Nuclear Physics
Page 6
II. ARTIFICIAL NUCLEAR TRANSMUTATIONS
We have thus described those processes in which the atoms themselves provide us with clues to the properties of their nuclei; now we shall proceed immediately to consider those experiments aimed at gaining more exact and detailed information concerning the nucleus, by means of interference from outside. Again, it was Rutherford who took the first step in this direction, too. He discovered the proper tool for the artificial transmutation of one atom into another—bombardment of atoms by alpha particles. In 1919, he achieved by this method the transmutation of an element: he turned nitrogen into oxygen. But let it not be assumed that this method offered the means of transmuting ponderable quantities. The transmutation affected a very small number of atoms only; but, of course, this does not detract from the fundamental significance of this discovery.
Rutherford found that a certain type of radiation, consisting of positively charged hydrogen atoms—nuclei of hydrogen—was emitted when nitrogen atoms were bombarded by alpha rays. The nucleus of a hydrogen atom is an elementary particle, and as we shall see later, it is—for this very reason—one of the most important fundamental building blocks of matter. Consequently, it has been named proton. When nitrogen atoms are bombarded by alpha particles—in other words, by helium nuclei—a proton is emitted occasionally by the nitrogen nucleus. The alpha particle remains in the nucleus. The laws of the conservation of mass and energy permit us to calculate what happens to the nucleus in such a case. The nitrogen nucleus has the mass number 14 and the nuclear charge number (atomic number) 7; its symbol, therefore, is 7N14. The mass number and charge number of the alpha particle are, respectively, 4 and 2, and its symbol is 2He4, as already mentioned; the symbol of the proton is 1H1. As regards mass, the nitrogen nucleus is changed by the absorption of an alpha particle and the loss of a proton, as follows:
14 + 4 − 1 = 17
But as regards its charge, the following equation applies:
7 + 2 − 1 = 8
Thus, a nucleus of mass 17 and nuclear charge (atomic number) 8 is formed. This nuclear charge number indicates that it is an oxygen atom, but the mass number 17 does not agree with the mass number of the common oxygen atom, which is 16. The truth is that this is a rare isotope of oxygen. We shall discuss such isotopes in due course. This transformation of 7N14 into 8O17 is the renowned first instance of the artificial transmutation of elements.
Such nuclear reactions can be represented by appropriate formulae. The formula of this particular reaction is:
Such nuclear transformations can be observed in the cloud chamber (Blackett). But since these are very rare phenomena, many thousands of photographs must be taken in order to be lucky enough eventually to see a nuclear transmutation take place.
In Figure 10 we see a large number of alpha particle tracks, going from right to left. But at one point, an individual diagonal track crosses the main path of the alpha particles, travelling upwards and slanting to the right. If we study the photograph more closely, we can detect a second very dense track, which starts out from the point of origin of the former and runs diagonally to the left sloping slightly downwards. This point is where a nuclear transmutation took place, produced by an alpha particle. These two tracks are: first, the path of the proton knocked out of the nucleus, and, second, the path of the transformed nucleus which received a powerful impact in this process.
These experiments indicate that protons are, in all probability, the fundamental building blocks of atomic nuclei, and this is reminiscent of Prout’s hypothesis that all atoms are formed from hydrogen.
Now in 1932 a particle of a previously totally unknown kind was discovered, which can be knocked out cf atomic nuclei by an analogous method. This discovery was the achievement of three scientists, Joliot, Curie and Chadwick, who followed a path first taken by Bothe in Germany. This particle, of practically the same mass as a proton, carries no electric charge, and therefore leaves no visible track in the cloud chamber. It was named neutron. The first nuclear reaction in which the emission of a neutron could be observed, was the transmutation of beryllium. Beryllium atoms, mass number 9, atomic number 4 (4Be9), were bombarded by alpha particles, 2He4; the product was a carbon nucleus of mass number 12 and atomic number 6. The following equations show the masses and charges (mass numbers and atomic numbers) of the particles involved:
9 + 4 − 12 = 1 and 4 + 2 − 6 = 0
Figure 10.—Transformation of a nitrogen nucleus into an oxygen nucleus.
As we see, in this process, a particle of mass number 1 and charge (atomic) number 0 is ejected—a neutron. We designate this particle by the symbol 0n1, and we can express this nuclear reaction (this is the name given to such processes) as well as those previously discussed, by formulae. We take into consideration also that in addition to the neutron a gamma-ray photon (its symbol is y) is frequently emitted, too. The formula of the nuclear reaction just mentioned reads, therefore:
4Be9 + 2He4 → 6C12 + 0n1 + γ
In such nuclear reactions it frequently happens that unstable atoms are produced; radioactive atoms, not occurring in nature, which change after a certain length of time into some stable atomic species. In the cases hitherto known, this process occurs with emission of electrons or positrons only. This fact completes our survey of the nature of the particles which issue forth from atomic nuclei, whether by spontaneous emission or due to outside interference.
