Proton-neutron interactions in the odd-odd nucleus 214Fr

Nuclear Physics A553 (1993) 519c-522c North-Holland, Amsterdam

NUCLEAR PHYSICS A

Proton-neutron interactions in the odd-odd nucleus 214Fr A.P. Byrne, G.D. Dracoulis, G.J. Lane, B. Fabricius, T. Kib~di, A.E. Stuchbery, A.M. Baxter and K. Schiffer Department of Nuclear Physics, RSPhysSE, Australian National University, PO Box 4, ACT 2601, Australia

Al~tract High spin states have been observed to excitation energies over 8 MeV in the odd-odd nucleus 214Fr using in-beam y-ray and electron spectroscopic techniques and heavy-ion induced reactions. Semi-empirical shell model calculations appear to provide a good description of the states observed, although some states expected within this model have yet to be identified.

1. INTRODUCTION The francium (Z=87) isotopes near the closed (N=126) neutron shell span two distinct regions of nuclear structure, with the lighter isotopes being well described within the shell model [1], with the hearer 216Fr (with N=129) identified [2/ as the lowest-mass corner of the region with reflection asymmetric shape. The structure of very high spin states with N_<126, is controlled by the excitation of neutrons from the closed core. These states become yrast both because of the attractive residual proton-neutron interaction and because of the mixing of states due to the ability of pairs of proton and neutron orbitals to couple to the octupole vibrational phonon (MPOC)[3]. In the odd-odd nucleus 214Fr (Z=87,N=127) valence ne utr ons are available immediately, so that MPOC effects are expected to be manifest at all levels. In addition, the presence of high-j neutron orbitals g9/2, i11/2 and J15/2 means that high a n g u l a r momentum states can be formed at relatively low excitation energies. 2. EXPERIMENTAL DETP, H-g AND CALCUIATIONS States in 214Fr have been populated using the 205TI(13C,4n) and 208pb(llB,5n) reactions. Experiments performed included 7-Y-coincidence, ¥-7-tcoincidence (using the CAESAR array), excitation function, a n g u l a r 0375-9474D3/$06.00 © 1993- ElsevierScience PublishersB.V. All rights reserved.

520c

A,P. Byrne et al, / Proton-~wutron inwractions in the odd-odd nucleus 2/4Fr

d~str~bution, pulsed-beam timing, conversion electron and TDPAD g-factor measurements. The level scheme deduced from the present work is shown in Figure 1.While the ~nultipolarity assignments for the individual transitions are in general firm, bc:~g based on the conversion electron and angular distribution data, the spin ass~gaments are guided by the results of the semi6559+~' I 610 --8145+~' 500 -----? 303

7546÷A'

"

735

7649+~'

~

7543+A' ?m = 5z',,s

I

1059 • - 6815+~'

395

337

130

m ~

6918+A

,~,.

5524+~

1316

r6478+A, "~,,, = 155 as

321

410

28 +

29 ÷ ---.,-644

,5oslo ~9,9+~ / 8!3m3" 28-

"• 'rl , 400

~?- lOOl 25+

~

~+ 2120"

19+.

. .'. . .1322 ....... ]

j

n

i 4,

332

~

•

3998+h

i

/

- , ~ 3214+~ , ~ " 3104~33089+~ "Y-~. -."~ _~.2904+A

-J-

,

~7~ " ~ 2 4

23~ 21+

I

. . . . , 246

.... :

515

|

m-.--

~

4928+~

~m+~ 3941+~

~_

3955,~

~--

~m = 11.5 ns

4o12+~

555

~m= 6 ns

B--2811+~

646

,IV,

~ 1

soo2+~

~---~"~-274 .... "'" "" 3057+~

[-

739

mz

. ,0? ~ .~ , l'"- ~ ' , ' . " ' + ~

858 "," m 7-446 _=_-........I~ ~ - - ~ - s ~ 5 ~

,

5334÷~

-i-/-~-92o7+~

1241 - - - - 1 ~ ' --~10i5~

4?05+~889

15" 13"

"

i

~_

I ........ ~. 12o . J. 51o ~s- - ....r'19"+~l

.~

I

'-1-]~, .......

/21,s+~ 1953

'!''~'"F-'1"7~-154:~

~

:.L517

135 m

-I599 -T

Tin= 15 Tin= 16 nS

13~a

Zm = 148

~:.

517 .

.

.

.

.

.

4~

.

(1-)

~l 0

214-

87e~127

Figure 1. Level scheme for 214Fr

~m = 7.2 ms

A.P. Byrne et al. I Proton-neutron interactions in the odd-odd nucleus 2t4Fr

521c

empirical shell model calculations. Specific uncertainties are discussed below. Semi-empirical and M P O C calculations have been Car..-d..edout in mariner described in references [1,3], where energies are expressed in terms of empirical single particle energies and recoupled empirical two-body interactions. Mixing within different couplings of a single configuration is included, however the calculations do not allow for mixing between configurations containing different orbitals.

