High spin states of 83Sr

Nuclear Physics A334 (1980) 71-79 ; © North-Holland Publishing Co ., Amrterdam

Not to be reproduced by photoprint or microfilm without written permission tlrom the publisher

HIGH SPIN STATES OF ' 3 Sr S. E. ARNELL, S. SJdBERG, ö. SKEPPSTEDT and E. WALLANDER Department of Physics, Chabners University of Technology, Göteborg

A. NILSSON Research Institute jor Physics, Stockhohn

and G. FINNAS

Âbo dkademi, .lbo

Received 12 March 1979 A6etract :'The reactions s° Kr(a, n) s 'Sr and °s Kr(a, 3n) ° 'Sr were used to populate excited states of high spin in s3Sr. The de~xcitation of these states was studied by in-beam ~-ray spectroscopy . A number of new high-spin states have been observed . The results are discussed within the framework of the Coriolis~oupling model and the cluster ore coupling model.

E

NUCLEAR REACTIONS s°Kr(a, n), E = 16-21 MeV, s ~Kr(a, 3n), E = 45 MeV ; measured E,., lY(E, t3Y ), yy-coin. °'Sr deduced levels, !, n. Enriched targets. Ge(Li) detectors.

1. Intmdactioo The present paper dealing with 38Sr~ s is part of a study of high-spin states in neutron-deficient Sr isotopes t " 2). A feature observed for the even Sr isotopes is a steady compression of the lowest level sequence as néutrons are removed from the closed neutron shell at N = 50 [refs. 3' 4)] (SrBg). Ogawa has reported') calculations of the energy spectra performed on the basis of a shell model where also the proton . orbitals are included . The transition of the energy level spectra of the even Sr isotopes from a shell structure to a rotation-like structure is reproduced . The main intention of the present work is to provide experimental knowledge of high-spin levels of e a Sr to be compared to those of the neighbouring even Sr isotopes and to calculations. Energy levels of'3Sr have earlier bcen studied via ß-decay of'3Y [refs. s .6)] and nuclear reactions' - ').

71

72

S. E. ARNELL et nl .

2. Experimeatal procedure 2.1 . REACTIONS AND TARGET ARRANGEMENTS

High-spin states of 83Sr were excited by the following reactions (Q-values from ref. 9)) 8°Kr(a, n)83 Sr, Q = -6.8 MeV, S2Kr(a, 3n)83Kr, Q = -25 .7 MeV. The (a, n) reactions were studied with a-particle beams of maximum 18 and 21 MeV delivered by the tandem generator of Uppsala University, Uppsala, and the cyclotron of the Physics department, ~bo Akademi, lobo, Finland, respectively . The study ofthe (a, 3n) reaction was performed at the 225 cm cyclotron at the Research Institute of Physics, Stockholm. The krypton target gases, enriched to 70 % in 8°Kr(24 %' S Kr, 6 % 82 Kr) and 71 % in 82Kr (24 % 8 °Kr), respectively, were enclosed in small volume perspex cells and separated from the main vacuum system of the beam tube by thin mylar (1 .5 mg/cmZ) or tantalum (6 .6 mg/cm2) entrance and exit windows. The gas handling system has been described elsewhere lo). 2.2 . MEASURING TECHNIQUES

