by Isaac Asimov
The cloud chamber has been modified in several ways since its invention, and “cousin” instruments have been devised. The original cloud chamber was not usable after expansion until the chamber had been reset. In 1939, Alexander Langsdorf, in the United States, devised a diffusion cloud chamber, in which warm alcohol vapor diffused into a cooler region in such a way that there was always a supersaturated region, and tracks could be observed continuously.
Then came the bubble chamber, a device similar in principle. In it, superheated liquids under pressure are used rather than supersaturated gas. The path of the charged particle is marked by a line of vapor bubbles in the liquid rather than by liquid droplets in vapor. The inventor, the American physicist Donald Arthur Glaser, is supposed to have gotten the idea by studying a glass of beer in 1953. If so, it was a most fortunate glass of beer· for the world of physics and for him, for Glaser received the Nobel Prize for physics in 1960 for the invention of the bubble chamber.
The first bubble chamber was only a few inches in diameter. Within the decade, bubble chambers 6 feet long were being used. Bubble chambers, like diffusion cloud chambers, are constantly set for action. In addition, since many more atoms are present in a given volume of liquid than of gas, more ions are produced in a bubble chamber, which is thus particularly well adapted to the study of fast and short-lived particles. Within a decade of its invention, bubble chambers were producing hundreds of thousands of photographs per week. Ultra-short-lived particles were discovered in the 1960s that would have gone undetected without the bubble chamber.
Liquid hydrogen is an excellent liquid with which to fill bubble chambers, because the single-proton hydrogen nucleus is so simple as to introduce a minimum of added complication. In 1973, a bubble chamber was built at Wheaton, Illinois, that was 15 feet in diameter and contained 7,300 gallons of liquid hydrogen. Some bubble chambers contain liquid helium.
Although the bubble chamber is more sensitive to short-lived particles than the cloud chamber, it has its shortcomings. Unlike the cloud chamber, the bubble chamber cannot be triggered by desired events. It must record everything wholesale, and uncounted numbers of tracks must be searched through for those of significance. The search was on, then, for some method of detecting tracks that combined the selectivity of the cloud chamber with the sensitivity of the bubble chamber.
This need was met eventually by the spark chamber, in which incoming particles ionize gas and set off electric currents through neon gas that is crossed by many metal plates. The currents show up as a visible line of sparks, marking the passage of the particles, and the device can be adjusted to react only to those particles under study. The first practical spark chamber was constructed in 1959 by the Japanese physicists Saburo Fukui and Shotaro Miyamoto. In 1963, Soviet physicists improved it further, heightening its sensitivity and flexibility. Short streamers of light are produced that, seen on end, make a virtually continuous line (rather than the separate sparks of the spark chamber). The modified device is therefore a streamer chamber. It can detect events that take place within the chamber, and particles that streak off in any direction, where the original spark chamber fell short in both respects.
TRANSMUTATION OF ELEMENTS
But, leaving modern sophistication in studying the flight of subatomic particles, we must turn back half a century to see what happened when Rutherford bombarded nitrogen nuclei with alpha particles within one of the original Wilson cloud chambers. The alpha particle would leave a track that would end suddenly in a fork—plainly, a collision with a nitrogen nucleus. One branch of the fork would be comparatively thin, representing a proton shooting off. The other branch, a short, heavy track, represented what was left of the nitrogen nucleus, rebounding from the collision. But there was no sign of the alpha particle itself. It seemed that it must have been absorbed by the nitrogen nucleus, and this supposition was later verified by the British physicist Patrick Maynard Stuart Blackett, who is supposed to have taken more than 20,000 photographs in the process of collecting eight such collisions (surely an example of superhuman patience, faith, and persistence). For this and other work in the field of nuclear physics, Blackett received the Nobel Prize in physics in 1948.
The fate of the nitrogen nucleus could now be deduced. When it absorbed the alpha particle, its mass number of 14 and positive charge of 7 were raised to 18 and 9, respectively. But since the combination immediately lost a proton, the mass number dropped to 17 and the positive charge to 8. Now the element with a positive charge of 8 is oxygen, and the mass number 17 belongs to the isotope oxygen 17. In other words, Rutherford had, in 1919, transmuted nitrogen into oxygen. This was the first man-made transmutation in history. The dream of the alchemists had been fulfilled, though in a manner they could not possibly have foreseen or duplicated with their primitive techniques.
As projectiles, alpha particles from radioactive sources had limits: they were not nearly energetic enough to break into nuclei of the heavier elements, whose high positive charges exercise a strong repulsion against positively charged particles. But the nuclear fortress had been breached, and more energetic attacks were to come.
New Particles
The matter of attacks on the nucleus brings us back to the question of the makeup of the nucleus. The proton-electron theory of nuclear structure, although it explained isotopes perfectly, fell afoul of certain other facts. Subatomic particles generally have a property visualized as spin, something like astronomical objects rotating on their axis. The units in which such spin is measured are so taken that both protons and electrons turn out to have spins of either +½ or –½. Hence, an even number of electrons or protons (or both), if all confined within a nucleus, should lend that nucleus a spin of 0 or of some whole number— +1, −1, +2, −2, and so on. If an odd number of electrons or protons (or both) make up a nucleus, the total spin should be a half-number, such as +½, –½, +1½, −1½, +2½, −2½, and so on. If you try adding up an even number of positive or negative halves (or a mixture), and then do the same with an odd number, you will see this is, and must be, so.
