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J Robert Oppenheimer, and the science behind the nuclear bomb

By, New Delhi
Jul 22, 2023 12:35 AM IST

J Robert Oppenheimer is known as the father of the atomic bomb. A science backgrounder on the making of the bomb

American theoretical physicist J Robert Oppenheimer, the subject of Christopher Nolan’s newest film, is known as the father of the atomic (nuclear) bomb. The story of the bomb, however, begins with the science behind it, which was uncovered some years before Oppenheimer (played by Cillian Murphy) headed the laboratory, part of the Manhattan Project, that developed the devastating weapon.

Cillian Murphy in a still from the film Oppenheimer,which is based on J Robert Oppenheimer. (AP) PREMIUM
Cillian Murphy in a still from the film Oppenheimer,which is based on J Robert Oppenheimer. (AP)

A nuclear bomb works on fission, or the splitting of atoms. Atoms bombarded with neutrons split into newer atoms, in the process releasing energy and more neutrons. The newly released neutrons would then strike more atoms and fission them, releasing still more energy and neutrons. This chain reaction ultimately releases a staggering amount of energy.

In the nuclear bombs dropped on Hiroshima and Nagasaki, the fission materials used were uranium-235 and plutonium-239. The science was new at the time the Manhattan Project was launched in 1941-42. The neutron had been discovered in 1932, and fission understood only in 1938-39. The story must, therefore, begin with the neutron.

The neutron and fission

Until the neutron was discovered, the atom was believed to contain only two kinds of particles, the positively charged protons in the nucleus and the negatively charged electrons orbiting it.

The neutron, which resides alongside the proton in the nucleus, was discovered by the English physicist James Chadwick, who would go on to win the Nobel Prize for Physics in 1935. It first emerged in experiments by other scientists, starting 1930, in the form of unexplained radiation when beryllium atoms were bombarded with alpha particles. Chadwick guessed that the radiation must be made of particles, determined their size, found they carried no electric charge, and concluded that these particles were of a new kind. This was the neutron.

The power that the neutron could unleash became apparent soon after. Since it had no charge, it could easily penetrate the nucleus of various atoms, and bombardment, in theory, could convert an atom of one element into an atom of the next higher element in the periodic table.

A team led by the Italian-American Enrico Fermi (played by Danny Deferrari in the film), then in his ancestral country, bombarded uranium with neutrons and thought they had created element 93. It was actually a product of fission, although they did not realise it then.

In 1939, the German scientists Otto Hahn and Fritz Strassman added non-radioactive barium to uranium, bombarded it with neutrons, and found that the barium contained some radioactivity. The riddle was eventually solved by the Austrian-born physicist Lise Meitner, Hahn’s former colleague, who fled to Sweden after Germany’s occupation of Austria. The radiation, she proposed, came from radioactive barium, which was formed by the bombardment of uranium.

This was counter-intuitive: how could barium be formed from the much heavier uranium? Meitner suggested that the bombardment spilt the uranium nucleus into two, forming barium and another element (later named technetium).

When the theory reached the Danish physicist Neils Bohr (played by Kenneth Branagh), he presented it at an international conference in Washington, and physicists soon embarked on this new line of experiments. It was not long before the idea of a chain reaction dawned on them. Bombardment of an atom with neutrons would release new neutrons, which would strike more atoms and release still newer neutrons, all the time releasing more and more energy.

The Hungarian-American physicist Leo Szilard (played by Máté Haumann) anticipated the consequences of such a weapon reaching Nazi Germany. Along with his colleagues Eugene Wigner and Edward Teller (Benny Safdie), they convinced the initially reluctant Albert Einstein (Tom Conti) to write to US President Franklin Roosevelt, urging that the US should try and develop such a weapon before the Nazis did.

The letter reached the President after World War II had begun in Europe. On December 6, 1941, Roosevelt commissioned a project, initially called the Manhattan Engineer District, for making an atom bomb. Japan, incidentally, bombed Pearl Harbor the very next day.

The Manhattan Project

Oppenheimer, then with the University of California, Berkeley, was among the scientists who joined the effort, which later came to be known as the Manhattan Project. Army officer Leslie Groves (played by Matt Damon), who was supervising military activities relating to the project, chose Oppenheimer to direct weapons development. Oppenheimer chose Los Alamos, New Mexico, for construction of his laboratory.

Among the challenges Oppenheimer and his team faced was finding the right element to be fissioned. Uranium was the obvious candidate, but the hitch was that the more abundant isotope, U-238, was more stable than U-235, which was less abundant. Bohr predicted that U-235 would yield more readily to fission, which was proved right in later experiments.

The laboratory needed large quantities of uranium, which would first need to be purified to ensure neutrons were not wasted. Scientists set about producing purified uranium, but less than 1% was U-235, which meant that the 99% U-238 would capture the neutrons without resulting in fission. Eventually, a process called gaseous diffusion was used to separate U-235 from U-238.

The next step was to moderate the speed of bombardment. Slower bombardment allows for better absorption of more neutrons. The experimenters eventually opted for carbon as a moderator. Subsequent experiments with carbon-graphite mixtures showed that a chain reaction was possible only if the lump of uranium was sufficiently large, and a uranium chain reactor of critical size was built at the University of Chicago.

As it turned out, U-235 was used in only one of the two bombs that were dropped. The other used plutonium-239, which had turned up in experiments bombarding U-238 with neutrons. Given its unstable nature, it was a candidate for fission, and was eventually produced at special laboratories for the purpose.

It was a plutonium-based bomb that was tested at Jornada del Muerto, New Mexico on July 16, 1945. This was the Trinity test, code-named by Oppenheimer. The first nuclear bomb to be used was uranium-based; called the Little Boy, it was dropped on Hiroshima on August 6 that year. The Fat Man, which was plutonium-based, was dropped on Nagasaki three days later.

Life after the War

Nolan’s film is based on a biography, American Prometheus: The Triumph and Tragedy of J. Robert Oppenheimer (2005), by Kai Bird and Martin J Sherwin, who look at Oppenheimer’s life, including outside the Manhattan Project.

After the War, Oppenheimer served on the advisory committee of the US Atomic Energy Commission (now the Department of Energy), which in 1949 recommended against building a hydrogen bomb. Such a bomb works on fusion, the opposite of fission; two atoms of hydrogen are made to combine into helium under high temperature, forming helium. The protons released in the process fuse with different hydrogen isotopes to create corresponding helium isotopes. This again is a chain reaction, which could result in a bomb much more powerful than those developed by the Manhattan Project.

Oppenheimer’s refusal to make a hydrogen bomb earned him enemies. He was accused of being a Soviet spy (he had past communist links) and had to face a review board, after which his security clearance was revoked, which blocked his access to classified documents. He continued teaching physics, and died in 1967.

In 2014, the records from the hearing were declassified. In 2022, the Department of Energy declared the proceedings unfair and reinstated Oppenheimer’s security clearance.

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