Properties of 81Kr levels populated in the decay of 81Rb isomers

Nreker Pwdes A28f (1977) 263-281 ; © NortA-Bo~P

Ca, A~~r~iaw

Na to b~ rspcoduesd b! o3otoprlat or sioee0lm withoYt wrlttw panaWoa hce iLs OeäitLtr

PROPERTIES OF °1Hr >< Fir~ POPULATED IN T]EIE DECAY OF °fRb ISOMERS J. LII'TILK t, K. KRI~TIAKOV~ tt and J. KRI3`I'IAK tt Joist Inatluue jar Nrelem" Rawa^dY, D~Gsa, USSR

Received 8 December 1976 (RevLed 30 March 1977) A6atraet : The ß+ decay of "Rb ground and isomeric °cafe° (4.58 h and 30.23 min) have beats studied with Oa(Il) detectors in both singly and coincidence modes. The decay of the "Rb isomer (T~ e 30 .23 f0.25 min) has been investigated for the ßnt time . The relative electron intemities of fourteen tramitions in "Kr have beau masund with a magnetic Si(I3) apedtometer sad as iron-free toroidal magnetic spectr+omefer. The internal oonvenion ooelscients have been determh~ed by the normalized electron-tom-ray ratio method. The mdtipolarities of the tramitioas were deduced. The spin-parity s~ignmmts have bem made from the consideration of the bgji values,thetramitionmdtipolarities andthey-raysbranching ratios. The rtruc0ure of some of the excited states in the "Kr nucleus is discored . B

RADIOACTIVITY""'~Rb[&om Zr(p, X~ B - 660 MeV] ; measured T4, Ey, yycoin, la : deduced Q, logJt. "Kr dednoed levels, J, s, It7C. Ge(Li) detector, magnetic 3i(Li) and iron-free magnetic spectrometers, mass-aeparatad ,comes.

1. I~rodacäou While the ß+-EC decay of ° tRb (4.58 h) has been the subject of several investigations l' 4), information concerning the ß+-EC dray of the °tRb 31.5 min isomer 4) as well as the properties of the levels in °tKr has remained sparse . One would expect that the properties of low-lying levels in the neutron-deficient 36~4s nucleus should be oompléx since this nucleus has S neutron holes in the N ~ 50 shell. The stradnre of the levels of odd-A nuclei in the vicinity of N or .Z a SO has bcen Csamined theorotic~ally using different approaches ranging from the shell mode] with a residua] interaction to the Coriolis-coupling mode] of the odd particle in the deformed nucleus, see refs. s,' °). In nuclei of mass number A sxs 81 the unislue-ptuity level oflarge spin] in the major shell (lg}) is being filled by neutrons. In these nuclei there is competition between spin j and spin (f-1) for the ground state. The simple shell model cannot aocouat for the low-lying (j-1) state and therefore such an extra low-lying state with spin J ~ j-1 and with the unique parity has been called the anomalous coupling state. The structure of such states is not clear but two theories [Paar 6) and Kurryama et aL')] have attempted to explain it thoroughly. Furthert The Facuhy of Mathemafia and Physic, G7~arles Uaivazstty, Prague, (~echoslovalva. tt 'The Imtitute of Physis, Sbvak Academy of Samoa, i3ratislava, Czechoslovakia.

263

264

~

J. LIPTAK st d.

more, Bohr and Mottelson 9) have pointed out ~a possible connection between the appearance of the (j-1) state as the ground state and the quadrupole deformation of the nucleus. In this connection it is thought worthwhile to continue the systematic investigation of the level structure of the odd-A Sr and Kr nuclei with mass numbers A = 77-85. 2. E~rlmental appantas and procedures The short- and long-lived s1Rb activities were prepared by the bombardment of a Zr target (0.1 mm thick), using 660 MeV protons in the external beam of the synchrocyclotron of the JINR in Dubna. The sources were prepared from a mixture of spallation products by electromagnetic mass separation, using a surface ionization source together with the "hot-solid-target" method lo). The internal conversion measurements were made with s1sRb sources prepared by the implantation of 81Rb ions into the Al backing. For the y-ray singles measurements a 41 curs coaxial Ge(Li) detector was used. The system resolution was 2.5 keV (FWHM) for the 1332 keV y-ray of s°Co. Two source-detector geometries were used : (a) close geometry, with a source on the top of the detector housing; (b) distant geometry, with the source at a distance 5.5 cm from the detector housing. The cascade-sum contribution for the stronger y-rays was estimated from the y-ray spectra taken at a distant geometry. Coincidence experiments were performed with 41 and 50 curs coaxial Ge(i i) detectors (FVPHM = 2.5 keV for the 1332 keV). Tie time resolution of the conventional coincidence system (2T) was about 100 nsec. Data were collected in a 4096 x 4096 channel matrix and stored on magnetic tape . After measurements, the tape was analysed using a program 11) written for a Hewlett-1?ackard 2116-C computer. The program sorted the data into spectra, each of which was coincident with a particular window. Coincidence windows were set on the full~nergy peaks of interest and on the background just above or below these peaks to account for events that are coincident with the Compton background . The relative intensities of the ß* decay of slur" aRb were determined by measuromenu of I,(511 keV) in the distant geometry . An Al absorber (3 mm thick) was placed on both sides of the 81Rb source . Two types of ß-spectrometers have been used to measure the eloctron spectrum of 81'Rb. The intensities of the low~nergy conversion lines (K 49 .5 and 64.5 keV) were measured by the iron-free ß-spectrometer with a toroidal magnetic field 1~). The momentum resolution of the ß-spectrometer was about 1 ~. The internal conversion of the higher-energy transitions was studied by a spectrometer which is a combination of the constant magnetic field (750 G) with the high-resolution 5i(Li) detector is). 3. Resalts

