Atomic Physics

Atomic Physics


– [Narrator] In the year
1826, Sir Humphry Davy, as president of the Royal Society, presented Mr. John
Dalton with a Royal Medal upon his development of the
theory of definite proportions, usually called the atomic
theory of chemistry. This presentation set an
official seal upon the emergence as a scientific theory
of an idea of matter which had existed for over
2,000 years as a belief, unsupported by experimental proof. (people applauding) In Dalton’s time, chemistry
was becoming an exact science, thanks to accurate methods
of weighing and analysis introduced in the 18th century. Dalton was able to work
out fairly accurately the relative combining
weights of many elements. In his new system of chemical philosophy, published in 1808, Dalton pointed out that these combining
weights, which are constant for each particular
compound, must be related to the weights of the ultimate particles, or atoms, of the combining elements. Thus the atomic theory was reborn. According to Dalton, matter
is composed of small particles which cannot be split up any smaller. These particles he called atoms. Our diagrams illustrate his ideas, but not, of course, the
real shapes of atoms. He believed secondly that all atoms of one element are exactly alike, and differ from the
atoms of other elements, and have a characteristic relative weight. Thirdly he believed that atoms are indestructible and cannot be created. Fourthly, that chemical combination occurs between small whole numbers of atoms. Such combinations of atoms
are now called molecules. In any given compound,
all molecules are alike. Dalton invented a system of symbols to represent the atoms of
the different elements. He used these to show how the atoms combined to form molecules. Though some of his
conclusions proved wrong, atomic theory was the
foundation upon which the chemistry of the
19th century was built. The painstaking chemical
analysis based on Dalton’s theory and research arising out
of Avogadro’s hypothesis regarding molecules and
gases enabled accurate atomic weights to be
assigned to the elements. A connection was sought
between atomic weights and chemical properties, and in 1869, this was supplied by
Mendeleev’s periodic table, which arranged all the elements
then known into families according to their atomic
weights and properties. Thus the column headed by
carbon contains a group of elements with thusly
similar properties. The next column contains another group of similar elements, and so on. Now from left to right,
the elements are arranged in ascending order of atomic weight. The whole table fits
together like a puzzle. The properties and place in
the table of unknown elements were predicted, and the
discoveries of later years proved the accuracy of these forecasts. The decomposition of solutions
by an electric current was used as a means of chemical analysis in the early 19th century. By careful measurement,
Michael Faraday was able to show that in such
cases, a definite quantity of electricity was associated
with definite amounts of the elements produced
at the electrodes, that is, with definite numbers of atoms. Hence he expected that electricity itself might be atomic by nature. It was established that there
were two kinds of electricity. Unlike charges attracted each other. Like charges repelled each other. By convention, these two
kinds of electric charge became known as positive and negative. In a solution like common salted water, the sodium chloride exists as ions, or charged atoms, of sodium and chlorine. When the solution is electrolyzed, the positive sodium ions
travel towards the cathode, and the negative chlorine
atoms to the anode. The work of many scientists
on liquids and gases led to the theory that in all matter, molecules are in a state
of perpetual motion. The botanist Robert Brown
was the first to see in his microscope what is
now called Brownian motion. This is the intricate motion
imparted to find particles in suspension in a liquid
by the impact of molecules. One can imagine the tiny
particles being pushed about by these ceaselessly active molecules. Each molecule is so small that
27 million million million are contained in one cc of gas at nought degrees centigrade
at normal pressure. The thinnest soap bubble that
can be made by ordinary means is about 1/100,000 of a centimeter thick. In this space, about 1,000
molecules could lie side by side. Each of these molecules would be three or more times the size of a small atom. This much was known at the
close of the 19th century, which Lord Rutherford
sums up in these words: – At the close of the 19th century, the labors of the chemist
had resolved the matter of our material world into
80 or more distinct elements, and the atoms of theses
elements appear to be permanent and indestructible by the
forces in their command. – [Narrator] The physicists
of the 19th century were not less active than the chemists. Faraday investigated the conduction of electricity by liquids. Later scientists investigated the passage of electric currents through
gases at low pressures with this kind of apparatus. Air is pumped from the tube until a luminous discharge
passes from cathode to anode. As evacuation proceeds, brilliant rays are seen streaming from the cathode. The rays become invisible
as pressure is reduced, but they still cause a faint green glow on the walls of the tube. This bell-shaped vessel shows the behavior of the cathode rays more clearly. At first the current
passes from the cathode to the edge of a fluorescent
screen beneath it. Next, the glow of the cathode rays is clearly seen around the cathode itself. As the pressure continues to
fall, the rays will disappear. But their presence is revealed
by a powerful green glow when they strike the fluorescent screen. These cathode rays can be made to cast a shadow of an object in their path. This proves they travel in
straight lines from the cathode. The rays are capable
of transmitting energy. The little vanes in this
tube are turned by energy supplied from the motion
of the tiny particles rushing up from the
glowing cathode beneath. It can be proved that the
rays are negatively charged. A wire connected to a plate
on which the rays fall will discharge a positively
charged electroscope. In 1897, Sir JJ Thomson
devised an apparatus to measure the ratio of charged
mass of these particles, which had been named electrons. He was able to make an estimate of the actual mass of the electron. In this apparatus, a luminous spot showed where the electron beam
from the cathode fell on the fluorescent coating
on the end of the tube. This beam could be
deflected in one direction by an electrostatic field
between charged plates, and in the other direction,
by a magnetic field. These fields could be adjusted
to balance each other, and from their strengths, the velocity of the particles
could be calculated. Then, from the strength of
the magnetic field alone and the deflection caused by it, the ratio of charge to
mass could be found. From experiments such as these, the mass of the electron
has been estimated at 1/1,8040 of the mass
of the hydrogen atom. Sir JJ Thomson’s own
