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Levels of 185Ir excited in the (α, x n) reactions
1 .E.1 : 3.A ~
Nuclear Physics A325 (1979) 445-462; © North-Holland Publishing Co., Amsterdam Not to be reproduced by photoprint or microfilm without written permission from the publisher
LEVELS OF test. EXCITED IN THE (a, xn) REACTIONS t S. ANDRÉ, J. GENEVEY-R1V1ER and J. TREHERNE Institut des Sciences Nucleaires (IN1P3, USMG), 53, Avenue des Martyrs, 38026 Grenoble-Cedex, France R. KACZAROWSKI, J. LUKASIAK and J. JASTRZFrBSKI Institutefor Nuclear Research, 05-400 S`K~ierk, Poland and C. SCHllCK Nucléaire Centre de Spectrométrie et de Spectrométrie de Masse (IN1P3), 91406 Orsay, France Received 8 March 1979 Abstract : Level properties of's'lr were investigated by in-beam y-spectroscopy using (a, 4n) and (a, 6n) reactions with E, ranging from 41 to 77 MeV. A well-developed system of levels built on the h9 ~ Z subshell is excited. A band built on the tt- isomeric state is also observed . In distinction from heavier Ir isotopes, only a single positive parity band is developed to high-spin states . Two isomers with T1 ~ 2 = 120 ns and 40 ns are found for excitation energies above 2.2 MeV. The experimental results are discussed in terms of rotational models including the Coriolis coupling and providing for a stable triaxial shape of the's'Ir nucleus. E
NUCLEAR REACTIONS 's'Re(a, 4ny), ' s 'Re(a, 6ny), E = 41-77 MeV ; measured Er, a(E, Er , 0), yyt~oin, y(t) . 's'Ir deduced levels, J, rz, T,~Z . iesmlr deduced T,~Z. Enriched targets.
Ir,
1. Illti'Odlleilll~ Preliminary data on level properties of ' e sIr were obtained from the radioactive decay of tesPt (ref.')) . The latter work was continued by the ISOLDE-2 group Z) and the results are published in the preceding paper 3). In-beam studies of this isotope were initiated by the Grenoble-~wierk collaboration and partial results have already been published 4-6). These papers considered mainly the ground state band and the yrare one developed up to the levels with spins i- and ~-, respectively . In particular, in ref. e) the variation of the g.s.b. moment of inertia versus the square of angular velocity was investigated and discussed. The present paper summarizes the level properties of tasi r studied in the (a, 4n) t This work was done within the Grenoble-Swierk collaboration. 445
446
S . ANDRÉ et at .
and (a, 6n) reactions. The level systems built on the h~ and h,~ subshells are extended as compared with our preliminary results. Beside the previously known ~ - isomeric state at 646 keV [ref. 1)], the existence of two other high-spin nanosecond isomers for excitation energies above 2 MeV has been established. A system of levels forming a positive parity band is presented. Attempts to deduce the shape of the nucleus from the analysis of the h.~ band in terms of the rotational models with or without the assumption of the axial symmetry are presented. Trends resulting from the analysis of odd-A Ir nuclei (A = 183-191) are discussed. 2. Experimental proced~ue and results The experiments have been carried out using the a-particle beam of the Grenoble variable energy cyclotron. The ' a s Ir nucleus was produced in the 1 a sRe(a, 4n) and l8'Re(a, 6n) reactions. Metallic Re powder targets approximately 20 mg/cm Z thick were used, deposited by centrifuging on a 1 mg/cmZ mylar backing. The target material was enriched in 'e'Re to 99 .2 ~ and in iesRe to 96 .7 ~. Measurements of the excitation.function were performed at Ea = 41, 47, 51 and 56 MeV (iesRe target) and at Ea = 72 and 77 MeV (18'Re target) with two Ge(Li) detectors (one of 2 .5 cm a and another of 10 ~ efficiency). For each energy the spectra were measured during and between the beam bursts. The beam energy corresponding to the cross section maximum for the (a, 4n) reaction was found to be 51 MeV. At this energy the y-ray angular distributions were measured independently by two Ge(Li) detectors placed at angles of 25°, 37°, 52°, 70° and 90° with respect to the beam direction. The normalization procedure was similar to that used in our previous work'' 8). Table 1 lists the properties of the y-lines attributed to the'BSIr nucleus and fig. 1 shows examples of the spectra. The three-parameter EYEYt coincidences were measured at bombarding energies of 51 and 77 MeV for the (a, 4n) and (a, 6n) reaction, respectively . The data were recorded event by event on magnetic tape . Each event was labelled with a bit set 0 if the "start" y-ray was detected during the beam pulse and 1 otherwise. The size of the coincidence matrix was 2048 x 1024 x 128. The data were sorted off-line using the method described in ref. 9). Fig. 2 shows examples of the coincidence spectra. The timing measurements were performed in two ways : the time distributions were measured with respect to the beam bursts or extracted from the y-y-t experiment . Fig. 3 shows the time distribution of several transitions. Three isomers with half-lives in the nanosecond range were found. The measured half-life' of the first one was T~ = 21 .5 f 2.0 ns, which is in agreement with the previous result 1). Two new isomers of high excitation energy were observed whose half-lives were 120 + 20 ns and 40 + 10 ns.
LEVELS OF 'eslr (Il)
447
W Z Z 6 2 U W O. N FZ O U
W Z Z
= U O .10E04 C W S N N Z O U
O .10E02
0
200
400
600
800 1000 CHANNEL NUMBER
1200
1400
1600
Fig. 1 . Examples of y-ray spectra: A - coincident with beam bursts ; B - measured between beam bursts. 1'he spectra were obtained during irradiation of the 'esRe target with a-particle of 51 MeV.
S . ANDRÉ et a! .
448
W
W Z Z
300
400
500 600 CHANNEL NUM~_R
700
800
900
1000
W
300
400
500 600 700 800900 CHANNEL NUMBER
1000 1100 1200 1300 14001500
Fig . 2 . Examples of coincidence spectra.
449
LEVELS OF ' BS Ir (11) NF Z 0u
30000-
99- 7 keV Tt,.~ ~40 ns
40
_ .~~r
20
30
10000 = . .. 5000TIME Cns)
10
138-6 + 137. 2 kèJ
ôV = Tth
N
10000-~
40 ns
50001
O i I
40
I
30
NZ
ôu
I
10
20
" , 2000L
TIME Cns) 2000
2566 keV Tt,~ ~120 ns
1000 =
"
..
ô mz
I
30 Nr
I
I
I
20
641 .0 keV Tthti21 ns
~
I
10
I
300
t
TIME Cns) 1000
..,
500
0
200-
m z
40
I
30
~
20
I
10
I
I
TIME Cns)
I
Fig . 3 . Time distributions of selected ;-transitions .
3. The level scheme of tssU
The level scheme of taslr is presented in fig . 4, and several selected features of this scheme are discussed below. 3 .1 . THE GROUND STATE ROTATIONAL BAND AND RELATED LEVELS
The measured ground state spin of tasl r was found to be ~ and interpreted as the lowest member of the rotational band built on the ~ - [541] Nilsson orbital to. tt ). In our study a strong cascade of the stretched E2 transition is observed . In accordance with the relative yields, excitation functions and angular distributions (table 1) these transitions were used to construct a decoupled g.s.b. The highest certain spin value in this band is -~ for the 3469.0 keV level. No definite conclusion
S . ANDRÉ et al.