The y-radiation was detected with large coaxial and small planar high-resolution Ge(Li) detectors. Energy and intensity calibration was obtained by use of a ' S2Eu source supported by the well-known y-lines from 18F and 19F. The slope of the curve giving the y-ray intensity of a given line as a function of the a-particle energy is useful in limiting the spin value of the level from which the y-line originates, as an increase in a-particle energy increases the average angular ma mentum brought into the compound nucleus system . This favours a relatively larger population of high-spin states at higher a-particle energies i') . In the present study of the excitation functions the detectors were placed at an angle of 90° with respect to the beam direction in order to avoid Doppler shins. Two-dimensional y-y coincidence experiments using X-ray detectors as well as large Ge(Li) detectors were performed. The time window of the coincidence circuit was 30 ns. Data were collected with a 4000 x 4000 channel system and stored on magnetic tape. The angular distributions of the y-rays relative to the incident beam direction were measured at seven angles between 0° and 90° at an incident a-particle energy of 21 MeV and at five angles between 30° and 90° at 45 MeV incident a-particle energy . Two Ge(Li) detectors were used, one of which could be rotated around the target centre, while the other one was in a fixed position to monitor the reaction . The expression W(6) = 1 +a 2P2(cos B)+a 4P4(cos B),

was fitted by a least-squares analysis to the experimental angular distributions, The caeffscients a s and a~, depend on the spins of the initiat and final states as well as on the mixing ratios ~x). 3. Facperint~nt~ rexulta

Twa singles y-ray spectra obtained from the a°Kr(a, n)g~Sr and sz~a, 3njs~Sr reactions at lß and 45 MeV, respectïvely, are spawn in figs. l and 2. The y-ray lines assigned to s3Sr are listed ïn table l. The results of the y-y coincidence ara summarized in table 2. The axcîtation functions far the strangest y-transitions from the (a, n) reaction are presented in fig. 3. The results of the angular distribution measurements are accounted far in table l.

Fig. i . Gamma-rsy spc~trum recardal at eoKr

4Q° bY a ôQ cm' Ge(Li} daector during bombardmait of 8 MtY x*particlss.

t

~aawa

Fig. 2. Gamma-ray spectrum rocordat at Br = 94° by a 64 cm' Cit{Li) äctazor du:sag bamlaardment of es~. ~, qg MeY a-particies.

74

S. E. ARNELL et al.

103 r c e

c 0

10`

18

19

E,~IMaV!

21

Fig. 3. Yield functions of y-rays excited by the a °Kr(a, n)s'Sr reaction, recorded at BY = 90°.

A comparison of the results from the y-spectrum obtained at 18 MeV a-energy with the y-transitions obtained from the 83Y y-decay studies' " 6) showed that a number of weak reaction lines could be fitted as transitions between known 83 Sr levels . As the ground state of 83Y has a spin value of ~, the 83 Sr levels, directly populated in the ground-state ß-decay of 83 Y, can be expected to have not too low spin values . All the y-lines in common, however, with the exception of the 259 and 859 keV lines, give insignificant contributions to the y-spectrum obtained at higher a-energies. T~st~ 1 Gamma-ray transitions in s3 Sr observed in °°Kr(a, n) and ° ~Kr(a, 3n) reactions Energy (IceV) 35 .4 259.1 527.9 679.8 764.4 800.4 858.5 875.0 945.9 1077 .1 1129 .4 1213 .3

Intensity E.a

= 21 MeV 45 MeV 18 20 d 20 38 100 14 35 7

5 11 10 7 15 31 100 7 55 27 10

Ass;gnment 35-0 259-0 3645-3117 1574-894 800-35 800-0 894-35 910-35 1856-910 1988-910 3117-1988 (2107-894)

I,-I~ ~ +_ ~+ }-- }+ ~+~+ ~+,~ +-~+

u + , ~ - ~+

~+, ~. -}+ ~+- ~+ ~+- ~+ ~+-~+ ~+-~+ ~+-,~+ -

A~

A,

-0.95(8) -0 .91(6)

0 0.02(2)

0.47(7) -0 .95(9) 0.30(4) -0 .77(10) 0.39(4) 0.25(6)

-0 .12(11) 0.12(6) -0.08(6) 0.21(13) -0.13(6) -0.26(8)

Multipolarity

M1/E2 M1/E2 E2orM1,El M1/E2 E2 M1/E2 E2 E2

75 Test .e 2

Summary of coincidence results for °'Sr obtained from the °°Kr(a, n) reaction (Es = 18 MeV) and the °'Kr(a, 3n) reaction (E, = 45 MeV) Gate 35 528 680 859 875 946 1077 1129

Coincidence with 405

(x)

528

(x)

680

764

859

875

x

x

x

x x

x

x

x x x

x (x)

946

x

1077

1129

x

(x)

x

(x)

x

(1213)

(x)

x

Energies in keV.

