Asimov's New Guide to Science

by Isaac Asimov

  For a moment, there may be an electron-positron association—a situation in which the two particles circle each other about a mutual center of force. In 1945, the American physicist Arthur Edward Ruark suggested that this two-particle system be called positronium, and in 1951, the Austrian-American physicist Martin Deutsch was able to detect positronium through the characteristic gamma-radiation it gave up.

  However, even if a positronium system forms, it remains in existence for only a 10-millionth of a second, at most. The dance ends in the combination of the electron and positron. When the two opposite bits of matter combine, they cancel each other, leaving no matter at all (mutual annihilation); only energy, in the form of gamma rays, is left behind. Thus was confirmed Albert Einstein’s suggestion that matter could be converted into energy and vice versa. Indeed, Anderson soon succeeded in detecting the·reverse phenomenon: gamma rays suddenly disappearing and giving rise to an electron-positron pair. This is called pair production. (Anderson, along with Hess, received the Nobel Prize in physics in 1936.)

  The Joliot-Curies shortly afterward came across the positron in another connection, and in so doing, made an important discovery. Bombarding aluminum atoms with alpha particles, they found that the procedure produced not only protons but also positrons. This in itself was interesting but not fabulous. When they stopped the bombardment, however, the aluminum kept right on emitting positrons! The emission faded off with time. Apparently they had created a new radioactive substance in the target.

  The Joliot-Curies interpreted what had happened in this way: When all aluminum nucleus absorbed an alpha particle, the addition of two protons changed aluminum (atomic number 13) to phosphorus (atomic number 15).

  Since the alpha particle contained four nucleons altogether, the mass number would go up by four—from aluminum 27 to phosphorus 31. Now if the reaction knocked a proton out of this nucleus, the reduction of its atomic number and mass number by one would change it to another element—namely, silicon 30.

  Since an alpha particle is the nucleus of helium, and a proton the nucleus of hydrogen, we can write the following equation of this nuclear reaction:

  aluminum 27 + helium 4 → silicon 30 + hydrogen 1

  Notice that the mass numbers balance: 27 plus 4 equals 30 plus 1. So do the atomic numbers, for aluminum’s is 13 and helium’s 2, making 15 together, while silicon’s atomic number of 14 and hydrogen’s 1 also add up to 15. This balancing of both mass numbers and atomic numbers is a general rule of nuclear reactions.

  The Joliot-Curies assumed that neutrons as well as protons had been formed in the reaction. If phosphorus 31 emitted a neutron instead of a proton, the atomic number would not change, though the mass number would go down one. In that case the element would remain phosphorus but become phosphorus 30. This equation would read:

  aluminum 27 + helium 4 → phosphorus 30 + neutron 1

  Since the atomic number of phosphorus is 15 and that of the neutron is 0, again the atomic numbers on both sides of the equation also balance.

  Both processes—alpha absorption followed by proton emission, and alpha absorption followed by neutron emission—take place when aluminum is bombarded by alpha particles. But there is one important distinction between the two results. Silicon 30 is a perfectly well-known isotope of silicon, making up a little more than 3 percent of the silicon in nature. But phosphorus 30 does not exist in nature. The only known natural form of phosphorus is phosphorus 31. Phosphorus 30, in short, is a radioactive isotope with a brief lifetime that exists today only when it is produced artificially; in fact, it was the first such isotope made in the laboratory. The Joliot-Curies received the Nobel Prize in chemistry in 1935 for their discovery of artificial radioactivity.

  The unstable phosphorus 30 that the Joliot-Curies had produced by bombarding aluminum quickly broke down by emitting positrons. Since the positron, like the electron, has practically no mass, this emission did not change the mass number of the nucleus. However, the loss of one positive charge did reduce its atomic number by one, so that it was converted from phosphorus to silicon.

  Where does the positron come from? Are positrons among the components of the nucleus? The answer is no. What happens is that a proton within the nucleus changes to a neutron by shedding its positive charge, which is released in the form of a speeding positron.

