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the mode. The monopole (L=O) mode is a spherically symmetric oscilla-
tion or compression of the nucleus; the dipole (L=1) is pictured as a
motion in which the neutrons and protons oscillate in bulk against each
other while the quadrupole mode is an oscillation of the spherical
nucleus to oblate shape then to prolate shape. Oscillations with L ' 3
are, of course, possible but are not shown. The nuclear fluid has
neutron, proton, "spin-up" and "spin-down" components and hence, for each
multipolarity (L) there are four possible combinations of these com-
ponents. Modes in which neutrons and protons oscillate in phase are
characterized as isoscalar modes (denoted as T=O here) while those modes
in which the neutrons and protons oscillate out of phase are called iso-
vector (T=1). Similarly, spin-up and spin-down nucleons oscillating in
phase yield S=O modes while the so-called spin-flip modes (S=1) are pro-
duced by spin-up and spin-down nucleons oscillating out of phase. The S=0
oscillations are the electric modes while the S=1 oscillations are the
magnetic modes. The well known GDR is an example of an L=1, T=1, S=O mode
of excitation, while the first 2+ level of nuclei are examples of L=2,
T=O, S=O modes.
The collectivity of these modes (i.e., of the nuclear states that
are the observable manifestation of the various modes) can be deduced by
studying the transition rates for their electromagnetic excitation (or
de-excitation) or by measuring the cross sections for their excitation
via direct reactions such as inelastic scattering. Useful "benchmarks"
for comparison are the single particle transition rates and sum rules.
While the former provide an estimate for a single nucleon promoted fromI
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Bertrand, F.E. Excitation of giant resonances via direct reactions, article, January 1, 1982; Tennessee. (https://digital.library.unt.edu/ark:/67531/metadc1055749/m1/4/: accessed July 18, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.