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Oqnf-9203164—1 De92 016800 Exotic Nuclear Shapes And OQNF-9203164—1 DE92 016800 EXOTIC NUCLEAR SHAPES AND CONFIGURATIONS THAT CAN BE STUDIED AT HIGH SPIN USING RADIOACTIVE ION BEAMS J. D. Garrett Physics Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831-6371, USA ABSTRACT The variety of new research possibilities afforded by the culmination of the two frontier areas of nuclear structure: high spin and studies far from nuclear stability (utilizing intense radioactive ion beams) are discussed. Topics presented include: new regions of exotic nuclear shape (e.g. superdeformation, hyperdeformation, and reflection-asymmetric shapes); the population of and consequences of populating exotic nuclear configurations; and complete spectroscopy (i.e. the overlap of state of the art low- and high-spin studies in the same nucleus). Likewise, the various beams needed for proton- and neutron-rich high spin studies also are discussed. DISCLAIMER This report was prepared as an account of work sponsored by an agency of the United Slates Government. Neither the United States Government nor any agency thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsi- bility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Refer- ence herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, rccom- mendaiiun, or favoring by the United Stales Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily slate or reflect those of the United Stiles Government or any agency thereof. DISTRIBUTION OF THIS DOCUMENT IS UNLIMITED "Tra MIDRMMII mtnuacrpt hat bMn •uihorxj by * contitcin ot th* US Govtmrram undw coniiaci No OE- AC0584OR7I40O AecoroVgrr. ttw US Govwnmtm win • naniiduMv*. ror>Hv ttn tctnx to (x*** or raproduc* the pubMwd farm ol KM tonBfcuton or Mow oltwrt to <k> to. lor U S Govwnmant P<«P I. Introduction Probably today's two most exciting frontiers of nuclear structure are rapidly-rotating nuclei and nuclei far from stability. The current renaissance in both frontier areas is based on recent technical innovations: to wit, the advent of large Complon-suppressed germanium detector arrays (e,g. EUROGAM and GAMMASPHERE) based on the ability to grow large crystals of high-purity germanium'1, and new breakthroughs in producing and accelerating high-intensity beams of radioactive ions, the subject of the last day of this workshop. Therefore, it is imperative at such a gathering as this to consider the culmination of these two initiatives, high- spin research possibilities afforded by intense beams of radioactive ions. The large angular momentum imparted to the residues of heavy-ion induced fusion-evaporation reactions produces two major nuclear structure effects: (i) a stabilization of highly-deformed nuclear shapes; and (ii) a major rearrangement of the nuclear single-particle states generated by the Coriolis and centrifugal forces acting on the nucleons due to the rotation of the nucleus. The increased stability of superdeformed nuclei, well known from microscopic calculations 2 3 (see e.g. - !), can be illustrated by plots of Ex(i) for the yrast states (i.e. the state of lowest energy for each angular momentum) of different deformations, see Figure 1. Though the rotational sequence based on the superdeformed intrinsic slate at low angular momentum occurs at a high excitation energy, it becomes the lowest decay sequence at the largest angular momentum. The superdeformed nucleus has a larger moment of inertia, hence a smaller slope on the EX(I) plot. The modification of the single-particle spectrum of states by rotation is illustrated in Figure 2. The Coriolis plus centrifugal term in the single-nucleon hamiltonian'°l (= -co ji. where a) is the angular frequency of rotation, and j] is the component of the intrinsic nucleonic angular momentum along the rotational axis) can produce perturbations as large as those resulting from nuclear deformation. Such rotationally-induced single-particle effects are the bases of a variety of high-spin phenomena as diverse as "backbending" (band crossings)1 '«l2l, loss of pair t 30- > / 20- jf/ B if/ C Superdeformed c o band r S /' Low deformation , />' band 4, a ^^l fl) it 10- ,A" sp\ o X M UJ ^* ^—^ *Oblate states 0- *~™ -r~—: 1 0 10 20 30 40 50 60 Spin(ti) 152 Figure 1. Plot of EX(I) for yrast states of Dy (refs. 4,5) corresponding to shell-mode!, normally-deformed, and superdeformed configurations. This figure is taken from ref. 6 which is an adaptation of a similar figure in ref. 5. 