PHYSICAL REVIEW C NOVEMBER 1997VOLUME 56, NUMBER 5

Octupole states in 196Pt„p,p8g…

J. K. Jewell, P. D. Cottle, K. W. Kemper, and L. A. RileyDepartment of Physics, Florida State University, Tallahassee, Florida 32306-3016

~Received 7 March 1997!

Possible octupole states in196Pt were studied via the196Pt(p,p8g) reaction with 12.7 MeV protons. Theresults considerably strengthen the argument made previously that the low-energy octupole state in196Pt isquite fragmented. Furthermore, one of the possible mixed symmetry states previously identified in this nucleusis observed in the present experiment, providing evidence that it hasJp521. @S0556-2813~97!00611-0#

PACS number~s!: 23.20.Lv, 27.80.1w, 21.10.Re, 21.60.Ev

s3

3c

-hedeV

e

ic

--

dop-

e

e

itye,n-

e

ofid

ra-

ro-

edyglec-ed

tecttion

90°on iser-abled tom-

ron

en-tes

le

ing

ringalso

I. INTRODUCTION

The behavior of octupole states in the stable Pt isotopeanomalous in several respects. While the energies of1

2

states in other mass regions vary smoothly@1#, there is a gapof approximately 1 MeV between the energies of the1

2

states in the even mass Pt isotopes and the collective opole states in198,200,202,204Hg ~see Fig. 1!. However, Cottle,Stuckey, and Kemper@2# pointed out that strong fragmentation of the low-energy octupole state might explain tanomaly in the behavior of the 31

2 states. They reanalyzethe angular distributions measured in the 35 M194,196,198Pt(p,p8) reactions by Deasonet al. @3#, and pro-posed that several states between 2 and 3 MeV which wstrongly populated in those reactions haveJp532. If thesystematic behavior of the octupole strength centroid, whis defined as

C5@SEiB~E3;0gs1→3i

2!#/@SB~E3;0gs1→3i

2!# ~1!

is examined~instead of just the 312 states!, a smooth system

atic trend is observed~see Fig. 1!, and the anomalous behavior of the 31

2 states is explained. In addition, it was found@4#that strong octupole fragmentation, such as that proposeoccur in 194,196,198Pt, is characteristic of nuclei conforming tthe O~6! dynamical symmetry of the interacting boson aproximation ~IBA !, of which 196Pt is the best-known example@5#.

However, the (p,p8) angular distributions of the possibl32 states analyzed in Ref.@2# were not able to definitivelydistinguish between 32 and 41 spin assignments. In thpresent work, we reportg-ray data from a196Pt(p,p8g) ex-periment and compare it with the196Pt(n,n8g) data ofDiPreteet al. @6# to provide an unambiguous spin and parassignment of 32 for one of the states analyzed by CottlStuckey, and Kemper@2#. We also argue that the assigments for other high-lying states discussed in@2# are identi-cal. In addition, we observe one of the states recently msured by von Brentanoet al. @7# in a search for mixedsymmetry states. The observation of this state in the (p,p8)reaction favors a 21 assignment for this state.

II. EXPERIMENTAL PROCEDURE AND RESULTS

The nucleus196Pt was excited via inelastic scattering12.7 MeV protons. The beam was produced with the Flor

560556-2813/97/56~5!/2440~5!/$10.00

is

tu-

re

h

to

-

a-

a

State University Super FN Tandem Van de Graaff Acceletor. The metallic Pt target was 2.03 mg/cm2 thick and en-riched to 97% in196Pt. Coincidences between scattered ptons and g rays were used to identifyg deexcitationsfollowing inelastic scattering. Four Compton-suppressn-type HPGeg-ray detectors having 20% relative efficiencand 2.5 keV resolution at 1.33 MeV were placed at an anof 90° to the beam direction. A silicon surface barrier detetor of 100 mm2 area and 1.5 mm thickness was also placat 90°, and at a distance of 6 mm from the target to descattered protons. The particle detector yielded resolu~full width at half maximum! of 150 keV, allowing moderateselection of excited states. The proton detector angle ofwas selected because the elastic scattering cross sectimuch smaller than at more forward angles, while the diffential cross sections for inelastic scattering were comparto those at forward angles. The silicon detector was coole220 °C to minimize the resolution effects of neutron daage.