450
TAHLE 1
Summary of y-ray data obtained from the 'e s Re(a, 4n) reaction E~ (keV) a)
Iq Ea =51MeV
AZ
A,
84 .0 92 .1 97 .4 99 .7 105 .E 106 .9 112 .7 114 .0 121 .8 125 .4 126.9 137.2 138.E 141 .2 152 .8 155 .7 161 .9 165 .7 169 .3 170 .2 178 .7 182 .9 184.2 185 .9 187 .4 201 .7 202 .8 205 .2
1 .5 (2) 1 .9 (2) 1 .7 (2) 4 .6 (3) Y 2 .4 (2) 1 .3 (2) 1 .3 (2) 3 .9 (2) 2.0 (2) 4 .2 (3) 1 .8 (2) 6 .2 (6) `)Y 6 .4 (6) Y 5 .9 (4) 100 (4) 5 .0 (8) 0.3 (2) 1 .6 (6) 1 .6 (3) 5 .6 (4) 2.4 (5) 4 .4 (5) 9 .2(10) `)Y 3 .8 (6) 4 .0 (4) 2 .7 (3) 3 .9 (4) 10.3 (5)
-0 .13 (8)
0 .06(14)
-0 .01 (3) -0 .39 (6) 0 .14(10) -0 .29(12)
-0 .07 (6) 0 .00 (9) -0 .21(15) 0 .42(21)
- 0.57 (9) 0.02 (6) - 0.15(12) -0 .12 (5) -0 .31 (6) -0 .06 (7) 0 .28 (2) -0 .54(12) -0 .9 (4) -0 .8 (4) 0 .2 (2) -0.60 (6) -0.33 (8)
0.47(13) -0.13(12) 0.14(16) 0.09 (8) 0.1E (9) 0.08(13) -0 .07 (3) 0 .07(20) 0 .05(66) -0 .1 (6) 0 .9 (3) 0 .09(10) -0 .25(12)
0 .28 (4)
0.28 (6)
(2) (1) (9) (4)
0.2 (3) -0.05(17) -0 .11(14) -0 .17 (6)
210 .3 212 .E 214.4 218 .5
3 .0 (8) 12.1 (5) 4.2 (3) 8.7 (9)`)Y 7.2 (9) °) 8 .2 (6) 43 .5(17) Y
0 .0 (3) 0 .05 (8)
0 .4 (4) 0 .04(13)
0.05 (4)
0 .00 (7)
-0.01 (5) 0.01 (2)
-0.01 (9) -0.04 (4)
-0 .18 (4)
0 .03 (6)
-0.17 (8) 0.11 (5) 0.01(10) 0 .21(10)
0 .02(10) 0 .25(10) -0.08(16) -0.47(16)
226 .3 229 .E 230 .0 °) 231 °) 247 .5 256 .E 263 .9 267 .0 277 .8
5 .9(10) °) 7.3(10) °)Y ~ 7 .6 (5) Y 2 .5 (5) °) 6 .3 (6) 3 .3 (8) `) 13
.1
0 .4 -0 .7 -0 .04 -0 .14
lnitial Final level (keV) level (keV) (or reaction assignment) 418 .7
335 .2 °) 4n 4n 2614 .0+X 2514.3+X°) 229 .E °) 335 .2 442 .2 335 .2 °) ') 1734.8 1622 .4 442 .2 °) 556.0 1856.E 1734 .8 ') 4n+' es Re 4n + R'eslr 556 .0 418 .7 °) 2295 .8+X 2157 .2+X ° ) 696 .7 556 .0 °) ') 158 .E 5 .8 1856 .E ') 2012 .3 496 .E 335 .2 °) 3n+(4n) (4n) 2182 .5 2012 .3 ') 4n 4n 696 .7 °) 880 .9 (4n) 4n 4n 1745 .4 °) 1948 .2 °) 1086 .1 880.9 2392.8 ') 2598 .0 2153 .0 1948 .2 °) 2182 .5 ') 2392 .8 442 .2 229 .E °) 1530.8 °) 1745 .4 2514.3+X 2295 .8+X °) °) 1304.5 1086.1 I304.5 °) 1530.8 0 °) 229 .E 1900 .E 1670 .E °) 3171 .E+X 2940 .4+X °) 4n ') 1192 .2 ( 944 .E 2148 .1 1900 .E °) 2157 .2 1900 .E °) 1779.1 1515 .2 ') 229 .E °) 496.E 696 .7 418 .7 °) 1734 .8 ') 2012 .3 (1130.1 852 .3) ')
LEVELS OF 'aslr (II)
451
T~at .e I (continued) E~ (keV) °) 290 `) 290 .2 291 .0 `) 297 .8 307 .4 312 .E 313 .E 317 .E 323 .0 325 .2 326 .4 335 .2 351 .E 361 .8 363 .E 370 .2 380 .E 390 .0 407 .2 4p9 .0 ~) 413 .1 414.0 `) 416 .8 417 .E } 418 .7 423 .E 427 .1 434.9 441 .1 444.8 457 .2 459 .9 465 .2 469 .8 486.0 492 .3 514.0 `) 521 .7 542.9 545 .5 557 .9 559.7 570.7
iY E,=51MeV
151 (6)
Y
8 (2) 5 .0 (4) 3 .2 3 .9 2 .0 4.5 7 .0 8 .1
(4) (4) (6) (4) (6j (6)
8.7 (6) 1 .5 `) 4 .4 (6) 5 (1) 1 .4 (6) `) 3 .8(13) 2 .6 (4) `) 3 .2 (5) 19 .2(10) Y 4 .9 (8) 104 (4)
Y
A2
A4
0 .28 (2)
-0 .11 (3)
0 .18 (6) -0 .43(11)
0 .39 (9) -0 .18(17)
0.24(17) 0.52(16) 0.2 (1) 0.15(15) 0.25(10) 0.34 (9)
-0 .04(26) 0 .12(24) 0.1 (3) -0.08(20) 0.04(15) -0.08(13)
-0 .03 (6) 0 .1 (2) 0 .01(13) 0 .09(11) 0 .32(10)
0.02 (9) 0 .7 (4) -0 .1 (2) -0.21(IS) -0 .12(15)
0 .29 (6)
-0 .21 (9)
0 .15(17)
-0 .39(26)
0.30 (3)
-0.14 (4)
7 .5(10) 6 .1 9.3(10) 2.9 (7) 11 .E (7) 7.6 (4) `) 14 .0 (7) 1 .8 (5) 8 .6 (6) 2 .9 (4) 2.5 (4) 5 .2 (4) 3 .8 (6) 61 .6(25) 4 .4 (6) 5.915) 4 .8 (5) 3 .4 (8) 2.1 (6) `) 3 .7 (6) `)
0.33(12) 0.2 (4)
- 0.30(17) -0 .4 (6)
0 .09 (7)
-0 .47(10)
-0.31 (7) 0.26(16) 0.14(19) 0 .49 (9) -0 .03(20)
-0.20(10) -0.12(26) -0.02(36) -0.32(13) 0 .01(30)
0 .32 (3)
-0.12 (5)
-0 .13(14) 0.30(10)
-0 .12(20) 0.04(14)
0 .31(10)
-0.15(14)
Initial Final level (keV) level (keV) (or reaction assignment) 465 .7 ') 158 .E ') 1856 .E °) 646 .8 ') 158 .E ') 448 .8 ') (4n) 648 .8 335 .2 n) 2614.0+X 2295 .8+X b) 1192 .2 ') 1515 .2 556.0 880.9 n) 2940.4+X 2614.0+Xb) 3630+X 3304.0+X b) 2182 .5 1856 .E ') 335 .2 0 b) 1779 .1 ') 2130 .7 696 .7 335 .2 n) 3304 .0+X 2940 .4+X b ) 1530 .8 1900 .E n) 1017 .0 646 .8 ') 1510.7 1130 .1 ') 2012 .3 ') 2392 .8 1086.1 696 .7 b) 1163 .9 755 .9 ') 2153 .0 1745 .4 n) 861 .9 448 .8 ') 2182 .5 ') 2598 .0
755 .9 448 .8 2148 .1 944 .E 465 .7 755 .9
1948 .2 418 .7 1304.5 (2828.0 1745 .4 1530.8 (1622.4 465 .7 (1998.0 1622 .4 1677 .E 1383.E
1530.8 0 880.9 (3n+4n) 2392.8) 1304.5 1086 .1 1163 .9) 5 .8 4n+3n 3n+(4n) 1510 .7) 1130 .1 1163 .9 861 .9
b) n) b) ') n) b) ') ') ') ') ') ')
4n 64b .8 ') 1192 .2 3171 .E+X 2614 .0+X ") 755 .9 ') 1315 .8 1515.2 944 .E ') 1734 .8 1163 .9 ')
S . ANDRÉ et al .
452
T~HI~ 1 (continuel)
Er (keV) a) 574 .0 580 .8 584.5 587 .1 597 .3 600.5 603 .8 60E `) 617 .7 641 .0 666 .8 680 .2 690 ~) 693 .E 700 .9 715 .1 718 .2 728 .4 741 .E 759 .E 766 .8 815 .7 822.9 835 .2 843 .2 846 .5 867 .2 891 .2 894.8
Ir E,=51MeV 3 .2 (6) 3.5 (5) 3.1 (6) 3.4 (5) 19.7(10) 5 .9(10) 4.2(10) 30.4(15) 19 .4 (9) 3 .1 (6) 2.4 (5)
Y Y
Y
11 (2) 14.3 (9) 11 (1) 11 .6(10) 8.7(10) 1 .2 (4) 1 .6 (3) 5 .2 (5) `) 7.7 (5) 1 .5 (5) `) 4.1 (8) 9.5(19) 7.8(12) 3.2 (4) 1 .8 (3) 2.7 (4)
898 .9 961 .4 971 .5 994,4 1007 .8 1014 .4 1058 .0 1061 .9 1136 .1 1144 .E 1150 .7 1155 .4 1167 .0 1173 .E 1286.2
3.5 5.4 7 .1 3 .8 2 .3 8 .3 1 .0 6 .3 2 .5 1 .6 1 .9 2 .5 1 .6 15 .E 6.8
1355 .5 1389 .E 1430.E 1439 .3 1451 .8
1 .6 (4) 2 .7 (4) 1 .6 (4) 1 .6 (4) 20 .1(10)
AZ
0.6
(2)
Y
-0 .3
(2)
0.43(15) -0.22 (7) 0.3 (2) 0.2 (2)
-0.09(27) 0.09 (8) 0.8 (3) 0.4 (4)
0.35 (3) -0.13 (4)
-0 .15 (6) 0 .02 (5)
0 .22 (8) -0.55(10)
-0.13(10) 0 .31(10)
0.15 (8) -0.96 (8) -0.35(25)
-0.17(11) 0.22(11) 0.3 (4)
0.08(17)
0 .33(22)
0.47 (8) -0.47 (8)
(4) (5) `) (5) (4) (4) (5) (3) (5) (5) (5) Y (5) (5) (4) (9) Y (5)
Initial Final level (keV) level (keV) (or reaction assignment)
A4
0 .7 -0 .6
(1) (2)
-0.50(24)
0 .17(32)
-0.76(20) 0.02 (8) -0.65(15)
0.2 (2) 0.45(10) 0.20(14)
-1 .05(13)
0 .14(17)
0 .35 (S) -0.14(20) .