  Now the emission of beta particles—the puzzle we encountered earlier in the chapter—can be explained. This comes about as the result of a process just the reverse of the decay of a proton into a neutron: that is, a neutron changes into a proton. The proton-to-neutron change releases a positron; and, to maintain the symmetry, the neutron-to-proton change releases an electron (the beta particle). The release of a negative charge is equivalent to the gain of a positive charge and accounts for the formation of a positively charged proton from an uncharged neutron. But how does the uncharged neutron manage to dig up a negative charge and send it flying outward?

  Actually, if it were just a negative charge, the neutron could not do so. Two centuries of experience have taught physicists that neither a negative electric charge nor a positive electric charge can be created out of nothing. Neither can either type of charge be destroyed. This is the law of conservation of electric charge.

  However, a neutron does not create only an electron in the process of producing a beta particle; it creates a proton as well. The uncharged neutron disappears, leaving in its place a positively charged proton and a negatively charged electron. The two new particles, taken together, have an over-all electric charge of zero. No net charge has been created. Similarly, when a positron and electron meet and engage in mutual annihilation, the charge of the positron and electron, taken together, is zero to begin with.

  When a proton emits a positron and changes into a neutron, the original particle (the proton) is positively charged, and the final particles (the neutron and positron), taken together, have a positive charge.

  It is also possible for a nucleus to absorb an electron. When this happens, a proton within the nucleus changes to a neutron. An electron plus a proton (which, taken together, have a charge of zero) form a neutron, which has a zero charge. The electron captured is from the innermost electron shell of the atom, since the electrons of that shell are closest to the nucleus and most easily gathered in. As the innermost shell is the K-shell (see chapter 6), the process is called K-capture. An electron from the L-shell then drops into the vacant spot, and an X ray is emitted. It is by these X rays that K-capture can be detected. This was first accomplished in 1938 by the American physicist Luis Walter Alvarez. Ordinary nuclear reactions involving the nucleus alone are usually not affected by chemical change, which affects electrons only. Since K-capture affects electrons as well as nuclei, the chance of its occurring can be somewhat altered as a result of chemical change.

  All of these particle interactions satisfy the law of conservation of electric charge and must also satisfy other conservation laws. Any particle interaction that violates none of the conservation laws will eventually occur, physicists suspect, and an observer with the proper tools and proper patience will detect it. Those events that violate a conservation law are “forbidden” and will not take place. Nevertheless, physicists are occasionally surprised to find that what had seemed a conservation law is not as rigorous or as universal as had been thought—as we shall see.

  RADIOACTIVE ELEMENTS

  Once the Joliot-Curies had created the first artificial radioactive isotope, physicists proceeded merrily to produce whole tribes of them. In fact, radioactive varieties of every single element in the periodic table have now been formed in the laboratory. In the modern periodic table, each element is really family, with stable and unstable members, some found in nature, some only in the laboratory.

  For instance, hydrogen comes in three varieties. First there is ordinary hydrogen, containing a single proton. In 1932, the chemist Harold Urey succeeded in isolating a second by slowly evaporating a large quantity of water, on the theory that
he would be left in the end with a concentration of the heavier form of hydrogen that was suspected to exist. Sure enough, when he examined the last few drops of unevaporated water spectroscopically, he found a faint line in the spectrum in exactly the position predicted for heavy hydrogen.

  Heavy hydrogen’s nucleus is made up of one proton and one neutron.

  Having a mass number of two, the isotope is hydrogen 2. Urey named the atom deuterium, from a Greek word meaning “second,” and the nucleus a deuteron. A water molecule containing deuterium is called heavy water. Because deuterium has twice the mass of ordinary hydrogen, heavy water has higher boiling and freezing points than ordinary water. Whereas ordinary water boils at 100° C, and freezes at 0° C, heavy water boils at 101.42° C and freezes at 3.79° C. Deuterium itself has a boiling point of 23.7° K as compared with 20.4° K for ordinary hydrogen. Deuterium occurs in nature in the ratio of 1 part to 6,000 parts of ordinary hydrogen. For his discovery of deuterium, Urey received the Nobel Prize in chemistry in 1934.

