MARIA GOEPPERT MAYER
Discovery of the Nuclear Shell Model
While carrying on her work at Argonne, she continued her voluntary role at
The University of Chicago by lecturing to classes, serving on committees,
directing thesis students, anal participating in the activities at the Institute for
Nuclear Studies (now known as the Enrico Fermi Institute). The University
had. pulled together in this Institute a stellar assembly of physicists and
chemists, including Fermi, Frey, and Libby, as well as Teller and the Layers.
Greg or Wentzel joined the faculties of the Physics Department and Institute
later, and the families quickly became very close, one outcome being the
joining of the families by marriage of Maria Ann to the Wentzels' son.
Subrahmanyan Chandrasekhar, who had been on the faculty of the Astronomy
Department for many years, also joined the Institute. A stream of young and
very bright physical scientists poured into the Institute, and the atmosphere was
stimulating to the extreme. To add to this exciting atmosphere, which in some
ways must have been reminiscent of Goettingen in the early days, her former
teacher and friend, James Franck, was already a member of the University's
Chemistry Department. |
The activities in the Institute reflected the interests of the leading lights,
interests that were very broad indeed, ranging from nuclear physics and
chemistry to astrophysics and from cosmology to geophysics. The
interdisciplinary character of the Institute was well suited to the breadth of her
own activities over the past, so that her Chicago years were the culmination of
her variety of scientific experience. In keeping with this, she turned her
attention at first to completing and publishing some earlier work in chemical
physics, including work with Jacob Bigeleisen on isotopic exchange reactions.
Bigeleisen had collaborated with her in other work at Columbia University and
at this time was a fellow of the Institute. At the same time, she began to give
attention to nuclear physics.
Among the many subjects being discussed at the Institute was the question of
the origin of the chemical elements. Teller was particularly interested in this
subject and induced Maria Mayer to work with him on a cosmological model of
the origin of the elements. In pursuit of data required to test any such model,
she became involved in analyzing the abundance of the elements and noticed
that there were certain regularities associating the highly abundant elements
with specific numbers of neutrons or protons in their nuclei. She soon learned
that Walter M. Elsasser had made similar observations in 1933, but she had
much more information available to her and found not only that the evidence
was stronger but also that there were additional examples of the effect. These
specific numbers ultimately came to be referred to as "magic numbers," a term
apparently invented by Eugene Wigner.
When she looked into information other than the abundance of the elements,
such as their binding energies, spins, and magnetic moments, she found more
and more evidence that these magic numbers were in some way very special
and came to the conclusion that they were of great significance for the
understanding of nuclear structure. They suggested the notion of stable "shells"
in nuclei similar to the stable electron shells associated with atomic structure,
but the prevailing wisdom of the time was that a shell structure in nuclei was
most unlikely because of the short range of nuclear forces as compared to the
long-range coulomb forces holding electrons in atoms. There was the further
difficulty that the magic numbers did not fit simple-minded ideas associated
with the quantum mechanics of shell structure.
Maria Mayer persisted in checking further evidence for shell structure, such as
nuclear beta-decay properties and quadrupole moments, and in trying to find an
explanation in terms of the quantum mechanics of the nuclear particles. In this
she was I greatly encouraged by Fermi and had many discussions with I him.
She was also strongly supported by her husband, who acted, as a continual
sounding board for her thoughts on the subject and provided the kind of
guidance that could be expected from a chemist who, in many ways, was better
equipped to deal with phenomena of this kind than a physicist. The systematics
of regularities in behavior with which she was faced had great similarity to the
systematics in chemical behavior that had led to the classical development of
valence theory in chemistry, and whose fundamental explanation had been
found in the Pauli Exclusion Principle.
It was Fermi who asked her the key question, "Is there any, indication of spin-
orbit coupling?" whereupon she immediately realized that that was the answer
she was looking for, and thus was born the spin-orbit coupling shell model of
nuclei.
Her ability to immediately recognize spin-orbit coupling as I the source of the
correct numerology was a direct consequence of her mathematical
understanding of quantum mechanics and especially of her great facility with
the numerics of the representations of the rotation group. This ability to
instantly identify the key numerical relationships was most impressive, and
even Fermi was surprised at how quickly she realized that his question was the
key to the problem.*
* Joseph Mayer gives the following description of this episode: "Fermi and
Maria were talking in her office when Enrico was called out of the office to
answer the telephone on a long distance call. At the door he turned and asked
his question about spin-orbit coupling. He returned less than ten minutes later
and Maria started to 'snow' him with the detailed explanation. You may
remem- ber that Maria, when excited, had a rapid fire oral delivery, whereas
Enrico always wanted a slow detailed and methodical explanation. Enrico
smiled and left: 'Tomorrow, when you are less excited, you can explain it to
me.'
-
While she was preparing the spin-orbit coupling model for publication, she
learned of a paper by other physicists presenting a different attempt at an
explanation and, as a courtesy, she asked the Editor of the Physical Review to
hold her brief Letter to the Editor in order that it appear in the same issue as
that paper. As a result of this delay, her work appeared one issue following
publication of an almost identical interpretation of the magic numbers by Otto
Haxel, J. Hans D. Jensen, and Hans E. Suess. Jensen, working completely
independently in Heidelberg, had almost simultaneously realized the
importance of spin-orbit coupling for explaining the shell structure, and the
result had been this joint paper.
Maria Mayer and Jensen were not acquainted with one another at the time, and
they did not meet until her visit to Germany in 1950. In 1951 on a second visit,
she and Jensen had the opportunity to start a collaboration on further
interpretation of the spin-orbit coupling shell model, and this was the
beginning of a close friendship as well as a very productive scientific effort. It
culminated in the publication of their book, Elementary Theory of Nuclear
Shell Structure (1955). They shared the Nobel Prize in 1963 for their
contributions to this subject.