-0 .05 (9) 0 .4 (4)
-0.77(26)
-0.98(43)
0 .34(15)
0 .05(30)
(4n) (4n) 1670 .E 1086 .1 n) 1779 .1 1192 .2 ') ') 755 .9 158 .E 4n 4n 2282 .5 1677.E y) ') 2001 .3 1383.E 646.8 5 .8 ') (4n) 2962 .7 2282.5 ') 3304.0+X 2614.0+X b) ') (852.3 158.6) 2702 .2 2001 .3 ') 1163 .9 448.8 ') Sn+(4n) 3n+4n 4n (3n+4n) 3469 .0 2702 .2 ') ') 1677.E 861 .9 (4291 .9 3469.0) ')
(852.3 1315.8
(4n) (4n)
5.8) 448.8 (4n) 2278.3 1383.E (4n) 2282.5 1383.E 2001 .3 2962 .7 1130 .1 158.E 1856 .E 861 .9 4n (4n)+ 22A1 4n 1510 .7 448 .8 861 .9 1998 .0 1900 .E 755 .9 (~) 1622.4 1734.8 (2148 .1
1900 .E
(4n) 448 .8 448 .8 861 .9) (4n) (4n) (~) (4n) 448 .8
') ') ') ') ') ')
') ')
')
LEVELS OF 'esIr (Il)
453
can be drawn as to the 4291 .9 keV level spin. The angular distribution of the 823 keV line is inconsistent with the expected E2 multipolarity, but this transition is contaminated by a radioactivity line. Two d7 = 2 sequences of levels decaying mainly to the g.s.b. are observed . The yrare band has the lowest observed level (IR = -) at 465.7 keV. The lowest state of the other band at 852.3 keV is proposed tentatively because both the feeding aid the de-exciting transitions are complex. Another band built on the 1622 .4 keV level has also been identified . The decay of this level to the g.s.b. and to the yrare band, together with the angular distribution of the 1173 .6 keV transition suggest Ix = ~- or i+ . 3.2. THE 21 .5 ns ISOMER LEVEL SYSTEM
In the delayed coincidence gate set on the 641 .0 keV isomeric transition the 247.5, 297.8, 323.0 and 545.5 keV transitions are clearly seen . The intensities of the 247.5 keV and 297.8 keV trânsitions (both complex) have been extracted from this delayed gate by comparing with the intensity of the 323 keV line. These data together with the prompt coincidence results allowed us to establish the existence of the 1192.2 keV and 1515 .2 keV levels. The quadrupole character of the 545 .5 keV and 570.7 keV transitions is suggested by the angular distribution data. Therefore spins and parities of ?- and - are proposed for these levels, in agreement with the systematics of the low-lying states in the h~ level system in odd-A lr isotopes . 3.3 . THE 120 ns AND 40 ns ISOMERIC STATES
The existence of two interconnected isomers was deduced from the timing and delayed coincidence measurements . The connection of the lower isomer with the g.s.b. is inferred from the prompt coincidences measured between the beam bursts . In the gate set on the 152.8 keV line we observe the 290.2, 1451 .8, 247.5 anti 256.6 keV transitions. From the time distributions of these lines we get the half-life of 120 ns for the lower isomer . No isomeric transition with half-life of 120 ns and energy above 80 keV was found. The energy of this isomer should therefore be close to 2200 keV. This isomer is also connected to the 1856.6 keV level of the band Only transitions occuring in 'sslr are listed.A complete list of y-rays as observed from bombardment of iesRe by a-particles can be obtained from the authors on request. ') See fig. 4A . n) Sce fig. 4B . `) The data for transition energies and intensities are taken from coincidence spectra only. °) The energy errors are about 0.1 keV for strong aced well resolved transitions and can attain 0.5 keV for weak or badly resolved lines. Y : transitions with a distinctly delayed component.
454
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Fig.4. Proposed levelschemefor'°sIr :A-negativeparityatates ;B-highapinisomersandpositiveparity state. The transitions marked with an asterisk are placed more than once in the level scheme.
LEVELS OF 'Belt (II)
45 5
built on the 1622.4 keV state. In the prompt coincidence gate set between the beam bursts on the 1173 .6 keV line we observe the 290.2 keV transition with an intensity much higher than that of the 152 .8 keV one, which indicates that the connecting transition has an energy close to 290 keV. In addition, in this gate we also see the 121 .8 keV line. The levels between the isomers were established on the basis of the delayed coincidences (start gates set on 99 .7, 138 .6 and 218 .5 keV lines) and from the prompt coincidences measured between the beam bursts . From the time distribution of these transitions the half-life of 40 ns is deduced for the upper isomer. For the 99 .7 keV line no prompt component was found, and therefore this transition should be the isomeric one. The time distribution of the 413.1 and 521 .7 keV lines shows a weak component close to 40 ns. The upper isomer may also be connected to the g.s.b., but the search for the connecting transition was unsuccessful . The levels above the higher-lying isomer are established on the basis of the delayed coincidences (stop gates set on 99 .7, 138.6 and 218 .5 keV lines) and from the prompt coincidences measured during the beam bursts . 3 .4 . POSITIVE PARITY STATES
A system of levels connected with the negative parity states only by transitions to the ground state has been established. Low-spin states of this system were also observed in the radioactive decay of iaspt and they were assigned s) positive parity . The high-spin states are deduced in the present work from coincidence data. This level system is connected with the 120 ns isomeric state through weak 370.2, 230 and 584.5 keV transitions. An interesting feature of the positive parity band is the weak side feeding up to several highest .levels. 4. Discussion The characteristic features of the level structure, previously identified e-8) by in-beam studies in heavier odd-A Ir isotopes, are also observed in issIr . The main differences are attributed to the higher quadrupole deformation which was calculated' 1) as e2 = 0.22 for iesIr and eZ = 0.18, 0.16 and 0.15 for ' e'Ir, '89Ir and ' 9' Ir, respectively . The deformation change has in this mass region a strong effect on the relative position of the single-particle states . This dependence is reflected in the energies of the lowest states of the rotational bands experimentally observed in odd-A Ir nuclei (see fig. 5). The h~ g.s.b., with a characteristic decoupled structure as well as the lateral band, develop to much higher spin states than those previously observed for heavier odd-A Ir nuclei . We hoped therefore that the h~ system in iesIr may be used as a good test for comparing various theoretical approaches for the description of this
456
S . ANDRÉ et al. E [MeVJ
77
sva uz.-tu»
Ir
nn fln-LSGÖ]
r /
3n.
ii
fl
/
OS rn n
slz fn -tsf 0 Lsn,rn~ssn3/a,f n 183 185
v
,\
L~ __
187 189 MA55 NUMBER
nd __
191
--
snsn~ozi
193
Fig . 5. The experimental energies of lowest states of various rotational bands in odd-A Ir isotopes . Each state is labelled by IK"[Nn~A], where the Nilsson quantum numbers refer to the main component of the intrinsic wave function .
system . Comparison of two of the models applied, one assuming axial symmetry (subsect . 4.1) and one without this assumption (subsect. 4.2), is presented below . The second negative parity level system built on the h,~ orbital is less strongly excited than in heavier lr isotopes . This is due to the relatively higher excitation energy of the i- isomer (cf. fig. 5). In the present (a, xrt) study of lsslr we did not observe low-spin states of this system (except the level at 1017.0 keV) . These states are, however, fed by the radioactive decay of'BSPt and discussed in the ref. 3). The levels of the dI = 1 band built on the 2-- isomer are more regularly spaced than in heavier Ir isotopes . As discussed previously a) in the framework of a triaxial rotor model, such a change in the level spacing is expected for a decreasing nonaxial deformation. In ie'Ir and 189Ir the observed positive parity states were reasonably well described') as two Coriolis coupled rotational bands built on ~+ [402] and ~}+ [400] Nilsson orbits . Low-spin states of these two bands were also identified a) in laslr from the radioactive decay of isspt. However, only one dl = 1 band is strongly excited in the (a, xn) reactions. The attempts of describing this band by way of simple Coriolis calculations are presented below (subsect. 4.3). The lack of unambiguous spin-parity assignments to the 120 ns and 40 ns isomeric states as well as to the 1622.4 keV state does not allow for a meaningful discussion of these levels .
