PHYSICAL REVIEW C VOLUME 39, NUMBER 3

Alternating parity structure in doubly odd 2'8Ac

MARCH 19S9

M. E. Debray, M. Davidson, A. J. Kreiner, J. Davidson, G. Falcone, D. Hojman, and D. SantosDepartamento de Fisica, Comision Nacional de Energia Atomica, RA-1429 Buenos Aires, Argentina

(Received 9 November 1988)

States in doubly odd ' Ac have been studied using in-beam a-, y-, and e -spectroscopy tech-niques mainly through the ~ 9Bi(' C, 3n) fusion-evaporation reaction. ' Ac shows a band struc-ture, with interleaved states of alternating parities connected by enhanced B(E1) transitions,which is strikingly similar to the one in its isotone ' Ra.

Heavy transitional translead nuclei lying between thedoubly magic Pb and the deformed Th-U region haverecently attracted the interest of several high-spin spec-troscopy groups. ' This interest mainly derives from thefinding of phenomena which seem to indicate that areflection asymmetric degree of freedom is playin~ an im-portant role. ' Quite a number of even-even' andodd-mass nuclei of this part of the chart of nuclideshave already been studied revealing structures with possi-bly both quadrupole and octupole (or more generally oddmultipole) collectivity. On the other hand, very little isknown on doubly odd nuclei in this region and it is impor-tant to investigate if they consistently fit into the same pic-ture. As a step in this direction and within the frame ofour program' to study collective phenomena in doublyodd nuclei, we present here first results obtained for ' Ac.

This nucleus was produced through the Bi(' ' C, 3nand 4n) fusion-evaporation reactions with the TANDARaccelerator of the Argentine Atomic Energy Commission.

' Ac is in the region of the shortest-lived a emitters ofthe whole chart of nuclides and hence allows the use ofpowerful in-beam a spectroscopy techniques in addition tomore conventional y and conversion electron (e ) mea-surements. Actually, the only previously known informa-tion on ' Ac was the existence of an a-emitting state(E,-9.21 MeV) of T~g2=1. 12 ps (Ref. 3). This state ismost likely of low spin [I (1 )] since it decays ex-clusively to the (1 ) ground state of ' Fr with a hin-drance factor which fits the systematics. Most of the ex-periments were performed with the Bi(' C, 3n) reac-tion which gives a better peak-to-background ratio (thisbackground coming mainly from fission). In addition toy-ray excitation functions, cross-section measurementsdetecting the ground-state a decay were performed in the64- to 82-MeV bombarding energy range. The productionof ' Ac maximizes at =68 MeV with a cross section of160~ 40 mb. The fission limitation of the fusion-evaporation cross section is already important, but not asstrong as in the compound nuclei of Th and heavier ele-ments. Fi ure 1 shows an in-beam a-particle spectrum at70-MeV ' C bombarding energy (at which higher spins in

' Ac are favored) measured with several large area(=10 cm ) surface barrier Si detectors looking at an Alcatcher placed =5-cm downstream from the target inwhich the recoils are implanted, in coincidence with y raysmeasured at the target position with a 40% HpGe detec-tor. The main competing channel at this energy is the 4n

104.,: T„, ('"Ag) =(70+ 6)ns '

103,- (B)

104.—:T,("'Ac) (1.31+0.12)jjs:

Vl

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Ui .~

101 I

3 2 1

t g Ipsj

102 s . i . t. . i ~

3 2 1 0t It' f psj

30

E [Mevj(A) (B)

FIG. 1. The lower part shows an in-beam a-particle spectrumin coincidence with y rays for the 209Bi(' C, 3n) reaction at 70MeV. The upper part shows the time distributions of the a par-ticles coming from the ground-state decay of (A) 2'sAc and (B)

'7Ac. The more prompt component in (A) comes from the2i5Fr a particle tail in the 2isAc peak.

leading to ' Ac which subsequently decays to the groundstate of ' Fr (Ref. 11). The a line coming from theshort-lived ground state of ' Ac provides us with an idealsignal to select and unambiguously identify the prompt y(and e ) radiation which belongs to ' Ac. The E -E„-t,„experiment yields a set of new lines in 2' Ac and alsoanother value for the half-life of its ground state:T&F2 =1.31+'0.12 ps (see Figs. 1 and 2). The consistencyof this procedure has been checked, making use of the pre-vious knowledge' on ' Ac (here we have obtained ahalf-life of 70+ 6 ns in excellent agreement with the pub-lished ' value of 69 ~ 4 ns). A y- y coincidence experi-

~9 1193 1989 The American Physical Society

1 194 M. E. DEBRAY et aI.

