All right, you did well on the exam.
Class average was sixty-two. I always aim for sixty-five,
so I was very happy. Eleven students scored a
hundred. I believe that my exam review
was extremely fair. According to some instructors,
perhaps even too close for comfort.
I did a problem with parallel resistors and a battery.
I applied Gauss's Law for cylindrical symmetry.
I spent quite a bit of time discussing where charge
occurs and where charge can not be located on conductors and I
hit the idea of capacitors and dielectrics also quite hard.
I prefer not to think about a rigid division between pass and
fail, but I'd rather tell you that all of you who scored less
than forty-seven, in my book, are sort of in the
danger zone. Now, that doesn't mean that
you're going to fail the course, nor does it mean that you will
pass the course if you scored seventy.
But those people are in the danger zone.
I think you should talk to your instructor, and I would advise
those people also to make frequent use of our tutors.
Two exams to go, plus the final.
Today I'm going to uncover a whole new world for you and you
will see how eight oh two comes in there in a very natural way.
The Lawrence force F is the charge times the cross product
of the velocity of that charge and the B field that the charge
experiences. If I have here a positive
charge plus Q and it has a velocity V in this direction,
and the magnetic fields would be uniform and coming out of the
blackboards, there's going to be a force on this charge according
to this relationship and the force
is then like so. Perpendicular to V,
perpendicular to B. In this case the charged
particle is going to go around in a circle .
The Lawrence force can not change the speed,
can not change the kinetic energy, because the force is
always perpendicular to the velocity but it can change the
direction of the velocity. And so, what you're going to
see is that the charged particle will
go around into a perfect circle if the magnetic field is
constant throughout. And the radius of this circle
can very easily be calculated using some of our knowledge of
eight oh two. The force is QVB because I
chose B also perpendicular to V, and so there is no sign of the
angle between them. This one, and this now has to
be the centripetal force that we
encountered in eight oh one, which is MV squared divided by
R, M now being the mass of this particle.
And so you'll find now that R equals MV divided by QB.
And this, by the way, I want to remind you,
is the momentum of that particle.
If you look at this equation, it's sort of pleasing.
If the charge is high then the
Lawrence force is high so the radius is small.
If the magnetic field is high then the Lawrence force is high
so the radius is small. If the mass of the particle is
high, there is a lot of inertia and so it is very difficult to
make it go around, so to speak,
so a very high mass, you expect a very high radius.
And so that looks all intuitively quite pleasing.
Let's do a numerical example.
I take a proton, P stands for proton,
and I take a one MEV proton. It's the same I took during my
test review. One MEV means that the kinetic
energy is one MEV is the charge times the potential difference
over which this proton was accelerated, in this case,
delta V would be one million volts.
And this now equals one-half times
the mass of that proton times the velocity squared.
In this case, if I have the one MEV,
so it is a million volts, you will find that this is one
point six times ten to the minus thirteen joules.
I gave you there the charge of the proton, you multiplied it by
a million, and this is the energy.
And so now you can calculate the velocity because you know
the mass of the proton. I gave
you that too, there.
And so you will find exactly what you found during my test
review, one point four times ten to the seventh meters per
second, which is five percent of the speed of light,
comfortably low so we don't have to make any relativistic
corrections. If this proton now enters a
magnetic field B, which is one Tesla,
then by using the equation I have up there,
you know the mass of the proton,
we just calculated the velocity.
You know the charge of the proton and you know the B field.
You will find that R is oh point one five meters,
which is fifteen centimeters, just a numerical example.
It is more common, or at least often done,
to eliminate out of that equation there the velocity and
replace it by the potential difference,
capital V, over which we accelerate these particles.
