ATOM SMASHING & DISCOVERY OF THE NEUTRON

[Music] from 1920 onwards many other scientists pursued researches in the field opened up by lord rutherford's transmutation of 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 improved apparatus developed gradually in chambers such as this the amount of ionization produced by a 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 when a potential difference 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 a 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 micro window to be amplified until it could operate a kind of counter then geiger and miller 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 large speaker 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 [Music] with such sensitive apparatus atomic disintegration and transmutation can be closely studied 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 thompson'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 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 botha and becca 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 frederick and madame giulio curie measured the penetrative 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 jolyokuri 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 right grains have on the billet 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 glances 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 jolly old curies and sir james chadwick the neutrons at once explain the existence of isotopes chlorine for instance has an atomic number of seventeen that is there are 17 electrons circling around a nucleus in which there are seventeen protons but chlorine has two isotopes the atomic weight of one is thirty five 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 for our experiments in transmutation intense beams of protons and alpha particles while the discovery of heavy hydrogen has given us a new projectile of remarkable efficiency in transmuting atoms 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 obtain significant results by bombarding lithium the machine you see is a descendant of their original set irons are produced inside the large mushroom shaped insulator and are accelerated down the rear most 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 a target in the science museum at south kensington is preserved the observation cabinet and accelerating tube of the original voltage quadrupler used by cockraft 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 are but different manifestations of the same thing a somewhat unfamiliar conception for the average mind furthermore the equation e is equal m c square in which energy is part 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 with the versa the mass is energy were in fact equivalent according to the formula mentioned above this was demonstrated by cocra and walton in 1932 experimentally in quiet non-committal terms the scientists summed up their results thus it seems not unlikely that the lithium isotope of mass 7 occasionally captures a proton and the resulting nucleus of mass 8 breaks into two alpha particles each of mass 4 and each with an energy of about 8 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 or 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 8.0 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 3 times 10 to the 10th 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 eight and a half 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 election votes measured in eggs 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 twenty seven point six times ten 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 had 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 8 million election votes can be obtained on this machine and in the united states elections of 100 million electron volts have already been produced giant synchrotrons are now being built to accelerate positive ions hidden behind thick 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 a wave guide eventually this machine will hurl electrons at its target with an energy of 5 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 fender graph 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 the potential of two million volts to be built up most impressive of all in their suggestion of a welsian world are these million and two million vote generators in the cavendish laboratory at cambridge you