  The deuteron turned out to be a valuable particle for bombarding nuclei. In 1934, the Australian physicist Marcus Lawrence Elwin Oliphant and the Austrian chemist Paul Harteck, attacking deuterium itself with deuterons, produced a third form of hydrogen, made up of one proton and two neutrons.

  The reaction went:

  hydrogen 2 + hydrogen 2 → hydrogen 3 + hydrogen 1

  The new “superheavy” hydrogen was named tritium, from the Greek word for “third,” and its nucleus is a triton. Its boiling point is 25.0° K, and its melting point 20.5” K. Pure tritium oxide (superheavy water) has been prepared, and its melting point is 4.5° C. Tritium is radioactive and breaks down comparatively rapidly. It exists in nature, being formed as one of the products of the bombardment of the atmosphere by cosmic rays. In breaking down, it emits an electron and changes to helium 3, a stable but rare isotope of helium, mentioned in the previous chapter (figure 7.3).

  Figure 7.3. Nuclei of ordinary hydrogen, deuterium, and tritium.

  Of the helium in the atmosphere, only about 1 atom out of 800,000 is helium 3, all originating, no doubt, from the breakdown of hydrogen 3 (tritium) which is itself formed from the nuclear reactions taking place when cosmic-ray particles strike atoms in the atmosphere. The tritium that remains at anyone time is even rarer. It is estimated that only 3½ pounds exist all told in the atmosphere and oceans. The helium-3 content of helium obtained in natural gas wells, where cosmic rays have had less opportunity to form tritium, is even smaller in percentage.

  These two isotopes, helium 3 and helium 4, are not the only heliums. Physicists have created two radioactive forms: helium 5, one of the most unstable nuclei known; and helium 6, also very unstable.

  And so it goes. By now the list of known isotopes has grown to about 1,400 altogether. Over 1,100 of these are radioactive, and many of them have been created by new forms of atomic artillery far more potent than the alpha particles from radioactive sources which were the only projectiles at the disposal of Rutherford and the Joliot-Curies.

  The sort of experiment performed by the Joliot-Curies in the early 1930s seemed a matter of the scientific ivory tower at the time, but it has come to have a highly practical application. Suppose a set of atoms of one kind, or of many, are bombarded with neutrons. A certain percentage of each kind of atom will absorb a neutron, and a radioactive atom will generally result. This radioactive element will decay, giving off subatomic radiation in the form of particles or gamma rays.

  Every different type of atom will absorb neutrons to form a different type of radioactive atom, giving off different and characteristic radiation. The radiation can be detected with great delicacy. From its type and from the rate at which its production declines, the radioactive atom giving it off can be identified and, therefore, so can the original atom before it absorbed a neutron. Substances can be analyzed in this fashion (neutron-activation analysis) with unprecedented precision: amounts as small as a trillionth of a gram of a particular nuclide are detectable.

  Neutron-activation analysis can be used to determine delicate differences in the impurities contained in samples of particular pigments from different centuries and, in this way, can determine the authenticity of a supposedly old painting, using only the barest fragment of its pigment. Other delicate decisions of this sort can be made: even hair from Napoleon’s century-and-a-half-old corpse was studied and found to contain quantities of arsenic—though whether murderous, medicinal, or fortuitous is hard to say.

  PARTICLE ACCELERATORS

  Dirac had predicted not only an antielectron (the positron) but also an antiproton. To produce an antiproton, however, would take vastly more energy. The energy needed was proportional to the mass of the particle. Since the proton was 1,836 times as massive as the electron, the formation of an antiproton called for at least 1,836 times as much energy as the formation of a positron. The feat had to wait for the development of a device for accelerating subatomic particles to sufficiently high energies.

  At the time of Dirac’s prediction, the first steps in this direction had just been taken. In 1928, the English physicists John Douglas Cockcroft and Ernest Thomas Sinton Walton, working in Rutherford’s laboratory, developed a voltage multiplier, a device for building up electric potential, which could drive the charged proton up to an energy of nearly 400,000 electron volts. (One electron volt is equal to the energy developed by an electron accelerated across an electric field with a potential of 1 volt.) With protons accelerated in this machine they were able to break up the lithium nucleus and, for this work, were awarded the Nobel Prize for physics in 1951.

