Rays and Particles
Michael Fowler, University of Virginia
Sparks in Gases
Electric discharges were passed through rarefied gases in the time of Benjamin Franklin - His friend William Watson, an Englishman, remarked: "It was a most delightful spectacle, when the room was darkened, to see the electricity in its passage." (This was in the 1740's.) (Pais page 79) The experimental arrangement was to have two small metal plates inside a glass container from which the air was gradually removed by a pump. The plate connected to the negative side of the electricity supply was called the cathode, that to the positive side the anode. As the pressure was lowered, a spark which fattened into a purplish glowing, writhing snake appeared going from the cathode to the anode. Lowering the pressure further, as Faraday noticed in the 1830's, a dark space opened up near the cathode, now called the Faraday Dark Space.
The big breakthrough came in the 1850's when Geissler invented a much better (mercury) pump. In 1869, Hittorf found that in a very good vacuum (0.01 mm Hg), the Faraday dark space expanded to fill the whole tube, and the cathode emitted rays that caused the glass to glow where they hit. In 1876, Goldstein called them "cathode rays". In 1879, an Englishman, William Crookes, declared that they must be particles of some sort, and demonstrated that they traveled in straight lines by inserting a Maltese cross in the tube, which cast a sharp shadow on the end of the tube, a demo still in common use 120 years later!
Were these cathode rays waves or particles? Hertz, with his student Lenard, discovered in 1891 that the rays could penetrate a thin aluminum plate, and detected them in the air just outside the tube. Hertz found experimentally that he could not deflect the rays with an electrostatic field applied from outside the tube, and they didn't affect a compass, so he concluded (wrongly) that they must be waves, not particles.
Thomson Discovers the Electron
For some odd reason, all the Germans physicists thought the cathode rays were waves, the English thought they were particles (presumably ions of some kind). In 1897, J. J. Thomson announced at the Royal Institution that they were negatively charged particles, and he'd measured e/m, the ratio of charge to mass for a single particle. He achieved this with a specially designed cathode ray tube, the precursor of the tube in a present-day TV set. The cathode was at the far narrow end of the tube, the anode a few centimeters away, with a small hole in its center. The rays went from the cathode to the anode, but a narrow beam continued through the hole in the anode to the glass at the other end of the tube, where their impact caused the glass to glow in a small central spot. Thomson also added inside the tube two parallel plates arranged so that the narrow beam of cathode rays went between them on its way to the end of the tube. He found that electrically charging these plates caused the beam to deflect, so the glowing spot moved from the center of the end of the tube. (This was different from Hertz' result. Possibly Hertz' externally applied field was shielded by space charge on or near the walls of the tube.) Thomson also added magnets to give a uniform magnetic field in the region between the plates. By adjusting this magnetic field strength to cancel the deflection of the rays caused by the electric field, he was able to measure the speed of the rays, because balancing these two forces
the charge cancels, and the speed is simply the ratio of electric field strength to magnetic field strength.
Having determined the horizontal speed, Thomson was able to deduce the charge to mass ratio by measuring the deflection in the electric field alone. In this situation, the trajectory of the particle is identical to that of a ball, initially thrown horizontally, veering downwards in a parabolic path. The downward (or upward, depending on the sign of the plates' charge) force is eE, and it operates for a time L/v, where L is the length of the plates. On emerging from the region between the plates, the electron moves in a straight line.
The value of e/m that emerged from this experiment was a complete surprise. It was about 2,000 times the value for the hydrogen ion, which has the largest value of any ion. Once it had been established that the cathode rays were not just uncharged electromagnetic waves, it had been assumed that they were "molecular torrents" - a flood of the same kinds of ions found in electrolysis. Now, apparently, the cathode rays were particles smaller than the smallest atom! This was the first hint that atoms might be made up of smaller entities.
Another important point is that Thomson found this ratio e/m to be independent of the material the cathode was made of. The cathode rays could have been fragments of atoms which were different for different atoms, but evidently this was not the case. Furthermore, he found in 1899 that photoelectrically produced particles had the same e/m, so were probably the same particles.
For the photoelectrically produced particles, Thomson was able to find the charge e, working with his student C. T. R. Wilson, in a cloud chamber experiment. They introduced photoelectrons into a supersaturated vapor. Tiny droplets formed around each free electron. Thomson and Wilson estimated the total number of droplets formed by counting, then found the total charge by collecting the droplets on an electroscope. Their results were off by almost 40%, but close enough to suggest strongly that the particles had the same magnitude charge as the hydrogen ion. This confirmed that the charged particles were much lighter than atoms. (They could have been the same mass as atoms, but carrying a huge charge. This unlikely possibility was now ruled out.)
