Germany, 1927-33
London and Denmark, 1933-39
England during the War, 1939-43
Los Alamos, 1943-46
Harwell & Cambridge, 1946-72
Retirement, 1972-79


Robert Otto Frisch

Otto Robert Frisch was born in Vienna on 1st October 1904. His father was a printer and his mother a gifted musician. He showed an early gift for mathematics, but in 1922 entered the University of Vienna to study physics, feeling that a career in mathematics would be too dry and abstract.

In the 1920s Austrian physics courses did not offer a bachelor's degree, so Frisch graduated with a Dr Phil. in 1926. He then spent a year in a private laboratory that manufactured X-ray dosimeters, devices that measure the amount of radiation a person has absorbed.

Germany, 1927-1933

In 1927 Frisch went to Berlin to work at the Physikalisch-Technische Reichsanstalt, the German government's national physics laboratory. He lived in the suburb of Dahlem near his aunt Lise Meitner, and was able to attend lectures by eminent physicists at the University of Berlin.

In 1930 Frisch left Berlin to move to Hamburg. He worked as an Assistant ('a pair of hands') to Otto Stern, who later received the 1943 Nobel Prize in Physics. Frisch studied molecular beams and developed a 'beam chopper', a device which selected atoms of a certain speed by passing them through rotating slotted disks.

Frisch also studied the response of atomic angular momenta to changing magnetic fields, the reflection of atomic beams from crystal surfaces, and helped in the discovery of the proton's magnetic moment.

In 1933 Nazi Germany introduced racial laws which forced both Frisch and Stern to leave Hamburg. Stern arranged for Frisch to join Patrick Blackett at Birkbeck College, London.

London and Denmark, 1933-1939

Blackett's laboratory was unequipped for molecular beam work, as Blackett's interests lay in developing the cloud chamber. Frisch and many others began studying the artificial radioactivity that had just been discovered by the Joliot-Curies. Frisch developed a device for rapidly moving a sample from a radioactive source to the vicinity of a cloud chamber, and used it to discover two new radioactive isotopes.

In 1934 Niels Bohr invited Frisch to join his Institute of Theoretical Physics in Copenhagen. He continued the work he had begun in London, discovering a further two new isotopes, before becoming interested in the collisions between neutrons and nuclei.

Frisch became involved with explaining nuclear fission in 1938, while spending Christmas with his aunt Lise Meitner. She had received a letter from Otto Hahn reporting a collision between uranium nuclei and neutrons could produce barium, an element of about half the atomic mass of uranium. It had previously been thought that these reactions would only generate products with roughly the same atomic mass as the bombarded elements.

Frisch and Lise Meitner realised that the impact of a neutron must have distorted the uranium nucleus in such a way that it became elongated. Nuclei contain protons whose positive electric charges try to repel each other, but the nucleus is held together by a strong surface tension. If the drop became elongated the electric forces could dominate, allowing the nucleus to tear in two. Frisch suggested the term 'fission' to describe this splitting of a heavy nucleus into two pieces of roughly equal size.

The mass of the two fission fragments is very slightly less than the mass of the uranium nucleus. According to Einstein's famous equation, this mass is equal to the energy acquired by the fragments. Frisch and Meitner calculated that the energy would be surprisingly large at 200 MeV.

Frisch and Meitner wrote up these conclusions in a letter to Nature magazine. Frisch then did an experiment to detect the fission fragments, which he managed in only two days.

By 1939 it was clear that a war was about to begin, and that Denmark would fall to German forces. Frisch made it clear to all his English visitors that he would like to leave Copenhagen, and eventually Mark Oliphant invited Frisch to the University of Birmingham.

England during the War, 1939-1943

Frisch visited Birmingham for the summer, intending to return to Copenhagen for his belongings, but the war began during his visit. Since Austria had been annexed Frisch was now a German citizen, and if he risked leaving England he may not have been allowed to return. Frisch was given a temporary appointment as a teaching assistant, but as a foreigner he wasn't allowed to participate in Oliphant's radar research.

Frisch remained focussed on nuclear fission. Niels Bohr had observed that the fission of uranium was due entirely to a rare isotope, uranium-235. This was reassuring, since the fission process was known to emit secondary neutrons. If these neutrons could initiate another fission process, a chain reaction could occur, and the energy released could be used as a devastating new weapon.

Fortunately Bohr's discovery suggested that such an explosive chain reaction was impossible in natural uranium. The fissionable uranium-235 isotope made up only a small proportion of the metal, and the number of emitted neutrons reacting with this small proportion would be far too small to establish the chain reaction.

Frisch believed Bohr's conclusion, but wanted to be sure that fission was due entirely to the light isotope. He needed to compare reactions between the two isotopes, so he tried to separate them through thermal diffusion. He was unsuccessful, but began to wonder what would happen if the light isotope could be separated off in large quantities.

Working with Rudolf Peierls, Frisch estimated the amount of pure uranium-235 that would be needed to sustain a chain reaction. To their surprise they found the answer was 'about a pound' (less than half a kilogram). This was much smaller than they had expected, and it made the development of a fission weapon a frightening possibility.

With Oliphant's help, Frisch and Peierls reported their finding to Henry Tizard, who advised the government on scientific problems concerned with warfare. The work was seen to be of particular importance, so Frisch continued to concentrate on problems related to atomic energy. Since Birmingham was fully occupied with essential work on radar, Frisch moved to join James Chadwick at Liverpool in August 1940.

