The 153Dy level structure has been studied through the 124Sn(34S, 5n) reaction. The low-spin collective character of 153Dy gives way to single particle yrast configurations at I &gt; 41 2; an I= 47 2 isomer at 5591 ke V shows the single particle character (and overall oblate shape) to be well established at that spin. Nuclear shape charges in Dy nuclei are discussed

Broda, R.

Chowdhury, P.

Chung, Y.H.

Daly, P.J.

Helppi, H.

Janssens, R.V.F.

Khoo, T.L.

Kortelahti, M.

Kühn, W.

McNeill, J.

Pakkanen, A.

Carolina Digital Repository

Volume 131B, number  4,5,6 PHYSICS L E T T E R S  17 November  1983 LEVEL STRUCTURE OF lS3Dy AND THE COMPETITION BETWEEN COLLECTIVE AND FEW-PARTICLE EXCITATION MODES IN Dy NUCLEI ~ M. K O R T E L A H T I  1, R. B R O D A  2, Y.H. C H U N G ,  P.J. DALY,  A. P A K K A N E N  1 Chemistry Department, Purdue University, West Lafayette, IN 47907, USA H. HELPpI3 ,  j. McNEILL,  and P. C H O W D H U R Y  4 R.V.F. JANSSENS, T.L. K H O O  and W. K U H N  5 Physics Division, Argonne National Laboratory, Argonne, IL 60439, USA Received 31 May 1983 The  ~53Dy level structure has been studied through the 124Sn(348, 5n) reaction. The low-spin collective character of ~53Dy gives way to single particle yrast configurations at I > 41/2: an I = 47/2 isomer at 5591 keV shows the single particle character  (and overall oblate shape) to be well established at that spin, Nuclear shape changes in Dy nuclei are discussed. Nuclei with N = 82-86 are spherical in the ground state but become oblate at high spin along the yrast line as successive high-j nucleons align their spins along the symmetry axis. In contrast, the yrast states of nuclei with N ~> 90 are prolate even at high angular momenta .  One consequence is the absence of nanosecond isomers of high multiplicity in nuclei with N ~> 90 [1]; the shell effects which drive nuclei prolate are apparently still dominant  up to the highest observed spins. The level spectra of the transitional nuclei with N = 87 and 88 feature low-lying collective band structures indicative of small prolate deformation.  It is in these soft nuclei, where prolate driving shell effects are barely established, that one might hope to "~'Work supported by the U.S. Depar tment  of Energy. i Permanent  address: University of Jyvfiskylfi, Finland. 2 Permanent  address: Institute of Nuclear Physics, Cracow, Poland. 3 Permanent  address: Lappeenranta  University of Tech- nology, Finland. 4 Present address: Niels Bohr  Institute, RisfL Denmark .  5 Present  address: University of Giessen, West  Germany.  0 I)31-9163/83/(~(J00-(1000/$03.00 © 1983 North-Holland observe the prolate to oblate shape change with increasing spin suggested by liquid drop con- siderations [2]. To explore these possibilities, we recently studied the high-spin level structure and lifetimes of the N = 88 nucleus ~SaDy by means of the (34S, 4ny) reaction. The results [3] in- dicated a transition above spin 32 in the character of the yrast states from collective to few-particle, and gave evidence for an evolution in the nuclear shape along the yrast line from prolate through triaxial to oblate. Since one could expect a similar transition to be mani- fested in the high-spin level spectrum of the neighboring N = 87 nucleus ~53Dy, we have now studied that nucleus in detail to learn more about how the nuclear shape changes in this region depend on neutron number  N and spin 1. The levels of ~53Dy have previously been studied in (a, xn7) reactions. Kleinheinz et al. [4] used the (a, 3n) reaction to locate levels up to I - 33/2, including AI = 2 bands built on weakly deformed i13/2, h9/2, and f7/2 neutron in- 305 Volume 131B, number 4,5,6 PHYSICS LETTERS 17 November 1983 trinsic states and a AI = 1 band built on a strongly deformed vhnn state. Subsequent (a, 5n) and (a, 6n) measurements  by Jansen et al. [5] generally confirmed the (a, 3n) results and extended them to slightly higher spins. However ,  there are some significant disagree- ments between the conclusions of the two groups. Particularly, a cascade including prom- inent 441,481, and 552 keV transitions was observed with too much intensity in the (a, 5n) reaction to be compatible with its earlier placement  [4], and it was relocated in the level scheme proposed in ref. [5]. We have per formed an extensive set of in- beam y-ray spectroscopy experiments  using the U4Sn (34S, 5n) reaction induced by 145-165 MeV ~S beams from the Argonne  Tandem-Linac.  They included excitation function, angular dis- tribution, and electronic timing measurements ,  as well as comprehensive y-y  coincidence measurements  using three Ge detectors and a large NaI  sum-spectrometer .  