24

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CHANNEL NUMBER

600

FIG. 2. y-ray spectrum in coincidence with the a-particleground-state decay line of ' Ac [gate (A) of Fig. 11.

ment using two HpGe detectors of 30'Fo and 40% efficiencyplaced face to face at 90' to the beam direction allowed usto construct the partial-level scheme shown at the centerof Fig. 3. The relative parity of the states follows from theconversion coefficient determinations for the transitionsconnecting them. In this mass region it is a relatively sim-

pie matter to distinguish between E 1 and M 1 transitions.These measurements were made with a BaFe permanentmagnet minorange spectrometer in conjunction with aL1V2-refrigerated Si(Li) (200 mm x 2 mm) detector.

Now the tentative spin-parity I'-(9 ) assignment tothe "band-head" state in ' Ac has to be briefiy discussed.There is a systematic occurrence " of two a-emitting iso-mers in many doubly odd nuclei of this region, namelyI' (1 ) and (9 ) states which mainly arise from thecoupling of the two lowest-lying single-particle orbits(xh9/2 and vg9/2) above the double-shell closure at Z -82and N 126. The splitting between these two states de-creases steadily " from ' Bi, where it is 27 1 keV, to itsisotone ' Ac, where ' it has most likely collapsed to 37keV as the h9/2 shell is being filled and the proton changesfrom particle to a mixed particle-hole (quasiparticle)character. These states are certainly purest in terms ofthe xh9/2 vg9g configuration for ' Bi and the (particle-particle) matrix elements of the p-n force can be extract-ed from the lowest-lying I J 0,1,. . . , 9 Inulti-plet (where J is the two-particle angular momentum) giv-ing ' the typical inverted-parabola shape for the matrixelements VJ &(nh9/2 vg9/2) J [ V~, ( ( )J&. In ' Bi thelowest-lying members of this multiplet are" the 1

(ground state), the 0 (46.5 keV), and the 9 (271 keV).Moving up in proton number to the isotone ' At, onefinds that the gap between the I and 9 has dimin-ished ' to 223 keV and furthermore that the 2 and 3states have moved into the gap and that the 5 state has

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(see text).

ALTERNATING PARITY STRUCTURE IN DOUBLY ODD ' Ac 1195

come down to 275 keV ( in addition, it has been suggestedthat the 7 lies just above the 9 state'5). This patternalso repeats itself for the known N 129 isotones (Ref. 11for ' Bi and Ref. 16 for ' At). In contrast to this situa-tion, no a-emitting high-spin isomeric state seems to existin ' Ac. One can envisage a situation in which, for in-stance, the 5 and the 7 cross the 9 state and hence itmay decay to the 1 ground state through a cascade ofvery low-energy highly converted transitions loosing itsa-emitting character. An example of such a low-energyE2 transition is the 5 3, 70-keV isomeric (T~t2-32ns) line' in ' At. The Weisskopf estimate for the half-life of a low-energy (& 100 keV) E2 transition correctedfor internal conversion reaches" an approximately con-stant value of less than 100 ns. Hence the lifetime for theground state of ' Ac as obtained here may be somewhatenlarged with respect to the true value. Since the 9state is expected to be low lying, it will be on the yrast linereceiving strong feeding in the heavy-ion-induced reac-tion (the maximum angular momentum brought into thecompound system by the 70-MeV ' C beam is about246). As the h9t2 proton quasiparticle character becomesmore pronounced, the residual p nforce is-expected to di-minish' because it becomes an average between theparticle-particle (attractive) and the particle-hole (repul-sive) matrix elements. Actually, if the g9t2 neutron can beconsidered predominantly particle (namely, u„ 1, U„=Oin terms of the usual BCS occupation amplitudes), andthe proton is approximately at midshell (which meansu&=U~), the expression' for the effective p-n interactionbecomes VJ u~VJ+U~VJ ', where VJ ' is the particle-hole matrix element. Since VJ tends to be of similarmagnitude but opposite sign than VJ, the Jmultiplet tendsto become degenerate.