LEVELS OF 'e'lr (II)
45 7
4.1 . ANALYSIS OF THE h 9 ~ z LEVEL SYSTEM ASSUMING AXIAL SYMMETRY
As in our previous work') an analysis of the h~ bands in tenors of the Nilsson model with hexadecapole deformation, pairing interaction and Coriolis coupling including also the ft subshell was performed. The calculation method and computer program is described in ref. ' z) . Here we present only the options used and describe the input parameters . The inertia parameter was allowed to vary with the level spin I according to the formula
1`)
~z
~z
(1)
_~ . +BI(I+ 1). 0
As suggested in ref. ia), the empirical attenuation coefficient of the non-diagonal matrix elements was taken as axaK . with ax = 1-aoe-~Ex, where EK is the band-head energy . All negative parity single particle states close to the Fermi surface for Z = 77 and expected to have a non-negligible influence on the ~ [541] band were included in the diagonalized matrix ; that is all the Nilsson orbits from the h~ subshell and the two lowest states from the f~ one, i.e., -~ [530] and ~ [521] . The solution was obtained by an iterative procedure, the secular equation being solved with the following free parameters : the Fermi energy ~., ~ /2.ß o and B (see eq. (1)), a o and ß (see eq. (2)), the decoupling parameter a 54 , of the ~ [541] band and the energies Essz and E 53 ~ Of the ~ [532] and ~ [530] single-particle states . All other quantities were calculated from the Nilsson model for the deformation parameters E z = 0.22 and Ea = 0.06. Twenty three negative parity levels known from our study and from the radioactive decay of gsPt (ref. were included in the fitting procedure. The fit obtained is satisfactory (see fig. 6). The results of calculations show that the main components of the wave function are ~ [541] for the ground state band and ~ [532] for the band built on the -, 465.7 keV state (both orbits from h~ subshell) as well as ~ [530] (f~ subshell) for the band built on the 852.3 keV state.
-
-
-
-
1
3))
u -,
z
-
-
-
-
4 .2 . ANALYSIS OF THE h9 ~z LEVEL SYSTEM ASSUMING A STABLE TRIAXIAL DEFORMATION
The previously described Coriolis calculations assumed the axially symmetric shape of the nucleus and treated the non-axiality as a perturbation only . In the recent years an alternative description of the odd-A nuclei in transitional regions was proposed ' S). In this approach a rigid non-axial shape of the nuclear potential
45 8
S. ANDRÉ et al.
r c~
W
Z
W Z
O Q
FU X W
Fig. 6. Comparison of the analyses of the h9rz band in terms of the Coriolis and extended asymmetric rôtor (EAR) models. The best fit parameters are x = 6.171iuE, A~/2J o = 17 .0 keV,B, _ -5 .8 eV, a = 0.135, ß = 10,0, es~z = 6.260iui~, esao = 6.353, as = 4.80 (Coriolis) and ~ = 0.342, a =. 0.914, A~/2J = 15 .3 keV (EAR). The levels marked "f" originate according to the Coriolis calculations from the f~~Z subshell . is assumed. The nonaxial rotor-plus-particle model was previously applied to a number of odd-A Ir nuclei for level systems built on the h t [ref. 's)] and
LEVELS OF ` 8 'lr (11)
459
h,~ [refs. ' " a " 1 s)] subshells. Fig. 6 shows the level energies of ' a S Ir calculated with the extended version 16) of this model (EAR). In this version the influence of the odd particle on the properties of the even-even core (the change of the moment of inertia) and the attenuation of the Coriolis matrix elements are taken into account, leaving 6 z/2.ß and a as free parameters . Two other parameters were allowed to vary, namely the Fermi energy ~, and the ydeformation. The energy gap was obtained from the masses of neighbouring nuclei . Both calculations, assuming the axial and non-axial shapes of the nucleus, reproduce the experimental level energies in a comparable way. It is worth noting that when only the h~ subshell is considered, y-must differ from zero to explain the low excitation energy of the band built on the z-, 852.3 keV state. However, this band is also well accounted for in the axial hypothesis ifboth the h~ and f~ subshells are considered . The above results show that the level energies of the h t system currently observed in this mass region cannot be used to prove the existence of a rigid non-axial shape of the odd-A nuclei . A similar conclusion, based on a smaller number of observed levels, was reached for' 8'Ir in our previous paper'). 4.3 . POSITIVE PARITY BAND
For s2 = 0.22, as expected 11) for the 'gSIr nucleus, the positive parity single particle states close to the Fermi surface are ~+[402], ~+[400] and the two states from the i,~ subshell : ~} + [660] and ~ + [651] . A band originating from the i,~ subshell was identified in 18SAu [ref. ")] and revealed a decoupled level pattern. No such structure is observed in the'sSIr case, where the dI = 1 band is developed up to the state with I = i. The Nilsson ~ +[400] and ~ + [660] states "cross" for fixed eZ = 0.22 for E 4 close to the value expected 1~ for 'B SIr. At the "crossing point" the Coriolis matrix elements and decoupling factors change abruptly their values . This can lead to the fact that the rotational bands calculated for e4 close to the crossing point may have a very different character from those for e4 away from this point. To check this possibility the Coriolis coupling calculations taking into account the s~, d~ and i,~ subshells were performed for e4 = 0.020, 0.035 (approximate position of the "crossing point") and 0 .060. Since the quantitatively good fit to the level energies was not our main goal, we did not attempt to find the set of best-fit parameters using, e.g., the method described in subsect. 4.1 for the h~ system . Instead, we performed the simple calculations assuming the inertial parameter in the form ~ z /2J = 16-O.OOSI(1+ 1) keV and taking the attenuation of Coriolis matrix elements a = 0.72 consistently with the values obtained from other isotopes . The Fermi energy ~, was set equal to that of the ~- [541] state. The results of such a calculation are presented in fig. 7. One should note that the yrast sequence for I Z ~ departs from dl = 2 character only at (and presumably
460
S. ANDRÉ et al. CORIOLIS MIXED st,~,d~ ANO i~ BANDS E 2 ~ 0.22
E [Me1/)
.5 0
0.020 0
0.035
e~
0.060
Fig. 7. The ! Z ~ levels of the positive parity band resulting from the Coriolis interaction of orbits from the s,~2 , d 3 ~2 and i,3rs subshells. The value e4 = 0.035 corresponds to the approximate position of the "crossing point" of } + [400] and }`[660] orbitals .
close to) the crossing point of the ~+[400] and ~+ [660] Nilsson states . The structure of low-spin levels is similar in all cases . In particular, a ~+ and a ~+ state, originating from the i,~ subshell, appear at relatively low excitation energies . An alternative explanation of the dI = 1 positive parity band would be the lack of influence of the i,~ subshell obtained by shifting up its energy by about 300 keV (or more). Such a shift should not to be regarded as unphysical, as the Nilsson model fails to reproduce precisely the single-particle energies in the spherical limit. In such a case a d7 = 1 band results either from the ~+[400] or the ~+ [402] orbit, but the existence of three low-lying ~+ and three ~+ levels could not be accounted for. Under this assumption the observation of only one positive parity band in the (a, xn) reaction, as distinct from heavier Ir isotopes, would also need explanation. 5. Conclos3oos Five odd-A Ir nuclei (A = 18191) were studied recently using in-beam y-ray techniques . From these studies a systematic trend in level properties as a function of the neutron number (or deformation) emerges. The level system built on the h t orbital is most strongly excited in light isotopes For which the levels of this orbital form the yrast band. This band has a characteristic rotation aligned structure as expected for orbitals lying close to the Ferrai surface
LEVELS OF 'sslr (11)
46 1
and having high-angular momentum j and small projection é2 . In taslr numerous low-spin states of the h~ system are also known from radioactivity studies . Therefore a detailed calculation of level energies in the framework of the rotational model with and without the assumption of axial symmetry of the nuclear potential was performed for this nucleus. The results appear to be insensitive to these assumptions, which indicates that the h~ level system energies cannot be used for testing the shape of odd-A lr nuclei . The level system built on the z- isomer belonging to the h,~ orbital was relatively weakly excited in taslr but reveals an appreciable feeding intensity in heavier lr isotopes . The energy levels of this system in heavier isotopes cannot be described by the rotational model with the assumption of axial symmetry . In our previous work it was shown that the triaxial rotor-plus-particle model was able to account for the main experimental features of these levels . Another possible way of explaining this system may be the particle-vibration coupling model, successfully applied in this region for odd-A Au and Hg nuclei . The positive parity level system observed in taslr from the radioactive decay of tespt develops as â single band in (a, xn) reactions. This behaviour differs from than in heavier Ir isotopes where two bands built on the ~ + [402] and ~+ [400] Nilsson orbits were