Thomson had discovered the electron. (Actually, the word electron was first used in 1891 by Stoney, an Irish physicist, to denote the charge on an ion, which he estimated from electrolysis and Avogadro's number.) The emerging picture was that the atom, known of course to be electrically neutral, contained negatively charged particles, the electrons, which could be removed in various ways, leaving a positively charged ion which contained almost all the mass of the original atom. To put it in his own words, in 1899: "Electrification essentially involves the splitting up of the atom, a part of the mass of the atom getting free and becoming detached from the original atom."(Pais p 86). The word atom comes from the Greek, and means that which cannot be cut up. ("tom" is the same root for cut that appears, for example, in appendectomy). Thomson was the first to talk about splitting atoms.
It had long been known, of course, that in electrolysis of water, for example, by passing an electric current through it, the molecules were somehow split into hydrogen ions, which carried a positive charge and moved to the cathode, where they received a negative electric charge from the battery and appeared as hydrogen atoms combined into hydrogen molecules; and negatively charged oxygen ions, appearing in the same way as oxygen gas at the anode. The new insight was that these ions were atoms having an excess or a deficiency of electrons, and the electrons were particles whose charge and mass was now known.
But actually in recounting this tale we skipped over an even more important related development in 1895. (Pais p 36) Wilhelm Roentgen was a 50 year old Director of a Physics Institute, a position that included generous family living quarters, including for example, a well stocked wine cellar. He was checking out the work of Hertz and Lenard on cathode rays, and in particular was interested in Lenard's discovery that the rays penetrated a little way into the air if the Crookes' tube had an aluminum window. He had covered his Crookes' tube with black cardboard, presumably (my guess) so that he could detect the rays in the air without the distracting glow from the tube itself. He darkened the room to check that no light was getting through, and suddenly (from an unnamed biographer quoted in the Project Physics Text) "noticed a weak light shimmering on a little bench nearby. Highly excited, Roentgen lit a match, and, to his great surprise, discovered that the source of the mysterious light was a little barium platinocyanide screen lying on the bench." This little screen had been made to detect ultraviolet light, which causes barium platinocyanide to fluoresce. But if the Crookes' tube was producing any ultraviolet light, it wouldn't make it through the cardboard, and the cathode rays themselves only traveled a few centimeters in air. The little screen was a meter away from the tube.
Roentgen concluded that the Crookes' tube must be emitting some new radiation of an unknown type. To reflect his bewilderment, he dubbed the new radiation "x-rays". He couldn't believe it. He checked and rechecked. He told his wife that when he announced it, everybody would say "Roentgen has probably gone crazy". Before publishing his results, he spent seven weeks investigating the properties of these x-rays. He had already found that ordinary film was sensitive to them. He found lead was a good shield, and, by using it to shield different parts of the Crookes' tube, established that the x-rays originated in the part of the glass that fluoresced where the cathode rays hit it.
He found that the x-rays traveled in straight lines, and (unlike the cathode rays) were not deflected by magnetic fields. He found they passed through flesh almost unimpeded, but bone cast a shadow. By having his wife place her hand between the point source of x-rays on the Crookes' tube and some unexposed film in a box, then developing the film, he took a picture of the bones in her hand. On January 1, 1896 he announced his findings, complete with the bone picture, to many of his colleagues, and remarked to his wife "Now the fun begins".
The x-ray story is a good antidote to the myth that new scientific discoveries are developed and utilized much more rapidly than they used to be. On the 25th of January, a young research student in Cambridge, Ernest Rutherford, wrote of his Professor (J. J. Thomson, Pais page 39): "The Professor, of course, is trying to find out the real cause and nature of the waves, and the great object is to find the theory of matter before anyone else, for nearly every Professor in Europe is now on the warpath." Meanwhile, in the USA, the New York Times reported on the 9th of February: "the wizard of New Jersey (Edison) will try to photograph the skeleton of a human head next week." (He didn't succeed.) Already in 1896 several hospitals had x-ray facilities, and x-ray photographs were ruled as acceptable evidence in courts in France, England and the USA. Before the end of the year, the dangers became apparent. A Columbia professor who had been giving demonstrations at Bloomingdale's suffered severe skin damage. Over the next five years, many doctors and patients suffered horribly, and patients began to sue successfully. By 1903, lead-impregnated rubber shielding devices were being used. (Pais, page 96.)
What are X-rays?
All the activity described above did little to clarify the actual nature of x-rays. Roentgen himself found they were undeflected by a magnetic field, so were not charged particles like the cathode rays. On the other hand, they didn't exhibit any diffraction phenomena, so did not seem to be waves. Roentgen, and independently Thomson, found that the x-rays were ionizing radiation - as they passed through air, ions were created. A gold leaf electroscope exposed to the x-rays would lose its charge, as the newly created ions were attracted to the charged leaves.
