by Isaac Asimov
Meanwhile the acceleration of electrons had been getting separate attention. To be useful in smashing atoms, the light electrons had to be raised to much higher speeds than protons (just as a ping-pong ball has to be moving much faster than a golf ball to do as much damage). The cyclotron would not work for electrons, because at the high velocities needed to make the electrons effective, their increase in mass was too great. In 1940 the American physicist Donald William Kerst designed an electron-accelerating device which balanced the increasing mass with an electric field of increasing strength. The electrons were kept in the same circular path instead of spiraling outward. This instrument was named the betatron, after beta particles. Betatrons now generate electron velocities up to 340 Mev.
They have been joined by another instrument of slightly different design called the electron synchrotron. The first of these was built in England in 1946 by F. K. Goward and D. E. Barnes. These raise electron energies to the 1,000 Mev mark, but cannot go higher because electrons moving in a circle radiate energy at increasing rates as velocity is increased. This radiation produced by an accelerating particle is called Bremsstrahlung, a German word meaning “braking radiation.”
Taking a leaf from the betatron and electron synchrotron, physicists working with protons began about 1947 to build proton synchrotrons, which likewise kept their particles in a single circular path. This helped save on weight. Where particles move in outwardly spiraling paths, a magnet must extend the entire width of the spiral to keep the magnetic force uniform throughout. With the path held in a circle, the magnet need be only large enough to cover a narrow area.
Because the more massive proton does not lose energy with motion in a circular path as rapidly as does the electron, physicists set out to surpass the 1,000-Mev mark with a proton synchroton. This value of 1,000 Mev is equal to I billion electron volts—abbreviated to Bev. (In Great Britain a billion is a million million, so Bev does not mean the same thing as in the United States; for 1,000 Mev the British use the shorthand Gev, the G from giga, Greek for “giant.”)
In 1952, the Brookhaven National Laboratory on Long Island completed a proton synchroton that reached 2 to 3 Bev. They called it the cosmotron, because it had arrived at the main energy range of particles in the cosmic rays. Two years later, the University of California brought in its Bevatron, capable of producing particles of between 5 and 6 Bev. Then, in 1957, the Soviet Union announced that its phasotron had got to 10 Bev.
But by now these machines seem puny in comparison with accelerators of a newer type, called the strong-focusing synchrotron. The limitation on the bevatron type is that particles in the stream fly off into the walls of the channel in which they travel. The new type counteracts this tendency by means of alternating magnetic fields of different shape which keep focusing the particles in a narrow stream. The idea was first suggested by Christofilos, whose “amateur” abilities outshone the professionals here as well as in the case of the Christofilos effect. This, incidentally, further decreased the size of the magnet required for the energy levels attained. Where particle energy was increased fiftyfold, the weight of the magnet involved was less than doubled.
In November 1959, the European Committee for Nuclear Research (CERN), a cooperative agency of twelve nations, completed in Geneva a strong-focusing synchrotron which reached 24 Bev and produced large pulses of particles (containing 10 billion protons) every 3 seconds. This synchrotron is nearly three city blocks in diameter, and one round trip through it is two-fifths of a mile. In the 3-second period during which the pulse builds up, the protons travel half a million times around that track. The instrument has a magnet weighing 3,500 tons and costs 30 million dollars.
The advance continued. Higher and higher energies were sought in order to produce more and more unusual particle interactions, forming more and THE PARTICLES 309 more massive particles, and learning more and more about the ultimate structure of matter. For instance, instead of accelerating a stream of particles and having them collide with some fixed target, why not set up two streams of particles, circling in opposite directions in storage rings, where the speed is simply maintained for some period of time. At appropriate times, the two streams are so directed that they will collide with each other head on. The effective energy of collision is four times that of either colliding with a fixed target. At Fermilab (Fermi National Accelerator Laboratory) near Chicago, an accelerator working on this principle went into operation in 1982 and should reach 1,000 Bev. It is called the Tevatron, the T standing for “trillion,” of course. Other accelerators are being planned that may eventually reach as high as 20,000 Bev.
