Worlds Within Worlds: The Story of Nuclear Energy, Volume 3 (of 3) Nuclear Fission; Nuclear Fusion; Beyond Fusion — Key Ideas to Explore

had found that slow neutrons, which had very little energy, were easily absorbed by atomic nuclei—more easily than fast neutrons were absorbed, and certainly more easily than charged particles were.

Often what happened was that the neutron was simply absorbed by the nucleus. Since the neutron has a mass number of 1 and an atomic number of 0 (because it is uncharged), a nucleus that absorbs a neutron remains an isotope of the same element, but increases its mass number.

For instance, suppose that neutrons are used to bombard hydrogen-1, which then captures one of the neutrons. From a single proton, it will become a proton plus a neutron; from hydrogen-1, it will become hydrogen-2. A new nucleus formed in this way will be at a higher energy and that energy is emitted in the form of a gamma ray.

Sometimes the more massive isotope that is formed through neutron absorption is stable, as hydrogen-2 is. Sometimes it is not, but is radioactive instead. Because it has added a neutron, it has too many neutrons for stability. The best way of adjusting the matter is to emit a beta particle (electron). This converts one of the neutrons into a proton. The mass number stays the same but the atomic number increases by one.

The element rhodium, for example, which has an atomic number of 45, has only 1 stable isotope, with a mass number of 103. If rhodium-103 (45 protons, 58 neutrons) absorbs a neutron, it becomes rhodium-104 (45 protons, 59 neutrons), which is not stable. Rhodium-104 emits a beta particle, changing a neutron to a proton so that the nuclear combination becomes 46 protons and 58 neutrons. This is palladium-104, which is stable.

As another example, indium-115 (49 protons, 66 neutrons) absorbs a neutron and becomes indium-116 (49 protons, 67 neutrons), which gives off a beta particle and becomes tin-116 (50 protons, 66 neutrons), which is stable.

There are over 100 isotopes that will absorb neutrons and end by becoming an isotope of an element one higher in the atomic number scale. Fermi observed a number of these cases.

Having done so, he was bound to ask what would happen if uranium were bombarded with neutrons. Would its isotopes also be raised in atomic number—in this case from 92 to 93? If that were so it would be very exciting, for uranium had the highest atomic number in the entire scale. Nobody had ever discovered any sample of element number 93, but perhaps it could be formed in the laboratory.

In 1934, therefore, Fermi bombarded uranium with neutrons in the hope of obtaining atoms of element 93. Neutrons were absorbed and whatever was formed did give off beta particles, so element 93 should be there. However, four different kinds of beta particles (different in their energy content) were given off and the matter grew very confusing. Fermi could not definitely identify the presence of atoms of element 93 and neither could anyone else for several years. Other things turned up, however, which were even more significant.

Before going on to these other things, however, it should be mentioned that undoubtedly element 93 was formed even though Fermi couldn’t clearly demonstrate the fact. In 1939 the American physicists Edwin Mattison McMillan (1907- ) and Philip Hauge Abelson (1913- ), after bombarding uranium atoms with slow neutrons, were able to identify element 93. Since uranium had originally been named for the planet, Uranus, the new element beyond uranium was eventually named for the planet Neptune, which lay beyond Uranus. Element 93 is therefore called “neptunium”.

What happened was exactly what was expected. Uranium-238 (92 protons, 146 neutrons) added a neutron to become uranium-239 (92 protons, 147 neutrons), which emitted a beta particle to become neptunium-239 (93 protons, 146 neutrons).

In fact, neptunium-239 also emitted a beta particle so it ought to become an isotope of an element even higher in the atomic number scale. This one, element 94, was named “plutonium” after Pluto, the planet beyond Neptune. The isotope, plutonium-239, formed from neptunium-239, was only feebly radioactive, however, and it was not clearly identified until 1941.

The actual discovery of the element plutonium came the year before, however, when neptunium-238 was formed. It emitted a beta particle and became plutonium-238, an isotope that was radioactive enough to be easily detected and identified by Glenn Theodore Seaborg (1912- ), and his co-workers, who completed McMillan’s experiments when he was called away to other defense research.

Neptunium and plutonium were the first “transuranium elements” to be produced in the laboratory, but they weren’t the last. Over the next 30 years, isotopes were formed that contained more and more protons in the nucleus and therefore had higher and higher atomic numbers. At the moment of writing, isotopes of every element up to and including element 105 have been formed.

