The decay scheme of 228Fr and K = 0 BANDS in 228Ra

Nuclear Physics A383 (1982) 1-10 Q North-Holland Publishing Company

THE DECAY SCHEME OF 2~Fr AND K= 0 BANDS IN Z~Ra E. RUCHOWSKA Institute of Experimental Physics, University of Warsaw, 00-681 Warsaw, Poland and Ktrnfysisch Vtrsneller Institutt, Universityy of Groningen, 9747 AA Groningen, 77u Netherlands W. KURCEWICZ

Institute of Experimental Physics, University of Warsaw, 00-681 Warsaw, Poland and GSI, D-6100 Darmstadt, Federal Republic of Germany N. KAFFRELL`

Institute of Nuclear Chtmistry, University of Mainz, D-6500 Mainz, Fedeaa! Republic of Germany T. BJÖRNSTAD

Institute of Physics, University of Oslo, Blindtrn, Oslo 3, Norway and

G. NYMAN Departmtnt of Physics, Chalmers University of Technology, S-41296 Gôteborg, Sweden Received 11 November 1981 (Revised 10 February 1982) Abetrad: The y-rays following the ß - -decay of 2asFr have been studied by means of y-ray singles including multi-spectrum analysis and y-y coincidence measurements using Ge(Li) spectrometers. Most of the observed y-transitions could be placed in the level scheme of zzsRa. The accuracy the energy of the fast-excited state in ZZSRa has been improved and 35 new excited levels have been established, 11 of them grouped into the ground-state band, the low-lying K~ = 0- bend with the head at 474.14 keV and two excited K~ = 0+ bands with heads at 721.17 end 1041 .9 keV. Candidates for two close-lying K~ = 2 + bands have also been found. It is concluded that the ground state octupole deformation, if any, is less pronounced in 22sRa than in lighter radium isotopes . E

RADIOACTIVITY 22sF.r [from 23sU(p, Snfip), mass separation]; measured E I yy-coin, T1~2 ; log ft. ZZa Ra deduced levels, l, a, Ge(Li) detectors.

1. Introduction Recent studies of K~ = 0- excitations in 224,226Ra led to the conclusion that the most likely explanation for the occurrence of these excitations, at unusually low energies in even-even nuclei with neutron number close to 136, is the existence ` Supported by the German BMFT . July 1982

2

E. Ruchowska et al. /

ZZBFr

of a non-zero octupole deformation of their ground states [ref. t) and papers quoted thereinj. Also, first-excited 0+ states were considered to be related to octupole excitations . The occurrence of octupole deformation was reproduced recently by theoretical calculations of potential energy surfaces for nuclei in this region 2'3) . In the present work we have undertaken investigations of the ZZBFr decay scheme searching for K = 0 bands in ZZgRa. The K~ = 0- band has been found to have its head at an energy of 474.14 keV. Two K~ = 0 + bands with heads at 721 .17 and 1041 .9 keV have been established. In addition, two close-lying K~ = 2+ bands have been found. An explanation for the abnormal branching ratios observed for El transitions deexciting the K~ = 0- band is proposed and the possible existence of 100

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zzsFr singles y-ray spectrum measured with a 32 cm' Ge(Li) detector . Fig. 1 . Low-energy part of the Measuring cycle: 70 s collection, 70 a counting . For the y-lines not belonging to the decay of zasFr a nuclide assignment is given or the energies are put in parentheses.

E. Ruckowaka et al. / zzeFr

a ground state octupole deformation in were presented in ref. 4).

ZZBRa

3

is discussed. First results of this study

2. Experimental procedure and teealte ZssU ZZBFr induced by The activity was produced in a spallation reaction of MeV protons from the CERN synchrocyclotron and extracted from other 600 3.6). reaction products by the ISOLDE II on-line mass separator The experimental set-up and experimental procedure are described in ref.'). ZZBFr Two independent experiments were performed. In the second experiment the target temperature was kept higher to avoid formation of molecules with mass number 228 of Ba isotopes with fission products from the uranium target 1).

