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Opera, that's a good one. That's good. It was opera,
yes. What opera? No, it wasn't Wagner, but that's a good guess.
It's certainly a reasonable style, way over the top. That was Maria
Callas singing La Mamma Morta from Andrea Chenier,
So, I thought that was a good match. The thing's best in class. That's
which was written in 1895, and premiered in the spring of 1896
exactly at the time when the world was going nuts over this mysterious
form of radiation that can see inside the human body.
Maria Callas. This is opera for those of you who don't like opera.
And this is playing. It's a fantastic piece,
This, by the way, some of you may recognize if you saw the movie
Philadelphia. This is the piece that's playing when the Tom Hanks
character visits the loft, or excuse me, the Denzel Washington
character visits the loft of the Tom Hanks character.
way over the top. And this is Maria Callas who is the woman that
restored melodrama to opera. It's fantastic. So go and listen
to it. Enjoy it. And, there's Bertha Roentgen's hand,
reminding us of the fact that you always irradiate the one
you love. [LAUGHTER] All right, so I have a few
announcements. Next week, a week from today,
there will be no lecture, instead, celebration of learning.
And, we'll go back to the early part. So, I think last time we said up
Celebration number two, Wednesday the 27th, coverage through
today's lecture. We'll go through the generation of
fair game. We started talking about the properties of ionic compounds
x-rays but we won't touch anything to do with the use of x-rays and
and electron transfer, octet stability. So, we'll go
indexing crystals, Bragg's law, any of that stuff.
through all of these. We've covered a fair bit of ground,
So, anything we talk about today I think is fair game.
and will end with x-ray spectra, but not Bragg's Law.
OK, second thing I want to remind people that I had a staff meeting
until the 22nd. So, I'm going to say the 22nd is
yesterday, and many of my recitation instructors said that some of you
are puzzled to learn that there's more than one book,
that the text is, in fact, three volumes.
And right now, the x-ray readings are coming from
this volume here called the course supplement. So,
Last point that came out of the staff meeting last night,
you need to be aware of that, and also some of the best material
a number of people are becoming tardy about taking the weekly
is in the archive notes on the Web, and also all of these images that I
quizzes. You are allowed to miss the weekly quiz for either health
show, they get posted as well. So, some of you are feverishly
reasons or some crisis in the family, but not because she just decided to
taking notes. And that's good, but if you miss something, you can
sleep in or you decided you'd like to take it at a later time.
go and take it off the website as well.
So, I'm giving the TAs discretion to deny you the right to take a make
up unless it's proper. I'm not asking that you come with a
medical certificate or anything; we'll take you on your honor.
But I do expect you to take those tests when they're offered.
If you can't, take it on the day, you can take a makeup up to one week.
After that, we want to close the books on it. And,
So, I know that recitation instructors are announcing this,
we're not going to allow you, as is the practice in other classes,
but I want you all to hear it from me as well. It's good for your
to throw away your bottom two or three scores. If you don't take
them, you'll get zeros, and that gets averaged in to your
mental health to take those tests, you know? It keeps you sharp. So,
homework grade. I want you to have the discipline
last day we talked about Roentgen and the discovery of x-rays.
of weekly homework and weekly homework quizzes.
Roentgen was working in this laboratory studying gas discharge
tubes. And, his special take was high-voltage and low pressure.
And, under these circumstances, he was generating, unbeknownst to
him until the night of November 8, 1895, photons of wavelength
approximately one angstrom. And, we looked at the relevant
physics and concluded that we could explain the generation of x-rays by
this energy level diagram, which is the energy level diagram of
the target. This is the anode.
Remember the gas discharge tube has a cathode, which is charged
negatively? And, electrons are made to boil off the
So, this is the energy levels of the element here in the target.
cathode, accelerate from rest, and they crash into the anode over
And, we reasoned that these incident electrons coming with
here. And, we think when these electrons crash into the anode,
kinetic energy imparted by acceleration voltages in the range
they cause a set of operations that ultimately result in the admission
of tens of thousands of volts have enough energy to come in and
of photons in the x region of the spectrum.
actually dislodge inner shell electrons. And,
in the extreme, N equals one, K shell electrons.