Typical y-ray single spectra obtained for the e i m, sRb decay are shown in figs . 1 and 2. The y-rays were assigned to their respective parents by following their decay

uKr LBVELS POPULATED

26 5

over a period of 1S h. A total of 48 and 44 y-rays were identified as being due to the decay of el'Rb and simRb, respectively. The energies of the most intense peaks in the 81'Rb spectrum were taken from the works of Broda et al. i ) and Waters et al. a). Some peaks fromthe decay of admixtures of the BaRb nuclei were used, too. These energies were taken as internal standards to determine the energies of weaker peaks. The peak positions of y-rays were obtained with a computer program' 1) which fitted a Gauasian curve to the data points of the photopeak. The energy calibration was determined by the least-square fit of the nth degree polynomial to positions of the y-rays taken as reference energy points . The fourth-degree polynomial was found to give the best fit. The relative intensities of y-rays were obtained from the peak areas determined from the area of a computer-fitted Gaussian curve. The relative efficiency curve ü) was obtained using s4Mn, ss~~ eo~~ ~s~~ isaEu~ is9~ and ieaTa standard sources. The y-ray energies and intensities together with their relative decay mode, are summarized in table 1, in which the results of Broda et al. i) and those Waters et al. a) are included for comparison. A summary of the results of the y-y coincidence experiments is given in table 2. Figs. 3, 4 and S show several of the coincidence y-ray spectra. The observation of a y-ray line in the spectrum coincident with a gating y-ray is indicated in table 2 by the symbol y or the value of its coincidence intensity calculated from the gated y-ray spectra. Besides this, the y-ray intensity calculated from the proposed decay scheme is given to show the completeness of the scheme. It is impractical to discuss in detail the extensive data of table 2. Instead, we would like to point out some ca.9es where an explanation is needed . In the 456.9 keV gate the observed intensity of the ?44.3 keV transition is smaller than the calculated one. The same holds for the 446.3 keV transition in the 602.3 keV gate. Similar discrepancies appear in the 643.6 keV gate for 368.3 and 463.3 keV transitions. At present, no explanation of these discrepancies is available. Generally, good agreement was obtained between experimental coincidence intensities and intensities calculated from both proposed decay schemes. This fact supports our point of view that all observed y-transitions are placed correctly, apart from those mentioned above. The measured y-ray intensities from the decay of el'Rb are in good agreement with the values published by Broda 1) and Waters et al. a). The only inconsistency found is a large value of the 190.4 keV transition intensity as reported by Broda et al. i) which disagrees with both our observation and that by Waters et al. a). In order to examine the possibility that our low value of the intensity of the 190.4 keV y-ray could be caused by loss of the 13 sec 81Kr isomer from the source due to escape of krypton gas, we repeated the experiment with the el'Rb source packed in the polyethylene bag. We obtained the same value of the intensity of the 190.4 keV y-transition . The reason for the disagreement between the present result and that given in ref. 1) is not known to the authors.

F.aeraies and relative intenaides of y-transitions fmm tlee'1"""A6 -s'lär decay (i) T~ ~ 4.38 h 1

(key this work

49.Sf0.1 °) 64.Sf0.4 190.4f0.2 218.8 f0.6 244.3f0 .2 .5 266 .2f0 283.1 f0.5 °) 319.5 f0.4 339.4~0 .4 337.7 t0 .2 386.Of0.3 389.Of0.2 399.7 f0.5 446.3f0.1 456.9 f0.1 476.8f0.1 499.4f0.2 510.4f0.3 ") 511 ") 537.E f0 .1 537.6f1 .0°) 545.9 f0.1 368.9f0.1 6023f0.3 608.5f0.2 689.9t0.3 701 .Sf0 .3 729.1 f0.1 758.3f0.2 7823 f0.3 803.Sf0.2 834.8f0.2 903.2f0.6 9125f0 .6 968.4f0.9~) 977.1 f0 .2 1041 .1f0.2 1048 .4 f0.3 1069.3f0.3 1087.7f0.5 lo9o.4fas 1108 .Of0 .2 1136.4tQ4 1363 .8f0 .6 1381 .5f0.3 1427.8fa2 1487.4f0.5 1536.0 f0.8 15S4.9f0 .3 1874.Of0 .4

this work 29E 0.7 24E 0.3 2760 f 60 0.8 f 0.2 13 .3E 0.4 1.6E 0.2 1.9 f 0.4 ") 1.9 f 0.2 25 f 0.3 326f 1.0 3.6E 0.4 19.6E 1.0 1.1 f 0.3 1000 f 30 130 f 4 225E 0.6 3.1 f 0.2 230 f 40') 2670 f 110 %.0 f 6.6 8 f 3°) 20.3 f 0.6 23.1E 0.7 22E 0.1 11 .1E 0.3 1.3E 0.1 23E 0.8') 127f 0.4 2.2E 0.1 0.6 f 0.1 35.9E 1.0 33.0E 1 .0 0.2E al 0.2E 0.1 < 0.1 24.3 f 0.8 23.0E 1.3 20 f 0.2 26E 0.1 0.3E 0.1 ~ o.5f o.l 2.2E 0 .1 O.S f 0.1 0.2E 0.1 0.4E 0.1 1.4E 0.1 0.4E 0 .2 OZ f 0.1 1.8E O.Z 0.6E 0.1

Relative intensity Hroda at d. ") 3300 f200

Waters st d.')