And so, what you can do, you can replace this V by using
the equation I have there, the one half MV squared,
so we have that one-half MV squared equals Q times delta V,
but I will write for that just a capital V, and I substitute
this V now in here, and so I now longer see the
velocity but I now see this potential
difference. In the case of that proton,
this V would be a million and you will find then that R is
then the square root of two M times that capital V divided by
Q B squared. And so the two equations are of
course the same physics, but it's different
representation. If you put in for V now ten to
the sixth mass of the proton, charge of the proton,
and one Tesla field, of course you find exactly the
same point one five meters. Now this is all nice and dandy,
but this works as long as the speed is much smaller than the
speed of light. If that's no longer the case,
then we have to apply special relativity and that is not part
of this course but I would like to briefly touch upon that
today. I can show you how things go
sour because suppose we have a five hundred kilo electric volt
electron. So that means that in this
equation here, the V is five hundred thousand,
the Q is the charge of the electron, M is now the mass of
the electron, and if I apply that equation I
find that V is four point two times ten to the eighth meters
per second and that is larger than the speed of light,
so that's clearly not possible. The actual speed,
if you make relativistic corrections, is two point six
times ten to the eighth meters per second.
And although I don't expect you to be able to make those
relativistic corrections, I will make them today and you
will see why I have to,
and I want to show you that in fact this is not all that
difficult even though I will not hold you responsible for these
equations. So what I have here is now
kinetic energy, is again QV,
that's not changing, but is no longer one-half MV
squared but it is gamma minus one times MC squared,
and gamma is defined there -- it's called the Lorenz Factor,
and so if you know now for the electron that capital V is five
hundred thousand, you can calculate what gamma is
from the first equation and then you go to the second equation
and you find what the speed is, and you will see then that you
never find a speed larger than the speed of light.
And so we now have to make the correction also for the radii
and those corrections become again relatively easy.
This now requires a factor gamma and you see that on the
upper blackboard there, and this too now has to be
replaced by this gamma plus one and then everything is OK.
So I don't expect you to know this, but I don't want you to
think that all these relativistic corrections come
out of the blue, nor do I want you think that it
is very difficult. It really isn't.
The equations are extremely straightforward.
So I want to show you now some of the results that we just
discussed. The one MEV proton and the five
hundred KEV electron, this is on the Web.
You can click on Lecture Supplements and you can make
yourself a hard copy. So here you see the kinetic
energy, one MEV proton. Notice the speed that we
calculated there is non-relativistic,
gamma is very close to one. You don't have to make a
correction. And in a one Tesla field you
get a radius of fifteen centimeters, which we just
calculated. If you go to a fifty MEV
proton, it's sort of in the borderline between relativistic
and non-relativistic. It's still non-relativistic
enough, and if it is non-relativistic you can clearly
see here that the radius goes with the square root of capital
V. And for fifty MEV,
capital V is fifty million, and for one MEV,
capital V is one million. And since it goes with the
square root of V, you expect roughly the radius
to be the square root of fifty times larger which is seven,
and indeed, you see that. So you see, from fifteen
centimeters the radius goes to about one meter.
Um, here is our five hundred KEV electron,
and notice that I did the calculation correctly.
This is relativistically corrected now.
You get your two point six times ten to the eighth meters
per second by applying the formalism that you see there.
I will leave this here throughout this lecture,
because I will return to this several times.
I want to show you a a cute demonstration.
I have an, er, electron gun here and the
electron gun comes like so.
This is the velocity of the electrons.
I put a minus sign there to remind you that they are
electrons. If electrons go in this
direction, the current goes in that direction.
And so if now I have a magnetic field which, let's assume the
magnetic field is in the blackboard.
This is B. Then I cross B is the direction
of the force. I is in this direction,
B is in the blackboard. So if I'm not mistaken,
I think the force is in this direction and so you will see
that it starts to bend in this direction.
If you change the direction of the magnetic field,
the magnetic field is coming out of the blackboard,
then the electron will go in this direction,
and I can show you that here. It is not too different from
the distortion experiment I did when I had the television
program there and I had the strong magnet
and we distorted the image, but this of course is a little
bit more controlled. So, we're going to see the
image there and we want to make it quite dark in the room.
Mmm. And turn on the electron gun.
So you see, the electron gun it strikes a fluorescent screen and
that's how you can see it, and I have here a bar magnet
and if I hold the bar magnet behind it then I can create more
or less situations like this. I can flip over the magnet and
then the direction of bending should change,
so here I come with the magnet, and you see,
curve up the electrons. I turn the magnet over and I
come in again and they curve down.
Very straightforward, very simple.
