How did atomic physicists split the atom and When?

In the early days of atomic physics, the overriding concern was the search for knowledge, pure and simple, unencumbered by the controversial moral and political ramifications that since 1945 have surrounded the atomic bomb.

In the forefront of the drive to reveal the secrets of the atom was one of the greatest physicists of all time: Ernest Rutherford (later Lord Rutherford), the exuberant and highly intuitive son of a Scottish immigrant to New Zealand.

Rutherford employed simple, direct methods that, by virtue of his farsightedness, led him to ground-breaking results. In 1911, for instance, he discovered the nucleus of the atom simply by intercepting a flow of alpha particles with a thin sheet of gold foil.

These particles are one of three types of radiation shown by the Curies to be emitted from radium. The alpha particles, identical to helium atoms minus their electrons, are electrically charged. When some of the high-speed particles bounced straight back from the foil, Rutherford reasoned that they were being deflected from hard, massive units within the atoms: the nuclei.

Using alpha particles again in 1919, Rutherford achieved the first transmutation of elements, which led directly to the splitting of the atom. He issued his alpha particles (helium nuclei) down a glass tube filled with nitrogen gas.

To his amazement, he detected hydrogen nuclei (later called protons) at the other end, although this element had not originally been present in the tube. His projectiles had actually succeeded in dislodging these protons from the nuclei of the nitrogen atoms. This meant that those highly concentrated nuclei were not indestructible, that physicists might indeed probe their nature and structure. Rutherford noted, furthermore, that the proton that was released had greater energy than the original alpha particle. Here was the first clue to the release of atomic power.

Rutherford theorized that if alpha particles could cause transmutation, other particles might be capable of doing the same, given that they had a comparable amount of energy. He decided to build a machine to accelerate protons and conceived of a positive-ray tube requiring 1 million volts. But in 1930 electrical technology was insufficient to generate and maintain such high voltage. The ingenious physicist therefore suggested to his associates, John D. Cockroft and E.T.S. Wilson, that they attempt the experiment with already existing equipment and relatively low-voltage particles.

These two men worked with lithium, a very light, common element. They built a proton gun, containing a hot filament that emitted electrons. Accelerated by a positive voltage, the electrons shot through a hole to a section of the gun where they struck atoms of hydrogen and caused ionization. A strong negative voltage of 150,000 volts was then applied. This attracted the positively charged protons and repelled the electrons back toward the other end of the gun.

Some of the protons passed out of the gun as a beam of 150,000-volt energy particles. Now this beam was directed at the lithium. The protons bombarded the lithium nuclei and combined to raise their atomic mass (i.e., weight) from 7 to 8; the protons also increased lithium’s atomic number (the number of electronic units of positive electrical charge in the nucleus) from 3 to 4. This was too much for the lithium nucleus to bear, and it immediately exploded, splitting precisely in two.

The result: two helium nuclei of atomic weight 4 and nuclear charge 2—alpha particles. Although the energy input was only 150,000 volts, each of the new man-made alpha particles was emitted with a dazzling energy of 8.5 million volts. Not all the protons succeeded in finding their mark, of course. The lithium nuclei were tiny targets, and the vast majority of protons glanced off to the side, missed entirely, or just lost their energy from hitting other electrons. Nevertheless, the proton gun worked and for the first time the atom was split under man’s direct control. The normally self-effacing Cockroft, overwhelmed by the phenomenon, walked through the streets of Cambridge, England, exclaiming to friends and strangers alike, “We’ve split the atom.”

Nineteen thirty-two was also the year of discovery of the third subatomic particle, which Rutherford had predicted some years before. Through exhaustive work night and day over a period of only three weeks, James Chadwick of England proved the existence of the neutron, a particle with mass but no electrical charge. The latter characteristic was highly significant when it came to splitting the atom. Because the neutron is neutral, it can approach the positively charged nucleus without being repelled, and do so even at slow speeds. Positively charged particles, on the other hand, can only get close to the nucleus if fired at tremendous speeds, sufficient to overcome the electric-charge repulsion. It was time to challenge increasingly heavier atoms than the volatile lithium.

Enrico Fermi, a Nobel Prize–winning physicist at the University of Rome, was the first to realize that the neutron could actually be captured and absorbed by the nucleus of an atom, thereby adding to its mass.

Fermi found that a fast stream of neutrons resulted from the impact of alpha rays from radium on metallic beryllium. He then slowed the rapidly moving neutrons by having them pass through blocks of carbon. He subsequently made many new artificial atoms by exposing various elements to the intruding neutrons. One of these elements was uranium, and the effects of exposing it were puzzling.

The resulting new element produced an abnormal amount of radiation. The nuclei were very heavy—heavier than those of uranium, which is the heaviest natural element on earth. Because they were so heavy, Fermi reasoned, they were unstable; they disintegrated, emitting the radiation he had witnessed. The chemistry of the new atoms did not seem to meet the characteristics of any known element. Thus a number of scientists (Fermi himself was skeptical) believed that new elements, called transuranic, had been discovered. It is terrifying to think how close Fermi and his associates came to blowing themselves up along with a good part of Rome. Had they realized what was in fact before them, the atomic bomb might have been developed as early as 1934, giving Hitler plenty of time to follow suit or, on the other hand, discouraging him from launching World War II at all.

When chemist Otto Hahn of the Kaiser Wilhelm Institute in Berlin repeated Fermi’s experiment, his results were the same. But on careful examination of the end products, he found that the common isotope uranium-238, with its 92 protons and 146 neutrons, had become barium, with a much lighter nucleus of 138, comprising 56 protons and 82 neutrons.

It was the persistent, clear thinking of Lise Meitner that penetrated the enigma, which in retrospect should have been obvious from the start. Meitner was Hahn’s associate, a Jew who fled Austria in 1938 and arrived in Goteborg, Sweden, where she was joined by her nephew, physicist Otto Frisch.

Until then, everyone had assumed that the very heavy nucleus of uranium was stable, that despite its many protons, which repelled each other, there were even more neutrons, which kept the whole unit together. At most, small fragments could be broken off, charged particles like the alpha particles Rutherford had used.

Meitner reflected on the analogy of an atom to a drop of water, postulated by the famous Danish physicist Niels Bohr. Just as internal stress can cause a liquid drop to split into two smaller drops, so, Meitner realized, the bombardment of the uranium nucleus with neutrons overloaded the nucleus and caused it to break apart.

Unable to incorporate the neutrons, the nucleus became unstable and divided, the two new nuclei still containing electrical charges of the parent. One part was barium, with its 56 electronic units; the other was found to be krypton, with 36 units. These charges repelled each other, as was expected, but the force with which they did so proved astounding. Using Einstein’s theoretical relationship between mass and energy and precise atomic measurements, Meitner calculated the energy released by the split to be 200 million electron volts, by far the greatest energy source known to man. Uranium fission had been discovered.

Initially, the discovery was not thought to be of much practical use. The splitting of a uranium atom was exceedingly rare, and it was too difficult to sustain a prolonged fire of neutrons.

Nevertheless, scientists throughout the world were excited. Niels Bohr and John A. Wheeler of Princeton theorized that when the uranium atom splits, it releases neutrons, which could go on to break up other nuclei. This would cause a chain reaction, an increasingly fast series of fissions. By March 1939 physicists accepted Bohr’s theory and his insistence that the rare isotope uranium-235, if collected in large enough quantities, could create a new and massive bomb.

The search was on, the wheels of the United States government were set in motion, and the Manhattan Project got under way.