Singles y-ray and yy coincidence measurements were carried out simultaneously s using two Ge(Li) detectors of 32 and 74 cm active volume and FWHM at 1332 keV ZZBFr of 1 .8 and 2.1 keV, respectively . The y-ray spectrum of for energies below 960 keV is shown in fig. 1. In order to assign the stronger y-transitions to the ZZBFr decay multispectra measurements were also performed. The obtained half-life of Ti~z = 38 t 1 s is in agreement with a previous result') . For the energy calibration ZZBFr was measured simultaneously with a lb°Tb source, the latter being used as calibration standard . Precisely known y-lines from natural ZZBFr background and from 9° ICr as well as a [ref. B)] cross contamination were also used for calibration purposes . The efficiency calibration of the detectors was S6 obtained using sources of Co, BB Y, 166~Ho and' 69Yb . The complete list of energies and intensities of y-rays following the ZZBFr decay obtained in the two runs is available as a report 9) . Also a summary of the coincidence results combined from two runs with total numbers of 5.3 x 10 3 and 3.5 x 106 coincident events is presented in ref. 9). In this paper as examples, two of the coincident spectra are shown in fig. 2.

3. Decay scheme More than half of the y-transitions found to follow the ZZBFr ß-decay were placed in the decay scheme on the basis of coincidence results and energy fits . While fig. 3 shows only the low-energy part of the decay scheme, information on the highenergy part can be found in the tabulations of ref. 9). From the 36 excited levels established in the present study only the first one at 63 .83 keV (instead of 59 keV as given in ref.' °)) was known previously. Recently three other excited states in ZZBRa, 700 t 40, 1070 t 60 and 1390 t 60 keV, were reported l ') to be populated 6Li)ZZBRa in Z3ZTh(d, reactions. The first two may correspond to our levels at 721 .17 and 1041 .9 keV.

4

E. Ruchowska et al.

/ Z~Fr

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Fig. 2. Gamma-ray spectra in coincidence with the 332.9 and 613.1 keV transitions in zisRa. The appropriate spectra coincident with the Compton distribution close to the gated peaks have been subtracted . Total transition intensities were determined from y-ray intensities using theoreti12). cal conversion coefficients The ß-branching to the individual levels resulted ZZBRa from intensity balances under the assumption of no ß-feeding of the ground state. The log ft values were determined using the "f"-tables of ref. 13), the half-life of 38 ~ 1 s and a Qa value of 3480 t660 keV. This Qa value resulted from the z2aF" ts)] . difference of mass-excess values of r [ref .' 4)] and ZZBRa [ref .

For the 140.9 keV y-transition an experimental K-conversion coefficient was evaluated from the ratio of the total K X-ray intensity and the 140 .9 keV y-ray intensity obtained from the coincidence spectra gated by the 694.2, 865 .8, 904.4 and 1145 .0 keV y-lines (see fig. 3) . The resultant average value of 0.5 t 0.2 for the

5

E. Ruchowska et al. / ZZaFr

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K-conversion coefficient compared to the theoretical values of 0 .17, 0.29 and 5 .0 for El, E2 and Ml multipolarities, respectively, suggests an E2 assignment to the 140.9 keV y-transition . Assuming an E2 multipolarity for this transition and taking the y-ray intensity ratio of this and the 63 .8 keV y-line observed in the spectrum gated by the 332.9 keV y-transition, one can evaluate the conversion coefficient for the 63 .8 keV transition . By comparing the result to the theoretical values of 0.37, 82 .3 and 12 .1 for El, E2 and M1 multipolarities, respectively, the experimental value of 133 t 40 suggests E2 multipolarity also for this y-transition . These ZZBRa. results allowed an I~ assignment of 2+ and 4+ to the first excited levels in Suggestions on spins and parities of higher excited states were made using level systematics in neighbouring even~ven nuclei . The evaluation of the coincidence spectra gated by the 782.3, 835 .0 and 846.2 keV y-transitions has shown an excess of K X-ray intensities . This led to the introduction of the 167.1 keV EO transition and an EO admixture to the 171 .4 keV transition in the level scheme shown in fig. 3. For the 167.1 keV transition, deexciting the 1013 .3 keV level, the total intensity was determined from the formula : K mx K X-rays associated with an EO transition, the mean For Imo, the intensity of intensity value obtained from the coincidence spectra gated by the 782.3 and 846.2 keV y-lines was taken. The K-fluorescence yield ~r K = 0.97 resulted from the ie) interpolation of data given in ref. . The (L+M+ ~ ~ ~)/K probability ratio of 0.33 was evaluated by extrapolation of reduced probability values for internal conversion of EO transitions given in ref. l') . The total intensity of the 171 .4 keV transition which deexcites the 1070 .3 keV level, was evaluated from its y-ray intensity and total conversion coefficient equal to 16 t 7. The latter value was obtained from the K-shell conversion coefficient evaluated from the y-ray spectrum gated by the 835 .0 keV y-line, and - to account for higher shell contributions - multiplied by the (K+L+ ~ ~ ~)/K probability ratio obtained by extrapolation of data from ref. ") . A general conclusion is that each of the two pairs of states conßected by the 167.1 and 171.4 keV transitions should have the same spins, parities and K-values . They may be states belonging to two K~ = 2+ rotational bands (see fig. 3) analogous to those observed in 23°'Th l II [ref .' B)], the isotone of ZZBRa. Our results revealed strong ß-feeding of 1-, 2+, 3 - and 4+ excited levels . According to the selection rules for ß-decay, states with such a wide range of spins cannot be populated with a comparable probability starting from one state of the initial nucleus. The existence of an isomer which would have a half-life very close to that of the ZZBFr ground state cannot be excluded, but it seems not to be very likely . Therefore, to explain this inconsistency we postulate a 63 .4 keV transition between the 3- and 1- levels . Under the assumption that there is no direct ß-decay to the 1- level, we would obtain for the y-ray intensity of this transition a value which is comparable with the intensity error of the 63 .8 keV transition and is too small to be seen in our measurements .