In the (34S, 5n) reaction we observed two strong parallel cas- cades of mainly stretched E2 transitions, one of which consisted of the known y-rays in the i13/2 band and some higher lying transitions. The other strong cascade included the 441,481, and 552 keV y-rays as well as many other transitions not observed in the (a, xn) studies; connections between the members  of this new band and the lowest transitions in the i13/2 band were clearly indicated by the coincidence data. A high-lying 2.3 ns isomer was found to de-excite predom- inantly through the new band with much weaker  decay pathways feeding all known members  of the i13/2 band. In determining the relative positions of the two bands, we per- formed quantitative analyses of the coincidence intensities and also took account of the reported (a, xn) data. Crucial steps in the construction of the level scheme (fig. 1) were the introduction of the 1601 keV level, de-exciting by three bran- ches, and the placement  of the weak 441, 394, and 342 keV interband transitions. The 2.3 ns isomeric state is also firmly located, with the required connections to the vi13/2 band iden- tified. An analysis of the delayed y-ray in- tensities also showed satisfactory intensity 8452 (61/2) _ _  7954 (59/2) 7065(55/21 _ _  6228(51/~ 2 518.0 869.5 857.4 61/2 - -  8638 608. I 5 7 / ~  _ _ _  8050 10299 53/2 - -  7000 51/2 281.5 6718 957.7 5~  47/2 I-I 5378 43/2 11 213.9 5207 ~ 170.3 41/2. 256.6 4783 41/2. 7 2 ~ 3 5 8 . 4  654.0 4487 -- 4064 37/2* 719.0 /423.4 674.5 3389 33/2* 627.2 _ 636.3T/2=2.3n s 4 ~  5761 41/21 }~ t 155.~ 37/2 137/~i 657.9 3829 33/2, 33/2- ~3~5 65c36 668.9 _29/2 3170 ; 29/2 3075 5245 717.6 5744 2762 29/2" 646.6 t-I 551.8 25/2 l 581,6 342.1~ / 25/2" o~ ,.oc-~l 480.8 2181 532.2 393.-/ ,7/2,-,j 44,.3 1648 21/2" l l60 17/2", 488.3 440.7/ 560.41 15 " - - 1 6 0 ,  28.  1273 1 0 4 0 ~ - -  ? 1656.0 71215/2. ] 447.8 328l/ 4 0 & 7 ~ 1  ' ~ _  637 75.6 ~'~ I636.7 0 7/2 1295.9 IIW P53D,, 66 Y87 Fig. 1. The level scheme of 153Dy. The widths of the tran- sition arrows are proportional to the singles intensities at 165 MeV beam energy. balance throughout the scheme. The transition ordering above the isomer is not certain, espe- cially for the 636 keV transition, which forms part  of an unresolved multiplet with complex coincidence relations. The spin-pari ty assignments shown are based on the angular distribution results, which in- dicate stretched E2 character for most of the strong transitions. (The A2 coefficients of tran- sitions below the 2.3 ns isomer were observed to be at tenuated by variable amounts depending on the ratio of prompt- to-delayed feeding.) For the 133 keV transition, M1 character was deduced from delayed intensity balance. Un- fortunately, the parity of the right-hand band (fig. 1) could not be settled, since an uncertainty in the assignment of the 1040 keV level prop- agates all the way up to the isomeric level; the available evidence distinctly favors negative 306 Volume 131B, number  4,5,6 PHYSICS L E T T E R S  17 November  1983 parity, and we shall refer to this as the negative parity band. The lower portions of the ~53Dy and ~54Dy schemes broadly resemble one another, each with two main de-excitation pathways, one in- volving even parity, the other odd parity states. While the 153Dy t'i13/2 band and the 154Dy S-band are related, it is not clear whether the negative parity band in ~53Dy is a structural counterpart  of the extensive band built on the 3 octupole in l~4Dy. The uil3/2 band exhibits collectivity in smoothly increasing transition energies, giving a straight line on a plot of the angular momentum L versus hto (fig. 2). We note that the data for the S-band in 154Dy also lie along a straight line which passes close to the origin, in this way approximating the behavior of a perfect rotor, but the full implications of these observations are not yet understood. That  the negative parity Ix x) i  24 153Dy 20 16 12 8 4 0 32 154Dy 28 24 2o 1t5 12 8 4 q 6.~ • positive parity band x negative parity band i i h i ~ y , / x' , positive ×" parity band ~"~ x negative ~/ parity band o'.2 o13 o14 o'.s hoJ (MeV) Fig. 2. Plots of Ix, the angular  m o m e n t u m  along the rotation axis, versus rotational frequency for the positive and nega- tive parity bands in 153Dy and 154Dy. bands in 153Dy and 154Dy have much less regular structures is also clearly illustrated in fig. 2. The population of the two main 153Dy bands drops sharply at high angular momenta,  and no band members with I > 41/2 could be identified. In the region around 5 MeV excitation, where the I = 41/2 band members occur, the level scheme is extremely complex with several com- peting de-excitation pathways consisting of cas- cades of dipole and quadrupole transitions with highly irregular energies. These observations show that the low-spin collective character of 153Dy gives way to single-particle configurations at higher spins. The complexity of the level scheme around 5 MeV indicates that the collec- tive and single particle modes of generating angular momentum here compete