The beam energy was chosen to minimize the neutflux from (p,n) and (p,2n) reactions while still maintaininga practical yield of inelastic scattering events. The beamergy used in the present work is similar to that used by Yaet al. @8# ~12.3 MeV! in a study of the198Pt(p,p8g) reaction.

The particle-g coincidence condition required a particevent followed by ag event from any of the four HPGedetectors within a 200 ns time window. Only events meet

FIG. 1. Energies of 312 states~filled shapes! of Pt and the col-

lective octupole states in Hg and Pb showing anomalous loweof the 31

2 state energy in the even mass Pt isotopes. The figureshows the correction of this anomaly by including higher-lying 32

states in the calculation of octupole centroid energy~open shapes!.

2440 © 1997 The American Physical Society

ci-

rtthdi

btiodkwp

on

wsatprth

eit

.

t

ere

ngofmre

ave

s

er

bynd

uchtes

f

l

5

56 2441OCTUPOLE STATES IN196Pt(p,p8g)

this condition were recorded. To further identify true coindences, the time difference between the particle andg-rayevents was also recorded via a time-to-amplitude conve~TAC! so that random coincidences which resulted fromlarge yield of (p,n) and (p,2n) reactions could be subtractefrom the coincidence spectra. The TAC spectrum, whichshown in Fig. 2~a!, clearly demonstrates the need for sutracting random coincidences. The effect of the subtracof random coincidences is dramatic, and is also illustrateFigs. 2~b! and 2~c!. In particular, the elastic scattering peain the proton spectrum, which is dominant in the raparticle-g coincidence spectrum, is almost completely supressed when random coincidences are subtracted.

In Fig. 3, the 2245 keV state is examined by gatingboth g-ray and particle spectra. The spectrum ofg rays incoincidence with protons from the 2245 keV state is shoin Fig. 3~a!. The 2245 keVg ray can be seen clearly in thispectrum. A state at 2262 keV, which cannot be separfrom the 2245 keV state in the proton spectrum, decaysdominantly to the 1969 keV state, which then decays toground state via the 1969 keVg ray which is also seen inthis spectrum. In Fig. 3~b!, the proton spectrum gated on th2245 keVg ray is shown. Once again, the spectrum is quclean, and only the 2245 keV proton group can be seen

FIG. 2. ~a! TAC spectrum;~b! Spectrum of protons from alg-proton coincidences recorded;~c! Proton spectrum with timebackground~random coincidences! subtracted.

ere

s-nin

-

n

ede-e

e

Table I lists the states andg rays observed in the presenexperiment; the relative intensities ofg rays deexciting aparticular state are also provided. All the states seen hhave been observed in previous work~for example, see@6#!.However, the selectivity of the proton inelastic scatterireaction and the information provided by the observationthe g rays allow us to draw new physics conclusions froour data. In particular, the data allow us to infer structuinformation about the 2245 and 2431 keV states, which hpreviously been associated with scissors modes@7# andanomalous octupole vibrations@2#, respectively. These stateare discussed in the following sections.

III. HIGH-LYING OCTUPOLE STATES IN 196Pt

Following the proposal by Cottle, Stuckey, and Kemp@2# that Deasonet al. @3# had observed 32 states near 2.5MeV in 194,196,198Pt, DiPreteet al. @6# used the196Pt(n,n8g)reaction and the superior energy resolution providedg-ray detection to identify all states which could correspoto possible 32 states seen in the196Pt(p,p8) experiment. Inaddition, DiPreteet al.measuredg-ray angular distributions,excitation functions, and decay patterns to obtain as mspin and parity information as possible. They found 12 stawhich could correspond to the four possible 32 states iden-tified in Ref.@2#. These states all fall within 15 keV of one o

FIG. 3. ~a! The spectrum ofg rays in coincidence with protonsfrom the 2245 keV state;~b! the proton spectrum gated on the 224keV g ray.

i

dy

tipve

hatects

e

c-

ys

ndsel.by

een

re-

f

heth isates.ree

tof

le

ed

2442 56JEWELL, COTTLE, KEMPER, AND RILEY

the energies assigned by Deasonet al. @3#, and the spin andparity information available for each of these 12 statesconsistent with either aJp532 or 41 assignment. For nineof these states, the data available from the (n,n8g) reactionare consistent with both 32 and 41 assignments.