The linear accelerator, or linac, has also undergone a revival. Improvements in technique have removed the difficulties that plagued the early models. For extremely high energies, a linear accelerator has some advantages over the cyclic type. Since electrons do not lose energy when traveling in a straight line, a linac can accelerate electrons more powerfully and focus beams on targets more sharply. Stanford University has built a linear accelerator 2 miles long which can reach energies of perhaps 45 Bev.
With merely the Bevatron, man at last came within reach of creating the antiproton. The California physicists set out deliberately to produce and detect it. In 1955, Owen Chamberlain and Emilio G. Segrè, after bombarding copper with protons of 6.2 Bev hour after hour, definitely caught the antiproton—in fact, sixty of them. It was far from easy to identify them. For every antiproton produced, 40,000 particles of other types came into existence. But by an elaborate system of detectors, so designed and arranged that only an antiproton could touch all the bases, they recognized the particle beyond question. For their achievement, Chamberlain and Segrè received the Nobel Prize in physics in 1959.
The antiproton is as evanescent as the positron—at least in our universe. Within a tiny fraction of a second after it is created, the particle is snatched up by some normal, positively charged nucleus. There the antiproton and one of the protons of the nucleus annihilate each other, turning into energy and minor particles. In 1965, enough energy was concentrated to reverse the process and produce a proton-antiproton pair.
Once in a while, a proton and an antiproton have only a near collision instead of a direct one. When that happens, they mutually neutralize their respective charges. The proton is converted to a neutron, which is fair enough. But the antiproton becomes an antineutron! What can an antineutron be? The positron is the opposite of the electron by virtue of its opposite charge, and the antiproton is likewise “anti” by virtue of its charge. But what gives the uncharged antineutron the quality of oppositeness?
PARTICLE SPIN
Here we must bring up the matter of particle spin again, a property first suggested, by the way, in 1925, by the Dutch physicists George Eugene Uhlenbeck and Samuel Abraham Goudsmit. In spinning, the particle generates a tiny magnetic field; such fields have been measured and thoroughly explored, notably by the German physicist Otto Stern and the American physicist Isidor Isaac Rabi who received the Nobel Prizes in physics in 1943 and 1944, respectively, for their work on this phenomenon.
Those particles—like the proton, the neutron, and the electron—which have spins that can be measured in half-numbers can be dealt with according to a system of rules worked out independently, in 1926, by Fermi and Dirac. These are therefore called Fermi-Dirac statistics. Particles that obey these are fermions, so that the proton, the electron, and the neutron are all fermions.
There also exist particles whose spin can be expressed as whole numbers. They can be dealt with by another set of rules devised by Einstein and by the Indian physicist Satyendranath Bose. Particles that follow the Bose-Einstein statistics are bosons. The alpha particle, for instance, is a boson.
These classes of particles have different properties. For instance, the Pauli exclusion principle (see chapter 5) applies not only to electrons but to all fermions. It does not, however, apply to bosons.
It is easy to understand how a charged particle sets up a mag
netic field, but not so easy to see why the uncharged neutron should. Yet it unquestionably does. The most direct evidence is that when a neutron beam strikes magnetized iron, it behaves differently from the way it does when the iron is not magnetized. The neutron’s magnetism arises from the strong probability that (as we shall see) the particle is made up of other particles that do carry electric charge. These cancel each other out over the neutron as a whole but somehow manage to set up a magnetic field when the particle spins.
In any case, the spin of the neutron gives us the answer to the question of what the antineutron is. It is simply a neutron with its spin direction reversed; its south magnetic pole, say, is up instead of down. Actually the proton and antiproton and the electron and positron show exactly the same pole-reversed phenomenon.