Also from the point of view of the particle-core(quadrupole-quadrupole) coupling, the 9 state is expect-ed to lie low in energy. The coupling of the h9/2 proton tothe core is weak since its quadrupole moment is quenched(by the u -v factor) due to its quasiparticle character.This means that the orientation of the proton angularmomentum j~ with respect to the core spin R is energeti-cally indifferent, which leads to the quasidegeneracy ofthe first —", and —", states ' in ' ' Ac. On the otherhand, the neutron Fermi level lies at the beginning of theg9/2 shell. This clearly privileges the aligned coupling ofj„and R which maximizes the overlap between particleand core quadrupole moments (in a deformed shell modellanguage this corresponds to a decoupling situation). Thismeans that, for instance, the "unfavored" '2' state ispushed up in energy with respect to the aligned —",

+ state,being consistent with its nonobservation in ' Ra. Hence

the yrast states in the doubly odd nucleus ' Ac will be ofthe type I j~+j„+R while the unfavored ones of thetype I (j~ —1)+j,+R should lie very near in energySimilar arguments should also hold for the octupole-octupole coupling.

Figure 3 shows a striking similarity between the levelschemes of ' Ac and 2' Ra (Ref. 5), suggesting that theaddition of the odd h9t2 proton does not significantlyinfluence the structure already developed in ' Ra, whichundoubtedly displays collective features. In particular,the excitation energies of the first negative parity states inboth nuclei are identical. This speaks strongly in favor ofa collective interpretation of these states.

In fact, the transition between a spherical shell model(or single particle) and a collective regime' ' seems tooccur precisely at N 129. The even N 128 isotones,

' Ra (Ref. 20) and ' Th (Ref. 7), have R4( E4+/E2+)ratios less than the critical value 1.82 in a Mallmannplot' ' ' ' showing a compression of the transition ener-gies as one goes up the ground-state band, while z'7Ac isessentially an hst2 proton weakly coupled to ' Ra. Onthe other hand, the even N 130 isotones, ' Ra (Refs. 1

and 2) and Th (Ref. 7), clearly show collectivefeatures; their R4 ratios lie beyond R4 1.82 and Rs andRs fall nicely on the collective branch of the variable mo-ment of inertia curves. ' ' ' ' The development of quadru-pole collectivity is accompanied by the appearance of in-terleaved negative parity states most likely connected tothe presence of reflection asymmetry. s It is interestingto note that the first transition energy both in ' Ra (540keV) and ' Ac (507 keV) coincides with the average ofthe first transition energies of its neighboring isotopes,namely (688+ 390)/2 =539 keV and (661+355)/2 508keV, respectively.

The 8(E1)/8(E2) ratios found here in ' Ac[(1.7+0.3) x10 fm ] lie, on average, between thoseof ' Ra and 2' Ac [(1.1+ 0.2) and (3.2+ 0.3)xl()fm, respectively].

Summarizing, excited states of ' Ac have been studiedthrough a complete set of combined a, y, and e spectros-copy measurements. This work has revealed the existenceof a band structure with interleaved states of alternatingparities connected by enhanced B(EI ) transitions, whichis strikingly similar to the one in its isotone ' Ra. Bothnuclei seem to be right on the edge of a phase transition todeformed, reflection asymmetric shapes.

This work was supported in part by Consejo Nacionalde Investigaciones Cientificas y Tecnicas (CONICET),Argentina.

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1196 M. E. DEBRAY etal.

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