description of this tiny object has been preserved for us. – Could anything at first
sight seem more impractical than a body which can
only exist in vessels from which all but a minute fraction of the air has been extracted, which is so small that its mass is an insignificant fraction of the mass of an atom of hydrogen, which itself is so small that a crowd of these atoms, equally numbered to the
population of the whole world, would be too small to have been detected by any means then known to science. – [Narrator] Thus the existence
of a negatively-charged, subatomic particle, the electron, was established beyond all doubt. It was even found possible to photograph the track of an electron. This was done by an ingenious apparatus invented by Professor CTR Wilson. During the War, most people saw the vapor trails left
by high-flying aircraft. Wilson, while investigating the ionization of gases by charged particles,
built a chamber in which such particles could be
made to leave a vapor trail. It consisted of a glass-topped chamber filled with moist air. The bottom of the chamber is really a piston head, supported
by the air beneath. Connected by a valve to the main apparatus is a large vacuum chamber. When the valve is opened,
the air beneath the piston is sucked into the vacuum,
and the piston drops sharply, creating an expansion of the moist air so that a cloud is formed. Now a charged particle from a
source of radiation entering the chamber will ionize
the gas along its path. Because of the certain
expansion, drops of moisture will condense on the
charged atoms or ions along the path of the radiation
and lead a visible trail. This is photographed by a
synchronized electric flash. This apparatus, much improved
since the original model, has proved invaluable in atomic research. Today, the electron is a commonplace. Any metal raised to red
heat gives off electrons. The stream of electrons
from this hot filament can discharge an electroscope. This is made use of in the radio valve, where the electrons from the hot filament can be used to send and detect signals broadcast over long distances. On this radar screen, electrons
act as the eyes of a ship as it makes its way down the
crowded river to the sea. And on the television screen, electrons paint pictures
for our amusement. – The electron owes its practical utility to its smallness. It might parody Shakespeare to say, “My use is great because I am so small” – [Narrator] The discovery of
electrons raised big problems. Ordinary matter is neutral. For instance, it has no
effect on an electroscope. Yet the cathode in this discharge tube can give off negative
particles much smaller than atoms which can
turn the disc beneath. Some sort of electrical
equilibrium has been upset by this emission of negative
electrons from neutral matter. Surely positively-charged particles must have been left behind. Investigation of the so-called canal rays, first discovered by Goldstein in 1886, proved that this conjecture was true. If a series of small slits
be pierced in the cathode of a gas-filled tube, a luminous
ray can be seen streaming in the opposite direction
to the cathode rays. These rays were subsequently
found to be positively-charged. Sir JJ Thomson used this
discovery to build an apparatus in which the mass of the
rays could be measured. A fine beam of positive rays were directed onto a photographic plate. A magnetic field produced a deflection of the beam horizontally. Similarly, an electrostatic field produced a vertical deflection. In both cases, the different velocities of individual particles produced
a line instead of a spot. If both fields were
applied simultaneously, the particles fell on
the plate in a parabola. The position of this parabola in relation to the that of the undeflected spot, was determined solely by
the ratio of mass to charge. Thomson was able to calculate
the mass of these particles. He found that each particular
kind of positive ray consisted of charged atoms of the gas
present in the discharge tube. Under certain conditions,
the different types of glow due to positive and cathode
rays can both be seen. These phenomena could only be explained by supposing that the
neutral atom contained both positive and negative
charges in equilibrium. The stream of electrons from the cathode detaches other electrons from the atoms of the gas through which the pass. These atoms immediately become positively charged and
rush towards the cathode. If the cathode is pierced,
they emerge as positive rays. The sequence of events
resembles electrolysis, and these charged atoms
of gas are called ions. A gas in which such conditions arise is said to be ionized. In 1896, Rontgen found that the green glow excited by cathode rays striking the glass was itself giving off rays which caused a fluorescent screen to become luminous. The rays could penetrate light metals, but not dense material like lead. The center piece of metal here is lead. The outer ones are aluminium. Lead stops the rays completely, but the lighter elements are penetrated and cast a lighter shadow. These powerful rays
became known as X-rays, and doctors soon found that
they could use shadow graphs made with X-rays to see
inside the human body. Dentists were able to study the
internal structure of teeth. And industrial X-ray
machines, such as this one, have been developed to
search for hidden defects. But in 1896, inquiry
turned to the fluorescence produced by X-rays in certain minerals. The french scientist
Becquerel investigated whether fluorescent substances could
in turn give off X-rays. He placed a metal medallion
on top of an unexposed photographic plate still
wrapped in blank paper. Next he shook onto blank paper a quantity of a fluorescent salt of uranium, and placed it over the medallion. Sometime later, he developed
the plate and made a print. Radiation from the uranium had produced an image of the medallion. Becquerel had discovered radioactivity. This radiation was
discovered to be a property of any mineral containing uranium. Among such minerals are autunite, torbernite, and, in particular, pitchblende, which was found to be more
active than uranium itself. The radioactivity in this pitchblende ore has revealed itself by its
action on a photographic plate. Pioneers in this new
field of investigation were Marie Curie and her husband Pierre. By sheer hard labor, she
succeeded in isolating from many tons of pitchblende
residue a fraction of a gram of an intensely
radioactive substance. An exhaustive series of
measurements and tests left no doubt that a new
element had been discovered, more radioactive than any before known. The Curies called it radium, and devoted the rest of their lives to the study of radioactivity. The power of this radiation
from radium is easily shown. The scrap of radium embedded
in this lead capsule ionizes the air to such an extent the charge on the electroscope leaks away. The complex nature of this radiation from radioactive substances
was revealed by recording on a photographic plate its
behavior in a magnetic field. A fairly weak magnetic
field produced a deflection of light charged particles
called beta rays. A very strong field
caused further deflection of the beta rays, and in