2745 f70

16.7E 0.8

ll .St 1.3

33 .2 f

289f 1.7

1 .7

19 .7E 1.0

1000 123.9 f 7.2 21 .0E 0.2 14.0E 4.0 110 f 44°) ~ 2880 f130

20 .9E 3.8

1000 f19 116.2 f 3.8 221E 26 2851 f98

95.1 f 5.2

98 .7 f 26

23.3 f 21 .8E 20E 10.4E

17.9 f 23 .4E 0.4E 13 .2E

1.2 1.1 .3 0 0 .6

0.9 0~9 0.4 0.4

20E 0.6 11 .4 f 0.6 23E 0.3

3.7E 0.3 123 f Q9 23E 0 .2

31 .0E 1.5 322E 1.5

33 .6E 1.3 33.2t 1.3

0.6E 0.2 20.1 f 1.2 21 .6E 1 .2

20.8 f 20 21 .3E 0.8

28E 0 .2 0.9E 0.2 ~

2.8E 0.7 1.2E 0.4

1.4E 0.3

27E 0.3

1.4E 0.3

1.Sf 0.3 1.8E 0.2

uKr LEVELS POPULATED T~.s 1 (confirmed) Cu) T} m 30.25 min

Energy (loeV) this work

Relative intercity this work

Sne;rey (loeV) this work

49.Sf0.1 86.2f0.2 h .2') 190.4f0 266.2f0.5') 36B.3f0.3 446.3 f0.1 ') 456.9f0.1') 463.3f0.3 465.Sf0.3 499.4f0.2 511 °) 548.9f0.1 SS1 .Sf1.5 °) 643.6f0.1 643.6f1.5°) 657.Sf0.2 682.3f0.1 729.2f0.8°) 732.1 f0.2 761.9f0.2 824.2f0.S 873.8f0.3

660 f 3S 4630 f200 12.1E 1.2') se 0.1') 9.2~ 0.5 18.0f 20') 7.4E 1.4') 18.0E 2.0 18.0E Z.0 25.0E 1.5 1810 f100 90.0f 5.5 S f 2°) 98.4E 4.2 1.6~ 0.8°) 11 .7E O.S 42.0E 1.8 28 f 2 18 f 1 8.Of 0.6 13 f 1 7.7E 0.6

885.Of0.2 932.4f0.2 981.6f0.2 1011 fl 1014.4f0.4 1087 f1 1099.9f0.2 1136 fl 1137.Of0.4 1194.6f0.2 1206.0f1.S 1286.9f0.4 1297 .Of0.4 1633.2f0.S 1638.4f0.4 16827f0.4 1687.9f0.4 1694.4f0.4 1732 f1 1743.Sf0.3 1781.8f0.5 1853 fl 1902.6f0.7

267

Rehtdve intensity this work 32.7f1 .4 32.7f1.4 24.9f1 .4 ~ 3 10.3f0.7 10.2f1.5 64.2f27 3.6f0.7 6.Of0.6 94.Sf3.5 ~ 1 S.9f0.4 6.3f0.4 6.Of0.4 10.8f0.7 13.1f0.8 9.1f0.7 13.6f0 .8 ar 1 48.4f2.1 4.7f0.4 0.9f0.2 4.1 f0.4

,) lief: i). ') Ref.') . °) Valve determined from the decay of ~'°Rb. °) Values determined on basis of y~y coiacidanoe measlu+emants. °) T]le annihilwti pn ~OSk. 7 Published by Hroda st al.') only" ~) The isomeric transition of s1Rb. ') Value determined from the decay ofs"Rb. ') The intercity has bean determined from the y-y coincidence measurement and the level scheme. The relatively strong 510.4 keV q-transition, maskedcompletely inthe single spectra by the strong A*,n;h;t~on line, was observed in coincidence measuroments . The existence of this transition is displayed very clearly in the 977.1 keV gate (see fig. 4). The annul+ ;let;on q-ray is absent in this spectrum because the 1677.9 keV level, depopulated by 977 .1 keV transition, cannot be reached in ß+ decay (the decay energy is too low). The electron spectra obtained by both ß-spectrometers are shown is figs. 6 aad 7. The internal conversion electron intensities were determined relative to the valtu of 2.4 for the K-electron peak of the 4415.3 keV transition. These intensities are given in table 3 along with the relative photon intensities, the dedt>
268

Energy

J. LIPTAK

a d.

T~ 2 Results ofy-ray wincidenoe meaaarements associated with the decay of~'"" "Rb exp.

Intensity

talc .