zzeF .r E. Ruchowska ct aL /

7

. DIBCUBS~Ou

ZZBRa Eleven of the 36 excited states in established in this study have been grouped into four K = 0 rotational bands. These include the ground-state band with a moment-of-inertia parameter A equal to 10 .64 keV, the K~ = 0- band with its head at 474.14 keV and an A-parameter equal to 6.34 keV, and two KA= 0+ bands with heads at 721 .17 and 1041 .9 keV and A-parameters of 8.26 and 7.57 keV, respectively . In addition, we obtained evidence for two close-lying K~ = 2+ bands with heads at 846.2 and 1013 .3 keV, connected by rather strong EO transitions . In the following we will restrict our considerations to the existence ZZBRa, of a stable octupole deformation in branching ratios for E1 transitions and the nature of the excited 0+ states . 4.1 . THE GROUND~TATE DEFORMATION AND K°=0- EXCITATIONS IN zzBRa From the analysis of E4+/EZ" energy ratios calculated for the members of the ZZBRa, zz6Ra ground-state band in and ZZBRa (2 .97, 3.13 and 3.21, respectively) it may be concluded that the latter nucleus is a better rotator than the lighter radium isotopes . This is consistent with the values of the quadrupole deformation parameter i9) . The values ß2, calculated from experimental quadrupole moments given in ref. zza.226 .ZZSRa for isotopes are 0.18, 0.20 and 0.22, respectively . ZZ° ZZ6Ra As was concluded in refs. l'4) the Ra and nuclei, in addition to a quadrupole deformation, also possess a non-zero octupole deformation in their ground states . This result is supported by recent theoretical calculations of the potential energy surfaces of nuclei in the Ra-Th region Z"s). ZZBRa 6R In the case of ZZBRa, similar to and ZZ a , a difference in the A-parameters A of the K = 0+ ground state and K~ = 0- bands is observed . As in ref. 1), the Coriolis interaction might explain this difference . This approach gives good results in reproducing the branching ratios for El transitions (see next section) . The energy splitting of 461.5 keV between the K~ = 0- and K~ = 0+ bands estimated from the ZZBRa ZZB.ZZ6Ra. data is twice that obtained in This suggests that the octupole deformation, if any, is smaller for ZZBRa than for lighter radium isotopes. Recently a new interpretation for low-lying 0- excitations was proposed by Iachello and Jackson Z° ). They claim that the occurrence of these excitations is connected with a-clustering effects in heavy nuclei . Unfortunately, no numerical results from their model are yet available. 4.2 . BRANCHING RATIOS AND CORIOLIS COUPLING OF THE NEGATIVE-PARITY BANDS The experimental branching ratios obtained for El transitions connecting the K~ = 0- states with the members of the ground state and the 721 .17 keV 0+ bands are given in column 6 of table 1. They are found to be in disagreement with

