The average g factors of high spin, high-excitation energy, quasi continuum structures in 194,193Hg were measured by observing the precessions of the angular distributions of γ-ray transitions in several normal-deformation bands that coalesce in the decay of the entry distribution of states. The average g factors of the states leading to the three main bands in the 193,194Hg isoles were: 〈g(193Hg)〉 = +0.19(1) and 〈g(194Hg)〉 = +0.26(1), respectively. These average g factors are smaller than the average of the g factors of the high energy states in the three superdeformed bands of 194Hg, 〈g(SD;194Hg)〉 = +0.41(8). While the nucleus in the superdeformed well behaves like a rigid rotor, the present results demonstrate the important role played by multiple, quasi particle neutron configurations in the structure of normal-deformation, highly-excited nuclear states

Benczer-Koller, N.

Broude, C.

Cizewski, J.A.

Hass, M.

Holden, J.

Janssens, R.V.F.

Kumbartzki, G.

Lauritsen, T.

Lee, I.Y.

Macchiavelli, A.O.

Mayer, R.H.

McNabb, D.P.

Satteson, M/

Weissman, L.

Carolina Digital Repository

14 January 1999Ž .Physics Letters B 446 1999 22–27Single particle signatures in high-spin, quasicontinuum states in193,194Hg from g-factor measurementsL. Weissman a,1, R.H. Mayer b,2, G. Kumbartzki b, N. Benczer-Koller b,3, C. Broude a,J.A. Cizewski b, M. Hass a, J. Holden b, R.V.F. Janssens c, T. Lauritsen c, I.Y. Lee d,A.O. Macchiavelli d, D.P. McNabb b,4, M. Satteson b,5a Weizmann Institute of Science, RehoÕot 76100, Israelb Department of Physics and Astronomy, Rutgers UniÕersity, New Brunswick, NJ 08903, USAc Physics DiÕision, Argonne National Laboratory, Argonne, IL 60439, USAd Nuclear Science DiÕision, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USAReceived 17 November 1998; revised 25 November 1998Editor: J.P. SchifferAbstractThe average g factors of high spin, high-excitation energy, quasicontinuum structures in 194,193Hg were measured byobserving the precessions of the angular distributions of g-ray transitions in several normal-deformation bands that coalescein the decay of the entry distribution of states. The average g factors of the states leading to the three main bands in the193,194 ² Ž193 .: Ž . ² Ž194 .: Ž .Hg isotopes were: g Hg sq0.19 1 and g Hg sq0.26 1 , respectively. These average g factors aresmaller than the average of the g factors of the high energy states in the three superdeformed bands of 194Hg,² Ž 194 .: Ž .g SD; Hg sq0.41 8 . While the nucleus in the superdeformed well behaves like a rigid rotor, the present resultsdemonstrate the important role played by multiple, quasiparticle neutron configurations in the structure of normal-deforma-tion, highly-excited nuclear states. q 1999 Elsevier Science B.V. All rights reserved.PACS: 21.10.Ky; 27.80.qwKeywords: g factors; High spin; Normal deformation structures; 193,194 Hg1 Present address:Instituut voor Kern- en Stralingsfysica,Katholieke Universiteit, Leuven, B-3001 Belgium.2 Present address: School of Chemical Engineering, PurdueUniversity, West Lafayette, IN 47907.3 E-mail: nkoller@physics.rutgers.edu4 Present address: Lawrence Livermore National Laboratory,Livermore, CA 94551.5 Present address: Interventional Innovations Corp., St. Paul,MN 55113.The light 190 – 196 Hg isotopes exhibit a variety ofcoexisting nuclear shapes ranging from weakly-de-formed oblate spheroids to superdeformed prolateellipsoids. Both non-collective neutron excitationsand high-J, high-K proton-hole configurations havew xbeen observed 1–8 . Most of these structures decayby E2 transitions, but the highest energy level se-quences consist of mixed dipole and quadrupole0370-2693r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved.Ž .PII: S0370-2693 98 01514-7( )L. Weissman et al.rPhysics Letters B 446 1999 22–27 23transitions. These observations have been confirmedby cranked Nilsson-Strutinsky calculations and totalRouthian surface calculations based on a deformedWoods-Saxon potential and the Strutinsky shell-model correction formalism with a monopole-pairingw xinteraction 9,10 . The highest energy bands ob-w xserved 5,6 associated with the presence of quasipar-ticle states involving valence neutrons in i , f13r2 5r2and p orbitals and protons in h and d3r2 11r2 3r2orbitals, support the view that multiple particle exci-tations play a dominant role, even at very high spins.However, the nature of unresolved states at yethigher excitation energy, decaying via a quasicontin-uum g spectrum has been characterized in a feww xcases only 11 . In general, the quasicontinuum spec-tra exhibit strong E2 characteristics and are thoughtto be mainly collective. In the mass As150 region,these strong quadrupole collective excitations feedinto states decaying via a mix of dipole andw xquadrupole transitions 11 . Fluctuation analysis ofg–g energy spectra have provided evidence ofw xdamped rotational motion at very high energy 12 .However, g-factor measurements can most sensi-tively determine the single-particle couplings to thecollective motions, as g factors distinguish uniquelyproton and neutron contributions to the total angularmomentum.Magnetic moment determinations of discrete, pi-cosecond, high spin states in nuclei produced infusion-evaporation reactions are scarce because ofthe experimental difficulties associated with thesew xmeasurements 13–16 . Measurements of magneticmoments of states in the pre-yrast continuum arew xeven scarcer 13,17–20 . The experimental signal, inthe form of a precession of the angular distributionof decay g rays, is very small. For such cases, theprecession can be detected via the use of the tran-sient magnetic field which originates from rapidlyfluctuating hyperfine fields acting on swift ionsw xtraversing a ferromagnetic material 21 . The mainprocesses which take place after a fusion evaporationreaction together with a schematic view of the kine-matics of the reaction products inside the target aresketched in Figs. 1 and 2.Under the experimental conditions presented here-after, the final nucleus is populated, after fast evapo-ration of a few neutrons, in states that exhibit a broaddistribution in excitation energy and spin and that,Fig. 1. Schematic illustration of the g –decay history of 194 Hgreaction products.furthermore, are spin-aligned by the reaction. Thevery high level density in the vicinity of the entrypoint results in a quasicontinuum spectrum of unre-solved g-ray cascades that proceed via numerousdecay paths. These paths eventually merge into a fewdistinct bands and finally end up decaying into theyrast levels. The higher, unresolved states are popu-lated during the traversal through the gadoliniumferromagnetic layer, where the transient field is oper-ational. The precessions of the nuclear spins in thetransient field are subsequently probed by observingthe precession of the discrete, low-lying transitionsthat carry the final alignment of the nucleus as itemerges from the ferromagnetic layer. It is possible,by using a large Ge array and applying selectedgating conditions to high-multiplicity events, to iso-late specific decay paths in the g cascades anddetermine the average g factors of the states leadingto these lower-energy discrete bands. The absolutevalues of resulting g factors depend on thew xparametrization of the transient field 21 , but therelatiÕe g factors of different ensembles of states areindependent of the uncertainties in the transient hy-perfine field.( )L. Weissman et al.rPhysics Letters B 446 1999 22–2724Fig. 2. Schematic time history of g –decays inside the target.This paper presents the first experimental mea-surement of the g factors of the high spin, quasicon-tinuum distribution of states in 193,194 Hg. A precursorexperiment designed to test the feasibility of tran-sient field measurements with large g-ray arrays wascarried out on 193Hg at the Eurogam array. Theanalysis of data, albeit of considerably less statisticalsignificance than the data obtained in this experi-ment, confirmed the applicability of the techniquew x22 .The experiment was carried out at the 88’’ Cy-clotron at Lawrence Berkeley National Laboratoryw xwith the Gammasphere array 23 which had, at thetime, ninety Compton-suppressed Ge detectors. The150 Ž48 .193,194reaction Nd Ca,5n-4n Hg at 203 MeV wasused to populate high-spin levels and to provide therecoiling Hg nuclei with sufficient velocity to tra-verse