And when that happens, there's vacancies that invite a
cascade. So, we have a cascade of electrons in the anode target from
higher levels down to lower levels. And, as you know, going back to the
And, such a photon was termed a K alpha photon, K because the photon
early part of 3. 91 when electrons move from
high-energy to low energy, photons are emitted. And these
photons are the source of the x-rays. These are the x-rays here.
So, we saw that if we had a vacancy in the k shell,
we could get electrons moving from L shell down to K shell.
came from an electron cascade into the K shell, alpha because it came
And, even just to complete the picture, I put K gamma.
from one shell above K. There's a slight chance that the
electron may fall from N equals three down to N equals one.
That's a greater energy difference. And, we will get a K photon, but K
beta, K beta indicating that the photon fell to K from two levels up.
Well, if there is vacancies in the K shell, if we have enough energy to
dislodge electrons from K shell, we certainly have enough energy to
This will be an L beta photon, and thus was the way we left it last
dislodge electrons from L shell. Vacancy here would invite a cascade
day. And, the last thing was that the instant energy values here,
from N equals three to N equals two. This would be L alpha because L is
the destination shell, alpha meaning you came from one
and therefore, the instant wavelengths are determined by the
shell above. And, in the event that you fall from two
chemical identity of the target. If I change the composition of the
shells above, N equals four down to N equals two.
target, I have a different internal energy structure,
and I'll get a different set of emissions.
So, that's what was left at the end of last day. So,
now the question is, is there a quantitative relationship
between any of this, specifically, is there a
He was a graduate student working under Rutherford in 1913,
quantitative relationship between the chemical identity of the target
and any of these wavelengths that are observed? And the answer is yes.
And for that, we go back to Manchester and the
person of a young graduate student at the name of Henry Moseley.
1914. And, he was conducting a study, what we call a systematic
study. For those of you who intend to become graduate students,
I want to let you know this is a very dangerous word.
What systematic means, it's polite talk for tons and tons
of measurements, systematic. His systematic study
identity of the target, and measuring the spectrum,
was to take the x-ray generator and change the chemical identity
the emission spectrum for 38 elements. He started as low as
of the target. And, he conducted a study of no
fewer than 38 elements, systematically changing the chemical
along the way. And, he measured the wavelengths of
aluminum and went all the way up to gold, stopping 38 places
these x-rays, and he found a pattern. What he found was that as the
molecular weight or as the atomics, and we are just dealing with
And, here is the plate from one of his papers. This is a beautiful
elements, as the atomic weight rose, the wavelengths fell. So, I'm
comparing apples and apples. So, let's compare all of the K
alpha lines. So, the lambda, K alpha,
would fall as the heavier and heavier elements were used.
piece of work. These are the photographic plates
along the lines of the Balmer series, only this is from the x region of
the spectrum. So, here's calcium, titanium,
vanadium, chromium, manganese, iron, cobalt, nickel, copper, and
brass. Here's where he's going.
There's calcium; didn't have scandium. It was too rare,
and still to this day, very, very expensive. So, now he's moving
across the transition metals. He gets to copper, and he can't use
"The author intends to make a general survey of the principal
zinc for reasons I'm going to show you very shortly,
namely that the energy in that tube is so high that he's going to melt
the zinc. So, he says instead I'll use brass which
is an alloy of copper and zinc. And, so this is the data from his
study. And, so here's the first page of the paper.
types of high-frequency radiation, examine the spectra of the few
elements in greater detail. The results already obtained show
that such data have an important bearing of the question on the
internal structure of the atom and further support the views of
Rutherford and of Bohr. This is what he's putting up here
for us. And, what I want to show is that when he took his results this
is what he found. He tried to get a better
functionality of this relationship between wavelengths and atomic
And, in fact, the square of the proton number he got a linear
weights. And here's what he found worked best. He found that if he
plotted not the wavelength but the wave number, which we've seen before,
the wave number as a function of proton number.
relationship. So, and let's just say this would be L
alpha, L beta, K alpha, K beta.
Let's say you are at copper. So, copper would be 29 squared.
So, he would get data for the L alpha line of copper,
the L beta line of copper, K alpha line of copper, K beta line
of copper. He might not get all four lines for
every element, that he would take sets of data and
plot them out. And he found that if he plotted the
reciprocal of the wavelength versus the square of the proton number,
the points lay on a line. So, over here would be aluminum,
and over here would be gold. And, this was extremely important.