the opposite direction, a deflection of heavier charged particles, the so-called alpha rays. There remained a penetrating
radiation unaffected by the magnetic field, the gamma rays. By 1902, the alpha ray was known to be a type of positive ray, the beta ray to consist of electrons,
and the gamma rays to be electromagnetic
waves, similar to X-rays. Curie and Laborde discovered
that the rays from radium can manifest their energy
in the form of heat. Here are two dual flasks in
which are tubes containing a radium salt on the left, and
a barium salt on the right. The barium flask also contains
a coil of known resistance, which can be used to raise
the temperature in the flask. Both flasks have thermometers. At the start of the experiment, the left-hand thermometer
shows a higher reading owing to the heat from the radium. Next, an electric current is
put through the heating coil. The heat from the radium is calculated from the amount of
electricity used to raise the barium to the same
temperature as the radium. Such experiments gave an inkling of the great energy locked in the atom. Three years after the
discovery of radioactivity by Becquerel, Rutherford
enclosed a radioactive gas called radon in a thin-walled glass tube, which was surrounded by an outer tube from which all the air had been exhausted. The alpha rays coming from the gas penetrated the thin glass and collected in the outer tube. After a few hours it was found that the outer tube contained the gas helium. Therefore, the alpha particles
were atoms of helium, and this was the first direct evidence of one kind of atom actually
changing into another. – A great change in our ideas
resulted from the discovery of the electron and of the
spontaneous radioactivity observed in the heavy
elements uranium and thorium. Soddy and I were able to show in 1903 that radioactivity was
the sign and measure of the instability of atoms, and that the atoms of uranium and thorium were undergoing a series of
spontaneous transformations. – [Narrator] Uranium, for instance, during its transformation to lead, gives rise to a whole new
series of unstable elements, which ultimately break down into lead. The rate of change from
one element to another may vary from a fraction of a
second to millions of years. Only a few elements are
normally radioactive. The majority are completely stable. – The next problem was to
examine where the means could be found to break up the stable
elements by artificial methods. Before this could be attempted
with any chance of success, it was necessary to have
a clearer conception of the structure of atoms. The idea of the nuclear
structure of atoms, which I suggested in 1911, has proved very useful for this purpose. – [Narrator] Experimental evidence supported Rutherford’s idea. Alpha particles from a radioactive source, if limited by a thin slit, will throw its image sharply
on a fluorescent screen. A sheet of gold foil placed
in the path of the rays causes a blurred image of the slit, owing to the scattering of alpha particles by the heavy atoms of gold. Geiger and Marsden investigated the nature and extent of this scattering. In the apparatus first used,
the source and scattering foil were stationary inside
an evacuated metal box, while the fluorescent screen
with a microscope attached was free to move on a
graduated circular platform. Scattering of alpha particles
continued to be observed as the screen and eyepiece rotated through an increasingly large angle. Only a powerful field
of force could produce such deflections of
heavy charged particles. Rutherford visualized a
small particle in the center of the atom carrying a
heavy positive charge. Since the majority of alpha
rays pass straight through, he deduced that these positive particles were small and widely separated. In 1911, he propounded his
theory of the nuclear atom. The atom, he said, had a small,
positively-charged nucleus. Around this the electrons revolved. The charge on the nucleus exactly offset the charges on the electrons. The Danish physicist Niels Bohr developed Rutherford’s theory. Here are some models
of the elaborate orbits for planetary electrons
which Bohr and others worked out by using the quantum theory and certain bold assumptions of their own. This is the atom of lithium,
one of the lighter elements. And this is the simplest
element of all, hydrogen, with one electron
revolving around a nucleus. They first worked out possible orbits for the singular electron
of the hydrogen atom. They varied from circles
to various ellipses. The conception became
more difficult when it was realized that these elliptical orbits themselves rotated, something like this. The structure of the atom
increases in complexity as we pass from the simple hydrogen atom to helium, with its two electrons
in the same ring or shell, lithium, with a third
electron in an outer shell, beryllium, with four electrons, and so on. Wilson’s cloud chamber provided experimental proof of
the nuclear atom theory. Here are the tracks of alpha particles at the moment of formation. Each of these tracks marks the path of a single alpha particle, a helium atom stripped of its electrons. Photographs show that
the tracks are straight and heavily defined
because of the velocity and relatively large
mass of these particles. Yet some alpha tracks showed
abrupt kinks caused by some force powerful enough to deflect
the heavy alpha particle. These kinks show where an atomic nucleus has deflected the alpha particle. Other photographs show tracks
ending with little forks. It was realized that these
were made by particles after direct hits had been
scored on atomic nuclei. Billiard balls can be used
to illustrate a collision between particles of
roughly the same mass. The white ball represents
an atomic nucleus, the black an alpha particle. When the black ball strikes the white, both are deflected and branch
off on different tracks. This is the sort of thing the cloud chamber photograph shows. The existence of the atomic
nucleus having been proved, the next thing was to find out the magnitude of the charge on it. To see how this was done, we
must turn again to X-rays. We have already seen how X-rays are caused when electrons fall upon any substance. X-ray tubes were built with a target on which the electron beam fell, causing X-rays to be given
off at convenient angles. About 1912, it was found that X-rays could be diffracted by a crystal, just as light is diffracted
by a ruled grating. HG Mosley used this discovery
to build an apparatus to obtain X-ray spectra
of different elements. The electron beam was directed
downwards from the cathode onto the target carried on
a little trolley beneath. X-rays were given off from the target. A crystal enclosed in a
vacuum chamber was so arranged that the X-rays, passing through a slit, struck the face of the crystal. From the crystal face, an X-ray spectrum was thrown onto the photographic plate. Mosley found that each
element used as a target gave off radiation which included an X-ray of a characteristic wavelength. The X-ray spectra showed
that this characteristic wavelength increased smoothly throughout the period system
according to a simple law. The wavelength could be