266.2 456.9

Y 11

0.2 13.1

446.3

29.2

32.6

608.3

3.1

3.6

446.3

19.2

19.6

64.5 283.1 319.3 339.4 357.7 389.0

Y 1.9 Y 3.2 32 .6 `) 20 .6

0.25 2.5 32.6') 19.6

244.3 537.6 568.9

11 .5 92 25.8

13 .1 95 24.7

49 .5 499.4

Y S.0

2.3 . 4.9

244.3 266.2

0.4 1.3

1.2

476.8

16.5

17.5

266.2

0.6

0.3

446.3

1 .5

Z.2

386.0 1069 .3

3.3 2.2

3.6 2 .6

499.4

0.3

0.26

319.5 399.7

1.8 0.7

1.7 1.0

729.1

1.4

1.8

244.3 436.9

1.3 1.2

446.3

24.8

2.4

23.0

Enorey (keV)

T} = 4.58h gate 244.3 gate 357.7

~P.

Intensity

talc.

337.6 977.1

0.7 1.3

0.3 1.4

602.3 701 .5 977.1 1041 .1 1108 .0 1427.8

2.1 2.3 0.9 25 .7 2.5 1.2

2.2 23 .p 2.2 1.4

782.5 977.1

0.6 1.8

0.3 1.4

548.9

17 .2

17.5

456.9

%

95

gato 386.0 gate 389.0 gate 446.3

gate 456.9

gate 476.8 gate 537.6 gate 348.9 gate 368.9 gate 602.3 gate 608.5 gate 689.9 gate 729.1

689.9

1.0

1.0

456.9

24.8

24.8

1136 .4

Y

0.3

348.9

1 .2

1.0

758.3

1.9

1.8

gate 758.3 gate 977.1 gate 1041 .1

510.4

23

TADI$ 2 (oontinued) Beer® (keV

exD.

Intensity

608.5

2.6

2.6

446.3

1.9

2.2

446.3

1.3

1 .4

368.3 499.4 682.3 885.0 932.4 463.3 643.6

18

3.4 10 4 S 11

368.3 446.3 436.9 463.3 348.9

6.4 18 .3 3.3 1S 4

9.2 18.0

499.4

3.2

2.6

49.5 368.3

Y 4.9

465.3 348.9

10 8

932.4

9

49.5

Y

49.5

Y

761.9

Y

4.5

gate 456.9

sate 643.6

gate 637.5 sate 682.3

1157 .0 1194.E 1633 .2 1638 .4 1694.4 643.6

64.2

49.5 499.4

Y 16

20

436.9

8

5.9

Y Y Y Y Y 18

18

4

5.9

8.8 12 TO

9.1 73.3

531.5 643.6 682.3 732.1 1099.9

6 2.6 6.6 4.4 64.2')

6.4 2.8 64.2')

548.6

7.0

9.1

643.6

4.3

6.4

sate (729.2-F732.1) 643.6 1014.4 sate T61.9 981.6 sate 883 .0

gate 981 .6

60

Y - coincideaoe.

sate 446.3

sate 932.4

643.6

talc.

Bate 1427.8

18 .0

6.4

Intensity

Bate 1108 .0

1286.9 gate (5489-~SSI .s) 637.5 729.2 1194.6

465.3 548.9 SS1.S 643.6

eap.

Qate 1069.3

T~ - 30.23 min Bate 49 .3

Y Y Y Y Y 14

BaeriY (loeV)

talc.

4 10

2.8 10.3

4.2

3.5

7619

4

4.3

548.9

73

73 .5

gate 1099 .9 sate 1194.6 sate 1286 .9

") I~itia aormalimed .

ro

J. LIPT~IC er .~.

Fib. 1. Low-a~ner~y part of the ~l"""Rb g ray epxcrum. The aPper :pectrnm belongi to ~`~Rb aad the lower one to ~l "+~Rb.

version coefficients ~ (ICC a~), tho theoretical ICC values and the multipolaritios of the transitions . The assumption of Ml multipolarity for the 446.3 keV transition is based on the fact that an E1 or E2 assignment for the multipolarity of 446.3 beV transition leads to unreasonable ae values for the other transitions. Moroover, this assumption is confirmed by the data fromthe Lemming compilation r'). The theoretical values of the internal conversion ooefficiants given in table 3 were obtained through graphical interpolation from the tables of Hager and. SeltLrer r°). The Ml character of the 49.5 keY transition as determined from a~ value has been confirmed by an independent method: The intensity balaaco analysis on the 49 .5 keV level provided the value of total ICC arr,, m 0.76 f0.30 as compared with the theoretical values aT = 0.93 for M1 type transition aad ar = 12.6 for E2 type transition t°) . It follows, that the 49:5 keV transition is M1. 4. Decay scheme

The decay schemes of °rm" °Rb have been constructed on the basis of >~ coincidence results, taking into account the intensities and energies of the relevant y-transitions. The results obtained in the (d, p) t4 ) and (a, xn) i9) reactions have bas used, too . The log ft values for ß + -EC feeding of levels in °1 Kr have bcen determined using the half-lives, the theoretical calculation of the EC/ß + ratio t s), the intensities of the

uKr LEVELS POPULATED

10~

2400

2600

2900 3000 CHANNEL NUMBER

271

3200

3400

Fig. 2. High-0neryy part of the sl'""Rb y-ray spectram. The upper sPe~r~ belongs to w+sRb and the lower one to "sRb. Meaning of the symboh used : a v y-liae belongs to the decay of s'Rb ; ~ e a real sam peak of cascade tramitions ; E~ = arandom sum peak.