A number of these new elements have been named for some of the scientists important in the history of nuclear research. Element 96 is “curium”, named for Pierre and Marie Curie; element 99 is “einsteinium” for Albert Einstein; and element 100 is “fermium” for Enrico Fermi.

Element 101 is “mendelevium” for the Russian chemist Dmitri Mendeléev, who early in 1869 was the first to arrange the elements in a reasonable and useful order. Element 103 is “lawrencium” for Ernest O. Lawrence. “Rutherfordium” for Ernest Rutherford has been proposed for element 104.

And “hahnium” for Otto Hahn (1879-1968), a German physical chemist whose contribution we will come to shortly, has been proposed for element 105.

Neptunium, however, was not the first new element to be created in the laboratory. In the early 1930s, there were still 2 elements with fairly low atomic numbers that had never been discovered. These were the elements with atomic numbers 43 and 61.

In 1937, though, molybdenum (atomic number 42) had been bombarded with neutrons in Lawrence’s laboratory in the United States. It might contain small quantities of element 43 as a result. The Italian physicist Emilio Segrè (1905- ), who had worked with Fermi, obtained a sample of the bombarded molybdenum and indeed obtained indications of the presence of element 43. It was the first new element to be manufactured by artificial means and was called “technetium” from the Greek word for “artificial”.

The technetium isotope that was formed was radioactive. Indeed, all the technetium isotopes are radioactive. Element 61, discovered in 1945 and named “promethium”, also has no stable isotopes. Technetium and promethium are the only elements with atomic numbers less than 84 that do not have even a single stable isotope.

The Discovery of Fission

But let us get back to the bombardment of uranium with neutrons research that Fermi had begun. After he had reported his work, other physicists repeated it and also got a variety of beta particles and were also unable to decide what was going on.

One way to tackle the problem was to add to the system some stable element that was chemically similar to the tiny traces of radioactive isotopes that might be produced through the bombardment of uranium. Afterwards the stable element could probably be separated out of the mixture and the trace of radioactivity would, it was hoped, be carried along with it. The stable element would be a “carrier”.

Among those working on the problem were Otto Hahn and his Austrian co-worker, the physicist Lise Meitner (1878-1968). Among the potential carriers they added to the system was the element, barium, which has an atomic number of 56. They found that a considerable quantity of the radioactivity did indeed accompany the barium when they separated that element out of the system.

A natural conclusion was that the isotopes producing the radioactivity belonged to an element that was chemically very similar to barium. Suspicion fell at once on radium (atomic number 88), which was very like barium indeed as far as chemical properties were concerned.

Lise Meitner, who was Jewish, found it difficult to work in Germany, however, for it was then under the rule of the strongly anti-Semitic Nazi regime. In March 1938 Germany occupied Austria, which became part of the German realm. Meitner was no longer protected by her Austrian citizenship and had to flee the country and go to Stockholm, Sweden. Hahn remained in Germany and continued working on the problem with the German physical chemist Fritz Strassman (1902- ).

Although the supposed radium, which possessed the radioactivity, was very like barium in chemical properties, the two were not entirely identical. There were ways of separating them, and Hahn and Strassman busied themselves in trying to accomplish this in order to isolate the radioactive isotopes, concentrate them, and study them in detail. Over and over again, however, they failed to separate the barium and the supposed radium.

Slowly, it began to seem to Hahn that the failure to separate the barium and the radioactivity meant that the isotopes to which the radioactivity belonged had to be so much like barium as to be nothing else _but_ barium. He hesitated to say so, however, because it seemed unbelievable.

If the radioactive isotopes included radium, that was conceivable. Radium had an atomic number of 88, only four less than uranium’s 92. You could imagine that a neutron being absorbed by a uranium nucleus might make the latter so unstable as to cause it to emit 2 alpha particles and become radium. Barium, however, had an atomic number of 56, only a little over half that of uranium. How could a uranium nucleus be made to turn into a barium nucleus unless it more or less broke in half? Nothing like that had ever been observed before and Hahn hesitated to suggest it.

While he was nerving himself to do so, however, Lise Meitner, in Stockholm, receiving reports of what was being done in Hahn’s laboratory and thinking about it, decided that unheard-of or not, there was only one explanation. The uranium nucleus _was_ breaking in half.

