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Near-Yrast Spectroscopy of Rare-Earth Nuclei. Yrast Isomerism and Bandcrossings

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Near-Yrast Spectroscopy of Rare-Earth Nuclei. Yrast Isomerism and Bandcrossings NEAR-YRAST SPECTROSCOPY OF RARE-EARTH NUCLEI. YRAST ISOMERISM AND BANDCROSSINGS STEFAN G. JONSSON Department of Physics University of Lund Lund Sweden 1983 COSMIC AND SUBATOMIC PHYSICS REPORT LUNFD6/(NFFK-7030)1-24(1983) ISSN 0348-9329 NEAR-YRAST SPECTROSCOPY OF RARE-EARTH NUCLEI YRAST ISOMERISM AND BANDCROSSINGS BY STEFAN G. JÖNSSON FK AKADEMISK AVHANDLING SOM FÖR AVLÄGGANDE AV FILOSOFIE DOKTORSEXAMEN VID MATEMATISK-NATURVETENSKAPLIGA FA- KULTETEN VID UNIVERSITETET I LUND KOMMER ATT OFFENT- LIGT FÖRSVARAS A FYSISKA INSTITUTIONENS FÖRELÄSNINGS- SAL B FREDAGEN DEN 20 MAJ 1983 KL. 10,15, TYP AV DOKUMENT C Ansökan G Tidskriftsartikel DOKUMENTBETECKNING/COOEN Ö Doktorsavhandling • Reserapport D Konferensuppsats D Examensarbete D Delrapport • D Kompendium O Slutrapport LUNFD6 / (KFFK-7030 ) 1 -;- i, ( 1 y'-,3 ) AVDELNING/INSTITUTION Department of Physics University of Lund Sölvegatan 1*+; S-223 6? LUND, Sweden FÖRFATTARE Jonsson; Stefan G. DOKUMENTTITEL OCH UNDERTITEL Near-Yrast Spectroscopy of Rare-Earth Nuclei. Yrast Isomerism and Bandcrcssings SAMMANFATTNING lb3 163 167 High-spin states in the rare-earth nuclei Er, - ^Yb and Lu were populated in heavy-ion fusion reactions. The half-life of an isomerie state in 153Er was measured and its spin and parity determined as 27/2*. The low lying levels were explained as configurations invol- ving three valence neutrons in the f7/2, n9/2 and i 13/2 orbitals and an octupole state. In 166Yb, particle-rotor calculations reproduce accurately energies in the g-band and S-band and observed branchings from levels in these bands. In lf)3-167Yb CSM calculations and constructed multiple-quasineutron routhians are compared with the experimental ones. Systematics of crossing frequencies and signature splitting are summarized and discussed. The B(M1) values are calculated for odd-N Yb nuclei and compared with ^article-rotor calculations. 165 The crossing fr - j( ncies for bands in Lu were determined and their variations discussed. NYCKELORD isor<: •. routhian, alignment, bandcrossing, crossing frequency, signature, particle-rotf.v ':jdel, cranked shell model (CSM). DOKUMENTTITE JU UNDERTITEL - SVENSK.ÖVERSÄTTNING AV UTLÄNDSK ORIGINALTITEL Nara-yras•yrast s ?• 'croskopcroskopi for kärnor i sällsynta joräartsomradet. Yrast isomerii -.ch bandkorsningar. TILLAMPNINGSOM» ADE Hög-spinn til..tand för kärnor i sällsynta jordartsområdet har studerat;;. Kärnorna exciteras til' höga spinn i kompound kärnreaktioner. Gamma strålning detekteras med Ge(Li) de' ktorer. Resultat av experiment jämförs med teoretiska modeller. niwuwnu isomeri, bandkorsning, korsnmgsfrekvem;, tungjonsreaktion, noga .spinn gamma strålning UTGIVNINGSDATUM ANTAL SID (inkl bilagor) SPRÅK ir 1983 [min 05 J ]8% D svenska B engelska D annat ÖVRIGA BIBLIOGRAFISKA UPPGIFTER ISSN ISBN PRIS Mottågamm acceuion$numm»r I, the undersigned, being the copyright owner of the abstract, hereby grant to all reference sources permission to publish and disseminate the abstract. Date Signature If i 3-03-25 Errata Page Replace By Paper I 167 ,, fig. 8 25/2 25/2* Paper II 330 , + 1 (h11/2) 330 , ref. 2 23.000 24,158 330 , ref. 14 23,000 24,164 330 , ref. 14 Milsson Nilsson Paper III 140 ,, fig. 7a 39/2"" 31ll' 155 , wrthin within Paper IV 283 , fig. 10 1/2 1/2" NEAR-YRAST SPECTROSCOPY OF RARE-EARTH NUCLEI. YRAST ISOMERISM AND BANDCROSSINGS STEFAN G. JONSSON NEAR-YRAST SPECTROSCOPY OF RARE-EARTH NUCLEI. YRAST ISOMERISM AND BANDCROSSINGS BY STEFAN G. JÖNSSON This thesis contains the following six papers in the field of near-yrast spectroscopy: I High-spin states and yrast isomerism in Er Nucl. Phys. A381 (1982) 155 (Lund-Copenhagen collaboration) 166 II High-spin band structure in Yb Physica Scripta 2£ (1981) 324 (Lund-Copenhagen collaboration) III Near-yrast spectroscopy of Yb Nucl. Phys. A382 (1982) 125 (Lund-Copenhagen collaboration) 163 IV High-spin properties of Yb; Band crossings and signature-splitting in Yb nuclei Nucl. Phys. A394 (1983) 269 (Copenhagen-Lund collaboration) V Near-yrast spectroscopy of Yb and neighbouring nuclei Cosmic and Subatomic Physics Report LUIP 8304 LUNFD6/(NFFK-7027) 1-39 (1983)ISSN-O348-9329 (Lund-Copenhagen collaboration) VI Experimental results on the study of high-spin states in Lu Cosmic and Subatomic Physics Report LUIP 8305 LUNFD6/(NFFK-7028) 1-21 ( 1983) ISSN-0348-9329 (Lund-Copenhagen-Oslo collaboration) The published material is printed according to written permission from North Holland Publishing Company, Amsterdam and The Royal Swedish Academy of Sciences, Stockholm 1. Introduction The papers presented here deal with high-spin states in rare-earth nuclei. The most efficient way of reaching high-spin states in these nuclei is by heavy-ion (HI) fusion reactions. The nuclei investigated, Er, Yb and Lu were produced in (Hl.xn) reactions using the C and 0 beams from the Niels Bohr Institute tandem accelerator in Riso. The beam energies were kept close the the maximum available ones, about 65 MeV for 12C and 85 MeV for 16~180. Isotopically enriched . 