III. THE BUILDING BLOCKS OF ATOMIC NUCLEI
With the knowledge which we have gained we can now proceed to consider the question, which of the elementary particles can be regarded as the ultimate building blocks of atomic nuclei. Let us, first of all, enumerate these particles once again? The proton and the neutron, the electron and the positron, the neutrino, and the gamma-ray photon. There are, furthermore, the alpha particles. But in view of the mass and charge numbers of the latter, we may assume that they are not elementary particles at all, but composite structures. However, our list of elementary particles is still incomplete. In the first place, it does not include the antineutrino, which is the opposite number of the neutrino, as the positron is the opposite number of the electron. While it has the same near-zero mass and zero charge, it differs from the neutrino as regards a property which is present in all elementary particles, and which we have not mentioned as yet: Its spin (or as it is called technically, its angular momentum) is opposite in direction for a given direction of the magnetic moment. Many elementary particles behave mechanically like small spinning tops. But their angular momenta can have only certain definite values, which can be accounted for by quantum mechanics. In the cases of the elementary particles with which we are concerned here, this value is, generally, /2 or h; this is an abbreviated symbol for h/2π, and h is Planck’s constant. As a general rule, the effect of this spin moment is that the particles possess a magnetic moment, in other words, they act like small magnets. In the case of heavier particles, this magnetic moment is measured in terms of a unit called nuclear magneton (n.m.), while for lighter particles a larger unit, called Bohr magneton (B.m.) is used. (See Table I.)
There is still another elementary particle we must mention; it is called meson, for its mass is between that of an electron and that of a proton (the Greek word ‘meson’ means ‘the middle one’). Mesons can be observed in cosmic radiation, and before we go into further details about their properties, we must first say a few words about the nature of cosmic radiation.
As a result of the studies of Hess and Kohlhörster, we have known for almost forty years that a faint, continuous, extremely penetrating radiation reaches the earth from outer space, and in the atmospheric envelope of the earth it releases every possible sort of secondary radiation, akin in character to radioactive radiation. However, it is only since 1947 that we have known anything at all about the cause of this remarkable phenomenon. This knowledge is the fruit of the work of Forbush and Ehmert, who were able to prove that when certain eruptions take place on the surface of the sun, the cosmic radiation inc
ident on the earth suddenly gains in intensity. Thus, cosmic radiation is probably produced by big, periodically changing electromagnetic fields on the surfaces of the stars, and (according to Unsöld), on the surfaces of the many red stars in particular, or in intersteller space. These electromagnetic fields can assume very large magnitudes and accelerate charged particles to extremely high velocities (according to Bagge and Biermann, especially when a strong spot activity exists on the stellar surface). The hydrogen nuclei are the ones that are accelerated this way in the first place, for the stars consist mostly of hydrogen, but this effect is imparted also to the nuclei of heavier atoms; they leave the stars and speed through cosmic space as the primary cosmic radiation. The particles hit the atmosphere of the earth from outer space, with kinetic energies ranging mostly from 109 to 1010 electron-volts. But particles of energies up to 1016 electron-volts have also been observed. In the earth’s atmospheric envelope, particles of such enormous energy produce not only atomic decay but also nuclear transmutations of all sorts, creating various kinds of elementary particles in so doing.
This is how the elementary particle just mentioned, the meson, was discovered (Anderson). The importance of this particle for nuclear physics is still uncertain. We know that its mass is approximately two hundred times the mass of an electron, that it carries one elementary charge of electricity, and that there are both positive and negative mesons. Beyond this, not much is known about its properties as yet. In 1947, Powell discovered another elementary particle, the mass of which is about three hundred times the mass of an electron. It is called heavy meson or π-particle. The researches of Leprince-Ringuet, Rochester and Butler have shown the probability of the existence of a still heavier elementary particle, of approximately nine hundred times the electronic mass.
Table Ic contains a list of all the above-mentioned elementary particles and their respective properties. By calling them ‘elementary particles’, we mean that they are not composed of still smaller particles, in contrast to the chemical atoms which obviously can be split up into component parts. But this does not indicate by any means that these elementary particles cannot be transformed. On the contrary, transformability is a characteristic of elementary particles. A photon can change into an electron plus a positron, and conversely, a photon can originate from an electron and a positron. But it would be wrong, or at least not advisable, to say therefore that a photon is a combination of an electron and a positron. For, conversely, a photon can be the product of an electron, for instance, when the electron jumps from one state to another. Moreover, a proton can change into a neutron and a positron, or a neutron into a proton and an electron. But one can hardly say that a proton is made up of a neutron and a positron. All these are true elementary particles, of which convertibility happens to be one of the characteristic properties.