In the present (p,p8g) study, one goal was to identifywhich four of the 12 candidate states identified in Ref.@6#were actually populated in the (p,p8) study. It is possiblethat more than four of the candidate states were populatethe previous (p,p8) study of Ref.@6#. Because the energresolution in the study of Deasonet al. is 2–8 keV, protonpeaks interpreted as single states may, in fact, be mulstates which were unresolved in that experiment. Howewe see no direct evidence from the present (p,p8g) experi-

TABLE I. Energy levels andg-ray transitions observed inEp512.7 MeV 196Pt(p,p8g) reaction.

Elevel ~keV! Eg ~keV! Branching ratio Ef ~keV! I ip→I f

p

355.7~2! 355.7~2! 0 21→01

688.7~2! 332.9~2! 356 21→21

876.8~3! 521.1~2! 356 41→21

1015.0~2! 326.3~2! 689 31→21

1402.7~3! 1047.0~3! 356 01→21

1447.0~2! 758.4~2! 12~4! 689 32→21

1091.1~3! 88~17! 356 32→21

1535.8~3! 847.1~3! 17~9! 689 41→21

521.0~4! 83~11! 1015 41→31

1677.2~5! 1677.3~6! 68~45! 0 21→01

1321.4~5! 32~81! 356 21→21

1847.3~5! 1491.6~5! 356 21→21

2047.0~4! 1358.3~4! 689 21→21

2093.0~3! 1737.3~3! 356 ~21)→21

2245.6~7! 2245.6~7! 0 21→01

2262.4~7! 1246.7~7! 67~15! 1015 21→31

1573.5~7! 33~18! 689 21→21

2423.4~4! 976.7~3! 52~13! 1447 32→32

2067.6~7! 48~22! 356 32→21

s

in

ler,

ment of any of this occurring. Furthermore, we assume tinelastic scattering of the 12.7 MeV protons used here selstates in196Pt that are also seen in the 35 MeV (p,p8) reac-tion of Ref. @3#.

We are able to identify with certainty only one of thproposed octupole states previously seen in the (p,p8) reac-tion of Ref. @3#. Gating on protons in the vicinity of 2431keV excitation energy~the energy reported by Deasonet al.!reveals the 2067.6 keVg ray reported by DiPreteet al. @6# todeexcite the 2423.4 keV state, as shown in Fig. 4~a!. Gatingon the 2067.6 keVg ray returns a peak in the proton spetrum corresponding to the correct excitation energy@Fig.4~b!#. Figure 4~c! shows that the 2423 keV state also decaby emission of a 976.7 keVg ray to the lowest-lying 31

2

level. This decay has not been reported previously and lecertainty to the placement of the 2423.4 keV energy levThe placement of the 976.7 keV transition is confirmedthe proton spectrum gated by thisg ray @Fig. 4~d!#. A protonpeak corresponding to the 2423.4 keV state can be sclearly.

Of the 12 candidate states identified in Ref.@6#, the 2423keV state is one of the three for whichJp541 is excludedby g-ray angular distribution measurements. We can thefore unambiguously assignJp532 to the 2423 keV levelbased on the proton angular distributions of Ref.@3# whichallow for only aJp532 or 41 assignment. Identification othis high-lying 32 octupole state in196Pt considerablystrengthens the argument made in Ref.@2# for a high degreeof fragmentation of the low-energy octupole state. Tpresent study shows that the low-energy octupole strengshared by at least two states, the 1447 and 2423 keV stHowever, Cottle, Stuckey, and Kemper proposed that thadditional levels~2608, 2638, and 2707 keV! seen in the 35MeV (p,p8) experiment@3# also carry a significant amounof the octupole strength. Since the angular distributionsthese states in the 35 MeV (p,p8) experiment are nearlyidentical to that of the 2423 keV level, it is highly probabthat these three states also haveJp532.

