9.7 Some Nuclear History
Nuclear spectroscopy began, of course, with the discovery of the nucleus,
described by Rutherford in his 1911 paper on the structure of the atom
[457]. In an experiment outlined in many freshman chemistry books, it was
observed that α-particles impinging on a thin gold foil were occasionally
scattered back at large angles. From this it could be deduced that the atom
was not a “plum-pudding” of electrons immersed in positive charge, but,
instead, the positive charge was concentrated in an extremely dense
“nucleus.” In fact, the density was such that a common analogy portrayed
the nucleus in the atom like a “fly in a cathedral” [458]. What is not
commonly appreciated is that the person who actually did the experiments
was a 20-year-old undergraduate student, Ernest Marsden, who observed
the backscattering events with his own eyes, one at a time in a dark room, as
flashes from a ZnSe screen [458,459].
Over the next 40 years, the properties of the nucleus as a composite of protons
and neutrons were gradually defined, and a number of experiments
observed resonant scattering of γ-rays from nuclear excited states, analogous to the scattering of visible light by electronic excited states
[461]. However, because of the much higher energy of γ-rays, the recoil
energy for atoms in the gas phase or solution is significant, and it was
necessary to use high-velocity radioactive sources to achieve the appropriate Doppler shifts. Success by this method came in 1950, when Moon used
a centrifuge rotor plated with a 411 keV
198
Au source spinning at
~800 m s
À1 to observe nuclear resonance scattering in liquid
198 Hg
[462]. Since most scientists hesitate to have radioactive sources flying in
their labs faster than the speed of sound, something had to change to
increase the popularity of this experiment.
Mossbauer Effect. In 1957 Rudolf Mössbauer, a graduate student under
Professor Maier-Leibnitz at the Max-Planck Institute in Heidelberg, set
out to investigate the nuclear resonance scattering from
191 Ir. He began at
room temperature, and since the recoil energy of a free
191 Ir nucleus is
E R ¼ 0.047 eV, while Doppler broadening at room temperature is ~0.1 eV,
he did observe some resonant scattering. Mössbauer then cooled both his
source and absorber, to reduce the Doppler broadening, and he expected a
sharp decrease in signal. Instead, he observed a rapid increase in effective
cross section (Fig. 9.22). He had discovered the recoil-free emission and
absorption of γ-rays from nuclear transitions [463–465], for which he was
awarded the Nobel Prize in Physics 3 years later at the age of
32 [460]. Within a year of Mössbauer’s first publication, efforts were
under way at Harvard, Harwell, Los Alamos, and Argonne National Lab
(continued)
9.7 Some Nuclear History
253
Nuclear spectroscopy began, of course, with the discovery of the nucleus,
described by Rutherford in his 1911 paper on the structure of the atom
[457]. In an experiment outlined in many freshman chemistry books, it was
observed that α-particles impinging on a thin gold foil were occasionally
scattered back at large angles. From this it could be deduced that the atom
was not a “plum-pudding” of electrons immersed in positive charge, but,
instead, the positive charge was concentrated in an extremely dense
“nucleus.” In fact, the density was such that a common analogy portrayed
the nucleus in the atom like a “fly in a cathedral” [458]. What is not
commonly appreciated is that the person who actually did the experiments
was a 20-year-old undergraduate student, Ernest Marsden, who observed
the backscattering events with his own eyes, one at a time in a dark room, as
flashes from a ZnSe screen [458,459].
Over the next 40 years, the properties of the nucleus as a composite of protons
and neutrons were gradually defined, and a number of experiments
observed resonant scattering of γ-rays from nuclear excited states, analogous to the scattering of visible light by electronic excited states
[461]. However, because of the much higher energy of γ-rays, the recoil
energy for atoms in the gas phase or solution is significant, and it was
necessary to use high-velocity radioactive sources to achieve the appropriate Doppler shifts. Success by this method came in 1950, when Moon used
a centrifuge rotor plated with a 411 keV
198
Au source spinning at
~800 m s
À1 to observe nuclear resonance scattering in liquid
198 Hg
[462]. Since most scientists hesitate to have radioactive sources flying in
their labs faster than the speed of sound, something had to change to
increase the popularity of this experiment.
Mossbauer Effect. In 1957 Rudolf Mössbauer, a graduate student under
Professor Maier-Leibnitz at the Max-Planck Institute in Heidelberg, set
out to investigate the nuclear resonance scattering from
191 Ir. He began at
room temperature, and since the recoil energy of a free
191 Ir nucleus is
E R ¼ 0.047 eV, while Doppler broadening at room temperature is ~0.1 eV,
he did observe some resonant scattering. Mössbauer then cooled both his
source and absorber, to reduce the Doppler broadening, and he expected a
sharp decrease in signal. Instead, he observed a rapid increase in effective
cross section (Fig. 9.22). He had discovered the recoil-free emission and
absorption of γ-rays from nuclear transitions [463–465], for which he was
awarded the Nobel Prize in Physics 3 years later at the age of
32 [460]. Within a year of Mössbauer’s first publication, efforts were
under way at Harvard, Harwell, Los Alamos, and Argonne National Lab
(continued)
9.7 Some Nuclear History
253