In 1899, Haga and Wind noticed a slight broadening of an x-ray beam after it passed through a slit a few thousandths of a millimeter wide. This could be from diffraction if the wavelength were of order 10-10 meters. This problem was not resolved conclusively until 1912, when Laue made the observation that since the wavelength of x-rays was apparently similar to the distances between planes of atoms in a crystal, perhaps a crystal would act as a diffraction grating for x-rays. This turned out to be correct, and in fact is now the standard way of finding crystal structure.
It was concluded, then, that x-rays were "ultra- ultra violet light", as one French physicist had put it in 1896.
Once the usefulness of x-rays was established, techniques for producing them evolved rapidly. It was found that they were produced far more copiously if the cathode rays impinged on a piece of heavy metal, such as Molybdenum, rather than glass. The physical picture of x-ray production was that the electrons radiated as they suddenly decelerated on hitting the target, unloading their kinetic energy as radiation (plus some heating of the target). This deceleration radiation is called bremsstrahlung in German, and this word is sometimes used to describe it.
X-rays come in quanta
Since crystals act as diffraction gratings for x-rays, it is possible to map out a spectrum of the x-rays. (Of course, this was not done until after 1912). The most important finding on doing this was that there was a maximum frequency (minimum wavelength) at which x-ray production stopped, and this maximum frequency depended on the voltage applied to the cathode ray tube in a simple way:
The explanation is of course very simple - each electron in the cathode rays gains a kinetic energy of eV on accelerating down the tube before crashing into the target. Therefore, this is the most energy that can be emitted as it loses kinetic energy in the crash. If this all goes into one photon, the frequency will be fmax. So the cutoff in the x-ray spectrum provides another way to measure Planck's constant.
To return now to 1896, at first nobody had any idea how the x-rays were being generated, but Roentgen had clearly established that they came from the bit of Crookes' tube glass that was fluorescing. A French physicist, Henri Becquerel, attended a meeting of the French Academy of Sciences on January 20, 1896, three weeks after Roentgen's first announcement. Two French physicians were already showing hand x-rays.
It occurred to Becquerel that any substance that fluoresced intensely might also emit x-rays. It happened that fifteen years previously he had worked with a substance that fluoresced strongly when exposed to sunlight, so he wondered if it might also be emitting x-rays, previously undetected.
To find out, he placed a sheet of it in the sun, lying on top of a cardboard box containing unexposed film with a small metal object above it. After a day's exposure to the sunlight, which caused fluorescence, he took out and developed the film. Sure enough, it was exposed except in the shadow of the metal object, which appeared in silhouette. He decided to check, and set up the same arrangement again, with a small metal cross above the film. Unfortunately, however, it continued cloudy, so he kept the package in a closet waiting for the sun to reappear. After several days Becquerel grew impatient, and, possibly not having enough to do, decided to develop the film, perhaps to check if some light had leaked in. To his utter astonishment, he found the silhouette of the cross on the film just as distinctly as before!
Question for the reader: before reading on, can you figure out any possible explanation for this surprising discovery?
Evidently, the recent exposure to sunlight had not been a crucial element in the production of the mysterious radiation that had exposed the film. Becquerel theorized that maybe the substance continued to manufacture rays long after the fluorescence died away. However, it soon became clear that in fact the fluorescence itself was irrelevant.
It happened that the fluorescent substance Becquerel used for his experiment contained uranium. After extensive experimenting with various substances, he concluded by May that the radiation came from any substances containing uranium, whether they fluoresced or not. He found the intensity of the radiation increased with the amount of uranium present, and did not appear to change in intensity with time, or temperature, or chemical action. Like x-rays, the radiation was ionizing. If a piece of uranium is held near an electroscope, the electroscope discharges. Unlike x-rays, though, these rays couldn't be turned off.
Marie Curie Investigates Becquerel's Rays
In contrast to all the hoopla over x-rays, Becquerel's rays had little public impact, at least at the beginning. Fortunately, they did attract the attention of two very talented young researchers, Marie Curie in Paris and Ernest Rutherford in Cambridge. Marie worked with her husband Pierre, carefully measuring the amount of radiation from different amounts of uranium compounds. They measured the radiation by using two parallel metal plates differing in potential by 100 volts, connected by a sensitive electrometer (basically an infinite resistance voltmeter, a calibrated electroscope). A thin layer of the uranium compound under examination was then put on the top plate, and the rate at which charge was lost was measured - the ionizing radiation caused the charge leakage. Marie concluded that the radiation intensity was just proportional to the number of uranium atoms present, so the radiation originated inside the uranium atom. This conclusion was harder to reach than it sounds, because some fraction of the radiation is absorbed in the material itself. This is why she used a thin layer, rather than a piece of the material under investigation.