A search forg rays connecting the other states report

3.6n-

3d

-

FIG. 4. ~a! A portion of the spectrum ofgrays in coincidence with protons from the 242keV state showing the previously reported 2067keV g decay;~b! the proton spectrum gated othe 2067.6 keVg ray showing the 2423 keV energy level; ~c! A portion of the spectrum ofgrays in coincidence with protons from the 242keV state showing the previously unreporte976.7 keV decay to the 31

2 level, and the subsequent decays from the 31

2 level; ~d! the protonspectrum gated on the 976.7 keVg ray showingthe 2423 keV level.

a

reeev

th

te

reharuthe

xce

te

—dth

o

t

ct

ig

s

rtheV

thctretro

leta

r t

ar

red

r-

er-pes,res-w-lei,

tate

veag-r-

ionc-

o-

neilar

-

enral

me-y

ed 3.5

,or a

t-

itheVude

ow

it1

the

56 2443OCTUPOLE STATES IN196Pt(p,p8g)

by DiPreteet al. with the 211 , 22

1 , 312 , and 41

1 states wasconducted by gating on the proton spectrum between 2.32.8 MeV excitation energy and examining the resultingg-rayspectra, but none were observed. Furthermore, gates weon theg-ray spectrum at the energies corresponding to thtransitions, but the resulting proton spectra showed nodence for peak structure at the correct energies.

There are several possible explanations why the ohigh-lying octupole states proposed in Ref.@2# were not ob-served in the present experiment. First, two of the sta~2608 and 2707 keV! have smallerB(E3) values@2.1 and3.2 Weisskopf units~W.u.! compared to 4.4 W.u. for the2423 keV state# and therefore are likely to have been modifficult to observe for that reason. Second, it is possible tthe 2423 keV state was more isolated in the proton spectthan the 2608, 2638, and 2707 keV states, and thereforeits g-ray spectrum was cleaner, making the weak 2067 kg ray easier to see than the high-energy transitions deeing the other three states. The resolution of the proton sptrum in the present experiment was not sufficient to demine whether this was true.

The observation of the 2423 keV state in196Pt is reminis-cent of the observation of the 2603 keV state in198Pt byYateset al. @8# using the (p,p8g) reaction. They identifiedthis state via its decay to the 31

2 state at 1680 keV, andproposed — based on the occurrence of this transitionthat the 2603 keV state hasJp532. They also associatethis state with the 2611 keV state strongly populated in35 MeV 198Pt(p,p8) reaction by Deasonet al. @3#. Subse-quently, Cottle, Stuckey, and Kemper@2# proposed five high-lying octupole states in198Pt ranging in energy from 2441 t2826 keV, of which the strongest@B(E3)56.7 W.u.# is the2611 keV state. The angular distributions of these five stain the 35 MeV (p,p8) data of Deasonet al. @3# are nearlyidentical to those of the high-lying octupole states in196Pt. Intotal, all this evidence provides a strong argument that opole fragmentation occurs in198Pt as well as196Pt. Further-more, Cottle, Stuckey, and Kemper also proposed three hlying octupole states in194Pt @including a state withB(E3)55.5 W.u. at 2543 keV# that have angular distributionnearly identical to the corresponding states in196,198Pt.

Ponomarevet al. @10# proposed 32 assignments for the2423 and 2638 keV states in196Pt on the basis of thei196Pt(e,e8) experiment. Since electron scattering probesrole of the protons in the octupole vibrations, and 35 Mproton scattering primarily probes the neutron role@9#, theavailability of both (e,e8) and (p,p8) data for the 2423 and2638 keV states~as well as for the 31

2 state at 1447 keV!provides the opportunity to compare the amplitudes ofproton and neutron oscillations in the three strongest opole states in196Pt. In the simple isoscalar collective pictufor low-energy octupole states, the amplitudes of the neuand proton oscillations~that is,bn andbp) are equal. How-ever, if bn and bp are different it suggests that the simpisoscalar collective picture is not adequate, and that the smay be better described as a quasiparticle excitation. Fothree strong 32 states in196Pt ~1447, 2423, and 2638 keV!,the (e,e8) and (p,p8) data are consistent with isoscalstates in whichbn and bp are equal. TheB(E3;0gs

1→32)values obtained@2# with 35 MeV (p,p8) @0.099~20! e2b3,

nd

setsei-

er

s

tmat

Vit-c-r-

e

es

u-

h-

e

eu-

n

tehe

0.070~14! e2b3, and 0.070~14! e2b3 for the 1447, 2423, and2638 keV states, respectively# are equal — within experi-mental uncertainty — to the corresponding values measuwith (e,e8) @0.111~11! e2b3, 0.087~14! e2b3 and 0.072~13!e2b3, respectively#. These results lend support to the intepretation of these 32 states as collective excitations.