OK.
There is a fantastic way in physics that we can separate
isotopes from one and the same element.
If we, for instance, take uranium,
now uranium, when you find it,
is for ninety-nine point three percent uranium two
thirty-eight. That means it had ninety-two
protons, otherwise it wouldn't be uranium,
and it has hundred and forty-six neutrons,
ninety-nine point three percent.
Point seven percent is uranium two thirty-five.
Again, ninety-two protons, otherwise it wouldn't be
uranium, but only a hundred and forty-three neutrons,
and that you'll find in nature for oh point seven percent.
So you go to a chemist and you give a chemist a little bit of
uranium and you say would you please separate these two
isotopes for me, and he of course would laugh at
you and he would say go fly a kite because the chemical
properties are exactly the same for the two because uranium is
uranium. Neutral uranium here has
ninety-two electrons and neutral uranium here has ninety-two
electrons, so there's no way that they could separate those.
And I will show you now how they can be separated with what
we call a mass spectrometer. You heat the uranium so that it
ionizes. Let's assume it's ionized once
so it loses one electron,
so it's positively charged with one unit charge,
one of those charges that you see here.
And we now accelerate it over certain potential difference so
these uranium atoms, two thirty-five and the two
thirty-eight get a certain speed and they come in here with this
speed V, so they're positively charged and let's assume that we
have a magnetic field that is uniform and that
is in this direction, comes out of the blackboard.
So what will happen is that these charged particles which
are positively charged now, one unit charge,
are going to go around a circle and hit here.
It is the radius. But if you look here at these
equations so you will see that the radius is proportional with
the square root of the mass of the particle and the mass of two
thirty-eight is one point two percent larger than the mass of
two thirty-five. And so with one point two
percent larger, since we have the square root
there, we see here the square root, we accelerate them over
the same potential difference, so this one doesn't change.
This is the only thing that changes.
So then you expect an oh point six percent change in radius and
so the two thirty-eight will end up
here. I exaggerate that very highly.
And the two thirty-five will end up here.
The two thirty-eight has a larger radius because it has a
larger mass, and you see that here.
There's no change in B, there's no change in Q,
and there's no change in capital V.
We accelerate them over the same potential difference.
And so if the radius, for instance,
were one meter of this mass spectrometer then the difference
here -- remember this is two R -- the difference would come out
to be about one point two
centimeters, and so you have a collector here where you collect
your two thirty-eight nuclei atoms and here you collect your
two thirty-five and that is the idea behind a mass spectrometer.
Why did I choose this particular example?
Well, this example changed our world and it made history.
Uranium two thirty-five was needed by the Americans to build
an atomic bomb to end the Second World War.
This is- this was done under the famous Manhattan Project.
And Ernest Lawrence of Berkeley built mass spectrometers which
were able to separate uranium two thirty-five from two
thirty-eight. In the beginning,
it went very slowly, about one hundred micrograms
per day. But a few kilograms was
required for an atomic bomb. They finally managed to get up
to one gram per day and in combination with other
separation techniques such as the gas
diffusion techniques which I will not discuss here now they
managed to get a few kilograms and they dropped a bomb on
Hiroshima on August sixth nineteen forty-five and three
days later a bomb was dropped on Nagasaki.
The Japanese surrendered and it was the end of World War Two.
It's a good thing that there are many peaceful applications
nowadays of mass spectrometers, particularly in the medical
area. People sometimes require
radiation and they need radiation from a particular
radioactive isotopes, but you don't want the other
isotopes from the same element and so you separate them then
with a mass spectrometer. It's a whole industry,
very important industry. And I would like to address
the issue how you accelerate protons to extremely high
speeds, almost approaching the speed of light.
And that is also something for which Ernest Lawrence is
credited. In the early days it was done
in a cyclotron, which I will describe to you
now. The cyclotron consists of a
chamber which is called a D. This is one D and here's
another D. These are conducting chambers.
If you look from the side it would look like so.
This is the left chamber and this is the right chamber and
all of this is in vacuum and let's assume that we have a
magnetic field coming out of the board like so.
Let's revisit our one MEV proton.