The 894 keV state . The coincidence measurements showed the 859 keV line to be in coincidence with the 35 keV line deexciting the first excited state (Jx = ~+) which is in accordance with the recent work by Liptak et al. ') where this line deexcites a state at 894 keV. Our angular distribution measurements give preference for a J -~ J-1 dipole type of transition with considerable contributions of quadrupole radiation, indicating a spin-parity value of for the 894 keV state. The 910 keV state. A new strong line at 875 keV appears in both the (a, n) and the (a, 3n) reactions: Intensity arguments and coincidence relationships show that the 875 keV transition is the second member of an yrast cascade to the ground state via the 35 keV, ~+ state. The angular distribution measurements show that the 875 keV transition is a pure quadrupole transition having a full Doppler shift. An upper limit of0.5 ns for the half-life of the 910 keV state is deduced from this observation. As all known M2 transitions seem to be inhibited by at least a factor of ten relative to the Weisskopf estimate t3 ~ t °), which in this case gives T} .. 26 ns, we infer E2 radiation and an assignment of ~+ for the 910 keV state. The coincidence measurements reveal one cascade of three y-lines and one or two single lines feeding this state. The 800 keV state. The state was found in ß-decay work (6) but no definite spin assignment was given, ~+, ~* and u+ all being possible. The angular distribution of the 800 keV line is also indecisive, as are the excitation functions. The IS74 keV state. The state decays to the 894 keV ~+ state by mixed dipolequadrupole radiation. The angular distribution gives the two possibilities ~+ and ~+, the former assignment being preferred from the excitation function of the 680 keV line. The 1856 keV state. This state decays to the 910 keV state by mixed M 1 /E2 radiation. Considering also the yield function ~+ is a probable assignment for this state. The 1988 keY state. This state is strongly populated in the (a, 3n) reaction and

u+

76

S. E. ARNELL et al .

decays by a pure quadrupole transition of 107? keV energy to the ~+ state at 910 keV. The lifetime of the 1988 keV state is estimated to be less than 0.5 ns which excludes M2 type of the 1077 keV transition . Therefore we give the 1988 keV state a ~+ assignment, in agreement with the yield function of the 1077 keV line. The 3117 keV state is deexcited by à 1129 keV transition which is only observed in the (a, 3n) reaction, indicating a high spin of this state. The a Z and a4 coefficients of the 1129 keV line are in agreement with those of pure quadrupole radiation, and the line shows a full Doppler shift implying a lifetime of the 3117 keV state of < 0.5 ns. Our assignment of this state is thus ~+ . The 3645 keV state. The 528 keV transition deexciting this state is of dipole type with a large quadrupole admixture. The transition is only observed in the (a, 3n) reaction . The spin value of the state is probably ~+ . 4. Discu:~don The very low-lying level structure of the five middle-shell N = 45 nuclei 32Ge, ââ~+ 36~+ 3ésr and âôZr is similar and consists of a ~+ state, which is the ground state (' 9 Se, et Kr, 83 Sr) or probably the ground state ("Ge, e sZr), a i+ state, and an isomeric ~- state indicating the proximity of the p} and g orbitals. The smallest energy separation of the ~ and ~ states probably occurs in ~ 3 Sr (35 keV) . The yrast states up to high spin values of these isotonic nuclei are known only in

6+

0+ %5

]f~ rib

Fig. 4. The level scheme of °'Sr observed in this work .