y-transitions and Qsc values : The Qac = ?.260t 30 keV value has been taken from the Wapstra and Gove compilation ts~. 4.1 . DECAY OF s 1 sRb(T~ s 4.Sg h)

The proposed decay scheme of the °; Rb ground state is shown in fig. 8. Here, all 15 excited levels arc introduced on the basis of yy+ coincidence results. Five of these levels are suggested for the $rst time. Comparison ofour results with the decay scheme

272

J. LIPTAR et d.

R1Nflfl1 .~f1 Hai7YflY

~'Kr LEVEL3 POP[JLATED

273

.8 a 8

8

274

J. LIPTfiK et d.

150 75 0 5 0 z 5 vô 0 0 ~ 30 w m ~ Z 0 10 5 0 20 10 10 0

~ATE

500

1000 CHANNEL M

mv

10y1 .1

v M ,vo v

162I " 8 12 ~6~

~1~_. .~_- . .

v

v

oi ~ 19 .6

ao

m ~n

M

m

11

v

1os9 .e

Lt ~,

hWiriW

1 69.3 FiB. 5. See $Y. 4.

W L.

m o r..~~ :4 L .. _L~_

Vars 3 Internal coavendon ooe®cients for y-transition in "Kr I1~eVl

49.5 64.3 244.3 357.7 389.0 446.3 456.9 510.4 337.E 548.9 568.9 803.3 834.8

Relative Y-~~tY

Normalized 1, x 101

lüp value act x 10'

Theo- a~ x 10' value') retical El M1 E2

2.9f 0.7 300 f 10 1000 f400 580 860 9400 400 f100 260 410 3800 2.4f 0.3 95 f 9 13.3E 0.4 26 f 2 20 f 3 5.6 11 31 4.3 f 1.3 2.0 32.6E 1.0 14 f 4 4.4 8.4 19.6E 1.0 7.2E 2.3 3.7 f 1.2 1.6 3.4 6.2 2.4 °) 2.4 1000 240 ~) 130 f 4 16 f 4 1.2 f 0.4 1.1 2.3 3.6 2.2 f 0.6 0.82 230 f40 Sl f 7 1.8 2.7 96 f 7 20 f 4 2.1 f 0.6 0.76 1.6 2.25 20.3f 0.6 4.6f 0.9 2.2 f 0 .6 .7 0 1.47 2.1 25.1f 0.7 3.4~ 1.0 1.3 f 0.5 0.63 1.33 1.84 33.9f 1.0 2.3f 1.0 0.70 f 0.28 0.29 .62 0 0.76 33.0f 1.0 2.2f 1.0 0.61 f 0.29 0.27 0.56 0.6B

") From raf. l'). ") Assumed valse for normalization. °) Theomtical at value for MI mdtipolarity. .

Deduoed mdtiPo~tY Ml -i-<6 ~ ld2

M1-~-< 3 ~ B2 MI-I-E2 M1 M1

M1 °) 131

Ml, l32 E2, M1 Ml, EZ M1,132 M1,13Z

M1,132

s1Kr LBVELS POPLTLATSD

275

e

N

z 80

Y Y

~60 a: W

j~

Y

,

Y~ i. ; , i w:.

70 ~ .~l ~.riY

Ii

em

u 600 2200 2300 CHANNEL NUMBER PLy. 6. Electron spectrom from the decay of ~' "Ab measured with the iron-fi+ee ßßpectrometer . 500

4.tf/t N F z

ô3]0~ 6 .1 O K

ô O

-Y

Ô

m2 I z

m

~ N

n

~ N y

~ O l+l Y

p

m x

Y~V mv O . ~J

a O N

1

m Y v ~ Or O O m 1~Y,i

~ooo

moo

Y

fl I

0

J iJ~ 40o

n

eoo

CHANNEL

NUMBER

F~~. 7. Electron spectrum from the decay of "~Rb measured with a Si(Li) detector .

proposed by Broda et d. t) shows that the levels at 1108.1,1280.2 and 1883 keV cannot be confirmed. The y-q coincidence measurements show that the rekvaattransitions of 499.4, 602.3, 1108.0 and 127.8 keV mast be placed differently than they are in ref. t). All of the rest of Broda's proposed levels are confirmed by our results, but this is not the case as far as the positions of y-transitions are concerned. The 190.4 keV transition is an E3 transition ta) . The theoretical value of ar(E3, 190.4 keV) = d.a9 [ref. ia)] was used to calculate the total intensity of this transition . The total intensity of the ß + decay branch was calculated from the decay scheme ~g Qsc ~ ~~ keV and the theoretical dependence of the EC/ß + ratio on the energy ~ s) . An excess of intensity of the 511 loot/ annihilation p-ray was found from the comparison of the calculated ß+ intensity with the ekperimental quantity . However, a revised valuo of the dray energy, Qsc ~ 2290±âs keV, can explain this difference. The deduced Qec value is in nice agreement with the tabulated one (Qsc ~ 2260 t~