specified by a number, Q, which increased by one each time he passed from one element to the next. The elements had already
been arranged in a table according to their
increasing chemical mass. Mosley related his Q numbers
to these atomic numbers. He deduced that there was in
the atom a fundamental quantity which increase by regular
steps from one element to the next, and that this quantity could only be the charge on
the positive nucleus. Thus, the number of each
element in the table, the atomic number, as it is called, acquired a new significance. It indicated the positive charge
on the nucleus of an atom, equal, though opposite
in sign, to the number of orbital electrons
in the particular atom. Of all atoms, hydrogen has
the simplest structure, a nucleus and one electron. This nucleus, with its
single positive charge, was christened by Lord
Rutherford the proton. At this time, Rutherford was investigating the possibility of actually
transforming atoms. – It became clear that
to obtain the veritable transformation of an atom,
it was necessary to change the charge or mass of a
nucleus, or both together. Now, the minute nuclei of
atoms are held together by powerful forces, and to
affect their disintegration, it seemed likely that a
very concentrated source of energy must be applied
to the individual atom. The bombardment of the
nuclei by the energetic alpha particles from radium appeared to be the most promising method
for such a purpose. – [Narrator] In 1919, Rutherford
found that nitrogen nuclei could be transformed by bombarding them with swift alpha
particles, and that protons were ejected at high speed as a result. In effect, he added
together helium and nitrogen and changed them into oxygen and hydrogen. This was transmutation, the
dream of the alchemists, to which the nuclear theory
had at last given substance. Most significant of all,
there could be no doubt that the proton, the hydrogen nucleus, was a fundamental particle in all atoms, the positive counterpart
of the negative electron. How is the atom built up
from these atomic bricks? Let us go back to our
table of the elements. The atomic numbers represent
the charge on the nucleus and also number the elements according to their increasing atomic weight. Now consider these atomic weights. One for hydrogen, four for helium, seven for lithium, and so on. Our figures are approximations
to the nearest whole number, but they show that in most
cases, atomic weights are at least twice as great
as the atomic number. How could this difference between mass and charge be accounted for? Consider the atom of one
element, say lithium. It has three electrons
circling round the nucleus. This nucleus, we know, must
have a positive charge of three. So we put three protons in the nucleus. Each has a charge of
one and a mass of one. But the atomic weight of
lithium is seven approximately. So the nucleus must have four more protons added to it to give a mass of seven. Now we have upset the
balance of the charge. Seven protons, seven positive charges. Three electrons, three negative charges. There is only one way to correct this, four electrons must be added. Now our charge is balanced. The negative charge from the
three electrons is balanced by the outstanding positive
charge of three on the nucleus. Such was the theory. But even in 1920, Rutherford suggested that there might be a neutral particle with the same mass as a
proton which could account for the extra mass without
affecting the charge. Ultimately this proved to be another instance of his amazing insight. But in the years between,
the drive to penetrate the secrets of the nucleus
continued unabated. From 1920 onwards, many other scientists pursued and opened up by Lord Rutherford’s transmuting nitrogen to oxygen. Steady accumulation of
data went hand in hand with improvement of apparatus. In this little instrument,
a needle carrying a speck of radium is fixed in front
of a fluorescent screen. Through a magnifying lens,
the screen can be seen to twinkle with tiny points of light. Alpha particles cause these scintillations as they strike the screen. Rutherford’s early experiments
were based on the use of such screens to observe
and count the scintillations. – Progress in our
knowledge of the mechanism of these transformations became more rapid when powerful electric
methods were developed to count automatically the swift particles ejected during these nuclear explosions. – [Narrator] Improved
apparatus to radioactive source could be measured. Inside the brass case is a single wire which serves as an electrode. Suppose that a radioactive
source at the bottom of the case is emitting alpha particles. This is established between
the wire and the outer case. The ionization caused by the
alpha particles is collected, constituting a current
to the central wire. Each alpha particle emitted ionizes the gas along its path into
positive and negative ions. Actually about 100,000 pairs of ions. These are pulled apart towards
the electrodes in the same way as the ions in the
solution being electrolyzed. The tiny charge collecting on
the central wire as a result of continuous alpha particle
emission can be measured. Development of radio
valves enabled the pulse of current from each single
alpha particle entering the chamber through a mica
window to be amplified until it could operate a kind of counter. Then Geiger and Muller
developed an ionization chamber which could multiply the original effect by a principle known as gas amplification. Here is a modern Geiger counter, capable of counting at the
rate of thousands per minute. Linked to it is an electric counter capable of high-speed recording. As the tiny speck of
radium fitted to this rod is brought near the tube, the
dials record the gamma rays from the disintegration
of single atomic nuclei. These gamma rays are actually penetrating the walls of the tube. But charged particles can be counted by using a suitable type of tube. The ionization effect can
be amplified and passed through a loudspeaker so
that each click represents the passage of a single
particle, or, as in this case, the ionization caused
by a single gamma ray. The same voltage will
operate an oscilloscope in which cathode rays trace on a screen the effect of these nuclear explosions. (equipment buzzing) Such sensitive apparatus, atomic disintegration and study, and accurate measurements obtained. About 1920, Aston, working
at the Cavendish Laboratory, designed this ingenious instrument, known as the mass spectrograph. Positive particles, or ions,
produced in the same way as in Professor Thomson’s apparatus, were allowed to pass
through an electric field, which deflected the particles at varying angles according
to their velocity. All particles of the same
mass could be brought to a common focus on a
photographic plate by means of a magnetic field of the
right shape and intensity. In effect, Aston could weigh
atomic nuclei very accurately. Work done with this apparatus showed that most of the elements
accepted as indivisible by the chemist were actually mixtures of two or more substances
of different atomic weights. The atomic weights of
the elements had been very accurately determined