This changed so much because look at what he's able to conclude here.
Let's read his paper here. "We have here a proof that there is
in the atom a fundamental quantity which increases by regular steps as
we pass from one element to the next. This quantity can only be the
And, people were perplexed by the fact that potassium is less massive
charge on the central positive nucleus of the existence of which we
than argon. But no one would put argon under sodium.
have already definite proof. And, remember, when Mendeleev
When Mendeleev's law is that you arrange things by atomic mass.
enunciated the law of periodicity, he said that the properties of the
Cobalt and nickel are reversed. What to do? Here's Moseley. "We
elements are a function of the atomic weight.
are therefore led by experiment to view that N, his capital N is what
we call proton number, is the same as the number of the
place occupied by the element in the periodic system."
"This atomic number, he's coining the term.
"This atomic number is then for hydrogen one, for helium two,
for lithium three, for calcium 20, for zinc 30, etc. We can
confidently predict that in a few cases in which the order of the
atomic weights A clashes with the atomic order of the periodic system,
the chemical properties are governed by N, while A itself,
probably a complicated function of N. This is brilliant.
This is a graduate student in 1913 correcting Mendeleev.
So, what are the implications? What are the implications? So,
First of all, corrected Mendeleev, not to say that Mendeleev was bad.
this proton number, we're not going to give it its due.
We are going to call it Z, the atomic number. This is the
Social Security number of every atom, the atomic number.
So, what's the significance of this paper?
Mendeleev was brilliant in what he did. But there were a few things
It's no longer a function of atomic mass as per Mendeleev,
that Mendeleev couldn't account for. So, now we know in the post-Moseley
world, that periodicity, that is, the periodic variation in
properties is not a function of atomic mass.
but rather a function of the atomic number. OK, and this now resolves
the problems with argon, potassium; cobalt, nickel; tellurium,
instances. They did find a number of incorrect values.
But these weren't among that group. And lastly, a pair that they
iodine, these three were known. These three were known. And,
wouldn't have known: uranium and neptunium are in the wrong order.
Mendeleev kept saying go and measure them again. The atomic
All right, so you say, OK, well that's cute. But let me show you
masses are wrong. And, he was right in certain
something else. By understanding that the atomic
number is the critical factor in determining where something belongs
in the periodic table, he was able to place the lanthanides.
If you look on the periodic tables from the early part of the 20th
century, they don't know where to put the lanthanides.
They are trivalent, so they tend to put them under aluminum,
maybe under scandium. But they don't know what to do with them.
So, with this, he was able to place the lanthanides,
and more importantly, he was able to predict that there
are 14 of them. There are 14 in all.
Now, how do you do that? It's really simple. Lanthanum was
discovered in 1839. Lanthanum was discovered in 1839.
It has an atomic mass of 138.91, and there were various other
about how many elements lie between lanthanum and lutecium?
It's anybody's guess. But, with Moseley, Moseley comes along
lanthanides that had the discovered. And, I'm going to put in lutecium,
and he says, this isn't the critical figure. The critical figure is the
which had been discovered only recently in 1907. And, its
atomic number. I'm going to tell you because we
atomic mass is 174.97. So what? What can I learn from this
can do the experiment. And, we can determine that
lanthanum has an atomic mass of 57, and lutecium has an atomic mass of
71. Now, I ask you, how many elements are there?
How many elements are there to be discovered? With the atomic number,
everything is resolved. In fact, once you know that the atomic number
of uranium is 92, you can say with surety that there
are 92 naturally occurring elements up to uranium.
They couldn't do this before Moseley. And,
he kept going. He didn't stop with this. Who else was in town
in 1913 in Manchester? Niels Bohr. So,
Moseley looks at these data, and he says, I wonder if I can fit
these data to a line? So, he said, well, Bohr is in the
building. I'll go talk to Bohr. And he says, why don't I use a
Rydberg-like equation? And this is what he writes.
through the origin. They don't go through the origin.
The nu bar varies with the square of the proton number in the
following manner: R, the Rydberg constant,
So, wave number goes as the square of the proton number,
one over N final minus one over N initial, each of them squared,
times Z squared. But he knew those lines don't all go
Moseley's law. This is Moseley's law.
