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Subatomic Physics

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Chapter 18<br />

Collective Model<br />

Although the shell model describes the magic numbers and the properties of many<br />

levels very well, it has a number of failures. The most outstanding one is the fact<br />

that many quadrupole moments are much larger than those predicted by the shell<br />

model. (1) It was shown by Rainwater that such large quadrupole moments can be<br />

explained within the concept of a shell model if the closed-shell core is assumed to<br />

be deformed. (2) Indeed, if the core is ellipsoidal it acquires a quadrupole moment<br />

proportional to the deformation. A deformation of the core is evidence for manybody<br />

effects, and collective modes of excitation are possible. The appearance of such<br />

modes is not surprising. Lord Rayleigh investigated the stability and oscillations of<br />

electrically charged liquid drops in 1877, (3) and Niels Bohr and F. Kalckar showed in<br />

1936 that a system of particles held together by their mutual attraction can perform<br />

collective oscillations. (4) A classical example of such collective effects is provided by<br />

plasma oscillations. (5) The existence of large nuclear quadrupole moments provides<br />

evidence for the possibility of collective effects in nuclei. From about 1950, Aage<br />

Bohr and Ben Mottelson started a systematic study of collective motions in nuclei; (6)<br />

over the years, they and their collaborators have improved the treatment so that<br />

today the model combines the desirable features of shell and collective models and<br />

is called the unified nuclear model.<br />

The salient facts can be discussed most easily by describing two extreme situations.<br />

Closed shell nuclei are spherically symmetric and not deformed. The primary<br />

collective motions of such nuclei are surface oscillations, like the surface waves on<br />

a liquid drop. For small oscillations, harmonic restoring forces are assumed, and<br />

equally spaced vibrational levels result. Far away from closed shells, the nucleons<br />

outside the core polarize the core, and the nucleus can acquire a permanent<br />

1 C.H. Townes, H.M. Foley, and W. Low, Phys. Rev. 76, 1415 (1949).<br />

2 J. Rainwater, Phys. Rev. 79, 432 (1950).<br />

3 J.W.S. Rayleigh, The Theory of Sound, Vol. II, Macmillan, New York, 1877, §364.<br />

4 N. Bohr, Nature 137, 344 (1936); N. Bohr and F. Kalckar, Kgl. Danske Videnskab. Selskab.<br />

Mat.-fys. Medd. 14, No. 10 (1937).<br />

5 Feynman Lectures, II-7-5ff; Jackson, Chapter 7.<br />

6 A. Bohr, Phys. Rev. 81, 134 (1951); A. Bohr and B.R. Mottelson, Kgl. Danske Videnskab.<br />

Selskab. Mat-fys. Medd. 27, No. 16 (1953).<br />

543

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