J. L1PT~IC et al.

276

0 2280 ks

stRb 084.5

OA58 5.9

V~wé~~

âôôddd :+nYln - 18779 1.28 woonnn n .e .rewwlsnnawn °OOOV~IfICI." V °L,~Fee .woe ° ddé d 4R4 ea e44i~ fi~i~i°I°IR`Se~owo ooo4R " oeddoeveoow~ddood,.,ii °. °`~~ oeen oneeaw ow9° w~^aY:~ oaeo,,,,,von °°°-e nn+é éé 1238 .7 0.38 YI,.,ee .~Fno we~ .e n woool" oavwowew ôrié~~ ge ^ enww ~ wno^ Inoo wna v w =o-1-~ 1 ~~j~ ~ Ü nn 1/2 wn °d '44 Z 978.1 _ ^~~~° 919 .6 0.27 w° .ree " O"f Y+n `r° oie alnn 1701 .0 (~l' S.0 ~I°nôô -~ 808.5 ( b .~0 0.4 z548 .9 5= ~~L 458.8 ;s/~" e O

t 49.5

ef Kr

36 ~5

0

5 .4

B.4 B.8 5.7 7.,~ 7.

y2+

85.3

5A

I~

E+/19L

l9 ft

7/2+

Fig. 8. Level scheme of "Kr populated in the decay of "~Rb. A dot indicates the ooincIdenoe relation. The intensities of the transitions are normalised on 100 decays of' 1 ~Rb.

keV) t e) but it is greater than Q$c = 2190 t 30 keV given in the paper of Hroda et al. t). The Spin and parity of the ground state of a 1 Kr is ~+ [ref. t'a]. The Ml character ofthe 49.5 keV transition (see table 3) as well as the ß + decay of et °Rb (J" _ ~+) t') to the 49.5 keV level gives ß = ~+ for this level. The same value has bean obtained by Chao et al. t ~) in the eoKr(d, p)B1 Kr reaction study [the values of level energies given by Chao et al. ta) must be shifted up about 49 keVj. The 190.4 keV level has 1" _ ~- as follows from the E3 character of the 190.4 keV ß+ transition to this level transition to the ground state (J" _ ~+) and the allowed t~) from the ground state of BtRb (J" _ ~-) . Again, the corrected results of Chao et al. t4) confirm this assignment . The value of ~ for 456.9 keV level is ~- as follows from the El multipolarity of t7) of the 456.9 keV transition to the ground state (J" _ ~+) and the E2 character 266 keV transition to 10:4 keV level (.1~ _ ~}-). The state at 548.9 keV is probably identical with the 551 keV state reported by Chao et al. ta). Their assignment of ~ _ ~+ for this state is not contrary to Ml and/or E2 character for the 548.9 keV ground-state transition . The most probable value of .T' for 608.5 keV level is ~+ or $t because relatively strong transition of 608.5 keV populates the ground state with the high value of Jx = ~+ .

The level at the energy of 636.7 keV has .T'ç = ~-. This spin and parity follows from the Ml character of the 446.3 keV transition, the allowed ß+ transition (log ft = 5.1)

uRr LEVELS 1?O1PiJLATSD

277

from the g.s. of e1Rb (J''=~ -) and the pray branching of 1099.9 keV level (,1;=~+). Since the 64 .5 and 244.3 keV transitions are both of magnetic dipole character we propose ~- or ~- as the possible spin and parity of 701 .0 keV level. As seen in fig. 8, the ECtransitions to the 994.2 and 1677 .9 keV levels have logft = 5.6 and 5.4, respectively. The allowed character of these indicates the negative parity and admits three spins, ~}, ~ and ~ for the 994 .2 and 1677.9 keV levels. In view of the branching ratios of the 357.7, 537.6 and 803.5 keV transitions we propose ~- or ~as the spin and parity of the 994.2 keV level . The intensity analysis of they-rays from the 1677.9 keV level leads to the suggestion that the quantum characteristics of this level are ~- or ~}- . The branching ratio of the 389.0 keV (Ml) and 834.8 keV transitions indicates that the most probable multipolarity of 834.8 keV transition is M1 . Therefore, we propose J" _ ~' for the 1025.5 keV level. 4.2. DECAY OF "`Rb (30.25 f0 .25 min)

The new value of the half-life of the s 1Rb isomer, T} = 30.25t 0.25 min, has been determined by measuring the intensity of the 86.2 keV isomeric transition as a function of time. The spin ~ ofthe isomeric state has been measured by the atomic beam method 1 ~). The E3 character i7) of 86.2 keV isomeric transition to 81sRb (~-) leads to positive parity for simRb . The total ICC of the 86 .2 keV transition a~,~ = 19.5, was determined from the Hager and Seltzer ICC tables ie) by interpolation. This value of err was used to calculate the isomeric branching ratio which is 2.2 ~. The values of logft were calculated using this branching ratio and Qsc = 2350 keV. As figs . 8 and 9 show, five levels excited in the decay of s1Rb ground state also have been seen in the decay of the,isomeric state of 81Rb, namely at 49.5 (~+), 190.4 (~}-), 456.9 (~-), 548.9 (~*) and 636.7 keV (~-). Similar arguments as for the 81'Rb decay [empirical rules for logft values s°), q-ray branching ratios] have been applied to the simRb decay study. It follows that the most probable spins and parities of the 731 .9, 1099.9 and 1743.6 keV levels are and ~+, respectively . The state at 934 .5 keV has .T` _ ~+, as follows from the study of the (ac, xn) reaction 1 g). The results of the (d, p) reaction 14) indicate .7s = ~+ for the 1099.9 keV level, in conformity with our suggestion. Three levels, at 873.8,1781.8 and 1902.¢ keV, haute been introduced on the basis of energy sums only . Sut the 873.8 keV state has been observed in the (a, xn) reaction 1 g). S. Dleco~bn The present study of °1Kr levels has established the properties of several states in this nucleus. Thus, for somo levels it has become possible to melee meaningful comparison between experimental results and theoretical model predictions.