observed . The structure of the taslr band was tentatively explained by simple Coriolis calculations and the assumption of such a deformation for which the ~+[400] and ~ + [660] orbits originating from the s~ and i,~ subshells lie very close to each other. The systematic studies of odd-A lr isotopes performed in the recent years have provided new information about high-spin states of these transitional nuclei . These data were largely used in the discussion of different nuclear models and, we hope, will prove their utility in the further development of these models . We are indebted to Dr. "R. Piepenbring for his comments and critical reading of the manuscript . Our thanks are also due to the cyclotron staff, to Mr . G. Margotton for his help in the electronics and to Mr. J. P. Richaud for preparing the targets. References 1) The ISOLDE Collaboration, report CERN 70-29 (1970) 2) The ISOLDE Collaboration, Proc. 3rd Int. Conf. on nuclei far from stability, Cargèse, 1976, p. 444 3) C. Schlick, J. Genevey-Rivier, V. Berg, A. Knipper, G. Walter, C. Richard-Serre and A. Hoglund, Nucl . Phys . A325 (1979) 421 4) S. André, J. Jastrzçbski, R. Kaczarowski, J. Lukasiak, J. Rivier, C. Sebille-Schlick and J. Treherne, Pros . Int. Symp. on highly excitod states in nuclei, Jûlich, 1975, p. 67 5) R. Kaczarowski, Ph .D . thesis, Rep. INR 1655/IA/Pl/B 1976 6) S. André, J. Genevey-Rivier, J. Treherne, J. Jastr~bski, R. Kaczarowski, J. ~.ukasiak, Phys . Rev. Lett. 38 (1977) 327 7) S. André, J. Boutet, J. Rivier, J. Trehtrne, J. Jastrz.~bski, J. l:.ukasiak, Z. Sujkowski and C. SebilleSchuck, Nucl . Phys. A243 (1975) 229
46 2 8) 9) 10) 11) 12) 13) 14) 15) 16) 17)
S. ANDRÉ et al. J. ~.ukasiak, Ph.D. thesis, 1978 G. Barbier, Nucl . Insu. 144 (1977) 561 H. Rubinsztein and H. M. Gustafsson, Phys . Lett . SSB (1975) 283 C. Ekström, H. Rubinsztein and P. Möller, Physics Scripts 14 (1976) 199 S. G. Nilsson, C. T. Tsang, A. Sobiczewski, S. Wycech, C. Gustafson, I. Lamm, P. Möller and B. Nilsson, Nucl . Physl A131 (1969) 1 R. Kaczarowski, Comp . Phys. Comm . 13 (1977) 63 S. A. Hjorth, A. Johnson and G. Ehrling, Nucl . Phys . A184 (1972) 113 J. Meyer-ter-Vehn, Nucl . Phys. A249 (1975) 111, 141 R. Kaczarowski and J. ~ukasiak, Proc . Int. Symp . on high-spin states and nuclear structure, Dresden, GDR, 1977, p. 45 A. C. Kahler, L. L. Riedinger, N. R. Johnson, R. L. Robinson ; E. F. Zganjar, A. Visvanathan, D. R. Zolnowski, M. B. Hughes and T. T. Sugihara, Phys. Lett . 72B (1978) 443
Nuclear Physics A325 (1979) 445-462; © North-Holland Publishing Co., Amsterdam Not to be reproduced by photoprint or microfilm without written permission from the publisher
LEVELS OF test. EXCITED IN THE (a, xn) REACTIONS t S. ANDRÉ, J. GENEVEY-R1V1ER and J. TREHERNE Institut des Sciences Nucleaires (IN1P3, USMG), 53, Avenue des Martyrs, 38026 Grenoble-Cedex, France R. KACZAROWSKI, J. LUKASIAK and J. JASTRZFrBSKI Institutefor Nuclear Research, 05-400 S`K~ierk, Poland and C. SCHllCK Nucléaire Centre de Spectrométrie et de Spectrométrie de Masse (IN1P3), 91406 Orsay, France Received 8 March 1979 Abstract : Level properties of's'lr were investigated by in-beam y-spectroscopy using (a, 4n) and (a, 6n) reactions with E, ranging from 41 to 77 MeV. A well-developed system of levels built on the h9 ~ Z subshell is excited. A band built on the tt- isomeric state is also observed . In distinction from heavier Ir isotopes, only a single positive parity band is developed to high-spin states . Two isomers with T1 ~ 2 = 120 ns and 40 ns are found for excitation energies above 2.2 MeV. The experimental results are discussed in terms of rotational models including the Coriolis coupling and providing for a stable triaxial shape of the's'Ir nucleus. E
NUCLEAR REACTIONS 's'Re(a, 4ny), ' s 'Re(a, 6ny), E = 41-77 MeV ; measured Er, a(E, Er , 0), yyt~oin, y(t) . 's'Ir deduced levels, J, rz, T,~Z . iesmlr deduced T,~Z. Enriched targets.
Ir,
1. Illti'Odlleilll~ Preliminary data on level properties of ' e sIr were obtained from the radioactive decay of tesPt (ref.')) . The latter work was continued by the ISOLDE-2 group Z) and the results are published in the preceding paper 3). In-beam studies of this isotope were initiated by the Grenoble-~wierk collaboration and partial results have already been published 4-6). These papers considered mainly the ground state band and the yrare one developed up to the levels with spins i- and ~-, respectively . In particular, in ref. e) the variation of the g.s.b. moment of inertia versus the square of angular velocity was investigated and discussed. The present paper summarizes the level properties of tasi r studied in the (a, 4n) t This work was done within the Grenoble-Swierk collaboration. 445
446
S . ANDRÉ et at .
and (a, 6n) reactions. The level systems built on the h~ and h,~ subshells are extended as compared with our preliminary results. Beside the previously known ~ - isomeric state at 646 keV [ref. 1)], the existence of two other high-spin nanosecond isomers for excitation energies above 2 MeV has been established. A system of levels forming a positive parity band is presented. Attempts to deduce the shape of the nucleus from the analysis of the h.~ band in terms of the rotational models with or without the assumption of the axial symmetry are presented. Trends resulting from the analysis of odd-A Ir nuclei (A = 183-191) are discussed. 2. Experimental proced~ue and results The experiments have been carried out using the a-particle beam of the Grenoble variable energy cyclotron. The ' a s Ir nucleus was produced in the 1 a sRe(a, 4n) and l8'Re(a, 6n) reactions. Metallic Re powder targets approximately 20 mg/cm Z thick were used, deposited by centrifuging on a 1 mg/cmZ mylar backing. The target material was enriched in 'e'Re to 99 .2 ~ and in iesRe to 96 .7 ~. Measurements of the excitation.function were performed at Ea = 41, 47, 51 and 56 MeV (iesRe target) and at Ea = 72 and 77 MeV (18'Re target) with two Ge(Li) detectors (one of 2 .5 cm a and another of 10 ~ efficiency). For each energy the spectra were measured during and between the beam bursts. The beam energy corresponding to the cross section maximum for the (a, 4n) reaction was found to be 51 MeV. At this energy the y-ray angular distributions were measured independently by two Ge(Li) detectors placed at angles of 25°, 37°, 52°, 70° and 90° with respect to the beam direction. The normalization procedure was similar to that used in our previous work'' 8). Table 1 lists the properties of the y-lines attributed to the'BSIr nucleus and fig. 1 shows examples of the spectra. The three-parameter EYEYt coincidences were measured at bombarding energies of 51 and 77 MeV for the (a, 4n) and (a, 6n) reaction, respectively . The data were recorded event by event on magnetic tape . Each event was labelled with a bit set 0 if the "start" y-ray was detected during the beam pulse and 1 otherwise. The size of the coincidence matrix was 2048 x 1024 x 128. The data were sorted off-line using the method described in ref. 9). Fig. 2 shows examples of the coincidence spectra. The timing measurements were performed in two ways : the time distributions were measured with respect to the beam bursts or extracted from the y-y-t experiment . Fig. 3 shows the time distribution of several transitions. Three isomers with half-lives in the nanosecond range were found. The measured half-life' of the first one was T~ = 21 .5 f 2.0 ns, which is in agreement with the previous result 1). Two new isomers of high excitation energy were observed whose half-lives were 120 + 20 ns and 40 + 10 ns.
LEVELS OF 'eslr (Il)
447
W Z Z 6 2 U W O. N FZ O U
W Z Z
= U O .10E04 C W S N N Z O U
O .10E02
0
200
400
600
800 1000 CHANNEL NUMBER
1200
1400
1600
Fig. 1 . Examples of y-ray spectra: A - coincident with beam bursts ; B - measured between beam bursts. 1'he spectra were obtained during irradiation of the 'esRe target with a-particle of 51 MeV.
S . ANDRÉ et a! .
448
W
W Z Z
300
400
500 600 CHANNEL NUM~_R
700
800
900
1000
W
300
400
500 600 700 800900 CHANNEL NUMBER
1000 1100 1200 1300 14001500
Fig . 2 . Examples of coincidence spectra.
449
LEVELS OF ' BS Ir (11) NF Z 0u
30000-
99- 7 keV Tt,.~ ~40 ns
40
_ .~~r
20
30
10000 = . .. 5000TIME Cns)
10
138-6 + 137. 2 kèJ
ôV = Tth
N
10000-~
40 ns
50001
O i I
40
I
30
NZ
ôu
I
10
20
" , 2000L
TIME Cns) 2000
2566 keV Tt,~ ~120 ns
1000 =
"
..
ô mz
I
30 Nr
I
I
I
20
641 .0 keV Tthti21 ns
~
I
10
I
300
t
TIME Cns) 1000
..,
500
0
200-
m z
40
I
30
~
20
I
10
I
I
TIME Cns)
I
Fig . 3 . Time distributions of selected ;-transitions .
3. The level scheme of tssU
The level scheme of taslr is presented in fig . 4, and several selected features of this scheme are discussed below. 3 .1 . THE GROUND STATE ROTATIONAL BAND AND RELATED LEVELS
The measured ground state spin of tasl r was found to be ~ and interpreted as the lowest member of the rotational band built on the ~ - [541] Nilsson orbital to. tt ). In our study a strong cascade of the stretched E2 transition is observed . In accordance with the relative yields, excitation functions and angular distributions (table 1) these transitions were used to construct a decoupled g.s.b. The highest certain spin value in this band is -~ for the 3469.0 keV level. No definite conclusion
S . ANDRÉ et al.