Actually, when one stopped to think of it (after getting over the initial shock) it wasn’t so unbelievable at that. The nuclear force is so short-range, it barely reaches from end to end of a large nucleus like that of uranium. Left to itself, it holds together most of the time, but with the added energy of an entering neutron, we might imagine shock waves going through it and turning the nucleus into something like a quivering drop of liquid. Sometimes the uranium nucleus recovers, keeps the neutron, and then goes on to beta-particle emission. And sometimes the nucleus stretches to the point where the nuclear force doesn’t quite hold it together. It becomes a dumbbell shape and then the electromagnetic repulsion of the two halves (both positively charged) breaks it apart altogether.

It doesn’t break into equal halves. Nor does it always break at exactly the same place, so that there were a number of different fragments possible (which was why there was so much confusion). Still, one of the more common ways in which it might break would be into barium and krypton. (Their respective atomic numbers, 56 and 36, would add up to 92.)

Meitner and her nephew, Otto Robert Frisch (1904- ), who was in Copenhagen, Denmark, prepared a paper suggesting that this was what was happening. It was published in January 1939. Frisch passed it on to the Danish physicist Niels Bohr (1885-1962) with whom he was working. The American biologist William Archibald Arnold (1904- ), who was also working in Copenhagen at the time, suggested that the splitting of the uranium nucleus into halves be called “fission”, the term used for the division-in-two of living cells. The name stuck.

In January 1939, just about the time Meitner and Frisch’s paper was published, Bohr had arrived in the United States to attend a conference of physicists. He carried the news of fission with him. The other physicists attending the conference heard the news and in a high state of excitement at once set about studying the problem. Within a matter of weeks, the fact of uranium fission was confirmed over and over.

One striking fact about uranium fission was the large amount of energy it released. In general, when a very massive nucleus is converted to a less massive one, energy is released because of the change in the mass defect, as Aston had shown in the 1920s. When the uranium nucleus breaks down through the ordinary radioactive processes to become a less massive lead nucleus, energy is given off accordingly. When, however, it breaks in two to become the much less massive nuclei of barium and krypton (or others in that neighborhood) much more energy is given off.

It quickly turned out that uranium fission gave off something like ten times as much nuclear energy per nucleus than did any other nuclear reaction known at the time.

Even so, the quantity of energy released by uranium fission was only a tiny fraction of the energy that went into the preparation of the neutrons used to bring about the fission, if each neutron that struck a uranium atom brought about a single fission of that 1 atom.

Under those conditions, Rutherford’s suspicion that mankind would never be able to tap nuclear energy probably still remained true. (He had been dead for 2 years at the time of the discovery of fission.)

However, those were not the conditions.

The Nuclear Chain Reaction

Earlier in this history, we discussed chain reactions involving chemical energy. A small bit of energy can ignite a chemical reaction that would produce more than enough energy to ignite a neighboring section of the system, which would in turn produce still more—and so on, and so on. In this way the flame of a single match could start a fire in a leaf that would burn down an entire forest, and the energy given off by the burning forest would be enormously higher than the initial energy of the match flame.

Might there not be such a thing as a “nuclear chain reaction”? Could one initiate a nuclear reaction that would produce something that would initiate more of the same that would produce something that would initiate still more of the same and so on?

In that case, a nuclear reaction, once started, would continue of its own accord, and in return for the trifling investment that would serve to start it—a single neutron, perhaps—a vast amount of breakdowns would result with the delivery of a vast amount of energy. Even if it were necessary to expend quite a bit of energy to produce the 1 neutron that would start the chain reaction, one would end with an enormous profit.

What’s more, since the nuclear reaction would spread from nucleus to nucleus with millionths-of-a-second intervals, there would be, in a very brief time, so many nuclei breaking down that there would be a vast explosion. The explosion was sure to be millions of times as powerful as ordinary chemical explosions involving the same quantity of exploding material, since the latter used only the electromagnetic interaction, while the former used the much stronger nuclear interaction.

The first to think seriously of such a nuclear chain reaction was the Hungarian physicist Leo Szilard (1898-1964). He was working in Germany in 1933 when Adolf Hitler came to power and, since he was Jewish, he felt it would be wise to leave Germany. He went to Great Britain and there, in 1934, he considered certain new types of nuclear reactions that had been discovered.

In these, it sometimes happened that a fast neutron might strike a nucleus with sufficient energy to cause it to emit 2 neutrons. In that way the nucleus, absorbing 1 neutron and emitting 2, would become a lighter isotope of the same element.