Frisch remained in Liverpool until late 1943. He continued to work on the nuclear cross-sections relevant to a uranium chain reaction, and developed a device to measure the isotopic composition of uranium based on its alpha-ray spectrum. Although Liverpool was in black-out and suffered frequent air raids, Frisch continued to work with good humour, obtaining permission from the local police station to work late in his laboratory!

In late 1943 it was decided that Britain's research into atomic energy should be combined with America's atomic weapons project, and Britain's leading scientists should relocate to the United States. Frisch could not enter America as a German, so he was hastily naturalised as a British citizen.

Los Alamos, 1943-46

On arrival in the United States, Frisch was assigned to the group working at Los Alamos. Pure fissionable uranium-235 and plutonium were now in production, and Frisch was studying neutron multiplication in these metals, trying to determine the exact quantities needed to sustain a chain reaction.

These experiments were very dangerous, as a slight change in the positions of the metals could begin a chain reaction, bathing the experimenter in a lethal dose of radiation. On one occasion Frisch accidentally began such a reaction simply be leaning over the sample - the small number of neutrons reflecting off his body were enough to start the reaction! Fortunately he realised what was happening and pulled the material apart in time.

Frisch suggested another dangerous experiment, allowing the Los Alamos group to get as 'near as they could possibly go towards starting an atomic explosion without actually being blown up'. He took an arrangement of uranium-235 that would actually explode, but left a big hole in its centre so that it wouldn't. The missing portion, a plug made exactly the right shape to fit the hole, was then dropped through the sample. The arrangement would become critical, but then the plug would then fall out the other side, and the reaction would abate.

This experiment had to be approved by a committee, and was nicknamed the 'Dragon Experiment' when Richard Feynman commented that it was 'like tickling the tail of a sleeping dragon'. When tickling the tail of a sleeping dragon you stand a chance of getting roasted! But the experiment was approved, and provided valuable information about the chain reactions.

There were so many Robert's at Los Alamos that Frisch used his middle name, Otto, while in America. When the scientists got a small radio transmitter working Frisch would play the piano in a weekly slot. He was referred to as 'our pianist', since his Austrian name might give away the research going on in the base.

When the first atomic bomb was tested at Trinity Frisch couldn't find his dark goggles so had to sit with his back to the explosion. He saw the first mushroom cloud, which looked 'a bit like a strawberry', or 'a red-hot elephant standing on its trunk'. When the bomb was dropped on Hiroshima Frisch was disturbed that most of his friends were celebrating. Few of the scientists at Los Alamos saw a need for the bomb dropped on Nagasaki.

Harwell and Cambridge, 1946-1972

In 1946, after the end of the war, Frisch returned to England to join the Atomic Energy Research Establishment in Harwell. He was appointed head of the Nuclear Physics Division, and used his influence to create an informal atmosphere, while leaving most of the administration to his deputy, Robert Cockburn.

While at Harwell, Frisch began to write books aimed at popularising science. His book, Meet the Atoms, was a guide to modern physics aimed at interested members of the public. In 1947 Frisch was offered the Jacksonian Professorship of Natural Philosophy at Cambridge, and elected to a Fellowship in Trinity College.

Frisch came to Cambridge ten years after the death of Ernest Rutherford, who had built up the Cavendish Laboratory as a world centre for nuclear physics. The new professor, Lawrence Bragg, was interested in different fields so on Frisch's arrival nuclear physics at Cambridge was in decline.

As a renowned nuclear physicist, Frisch may have been expected to attract more funding and expensive equipment for the group in Cambridge. This expectation may have imposed some strain on Frisch's work in the Cavendish, as he was not at all interested in fighting for grants. Most of his work focussed on his keen interest in gadgets and instrumentation.

He developed an early recording device called a kick-sorter, which kicked ball-bearings into different channels on an inclined board, depending on the strength of an input signal. Most of his time at Cambridge, however, was spent on developing track-measuring devices for use with bubble chamber photographs.

In 1969 he described the 'Sweepnik', a fast semi-automatic device for measuring paths of ionised particles on a bubble chamber photograph. Sweepnik projected a short rotating line of light through the film, and constantly adjusting its position using mirrors and an early computer. The device was successful enough to build commercially, and Frisch became the first chairman of Laser Scan Ltd., the company that sold Sweepnik worldwide.

Frisch continued to put effort into the popularisation of science, writing several successful books. Although none of them were ever best-sellers, Frisch always received a little fan mail, and felt that 'if just a few youngsters are attracted to physics by one of my books and become good scientists then it was well worth writing'.

Retirement in Cambridge, 1972-79

Frisch retired from the University in 1972. Since he had been born on the first day of the academic year he was able to work the whole year, considering himself to have a whole extra year before having to retire! After retirement he continued as chairman for Laser Scan, although would occasionally sleep through any meetings he found uninteresting.

Frisch was popular and friendly, and enjoyed making music. He had a great sense of family, and in 1948 his parents joined him in Cambridge. His aunt, Lise Meitner, co-writer of the letter to Nature on fission, also joined Frisch in Cambridge when she retired from research in Stockholm. In 1951 Frisch married Ursula Blau, a Viennese artist. They had a daughter and a son.

In 1979 an accidental fall put Frisch in hospital. A short time later, on 22nd September, he died, a week short of his seventy-fifth birthday.