So, it's Z minus some nonzero constant squared.
but there's a little bit of an offset. And this is called
So, it's a Rydberg type equation. It's essentially a Bohr adaptation
for the x-ray spectra that he had measured. And so,
for example, you can have a Lyman-like series.
The Lyman-like series would be two to one, right,
N equals two down to N equals one? So that means, that's all of the K
Well, he keeps going. Let's see what he does.
alpha. So, the wave number of any K alpha line is equal to,
if I plug in one over two squared minus one over one squared,
this will come out to give me three quarters times the Rydberg constant
times Z minus, pardon me, sigma squared.
and N for calcium is really 20, then K equals one.
He solves for this thing, solves for it. And look at this
He goes through, and he rewrote the thing in a slightly different form.
But ultimately, if you read the highlighted passage here,
hence the frequency nu varies as N minus K squared,
little sort of very delicate, extremely important statement.
But, very typical British understatement: "there's good reason
to believe that the x-ray spectra with which we are now dealing come
from the innermost ring of electrons" because he knows
he's way down inside. So, sigma equals one,
in this case, for K alpha. And, there is a Balmer like series.
This is to x-ray what Balmer was to hydrogen. And that would be lines
And, it turns out that sigma equals 7.4 for L alpha series.
three to two. And, so that would be by definition the L
alpha lines. And that would be one over three squared minus one over
Let's try to understand, what is this sigma? Where does it come from?
two squared, which gives you 536 times R times Z minus sigma squared.
Here's my K shell. This is K shell. Here's the L
So, this is really good. Now, let's go into the physics.
shell. Here's the M shell. Now, K shell, I've got maximum.
I've got two electrons max, filled. OK, L shell I've got eight
So, for that, let's look at this little cartoon.
electrons max. And, out here I've got 18 electrons
max. Let's look at, first of all, sigma equals one.
That involves transitions from L down to K.
So, for transitions from L down to K, normally I've got two electrons in
the N equals one shell. If I'm going to have cascade,
one of these is missing. There's a vacancy down here.
So, we say that the attractive force of nucleus is mediated.
So let's go up here. I've got one, two, three,
four, five, six, seven, eight. Now the eight of these electrons
are feeling the positive attraction of the nucleus mediated by the
Or, we say the electron screens the attractive force of the nucleus.
presence of the one electron. So, you see plus Z minus one.
So, the outer electrons are going to fall down, don't get the full brunt
of the positive force of the nucleus. And, there's only one electron here.
So, instead of Z, it's Z minus one. It makes sense.
Here, there's either one or two electrons. Suppose there's two here
It makes sense. And, how about, let's do the next one.
and there's seven here. That's nine. Suppose it is only
The next one would be from three to two. So, out here,
one here. Well, one plus seven is eight.
I've got electrons. If they are going to go from three to two,
But, maybe I've blown both of these out. Maybe I've blown more than one
I need at least one vacancy. I could have a vacancy here.
of these out so, you put it all together,
So, there's maybe, instead of eight electrons there is seven.
and it turns out that the number has to be somewhere on the order of what?
Eight, seven, something like that?
And, sure enough, the number is 7. .
So, there is a physical basis for the screening factor.
Sigma is termed the screening factor. The screening factor
defines a Z effective, not the Z. There's a Z effective.
It's what's mediated. And to show you, just to get a sense of how
powerful Moseley was, here's the thing. I told you that
I've had it drilled into my head that the lambda of copper K alpha is
1.5418 angstroms. There's not many things I know of
five significant figures. I know my Social Security number to
There's no question this man should have won the Nobel Prize.
nine significant figures. But I know this one to five
He should have won it right away. But he didn't. Mosley was very
significant figures. And, if you use Moseley's law,
politically active. And, he decided when World War I
lambda of copper K alpha for Moseley is 1.546 angstroms.
broke out to enlist in the British military. And,
And, the delta here is 0. %, one third of one percent.
Rutherford was furious. Rutherford called the Secretary of
War and tried to get Moseley a desk job and keep him in London.
And, Moseley refused. So, he ended up in the military,
and one of the major battles of World War I was the Battle of Suvla
Bay, which was part of the Gallipoli campaign, which was managed by none
other than Winston Churchill. This cost Churchill his political
capital. It took him years to recover from the debacle of the
1915, at the age of 27, Moseley was killed in action.