the ferromagnetic layer of gadolinium. Thetarget consisted of 1 mgrcm2 150 Nd evaporated on2.15 mgrcm2 rolled and annealed gadolinium. Alayer of 40 mgrcm2 of gold was evaporated on thedownstream side of the gadolinium layer to providea perturbation free environment for the stopped Hgions. The excited nuclei enter the ferromagnetic foilwith an average initial energy of 40 MeV and veloc-Ž . Ž .ity ÕrÕ s2.9 2.1%c , and exit from the foil0 inŽ . Ž .with an average velocity ÕrÕ s2.0 1.4%c ,0 outwhere Õ is the Bohr velocity. In this target, the Hg0ions enter the gadolinium foil at an average time of0.1 ps after production and exit on average 0.5 pslater. During the experiment, the beam intensity waskept at 2 pnA. The target was cooled to 77 K by aliquid-nitrogen transfer line which fits into the enclo-sure of Gammasphere. The gadolinium foil was po-larized by an external field of 0.06 T produced in thegap of a small electromagnet mounted outside thevacuum chamber. The current in the electromagnetwas reversed every five minutes. The magnetizationof the gadolinium foil, measured independently as afunction of temperature in an ac magnetometer, wasŽ .Ms0.182 4 T.The sorting of the multi-parameter coincidenceevents for a magnetic moment experiment requiresthat the spectra of g-ray transitions be generated foreach detector individually, gated by transitionsrecorded in all other detectors, as a function of themagnetic field direction, ‘‘up’’ or ‘‘down’’. Theangular distributions of the discrete transitions stud-ied can be determined by summing the spectra ofdetectors located in rings at the same angle withrespect to the beam direction. However, precessionmeasurements require the analysis of the spectra inindiÕidual detectors located at the Euler angles uand f with respect to the direction of the externalmagnetic field. Coincidence spectra were sorted sep-arately, for 36 detectors, by gating on two transitionswithin one of three bands corresponding to the maindecay branches ABC, ABE q ABCDE, ABF qABCDF for 193Hg and AB q ABCD, AF q ABCFand AE q ABCE for 194 Hg, where the notationw xrefers to the cranked shell model calculations 24w xused by Hubel et al. 3 . Specifically the following¨gating transitions were used in 193Hg: 617.9, 738.8,193.5, 480.6, 704.3 and 807.7 keV for the ABCband; 745.5, 205.1, 487.7, 639.6, 660.3, 512.8 and651.1 keV for the ABFqABCDF bands and 606.4,857.7, 375.3, 302.8, 573.0, 653.3, 523.2 and 873.1keV for the ABEqABCDE bands. The correspond-ing gating transitions in 194 Hg were: 565.0, 412.9,643.0, 743.6, 710.4 and 592.8 keV for the ABqABCD bands; 232.9, 544.6, 706.2, 485.2, 235.5 and( )L. Weissman et al.rPhysics Letters B 446 1999 22–27 25665.8 keV for the AEqABCE bands and 227.6,423.8, 611.2, 574.7, 236.3, 305.9 and 606.8 keV forthe AFqABCF bands.The coincidence events were ‘‘unpacked’’ as de-w xscribed in 25–28 and background spectra weredetermined for each band and subtracted by thew xtechnique described by Crowell et al. 26 . The angu-lar distributions of the g rays in the selected bandswere obtained from the analysis of the efficiency-corrected sum spectra of individual detectors locatedat the same angle with respect to the beam. Typicalspectra are shown in Fig. 3a and Fig. 3b.The detailed account of these measurements canw xbe found in 29 . The E2 transitions from differentbands in both nuclei exhibit similar angular correla-tion coefficients A and A since a typical stretched2 4g-ray cascade is governed by the initial nuclear spinalignment and results in identical angular distribu-tions for all members of the cascade. The fittedŽ . Ž .coefficients, A s0.31 1 and A sy0.15 2 , were2 4used for all E2 transitions in the precession analysis.The angular precessions Du were determined fol-lowing the standard procedure described inw x13,21,30 .The measured precessions of individual transi-tions were averaged over nine symmetrical combina-tions of four detectors. The g factors were extractedfrom the precessions through the relation,Dusy gm r" B Õ t ,Z dtŽ . Ž .