by many chemists. Very few of them could be expressed as complete whole numbers. Aston showed that chlorine, for instance, with its atomic weight of 35.5, is in reality a mixture
in a definite proportion of two forms of the same element, one with an atomic weight of 35, and one with an atomic weight of 37. Such different forms of the same element were called isotopes,
and subsequent research has shown that the number of elements of which isotopes exist
is very large indeed. Little by little, hundreds of
experiments all over the world built up the store of
knowledge about atomic nuclei. As time went by, experimental
evidence and theory both emphasized the increasing probability that there did exist in the
nucleus an uncharged particle of about the same mass as a proton. In 1930, Bothe and Becker in Germany announced that they had noted what seemed to be an unusually
penetrating gamma radiation, produced by allowing alpha particles from polonium to bombard beryllium. The radioactive polonium gave off alpha particles which struck the beryllium. Radiation of some kind
from the excited beryllium gave a high reading in the detector. Frederic and Madame Joliot-Curie measured the penetrating
power of the radiation by inserting screens
of varying substances. If these screens were of
suitable density and thickness, most of them reduced the amount of radiation reaching the detector. But when screens containing hydrogen in the form of paraffin wax were used, a heavy increase of
radiation was observed. In England, Sir James Chadwick confirmed the findings of the Joliot-Curies that this radiation consisted of protons. He suggested that the protons
were ejected from the paraffin by an uncharged particle of about the same mass as the proton. Imagine that this
billiard ball is a proton. No stream of light
particles like electrons, no known type of wave, could
have any more effect on such a body than these rice grains
have on the billiard ball. But another particle of
equal mass would share its energy on collision, just
as this second ball imparts half its energy to the
first as it pushes it away. A particle of unit mass,
but without electric charge, and so very difficult to detect. Chadwick’s reasoning
recalled Lord Rutherford’s suggestion of 1920, and
the idea of a nucleus composed of protons and
the uncharged particles, which were christened
neutrons, was accepted. The field of force around
the nucleus acts as a barrier against the penetrative
power of charged particles. Neutrons, however, being uncharged, can penetrate these barriers
and strike the nucleus. The majority of nuclei are
much heavier than the neutron. If collision occurs, the
neutron simply dances off and continues with a slight loss of speed. Now paraffin is made up
of carbon and hydrogen. Protons, or hydrogen nuclei,
are no heavier than neutrons. So in a collision, the
neutron is able to transmit a large amount of energy to the proton. This is the explanation
of the proton radiation observed by the Joliot-Curies
and Sir James Chadwick. The neutrons at once explain
the existence of isotopes. Chlorine, for instance,
has an atomic number of 17. That is, there are 17 electrons circling around a nucleus in which
there are 17 protons. But chlorine has two isotopes. The atomic weight of one is 35, and of the other, 37. If we add 18 neutrons to 17 protons, we get an atomic weight of 35. And in the 37 isotope, we
have to add 20 neutrons to get the correct atomic weight. An isotope of hydrogen,
called heavy hydrogen, has a nucleus consisting
of the single proton of ordinary hydrogen, plus one neutron. This nucleus, called a deuteron, has proved useful as a
particle for bombarding atoms. – It became clear that
to extend our knowledge, a more copious supply
of bombarding particles of different kinds was necessary. Charged atoms of various
sorts can be produced in vast numbers by the electric
discharge through gases, and then accelerated by
the use of high voltages. In this way, we have been able to obtain through our experiments in transmutation intense beams of protons
and alpha particles, while the discovery of
hydrogen has given us a new projectile of remarkable
efficiency in transmuting atoms. – [Narrator] Here, at
the Cavendish Laboratory in Cambridge, the experts
of this modern alchemy of transmutation go about their business. The purpose of these
huge electrical machines is simply to use electrostatic
forces to accelerate beams of charged particles
to very high energies. And with these atomic bullets, to bombard the atoms of various elements in order to see what changes
occur in their structure. This laboratory grew out of
the converted lecture room, where in 1932, Cockroft and
Walton operated one of the first successful machines to
produce high-speed particles, and obtained significant
results by bombarding lithium. The machine you see is a
descendant of their original set. Ions are produced inside the
large mushroom-shaped insulator and are accelerated
down the rearmost tower. The bombarding tube continues
down into the room beneath, where the stream of charged particles can be bent by a powerful magnetic field and directed towards the target. In the Science Museum of South Kensington is preserved the observation cabinet and accelerating tube of the original voltage quadrupler used
by Cockroft and Walton. Three cylindrical metal
electrodes are arranged vertically inside a glass tube where
a high vacuum exists. Protons emerged from a narrow positive ray canal at the top of the tube. As the protons descended, they passed through
intense electrical fields by which they were accelerated
and sharply focused. At the bottom of the tube, they struck the element set up as the
target for bombardment. Any particle ejected
from the target element passed through a mica window, and caused scintillations
on a fluorescent screen, which could be observed
through a microscope. With lithium as the target, a
startling result was obtained. Alpha particles of high
energy were ejected. This result received wide publicity. Through the press, the public
imagination was stirred by the vague implications of
the phrase, splitting the atom. But to understand the
significance of this result, it is necessary to
appreciate one of the most important aspects of Einstein’s
theory of relativity. Let us hear Professor Einstein himself. – It followed from the
special theory of relativity that mass and energy are both but different manifestations
of the same thing, a somewhat unfamiliar
conception for the average man. Furthermore, the equation
E is equal M C squared, in which energy is put equal to mass multiplied with the square
of the velocity of light, showed that very small amount
of mass may be converted into a very large amount
of energy, and vice versa. The mass and energy
were in fact equivalent, according to the formula mentioned before. This was demonstrated
by Cockroft and Walton in 1932 experimentally. – [Narrator] In quiet, noncommittal terms, the scientists summed
up their results thus. It seems not unlikely
that the lithium isotope of mass seven occasionally