E. Ruchowska et al. / zzsFr

8

TABLE 1 for Branching ratios the E1 transitions in zzsRa Initial level

Final level

Reduced branching ratios

energy (key

1,K;

energy (key

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Transition energy (key

474.14

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537.57

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474.0 410.40 473.7 332.91 451.2 244.4

1 1 .27 t0.24 1 0.73 t0.09 1 0.30 f 0.07

1 2.0 1 1 .33 1 1.20

770.70

2, 0+

880.30

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474.14 537.57 537.57 655.96

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296.53 233.25 342.88 224.35

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°) According to the Alaga rule . Based on the Coriolis interaction of the Ka =0 - and K° =1 - bands (see subsect . 4.2) .

theoretical intensity ratios given by the Alaga rule (column 7 of table 1) . This discrepancy may be caused by a strong Coriolis coupling of the K~ = 0- states with the K~ =1- band . When the Coriolis interaction is included, the K~ = 1- admixtures appear in the level wave functions of the K~ = 0- band . Then, the reduced matrix elements describing E1 transitions between this band and the ground-state band are given at) by the following formula : (IcI~(El)I~~) _ (2I;+ 1) t~2Moo[a (I;Ol0~If0) + b~ Moo (l' 1 l -1~IfO)] , where I; and If are spins of initial and final levels, a and b are the wave function mixing coefficients and Moo and Mot are intrinsic matrix elements, corresponding to the K= 0 and K =1 components of the initial level wave functions. Mixing coefficients may be evaluated independently from the perturbed level energies applying a standard perturbation procedure . The branching ratios are then simply given by squared ratios of these reduced matrix elements with one free parameter S=MoI /M~ to be adjusted . As we could not establish the K~ =1 - band in this study the mixing coefficients were obtained from perturbed energies of K ~ = 0- levels using a theoretical coupling matrix element with the value of -52 [I(I + 1)]~is calculated from eq. (6.294) in ref. az) . Results of our calculations are presented in the last column of table 1. Good agreement with experimental values is obtained when S is equal to -0.871 . The same procedure applied to E1 transitions connecting the K~ = 0+ band with

zasF.r E. Ruchowska et al. /

9

its head at 721 .17 keV and the K~ = 0- band gives good results when S is equal to 0.366 . In this calculation the same A-parameter as for the ground-state band has been used for the K~ = 0 - and K~ =1 - bands and the interaction with other negative-parity bands has been neglected. Abnormal branching ratios for E1 transitions were also observed in 23°'Th l II [ref. tß )]. In this case our procedure also gives good agreement with the experiment a l data. 4.3 . THE K ? = 0+ EXCITATIONS

Two excited K~ = 0 + bands with heads at 721.17 and 1041 .9 keV are proposed. The results of ref. tt) indicate that they are populated in the Zs2.Lh(d, 6L.i)ZZaRa reaction . These states may be related to the 0+ excitations, observed at energies close to 1 MeV in many other even nuclei in the actinide region . They have very characteristic properties, namely, they are fed with low hindrance factors in adecay23-ZS), are strongly populated in (p, t) reactions zb'2'), and are not observed (except in two special cases Zß'29)) in (t, p) reactions so"3t). Different explanations of their origin have been proposed, such as quadrupole-pairing vibrations 32) or ground state octupole deformation t). However, no fum explanation of the strange features of these excitations is available. The authors want to express their gratitude to Prof . J. Zylicz for stimulating discussions . Also discussions with Dr. G. Rohozinski and Prof . J. Jänecke are greatly appreciated. One of the authors (E .R .) would like to thank Prof . R. Siemssen for permitting the completion of the data evaluation using the Groningen University computer facilities . We are indebted to the members of the ISOLDE collaboration for generous support. References 1) W. Kurcewici, E. Ruchowska, N. Kaffrell, T. Björnstad and G. Nyman, Nucl. Phys. A356 (1951) 15 2) A. Gyurkovich, A. Sobiczewski, H. Nerlo-Pomorska, K. Pomoraki, Proc. 4th Int. Conf. on nuclei far from stability, Helsing0r 1981 ; CERN 81-09, 1981, p. 525 3) A. Gyurkovich, A. Sobiczewski, B. Nerlo-Pomorska and K. Pomorski, Phys. Lett. 105B (1981) 95 4) W. Kurcewicz, E. Ruchowska, N. Kaffrell, T. Bjôrnstad and G. Nyman, Proc. 4th Int. Conf . on nuclei far from stability, Helsing0r 1981 ; CERN 81-09, 1981, p. 649 5) H.L . Ravn, L.C . Carraz, J. Denimal, E. Kugler, M. Skarestad, S. Sundell and L. Westgaard, Nucl. Instr. 139 (1976) 267 6) L .C . Carraz, S. Sundell, H.L. Ravn, M. Skarestad end L. Westgaard, Nucl . lnstr. 158 (1979) 69 7) H.L . Ravn, S. Sundell, L. Westgaard and E. Roeckl, J. Inorg. Nucl . Chem . 37 (1975) 383 8) T. von Egidy et al., Nucl . Phys . A36S (1981) 26 9) E. Ruehowska, W. Kurcewicz, N. Kaffrell, T. Björnstad and G. Nyman, IKMZ-Report 82-1, Institute of Nuclear Chemistry, University of Mainz (1982) 10) D.J . Horen, Nucl . Data Sheets 17 (1976) 367