Battle of Gallipoli. The Battle of Suvla Bay took a
And, they don't give the Nobel Prize posthumously.
quarter of a million lives on the Allied side, and about a third of
So, no Nobel for this, one of the most brilliant pieces of
the million lives on the Turkish side, over half a million people
work in those early days of the 20th century. Bohr said that World War I,
killed in that campaign. And, on August 8,
he called World War I a horrible spectacle inflicting great
losses on humanity. But the number one loss to the world
was Henry Gwyn Jeffreys Moseley. So, that's why you won't see
Moseley's name among the physics Nobelists. Well,
OK, so we should have something that looks like this: K alpha,
this is brilliant work. Is there any data? What's the data?
Well, what should this look like? If we take this set of lines and we
make a spectrum, the spectrum will be intensity,
right, intensity versus some energy coordinate.
K beta, and then over here, L alpha, L beta. That's what it
should look like. And, so just to be clear,
And, so we say that this spectrum is characteristic of the target,
we have an emission spectrum that is quantized. It's quantized because
the energy levels are quantized. So, we have a quantized emission
and I saw an x-ray spectrum, and I saw that this was 1.54 Å,
spectrum. That's what we should expect to find.
It's a function of the atomic number of the specimen.
I go its copper because as Z changes, so does the element.
or of the anode. And, I told you last day, if I walked into a room
It's additive. Look down here at brass.
Brass is an alloy of copper and zinc. So, there's the copper lines.
And in brass, you see the copper lines, and you see the zinc lines.
So, by this technique, I could give you an alloy consisting of three
In fact, look at how sensitive this technique is.
elements. And, you could see from where those lines
are what the elements must be that are present in the alloy.
And, the relative intensity of the lines must be related to the
relative concentrations of this constituents that are present.
This is so important. It's amazing. All right,
so let's look at the data. Here's the data. This is data from
molybdenum. Now, remember, this is what we are
expecting, and this is what we've got. So, it's there's something
else going on here. It looks as though what we have is
a combination of, it looks like we've got a
combination of this plus this. So, this here is our characteristic
spectrum. It's as though our characteristic spectrum is imposed
upon something else. And this something else is
continuous, right? There's no break in this curve.
So, this is a continuous spectrum. This is a continuous spectrum.
There are various ways of describing the shape.
It's certainly asymmetric. But since we're in New England,
we call this whale shaped. It's a whale shaped. And,
So it's not quantized. And, furthermore, if you look at
it's got a very steep, it's got a vertical rise here.
I want you to note the shape. It's not just arbitrarily drawn.
this figure carefully, you'll see that there is a family of
It's a steep vertical rise, and then an asymptotic tail, and of
these. There is a family of these, and they are enveloping one another.
course a maximum skewed asymmetrically.
They envelop one another as shown here so that as the plate voltage
increases, the height increases and the minimum wavelength decreases.
So, if this is wavelength increasing from left to right,
What's the physics of the spectrum? Oh, by the way, and the highest one,
you see the K alpha, K beta designation? If you didn't
this means that energy is increasing from right to left,
know what they were, you just took the value of K alpha,
which makes sense. As you go to higher plate voltage, you
it looks like it's around, oh, about three quarters of an angstrom by my
go to higher energies. But, what's going on here?
eyeballing it. It looks like it's about three
quarters of an angstrom. And, if you take the calculation of
Moseley's Law for lambda of molybdenum K alpha.
You know that molybdenum is Z equals 42 for Moly. Z equals 42; plug them
in and you get 0. 2 Å. So, that seems to be OK.
So, in other words, Moseley is predicting the peaks.
OK, so what's going on here? How can we explain this? What's
And, the peak, by the way, is I'm drawing a straight line like
the underlying physics here? Well, let's take a look. Here's a
this. But in reality, these peaks have some finite width
body centered cubic crystal, as is molybdenum. So, let's say the
because real materials are not perfect. And so,
plane of the table is the anode. And, the electrons are zooming in
there's going to be some variation in the spacing and the energies.
from the cathode in the ceiling. So, the electrons are coming down.