Now as it happens, the nitrogen nucleus has an electric charge of +7 and a mass of 14. By the proton-electron theory, its nucleus must contain 14 protons to account for the mass, and 7 electrons to neutralize half of the charge and leave +7. The total number of particles in such a nucleus is 21, and the overall spin of the nitrogen nucleus should be a half-number—but it is not. It is a whole number.
This sort of discrepancy turned up in other nuclei as well, and it seemed that the proton-electron theory just would not do. As long as those were the only subatomic particles known, however, physicists were helpless at finding a substitute theory.
THE NEUTRON
In 1930, however, two German physicists, Walter Bothe and Herbert Becker, reported that they had released from the nucleus a mysterious new radiation of unusual penetrating power. They had produced it by bombarding beryllium atoms with alpha particles. The year before, Bothe had devised methods for using two or more counters in conjunction—coincidence counters. These could be used to identify nuclear events taking place in a millionth of a second. For this and other work, he shared in the Nobel Prize for physics in 1954.
Two years later the Bothe-Beeker discovery was followed by the French physicists Frederic and Irène Joliot-Curie. (Irene was the daughter of Pierre and Marie Curie, and Joliot had added her name to his on marrying her.) They used the new-found radiation from beryllium to bombard paraffin, a waxy substance composed of hydrogen and carbon. The radiation knocked protons out of the paraffin.
The English physicist James Chadwick quickly suggested that the radiation consisted of particles. To determine their size, he bombarded boron atoms with them; and from the increase in mass of the new nucleus, he calculated that the particle added to the boron had a mass about equal to the proton. Yet the particle itself could not be detected in a Wilson cloud chamber. Chadwick decided that the explanation must be that the particle had no electric charge (an uncharged
particle produces no ionization and therefore condenses no water droplets).
So Chadwick concluded that a completely new particle had turned up—a particle with just about the same mass as a proton but without any charge, or, in other words, electrically neutral. The possibility of such a particle had already been suggested, and a name had even been proposed—neutron. Chadwick accepted that name. For his discovery of the neutron, he was awarded the Nobel Prize in physics in 1935.
The new particle at once solved certain doubts that theoretical physicists had had about the proton-electron model of the nucleus. The German theoretical physicist Werner Heisenberg announced that the concept of a nucleus consisting of protons and neutrons, rather than of protons and electrons, gave a much more satisfactory picture. Thus, the nitrogen nucleus could be visualized as made up of seven protons and seven neutrons. The mass number would then be 14, and the total charge (atomic number) would be +7. What’s more, the total number of particles in the nucleus would be fourteen—an even number—rather than twenty-one (an odd number) as in the older theory.
Since the neutron, like the proton, has a spin of either +½ or –½, an even number of neutrons and protons would give the nitrogen nucleus a spin equal to a whole number, and fits the observed facts. All the nuclei that had spins that could not be explained by the proton-electron theory, turned out to have spins that could be explained by the proton-neutron theory. The proton-neutron theory was accepted at once and has remained accepted ever since. There are no electrons within the nucleus after all.
Furthermore, the new model fitted the facts of the periodic table of elements just as neatly as the old one had. The helium nucleus, for instance, would consist of two protons and two neutrons, which explained its mass of 4 and nuclear charge of 2 units. And the concept accounted for isotopes in very simple fashion. For example, the chlorine-35 nucleus would have seventeen protons and eighteen neutrons; the chlorine-37 nucleus, seventeen protons and twenty neutrons. They would both, therefore, have the same nuclear charge, and the extra weight of the heavier isotope would lie in its two extra neutrons. Likewise, the three isotopes of oxygen would differ only in their numbers of neutrons: oxygen 16 would have eight protons and eight neutrons; oxygen 17, eight protons and nine neutrons; oxygen 18, eight protons and ten neutrons (figure 7.2).
Figure 7.2. Nuclear makeup of oxygen 16, oxygen 17, and oxygen 18. They contain eight protons each and, in addition, eight, nine, and ten neutrons, respectively.
In short, every element could be defined simply by the number of protons in its nucleus, which is equivalent to the atomic number. All the elements except hydrogen, however, also had neutrons in the nucleus, and the mass number of a nuclide was the sum of its protons and neutrons. Thus, the neutron joined the proton as a basic building block of matter. For convenience, both are now lumped together under the general term nucleons, a term first used in 1941 by the Danish physicist Christian Moller. From this came nucleonics, suggested in 1944 by the American engineer Zay Jeffries to represent the study of nuclear science and technology.
This new understanding of nuclear structure has resulted in additional classifications of nuclides. Nuclides with equal numbers of protons are, as I have just explained, isotopes. Similarly, nuclides with equal numbers of neutrons (as, for instance, hydrogen 2 and helium 3, each containing one neutron in the nucleus) are isotones. Nuclides with equal total number of nucleons, and therefore of equal mass numbers—such as calcium 40 and argon 40—are isobars.