Suppose I release in this chamber
here a one MEV proton and I know the speed with which it
comes out, because the one MEV proton had a speed -- Oh,
you still see it there, one point four times ten to the
seven meters per second. We also know that in a one
Tesla field, let's make this one Tesla, that the radius is going
to be fifteen centimeters. You see it up there.
So what is this proton going to do?
It's going to do this. But when it gets there,
the potential difference is introduced between these two
D's, so that this is low pot- high potential and this is a low
potential. And so you're going to get an
electric field now in this gap in this direction and so this
proton is being accelerated. And let's suppose that the
difference in potential is twenty kilovolts.
Then this proton will gain in electric- in kinetic energy,
it will gain kinetic energy, twenty
kilo electron volts. That's the way electron volt is
defined. And so you start off with one
MEV, so when it has crossed this gap it is now one point oh
two MEV. Twenty KEV more.
The radius, now, is larger.
If capital V is two percent higher, and I go to this
equation, then the radius is one percent
higher and so when it comes out here and it makes a circle,
the radius now is one percent higher than fifteen centimeters.
But when it gets to this part of the D, this potential
difference is reversed and so the electric field is again in
this direction, in the direction of the proton
and so it is accelerated again by twenty kilo electron volts.
Now the radius, of course, is even larger and
so very gradually every time that it reaches the gap the
potential difference is changed in direction to accelerate the
proton and so it gradually spirals out, then,
to the largest radius that you have.
So during one full rotation it gains twenty kilo electron volts
once, and twenty kilo electron volts twice, so it gains forty
kilo electron volts. And so the electric fields are
doing the work. They accelerate the particles.
Magnetic fields can not accelerate.
Magnetic fields can change the direction but they can do no
work on the particles. So the magnetic fields confine
the particles. So let's assume we go twelve
hundred and twenty-five full rotations.
During each rotation the kinetic energy increased by
forty KEV. And so if you multiply the two
then you see now that the kinetic
energy of this proton increased by forty-nine million electron
volts, because it went twelve-hundred and twenty-five
times all the way around, and so now you have forty-nine
MEV plus the one MEV that you started with,
so now you have a fifty MEV proton.
You see the second line there? There we have that fifty MEV
proton that I discussed with you
earlier. In a one Tesla field now the
radius is one meter, so if this unit had a radius of
one meter that would be fine. By that time it would be all
the way near the circumference of this unit.
What is remarkable and not intuitive, that the time to go
around as long as we don't have to make relativistic
corrections, that the time for a proton to
go around is independent of its speed.
Not so intuitive, and you can see this very
easily because the time to go around is two pi R divided by
its speed. You see, the radius is
proportional to V. And so the time itself is
independent of V, because R itself is linearly
proportional with the speed and so that cancels and so you'll
find now that the time to go around is simply two pi times
the mass of that particle divided by QB.
And if you correct relativistically,
then you have to multiply by gamma, but if you stay
non-relativistic, then it's independent of the
speed of the protons. So if we stick to this
particular case of our one MEV proton that became a fifty MEV
proton going around twelve hundred and twenty-five times,
this time to go around once is only sixty-six nanoseconds,
so this is six point six times ten to the minus eight seconds.
Give you some feeling of how fast all this is going.
So if you go around twelve hundred
and twenty-five times, that would take only eighty
microseconds so in eighty microseconds does all of this
occur and that means you have to switch this field twice before
rotation, make sure that the e-field is in this direction,
but when the proton comes here the e-field has to be in that
direction. And so the switching frequency
which easily be calculated becomes about thirty million
times per second, about thirty megahertz.
And all of that takes place in eighty microseconds and you
create one MEV protons, you turn them into fifty MEV
protons. A mind-boggling concept,
but it works. Quite remarkable.
Now because of the relativistic corrections that you see here
with gamma, if you go to very high energies then the time is
not constant for a full rotation,
so you have to adjust now the frequency with which you switch
the potential between these gaps.
So if the time increases then this switching frequency has to
go down and we call those instruments synchrotrons,
or synchrocyclotrons. They have names.
So you synchronize now and correct for relativistic
effects. Modern accelerators have
constant radii. They are rings.