captures a proton. And the resulting nucleus of mass eight breaks into two alpha particles, each of mass four, and each with an energy of about eight million electron volts. This reaction, which you see
here, can be written down. We start with lithium of
mass seven, placing seven at the top left corner
of the lithium symbol. Its nuclear charge is three, so we add the figure three
at the bottom left corner. The lithium is bombarded by protons. That is, hydrogen is added rather forcibly to the lithium, so we put down plus H. Hydrogen has a mass of
one and a charge of one. As a result of the reaction, we get two alpha
particles of helium atoms, each with a mass of four
and a charge of two. Note that the two sets of numbers add up to the same on each side of the reaction. Seven plus one equals four plus four. Three plus one equals two plus two. The actual masses
concerned in the reaction are known from mass spectra. On the left they total 8.0241 mass units. On the right, 8.0056 mass units. There is a difference
of 0.0185 mass units. Measured in grams, this amounts to 3.07 times 10 to the minus 26. It is this amount of mass
which has been converted into the energy released in the reaction. In other words, the relationship between mass and energy is a constant, and is expressed in Einstein’s equation, E equals M C squared, in which energy is put equal to mass multiplied by the square
of the velocity of light. So, to find the energy released, we substitute for M the amount 3.07 times 10 to the minus 26. And for C squared, we substitute three times 10 to the 10, all squared, which is the square of the speed of light in centimeters per second. Thus, the energy released expressed in ergs is the product of these values. Namely, 27.6 times 10 to the minus six. Now the energy of each
alpha particle released in the reaction can be
directly calculated. It is 8 1/2 million electron volts. Or, more conveniently, 8.5 MeV. Thus in our equation we get E equals the sum of these energies. That is, 17 million electron volts. Measured in ergs, this becomes 27.2 times 10 to the minus six. Thus one calculation gives
27.2 times 10 to the minus six, and the other, 27.6 times
10 to the minus six, a sufficiently close approximation to show that Cockroft and Walton were correct, and that great energy can be
released by splitting the atom. Man has set his foot firmly on the road towards the unlocking of
great knowledge and power. All over the world, atomic physicists sought machines of
higher and higher voltage with which to reach a goal
that was still hidden. This machine is known as a synchrotron. In it, electrons are whirled
around at fantastic speeds by new methods of using
small high-frequency currents combined with powerful magnetic fields. High-speed electrons cause gamma radiation which is betrayed by
this sensitive detector. Even a thick screen is easily
penetrated by this radiation. Energies as high as eight
million electron volts can be obtained on this machine. And in the United States, electrons of 100 million electron volts
have already been produced. Giant synchrotrons are now being built to accelerate positive ions. Hidden behind six concrete
barriers to absorb the dangerous gamma
radiation is this latest type of linear electron accelerator,
in which electrons pass through a series of alternating
magnetic fields arranged in line along a corrugated
tube known as waveguide. Eventually this machine will
hurl electrons at its target with an energy of five
million electron volts. Older in conception, but
impressive in size and shape, is this electrostatic generator,
designed to accelerate positive particles such
as protons and neutrons. And this is one of the newest Van de Graaff machines
to be built in England. These are so called after
the American scientist who first designed them. Electric charge is sprayed
onto a moving belt inside this tower and carried to the
top of the accelerating tube, where the high potential thus
generated is used to drive the particles down towards
the target at great speed. When in operation, the machine is enclosed in a high-pressure casing
which permits a potential of two million volts to be built up. Most impressive of all in their suggestion of a Wellsian world are these million and two million volt generators in the Cavendish Laboratory at Cambridge. This is a cyclotron, one of the most famous
of these accelerators. The inventor was Professor EO Lawrence of the University of California. The accelerating chamber or tank is fixed in the middle of the apparatus between the poles of a very
powerful electromagnet. The principle is ingenious. The tank, which is completely evacuated, contains two hollow D-shaped electrodes to which a high-frequency
alternating current is fed. Minute quantities of the gas, which is to provide
the ions, are admitted. The ions are produced in
the center of the tank by electrons from the
small arc, or filament. Once produced, they are
sucked into one or other of the Ds by the powerful electric field. But this movement of the ions takes place between the poles of a big magnet. Consequently, the ions will travel with a circular motion
in the magnetic field. The alternating voltage
accelerates the particles each time they cross the
gap from one D to the other so that they are whirled
around faster and faster until finally they are
led off towards the target by means of a charge deflector plate. Here an element is being set up and bolted into the target chamber
ready for bombardment. Nowadays it is known
that almost all elements, if bombarded by neutrons, can be made radioactive
for varying periods. Irene and Frederic
Jolio-Curie first produced artificial radioactivity,
and Professor Fermi made brilliant advances in this field. As the operator moves off
towards the control panels, he passes through the water
screens, 3 1/2 feet thick, which protect the scientists
from the intense radiation produced by beams of
accelerated particles. Now he begins to switch on the complicated high-frequency installation
which feeds current to the Ds. To build and operate such machines, atomic physicists have become engineers, overcoming step by step
problems of high vacuum, screening from harmful radiation, construction of special
magnets, and so on. For the production of neutrons, deuteron beams are usually
found the most convenient. When the deuteron strikes the target, it splits up, liberating a neutron, which easily penetrates to the nucleus. At last the full power begins to flow. Through the observation window can be seen the arc in the center of
the tank, and round it, the pulsation of the
accelerating particles. Some idea of the neutron’s
effect on a nucleus can be given by this simple experiment. Imagine that the saucer
is the nucleus of an atom. One might think that the
neutrons would go straight in and out on the other side, like this. Now let us fill the saucer with marbles to represent the particles in the nucleus. If we let the neutron marble
roll down into the nucleus, it will jostle the others
but remain in the saucer. The nucleus has captured a neutron. It becomes unstable, and
gives off its excess energy as beta and gamma radiation. If the neutron marble enters
the saucer with a somewhat greater energy, one or more