While interacting boson approximation~IBA ! @4#,quasiparticle-phonon model~QPM! @10#, and Hartree-Fock@11# frameworks have been used for attempting to undstand the behavior of octupole states in the stable Pt isotoonly the IBA and QPM approaches have explained the pence of fragmentation. It has long been known that the loenergy octupole state is fragmented in well deformed nucsuch as those in the rare-earth and actinide mass regions~forexample, see@12,13#!. However, Zamfiret al. @4# demon-strated that fragmentation of the low-energy octupole salso occurs in nuclei which conform to the O~6! dynamicalsymmetry, of which196Pt is the best known example@5#.Such nuclei haveg-unstable, triaxial shapes and do not halarge stable deformations. The confirmation of octupole frmentation in196Pt given in the present work provides impotant support of this prediction of the IBA.

The QPM calculations of Ponomarevet al. @10# provideanother perspective on the origin of octupole fragmentatin 196Pt. Ponomarevet al. propose that the three strong otupole states~at 1447, 2423, and 2638 keV! arise from themixing of the one phonon octupole state with the two phnon @21

1^ oct#3 state and the two phonon@21

1^ 51

2#3 state,where the 51

2 state corresponds to the 25-pole phonon. TheQPM picture of octupole fragmentation in which the ophonon octupole state mixes with other structures is simto that given by the semidecoupled model@14,15#, whichYates et al. @8# qualitatively cite to explain octupole fragmentation in198Pt.

IV. MIXED SYMMETRY STATES IN 196Pt

While mixed symmetry ‘‘scissors mode’’ states have beobserved in well deformed and spherical nuclei in sevemass regions, the first possible observation of mixed symtry states in ag-soft nucleus was just recently reported bvon Brentanoet al. @7# in 196Pt. Using nuclear resonancfluorescence, they measured ten states between 2.2 anMeV and suggested that these states may haveJp511 andbe fragments of the orbitalM1 ‘‘scissors mode’’ state. Herewe argue that the present data for the 2245 keV state favJp521 assignment instead of 11.

As noted in Refs.@16–18#, the cross sections for populaing 11 states of a purely orbital~mixed symmetry! characterare vanishingly small for inelastic scattering of protons wenergies up to 200 MeV. Since we observe the 2245 kstate strongly in the present experiment, we can conclthat it is not a pure mixed symmetry 11 state. In addition, theenergy of the proton beam in the present work is so l~12.7 MeV! that even ‘‘spin’’-type 11 states are not likely tobe populated@19#. While the data of von Brentanoet al. @7#constrained theJp assignment of the 2245 keV state to 11 or21, we propose that 21 is the correct assignment sinceappears there is no mechanism for strongly populating a1

state with 12.7 MeV protons.A 21 assignment does not exclude the possibility that

i

e.Vd

so

in

keVnt-sent

y

u-ce

2444 56JEWELL, COTTLE, KEMPER, AND RILEY

2245 keV state is a mixed symmetry excitation. Iachello@20#predicted that the lowest mixed symmetry state in an O~6!nucleus like196Pt would haveJp521 and a significantE2transition to the ground state. Of the fully symmetric statesan O~6! nucleus, only one — the 21

1 state — is expected tohave a significantE2 matrix element with the ground statThe strength of theE2 transition connecting the 2245 kestate to the ground state~2.6 W.u., using the width measurein @7#! marks this state as a candidate for the 21 mixedsymmetry excitation.