°sSr 4,0

77

-123,25)

Ex (MeVI

-zt

3.0 -t~ -ts

2A

-13

1.0

-13 -11 -t t

aol J~

~~

-23 -2t

-t~ -t5 --{17) -t3.g ~7,111

~3 °~s~ J;

al< K

-It31

_~'

°Sz~ s~)], '9Se [ref. ~a)], °' ICr [ref. 's )], Fig. 5. Some excited states mainly of yrast character in "Ge [ref. a.~b )], (this work) and °s Zr [refs. °'Sr

MeV 3

N

r V

2

Y

t

0

Fig. 6. Level energies of some ICr nucld plotted against the feud ~ergies of thecorreaponding isotonic Sr nucld. For the add-N nucld the first ~ * level is taken as the origin . The lords of theeves-N nucld are in order : 0*, 2*, 2=,'4*, 6* . Legead : Triangles: N = 44 . Circles: N s 45 . Squares: N = 46 . For the two dotted circles our assignments are uncertain. All levels of fig. 4, except those at 259 and 2107 keV, are introduced into the diagram.

S . E . ARNELL et al.

78

BtKr [ref.' s)] and from this work in 83Sr (fig . 5). There is a striking similarity between 83Sr and 8 'Kr which is illustrated in fig. 6 where the experimental level positions of BtKr are plotted versus the corresponding positions in BaSr. A linear relationship is accurately obeyed by these levels, the slope being close to 1 .07 and the deviations .mostly less than 10 keV. One may conclude that, as long as the excitations can be built up without breaking proton pairs, the protons have an almost negligible influence on the details of the level structure . Models implying effective interaction shell-model calculations te) as well as quasiparticle-phonon coupling models t ') and the simple Nilsson model fail to explain the low-lying ~+ level. The Coriolis coupling model, however, incorporating a residual interaction of pairing type and prolate deformation has proved to resolve many of the difficulties associated with the level structure of the deformed part of the is-2°) . A shown by Heller and Friedman' 9) in their calculation of the'9Se s g~ shell energy spectrum along these lines, the Coriolis interaction destroys the simple band structure of the deformed Nilsson model. Coriolis mixing of the K = ~, -~ and ~ bands from the get shell depresses the ~+ state to become the ground state and gives rise to an ~+ state at low energy . According to the calculations by Heller and Friedman ' 9), however, the mixing also generates a low-lying ~+ state, i.e. a closely spaced ~+, ~+, ~+ triplet should be generated. No N = 45 nucleus seems to have such a close triplet. In fact, from the 2+ state is at cases where 1 values are known, "Ge [ref. 22)],'9Se [ref. z°)], the .first about 0.5 MeV. For the high-spin positive parity states (as well as low-lying states) a model comprisiflg a three-neutron-hole cluster coupled to a quadrupole vibrational field 21 z1 15 -.c / -

17 15

17

13.9

9

13 11 11 .7

13 11

9 7

5

EXP

..--

83S~

CALC .

9 7

Fig . 7 . Comparison between some observed positive parity feuds of s'Sr and those calculated by a corecluster model z').

has been applied to 8 'Sr by Beshai z3), who assumes the coupling of the neutron shell model configuration ((2pß , 2p ß, lf~ -2, (1g.tJ') to the nearby double-even Sr vibrator . Core excitations of up to three phonons are included in the calculations . Adjusting the hole-field coupling strength so as to get the experimental value for the ~+~+~ ~+~+ and ~+~+ energy separation, the level scheme shown in fig. 7 is obtained. In a tICr [ref. t s)] and '9Se [ref. Za)] a close-lying ~-+, ~+, ~+ . triplet is observed at an energy of about 1 MeV (fig . 5). Probably also in 83Sr an additional u+ state exists. A model reproduction of this triplet for the N = 45 system may require some coexistence phenomenon. Note added in proof After submitting this paper we learned that 83Sr has recently been studied by the'4Ge( 12C, 3n)83Sr reaction z') .

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