They're coming down, and smashing into this. And,
something's going on that causes the emission of x-rays
in all directions. And, we've just seen,
it's OK, you've seen in the central board, there, what those energies
should be, the discrete energies. But let's see what else is going on.
Let's zoom in at the free surface of the molybdenum.
Here's body centered cubic. And, the electrons are coming in.
Let's say electrons coming in from the top, and electrons charge
negatively. And, we know that this is net neutral.
But, there is an electron cloud. And so, we've seen Born repulsion in
the past. We're going to see it again. The mutual repulsion of the
electrons in the molybdenum atom with the incident electron that's
just arrived long distance from the cathode is going to cause some
repulsion. And it's going to be a deflection. Another electron might
But, when a charged particle accelerates, that causes emission of
come in a little closer. If it comes in closer, it might be
deflected more. Now, when a charged particle
changes direction, you know, any particle,
forget charged particle, a change in speed or a change in
direction represents an acceleration.
radiation. So, every one of these changes in
So, what kind of radiation? Every one of these comes off.
direction, change of direction we term acceleration.
That's point one. And, point two, charge accelerating
energy differences. Modest deflections,
produces radiation because it's an energy change.
You might even get one that comes in sort of a la Rutherford that comes
They give off a photon. And, different deflections mean different
in and gets turned around. So, what do we see? That could
explain what's going on here, where we have at the one extreme,
modest difference, intense deflection, intense difference.
this is very low. This is a high wavelength, low energy.
So, this is low energy; this is low angle deflection.
And, this is the most common. This is the most common angle.
And, over here is very high angle, and there aren't many of them. And
we can't predict what this line is. There's no way we can calculate
And, that one we can calculate, for that one we know that if we take
that curve except for one point on it. And, that's the remote
possibility that an electron comes in dead-on, stops,
is captured by the molybdenum atom, and gives up all of its kinetic
energy and converts it to photon energy.
the energy of the incident electron, the total energy. Total kinetic
energy is equal to what? The product of the charge on the
If we do that, we can invert this and get the value
electron times the plate voltage. The charge on the electron is the
elementary charge, and convert that into the energy of
the emitted photon, which is equal to hc over lambda.
for this point here, which is lambda of the shortest
And, if you plug in Planck's constant, speed of light,
wavelength, which is the wavelength represented by the conversion of the
total kinetic energy of the incident electron, lambda of the shortest
wavelength, then, will equal hc over e times V.
elementary charge, you end up with 12,400 over plate voltage in
angstroms. You know I am not fond of the nanometer.
I insist on using the angstrom in defiance of Systeme International.
here. There is another term for this phenomenon of changing
So, it's 12,400 divided by plate voltage. So, you can see that if
directions. And it comes from the idea that we are decelerating the
you have on the order of 10, 00 volts, 10,000 V will give you a
incident electrons, decelerating not by changing
shortest wavelength of 1. 4 angstroms, smack dab in the middle
velocity, but by changing direction. And, it's the German term
of the x region of the spectrum. And that's what you're seeing up
Bremsstrahlung. Brems means to brake as in putting
on the brakes of your car. Brems means to brake, and strahl is
So, the bremsstrahlung plus the characteristic spectrum give us what
the word for ray. Strahl means ray.
So, this literally means braking radiation. It's the radiation from
the braking of the incident electrons, braking radiation.
we see in the figure up here. Last topic I want to cover is
something on instrumentation and safety. A number of you approached
Well, the answer is it was unsafe, and many of them died as a result of
the radiation exposure because they didn't know at the time what was
me after the last lecture and asked, with incredulity, how could these
going on. So, I want to show you several things
people work with these generators exposed to x-rays?
that have been done in order to make things a little bit safer,
These were clear glass tubes emanating in all directions x-rays.
and also to be more efficient. And, what I'm going to show you is
the design by Coolidge in 1913. And Coolidge made the following
improvements. And, I think I've got a cartoon up here.
This is taken from one of your readings out of the supplement.
This is out of the Stout and Jensen book. So, this is figure 1.