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J. Jiinecke, F.D . Bewhetti, D. Overway, J.D . Cossairt and R.L. Sprosa, Phys . Rev. C23 (1981) 101 F. Râsel, H.M . Fries, K. Alder, H.C . Pauli, Atomic Data and Nucl . Data Tables 21 (1978) 291 B.S. Dzhelepov, L.N. Zyryanova and Yu.P. Suslov, Beta processes (Science Press, USSR, 1972) M. Epherre, G. Audi, C. Thibault, R. Klapisch, G. Huber, F. Touchard and H. Wollnik, Nucl . Phys . A340 (1980) 1 15) A.H . Wapstra and K. Bos, Atomic Data and Nucl . Data Tables 19 (1977) 177 16) W. Bambynek, H. Crasemann, R.W. Fink, H.U . Freund, H. Mark, C.D . Swift, R.E . Price and P.V . Rao, Rev. Mod. Phys . 44 (1972) 716 17) R.B . Begzhanov and V.M. Belen'kü, in Struktura Yadra, ed . R.B . Begzhanov (FAN, Tashkent, 1969) p. 256 18) W. Kurcewicz, K. Stryccniewicz, J. Zylicz, S. Chojnacki, T. Morek and I. Yutlandov, Acta Phys . Pol. B2 (1971) 451 19) A.S . Goldhaber and G. Stharfi-Goldhaber, Phys . Rev. C17 (1978) 1171 20) F. Iachello and A.D . Jackson, Phys. Lett. 108B (1982) 151 21) L. Kotbach and P. Vogel, Phys . Left . 32B (1970) 434 22) A. Bohr and B.R . Mottelson, Nuclear structure, vol. lI (Benjamin, Massachusetts, 1975) 23) W. Kurcewicz, N. Kaffrell, N. Trautmann, A. Plochocki, J. Zylicz, K. Stryczniewicz and I. Yutlandov, Nucl. Phys. A270 (1976) 175 24) W. Kurcewicz, N. Kafirell, N. Trautmann, A. Plochocki, J. Zylicz, M. Matin and K. Stryczniewicz, Nucl. Phys . A289 (1977) 1 25) W. Kurcewicz, E. Ruchowaka, J. Zylicz, N. Kafirell and N. Trautmann, Null. Phys. A304 (1978) 77 26) J.V. Maher, J.R. Erskine, A.M. Friedman, J.P . Schiffer and R.H. Siemasen, Phys . Rev. Left . 2S (1970) 302 ; Phys . Rev. CS (1972) 1380 27) A.M . Friedman, K. Katori, D. Albright and J.P . Schiffer, Phys . Rev. C9 (1974) 760 28) E.R. Flynn, G.L. Struble, R.G. Lamer and L.G . Mann, Phys . Lett . 67ß (1977) 158 29) R.E. Brown, J.A. Cizewski, E.R . Flynn and J.W. Sunier, Phys . Rev. C20 (1979) 1301 30) R.F . Casten, E.R . Flynn, J.D . Garrets, O. Hansen, T.J . Mulligan, D.R . Bès, R.A. Broglia and B. Nilsson, Phys . Left. 40B (1972) 333 31) B.B . Back, E.R . Flynn, O. Hansen, R.F . Casten and J.D. Garrett, Nucl . Phys. A217 (1973) 116 32) I. Ragnarsson and R.A . Brogue, Nucl . Phys. A263 (1976) 315