CHAPTER VI. Reactions Between Nuclei

Reactions Between Nuclei

The alchemists tried to transform one element into another artificially. They used heat, they used chemicals; they even used witchcraft. They failed. Their simplest method—to heat the substance in order to transform it—was really correct. The trouble was that their temperatures were too low by a factor of more than 10,000. What is needed, is a temperature of the order of tens of millions of degrees.

At such high temperatures two nuclei may occasionally approach each other in spite of the electrical repulsion between them. Sometimes they may even get close enough to each other to undergo a nuclear reaction. This, of course, happens with least difficulty if the nuclear charge is small. Hydrogen nuclei, which carry charge 1, participate in such reactions most easily.

In the interior of stars temperatures range from about 10 to 100 million degrees, and nuclear reactions do occur. The reaction responsible for the production of energy in the stars is:

4H¹ → He⁴ + energy

Four protons combine to make an alpha particle with a release of energy. Actually this reaction does not take place all at once but several steps are required. That energy should be released, one expects from the fact that the alpha particle is very stable. Any process in which light nuclei combine to form a heavier nucleus with a release of energy is known as “fusion.”

The particular fusion process that goes on in the stars releases its energy in many forms: as positrons, neutrinos, electromagnetic radiation, and motion of the reacting particles. The positrons also carry off the excess charge of the reaction.

The neutrinos fly through the star without interacting, carrying their energy away into outer space, probably never again to make contact with the material universe. The remainder of the fusion energy is deposited within the star’s interior, which is thus kept hot enough so that the fusion reaction can keep going. The name “thermonuclear” is appropriately applied to this type of reaction.

A lot of effort and imagination is being devoted to the problem of making a controlled thermonuclear reaction. The motivation for this project comes from the fact that good thermonuclear fuels, such as deuterium (H²), are abundant and cheap. There is enough deuterium in the oceans of the world to supply man’s energy needs for many millions of years. One difficulty, of course, is to find a container for the reaction.

Even under stellar conditions the rate of fusion reactions is not very great. It takes approximately a billion years for only one per cent of the nuclei to react. Consequently even higher temperatures than those found in stars are required to produce large amounts of energy in a short time. But no known materials can withstand temperatures of more than a few thousand degrees centigrade. One idea is to keep the “burning” fuel away from material walls by means of magnetic fields.

Is there a way to make nuclei react without the extreme temperatures needed in the thermonuclear reactions? What one is really trying to do is bring two nuclear particles into intimate enough contact so that the nuclear forces can act between them. There is no reason why one should not use a cold target material, which is bombarded from the outside by energetic nuclear projectiles, for example protons or alpha particles. The projectiles, if they are energetic enough, can overcome the electrical repulsion of the target nuclei, and they actually can penetrate. The resulting “compound” nuclei would either be unstable and disintegrate instantaneously, or else be almost stable (i.e., radioactive) and disintegrate after some period of time. In either case nuclei of new elements would probably be formed in the reaction. This procedure sounds simple, but it has its difficulties.

The main difficulty is that the nucleus is a very tiny target. Its area is about 100 million times smaller than the area of the atom as a whole. If a piece of matter is bombarded by an energetic particle, chance alone will determine whether the particle is directed toward a nucleus. To be sure, if the particle misses the nucleus of one atom, it still has the opportunity of hitting the nuclei of other atoms which may lie in its path. It does not have many such opportunities, however, because, being charged, it constantly interacts with the atomic electrons, which gradually absorb energy causing the particle to slow down.

As the particle slows down, its chance of hitting a nucleus decreases, even if it is heading directly toward one, because of the repulsion between its charge and that of the nucleus. Unless the particle has sufficient speed, it cannot overcome this repulsion.

Charged particles may be given the required speeds by accelerating them through large electric fields. If a unit charge is accelerated through a potential difference of one volt, it acquires an energy of one electron-volt. The energies required for nuclear bombardment are of the order of several million electron-volts, which can be provided by atom-smashing machines such as the cyclotron.

Even at such high energies very few of the nuclear projectiles actually find their way to a target nucleus. Most of them are slowed down by the electrons, wasting their energy in heating up the target material. Perhaps one particle out of a million will be lucky enough to induce a nuclear reaction.

If the purpose of the nuclear accelerating machines were to produce cheap energy, they would not be of much value. A nuclear reaction may typically release five to 20 million electron-volts of energy. But to obtain this reaction, a million particles had to be accelerated to energies of several million electron-volts. The recoverable and useable energy will be only a minute fraction of the total invested.

On the other hand, as a tool for scientific discovery, the atom-smashers have been of great importance. That one event in a million has given us much of our knowledge of nuclear physics.

The achievement of nuclear reactions by particle bombardment did not actually wait on the invention of man-made accelerating machines. Energetic alpha particles are available from the radioactive decay of heavy elements. In 1919 Ernest Rutherford used such radioactive elements as a source of alpha particles. The alpha particles were made to bombard ordinary nitrogen, causing the reaction:

He⁴ + N¹⁴ → O¹⁷ + proton (2 protons) (7 protons) (8 protons) (2 neutrons) (7 neutrons) (9 neutrons)

That is, an alpha particle plus a nitrogen¹⁴ nucleus react to produce a nucleus of (stable) oxygen¹⁷ plus a proton. Oxygen¹⁷ is a nucleus with 8 protons and 9 neutrons. The ordinary abundant form of oxygen has 8 protons and 8 neutrons. Natural oxygen contains a small amount of oxygen¹⁷.