  Meanwhile the American physicist Robert Jemison Van de Graaff was creating another type of accelerating machine. Essentially, it operated by separating electrons from protons and depositing them at opposite ends of the apparatus by means of a moving belt. In this way the Van de Graaff electrostatic generator developed a very high electric potential between the opposite ends; Van de Graaff got it up to 8 million volts. Electrostatic generators can easily accelerate protons to a speed amounting to 24 million electron volts (physicists now invariably abbreviate million electron volts to Mev).

  The dramatic pictures of the Van de Graaff electrostatic generator producing huge sparks caught the popular imagination and introduced the public to the atom smasher. It was popularly viewed as a device to produce “man-made lightning,” although, of course, it was much more than that. (A generator designed to produce artificial lightning and nothing more had actually been built in 1922 by the German-American electrical engineer Charles Proteus Steinmetz.)

  The energy that can be reached in such a machine is restricted by practical limits on the attainable potential. However, another scheme for accelerating particles shortly made its appearance. Suppose that, instead of firing particles with one big shot, you accelerated them with a series of small pushes. If each successive push was timed just right, it would increase the speed each time, just as pushes on a child’s swing will send it higher and higher if they are applied “in phase” with the swing’s oscillations.

  This idea gave birth, in 1931, to the linear accelerator (figure 7.4). The particles are driven down a tube divided into sections. The driving force is an alternating electric field, so managed that as the particles enter each successive section, they get another push. Since the particles speed up as they go along, each section must be longer than the one before, so that the particles will take the same time to get through it and will be in phase with the timing of the pushes.

  Figure 7.4. Principle of the linear accelerator. A high-frequency alternating charge alternately pushes and pulls the charged particles in the successive drive tubes, accelerating them in one direction.

  It is not easy to keep the timing just right, and anyway there is a limit to the length of a tube it is practical to make, so the linear accelerator did not catch on in the 1930s. One of the things that pushed it into the background was that Ernest Orlando Lawrence of the University of California conceived
a better idea.

  Instead of driving the particles down a straight tube, why not whirl them around in a circular path? A magnet could bend them in such a path. Each time they completed a half-circle, they would be given a kick by the alternating field; and in this setup, the timing would not be so difficult to control. As the particles speeded up, their path would be bent less sharply by the magnet, so they would move in ever wider circles and perhaps take the same time for each round trip. At the end of their spiraling flight, the particles would emerge from the circular chamber (actually divided into semicircular halves, called dees) and strike their target.

  Lawrence’s compact new device was named the cyclotron (figure 7.5). His first model, less than 1 foot in diameter, could accelerate protons to energies of nearly 1.25 Mev. By 1939 the University of California had a cyclotron, with magnets 5 feet across, capable of raising particles to some 20 Mev, twice the speed of the most energetic alpha particles emitted by radioactive sources. In that year Lawrence received the Nobel Prize in physics for his invention.

  Figure 7.5. Principle of the cyclotron, shown in top view (above) and side view (below). Particles injected from the source are given a kick in each dee by the alternating charge and are bent in their spiral path by a magnet.

  The cyclotron itself had to stop at about 20 Mev, because at that energy the particles were traveling so fast that the mass increase with velocity—an effect predicted by Einstein’s theory of relativity—became appreciable. This increase in mass caused the particles to start lagging and falling out of phase with the electrical kicks. But there was a cure for this, and it was worked out in 1945 independently by the Soviet physicist Vladimir Iosifovich Veksler and the California physicist Edwin Mattison McMillan. The cure was simply to synchronize the alternations of the electric field with the increase in mass of the particles. This modification of the cyclotron was called the synchrocyclotron. By 1946 the University of California had built one that accelerated particles to energies of 200 to 400 Mev. Later larger synchrocyclotrons in the United States and in the Soviet Union raised the energies to 700 to 800 Mev.