300 280 260 240 220 200 190 180 170 160
80 60 40 20
Continuous emission from a deuterium lamp observed with
a Fourier transform spectrometer at the National Institute of
Standards and Technology. In the 160 nm region are emission
lines of molecular hydrogen. The long wavelength cutoff near
286 nm is due to the falloff in the detector response in this region.
results showed that, at the wavelength that Bohr’s theory
predicted for the mass 2 isotope of hydrogen, there was a line
that grew in strength with the expected concentration of the
sample. There was no evidence for an isotope of mass 3. It
seems fitting that the discovery of deuterium took place using
the very same Balmer spectral lines of hydrogen that provided
key evidence for Bohr’s atomic theory.
Brickwedde’s samples were subsequently sent by Urey to
Walker Bleakney, an eminent mass spectroscopist at Princeton
University. Bleakney sought deuterium not in the mass 2 channel, which was notorious for isobaric interference, but in the
mass 3 channel where the molecular ion HD+ might be seen.
This was successful.
Consequences of the discovery
The discovery of deuterium was published in The Physical
Review on New Year’s Day, 1932. Just seven weeks later, James
Chadwick announced his discovery of the neutron: a neutral
particle with a mass very nearly equal to that of the proton.
In early June, Werner Heisenberg suggested that the neutron
and proton should be regarded as two alternative states of a
two-level quantum particle, which we now call the “nucleon.”
All nuclei would be made up only of these nucleons. If the
coordinates (particle identity, spatial coordinates, spin) of
any two particles in the nucleus would be interchanged, the
wave function of the nucleus would change sign—that is, the
nuclear wave function would be antisymmetric with respect
to the interchange of any two nucleons. We now call particles
that produce antisymmetric wave functions “fermions.” Fermions are particles (electrons, protons, neutrons) or composite
bodies (atoms or molecules) with half-integral spin. They obey
the Pauli exclusion principle.
This instantly solved several outstanding problems with the
previous nuclear-electron model. For example, the nucleus of
lithium- 6 (6Li) was then known from molecular spectroscopy
to be what we now call a boson, for which the wave function is invariant under the interchange of particles. Bosons
are particles or composite bodies with integral spin. Since the
nuclear charge of Li is Z = 3, in the old nuclear-electron model,
the nucleus of 6Li would have to contain six protons plus three
nuclear electrons, or a total of nine fermions. But the composite particle of an odd number of fermions must be a fermion,
not a boson. So the old model had to be discarded.
Randy Hulet’s group at Rice University made a dramatic
demonstration of the boson-fermion difference in the isotopes
6Li and 7Li in 2001. Their research involved creating a Bose-Einstein condensate of lithium atoms. It is not possible to
create a Bose-Einstein condensate with fermions because of
their adherence to the exclusion principle. Indeed, all Bose-Einstein condensates of atoms are done with bosons. Since
the 6Li atom has six fermions in its nucleus (three protons
and three neutrons) and three electrons in Bohr orbits, the
6Li atom constitutes a fermion. By the same token, the 7Li
atom constitutes a boson. When Hulet’s group cooled atoms
of these isotopes in the same magnetic trap, 7Li underwent
Bose-Einstein condensation, while 6Li did not.
The nucleon idea resolved all of the outstanding problems
in molecular spectroscopy, and the nucleon symmetrization
principle made possible a systematic classification of states of
nuclei which is valid to this day. It is a precursor to the standard model of particle physics.
Discovery to application
In 1934, Urey was awarded the Nobel Prize in chemistry for
discovering deuterium, and Chadwick received the Nobel Prize
in physics in 1935 for uncovering the neutron. Both discoveries were rapidly put to use. We now know that deuterium is
found in sea water at a level of about 1 atom of deuterium (D)
to 6,400 atoms of hydrogen (H). Working with Urey during
1932, Edward Washburn of NBS found efficient methods for
separating the deuterated (“heavy”) water from normal water
by electrolysis, thereby eliminating the need to pursue the
complex low-temperature road to obtaining deuterium.
Electrolysis techniques for deuterium separation were
implemented on an industrial basis in 1934 at the Norsk
Hydro hydroelectric plant in Rjukan, Norway. By 1935, Norsk
Hydro was shipping 99 percent pure heavy water at a cost of
$0.50/g. The easy availability of deuterium spurred the growth
of isotope chemistry, in which deuterium was substituted for
hydrogen in molecules to elucidate molecular structure and
Deuterium also turned out to have unique value for the
development of nuclear energy and weapons. It played a key
role in the Nazi nuclear program in World War II. The Norsk
Hydro plant in occupied Norway was the scene of protracted
struggle, as dramatized in the 1965 film The Heroes of Telemark.
When the United States decided to build a thermonuclear
weapon (the “hydrogen bomb”), its resident center of expertise