The proton-electron theory of nuclear structure left unexplained, just at first, the fact that radioactive nuclei could emit beta particles (electrons). Where did the electrons come from if there were none in the nucleus? That problem was cleared up, however, as I shall shortly explain.
THE POSITRON
In a very important respect the discovery of the neutron disappointed physicists. They had been able to think of the universe as being built of just two fundamental particles—the proton and the electron. Now a third had to be added. To scientists, every retreat from simplicity is regrettable.
The worst of it was that, as things turned out, this was only the beginning. Simplicity’s backward step quickly became a headlong rout. There were more particles to come.
For many years, physicists had been studying the mysterious cosmic rays from space, first discovered in 1911 by the Austrian physicist Victor Francis Hess on balloon flights high in the atmosphere.
The presence of such radiation was detected by an instrument so simple as to hearten those who sometimes feel that modern science can progress only by use of unbelievably complex devices. The instrument was an electroscope, consisting of two pieces of thin gold foil attached to a metal rod within a metal housing fitted with windows. (The ancestor of this device was constructed as long ago as 1706 by the English physicist Francis Hauksbee.)
If the metal rod is charged with static electricity, the pieces of gold foil separate. Ideally, they would remain separated forever, but ions in the surrounding atmosphere slowly conduct away the charge so that the leaves gradually collapse toward each other. Energetic radiation—such as X rays, gamma rays, or streams of charged particles—produces the ions necessary for such charge leakage. Even if the electroscope is well shielded, there is still a slow leakage, indicating the presence of a very penetrating radiation not directly related to radioactivity. It was this penetrating radiation, which increased in intensity, the higher Hess rose in the atmosphere. Hess shared the Nobel Prize for physics in 1936 for this discovery.
The American physicist Robert Andrews Millikan, who collected a great deal of information on this radiation (and gave it the name cosmic rays), decided that it must be a form of electromagnetic radiation. Its penetrating power was such that some of it could even pass through several feet of lead. To Millikan this suggested that the radiation was like the penetrating gamma rays, but with an even shorter wavelength.
Others, notably the American physicist Arthur Holly Compton, contended that the cosmic rays were particles. There was a way to investigate the question. If they were charged particles, they should be deflected by the earth’s magnetic field as they approached the earth from outer space. Compton studied the measurements of cosmic radiation at various latitudes and found that it did indeed curve with the magnetic field: it was weakest near the magnetic equator and strongest near the poles, where the magnetic lines of force dipped down to the earth.
The primary cosmic particles, as they enter our atmosphere, carry fantastically high energies. Most of them are protons, but some are nuclei of heavier elements. In general, the heavier the nucleus, the rarer it is among the cosmic particles. Nuclei as complex as those making up iron atoms were detected quickly enough; and in 1968, nuclei as complex as those of uranium were detected. The uranium nuclei make up only 1 particle in 10 million. A few very high-energy electrons are also included.
When the primary particles hit atoms and molecules of the air, they smash these nuclei and produce all sorts of secondary particles. It is this secondary radiation (still very energetic) that we detect near the earth, but balloons sent to the upper atmosphere have recorded the primary radiation.
Now it was as a result of cosmic-ray research that the next new particle—after the neutron—was discovered. This discovery had actually been predicted by a theoretical physicist. Paul Adrien Maurice Dirac had reasoned, from a mathematical analysis of the properties of subatomic particles, that each particle should have an antiparticle. (Scientists like nature to be not only simple but also symmetrical.) Thus there ought to be an antielectron, exactly like the electron except that it had a positive instead of a negative charge, and an antiproton with a negative instead of a positive charge Dirac’s theory did not make much of a splash in the scientific world when he proposed it in 1930. But, sure enough, two years later the antielectron actually turned up. The American physicist Carl David Anderson was working with Millikan on the problem of whether cosmic rays were electromagnetic radiation or particles. By then, m
ost people were ready to accept Compton’s evidence that they were charged particles, but Millikan was an extraordinarily hard loser and was not satisfied that the issue was settled. Anderson undertook to find out whether cosmic rays entering a Wilson cloud chamber would be bent by a strong magnetic field. To slow down the rays sufficiently so that the curvature, if any, could be detected, Anderson placed in the chamber a lead barrier about ¼ inch thick. He found that the cosmic radiation crossing the chamber after it came through the lead did make a curved track. But he also found something else. In their passage through the lead, the energetic cosmic rays knocked particles out of the lead atoms. One of these particles made a track just like that of an electron. But it curved in the wrong direction! Same mass but opposite charge. There it was—Dirac’s antielectron. Anderson called his discovery the positron. It is an example of the secondary radiation produced by cosmic rays; but in 1963, it was found that positrons were included among the primary radiations as well.
Left to itself, the positron is as stable as the electron (why not, since it is identical with the electron except for electric charge?) and could exist indefinitely. It is not, however, left to itself, for it comes into existence in a universe filled with electrons. As it streaks along, it almost immediately (say, within a millionth of a second) finds itself in the neighborhood of one.