V. SUMMARY

We have used the (p,p8g) reaction with 12.7 MeV pro-tons to study octupole fragmentation and a possible scis

C

.

en

R

Ws

.-DN

le-

ys

. C

.

n

rs

mode state in196Pt. The observation of the 2423 keV statethe present experiment confirms itsJp532 assignment andprovides strong evidence that the 2608, 2638, and 2707states haveJp532 as well, thus strengthening the argumethat the octupole state in196Pt is strongly fragmented. Furthermore, the appearance of the 2245 keV state in the preexperiment provides evidence that this state hasJp521. Wesuggest that this is the 21 mixed symmetry state predicted bIachello in an O~6! nucleus@20#.

ACKNOWLEDGMENTS

We thank N.V. Zamfir for his comments on the manscript. This work was supported by the National ScienFoundation and the state of Florida.

ov,

v.

ucl.

ys.

O.

.

D.

@1# R. H. Spear, At. Data Nucl. Data Tables42, 55 ~1988!.@2# P. D. Cottle, K. A. Stuckey, and K. W. Kemper, Phys. Rev.

38, 2843~1988!.@3# P. T. Deason, C. H. King, R. M. Ronningen, T. L. Khoo, F. M

Bernthal, and J. A. Nolen, Phys. Rev. C23, 1414~1981!.@4# N. V. Zamfir, P. D. Cottle, J. L. Johnson, and R. F. Cast

Phys. Rev. C48, 1745~1993!.@5# J. A. Cizewski, R. F. Casten, G. J. Smith, M. L. Stelts, W.

Kane, H. G. Borner, and W. F. Davidson, Phys. Rev. Lett.40,167 ~1978!.

@6# D. P. DiPrete, T. Belgya, E. M. Baum, E. L. Johnson, S.Yates, P. D. Cottle, M. A. Kennedy, and K. A. Stuckey, PhyRev. C48, 2603~1993!.

@7# P. von Brentano, J. Eberth, J. Enders, L. Esser, RHerzberg, N. Huxel, H. Meise, P. von Neumann-Cosel,Nicolay, N. Pietralla, H. Prade, J. Reif, A. Richter, C. Schgel, R. Schwengner, S. Skoda, H. G. Thomas, I. Wiedenho¨ver,G. Winter, and A. Zilges, Phys. Rev. Lett.76, 2029~1996!.

@8# S. W. Yates, R. Julin, J. Kumpulainen, and E. Verho, PhRev. C37, 2877~1988!.

@9# V. A. Madsen, V. R. Brown, and J. D. Anderson, Phys. Rev12, 1205~1975!.

@10# V. Yu. Ponomarev, W. T. A. Borghols, S. A. Fayans, M. N

,

.

.

.

..

.

Harakeh, C. W. de Jager, J. B. van der Laan, A. P. PlatonH. de Vries, and S. Y. van der Werf, Nucl. Phys.A549, 180~1992!.

@11# J. L. Egido, V. Martin, L. M. Robledo, and Y. Sun, Phys. ReC 53, 2855~1996!.

@12# K. Neergard and P. Vogel, Nucl. Phys.A145, 33 ~1970!.@13# P. D. Cottle and N. V. Zamfir, Phys. Rev. C54, 176 ~1996!.@14# S. W. Yates, J. C. Cunnane, R. Hochel, and P. J. Daly, N

Phys.A222, 301 ~1974!.@15# H. Toki, K. Neergard, P. Vogel, and A. Faessler, Nucl. Ph

A279, 1 ~1977!.@16# J. A. Carr, F. Petrovich, R. J. Philpott, M. J. Threapleton,

Scholten, and H. McManus, Phys. Rev. Lett.54, 881 ~1985!.@17# C. Djalali, N. Marty, M. Morlet, A. Willis, J. C. Jourdain, D.

Bohle, U. Hartmann, G. Ku¨chler, A. Richter, G. Caskey, G. MCrawley, and A. Galonsky, Phys. Lett.164B, 269 ~1985!.

@18# C. Wesselborg, K. Schiffer, K. O. Zell, P. von Brentano,Bohle, A. Richter, G. P. A. Berg, B. Brinkmo¨ller, J. G. M.Romer, F. Osterfeld, and M. Yabe, Z. Phys. A323, 485~1986!.

@19# F. Petrovich, J. A. Carr, and H. McManus, Annu. Rev. Nucl.Sci. 36, 29 ~1986!.

@20# F. Iachello, Phys. Rev. Lett.53, 1427~1984!.