on a roughly equal basis, giving rise to many alternative de- excitation possibilities. The existence of the 2.3 ns 5591 keV isomer with I = 47/2 shows that the single-particle character is already well established at that spin. The E2 transition speed is 10 WU, similar to the values observed [6,7] for aligned particle yrast isomers in 151Dy and 152Dy. With seven valence nucleons outside the ~46Gd core, there are many ways to generate I = 47/2 in t53Dy; from the systematics of yrast isomers in lighter Dy nuclei, it seems most likely that the 2.3 ns isomeric state has a seniority-five configuration involving two aligned h1~/2 protons and three active neu- trons. Whatever the configuration of the isomer, its main significance in the present context is that if manifests the occurrence in 153Dy of aligned particle yrast structures (and overall oblate shapes) similar to those observed in l~i'152Dy, whereas no high-spin yrast isomers are known in heavier Dy nuclei. In summary, this investigation has shown that 153Dy has collective character at low-spin, mar- ked by regular cascades of stretched E2 tran- sitions, but a change to single particle yrast excitations and irregular cascades with both dipole and quadrupole transitions occurs around I = 41/2. These single particle excitations are almost certainly aligned particle configurations, leading to an oblate nuclear shape. The present results and those of ref. [3] show 307 Volume 13lB, number  4,5,6 PHYSICS LETTERS 17 November  1983 I ( I+ l )  200 400 600 800 tO00 1200 15 Dy • 155Dy 55Dy 6 n s  3ns ' ° 5 0 0 Q 13 ns,,~ • 9.5 ns~ D c ,..* 2.3ns 6 0 n s  = • ° • ) F A 1.6ns - Z  r £ A • 152Dy A M  ~,  " 154Dy 156Dy A< 2 0 200 400 600 800 I000 1200 I(I*I) Fig. 3. Excitation energies E ,  of yrast levels in 151-1S6Dy plotted as a function of I ( I  + 1). The  upper  abscissa scale refers to the odd-A isotopes, the lower scale to the even-A isotopes. Half-lives of known yrast isomers are shown. The  data are from refs. [3,7,10-12] and the present  work. the Dy isotopes with N = 87 and 88 to be tran- sitional in two respects. Not only do they con- nect isotopes that have spherical or oblate shapes (N <~ 86) to those with prolate shapes characterized by pronounced collectivity (N I> 90), they also exhibit features of both groups. The transition from collective to single-particle character occurs at a higher spin in 154Dy (I > 32) than in 153Dy (I > 41/2), consistent with the tendency towards greater collectivity for higher neutron number. This is illustrated in fig. 3, where the energies of the yrast states of ~51-156Dy are plotted as a function of I ( I  + 1). In ~51Dy, even the lowest spin yrast states are of aligned particle character, and many yrast isomers are observed. In 152Dy, ~53Dy, and ~54Dy, the low- lying states are weakly collective, and the tran- sition from collective to single-particle charac- ter, usually accompanied by the appearance of yrast isomers, happens at spin values of ap- proximately 10, 41/2, and 32, respectively. In 155Dy and 156Dy, the yrast states are collective up to the highest observed spins and no yrast isomers occur. Angular momentum-induced shape transitions in the Dy nuclei have been predicted in theoretical calculations [8,9], in qualitative agreement with the experimental findings. The observed transitions, however, occur at lower spins than these calculations in- dicate. While the general features are now seen, important details still need to be understood, for example, how weakly collective (even vibra- tional) structures evolve into strongly collective ones represented by rotation of deformed shapes. We thank Peter Kleinheinz for additional in- formation about the (c~, 3n) results. References [1] J. Pedersen et al., Phys. Rev. Lett. 39 (1977) 990. [2] A. Bohr  and B.R. Mottelson, Phys. Scr. 10A (1974) 13. [3] A. Pakkanen et al., Phys. Rev. Lett. 48 (1982) 1530. [4] P. Kleinheinz et al., Nucl. Phys. A283 (1977) 189. [5] J.F.W. Jansen et al., Nucl. Phys. A321 (1979) 365. [6[ M. Piiparinen et al., Z. Phys. A290 (1979) 337. [7] T.L. Khoo  et al., Phys. Rev. Lett. 41 (1978) 1027; B. Haas  et al., Nucl. Phys. A362 (1981) 254. [8] S. A.berg, in: Elementary  modes  of excitation in nuclei, eds. A. Bohr  and R.A.  Broglia (North-Holland, Am-  sterdam, 1977). [9] C.G. Anderson  et al., Phys. Scr. 24 (1981) 266. [10[ D. Horn  et al., Phys. Rev, Lett. 50 (1983) 1447. [11] G. L0vhoiden et al., University of Bergen report (1980), unpublished.  [12] D. Ward et al., Nucl. Phys. A332 (1979) 433. 308 

Level structure of 153Dy and the competition between collective and few-particle excitation modes in Dy Nuclei

http://cdr.lib.unc.edu/downloads/3x816x465

Level structure of 153Dy and the competition between collective and few-particle excitation modes in Dy Nuclei

Abstract

Similar works

Full text

Available Versions

Carolina Digital Repository