High-spin Nuclear States and Related Phenomena

Abstract

Nuclear states with high angular momenta have in recent yearsbeen the subject of increasing theoretical and experimentalinterest. This is primarily due to the unique possibility such statesoffer of gaining new insight into nuclear structure and of testingtheoretical predictions. Systematic experimental studies havebecome feasible due to the improved technique of exciting high-spin states in nuclear reactions induced by accelerated heavy ions.Interesting properties of such an important concept as thenuclear moment of inertia, ℑ, have thus recently been revealedin studies of the rotational bands of deformed doubly even nucleiup to spin values as high as 18-22 units of ℏ. As is well known, thevalue of ℑ for good rotors first grows slowly and practicallylinearly as the squared rotational frequency, ω2, increases.Thereafter, in many cases, ℑ undergoes a sudden increase at acritical spin value, as was first observed [l] at the ResearchInstitute for Physics in Stockholm. In some cases the nuclearrotation actually slows down, giving rise to a "backbending"or an "S-shape", in an ℑ versus ω2 graph [2]. This phenomenonmay be interpreted as a consequence of the Coriolis anti-pairingeffect which was predicted already in 1960 by Mottelson andValatin [3]. With increasing rotational frequency the Coriolisforces tend to break the nucleon pairs, that is, to reduce andfinally to eliminate the effects of the pairing correlations. Theobserved behaviour of ℑ at high spin values is thus indicative ofa change in the intrinsic structure of the nucleus, analogous tothe phase transition from a correlated to a non-correlated stateknown from the theory of superconductivity.The effects of the Coriolis force acting on the valence nucleonsmoving in the rotating frame of the nuclear core are directlyobservable in odd-mass deformed nuclei. Large perturbations areobserved for rotational bands when these are populated up tolarge spin values, that is, to as much as 33/2 or more [41] Theperturbed band structure, including energy as well as dynamicproperties, can be empirically well described in terms ofeffective Coriolis coupling matrix elements which are smallerthan the theoretical ones by a factor of about 0.6-0.7 [4].Although the Coriolis coupling mechanism is qualitatively fairlywell understood, it seems that a consistent theory has yet to beformulated.High-spin nuclear states may in many cases be attributed to specific particle excitations which are remarkably pure in terms oftheir shell model classification, as is the case with certainmultiparticle configurations in spherical and deformed nucleiat comparatively low energy. As an example the recentlyobserved 3912 isomer in 211At at an energy of 4.816 MeV [5]can be mentioned. Due to their purity such states can be usedfor detailed nuclear structure studies. In particular, informationhas thus been gained on the effective electric charges [6] andgyromagnetic factors of the nucleons in heavy nuclei.The effective electromagnetic properties of nuclei are partiallydue to the polarization of the nuclear core caused by the oddnucleon or nucleons in orbitals near the Fermi level. Furtherpolarization phenomena are caused by meson currents or bynucleon resonances in the nucleus. The nuclear polarizationphenomena can be studied, e.g., through the properties of high-spin states as well as in experiments with muonic or hadronicatoms [7].The contributions to the Symposium on High-Spin NuclearStates and Related Phenomena, arranged at the ResearchInstitute for Physics in Stockholm between May 30 and June 3,1972, dealt with experimental and theoretical aspects of theproblems mentioned above. No conference proceedings wereplanned, but several papers related to the general theme of theSymposium have been submitted for publication in PhysicaScripta and are presented in this issue under the Symposiumtitle.