450
TAHLE 1
Summary of y-ray data obtained from the 'e s Re(a, 4n) reaction E~ (keV) a)
Iq Ea =51MeV
AZ
A,
84 .0 92 .1 97 .4 99 .7 105 .E 106 .9 112 .7 114 .0 121 .8 125 .4 126.9 137.2 138.E 141 .2 152 .8 155 .7 161 .9 165 .7 169 .3 170 .2 178 .7 182 .9 184.2 185 .9 187 .4 201 .7 202 .8 205 .2
1 .5 (2) 1 .9 (2) 1 .7 (2) 4 .6 (3) Y 2 .4 (2) 1 .3 (2) 1 .3 (2) 3 .9 (2) 2.0 (2) 4 .2 (3) 1 .8 (2) 6 .2 (6) `)Y 6 .4 (6) Y 5 .9 (4) 100 (4) 5 .0 (8) 0.3 (2) 1 .6 (6) 1 .6 (3) 5 .6 (4) 2.4 (5) 4 .4 (5) 9 .2(10) `)Y 3 .8 (6) 4 .0 (4) 2 .7 (3) 3 .9 (4) 10.3 (5)
-0 .13 (8)
0 .06(14)
-0 .01 (3) -0 .39 (6) 0 .14(10) -0 .29(12)
-0 .07 (6) 0 .00 (9) -0 .21(15) 0 .42(21)
- 0.57 (9) 0.02 (6) - 0.15(12) -0 .12 (5) -0 .31 (6) -0 .06 (7) 0 .28 (2) -0 .54(12) -0 .9 (4) -0 .8 (4) 0 .2 (2) -0.60 (6) -0.33 (8)
0.47(13) -0.13(12) 0.14(16) 0.09 (8) 0.1E (9) 0.08(13) -0 .07 (3) 0 .07(20) 0 .05(66) -0 .1 (6) 0 .9 (3) 0 .09(10) -0 .25(12)
0 .28 (4)
0.28 (6)
(2) (1) (9) (4)
0.2 (3) -0.05(17) -0 .11(14) -0 .17 (6)
210 .3 212 .E 214.4 218 .5
3 .0 (8) 12.1 (5) 4.2 (3) 8.7 (9)`)Y 7.2 (9) °) 8 .2 (6) 43 .5(17) Y
0 .0 (3) 0 .05 (8)
0 .4 (4) 0 .04(13)
0.05 (4)
0 .00 (7)
-0.01 (5) 0.01 (2)
-0.01 (9) -0.04 (4)
-0 .18 (4)
0 .03 (6)
-0.17 (8) 0.11 (5) 0.01(10) 0 .21(10)
0 .02(10) 0 .25(10) -0.08(16) -0.47(16)
226 .3 229 .E 230 .0 °) 231 °) 247 .5 256 .E 263 .9 267 .0 277 .8
5 .9(10) °) 7.3(10) °)Y ~ 7 .6 (5) Y 2 .5 (5) °) 6 .3 (6) 3 .3 (8) `) 13
.1
0 .4 -0 .7 -0 .04 -0 .14
lnitial Final level (keV) level (keV) (or reaction assignment) 418 .7
335 .2 °) 4n 4n 2614 .0+X 2514.3+X°) 229 .E °) 335 .2 442 .2 335 .2 °) ') 1734.8 1622 .4 442 .2 °) 556.0 1856.E 1734 .8 ') 4n+' es Re 4n + R'eslr 556 .0 418 .7 °) 2295 .8+X 2157 .2+X ° ) 696 .7 556 .0 °) ') 158 .E 5 .8 1856 .E ') 2012 .3 496 .E 335 .2 °) 3n+(4n) (4n) 2182 .5 2012 .3 ') 4n 4n 696 .7 °) 880 .9 (4n) 4n 4n 1745 .4 °) 1948 .2 °) 1086 .1 880.9 2392.8 ') 2598 .0 2153 .0 1948 .2 °) 2182 .5 ') 2392 .8 442 .2 229 .E °) 1530.8 °) 1745 .4 2514.3+X 2295 .8+X °) °) 1304.5 1086.1 I304.5 °) 1530.8 0 °) 229 .E 1900 .E 1670 .E °) 3171 .E+X 2940 .4+X °) 4n ') 1192 .2 ( 944 .E 2148 .1 1900 .E °) 2157 .2 1900 .E °) 1779.1 1515 .2 ') 229 .E °) 496.E 696 .7 418 .7 °) 1734 .8 ') 2012 .3 (1130.1 852 .3) ')
LEVELS OF 'aslr (II)
451
T~at .e I (continued) E~ (keV) °) 290 `) 290 .2 291 .0 `) 297 .8 307 .4 312 .E 313 .E 317 .E 323 .0 325 .2 326 .4 335 .2 351 .E 361 .8 363 .E 370 .2 380 .E 390 .0 407 .2 4p9 .0 ~) 413 .1 414.0 `) 416 .8 417 .E } 418 .7 423 .E 427 .1 434.9 441 .1 444.8 457 .2 459 .9 465 .2 469 .8 486.0 492 .3 514.0 `) 521 .7 542.9 545 .5 557 .9 559.7 570.7
iY E,=51MeV
151 (6)
Y
8 (2) 5 .0 (4) 3 .2 3 .9 2 .0 4.5 7 .0 8 .1
(4) (4) (6) (4) (6j (6)
8.7 (6) 1 .5 `) 4 .4 (6) 5 (1) 1 .4 (6) `) 3 .8(13) 2 .6 (4) `) 3 .2 (5) 19 .2(10) Y 4 .9 (8) 104 (4)
Y
A2
A4
0 .28 (2)
-0 .11 (3)
0 .18 (6) -0 .43(11)
0 .39 (9) -0 .18(17)
0.24(17) 0.52(16) 0.2 (1) 0.15(15) 0.25(10) 0.34 (9)
-0 .04(26) 0 .12(24) 0.1 (3) -0.08(20) 0.04(15) -0.08(13)
-0 .03 (6) 0 .1 (2) 0 .01(13) 0 .09(11) 0 .32(10)
0.02 (9) 0 .7 (4) -0 .1 (2) -0.21(IS) -0 .12(15)
0 .29 (6)
-0 .21 (9)
0 .15(17)
-0 .39(26)
0.30 (3)
-0.14 (4)
7 .5(10) 6 .1 9.3(10) 2.9 (7) 11 .E (7) 7.6 (4) `) 14 .0 (7) 1 .8 (5) 8 .6 (6) 2 .9 (4) 2.5 (4) 5 .2 (4) 3 .8 (6) 61 .6(25) 4 .4 (6) 5.915) 4 .8 (5) 3 .4 (8) 2.1 (6) `) 3 .7 (6) `)
0.33(12) 0.2 (4)
- 0.30(17) -0 .4 (6)
0 .09 (7)
-0 .47(10)
-0.31 (7) 0.26(16) 0.14(19) 0 .49 (9) -0 .03(20)
-0.20(10) -0.12(26) -0.02(36) -0.32(13) 0 .01(30)
0 .32 (3)
-0.12 (5)
-0 .13(14) 0.30(10)
-0 .12(20) 0.04(14)
0 .31(10)
-0.15(14)
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S . ANDRÉ et al .
452
T~HI~ 1 (continuel)
Er (keV) a) 574 .0 580 .8 584.5 587 .1 597 .3 600.5 603 .8 60E `) 617 .7 641 .0 666 .8 680 .2 690 ~) 693 .E 700 .9 715 .1 718 .2 728 .4 741 .E 759 .E 766 .8 815 .7 822.9 835 .2 843 .2 846 .5 867 .2 891 .2 894.8
Ir E,=51MeV 3 .2 (6) 3.5 (5) 3.1 (6) 3.4 (5) 19.7(10) 5 .9(10) 4.2(10) 30.4(15) 19 .4 (9) 3 .1 (6) 2.4 (5)
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898 .9 961 .4 971 .5 994,4 1007 .8 1014 .4 1058 .0 1061 .9 1136 .1 1144 .E 1150 .7 1155 .4 1167 .0 1173 .E 1286.2
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(4n) (4n) 1670 .E 1086 .1 n) 1779 .1 1192 .2 ') ') 755 .9 158 .E 4n 4n 2282 .5 1677.E y) ') 2001 .3 1383.E 646.8 5 .8 ') (4n) 2962 .7 2282.5 ') 3304.0+X 2614.0+X b) ') (852.3 158.6) 2702 .2 2001 .3 ') 1163 .9 448.8 ') Sn+(4n) 3n+4n 4n (3n+4n) 3469 .0 2702 .2 ') ') 1677.E 861 .9 (4291 .9 3469.0) ')
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5.8) 448.8 (4n) 2278.3 1383.E (4n) 2282.5 1383.E 2001 .3 2962 .7 1130 .1 158.E 1856 .E 861 .9 4n (4n)+ 22A1 4n 1510 .7 448 .8 861 .9 1998 .0 1900 .E 755 .9 (~) 1622.4 1734.8 (2148 .1
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LEVELS OF 'esIr (Il)
453
can be drawn as to the 4291 .9 keV level spin. The angular distribution of the 823 keV line is inconsistent with the expected E2 multipolarity, but this transition is contaminated by a radioactivity line. Two d7 = 2 sequences of levels decaying mainly to the g.s.b. are observed . The yrare band has the lowest observed level (IR = -) at 465.7 keV. The lowest state of the other band at 852.3 keV is proposed tentatively because both the feeding aid the de-exciting transitions are complex. Another band built on the 1622 .4 keV level has also been identified . The decay of this level to the g.s.b. and to the yrare band, together with the angular distribution of the 1173 .6 keV transition suggest Ix = ~- or i+ . 3.2. THE 21 .5 ns ISOMER LEVEL SYSTEM
In the delayed coincidence gate set on the 641 .0 keV isomeric transition the 247.5, 297.8, 323.0 and 545.5 keV transitions are clearly seen . The intensities of the 247.5 keV and 297.8 keV trânsitions (both complex) have been extracted from this delayed gate by comparing with the intensity of the 323 keV line. These data together with the prompt coincidence results allowed us to establish the existence of the 1192.2 keV and 1515 .2 keV levels. The quadrupole character of the 545 .5 keV and 570.7 keV transitions is suggested by the angular distribution data. Therefore spins and parities of ?- and - are proposed for these levels, in agreement with the systematics of the low-lying states in the h~ level system in odd-A lr isotopes . 3.3 . THE 120 ns AND 40 ns ISOMERIC STATES
The existence of two interconnected isomers was deduced from the timing and delayed coincidence measurements . The connection of the lower isomer with the g.s.b. is inferred from the prompt coincidences measured between the beam bursts . In the gate set on the 152.8 keV line we observe the 290.2, 1451 .8, 247.5 anti 256.6 keV transitions. From the time distributions of these lines we get the half-life of 120 ns for the lower isomer . No isomeric transition with half-life of 120 ns and energy above 80 keV was found. The energy of this isomer should therefore be close to 2200 keV. This isomer is also connected to the 1856.6 keV level of the band Only transitions occuring in 'sslr are listed.A complete list of y-rays as observed from bombardment of iesRe by a-particles can be obtained from the authors on request. ') See fig. 4A . n) Sce fig. 4B . `) The data for transition energies and intensities are taken from coincidence spectra only. °) The energy errors are about 0.1 keV for strong aced well resolved transitions and can attain 0.5 keV for weak or badly resolved lines. Y : transitions with a distinctly delayed component.