In our survey of these elementary particles, we must look for properties capable of giving us a clue as to which ones are to be regarded as true building blocks of atomic nuclei, and which play a different part. For instance, the fact that a particle can emerge from the nucleus, is still not a proof of its being an ultimate, fundamental component part of the latter. This is evidenced by a simple consideration of the extra-nuclear structure of the atom. It consists of electrons. Yet, occasionally, also other particles emerge from it—namely: photons. But since these make an appearance only when certain changes in state of the extranuclear atomic structure occur, they are not referred to as integral component parts of that structure. Thus we make a distinction between particles which are always present in the extranuclear atomic structure—which we designate as its true component parts—and those which are produced within it, occasionally, due to changes in state, and depart from it subsequently. In the case of the extranuclear atomic structure, the former are the electrons, and the latter are the photons. In a certain sense, though, one might of course say that the photons were already present in that extranuclear structure. For although the space between the electron shells is empty (fast particles can penetrate it without any difficulty), yet it contains something—the electric field, which, to use an analogy, acts like the mortar that binds the building blocks of the extranuclear structure of the atom to the nucleus. In the wave aspect, the emitted light is an electromagnetic wave, the energy of which is drawn from that of this field. Fundamentally, it is merely a question of terminology whether we call this field a kind of substance or a property of space, and in this sense the photons may be said to have already been present as a field in the extranuclear structure. Nevertheless, it is helpful to make a distinction between the true building blocks of the nucleus and the field that holds them together, although this distinction cannot be of any fundamental importance. In any case, with respect to the extranuclear structure of the atom, there is a good reason for referring solely to electrons as its elementary building blocks and crediting the field with the capacity of occasionally producing photons. The following distinction may be made between the two kinds of particles: whenever the extranuclear atomic structure is subjected to outside interference—as, for instance, to bombardment by electrons or photons—the result is either that an extranuclear, planetary electron is detached and hurled out of the atom, or that the extranuclear structure itself remains in an excited state, from which it will revert to its original state through the emission of a photon only. But whereas the electron is ejected in the very moment of the interference, the state in which the photon is formed and emitted does not ensue immediately; a certain length of time must always elapse first. This length of time is very short in absolute duration, but of considerable length relative to the time required for a complete revolution of a planetary electron around the nucleus. We may regard this as a characteristic which distinguishes true elementary particles from secondarily formed ones.
Proceeding from the same viewpoint, let us now examine those elementary particles which merit immediate consideration as building blocks of the nucleus. The nucleus, too, can be subjected to outside interference, in the form of bombardment by alpha particles or other elementary particles. As we have already seen, nuclear reactions may then take place, in which either a proton or a neutron is hurled forth. There are also instances where alpha particles are ejected. But it is certain that the latter are not true elementary particles. The ejection of a proton or neutron—like that of an extranuclear, planetary electron—usually occurs in the moment of interference. But in such a nuclear reaction an unstable, radioactive atom may be produced, which then changes further by a radioactive process. In these cases, only electrons or positrons, as well as neutrinos, are emitted. Like the naturally radioactive substances, these radioactive atoms have a definite decay probability or average life, which varies according to the individual instance. Thus, a longer or shorter period of time must elapse before the electron or positron is ejected, accompanied by a neutrino, as in the case of the emission of photons from the extranuclear atomic structure. On the other hand, there are also cases in which outside interference will result in the emission of a gamma-ray photon. Generally speaking, the time interval is long when measured in terms of nuclear frequencies, but extremely short in absolute duration, and it cannot be measured directly, but only by inference from other data. It occasionally happens, of course, that a gamma-ray photon is ejected in the very moment of the interference.
From the above findings we can conclude that only the protons and the neutrons may be regarded as true building blocks of the nucleus. This conclusion is very close to Prout’s old hypothesis. The mass of a neutron differs very little from that of a proton.
The ideas discussed are illustrated in the following table by a comparative survey of the conditions which obtain in the extranuclear structure of the atom, on the one hand, and those present in the nucleus, on the other hand.
This first horizontal row contains the fundamental building blocks, while the second one shows the field acting between them, and in the third one we find the particles emitted in the occasional changes of state
.
We have still to inquire into the nature of the force which binds these building blocks of the nucleus together. It would seem logical to assume that this field, like the one within the extranuclear structure, is electrical. It is, however, easy to show that electric forces alone would not be sufficient to provide an explanation of nuclear cohesion, for the simple reason that the strongest electric effects are due to the charges carried. by the protons, and these are forces of repulsion. Therefore, a further field of another type must be operative in the nucleus. For lack of a more exact knowledge of the nature of this field, let us first give it a name, and call it nuclear field. Of course, in addition to it, there exists also an electric field in the nuclei, since the protons carry electric charges.
In the extranuclear structure of the atom, changes in state are accompanied by the production of particles—photons—out of the energy of the electric field. In considering the nucleus, we shall, analogously, look for particles which come into being, in conjunction with changes in state, out of the energy of its two fields, and which occasionally—in excited states—detach themselves and are emitted. Again, photons alone can correspond to the electric field; in fact, it has been discussed that in nuclear transmutations gamma-ray photons are frequently emitted, which are simply photons of an extremely short wavelength. It is obvious, therefore, that the nuclear field must be correlated with the other particles which are emitted in nuclear transformations after the lapse of a certain period of time only—namely, the electrons, positrons and neutrinos. This analogy between the extranuclear structure of the atom and its nucleus gives us a relatively simple picture of the latter.