Ž .HNwhere the integration was carried over the time the193,194 Hg ion spends in the gadolinium foil. TheChalk River parametrization of the transient fieldw x Ž . Ž .31 , B Õ , Z s 154.7 = Z = ÕrÕ = exp0Ž .y0.135ÕrÕ =M and the Ziegler stopping powers0w x32 were used. Table 1 displays the resulting preces-sions for each of the groups of states decaying intoone of the main bands together with the correspond-ing average g factors.The region of excitations probed under the condi-tions of this experiment, determined from MonteFig. 3. Spectra of g-rays in coincidence with two cascade radia-. 193 .tions specifying the six bands of interest in a Hg and b in194 Hg, observed in one detector, in one field direction.( )L. Weissman et al.rPhysics Letters B 446 1999 22–2726Table 1Results of the precession measurements and average g factors forŽ . 193,194 Žthe main normal-deformation ND bands in Hg present. Ž . 194 wresults , and the three superdeformed SD bands in Hg Ref.w xx30Ž . ² :Bands Du mrad g193ND bands in HgABC 18.4"1.4 0.188"0.014ABEqABCDE 17.0"1.4 0.176"0.014ABFqABCDF 19.3"1.7 0.200"0.018194ND bands in HgABqABCD 24.6"1.5 0.255"0.016AEqABCE 24.9"2.5 0.258"0.026AFqABCF 26.0"2.1 0.270"0.022194SD bands in HgSD1 33"9 0.36"0.10SD2 36"18 0.41"0.20SD3 73"24 0.72"0.26Carlo simulations similar to the ones carried out in152 w x w x 192 w xDy 11,13 , Hf isotopes 17,18 , and Hg 33 ,lies 2–3 MeV above the yrast line, at spins If30y50". In both 193Hg and 194 Hg, the decay toward theyrast states proceeds via a variety of pathways. Theband selectiÕity aspect of the present measurementallows a comparison of the average g factors ofdifferent subsets of states in the quasicontinuumdistribution. Thus, the structural differences in en-sembles of excited states in this spin and energyregion can be specifically assessed.Three main conclusions can be drawn from the² :present measurements. First, the g factors corre-sponding to the different subsets of states within anyone nucleus are found to be essentially the same.This observation leads to the conclusion that, eitherthe entry distribution of states feeds uniformly thethree main lower paths and, hence, the average gfactors measured belong to one ensemble of states,or, that there are three distinct distributions of statesat high spin and energy which happen to have thesame average g factor. Assuming that the feedinginto the various bands is a statistical process, theformer interpretation is preferred.Second, the present results yield average g fac-tors in the odd 193Hg nucleus which are about 30%smaller than the ones in 194 Hg and highlight theimportance of the additional neutron alignment in thehigh-spin region. This result is consistent with the² Ž163,165 . :observed average g factors, g Hf sŽ . ² Ž162,164,166 . : Ž .q0.15 2 and g Hf sq0.22 1 , in thew xodd and even Hf nuclei, respectively 17,18 . Fur-thermore, the average g factors of the high spinstates in the even isotopes are also smaller than thevalue of the collective rotational estimate gfZrAŽ .Fig. 4 . This reduction of the g factors can beinterpreted in the context of the pattern of proton andneutron quasiparticle excitations predicted in variousw xcranking calculations 1–4,9,10,34 . At moderate ro-tational frequencies, the Ns114 quasineutron Rou-thians indicate that a large number of neutron or-bitals are available for multi-quasiparticle excita-tions. In contrast, because of the proximity of theZs82 shell closure, the Zs80 quasiproton Routhi-Ž .ans exhibit a large energy gap f 1 MeV in thesame frequency regime, making proton multi-particleexcitations less probable.Third, the average g factor of the highest-spinstates in the three superdeformed bands in 194 Hg,² Ž .: Ž .g SD sq0.41 8 has been determined in a studyw x Žcarried on the same data set 30 Table 1 and Fig..4 . This value is significantly larger than the averageg factors of the normal-deformation states in thequasicontinuum distribution. This result also agreeswith theoretical calculations that predict that thesuperdeformed nucleus is likely to behave like aw xrigid rotator with a g;ZrA 30 . Thus, the compar-ison of the present data for superdeformed and nor-mal deformation nuclei indicates that, while a nu-cleus in the superdeformed well resembles a classicalFig. 4. Summary of measured g factors for normal-deformedstates in 193Hg, and both normal-deformed and superdeformedw x 19430 states in Hg.