144,149,150,152,154, , 153 targets of Sm and Eu were used in the different experiments. The spins reached in the rotational nuclei (Yb and Lu) are usually of the order of 15-2 5"h for the different cascades depending on their intensity. This is high enough in frequency to make a systematic study of bandcrossings, due to the alignment of i ... neutrons, possible. During the years in which these studies were being carried out, there have been great improvements in both detector techniques and in heavy-ion accelerators. In our work this is best reflected by the introduction of an array of Ge(Li) detectors surrounded by a Nal(Tl) shield for the suppression of the Compton background ) for the study of Y~y coincidences. Recent detector set-ups like the crystal-ball, together with heavier ions are moving discrete Y~ray spectroscopy up in the spin region, approching 40Tt for the yrast sequence in rotational nuclei. 2. Background to the papers 2.1. Theoretical background Yrast states are the lowest excited states for a given angular momentum. Thus, the yrast line gives the lower limit for excited states, when energy is represented as a function of angular momentum. There is an upper limit of angular momentum that a nucleus can carry before fission takes place (see fig. 1). 60 -- Az\ 60, Z = 66 fission barrier __ i ,» '' •• -• - ~~~—' O> ///.' /////•• '///• v LLl 20 'fä *-- ' yrast line Sn *^ i 1 ML i 20 40 60 80 100 I Fig. 1. Excitation energy versus angular momentum diagram based on the liquid-drop model. The dotted curve gives the effective separation energy for removal of a neutron with angular momentum 1-6, which represents approximately the largest angular momentum for a bound nucleon in this nucleus. The fig. is from ref. 2). In a compound nucleus reaction, the angular momentum of the compound nucleus depends upon the beam energy and the target-projectile combination. The compound system has a high excitation energy and lies therefore far above the yrast line. The reduction of the excitation energy takes place usually first by evaporation of one or more neutrons, each carrying away about one unit of angular momentum* For lighter or neutron-deficient nuclei emission of charged particles, such as protons and a particles, may compete. When approaching the yrast line, the nucleus still carries large angular momentum. About one neutron separation energy above the yrast line the de-excitation proceeds through a series of y-rays. Most of the nuclear structure information comes from investigations of the y- ray spectrum. Basicly, there are two ways of building up angular momentum in nuclei. One way is by the spin-alignment of a few individual nucleons, when the angular momentum is directed along the symmetry-axis. In this case collective rotations do not contribute to the angular momentum. Thus, the total angular momentum is the sum of those of the individual nucleons. Changes in the total angular momentum are obtained by a rearrangement of nucleons in the single- particle orbits. Consequently the spacing of energy levels is rather irregular. The transitions connecting yrast levels are usually of dlpole or quadrupole character but sometimes even higher multipole transitions occur. The other way is by a collective rotation in nuclei rotating perpendicular to the symmetry-axis. These nuclei have prolate shapes and their excited states consist of collective bands obtained by small changes in the motion of a large number of nucleons. The levels within a band are regularly spaced and connected by collective qaadrupole transitions. In a rotating nucleus t>e Coriolis force counteracts the pairing force. Thus, the pairing correlations are reduced as the rotational frequency is increased. The Coriolis force acts most strongly on particles in high-j orbits. Therefore pairs in a hlgh-j orbit should be the first to be decoupled from the core, with increasing frequency, and their angular momenta aligned along the rotational axis. Looking at the rare-earth region, Z-64 and N-82 may be treated as magic numbers. When adding a few particles to this core, ,,Cd , one gets spherical or slightly oblate nuclei, whose excited states can be considered as excited single particle states. The decay pattern of these nuclei is therefore irregular and one can expect isomeric states with half-lives of nanoseconds or more. In a survey 3 experiment ) this was proven to be the case for several nuclei centered around N-84. However, very little information was available on the structure of these isomeric states. This initiated several studies on the structure of these nuclei, among them our study of Er. For heavier nuclei the shape of the nuclear ground state suddenly changes to prolate when going from N-88 to N-90. The prolate nuclei exhibit a series of rotational sequences» The level energies can in the first approximation be given by the formula ) where f* is the moment of inertia of the deformed nucleus and I is the angular momentum of the level.
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