marbles will be knocked out. Thus, neutron capture can eject particles from the nucleus and cause disintegration. Outside the laboratories,
the world was disturbed by the strident voices of evil men and the ever louder
rumble of approaching war. But in 1939, atomic scientists
made a fateful discovery. Professor Frisch, now at work in England, was closely concerned in this. – Early in 1939, Professor
Hahn and Dr. Strassmann in Germany found that uranium bombarded by neutrons gave rise to what
appeared to be an isotope of barium only about half
the atomic weight of uranium. Professor Lise Meitner,
who had previously worked with Hahn and Strassmann,
was present in Sweden. He told me of their puzzling discovery, and together we considered the possibility that the uranium nucleus
sometimes broke into two halves. We found that such a process,
although quite unexpected, was indeed compatible with what
we knew about atomic nuclei. If we were right, a great amount of energy had to be released in such a process. And I was able to show
this experimentally. – [Narrator] Hahn and
Strassmann’s evidence that a disintegration
of the unstable uranium gave rise to elements such
as barium, in the middle of the atomic table, was
purely chemical, and required a physical confirmation
before it could be accepted. The uranium nucleus is
unstable and radioactive, as is known from its ability
to emit an alpha particle. If a uranium nucleus could
indeed capture a neutron, becoming more excited and
splitting into two approximately equal fragments, it was to be
expected that a large amount of energy would be
released in such a process, the fission fragments being hurled apart with great velocity. Professor Frisch obtained
confirmation that this was so in an experiment essentially
similar to the one shown here. Uranium was placed in
the ionization chamber, which was placed over the
end of a bombarding tube through which high-energy
deuterons could be sent. The chamber was connected, as
you see, to an oscilloscope. The trace you see here is
made by alpha particles from the radioactive uranium. Now the high-tension set is switched on, and the uranium is bombarded by neutrons. On the screen of the oscilloscope, the fission pulses
reveal the large amounts of energy released when
uranium atoms are split. If each fission had to be
provoked by elaborate apparatus like this for accelerating
particles, there was no hope of deriving useful
energy from the process. But each fission was
found to be accompanied by the release of a few neutrons. One neutron had been used
to provoke the fission. The fission itself produces
more than one neutron, and is thus itself capable of provoking more than one fission, and so on There is thus the possibility of a self-sustaining chain reaction. The reaction can begin without any neutrons being supplied from outside, since a few fissions are always
taking place spontaneously. These would supply the fuse neutrons. While these facts were being investigated, the political high tension in Europe sparked over into actual war. Years of aggression had at last forced the Western democracies to challenge the attempt at fascist
domination of the world. Under wartime security,
research on the use of atomic energy was pushed ahead. What was the problem to be attacked? The possibility of a fast chain reaction suggested an atomic bomb. There are two isotopes which make up the bulk of natural uranium. The atomic weight of one is 238. Of the other, 235. Over 99% of natural uranium is U-238. U-238 undergoes fission only
with very fast neutrons. Many neutrons are
scattered by uranium atoms, and lose the speed
necessary to cause fission. So, a divergent chain
reaction will not take place in uranium which is predominantly U-238. U-235, on the other
hand, undergoes fission with neutrons of any velocity. It seemed possible that a
controllable chain reaction might be achieved by using slow neutrons to cause fission in U-235. Using pure uranium, containing both isotopes,
a pile was built, in which uranium rods
were embedded in a mass of pure carbon in the form of graphite. Neutrons produced in the
uranium were slowed down by collisions with the
light atoms of the carbon, the moderator, as it is called, giving them a chance to
cause fission in U-235. A slow controlled chain
reaction was achieved on December the 2nd, 1942. If natural uranium and a moderator were replaced by a small
amount of pure U-235, slow neutron reactions would give place to fast neutron reactions. Fissions would occur. But so many neutrons would
escape before hitting another nucleus that no chain
reaction would built up. But if the amount of U-235 became larger than a certain critical
size, a fast chain reaction would take place in less
than a millionth of a second. This would produce a violent explosion. A simple mechanism for an atomic
bomb thus suggests itself. Take two pieces of U-235, each smaller than the critical size, but which if placed
together exceed that size. They are driven together, and, so U-235 became bomb material number one, and its separation from
U-238 became a war priority. The behavior of U-238 in a pile
is also of great importance. Slow neutrons are sometimes
captured by U-238. No fission is caused, but
this nucleus becomes U-239. This is unstable. A beta particle is emitted,
causing a change to a new element with different chemical
properties, neptunium-239. This new element is also unstable. Another beta particle is given off, forming another completely
new element, plutonium-239. Now plutonium-239 has fission
properties similar to U-235, and will be built up by this process of neutron capture and
subsequent beta disintegration throughout the uranium in the pile. Being a different element,
it can be separated by chemical means from the uranium and used also for a bomb. So, plutonium is bomb material number two. Huge plants for the
production of pure U-235 and of plutonium arose
in the United States. By 1943, it had become
plain that Great Britain was fully extended in war production. In consequence, her nuclear
physicists crossed the Atlantic to continue with American
and Canadian colleagues the brilliant work already done. The main practical effort and most of its prodigious cost now fell
on the United States, where very active research
had been going on since 1939. The processes were
laborious, and at all times, elaborate precautions had
to be taken by workers on the atomic pile and in
the chemical laboratories because of the ever-present dangers from gamma and neutron radiation. Danger signs and warning signals,
lead and concrete shields, remote controls, detectors,
protective clothing, these were the everyday
equipment of the men who worked to make the
material for the bomb. At last, in the desert of New Mexico, the first atomic bomb was set off under conditions of great secrecy. Thousands of calculations, years of work, reached their climax in this
moment of dreadful splendor. Within a few weeks, the
bomb bursts upon Japan, and the news of it upon a
startled world, which has now seen this supreme destroyer in
action five times in all. We are aware now of the compelling urgency of the situation created by the bomb. We know that war must be banished, or else our civilization may die, as Hiroshima died on that