( )L. Weissman et al.rPhysics Letters B 446 1999 22–27 27rotational system, the structure of the nucleus inhigh-spin, normal-deformation states is governed bydistinct single-particle configurations.The continuous development and improved per-formance of large Ge arrays provides the possibilityof systematic studies of these phenomena. Futureexperiments using targets with a gap between a Ndtarget and a gadolinium foil in a plunger arrange-w xment 16 will allow variations in the transient-fieldtime window and, hence, to probe the evolution ofthe nuclear structure as the nucleus decays fromregions of high spin and excitation to the yrast line.AcknowledgementsThe authors would like to thank J. Dudek and W.Nazarewicz for stimulating discussions. Thanks aredue to L. Sapir and J. Greene for target preparation,to B. Elkonin for the design of the cooling system, toR. McLeod for assistance with the data acquisitionsystem and to the staff of the 88’’ Cyclotron for theirhelp during the experiment. This work was supportedin part by the US National Science Foundation, theIsrael – US Binational Science Foundation and theUS Department of Energy, Nuclear Physics Division,Ž .under contract Nos. W-31-109-ENG-38 ANL andŽ .DE-AC03-765F00098 LBNL .Referencesw x Ž .1 I.G. Bearden, Nucl. Phys. A 576 1994 441.w x Ž .2 D. Ye et al., Phys. Lett B 236 1990 7; Nucl. Phys. A 537Ž .1992 207.w x3 H. Hubel, A.P. Byrne, S. Ogaza, A.E. Stuchbery, G.D.¨Ž .Dracoulis, Nucl. Phys. A 453 1986 316.w x Ž .4 Y. Le Coz, Z. Phys. A 348 1994 87.w x Ž .5 J.K. Deng, Phys. Lett. B 319 1993 63.w x Ž .6 N. Fotiades et al., J. Phys G 21 1995 911; Z. Phys. A 354Ž .1996 169.w x7 R.V.F. Janssens, T.L. Khoo, Ann. Rev. Nuc. Part. Sci. 41Ž .1991 321.w x Ž .8 G. Hackman, Phys. Rev. Lett. 79 1997 4100, and refer-ences therein.w x Ž .9 T. Bengtsson, I. Ragnarsson, Nucl. Phys. A 436 1985 14.w x10 R. Wyss, J. Nyberg, A. Johnson, R. Bengtsson, W.Ž .Nazarewicz, Phys. Lett. B 215 1988 211; R. Wyss et al.,Ž .Nucl. Phys. A 503 1989 244.w x Ž .11 R. Holtzmann, Phys. Lett. B 195 1987 321, and referencestherein.w x12 S. Leoni, P. Bosetti, A. Bracco, T. Døssing, B. Herskind, E.Ž .Vigezzi, Z. Physik A 358 1997 157.w x Ž .13 M. Hass, Phys. Rev. C 44 1991 1397.w x Ž .14 E. Lubkiewicz, Z. Physik A 335 1990 369.w x Ž .15 U. Birkental, Nucl. Phys. A 555 1993 643.w x Ž .16 A. Jungclaus, Phys. Rev. Lett. 80 1998 2793.w x Ž .17 L. Weissman, M. Hass, C. Broude, Phys. Rev. C 53 1996151.w x Ž .18 L. Weissman, M. Hass, C. Broude, Phys. Rev. C 57 1998621.w x Ž .19 O. Hausser, Phys. Lett. B 144 1984 341.¨w x20 H.-E. Mahnke, O. Hausser, H. Grawe, H. Kluge, W. Semm-¨Ž .ler, Phys. Lett. B 134 1984 153.w x21 N. Benczer-Koller, M. Hass, J. Sak, Ann. Rev. Nucl. Part.Ž .Sci. 30 1980 53.w x22 N. Benczer-Koller et al., 5th International Spring Seminar onNuclear Physics, New Perspectives in Nuclear Structure,Ravello, Italy, May 22–26, 1995, World Scientific, 1996, p.493.w x Ž .23 I.Y. Lee, Nucl. Phys. A 520 1990 641c.w x Ž .24 R. Bengtsson, S. Frauendorf, Nucl. Phys. A 327 1979 139.w x25 C.W. Beausang, Nucl. Instrum. Methods Phys. Res. A 364Ž .1995 560.w x26 B. Crowell, M.P. Carpenter, R.G. Henry, R.V.F. Janssens,T.L. Khoo, T. Lauritsen, D. Nisius, Nucl. Instrum. MethodsŽ .Phys. Res. A 355 1995 575.w x27 G. Hackman, J.C. Waddington, Nucl. Instrum. Methods Phys.Ž .Res. A 357 1995 559.w x28 D.S. Haslip, G. Hackman, J.C. Waddington, Nucl. Instrum.Ž .Methods Phys. Res. A 345 1994 534.w x29 L. Weissman et al., in: Proceeding of SNEC98, 1998, Padova,Nuovo Cimento, in press.w x Ž .30 R.H. Mayer, Phys. Rev. C 58 1998 R2640.w x Ž .31 O. Hausser, Nucl. Phys. A 412 1984 141.¨w x32 J.F. Ziegler, J.P. Biersack, U. Littmark, The Stopping andRange of Ions in solids, Pergamon, New York, 1985.w x Ž .33 T. Lauritsen, Phys. Rev. Lett. 69 1992 2479.w x34 J. Dudek, private communication.

Single particle signatures in high-spin, quasi continuum states in 193,194 Hg from g-factor measurements

http://cdr.lib.unc.edu/downloads/b8515z626

Single particle signatures in high-spin, quasi continuum states in 193,194 Hg from g-factor measurements

Abstract

Similar works

Full text

Available Versions

Carolina Digital Repository