And so if you have a ring with constant radius,
the only way that you can keep the particles in the ring when
they have a low energy and when they have the high energy is by
gradually increasing the magnetic field.
So you start off with a weak magnetic field,
you go around huge circle, very large radius,
and you gradually increase the magnetic fields as you keep
accelerating them and by making the magnetic field go up just in
the right way, maybe all the way up to two
Tesla, you can keep them in that ring.
The first slide that I'd like to show you is the slide of an
ancient cyclotron that is actually a synchrocyclotron,
was built by Lawrence in Berkeley, and this was capable
of accelerating protons to seven hundred thirty MEV.
You see here a person to give you feeling for the size of
this instrument. Lawrence received the Nobel
Prize for Physics in 1939 for his
invention of the cyclotron. The next slide is Fermilab near
Chicago. This is one of these modern
accelerators also called sometimes colliders,
and this has a diameter of two point two kilometers,
and this instrument, this here, plans to accelerate
protons up to thousand GEV, G stands for giga,
giga is the same as billion. A thousand giga electron volts
would be ten to the twelfth electron
volts. The beams of high energy
protons are made to collide with other nuclei to uncover the
inner workings of nuclear physics.
The higher the energy of the protons, the larger is the
impact when the protons collide and the more one expects to
learn. By using ever-increasing
energies of the protons, which are nuclear bullets,
one explores unknown territory. In the news,
these colliders are often called
atom smashers. That is a flashier name which
appeals more to the general public who pays for all this
with their tax money. This research is a multibillion
dollar industry. The words atom smasher are
actually a misnomer. The colliders smash nuclei
which are ten thousand times smaller than atoms.
And the next slide shows you the tunnel of the largest ring
in the world which is in Geneva, at CERN which is a European
collaboration. This tunnel which already
exists for many years has a confluence of seventeen miles,
has a radius of four point three kilometers,
and in here are these protons being accelerated [inaudible] on
the high vacuum. And with very modern techniques
of superconducting magnets they can even go up now to about five
Tesla. And in this tunnel right now a
whole new experiment is under
development which is called the Large Hadron Collider which is
considered the Holy Grail for particle physicists and it's
hoped that that will go on the air in the year Two Thousand
Seven or so and it will accelerate protons to an
unprecedented energy that will give them kinetic energies of
seven thousand GEV, seven times ten to the twelfth
electron volts. So I would like to return to my
overhead there, so that you can see some of
that what we just discussed right there, thank you Tom.
So here we have Fermilab. You see a radius of one point
one kilometers. I showed you a picture from the
air and so they went up to one point five Tesla,
so that's the maximum magnetic field strength.
Get very close to the speed of light
by the way [laughter] and you see five hundred GEV protons.
And here you see the holy grail, the large hadron
collider, European collaboration in Geneva at CERN,
whereby you have the circumference of seventeen miles
and the magnetic fields that they hope to achieve going up to
five and a half Tesla using modern techniques of
superconductors. So if you want to go around in
those tunnels by the way, by the way, you need a
motorcycle, [inaudible] go seventeen miles around.
The goal of all this physics, of all these experiments,
is to enter new territory, to learn about these mysterious
nuclear forces and to see what is insi- inside protons and to
see what is inside neutrons. And with these experiments many
nuclear particles were discovered
whose existence was completely unknown previously.
Now comes the issue how can you see the results of these
collisions of these particles with very high energies.
Well, you can make the tracks of these particles visible.
In fact, today you will see them with your own eyes.
And in the old days this was done with cloud chambers and
that's the demonstration I will do today
for you. But nowadays they do them with
bubble chambers. Let's first understand the
principle. If you had a charged particle,
whether it is an electron or a proton or an alpha particle --
alpha particle is the nucleus of helium, it's two protons,
two neutrons. If it goes through the air it
makes ions and as it goes through the air and it makes
ions it slowly loses its kinetic energy and it finally
comes to a halt. If we take a ten MEV electron
and one atmosphere air it could go forty meters.
If you take a proton of ten MEV it would only go one meter
because the density of ions is higher because it has a higher
mass and if you take an alpha particle which has a higher mass
than a proton and it has a double charge of a proton,
then it would only go ten centimeters.