454
S . ANDRÉ et al. ___r__az91.9 1 i i '" 1 ?1 1 372 -~ 1e69~o
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Fig.4. Proposed levelschemefor'°sIr :A-negativeparityatates ;B-highapinisomersandpositiveparity state. The transitions marked with an asterisk are placed more than once in the level scheme.
LEVELS OF 'Belt (II)
45 5
built on the 1622.4 keV state. In the prompt coincidence gate set between the beam bursts on the 1173 .6 keV line we observe the 290.2 keV transition with an intensity much higher than that of the 152 .8 keV one, which indicates that the connecting transition has an energy close to 290 keV. In addition, in this gate we also see the 121 .8 keV line. The levels between the isomers were established on the basis of the delayed coincidences (start gates set on 99 .7, 138 .6 and 218 .5 keV lines) and from the prompt coincidences measured between the beam bursts . From the time distribution of these transitions the half-life of 40 ns is deduced for the upper isomer. For the 99 .7 keV line no prompt component was found, and therefore this transition should be the isomeric one. The time distribution of the 413.1 and 521 .7 keV lines shows a weak component close to 40 ns. The upper isomer may also be connected to the g.s.b., but the search for the connecting transition was unsuccessful . The levels above the higher-lying isomer are established on the basis of the delayed coincidences (stop gates set on 99 .7, 138.6 and 218 .5 keV lines) and from the prompt coincidences measured during the beam bursts . 3 .4 . POSITIVE PARITY STATES
A system of levels connected with the negative parity states only by transitions to the ground state has been established. Low-spin states of this system were also observed in the radioactive decay of iaspt and they were assigned s) positive parity . The high-spin states are deduced in the present work from coincidence data. This level system is connected with the 120 ns isomeric state through weak 370.2, 230 and 584.5 keV transitions. An interesting feature of the positive parity band is the weak side feeding up to several highest .levels. 4. Discussion The characteristic features of the level structure, previously identified e-8) by in-beam studies in heavier odd-A Ir isotopes, are also observed in issIr . The main differences are attributed to the higher quadrupole deformation which was calculated' 1) as e2 = 0.22 for iesIr and eZ = 0.18, 0.16 and 0.15 for ' e'Ir, '89Ir and ' 9' Ir, respectively . The deformation change has in this mass region a strong effect on the relative position of the single-particle states . This dependence is reflected in the energies of the lowest states of the rotational bands experimentally observed in odd-A Ir nuclei (see fig. 5). The h~ g.s.b., with a characteristic decoupled structure as well as the lateral band, develop to much higher spin states than those previously observed for heavier odd-A Ir nuclei . We hoped therefore that the h~ system in iesIr may be used as a good test for comparing various theoretical approaches for the description of this
456
S . ANDRÉ et al. E [MeVJ
77
sva uz.-tu»
Ir
nn fln-LSGÖ]
r /
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ii
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/
OS rn n
slz fn -tsf 0 Lsn,rn~ssn3/a,f n 183 185
v
,\
L~ __
187 189 MA55 NUMBER
nd __
191
--
snsn~ozi
193
Fig . 5. The experimental energies of lowest states of various rotational bands in odd-A Ir isotopes . Each state is labelled by IK"[Nn~A], where the Nilsson quantum numbers refer to the main component of the intrinsic wave function .
system . Comparison of two of the models applied, one assuming axial symmetry (subsect . 4.1) and one without this assumption (subsect. 4.2), is presented below . The second negative parity level system built on the h,~ orbital is less strongly excited than in heavier lr isotopes . This is due to the relatively higher excitation energy of the i- isomer (cf. fig. 5). In the present (a, xrt) study of lsslr we did not observe low-spin states of this system (except the level at 1017.0 keV) . These states are, however, fed by the radioactive decay of'BSPt and discussed in the ref. 3). The levels of the dI = 1 band built on the 2-- isomer are more regularly spaced than in heavier Ir isotopes . As discussed previously a) in the framework of a triaxial rotor model, such a change in the level spacing is expected for a decreasing nonaxial deformation. In ie'Ir and 189Ir the observed positive parity states were reasonably well described') as two Coriolis coupled rotational bands built on ~+ [402] and ~}+ [400] Nilsson orbits . Low-spin states of these two bands were also identified a) in laslr from the radioactive decay of isspt. However, only one dl = 1 band is strongly excited in the (a, xn) reactions. The attempts of describing this band by way of simple Coriolis calculations are presented below (subsect. 4.3). The lack of unambiguous spin-parity assignments to the 120 ns and 40 ns isomeric states as well as to the 1622.4 keV state does not allow for a meaningful discussion of these levels .
LEVELS OF 'e'lr (II)
45 7
4.1 . ANALYSIS OF THE h 9 ~ z LEVEL SYSTEM ASSUMING AXIAL SYMMETRY
As in our previous work') an analysis of the h~ bands in tenors of the Nilsson model with hexadecapole deformation, pairing interaction and Coriolis coupling including also the ft subshell was performed. The calculation method and computer program is described in ref. ' z) . Here we present only the options used and describe the input parameters . The inertia parameter was allowed to vary with the level spin I according to the formula
1`)
~z
~z
(1)
_~ . +BI(I+ 1). 0
As suggested in ref. ia), the empirical attenuation coefficient of the non-diagonal matrix elements was taken as axaK . with ax = 1-aoe-~Ex, where EK is the band-head energy . All negative parity single particle states close to the Fermi surface for Z = 77 and expected to have a non-negligible influence on the ~ [541] band were included in the diagonalized matrix ; that is all the Nilsson orbits from the h~ subshell and the two lowest states from the f~ one, i.e., -~ [530] and ~ [521] . The solution was obtained by an iterative procedure, the secular equation being solved with the following free parameters : the Fermi energy ~., ~ /2.ß o and B (see eq. (1)), a o and ß (see eq. (2)), the decoupling parameter a 54 , of the ~ [541] band and the energies Essz and E 53 ~ Of the ~ [532] and ~ [530] single-particle states . All other quantities were calculated from the Nilsson model for the deformation parameters E z = 0.22 and Ea = 0.06. Twenty three negative parity levels known from our study and from the radioactive decay of gsPt (ref. were included in the fitting procedure. The fit obtained is satisfactory (see fig. 6). The results of calculations show that the main components of the wave function are ~ [541] for the ground state band and ~ [532] for the band built on the -, 465.7 keV state (both orbits from h~ subshell) as well as ~ [530] (f~ subshell) for the band built on the 852.3 keV state.