. What I did is I turned it on its side so it would look like all of
our gas tubes. Every gas tube I've ever drawn for
He got the gas pressure way, way down. And, this did two things.
you has the anode on the right. So, just to orient it I lay it on
First of all, there's no glow in the visible. And secondly,
its side. And so, the anode is on the right.
it's a much higher efficiency because when you have glow,
The cathode is on the left. And now, what I want to do is
what it means is that some of those electrons leaving the cathode are
highlight what the improvements have been. So, the first thing that
not getting to the anode. They are colliding with gas
Coolidge did is he used a bona fide vacuum tube.
molecules and consuming their energy. So, by the time they get to the
anode, they don't have enough punch to kick out inner shell electrons.
So, the vacuum tube is an improvement. The second thing is
you have to rip the electrons out of the cathode. And the electrons are
the thermal energy then weakens the bonds. And so,
Now, how's he going to heat the cathode? Let's think about this.
hot cathode. What happens with a hot cathode? You know,
So, a torch won't work in a vacuum. So, I wonder, maybe if I passed a
bound. So, if you raise the temperature of the cathode,
it takes less Coulombic force to rip the electrons out.
You can't use a torch because you've got a vacuum inside.
current, and sure enough, what he does is he's got a second
power supply. You see, here's the anode over here.
Here's the cathode, and there's the 30,000, 40,000 volts between the
And, all it does is it makes this cathode filament through joule
cathode and the anode. And, the electrons boil off here
heating, raise its temperature. And that makes the electrons less
and zoom from left to right. And, he's got a second little
circuit here. You can put more than one electrical signal through an
tightly bound, so it's easier to boil them off.
item. So, let's put a second circuit through here.
So, the hot cathode, by raising temperature, reduces binding of the
And, this circuit is a little heater circuit.
electrons. And, you're trying to get rid of them.
The third thing he did was water cooling.
You've got all of these electrons. They've zoomed across at an
acceleration voltage of 30, 40, 50,000 volts. They're crash,
crash, crash, crash, hitting that copper or molybdenum anode.
Where is that kinetic energy going? Some of it's going into light, and
some of it's going into heat. And, the temperature of the anode
is rising to the point where you could melt this.
So, what Coolidge did is he put the anode on a hearth, and
the hearth is copper.. And underneath,
there's water running through copper tubing. So, by water cooling,
he's able to keep the anode cool, so, dissipate heat, and the second
If you've got continuous current, that means you've got continuous
thing, remember, Roentgen used pulse power.
It was eight times a second: bam, bam, bam, bam. If you've got water
x-ray emission. You don't want an x-ray flashbulb.
cooling, now you can go to continuous power,
You want an x-ray beam. And, the last thing he did is probably the
dissipated heat, and allowed for continuous current flow.
most important thing is he put lead shielding, which I've indicated in
yellow, he put lead shielding and beryllium windows.
Beryllium windows for efficiency, and lead shielding for safety.
OK, now let's think about it. Let's think about why we would
choose these. Why would you choose lead for the shielding and beryllium
Beryllium is the first metal that can be used sensibly in our
for the window? For that, I decided,
atmosphere. And, look at lead. Lead is down here.
why don't we go back to our friend the periodic table?
And remember, it's 1913 so forget about the actinides.
Look at where beryllium and lead lie on the periodic table.
So, you've got bismuth, polonium, and astatine. So,
Beryllium is the lightest practical element. Hydrogen and helium are
lead is effectively the heaviest practical element.
gases, and lithium as a metal is unstable in moist air.
Why would you choose this? Why would you choose lead as your
shielding? What do we know about the energy levels in lead?
First of all, there's lots of them because you have not
only K, L, M, and N. You've got n. You keep going,
oh. So, you've got plenty of levels. And, the levels are very closely
spaced. So, if you've got lots of energy levels,
and they're closely spaced, then that means when an x-ray comes
So, it moves you out of the x region. You still get radiation,
in, you'll have excitation and cascading down into much,
much longer wavelengths. So, because of the extra energy levels,
this causes the lead to act as a frequency shifter.
but it's not so toxic. I mean, you have conservation of energy here.
But, I mean, you don't have things coming at you into x region of the
spectrum. So, if there were a shortage of lead,
or, let's say, for the man or woman who has everything,
when he or she goes to the dentist, what would be a really chi-chi
material to make the vest out of? Mercury is a liquid at room
temperature, bad choice. Thallium: toxic. How about gold?
How about a vest made of gold? It would work. That would be a classy
vest. Now, how about up here? Beryllium window: beryllium is the
inverse. Beryllium has very few energy levels and they are far apart.