14 9 correlations'3. ), ,ne nuciear shrinking paradox !, and an enhancement of residual interactions'5! based on an increased overlap of nucleonic states9-16'. Radioactive ion beams will provide experimental access to a variety of even more exotic shapes and combinations of single-particle orbitals in nuclei far from stability. Some examples are described in Sections 2 and 3, respectively. Likewise, the possibility and consequences of comprehensive high spin studies of nuclei, whose low angular momentum properties also are well known, are discussed in Section 4. Whereas the neutron-rich portion of this research will require large ISOL facilities, for example, the proposed IsoSpin Laboratory30), nearly all of the proton-rich studies also can be made using "first generation" facilities, such as the Oak Ridge RIB Facility3U. DEFORMED Jg3v odd Figure 2. Comparison of the spectra of single-particle states from spherical, deformed, and rotating deformed nuclear potentials taken from refs. 7,8 and 9, respectively (see also figures 7 and 9). Because of the greater complexity of the single-particle states with increasingly- complex approximations to the nuclear potentials, only the subset of states indicated in the figure is shown for the deformed and rotational cases. 2. Kxotic Nuclear Shapes Accessible with Radioactive Ion Beams The observations of high-spin superdeformed stales (2:1 ratio of the major to minor axes) in A = 150 and 190 nuclei (refs. 5,17 and 18.19, respectively) and deformations intermediate between normal and superdeformed in A = 130 and 186 (refs. 17,20 and 21, respectively) have focused an intense interest on highly-deformed nuclear shapes. Such states correspond to secondary minima in the nuclear potential energy at large deformations resulting from major gaps in the single-particle levels22^*]. This striking illustration of the interpluy of single-particle CO 0 1 120 •s«u.ENETOr:i»7-«*7>; c«a6.t,.aoa; SCALE OSM»V 0J5 80 90 00 ISO CO NEUTRON NUMBER Figure 3. Predicted shell-energy corrections for a 2:1 ratio of the major to minor axes (i.e. prolate superdeformed nuclei) as a function of particle number and rotational frequency, to, for protons (top) and neutrons (bottom). Note that the proton calculations28' predate the discovery of high-spin superdeformed states. Though similar calculations for neutrons also exist predating high-spin superdeformed states, see e.g.28', later calculations6' are shown because of the availability of u superior figure. and collective degrees of freedom was developed-*! 10 describe isomerie fission of actinide nuclei25 26l Already in 1968 it was realized that such secondary minima should also exist in lighter nuclei; indeed the proton and neutron numbers of the occurrence of such shell corrections were predicted27!. However, it wasn't until 1986 that the technique of populating superdeformed minima at high spin and observing the gamma-ray cascade based on the rotating superdeformed intrinsic stale succeeded5' in establishing additional superdeformed states at high spin. Figure 3 shows calculations6'28' of the proton and neutron shell energy as a function of Z and N respectfully, and of the rotational frequency. The high-spin superdefonned states near A = 150 and 190 and the actinide fission isomers all occur at or near predicted minima in both the proton and neutron shell energy, see Figure 4. Other regions of the table of isotopes where the proton and neutron shell energies arc predicted to act coherently to produce additional minima in the potential energy at superdeformed deformations, also are indicated in this figure, as are the projected "new" nuclei •hat can be studied29' using the radioactive ion beams planned for the isoSptn Laboratory^'. Though a large number cf "new" neutron-rich nuclides become accessible to study with radioactive beams, these cannot be populated by heavy-ion fusion- evaporation reactions, the traditional technique for studying high-spin superdeformed states17). Therefore, new techniques are necessary to populate high-spin superdeformed states to the right of the valley of beta stable nuclei. For example. Coulomb excitation of neutron-rich radioactive beams can provide some high spin information, but this process has a vanishingly- smalt probability of populating superdefonned states. On the other hand, Coulomb excitation plus few-particle transfer may offer some chance of studying superdeformation in these neutron-rich nuclei; however, the associated cross sections also are small at very high spin32'. So far, the probability of populating superdeformed states has not been demonstrated by these mechanisms. Indeed, the known A = 150 and 190 high-spin superdeformed nuclei"-19! are on the proton- rich side of stable nuclei, see Figure 4. Although some of the best candidates33,34) of (j,e jjghj NWtt UN ftltVCII fSai P.M *NB HSSKWISBMERS- • Slat>!» Nutfat O New Nuclei suiiwn I wilt tSl fleams iiiiiii-l aaiiiiiiiiaift>Bka«aMaaftii* llllllllllllllllllNIIIII iiitii mMii »i> «••«a*tttt*itfe««* J«. a) a ai • ' • 1 tkltaftkBM* KMBi kBIKBBBBBIk kl n»* »i taa,a*»M»ai t«a» k a kia ^rSBaSiBHiaaaaSIS'a11! "•"*•*" ' "V "••••••{••••kkiai«a.a ••• i \tvM\\rrt""MaNJikBkaaakBa a • :•!•:.••:• . v4 . SHKM'J"" s's*::;::-'. -M. MICUI IN WHICH PnOlON COfintRTIWISACT CQUEREtmY • ^4*^N',i>iMii BttaaaaB •'• •> •••llii'.i) ~ " ».»-•• /J^'#rfA»rf?rjvi* • • aaa > <• aaaaHMH^1 Si-Ma :US'*Jtii'Vl>«j#Bi ••••BIIB44 t • m*iM^«H[i8 *'.t -- I 10*117 71-tt i «••••••••* Figure 4.
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