278

J. LIPTIi1K st al. 4 s

aa .ro `r ov-~ o, ~wn °aoo r ,mrw.irri~aioi~^!

Ttn=30.25min ô I~9/2+

n=2350 k~~~1/ °tRb

oar+ d°°~^~,; . . .. or ~~~,+~~

-1902 .E ~1781 .8 -1743.E 1887 .9 1682 .7 ~1206 .4

oa,., 19 1 .7 - öö v~ ô0~~-934â ~~~'~ 873.8 7 31 .9 , 638.7 ,~- 548 .9 ~~- 456.9 o . =190 .4 ~ 490.5

vi i 36'~~ B

0.005 0.00E 0 .32 0.021 0.02

6.6 8.7 .1 8.4 6.4

,

0.02

6.8

,

0.052 0.034 0.022 0.0 5

6.6 6.8 7.0 6.7

Ô~8

6.5

7l2*

1tt2

hlzi9 3i25~2+ 6i2t12-

~ii' Is

E+p*X lg ft

Fi;. 9. Level °theme of' 1Kr populated in the deatiy of "'Rb. A dot indicate° the coinctdanoe relation°. The intenaitia of the ttaasltiona in'1 Kr aro slum is the snit of 1000 decays of a' "Rb.

The Coriolis~oupling modal °), where the last impaired neutron ofthe lgt, shell is coupled to a rotating core and the Coriolis interaction is properly accounted for, correctly predicts the doublet of states with J~ _ ~+ and $+. Moreover, the ~+ state becomes the ground state of °1 Kr. Further, this calculation successfully predicts two low-lying levels with J~ _ ~+ and ~+ (at 800 and 1100 keY, respectively) which we observed at the excitation energy of 934:5 (~+) and 1099.9 keY (~+). On the other hand, the level with .T" _ ~+ is predicted to occur between the levels of the doublet with J; _ ~+ and ~±. Such a state has not been observed . But a more detailed wmparison of the Coriolis-coupling model with our experimental data is impossible as, to our knowledge, no calculation of the electromagnetic properties of °I ICr levels has been performed . Therefore, no definite conclusion about the da formation of the ° 1Kr nucleus can be made. However, the comparison of the calculated °) and the experimental states indicates positive deformation (prelate shape) of the °lICr nucleus if it is deformed. From the other point of view, the first excited state at 49.5 keV (~+) can be interpreted as a neutron state whose main component is a (n(lg~,)-s, v = 1) configuration (v is the seniority number). Such an interpretation is in accordance with (d, p) reaction data l') and it enables us to account for the structure of the ground state of ss~as with J; _ ~+ which belongi to the family of anomalous coupling states.

uKt LEVELS POPULATED

279

There are two theories which attempt to explain the strneture of tlu anomalous coupling state. The first is Alaga's model e) where a cluster of particles (or holes) is coupled to the vibrational field of the nucleus . The second, the modal of Kuriyama et al. ~) is basically the quasiparticle-phonon coupling model si) but it takes into accountthephonon dissociation into a pair ofquasiparticles, one of which reassociates with the odd quasiparticle . The experimental values of a=(49:5 keV) = 1 .0 f 0:4 and az = 0.7610.30 indicate the Ml multipolarity of the ground-state transition . The maximum F2 admixture is 4 l. It means that the ratio p of the probabilities of an M1 transition to an F.2 transition is larger than 24 . In the framework of Alaga's model, Paar e) has calculated the B(M1) and B(E2) values for the ~+ ~ ~+ transition in nuclei with Z = 47 (Ag). If we suppose that a similar calculation is valid for nuclei with N = 45 and 47 then Paar's reduced probabilities B(Ml) = 0.015 n.m.3 and B(E2) a 0.11 (e " b)~ (ref. 6 )] for the transition ~+ -~ ~+ enable us to calculate tho ratio p. The calculation of A, using the known relationship between the probability of q-transition and the reduced transition probability ~~), gives p = 100. Similar calculation of p was mado with the values of B(Ml) s 4 .5 n.m .~ and B(E2) = 13 x 10 - s° e2 " cm a given by Kuriyama et d. ~) for the ~+ -+ ~+ trSn31t10II m 36Kr4,. The value of D = 20 has boen obtained. pn the basis of such a calculation the conclusion about the structure of tho anomalous coupling state with J'° _ ~+ can be drawn, namely, this state contains considerable amount of In(g})- s, ~, 00~, In(gt)- s, ~r,12i and In(gt.) - ', ~i,12~ configurations [ref. 6)] (the basis states are I (lj) - ', J, NR~, where J is the angular momentum of the flue-hole cluster, N is the nug~ber and R the angular momentum of the phonons). According to the theory of Kuriyama et al. '), this state caa be created by the new type of collective excitation, the dressed k neutron quasipartick mode, as a bound state of k neutrons of the lgt, shell with seniority v = k (k = 3, 5). The calcuation of Kuriyama et al. ~) predicts the excited levels of °1Kr with J; _ ,~,+, ~+ and ~,+ atthe apprôximate energies of 1,1 and 1 .5 MeV, respectively. It is remarkable that two levels with such quantum characteristics have bean observed in the decay of si °" 'Rb, i.e. at 934.5 (~+) and 1099.9 keV (~}+). Similarly, as in the case of the °3 Sr nucleus sa), some levels (548 .9, 1014.4, 1099.9 and 1682.7 keV) are dominantly deexcited to the ground state with JR m ~+, whereas the levels from the second group (731 .9, 873.8 and 934.5 keV) are mainly deéxcited to the first excited state with JR a ~+ . But it is necessary to remark that this property of the levels is not so distinctive as in the case of the s3Sr nucleus. The large number of negative-parity states populated in the decay of °i ',Rb make it possible to draw some conclusions aboutthe validity of the Coriolis-coupling model e). The relatively low logft values of the 8''Rb ß+ decay to the levels at 190.4 (logft = 5.0), 636,7 (5.1), 701.0 (5.7), 994.2 (S.fi), 1025.5 (5.8) and 1677.9 keV (5:4) point to a similarity of the wave functions of these states (mainly 2pß and 2pß neutron hole