Antiparticles can undoubtedly combine to form antimatter, as ordinary particles form ordinary matter (figure 7.6). The first actual example of antimatter was produced at Brookhaven in 1965. There the bombardment of a beryllium target with 7 Bev protons produced combinations of antiprotons and antineutrons, something that was an antideuteron. Antihelium-3 has since been produced; and undoubtedly, if enough pains are taken, still more complicated antinuclei can be formed. The principle is clear, however, and no physicist doubts it. Antimatter can exist.
Figure 7.6. An atom of hydrogen and an atom of its antimatter counterpart, consisting of an antiproton and a positron.
But does it exist in actuality? Are there masses of antimatter in the universe? If there were, they would not betray themselves from a distance. Their gravitational effects and the light they produce would be exactly like that of ordinary matter. If, however, they encountered ordinary matter, the massive annihilation reactions that result ought to be most noticeable. It ought to be, perhaps, but it is not. Astronomers have not spied any energy bursts anywhere in the sky that can be identified unequivocally as the result of matter-antimatter annihilation. Can it be, then, that the universe is almost entirely matter, with little or no antimatter? If so, why? Since matter and antimatter are equivalent in all respects but that of electromagnetic oppositeness, any force that would create one would have to create the other, and the universe should be made of equal quantities of each.
This is a dilemma. Theory tells us there should be antimatter out there; and observation refuses to back it up. Can we be sure that observation is failing us? What about the cores of active galaxies and, even more so, quasars? Might those energetic phenomena be the result of matter-antimatter annihilation? Probably not! Even such annihilation does not seem enough, and astronomers prefer to accept the notion of gravitational collapse and black hole phenomena as the only known mechanism that would produce the required energy.
COSMIC RAYS
What about cosmic rays, then? Most of the cosmic-ray particles have energies between 1 and 10 Bev. This might be accounted for by matter-antimatter interaction, but a few cosmic particles run much higher: 20 Bev, 30 Bev, 40 Bev (see figure 7.7). Physicists at the Massachusetts Institute of Technology have even detected some with the colossal energy of 20 billion Bev. Numbers such as this are more than the mind can grasp, but we may get some idea of what that energy means when we calculate that the amount of energy represented by 20 billion Bev would be enough to enable a single submicroscopic particle to raise a 4-pound weight 2 inches.
Figure 7.7. Smashing of a silver atom by a 30,000-Bev cosmic ray. The collision of the cosmic particle with the silver nucleus produced ninety-five nuclear fragments, whose tracks form the star.
Ever since cosmic rays were discovered, people have wondered where they came from and how they arise. The simplest concept is that somewhere in the galaxy—perhaps in our sun, perhaps farther away—there are nuclear reactions going on which shoot forth particles with the huge energies we find them possessing. Indeed, bursts of mild cosmic rays occur every other year or so (as was first discovered in 1942) in connection with flares from the sun. What, then, of such sources as supernovae, pulsars, and quasars? But there is no known nuclear reaction that could produce anything like 20 billion Bev. The mutual annihilation of the heaviest nuclei of matter and antimatter would liberate speeding particles with energies of, at most, 250 Bev.
The alternative is to suppose, as Fermi did, that some force in space accelerates the cosmic particles. They may come originally with moderate energies from explosions such as supernovae and gradually be accelerated as they travel through space. The most popular theory at present is that they are accelerated by cosmic magnetic fields, acting like gigantic synchrotrons. Magnetic fields do exist in space, and our galaxy as a whole is thought to possess one, although this can at best be but 1/20,000 as intense as the magnetic field associated with the earth.
Traveling through this field, the cosmic particles would be slowly accelerated in a curved path. As they gained energy, their paths would swing out wider and wider until the most energetic ones would whip right out of the galaxy. Although most of the particles would never reach this escape trajectory, because they would lose energy by collisions with other particles or with large bodies, some would. Indeed, the most energetic cosmic particles that reach us may be passing through our galaxy after having been hurled out of other galaxies in this fashion.
