On Nuclear Dipole Vibrations

Abstract

The high frequency resonances recently observed for (y,n) reactions as well as photo-fission are interpreted in analogy with the "reststrahl frequencies" of polar crystals. The estimated frequencies are in good agreement with the experimental results. An interesting consequence of this interpretation is the conclusion that strong resonance scattering of 7-rays should take place at a frequency characteristic of the scattering nucleus. I T has been observed recently 1 that some nuclear photo-disintegrations, (y.n) reactions as well as photo-fission, exhibit a behavior which has the appearance of a high frequency resonance. Such a resonance was observed in carbon at 30 Mev, 1 in copper at 22 Mev, 1 in tantalum at 16 Mev, 2 and for photofission in thorium and in uranium at 16-18 Mev. 1 The suggestion was made that this phenomenon may be due to the interplay of increasing level density and increasing competition from other processes such as, for instance, the (y y 2n) process. Recent experiments 3 show, however, that in the case of Cu 63 as well as other nuclei the decrease in the (y,n) cross section in the high energy region is not compensated to any considerable extent by an increase in the (T,2W) cross section. It is, furthermore, desirable to explain why such different processes as (y,n) reactions and photo-fission should have maximum cross sections at so similar 7-energies.We propose to interpret these frequencies as resonances, somewhat different from those caused by definite nuclear levels, and analogous to the "reststrahl frequencies'' of polar crystals. We assume that the 7-rays excite a motion in the nucleus in which the bulk of the protons move in one direction while the neutrons move in the opposite direction. We shall call this motion the 1 G. C. Baldwin and G. S. Klaiber, Phys. Rev. 71, 3 (1947) and Phys. Rev. 73, 1156(1948). 2 J. McElhinney and A. O. Hanson, private communication. 3 M. L. Perlman and G. Friedlander, Phys. Rev. 74, 442 (1948). We are indebted to Dr. Friedlander for informing us of these results before publication. "dipole vibration." Such a vibration will have a high frequency as a result of the partial separation of the protons from the neutrons to which they are strongly bound. The maximum in the 7-absorption cross section will give rise to similar maxima for several nuclear processes in which 7-absorption is the first step. We shall see below that the magnitude of the integrated cross sections deduced from experiments is too great to be explained by the motion of a single proton or of a small fraction of the protons contained in the nucleus.The breadth of the resonance-so far only crudely known-is probably due to the transfer of energy from the orderly vibration described above into other modes of nuclear motion. Thus, the breadth is due to a process analogous to damping by friction. One may use a nuclear model in which the ordered dipole vibration of protons and neutrons in opposite directions correspond to well defined quantum states. Coupling with other degrees of freedom will b...

The decay time scale for highly excited nuclei as seen from asymmetrical emission of particles

“…This means that the PLS charge center can be shifted from the PLS center-of-mass. A similar process of a dipole charge polarization is well known in electromagnetic interactions with nuclei as a Goldhaber-Teller and Steinwedel-Jensen giant resonances [10,11]. Since the evaporation rate is very fast at high excitation energies, the charge distribution of the evaporated particle and the residue should correspond to this 'dipole' shift of the PLS charge distribution.…”

Couplings between dipole and quadrupole vibrations in tin isotopes

“…In a macroscopic approach, the isovector GDR is interpreted by a combination of the Steinwedel-Jensen model in which the total density is kept unchanged [51] and the Goldhaber-Teller model where proton and neutron fluids are incompressible [4]. It is obvious that the Steinwedel-Jensen model does not affect the quadrupole moment because any modification of the density of one isospin specie is exactly compensated by the other in every point of space.…”

Microscopic studies of two-phonon giant resonances

“…At this beam energy the monopole-dipole excitation is of order of 1 barn. The total cross section contains an interference between the amplitudes a (1) f i and a (2) f i . This is shown in Table 2 for 100A MeV for which the effect is larger.…”

“…However, in Ref. [42] the source for anharmonic effects were discussed and it was suggested that it should be very small, i.e., ∆ (2) …”

“…These observations were later confirmed by Baldwin and Klaiber (1947) with photons from a betatron. In 1948 Goldhaber and Teller [2] interpreted these resonances (named by isovector giant dipole resonances) with a hydrodynamical model in which rigid proton and neutron fluids vibrate against each other, the restoring force resulting from the surface energy. Steinwendel and Jensen [3] later developed the model, considering compressible neutron and proton fluids vibrating in opposite phase in a common fixed sphere, the restoring force resulting from the volume symmetry energy.…”