It's a very high density track that
you would get from an alpha particle.
And so one way you could see these tracks is using cloud
chambers, and a cloud chamber works in principle as follows.
You can just have a chamber in air, one atmosphere air,
which you put liquid alcohol in there, that's the way we will do
it, and you cool the bottom. You see one there,
which you will see a little later, and you cool the bottom
with solid CO two and then you get inside this chamber,
you get a temperature gradient and there's a layer there where
the alcohol should really condense
into little drops because it's that cold, but for reasons that
are complicated it doesn't do it quite yet.
We call that undercooled alcohol.
Even rain can be undercooled. Just below freezing point,
still liquid. When the moment it hits the
ground it will immediately become solid,
by the way. That's also undercooled liquid.
Now, here we deal with an undercooled vapor and so when
these ions are made by these charged
particles, these ions act as seeds for the drops,
in this case the alcohol drops, and you can literally with your
eyes, visually see these drops being formed.
I'd like to go through one numerical example and I want to
go to a five hundred KEV electron which you see there.
Um, notice that I have corrected the speed
relativistically, otherwise you would get this
ridiculous number that we calculated
earlier which is larger than the speed of light.
And suppose we have a one-tenths Tesla field.
Then the radius would be two point nine centimeters.
But after a while this electron will lose its energy and then
there comes a time that it has only hundred kilo electron volts
left. By that time,
the radius in a one-tenths Tesla field would now be one
point one centimeters and so when you look at cloud chambers
at the tracks of electrons and you have
magnets there, you will see the tracks being
curled up which of course is the result of the fact that the
radius gets smaller in time and since the magnetic field is
constant you can see then large radius here and as the kinetic
energy slowly decreases the radius gets smaller and smaller
and smaller. So let's look at a few more
slides. In 1932, Anderson noticed a
track in a cloud chamber which had the appearance of an
electron. It had the right mass,
it had the right charge, but the curve- the direction of
curvature was wrong. And so he concluded that it was
an electron which was positively charged, which is now called a
positron. And these positrons had been
predicted on purely theoretical grounds by Dirac,
and Anderson receives a Nobel Pri- [break in file] discovery
in Nineteen Thirty-Six, only four years after he
discovered the positron and Dirac had already received his
Nobel Prize in Nineteen Thirty Three for his theoretical work.
The bubble chamber is an advanced form of the cloud
chamber. In the bubble chamber,
liquid hydrogen is used and if now
the ions go through the ions become the seeds now for little
gas bubbles. So you have liquids which
really should have been gas but, uh, not quite and so now it
forms gas bubbles. So in a cloud chamber,
you're going to see the drops. In a bubble chamber,
you see gas bubbles but the idea is the same,
and Glaser who invented these chambers, is also from Berkeley
by the way, he got a Nobel Prize for that
in Nineteen Sixty. So let's look at the discovery
by Anderson. Here you see a positron coming
from above, and this positron has sixty-three MEV kinetic
energy and Anderson put some half a centimeter of lead in
there which was very clever, think about that.
When it comes out, the energy now is less,
because in the lead it produces a lot of ions and so it loses a
lot of kinetic energy and it comes out with roughly
twenty-three MEV. And why did he do that?
Because now he knows for sure that this particle came from
above, because when it loses energy the radius is smaller.
That's why he was sure that it was curved in the wrong
direction. If he didn't have the lead,
you never know whether the electron
came this way, in which case the curvature
would be perfect. But now he knows it comes from
above and if this had been an electron it would've curved this
way. So this is one of the early
discovery, cloud chamber photographed by Anderson.
And the next slide is a bubble chamber and we see here both
positron and an electron in a constant magnetic field and it
speaks for itself, notice that the curvatures are
exactly in opposite directions and you see this spiral
structure that I discussed with you as the electrons lose their
energy and since this is a bubble chamber,
which has an enormous density a thousand times higher density,
say, than air, these electrons don't travel
forty meters in these chambers. In air they would,
but in this case it is substantially less and so you
can roll them up nicely. And you can study them,
their momentum and their charge.