So that we don't want the x-rays absorbed as they are
going to the window. So, if you had a window that wasn't
crystalline beryllium, you run the risk of losing some of
your x-rays. So, your efficiency goes down.
This painting, anybody recognize this one?
So, you choose a low z element for the window, and the high z element
for the shielding for those reasons. It all goes back to the internal
structure energy levels. OK, well, let's take a look at
for an insurance company in Boston. Millet was very popular with Boston
another use of x-rays, x-rays in art.
The Angelus by Jean Francois Millet painted in 1857-1859 on commission
This painting was commissioned and hung here in Boston.
Brahmins because he painted rural life in 19th century France.
It's now at the Musee d'Orsay. Salvador Dali had to paint this as
part of his art education. He hated this painting, and in 1963,
it was hanging in the Louvre. And he had the painting x-rayed.
here, the little basket of potatoes was not the basket of potatoes in
He said there's something spooky about this painting.
the original painting. The original painting depicted the
Well, Dali had such authority that he could ask for the x-ray,
casket of a baby. This is showing the poverty,
and they x-rayed it. And they discovered that,
the futility, of peasant life in France. And, these people are so
indeed, the painting had been painted over. It's
poor that they just lost their baby. Now, you can imagine that this
not an art forgery. It turns out that this zone down
painting comes to Boston. They unveil it, and they say,
my God, we can't hang this in the lobby of an insurance company.
So, my theory is that they sent it back and Millet painted it over,
put a basket of potatoes. But, their heads are not bowed in the
form of saying a prayer thanking God for this miserable bounty of
potatoes. It doesn't make sense. So, x-rays figured out what was
going on. So then, Dali got a chance to paint.
And this is his revenge. This is his painting in 1935.
This is Dali, and this is his father. You notice in this case,
the man is taller than the woman. Here, the woman is taller than the
man. There's all kinds of Freudian stuff going on with where he's got
his hat placed, but I don't have enough time to go
into that. So, you can see. While we're talking
So, you see a bunch of Venuses facing forward,
about Dali, here's the hallucinogenic toreador.
And, I want to show you this because it hearkens back to crystal
structures. He went out one day in Manhattan to buy some Venus pencils.
They're pencils, but the Venus was the manufacturer.
See, I have the Venus de Milo on the pencil box.
and some facing back. Can you see the breast here is the
nose of the toreador? Here's it's mouth; does that help?
OK, those are the gadflies of St. Narciso, which is I think the patron
saint of Catalonia. You see, here's his cape,
and there's the bull. And, you can see symmetry planes here.
OK, there's the head of the thing, OK. There's the bull, yeah, yeah.
OK, now look, here's the symmetry. See, these Venuses are forward.
These Venuses are facing backward. What else do we have?
I don't know why it's doing this. OK, now, there's the bull. What's
the crystal structure? See, this is the life force leaving
the bull. What's the crystal structure? If you ignore color it's
simple cubic. You can even see symmetry here.
You see the shadow of the fly? You see these two atoms here, these
two atoms here? There's even something.
You see the shadow of the Venus actually looks a little bit like,
come on, come on, come on. It looks a little bit like this.
He really hated this. OK, we're going to show you one more and go
through this. Ah, this is his tribute to Watson and
And so, he has this group of people all arranged in cubic arrays with
Crick. Over here, you have the DNA double helix,
and what he's portraying here is that now that we have the capacity
to understand how life is encoded, replication, and so on, ironically,
humanity hasn't figured out how to stop killing itself.
muskets pointing at one another. Again, crystallography, you could
say actually this is simple cubic, but if you put somebody inside, it
would be body centered cubic. [LAUGHTER] On the last thing,
lithium, why is 7-Up called 7-Up? When it first came out it was
called lithiated lemon-lime soda. It contained lithium citrate in
large quantities. It was supposed to make you happy.
It was an anti-depressant. And, it turns out that some people have a
sodium deficiency. Lithium's a smaller ion.
If you're already sodium deficient, lithium gets in there, displaces a
sodium; you die. [LAUGHTER] So, after some tens of
people died, they took the lithium out around 1950.
So, you don't have lithium in your 7-Up today. I'll see you on Friday.