THE STRUCTURE OF THE NUCLEUS
Now that so much has been learned about the general makeup and nature of the nucleus, there is great curiosity as to its structure, particularly the fine structure inside. First of all, what is its shape? Because it is so small and so tightly packed with neutrons and protons, physicists naturally assume that it is spherical. The fine details of the spectra of atoms suggest that many nuclei have a spherical distribution of charge. Some do not: they behave as if they have two pairs of magnetic poles, and these nuclei are said to have quadrupole moments. But their deviation from the spherical is not large. The most extreme case is that of the nuclei of the lanthanides, in which the charge distribution seems to make up a prolate spheroid (football-shaped, in other words). Even here the long axis is not more than 20 percent greater than the short axis.
As for the internal structure of the nucleus, the simplest model pictures it as a tightly packed collection of particles much like a drop of liquid, where the particles (molecules) are packed closely with little space between, where the density is virtually even throughout, and where there is a sharp surface boundary.
This liquid-drop model was first worked out in detail in 1936 by Niels Bohr. It suggests a possible explanation of the absorption and emission of particles by some nuclei. When a particle enters the nucleus, one can suppose, it distributes its energy of motion among all the closely packed particles, so that no one particle receives enough energy immediately to break away. After perhaps a quadrillionth of a second, when there has been time for billions of random collisions, some particle accumulates sufficient energy to fly out of the nucleus.
The model could also account for the emission of alpha particles by the heavy nuclei. These large nuclei may quiver as liquid drops do if the particles making them up move about and exchange energy. All nuclei would so quiver, but the larger nuclei would be less stable and more likelyto break up. For that reason, portions of the nucleus in the form of the two-proton, two-neutron alpha particle (a very stable combination) may break off spontaneously from the surface of the nucleus. The nucleus becomes smaller as a result, less liable to break up through quivering, and is finally stable.
The quivering may result in another kind of instability, too. When a large drop of liquid suspended in another liquid is set wobbling by currents in the surrounding fluid, it tends to break up into smaller spheres, often into roughly equal halves. It was eventually discovered in 1939 (a discovery I will describe quite fully in chapter 10) that some large nuclei could indeed be made to break down in this fashion by bombardment with neutrons. This is called nuclear fission.
In fact, such nuclear fission ought to take place sometimes without the introduction of a disturbing particle from outside. The internal quivering shou
ld, every once in a while, cause the nucleus to split in two. In 1940, the Soviet physicists G. N. Flerov and K. A. Petrjak actually detected such spontaneous fission in uranium atoms. Uranium exhibits instability mainly by emitting alpha particles, but in a pound of uranium there are four spontaneous fissions per second while about 8 million nuclei are emitting alpha particles.
Spontaneous fission also takes place in protactinium, in thorium, and, more frequently, in the transuranium elements. As nuclei get larger and larger, the probability of spontaneous fission increases. In the heaviest elements of all it becomes the most important method of breakdown, far outweighing alphaparticle emission.
Another popular model of the nucleus likens it to the atom as a whole, picturing the nucleons within the nucleus, like the electrons around the nucleus, as occupying shells and subshells, each affecting the others only slightly. This is called the shell model.
By analogy with the situation in the atom’s electronic shells, one may suppose that the nuclei with filled outer nucleonic shells should be more stable than those whose outer shells are not filled. The simplest theory would indicate that nuclei with 2, 8, 20, 40, 70, or 112 protons or neutrons, would be particularly stable. This, however, does not fit observation. The German-American physicist Maria Goeppert Mayer took account of the spin of protons and neutrons and showed how this would affect the situation. It turned out that nuclei containing 2, 8, 20, 50, 82, or 126 protons or neutrons would then be particularly stable—as fitted the observations. Nuclei with 28 or 40 protons or neutrons would be fairly stable. All others would be less stable, if stable at all. These shell numbers are sometimes called magic numbers (with 28 or 40 occasionally referred to as semi-magic numbers.)