summer morning in 1945. The tale has been told in full by those who lived through it. A city destroyed in an instant. 60,000 people dead, many from flash burns, thousands more by blast, many from strange sicknesses
caused by gamma radiation. For a short while, the world
was content to accept the fact that atomic energy, by
destruction, had brought peace. Then came the question, can this power be controlled to serve mankind? Professor Einstein has
recorded his opinion. – To give any estimate
of when atomic energy can be applied to constructive
purposes is impossible. What now is known is only how to use a fairly large quantity of uranium. The use of quantities
sufficiently small to operate, say, in a car or airplane
is as yet impossible. Presumably, all materials
which may be used for such purposes will be among the heavier elements
of high atomic weight. Those elements are relatively scarce because of their lesser stability. So, though the release of atomic can be, and no doubt will be a
great boon to mankind, that may not be for some time. – [Narrator] The core of the problem is to turn the atomic pile
into a heat engine. The release of energy in a pile results in the generation of heat. If the reaction is allowed
to run fairly fast, a cooling system is
necessary to absorb the heat. To control the rate of
reaction, rods of cadmium or some such substance must be introduced which can absorb neutrons
if lowered into the pile. Now if the cooling fluid,
probably helium gas, were passed through a heat
exchanger containing water tubes, the water could be heated
up and turned into steam. Most of this part of the
installation would have to be screened off by heavy
concrete or water shields and operated by remote control. The steam could be used
to drive turbo generators to provide electricity in a
powerhouse or in a big ship. This adaptation of the
atomic pile as a source of heat energy is now being tackled in the United States and Great Britain. The use of radioactive byproducts from the Pile is also being investigated. Medical research is now
employing new tracer techniques based on radioactive elements. In this test, a radioactive sodium salt is being used to check
blood circulation time. The Geiger counter picks up the radioactivity from the sodium. Now the doctor prepares
to inject the sodium into a vein in the patient’s foot. The radioactivity will be
picked up by the counter as the sodium in the blood
reaches the patient’s groin. The injection is made, and the scientist starts his stopwatch. The radiosodium travels up
the leg in the bloodstream. The counter is shielded
from outside radiation by a lead block with a slot beneath. Now 10 seconds have passed and the counter reveals the
arrival of the radiosodium. From such experiments it is hoped to devise a test to help prevent some of the complications
which follow childbirth. Other possible sources of atomic energy are suggested by what is known of the generation of
solar and stellar energy. Under conditions of
tremendous heat and pressure, complicated nuclear
reactions are provoked, resulting in the building up
of helium nuclei from hydrogen. In this process, large amounts
of energy are released. Perhaps man may one day obtain energy by building up heavier
atoms from the lighter ones. Meanwhile, the representatives
of the United Nations in conference strive for some formula to banish the atomic bomb
and to permit the blessings of atomic energy to be
used for peaceful ends. Great Britain’s Research
Establishment at Harwell is under the direction of Dr. Cockroft. – You have seen the story of a remarkable scientific achievement, based
on the work of scientists of many nations, but owing most of all to the work of Rutherford and the school of nuclear physics which he developed. The final achievement of the
release of nuclear energy for war was due largely to the scientists and engineers of the United States, stimulated and helped by British physics. We have now the task of
using the immense power of nuclear energy for peaceful purposes. For the production of
radioactive materials, for medical and biological research, and for the generation of heat and power. With our present knowledge it is possible to design nuclear power
stations which would produce power at a moderate efficiency. Until we acquire operating
experience of these plants, we do not know how
economical they will be, nor are we sure of overcoming
all the technical difficulties which will occur in a large-scale development of nuclear power. Nevertheless, there is a real promise that over the next few decades, world power resources
can be greatly increased, and that the very great
benefits to be obtained will do much to increase
standards of living. It is the hope of every
scientist that nuclear energy will lead to the establishment
of a world organization for the effective control of all
weapons of mass destruction, and through this, to the abolition of war. – [Narrator] From Dalton’s
theory to atomic power, it is a long road, but
the landmarks are clear. With the atomic theory as a basis, a pretty complete picture of the elements of our material world was fitted together
during the 19th century. The discovery of electrons, the realization of the
nature of positive rays, the strange powers of X-rays,
revolutionized conception of the nature of matter,
and of the atom itself. The study of radioactivity,
first investigated by Becquerel, and then by the Curies,
led Lord Rutherford to make the greatest single
advance in atomic theory. He pictured a small heavy
nucleus in the atom, round which the electrons revolved. In 1919, Rutherford discovered
how to change one element to another by bombardment
with alpha particles. He suggested that the nucleus
of the atom might contain protons and uncharged particles
of about the same mass. In 1932, scientists could say
with certainty that the atom contained electrons, carrying
unit negative charges, and a nucleus built up of protons, carrying unit positive
charges, and of neutrons, uncharged particles
equal in mass to protons, whose existence was proved
by Sir James Chadwick. In 1932 also, Cockroft and
Walton split the lithium atom by bombardment with protons and found that mass could actually
be translated into energy in perfect accord with Einstein’s theory. Atomic disintegrations
provoked by great machines became a commonplace of
well-equipped physics laboratories. In 1939, uranium fission was first noted. The possibility of a
self-sustaining chain reaction suggested the wholesale release of energy. Controlled chain reactions were
achieved in the atomic pile. The imagination of the
world has been stirred by the prospect of
adapting such piles to use as sources of heat energy and as sources of radioactive materials
for use in medicine. In research laboratories,
the scientists are even now writing fresh pages in
this unfinished story. But overall, the smoke of the
atomic bomb hangs like a pall. If we are to reach the future
that promises so bravely, the peoples of the world must see that this new power is wisely used.

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