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.
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