3. DISCUSSION

3.1 L o w spin structure Prior to the present work only two low lying states, both a-decaying, had been assigned to 214Fr [4]. In the isotones 210Bi and 212At similar states have II been observed and assigned as 1- and 9- members of the nhg/2vgg/2 configuration. Shell model calculations for these isotones confirm the assignments, however for 214Fr the ~situationis not so clear, with all members (excluding the 0- coupling) of the nh~/svgg/2multiplet predicted to lie within 150 keV. Further, as the h9/2 orbital fills,the ordering of the 9- and 8- members is expected to be inverted, with the transition from a pure particle-particle interaction to a hole-particle interaction. Experimentally two states connected by a 45 keV, M1, transition are observed at the lowest excitation energies and we have assigned these as the 8- and 9- members of the multiplet. Three isomers are observed below 2 MeV, the lowest of these at 639 keV has a meanlife of 148 ns. The g-factormeasured for this state, 0.511(3),indicates a mixed structure. Many-particle octupole coupling calculations (.MPOC) predict ~ wavefunction with the dominant components 51% n[h~/2i13/2]vg9/2 + 31% n[hg/2 f7/2]vjls/2 + 10% n[h$/2f7/2]vgg/2@3-, with the g-factor, g=0.52, in good agreement with experiment. The two other isomers have been associated with statues of spins 15- and 14- and are likely to be due to couplings of the ~[h~/2]vg9/2 configuration, with some admixture from the ~[hg/2fT/2]vg9/2 configuration. 3.2 Intermediate spin structure 4 . In the neighbouring isotope 213Fr the 29/2 + state of the ~[h9/2113/2] configuration is isomeric. Semi-empirical shell model calculations predict the excitation energy of this state to within 15 keV [1]. The calculations for 214Fr, predict a 19 + state at 2083 keV, formed by the coupling of a g9/2 neutron to the 29/2 + state. Experimentally a state of spin 17+ is observed at 2165 keV, but there is no higher state within 600 keV. However, unlike in 213Fr, a close lying multiplet can now be formed, with the possibility of decay from the 19+ state to the 17 + state through a pair of M1 transitions. This could bypass the direct (isomeric) decay to the 16- state expected from simple weak coupling to 213Fr. Although no experimental evidence for these extra states was found, a concise

5 22c

A.P. Byrne et el. / Proton-neutron interactions in the odd-odd nucleus 214Fr

interpretation of the structure above this point is impossible without the addition of the two extra units of spin. Assuming a spin of 19 + the m a i n cascade proceeds through states which are readily interpretable in terms of weak coupling to states in 213Fr. In particular, the isomers a~,:J sI~ins 23 + and .5 27- are expected, arising from the n[h9/2i13/2137/2-vg9/2 and ~[hg/2i13/2145/2-vg9/2 configurations respectively. 3.3 High spin s t r u c t u r e Two isomeric states have been observed at high excitation energy. The lower of these at an excitation energy of around 6500 keV has a meanlife of 156 ns. A transition of energy 1068 keV and E3 character depopulates the uppermost isomeric state observed in the present work. The g-factor for the 156 ns isomer, g=0.677(4) is not consistent with any simple valence configuration and indicates the core-excited n a t u r e of the state. The actual location and spin of this state is as yet uncertain, with the 296 keV transition unlikely to be directly deexciting the isomer. In the neighbouring isotopes 213Fr and 212Fr isomers have been observed at similar spins [3,5J and have been interpreted as arising fi-om related configurations containing two valence • . 3 .2 neutron orbitals. In 214Fr the analogous configuration is ~[h9/2113/2J45/2-1 vgg/2il 1/2Pl/2, differing from the lighter isotopes only in the neutron hole orbital. Octupole vibrational coupling effects in such configurations causes them to be highly favoured, producing very long lived yrast traps. In the lighter isotopes the resultant isomers are in the microsecond region with meanlives of 4.5~s and 34bts for 213Fr and 212Fr respectively. In 214Fr the 33 + state is most favoured and predicted to occur at an excitation energy of 5888 keV, well below observed isomer. Given the success of the previous calculations, in which the excitation energies of these isomer were predicted within 100 keV, and the good agreement for the calculations in 214Fr at lower energies, it is unlikely that the 156 ns isomer is due to this configuration. Further, the calculations imply that the observed isomer, and many of the states populated in the path from this isomer to the 27- isomer, are non-yrast states, making configuration assignments difficult. Although no candidate for the yrast 33 + state has been identified, the time limitations used in the present study restrict the sensitivity to isomeric states with lifetimes shorter than a few microseconds. The calculations and results for the lighter isotopes indicate that a state with considerably longer meanlife may be present in 214Fr. Future studies to search for isomeric states in a much longer time regime are proposed. 1. 2. 3. 4. 5.

A.P. Byrne, et al. Nucl. Phys. A448 (1986) 137. M.E. Debrayet al. Phys. Rev. C41 (1990) R1895. A.P. Byrne et al. Phys. Lett. 217B (1989) 38, and references therein. D.F. Torgerson et al. Phys. Rev. 174 (1968) 1494. A.P. Byrne et al. Phys. Rev. C42 (1990) R6