Otto
Frisch was born into a Jewish family in
Vienna, Austria, in 1904. He studied science and received his doctorate
in 1926.
When
Adolf Hitler gained power in 1933 he
fled from Nazi Germany. He went
to the Institute of Theoretical Physics in Copenhagen where he worked
with Nils Bohr.
In
1939 Frisch wrote a paper with his aunt, Lise
Meitner, explaining the theory of uranium fission. In the paper
they argued that by splitting the atom it was possible to use a
few pounds of uranium to create the explosive and destructive power
of many thousands of pounds of dynamite.
Frisch
worked in Copenhagen until 1940 when he moved to England where he
worked on atomic research with James Chadwick
and Rudolf Peierls.
In
1943 Frisch
joined the
Manhattan
Project.
in the United States. Over the next two years he worked with Robert
Oppenheimer,
Edward Teller,
Enrico Fermi, Felix
Bloch,
David Bohm, James
Chadwick, James
Franck,
Emilio
Segre,
Eugene Wigner, Leo
Szilard and Klaus Fuchs in developing
the atom bombs dropped on Hiroshima
and Nagasaki.
After
the war Frisch was head of the Nuclear Physics Division at Harwell
before becoming professor of natural philosophy at Cambridge
University (1947-71). Otto
Frisch,
who wrote several books, including Atomic
Physics Today (1961), died on 22nd September, 1979.
(1)
Lise Meitner and Otto Frisch, Uranium
Fission (1938)
From chemical evidence, Hahn and Strassmann conclude that radioactive
barium nuclei (atom number Z = 56) are produced when uranium (Z
= 92) is bombarded by neutrons. it has been pointed out that this
might be explained as a result of a "fission" of the uranium
nucleus, similar to the division of a droplet into two. The energy
liberated in such processes was estimated to be about 200 Mev, both
from mass defect considerations and from the repulsion of the two
nuclei resulting from the "fission" process.
If this
picture is correct, one would expect fast-moving nuclei of atomic
number 40 to 50 and atomic weight 100 to 150, and up to 100 Mev
energy, to emerge from a layer of uranium bombarded with neutrons.
In spite of their high energy, these nuclei should have a range
in air of a few millimeters only, on account of their high effective
charge (estimated to be about 20), which implies very dense ionization.
Each such particle should produce a total of about 3 million ion
pairs.
By means
of a uranium-lined ionization chamber, connected to a linear amplifier,
I have succeeded in demonstrating the occurrence of such bursts
of ionization. The amplifier was connected to a thyratron which
was biased so as to count only pulses corresponding to at least
5 x 105 ion pairs. About 15 particles per minute were recorded when
300 milligram of radium, mixed with beryllium, was placed one centimeter
from the uranium lining. No pulses at all were recorded during repeated
check runs of several hours total duration when either the neutron
source or the uranium lining was removed. With the neutron source
at a distance of four centimeters from the uranium lining, surrounding
the source with paraffin wax enhanced the effect by a factor of
two.
It was
checked that the number of pulses depended linearly on the strength
of the neutron source; this was done in order to exclude the possibility
that the pulses are produced by accidental summation of smaller
pulses. When the amplifier was connected to an oscillograph, the
large pulses could be seen very distinctly on the background of
much smaller pulses due to the alpha particles of uranium.
By varying
the bias of the thyratron, the maximum size of pulses was found
to correspond to at least 2 million ion pairs, or an energy loss
of 70 Mev of the particle within the chamber. Since the longest
path of a particle in the chamber was 3 centimeters, and the chamber
was filled with hydrogen at atmospheric pressure, the particles
must ionize so heavily that they can make 2 million ion pairs on
a path equivalent to 0.8 cm of air or less. From this it can be
estimated that the ionizing particles must have an atomic weight
of at least about seventy, assuming a reasonable connection between
atomic weight and effective charge. This seems to be conclusive
physical evidence for the breaking up of uranium nuclei into parts
of comparable size, as indicated by the experiments of Hahn and
Strassmann.
Experiments
with thorium instead of uranium gave quite similar results, except
that surrounding the neutron source with paraffin did not enhance,
but slightly diminished the effect. This gives evidence in favor
of the suggestion that also in the case of thorium some, if not
all of the activities produced by neutron bombardment, should be
ascribed to light elements. it should be remembered that no enhancement
by paraffin has been found for the activities produced in thorium,
except for one which is isotopic with thorium and is almost certainly
produced by simple capture of the neutron.
Meitner
has suggested another interesting experiment. If a metal plate is
placed close to a uranium layer bombarded with neutrons, one would
expect an active deposit of the light atoms emitted in the "fission"
of the uranium to form on the plate. We hope to carry out such experiments,
using the powerful source of neutrons which our high-tension apparatus
will soon be able to provide.