The mineral pitchblende, rich in uranium oxide, was found to be more radioactive than pure uranium metal. Marie concluded that it must contain some other element that was more radioactive than uranium. In 1898, the Curies chemically separated out a new element they called polonium, after Marie's country of birth, Poland. They measured its radioactivity as four hundred times more intense than that from pure uranium. Six months later, they chemically separated out from pitchblende another radioactive substance more than one million times more radioactive than uranium. They called it radium. One gram of radium gives out enough energy to raise one gram of water from just above freezing to boiling in one hour. Where this energy could be coming from was a complete mystery. It was hard to imagine how the energy could be stores in the material - in a few days, radium gave off as much energy as the most potent chemical fuel, yet it kept on radiating year after year. One popular theory at the time was that space was full of waves of energy, and almost all materials were completely transparent to this sea of radiation, but radium, polonium and uranium absorbed some of this energy and re-emitted it.
Ernest Rutherford Investigates Becquerel's rays
Rutherford worked as a student with J. J. Thomson from 1896, studying the ionization of gases by x-rays in a quantitative fashion. When Becquerel announced his discovery of new rays, it was natural for Rutherford to investigate them as well. He began work in Cambridge, but then accepted a job at McGill University in Montreal. He needed some money because he wanted to get married. He wrote to his fiancee: "I am expected to do a lot of original work and to form a research school to knock the shine out of the Yankees!" (Pais page 60).
He carefully studied absorption of the Becquerel rays, and found that one component, which he called the alpha-rays, could be absorbed by a sheet of writing paper, or a few centimeters of air. In fact, Becquerel had not detected these alphas, because they were absorbed by the box containing the film. A second component, the beta rays, Rutherford found to be one hundred times more penetrating. He published this result in 1899. In 1900, Rutherford went home to New Zealand and got married. French physicists, meanwhile, kept up the pace of discovery. Villard found that radium emitted some far more penetrating radiation, which he christened gamma rays. These rays could penetrate several feet of concrete.
Identifying the Beta Rays
It was clear by 1900 from their deflection by a magnetic field that the beta rays were negatively charged particles. In that year, Becquerel found e/m for beta rays to be quite close to that for cathode rays, suggesting that they also were electrons. One difference between the beta rays and cathode rays was that the beta rays could be much faster - up to 95% of the speed of light. By 1902, a German physicist, Kaufman was writing: "for small velocities, ..[e/m for] Becquerel rays fits within experimental error with the value found for cathode rays." (my italics). The interesting historical point here is that physicists already expected that mass might change with speed. There were complicated (and wrong) electromagnetic theories about how the mass might vary, beginning with the idea that the kinetic energy of a charge moving through the ether included energy from the ether flowing around it - an idea based on earlier analyses of the motion of a sphere through a real fluid. Lorentz contraction effects were added to give yet more complicated formulas. However, in 1908, a German experimenter, Bucherer, claimed that the best fit to the e/m data was given by Einstein's much simpler recent formula for mass variation with speed. These experiments were difficult to do, and Bucherer's results were not universally accepted until about 1916, by which time others had found the same results.
Identifying the Alpha Rays
The less penetrating alpha radiation proved much more difficult to identify. At first the alphas were thought to be electrically neutral, because there was no observed deflection as they shot through a magnetic field. It became clear later that this lack of deflection was because the alphas were much more massive than the betas (electrons). In 1903, in a more careful experiment, Rutherford found the alphas were in fact deflected slightly, and in the opposite direction to the electrons, so they were positively charged. By 1905, he had found e/m, and concluded that if the charge on an alpha was the same as that on a hydrogen ion, the mass of the alpha was approximately twice that of the hydrogen atom. In 1908, he finally established that the alphas were helium atoms with two electrons missing, carrying charge 2e, and having mass four times that of the hydrogen atom.
Identifying the Gamma Rays
The very penetrating gamma rays, discovered in 1900, also took
years to identify. They were not deflected by a magnetic field.
Rutherford, in 1902, thought (wrongly) they might be extremely
fast beta particles. It gradually became clear that they were
akin to x-rays, but with much shorter wavelength. This was not
settled until 1914, when Rutherford observed them to be reflected
from crystal surfaces (Pais, page 62).
Books I used in preparing this lecture:
Inward Bound, Abraham Pais, Oxford, 1986 (my main source)
The Project Physics Course, Text, Holt, Rinehart, Winston, 1970
Great Experiments in Physics, Morris H. Shamos, Ed., Dover, 1959