium 218, this is the number of those
the second alpha, the one that came from the polonium 214 who's intrinsic half-life is
150
micro Serbs. But you observe a half-life more like 40 minutes. I'll let you know about
that. That's the right answer by the way.
Again, you say this is nuclear nuclear engineering who cares about alpha decay. I
think I've already mentioned alpha decay is use north side lots of devices one of which
is
a smoke detector. There everybody's got these at home. If you open them you'll find
sticker in there that says you have about a micro Curie of Americium, sitting there.
Americium senses whether there's smoke in the air. The idea is you have a detector
sitting close to the alpha emitter. If there's air between the detector and the alpha
source you see a certain current. If there's snowing particles in the air they stop the
alphas, the current reach your detector changes and the alarm goes off. These are
radioactive sources that you can buy, you don't need a license. When you're done using
them there's a special dispensation that allows us to throw these in the garbage because
even though there's some risk associated with doing that, the benefit they provide is so
much greater people have figured out it's worth the risk of throwing them in the garbage.
I think it's the only radioactive thing for which you're a Ludd to do that.
Another, use of alpha decays again was discussed a little bit on your exam. The fact
that if you have a big object, a macroscopic object condition Tang an alpha emitter,
because of short range of alpha, the alpha will stop inside that object. This is a people
let of flew ton yum 238. Plutonium, it's a ground stayed is to ground state alpha
emitter. The alpha decay goes almost entirely to the ground state of u234. The only
radiation emitted is is the alpha parliament this is a fairly lank chunk. The reason
glowing reds is basic the heat generated by alpha decays. There's enough energy in the
alpha decays to heat this up so it's red hot. That can be used for practical purposes in
what's called the radioisotope generator.
STUDENT: How big was that pellet.
PROFESSOR: So I don't know the donations here. I would guess centimeters. A few
centimeters. I'm not sure. I don't really know Leo. I think it's a few centimeter.
These things get used in radioisotope generator. If you go back to your maybe solid state
physics or I forgotten where you learn this, there's an effect known as the Seebeck effect
which is that if you have two different materials that are maintained at two different
temperatures, you can produce an electric current by connecting them to one another.
The
idea is you have one object here at one temperature and another material at a different
temperature and look up a electric circuit, an electric Cal current there flow threw that.
That's, the way you get it temperature difference is to use the heat produced by the
radioactive decay of some radioactive substance. In a lot of cases at that substance is
plutonium 238. The heat from here is used to heat up some object. You connect to another
object and you get electric Cal current going through it. In your exam question it had to
do with how much power you could generator from such a thing. The conversion of thermal
heat into electric Cal power is not 100 percent efficient. If you have a kilowatt, from
the radioactive decay maybe you get a thirds of that in terms of electric Cal power.
These are important if you're trying to produce electrical power in some remote
environment. , for example, if you're going to accepted a spacecraft out to study Saturn
which is what can a scene Cassini is doing. You don't run a extension cord out there.
both of those spacecraft have these objects on them. When they got launched there was
a
fair bit of concern about what with happen if the thing blew up on the launch pad because
there's on the orders of a kilogram of plutonium sitting on these. People worked hard to
make sure it was safe and everything behaved as planned and nothing happened that would
cause this to get dispersed. And this is the Curiosity Rover which you've seen pictures
of. Here the thermal electric generator. That's what provides the bulk of electrical
power to do those experiments.
STUDENT: How do they cool in space.
PROFESSOR: How do they cool in space. I'm sure the NASA engineers have thought of
that.
STUDENT: There's a cooling, they're just spinning off ir from a vast array of
panels.
PROFESSOR: Wouldn't want it to melt , for example, you're right. So there has to be
some control mechanism built in and people have done that. Other questions? Okay. I
think I'm going to quit here for today. You've got a homework assignment which you know
people are working on because people have asked me questions already. On Friday we'll
go
through the rest of the discussion of how you calculate alpha decay half-lives, which
is a function
of z nucleus and the Q value for the decay. But it all goes back to that coulomb penetrability
business. All right. We'll see you Friday.