Using uh, accelerators and cloud chambers and bubble
chambers, a whole new world of nuclear physics emerged.
Wow. And between Nineteen
Fifty-Eight and Sixty-Eight thirty new nuclear particles
were discovered. And MIT has always been on the
forefront of this research. Professor Sam King,
who is still at MIT, got the Nobel Prize in Nineteen
Seventy-Six. Steven Weinberg,
a theoretician who did his work while he was at MIT got his
Nobel Prize in Nineteen Seventy-Nine.
Jerry Friedman, still at MIT,
and Henry Kendall, got the Nobel Prize for their
work in Nineteen Ninety. And Clifford Shull got his
Nobel Prize in Nineteen Ninety-Four.
If I summarize the basic idea behind it, it is very relevant
to eight oh two, you accelerate these particles
using electric fields. That's the only way you can
accelerate them. Magnetic fields can only be
used to confine them. It can not change their kinetic
energy but kinetic electr- magnetic fields are crucial,
because that allows you as you gradually increase their speed
to confine them, either by a ring,
which is done nowadays, or in the old days in these
chambers of the cyclotrons and the synchrotrons.
And then we have the bubble chambers, in the old days the
cloud chambers, whereby you use magnetic fields
to get information on the radius of these particles as they are
being detected. And out of all these emerged a
whole new way of looking at our world and completely new ideas
about what makes the world tick. This is nothing short of a
revolution. And so now I want to enjoy with
you the last five minutes of this lecture by looking at a
cloud chamber and by looking at some of these tracks.
You're going to see a lot of electrons in there.
The walls of the cloud chamber are radioactive just like you
are radioactive. Your bones are radioactive,
your windows are radioactive. They emit electrons.
No protons, but they certainly emit electrons.
And we have in there a radioactive isotope,
a rod, which has thorium in it which
produces alpha particles. And so you're going to see
electrons which make beautiful spider web structures.
Please don't clean up yet, we have plenty of time.
To be precise, we have five minutes and
eighteen seconds left. So these electrons,
you will see them going like spiders through this chamber and
sometimes they change abrupt direction because they can
collide, particularly when they have low energy and then they
[break in file] so to speak,
and then occasionally we may see an alpha particle coming
from our radioactive thorium and that makes a very thick track.
And so let's try this. We have an expert here which is
Marcos who has not only borrowed these instruments,
and if you like it we may actually buy it,
it's not cheap, we may buy it,
but he borrowed it specially for you, for which I'm very
grateful, Marcos, and he also did quite a bit of
work to get the light just right.
It's not too easy to see those tracks.
So Marcos, you will get a chance to adjust the lights if
you want that. Um.
We're going to make it very dark and let's enjoy them,
this wonderful world, invisible world,
of nuclear physics. So here you see this th- the
rod with thorium so Marcos feel free to adjust the light if you
feel the need. I will go into the audience as
well and see whether we can
identify the electrons. Oh, there was an alpha
particle. So this is this rod and so the
bottom of this chamber is cooled with solid CO two.
Ah, there was an electron, very nice.
As I said, you know, they almost look like spider
webs. There was an electron here.
Also keep an eye on this rod and occasionally you will see a
very dense track which then indicates -- there was one,
that's an alpha particle.
There's a beautiful alpha particle.
Just enjoy this. You know, you're looking at a
world which is completely new. Think about it.
You're looking at the world of nuclear physics.
You're seeing individual electrons and occasionally you
see alpha particles. Here, there's one coming out.
And think about the physics what's going on here,
this alcohol which refuses to become drops and then these ions
say we force you to become uh -- here's an alpha particle,
I don't know what it's doing there.
So these, these, um, these ions force these,
um, these, this, this alcohol vapor to become
drops. That's an incredible,
complex picture that you're looking at.
It is amazing every time I see this.
It's absolutely fabulous. And all through these very
simple rules, now think about it,
we have the Lawrence force, that makes these particles go
around, electric fields that you can use them to accelerate
them, and then this subtle way that you can actually make them
visible, individual particles visible.
It is a new world. And the goal of my lecture was
to make you peek into this world which has revolutionized our
whole way of thinking. Thank you.