280

J. LIPT~K et a~

states). The calculations of Heller aad Friedman s) predict the states with J'' _ }', ~' and ~' at excitation energies of 0.5 and 1 .2 MeV above the ~' state. They do not predict more }' states in the region around 0.8 MeV. However, our results suggest a possibility of the existence of more than one state with J" _ ~', which have similar structure in the region of 0.5=1.0 MeV excitation energy. The results of Paar's calculations e) show one more ~' state at .approximately 0.8 MeV excitation energy . Moreover, if we take his calculated values of the reduced probabilities B(Ml, }z -. ~i) and B(E2, $s -. ~ ) for the ~s state at 994.2 keV excitation energy we are able to reproduce our experimental y-ray branching ratio for the 357.7 and 803.5 keV y-transitions. More detailed analysis of the decay modes of the excited levels with negative parity is difficult because we cannot identify our levels with the levels of Paar's calculations unambigously. auf there ie as indication that the excited states with n = -1 can be described as a neutron cluster coupled to the vibrating nucleus. The neutron cluster should be composed of the holes in the g}, p} and p} shells . In conclusion we summarize: (a) The negativo-parity states in ar Kr seem to be mainly neutron hole states. (b) The wave function of the first excited state at 49.5 keV has a great amount of the fin( lgl<)' a, v = 1J configuration . (c) At present, no definite conclusion can be made regarding the validity ofAlaga's [ref. 6)] or Kuriyama's') model as no detailed theoretical calculation of properties of nuclei with N = 45 exists . However, if our extrapolation from .Z = 47 to N = 45-47 is correct, then Alaga's model is more appropriate. (d) The calculation of the electromagnetic properties of the excited states with n = + 1 in the framework of the Coriolis~oupling model is needed to answer the question of the deformation of nuclei in the lg~ shell. The authors are grateful to V. Gorozhanldn for the help during the measurements of the internal conversion electron spectra and to Dr. $. Grigoriev for the useful discussions concerning the interpretation of the presented data. Re~fereaoes

1) R. làoda et al., Nucl. Plays. A316 (1973) 493 2) 3. L. Waleis et ah, Phys. Rev. Cy (1970) 2441 3) A. Li~Scholz sad H. làakhru, Pi~ys. Rev. 168 (1968) 1244 4) W. Do~ett, Thesis, University of California, 1936 S) L Talmi aad L Unna, Nucl. Phys. 19 (1960) 223 6) V. Paar, NucL Plays. A?11 (1973) 29 7) A. Kuriyama et aï., 3upp . Pmg. Phys . S8 (1973) 1 8) S. L. Heller and J. N. Friedman, Pl~yi. Rev. C10 (1974) 1309 9) A. Rohr aad & R. Mottelson, Mat. Pys. Madd. Dan. Vid. Selak. 27 (1933) na 16 10). C~. Beyàr st ob, to be publishod 11) M. Honusek, private communication 12) M. C3osior st d., Preprint JINR D6-7094, Dubna (1973) 167

"ICr LEVELS POPI;TLATED

281

13) C. Vybv et d., Preprint JiNR P6-9071, Dubna (197 14) J. Chan n d., Phys . Rev. Cü (1975) 1237 1S) B. S. Dïelepow n d., Heta processes (Nau]ra, Moscow, 1973) 16) A. H. Wapstra and N. B. Clove, Nucl. Data Tabka 9 (1971) 265 17) J. F. Lemming, Nncl . Data Sheets iS (1975) 137 18) R S. Halter and E. G Sehzer, N~1 . Data M (1968) 19) F. Ingebretsen n. d., A~ual report 1974, Research Institute for Physics, Stockholm, p. 99 20) J. Raman aad N. B. Clove, Phya. Rev. C7 (1973) 1995 21) L. 3. Kiaslinger and R A. 3orenseu, Rev. Mod . Phys. 35 (1963) 8S3 22) A. Rohr and B. R Mottelson, Nuclear stn>cture, vol. 1 (Heßjamin, NY, 1969) 23) J. Lipték, J. ICrütiak andK. Krlëtiakov8, Proprint JINR E6-9S94, Dabna (1976) ; Czech. J. Phys . B26 (1976) 1321