-
-
-
-
1
3))
u -,
z
-
-
-
-
4 .2 . ANALYSIS OF THE h9 ~z LEVEL SYSTEM ASSUMING A STABLE TRIAXIAL DEFORMATION
The previously described Coriolis calculations assumed the axially symmetric shape of the nucleus and treated the non-axiality as a perturbation only . In the recent years an alternative description of the odd-A nuclei in transitional regions was proposed ' S). In this approach a rigid non-axial shape of the nuclear potential
45 8
S. ANDRÉ et al.
r c~
W
Z
W Z
O Q
FU X W
Fig. 6. Comparison of the analyses of the h9rz band in terms of the Coriolis and extended asymmetric rôtor (EAR) models. The best fit parameters are x = 6.171iuE, A~/2J o = 17 .0 keV,B, _ -5 .8 eV, a = 0.135, ß = 10,0, es~z = 6.260iui~, esao = 6.353, as = 4.80 (Coriolis) and ~ = 0.342, a =. 0.914, A~/2J = 15 .3 keV (EAR). The levels marked "f" originate according to the Coriolis calculations from the f~~Z subshell . is assumed. The nonaxial rotor-plus-particle model was previously applied to a number of odd-A Ir nuclei for level systems built on the h t [ref. 's)] and
LEVELS OF ` 8 'lr (11)
459
h,~ [refs. ' " a " 1 s)] subshells. Fig. 6 shows the level energies of ' a S Ir calculated with the extended version 16) of this model (EAR). In this version the influence of the odd particle on the properties of the even-even core (the change of the moment of inertia) and the attenuation of the Coriolis matrix elements are taken into account, leaving 6 z/2.ß and a as free parameters . Two other parameters were allowed to vary, namely the Fermi energy ~, and the ydeformation. The energy gap was obtained from the masses of neighbouring nuclei . Both calculations, assuming the axial and non-axial shapes of the nucleus, reproduce the experimental level energies in a comparable way. It is worth noting that when only the h~ subshell is considered, y-must differ from zero to explain the low excitation energy of the band built on the z-, 852.3 keV state. However, this band is also well accounted for in the axial hypothesis ifboth the h~ and f~ subshells are considered . The above results show that the level energies of the h t system currently observed in this mass region cannot be used to prove the existence of a rigid non-axial shape of the odd-A nuclei . A similar conclusion, based on a smaller number of observed levels, was reached for' 8'Ir in our previous paper'). 4.3 . POSITIVE PARITY BAND
For s2 = 0.22, as expected 11) for the 'gSIr nucleus, the positive parity single particle states close to the Fermi surface are ~+[402], ~+[400] and the two states from the i,~ subshell : ~} + [660] and ~ + [651] . A band originating from the i,~ subshell was identified in 18SAu [ref. ")] and revealed a decoupled level pattern. No such structure is observed in the'sSIr case, where the dI = 1 band is developed up to the state with I = i. The Nilsson ~ +[400] and ~ + [660] states "cross" for fixed eZ = 0.22 for E 4 close to the value expected 1~ for 'B SIr. At the "crossing point" the Coriolis matrix elements and decoupling factors change abruptly their values . This can lead to the fact that the rotational bands calculated for e4 close to the crossing point may have a very different character from those for e4 away from this point. To check this possibility the Coriolis coupling calculations taking into account the s~, d~ and i,~ subshells were performed for e4 = 0.020, 0.035 (approximate position of the "crossing point") and 0 .060. Since the quantitatively good fit to the level energies was not our main goal, we did not attempt to find the set of best-fit parameters using, e.g., the method described in subsect. 4.1 for the h~ system . Instead, we performed the simple calculations assuming the inertial parameter in the form ~ z /2J = 16-O.OOSI(1+ 1) keV and taking the attenuation of Coriolis matrix elements a = 0.72 consistently with the values obtained from other isotopes . The Fermi energy ~, was set equal to that of the ~- [541] state. The results of such a calculation are presented in fig. 7. One should note that the yrast sequence for I Z ~ departs from dl = 2 character only at (and presumably
460
S. ANDRÉ et al. CORIOLIS MIXED st,~,d~ ANO i~ BANDS E 2 ~ 0.22
E [Me1/)
.5 0
0.020 0
0.035
e~
0.060
Fig. 7. The ! Z ~ levels of the positive parity band resulting from the Coriolis interaction of orbits from the s,~2 , d 3 ~2 and i,3rs subshells. The value e4 = 0.035 corresponds to the approximate position of the "crossing point" of } + [400] and }`[660] orbitals .
close to) the crossing point of the ~+[400] and ~+ [660] Nilsson states . The structure of low-spin levels is similar in all cases . In particular, a ~+ and a ~+ state, originating from the i,~ subshell, appear at relatively low excitation energies . An alternative explanation of the dI = 1 positive parity band would be the lack of influence of the i,~ subshell obtained by shifting up its energy by about 300 keV (or more). Such a shift should not to be regarded as unphysical, as the Nilsson model fails to reproduce precisely the single-particle energies in the spherical limit. In such a case a d7 = 1 band results either from the ~+[400] or the ~+ [402] orbit, but the existence of three low-lying ~+ and three ~+ levels could not be accounted for. Under this assumption the observation of only one positive parity band in the (a, xn) reaction, as distinct from heavier Ir isotopes, would also need explanation. 5. Conclos3oos Five odd-A Ir nuclei (A = 18191) were studied recently using in-beam y-ray techniques . From these studies a systematic trend in level properties as a function of the neutron number (or deformation) emerges. The level system built on the h t orbital is most strongly excited in light isotopes For which the levels of this orbital form the yrast band. This band has a characteristic rotation aligned structure as expected for orbitals lying close to the Ferrai surface
LEVELS OF 'sslr (11)
46 1
and having high-angular momentum j and small projection é2 . In taslr numerous low-spin states of the h~ system are also known from radioactivity studies . Therefore a detailed calculation of level energies in the framework of the rotational model with and without the assumption of axial symmetry of the nuclear potential was performed for this nucleus. The results appear to be insensitive to these assumptions, which indicates that the h~ level system energies cannot be used for testing the shape of odd-A lr nuclei . The level system built on the z- isomer belonging to the h,~ orbital was relatively weakly excited in taslr but reveals an appreciable feeding intensity in heavier lr isotopes . The energy levels of this system in heavier isotopes cannot be described by the rotational model with the assumption of axial symmetry . In our previous work it was shown that the triaxial rotor-plus-particle model was able to account for the main experimental features of these levels . Another possible way of explaining this system may be the particle-vibration coupling model, successfully applied in this region for odd-A Au and Hg nuclei . The positive parity level system observed in taslr from the radioactive decay of tespt develops as â single band in (a, xn) reactions. This behaviour differs from than in heavier Ir isotopes where two bands built on the ~ + [402] and ~+ [400] Nilsson orbits were observed . The structure of the taslr band was tentatively explained by simple Coriolis calculations and the assumption of such a deformation for which the ~+[400] and ~ + [660] orbits originating from the s~ and i,~ subshells lie very close to each other. The systematic studies of odd-A lr isotopes performed in the recent years have provided new information about high-spin states of these transitional nuclei . These data were largely used in the discussion of different nuclear models and, we hope, will prove their utility in the further development of these models . We are indebted to Dr. "R. Piepenbring for his comments and critical reading of the manuscript . Our thanks are also due to the cyclotron staff, to Mr . G. Margotton for his help in the electronics and to Mr. J. P. Richaud for preparing the targets. References 1) The ISOLDE Collaboration, report CERN 70-29 (1970) 2) The ISOLDE Collaboration, Proc. 3rd Int. Conf. on nuclei far from stability, Cargèse, 1976, p. 444 3) C. Schlick, J. Genevey-Rivier, V. Berg, A. Knipper, G. Walter, C. Richard-Serre and A. Hoglund, Nucl . Phys . A325 (1979) 421 4) S. André, J. Jastrzçbski, R. Kaczarowski, J. Lukasiak, J. Rivier, C. Sebille-Schlick and J. Treherne, Pros . Int. Symp. on highly excitod states in nuclei, Jûlich, 1975, p. 67 5) R. Kaczarowski, Ph .D . thesis, Rep. INR 1655/IA/Pl/B 1976 6) S. André, J. Genevey-Rivier, J. Treherne, J. Jastr~bski, R. Kaczarowski, J. ~.ukasiak, Phys . Rev. Lett. 38 (1977) 327 7) S. André, J. Boutet, J. Rivier, J. Trehtrne, J. Jastrz.~bski, J. l:.ukasiak, Z. Sujkowski and C. SebilleSchuck, Nucl . Phys. A243 (1975) 229
46 2 8) 9) 10) 11) 12) 13) 14) 15) 16) 17)
S. ANDRÉ et al. J. ~.ukasiak, Ph.D. thesis, 1978 G. Barbier, Nucl . Insu. 144 (1977) 561 H. Rubinsztein and H. M. Gustafsson, Phys . Lett . SSB (1975) 283 C. Ekström, H. Rubinsztein and P. Möller, Physics Scripts 14 (1976) 199 S. G. Nilsson, C. T. Tsang, A. Sobiczewski, S. Wycech, C. Gustafson, I. Lamm, P. Möller and B. Nilsson, Nucl . Physl A131 (1969) 1 R. Kaczarowski, Comp . Phys. Comm . 13 (1977) 63 S. A. Hjorth, A. Johnson and G. Ehrling, Nucl . Phys . A184 (1972) 113 J. Meyer-ter-Vehn, Nucl . Phys. A249 (1975) 111, 141 R. Kaczarowski and J. ~ukasiak, Proc . Int. Symp . on high-spin states and nuclear structure, Dresden, GDR, 1977, p. 45 A. C. Kahler, L. L. Riedinger, N. R. Johnson, R. L. Robinson ; E. F. Zganjar, A. Visvanathan, D. R. Zolnowski, M. B. Hughes and T. T. Sugihara, Phys. Lett . 72B (1978) 443