Later, in 1934, Irene Curie Joliot (the daughter of the discoverer of radium, Madame Curie) and her husband, Frederic Joliot, used naturally available alpha particles to make artificial radioactive nuclei for the first time. The reaction was:

He⁴ + aluminum²⁷ → phosphorus³⁰ + neutron (2 protons) (13 protons) (15 protons) (2 neutrons) (14 neutrons) (15 neutrons)

Phosphorus³⁰ is an unstable nucleus and emits a beta ray (a positron) to become silicon³⁰ (which is stable). The half-life for this decay is about 2.5 minutes. The Joliots’ reaction was the first instance in which man had produced radioactivity and known it. Actually cyclotrons had been producing radioactivity in good abundance for the preceding two years—but physicists had been unaware of this fact.

It is amusing that nature has also provided us with an atom-smashing machine and indeed one that produces far greater energies than any apparatus yet devised by man. This machine operates on the principle of fluctuating, turbulent magnetic fields in interstellar space. Cosmic particles—mainly protons, but also some alpha particles and even heavier nuclei—are accelerated by these changing magnetic fields and hurled occasionally into the earth’s atmosphere. The energies of these cosmic particles are enormous, ranging from billions of electron-volts to values a million times higher.

When a cosmic particle gets inside the earth’s atmosphere, it does not go far before colliding with a nucleus of nitrogen or oxygen. Out of this nuclear event emerge all the fundamental particles mentioned so far, and some others known as mesons. Mesons are particles which may be charged or neutral, and which have a weight a few hundred times that of the electron. Some of these particles are believed to be connected with the forces that hold the nucleus together.

The nuclear debris from the collision will itself be very energetic and will further disrupt other nitrogen and oxygen nuclei. There soon develops a cascade of electrons, positrons, mesons, neutrons, protons, and electromagnetic radiation moving toward the surface of the earth.

About once a second every square inch of the earth’s atmosphere receives such an energetic particle from outer space. The cascade that results carries penetrating radiations to the surface of the earth. All living organisms are constantly subjected to this radiation background. It is an important fact that the intensity of this radiation is reduced in its passage through the air, and inhabitants of Denver or Lima receive more cosmic radiation than the inhabitants of Los Angeles or New York.

Some neutrons made by collisions of the primary cosmic particles in the atmosphere may collide with nuclei of nitrogen. When this happens, the following reaction occurs:

nitrogen¹⁴ + neutron → carbon¹⁴ + proton (7 protons) (6 protons) (7 neutrons) (8 neutrons)

Carbon¹⁴ is a radioactive electron emitter with a half-life of 5,600 years. This half-life is long enough so that much of the carbon¹⁴ in the world today was probably made ten to twenty thousand years ago. Willard Libby studied this process in a careful and quantitative way, traced the history of the radioactive carbon from the atmosphere into living beings, and, by measuring the carbon¹⁴ content in historical remains, opened up a whole new branch of archeology.

Living organisms breathe in carbon (in the form of carbon dioxide) from the air. Most of this carbon is ordinary stable carbon¹²; a tiny fraction is radioactive carbon¹⁴. The organism is unable to distinguish between the two isotopes, and takes in carbon¹⁴ in the same ratio to carbon¹² as exists in the atmosphere. This ratio persists throughout the organism’s lifetime, but when the organism dies and no more carbon is assimilated, the ratio begins to decrease as the carbon¹⁴ nuclei gradually disintegrate. By observing the ratio of carbon¹⁴ to carbon¹² in fossil remains and other archeological objects, the date at which death occurred can be calculated. In this way the age of ancient Egyptian mummies has been found, and it has been shown that some sequoia wood is more than 1,500 years old. By measuring the carbon¹⁴ in trees that were killed by the last advance of glaciation, and looking into other remains of life from the last ice age, it has been possible to show that this last ice age occurred only 10,000 years ago—instead of 20,000 years, as had been previously believed. Carbon¹⁴-dating has therefore thoroughly revised our ideas about the rapidity with which the empires known to history have emerged from the most primitive conditions. A crucial part of the argument is that isotopes of the same element are chemically indistinguishable.

An alternative reaction which may occur when neutrons strike nitrogen, is

N¹⁴ + neutron → carbon¹² + H³ (7 protons) (6 protons) (1 proton) (7 neutrons) (6 neutrons) (2 neutrons)

H³, triton, is also radioactive, undergoing a beta decay to become He³ (2 protons and 1 neutron) with a half-life of 12.25 years. Tritons too can be used for dating old objects—for example, old wine. The water in the wine cannot be replenished with cosmic-ray tritons after the wine has been bottled. Thus fifty per cent of the tritons disappear every 12.25 years.

We have here two examples of nuclear reactions induced by neutron bombardment. Recalling the disadvantages of charged particles as nuclear projectiles for alchemists, it must surely seem that neutrons would be ideal for this purpose. Being chargeless, they are neither electrically repelled by the nuclei nor constantly slowed down by energy-losing collisions with the electrons. The fate of almost every neutron moving in a large piece of matter is eventual collision with a nucleus. Neutrons are ideal nuclear projectiles, except for one thing: they are hard to get.

Protons and alpha particles are found abundantly in nature as the nuclei of hydrogen and helium atoms. Neutrons, however, are not found in nature, and in the past have been made in nuclear reactions that were themselves initiated by charged particles. For example,

He⁴ + beryllium⁹ → C¹² + neutron (2 protons) (4 protons) (6 protons) (2 neutrons) (5 neutrons) (6 neutrons)

But now we encounter again the difficulty associated with charged particles. Only one alpha particle in a million undergoes a nuclear reaction to produce a neutron. The neutron, of course, makes a nuclear reaction every time. Over-all, then, we obtain two nuclear reactions per million nuclear projectiles, instead of one per million. With such methods we are not so much better off than the old alchemists. A cheap and plentiful source of neutrons would, however, put the alchemist in business. In this way one could make rare elements and radioactive isotopes, and what is more important, he would be able to utilize concentrated nuclear energy.