But what would happen if each of the 2 neutrons that emerged from the original target nucleus struck new nuclei and forced the emission of a pair of neutrons from each. There would now be a total of 4 neutrons flying about and if each struck new nuclei there would next be 8 neutrons and so on. From the initial investment of a single neutron there might soon be countless billions initiating nuclear reactions.

Szilard, fearing the inevitability of war and fearing further that the brutal leaders of Germany might seek and use such a nuclear chain reaction as a weapon in warfare, secretly applied for a patent on a device intending to make use of such a nuclear chain reaction. He hoped to turn it over to the British Government, which might then use its possession as a way of restraining the Nazis and keeping the peace.

However, it wouldn’t have worked. It took the impact of a very energetic neutron to bring about the emission of 2 neutrons. The neutrons that then emerged from the nucleus simply didn’t have enough energy to keep things going. (It was like trying to make wet wood catch fire.)

But what about uranium fission? Uranium fission was initiated by slow neutrons. What if uranium fission also produced neutrons as well as being initiated by a neutron? Would not the neutrons produced serve to initiate new fissions that would produce new neutrons and so on endlessly?

It seemed very likely that fission produced neutrons and indeed, Fermi, at the conference where fission was first discussed, suggested it at once. Massive nuclei possessed more neutrons per proton than less massive ones did. If a massive nucleus was broken up into 2 considerably less massive ones, there would be a surplus of neutrons. Suppose, for instance, uranium-238 broke down into barium-138 and krypton-86. Barium-138 contains 82 neutrons and krypton-86 50 neutrons for a total of 132. The uranium-238 nucleus, however, contains 146 neutrons.

The uranium fission process was studied at once to see if neutrons were actually given off and a number of different physicists, including Szilard, found that they were.

Now Szilard was faced with a nuclear chain reaction he was certain would work. Only slow neutrons were involved and the individual nuclear breakdowns were far more energetic than anything else that had yet been discovered. If a chain reaction could be started in a sizable piece of uranium, unimaginable quantities of energy would be produced. Just 1 gram of uranium, undergoing complete fission, would deliver the energy derived from the total burning of 3 tons of coal and would deliver that energy in a tiny fraction of a second.

Szilard, who had come to the United States in 1937, clearly visualized the tremendous explosive force of something that would have to be called a “nuclear bomb”. Szilard dreaded the possibility that Hitler might obtain the use of such a bomb through the agency of Germany’s nuclear scientists.

Partly through Szilard’s efforts, physicists in the United States and in other Western nations opposed to Hitler began a program of voluntary secrecy in 1940, to avoid passing along any hints to Germany. What’s more, Szilard enlisted the services of two other Hungarian refugees, the physicists Eugene Paul Wigner (1902- ) and Edward Teller (1908- ) and all approached Einstein, who had also fled Germany and come to America.

Einstein was the most prestigious scientist then living and it was thought a letter from him to the President of the United States would be most persuasive. Einstein signed such a letter, which explained the possibility of a nuclear bomb and urged that the United States not allow a potential enemy to come into possession of it first.

Largely as a result of this letter, a huge research team was put together in the United States, to which other Western nations also contributed, with but one aim—to develop the nuclear bomb.

Although the theory of the nuclear bomb seemed clear and simple, a great many practical difficulties stood in the way. In the first place, if only uranium atoms underwent fission a supply of uranium had at least to be obtained in pure form, for if the neutrons struck nuclei of elements other than uranium, they would simply be absorbed and removed from the system, ending the possibility of a chain reaction. This alone was a heavy task, since there had been so little use for uranium in quantity that there was almost no supply in existence and no experience in how to purify it.

Secondly, the supply of uranium might have to be a large one, for neutrons didn’t necessarily enter the first uranium atom they approached. They moved about here and there, making glancing collisions, and travelling quite a distance, perhaps, before striking head-on and entering a nucleus. If in that time they had passed outside the lump of uranium, they were useless.

Albert Einstein Old Grove Rd. Nassau Point Peconic, Long Island

F.D. Roosevelt, President of the United States, White House Washington, D.C.

Some recent work by E. Fermi and L. Szilard, which has been communicated to me in manuscript, leads me to expect that the element uranium may be turned into a new and important source of energy in the immediate future. Certain aspects of the situation which has arisen seem to call for watchfulness and, if necessary, quick action on the part of the Administration. I believe therefore that it is my duty to bring to your attention the following facts and recommendations: