Isomerism in neutron-deficient iridium isotopes alpha- and beta-decay studies of 171–175Ir

NUCLEAR PHYSICS A

Nuclear Physics A545 (1992) 646-664 North-Holland

Isomerism in neutron-deficient iridium isotopes* Alpha- and beta-decay studies of " t-t "Ir

W.-D.

Schmidt-Ott, I-I. Salewski, F. Meissner, U . Bosch-Wicke, P. Koschel and V. Kunze

IL Phvsikaliscl:es Institut, Universität Göttingen, D-3400 Göttingen, Germane

R. Michaelsen

Hahn-Meitner-Institut, D-1000 Berlin 31, Germant Received 18 October 1991 (Revised 18 March 1992) Abstract: The a- and ß-decays of "' - "S lr were measured in ' 4 'Pr( ;6 Ar, xn) reactions. New a-rays and a-coincident y-rays were found for "' - " 4 Ir, giving evidence for isomerism in these nuclei. The measured a- and /3-decays of "' -"~Ir are interpreted in terms of single-particle nuclear structure. E

RADIOACTIVITY '''- "S lr [from "' Pr( ;6Ar, xn)]; measured E , I , T,1,(a), E, l, ay-, yy-coin ; deduced Q,,, a-branchings. 168,1'`',"'',"' .""Re, "`',";'"SOS deduced levels, J, a. Si, Ge detectors.

l . Introduction

During the last years, fundamental decay properties of neutron-deficient isotopes For the lighter of these of hafnium-to-platinum have been investigated [refs. isotopes, a-decay is the prominent decay mode. The a-branchings are rapidly decreasing and .8-decay comes forward when proceeding towards the line of 13stability. Nevertheless, in many cases little was known on 13-delayed y-decays in this region of the isotopic chart. 4)] and ' 79,' K°Ir Recently we reported new results on the ,6-decays of "6, "'Ir [ref. [ref. K )] . In the course of these systematic investigations we discovered the ß-decays of the isotopes "2- ' 74 In First results on these isotopes, including their a- and ß-decays, have been presented [refs. 9- ")] and are given here in more detail . In addition to the a-decays with one a-line per isotope known so far [refs. ''- '4)], the measurements of "`" '74 Ir revealed complex a-decay schemes . For each of the Correspondence to: Dr. W.-D. Schmidt-Ott, II . Physikalisches Institut, Universität Göttingen, D-3400 Göttingen, Germany . This work has been funded by the German Federal Minister for Research and Tec`unology (BMFT) under contract number 06G6105 . 0375-9474/92/$05 .00

ou

1992 - Elsevier Science Publishers B.V. All rights reserved

W.-D. Schmidt-Oit et al. / Isomerism

64 7

isotopes "2-"4 Ir, two a-decay half-lives were measured, giving evidence for isomerism. The ßf3-delayed y-decay, which was seen for the first time [refs. 9_")], is correspondingly interpreted by the feeding of osmium daughter levels from two isomeric states . A similar situation was recently found in our study of the neighbouring rhenium isotopes [ref. '5 )]. For " 5 Ir, where only a-decay was known [tef.'6)], we found ß-delayed y-rays. For the lightest iridium isotope investigated in this work, "'Ir, a-delayed y-decay was measured . The measurement of coincident y-rays following the adopted a-decays of "' -"4 Ir [refs. ' 2-'4, ")] causes an increase of the "5 Ir, the level structure of the a-daughter nucleus Qa values of these nuclei . Also for "' Re [ref. 'K)] leads to an increase of the Q« value. 2. Experimental procedure

Monoisotopic 14' Pr targets of 2.3 mg/cm2 thickness were irradiated with 36Ar ions of 234 MeV primary energy and 5-6 particle - nA beam intensity at the VICKSI accelerator of the Hahn-Meitner-Institut in Berlin . The recoiling reaction products were thermalized in helium gas and collected with a heliumjet system combined with a fast transport tape [refs . ")] . Samples were periodically moved in front of energy- and efficiency-calibrated detectors. A cimuiar a-detector of 24 mm diameter and a thickness of 100 Rm and of 18% efficiency, and two high-resolution y-detectors of the type y/X and LOAX were used . The detectors were connected with standard coincidence set-ups. Coincidence data were stored in list-mode on magnetic tape. A coincidence efficiency of 91% was obtained for ay-coincidences with a range of 1 Rs of the time-to-amplitude converters . Each event was also tagged with the time elapsed since the last tape movement for the half-life analyis. Excitation functions of a- and y-rays were measured by inserting tantalum degrader foils of up to 4.4 mg/cm2 thickness in front of the target . Summing lines of a-rays and of K-, and L+ - - - conversion electrons from prompt transitions were used for conversion measurements. 3. Identification of a-rays and a-delayed y-rays

In order to assign new a- and y-rays several criteria were used. Excitation functions of measured a-rays determine the mass number. Element assignment is given if X-rays are observed in coincidence with the a-rays. Alpha-coincident -Y-rays may sometimes fit into the known level scheme of the daughter isotope and energy sums of a- and y-rays may correspond to another detected a-ray. A further identification sign is provided by the half-life measurements . Alpha-ray singles spectra measured at three different irradiation energies are displayed in fig. la-c. The earlier known, the new a-rays and summing lines are labeled. The a-emitters were produced by 2- to 6-neutron evaporation from the compound nucleus "' Ir. Four iridium a -rays are reported for the first time. In many cases, coincident y-radiation was observed. This is demonstrated in fig. Id, where

W.-D. Schmidt-Oit et al. / Isomerism

648

Soo

mY N tA

t N i

300

A

!

,-

2

Ô

P1

10

~o,

n N _n

8

~d

~

tn

W

i

i

NN O~ m tn

ui ~r

r ~O J z

!

400

Q 300 U W 100 0 z

0 200 U 160

tt ~ ~ô

~i

o

o f°

â

,*

c v;

Y_ ;

&n'O N~ t^~

ii ~o.o m

1

C

I/

m t

!

120

x

0

tq C

40

_

n

^

â

0

t ~r

.à

.t

t

tA~f+~

N ~

tri

Nfn~

~D ~Ô

tf~tf~

\

n~ .q .o tn

40 to

30 20

m. .o

0

O

2 400

1

2600

2eon

30 00

CHANNEL NUMBER

3200

Fig. 1 . Alpha-spectra observed after the irradiation of 'a' Fr with ; 'Ar . Beam energies at the target entrance are (a) 204, (b) 185, and (c) 175 MeV. Each spectrum was collected for -2 h with collection and measurement cycle times of (a) 4 s, and (b, c) 8 s. New a-rays are labeled with asterisks . (d) Alpha-spectrum measured in coincidence with 210 keV y-rays of "41r, recorded in 4 h.

W.-D. Schmidt-011 et a1. / Isomerism

400

600

800

CHANNEL NUMBER

64 9

1000

Fig. 2. Gamma-rays measured in coincidence with 5.478 MeV a-rays of " 4 Ir. The measuring time was ---4 h with an 8 s collection-measurement cycle.

the a-ray spectrum gated by a 210 keV transition following the '74 Ir decay is shown. In fig. 2, the y-ray spectrum measured in coincidence with the so far known 5 .478 MeV a-rays of '74 Ir exhibits rhenium KX rays and three y-rays. The small peaks to the right of 5 .478 MeV a-rays in fig. Ic are due to summing with Kconversion electrons of these transitions. These and the other results of the a-ray measurements are summarized in table 1 . The underlined a-ray energies are known with 3 keV accuracy [refs. 12-'4,'6,")] and were used as internal standards . The a -ray intensities given in table 1 were normalized to 100% for each decaying isotope . From the number of gating a-rays and the coincidence efficiency of 91% in our measurements, the KX-ray and y-ray intensities in table 1 were derived relative to the a-ray intensities . 4. Identification of /3-delayed y-rays Criteria similar to those given in sect. 3 apply to the assignments of new y-rays. In fig. 3, the A =173 assignment of 50 and 92 keV y-rays by comparison of their excitation functions with those of assigned a-rays is demonstrated . In this case, the respective level spacings of 50 and 92 keV in " ;Os are known from in-beam work [refs . '9,2°)] . The assignment of new y-rays to the decays of iridium isotopes was performed by the observation of coincidences with osmium KX rays following the electron capture decay or the conversion of fl-delayed y-rays. Half-lives of y-rays were measured and compared with the results of the a-decay studies . As an example, the decay curves of four y-rays assigned to the decay of 174 Ir are given in fig. 4.

W.-D. Schmidt-01i et al. / Isomerism

65 0

TABLE 1

171-175 In The a fine-structure intensities and the Alpha-rays and a-coincident y-rays of y-rays are normalized to 100% per a-decaying isomer intensities of a-delayed X-rays and

isotope

Coincident y-rays ")

a-rays E«, [MeV]

1 [%]

Ir

5.925

100

"`Ir

5.828

100

2.00)

5.510 (10)

100

4.4(3)

5.674

100

2.20(5)

5.416(10)

100

9.804)

171

17 ;Ir

"'Ir

175

Ir

T,12 [s]

5.478

88(2)

4.9(3)

5.316 (10)

12(l)

5.5(6)

5.275 (10)

100

9(2)

5.393

100

7.203)

Ey [keV]

lax, ly

ReKX 92

9505) 9(5)

ReKX 162.1(2) ReKX 89.7(4) 123 .2(2) 136 .3(4)

29(2) 44(4) 24(4) 10(3) 54(7) 10(3)

ReKX 136 .2(2)

41(3) 31(2)

ReKX 20.2(4) 190.2(2) 210.3(2) ReKX I27 .2 (4) 159.8(5) 190 .2(2) 210 .3(2) ReKX 31 .4(4) 193 .5(2) 224 .6(4)

9(l) 2.6(10) 17(2) 63(5) 5.6(8) 1 .3(3) 1.2(4) 1.5(5) 5.9(11) 8(3) 3 .6(16) 52(8) 35(7)

") Coincident y-rays with gate on a-radiation in the first column. The measured aycoincidence efficiency of 91 (6)% is respected .

The y-spectroscopic results are summarized in table 2. The uncertainties given in brackets represent lo-errors. The intensities of the 511 keV radiation in the respective decays were corrected for the separately measured annihilation efficiency of our detector geometry. 5. Spectroscopic results We report on the identification of new isomers in 172-174 Ir. Nuclear structure information is derived from the measured ay-, yX-, and yy-coincidence data. Corrections to the Q,, values are applied .

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651

500`

16-

Z _D

O U

1

10 L'~V 170

180

190

200

210

36Ar-BEAM ENERGY [MeV] Fig . 3. Excitation functions of 50, and 92 keV y-rays . For comparison, the yields of known a-rays of 172 Ir (5.828 MeV, squares), "; Ir (5.674 MeV, circles), and "4 Ir (5.478 MeV, diamonds) are given. 5.1 . THE DECAY OF "' Ir

In the 5.925 MeV line in fig. la summing of 5 .828 MeV a-rays with K-conversion electrons of the 162 keV transition (5.828+K162) and a-rays of "' Ir [ref. ")] are detected . Our decaytime of 1 .6 (2) s is explained by the two contributions . The earlier result for "' Ir is T112 =1 .5 (1) s [refs. ",'')]. Using the measured conversion of the 162 keV transition (cf. sect. 5.2) the portion due to the summing was derived leaving 45 (5)% of the 5 .925 MeV line to 171 In From the number of measured X-coincidences the X-rays from the K-conversion were subtracted . In table 1 the Re-K X-rays of the "' Ir decay are given. In coincidence with the 5.925 MeV a-rays we also saw the indication of 92 keV y-rays which are not present in coincidence with the a-rays of "`Ir. From the intensities a K-conversion coefficient 10 (8) was estimated . By comparison with calculated values 22) only the M1(E2) characterization is indicated. We searched for the ß-decay of "' Ir, but no y-rays were observed in coincidence with osmium KX rays, particularly no transitions from the »1OS in-beam study 2") . The a-branching ratio of "'Ir was estimated earlier 21) to be a/total-1 . 5.2. THE DECAY OF "`'Ir

The results for the strong 172Ir a-ray measured in this work agree with the known data E,r = 5 .828 MeV and T,/2 = 2.1 (1) s [ref. '2)]. Our half-life analysis yielded the value TI/2= 2 .0 (1) s for the 5 .828 MeV a-rays (see table 1) . In coincidence with the

W-D. Schmidt-Ott et al. / Isomerism

65 2

9(2) s

5,275 MeV

0

104L

LO

W 1031 a.

`i59keV

~~

l' --v-_

60(2)s1

276 keV

5 6 (3)s~

342 keV

5 0(4)sl

532 keV

4 8(5) s:

Z u

10`J

t

10 3

1----L--L _-L. . . -

0

2

4 6 TIME' (s]

1

8

L .

_. .

10

t

.-

Fig. 4. Decay curves of y-rays assigned to 174 1r, comparison with the decay curves measured for the "'Ir a-rays of 5.275, and 5.478 MeV.

5.828 MeV a-ray a 162.1 keV -y-ray was observed. Using the measured y-ray ail% KX-ray intensities given in table 1, the conversion coefficient aK = 0.69 (6) of the 162 keV transition was derived, indicating 48 (7)% M1 + 52% E2 multipolarity. The same ratio of multipolarity mixing was obtained from the intensities of 5.828 MeV and 5.828+ L162 line in fig. la. Thus, the summing contribution to the 5.925 MeV line could be derived. An a -decaying isomer with E,, = 5.510 MeV and a half-life of 4.4 (3) s was identified. Three -y-rays with energies of90,123, and 136 keV were detected in coincidence with these a-rays. Since the experimental KX-ray intensity in the a-gated spectrum is comparatively small (see table 1), the multipolarity of the intense 123 keV y-ray is El, and also the 90 and 136 keV transitions are of low multipole order. In the a-daughter ' 6"Re, no isomer was traced in a recent work [ref. '5 )] . We may therefore assume that both "'Ir isomers proceed via a- and y-decay to the ground state of ""Re as shown in fig. 5 . The isomerism in "`' Ir is plainly demonstrated by the two a-decay half-lives. It is less convincing from the ,8-decay data. Using the level information from in-beam investigations of the daughter "`'Os [refs.`",'`;)], the ground-state rotational band and negative-parity states at higher excitation energy might be reached in the,8 -decay of "'Ir with Qß --- 9.7 MeV [ref. '_4)] . In the present experiment, only the cascading transitions of the ground-state band were observed up to the 8 + state (see table 2).

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653

172 I r (71

2.0(1),

1525.2 1054.6 m cn

168 Re

606 .2

i--227.8 1720S Fig. 5. Decay scheme of

172

In

These measurements were performed at two irradiation energies and collectionmeasurement cycles of 4 and 8 s, respectively . Different relative y-ray intensities and half-lives were found for the strongest 228 and 378 keV transitions, indicating that both iridium isomers decay to states in 1720s . Respecting the conversion [ref. 22)] of the cascading E2 transitions, and assuming a feeding of the 6+ and 8+ states by the 2 s isomer and of the 2+ an_d 4+ states by the 4.4 s isomer, the short-lived portions in the 228 and 378 keV transitions were obtained . In fact, correcting our measured intensities for the different collectionmeasurement cycles, we derived the same production yield ratio for the 172Ir high-spin and low-spin isomers. In the actual measurements, the portions of 2 s and 4.4 s activities in the 228 keV transition are 38% and 62% for the 4 s cycle, and 27% and 73% for the 8 s cycle. The 378 keV transition exhibits 64% and 36% contributions of the 2 s and 4.4 s activity for the 4 s cycle, and 52% and 48% contributions for the 8 s cycles, respectively . With these numbers, the experimental y-ray half-lives from table 2 are well reproduced within an accuracy of -10% . In fig. 5 we present the decay scheme of 172 Ir. For both isomers, the a- and ,8-decay intensities are normalized to 100% . In order to derive spin assignments, one may assume allowed or first forbidden .8 -decays. The given ß-branchings feeding the ground-state band in 1720S may be due to higher-lying states and unobserved y-radiation. 5.3. THE DECAY OF

 31r

In the earlier work [ref. ")], a 5.674 MeV a-ray with a half-life of 3 (1) s was reported for 173 Ir. We have identified two a-emitting states with 2.20(5)s and 9.8 (14) s half-lives (cf. table 1). In coincidence with the known 5.674 MeV a-ray,

8

175

175

r

1741

1 "Ir

") h) `) d)

8

196

1"Ir

[s]

10(1) 14(1) 4.8(5) d)

393 .9 532 .4 100 11(3) 30(2)

530) 5.0 (4) d)

342 .3

105 .4 399.0

95(5) 5.6(3 ) d)

d)

276 .2

6.0(2)

100

7(21, 16(2) 25(2) 7.0(14) 16(2) <5

127 .6(5) 147.7 285 .0 294.9 296

158.6

100 7.6(5) d) 27 (2) 5.0(7) d)

43(4) 190)

448 .4 470.6(3)

49.6 91 .8

66(6) 2.6(2)

378 .4

3 .0(2)

TI/2 (S]

100

lyre,

227 .8

[keV] `)

Coincident -/-rays with gate on radiation in the 4th column. The intensities obtained in two irradiations were added. The errors are t0.2 keV, unless denoted otherwise . The decay curves of two irradiations were summed.

8

4

204

"! Ir

Tc.

EAr

[MeV

Isotope

]

185

185

196

EAr

[MeV]

8

8

8

Tc [S]

11(1) 12(1)

54(3)

100(5)

100

19(2) 24(2) 18(3) 3(2)

100 28(2)

30(4) 160)

50(5)

100

lyrel

Coincidences'-') E,[ I,]

ReKX [100], 399 [19(6)], 511 [84(20)]

ReKX [130(22)],159 [100], 511 [130(20)]

ReKX [110(7)], 276 (100], 342 [47(5)], 387[4(1)], 394 [8(2)], 511 [19008)], 532 [16(3)] ReKX [117(8)],159 [100], 342 [80(8)], 394 [12(2)], 511 [160(20)] ReKX [84(8)],159 (73(7)], 276 (1001, 394 [25(4)], 511 [142(20)]

ReKX [543001, 92 [100], 511 [<1] ReKX [60(7)], 50 [100], 80[<2], 86 [3(1)], 128 [4(1)], 296 [2], 511 [80(10)]

ReKX [118(20)] 378 [1001, 448 [54(15)1, 471 [18(6)], 511 [205(40)] 3.0(2) ReKX [78(17)], 228 [1001,448 [9600)], 471, 511 3.60)

Tuz [S]

TABLE 2 Gamma-rays following the ß-decays of 172- 1751r, measured at `°Ar beam energies EAr with a collection and measurement cycle T,,, energy Ey , relative y-ray intensity lyre,, and half-life T, 1Z . Intensities measured in coincidence spectra are given in square brackets

m

y 0

0

0

y 3 â

0

W.-D. Schmidt-011 et al. / Isomerism

65 5

a 136 keV y-ray was found. From the y- and X-ray intensities given in table l, the K-conversion coefficient of the 136 keV transition was derived to be aK =1 .37 (6), in accordance with a 63 (7)% M1 + 37% E2 multipolarity. The same multipolarity resulted from intensities in fig. lc of three lines, 5.674 MeV (L,), 5.674+K136 (L2), and 5.674+L136 (L3), namely L2/(L1 +L2 +L3)= EaK/ (1 + acat) and L 3 /(L, + L2+ L3) = Ea r+ . . ./ (1 + ac0J . For the electron detection we have used the efficiency E =18% of the a-detector. In-beam results are known for the nucleus "30s [refs. '9,2° )], in particular a 92 keV ground-state transition, which has also been observed in the a-decay of "'pt [refs. 6'9'`5)], and a coincident 50 keV El-transition [ref. 2°)]. For the cascade-emitting 141 keV level a lifetime of several microseconds was conjectured [ref. 2°)]. A rotational band feeds the 92 keV state by 128 and 296 keV transitions [ref.'`°)]. We have measured y-rays following the .8-decay of "3 Ir in two experiments with the same collection and measurement cycle time of 8 s. Both measurements revealed similar intensities for a series of y-rays. In coincidence with the 50 keV y-rays, only the 92 keV transition was seen. The non-observation of coincident annihilation radiation from the ß-decay can be explained by the lifetime of the 141 keV state. Using the measured KX-ray and 92 keV y-ray intensities, the K-conversion coefficient of the 92 keV transition was derived to be aK (92) = 5.7 (4). This result is in agreement with the analysis of the a-ray gated spectra from "'pt decay [refs. 6'9)], yielding aK (92) = 6.4 (4) . By this, the M1 multipolarity of the 92 keV transition is confirmed. With the coincidence gate on the 92 keV transition, we found in addition to the 50 keV y-rays and Os KX rays a 127 keV y-ray, annihilation radiation and an indication of further y-rays. Comparing the intensities of the 50 keV y-rays and 511 keV annihilation radiation in the calibrated annihilation geometry, a feeding of the 92 keV level of 25 (5)% from ~6-decay, and -75% from the 50 keV transition was derived. About one half of the measured KX rays is emitted in the electroncapture decay and in the conversion of the 127 keV Ml transition [ref. 2°)]. With these branchings, the intensity of the 50 keV y-rays and the corrected intensity of the 92 keV y-rays were used to estimate the conversion coefficient a = 0.7 (2) of the 50 keV transition, in agreement with an E1 characterization . Different decay times were measured for the 50 and 92 keV -/-rays (see table 2). "3 This fact can be explained by an unequal feeding from the two Ir isomers with 2.2 and 9.8 s half-lives . (Accidentally, none of the two decays was favoured in the experimental 8 s cycles of collection and measurement.) The shorter decay time of the 92 keV transition, which is fed by 127 keV transitions from a 219 keV state, and probably by 296 keV transitions from a 388 keV state, may therefore be related to the 2.2 s isomer. On the other hand, the longer decay time of the 50 keV y-ray is due to a larger contribution of the 9.8 s isomer. Recently, high-spin states in "3 Ir built on three band heads have been investigated [ref. 26)] . One of them is the z- [505] band head. If we identify our 2.2 s-isomer with the 2- band head, the feeding of the z- 92 keV level via 9 - and -'- states in "30s

W.-D. Schmidt-Ott et al. / Isomerism

656

[ref. `°)] can be explained . The feeding of the 141 keV state in 173 0s with a proposed ~+ assignment [ref. '`°)] could proceed from the other "3 Ir isomer with 9.8 s half-life. However, because of the lifetime of the 141 keV state, no y-rays in coincidence with the 50 keV transition were traced. The separation of the 92 keV intensity into -25% feeding of the 2.2 s decay and -75% of the 7.6 s decay (i.e. the actual half-life of the 50 keV y-ray) yields, by adding two experimental decay curves with 2.2 and 7.6 s half-life, in our 8 s measurement cycle, a decay time of 5 s, which agrees with the experimental value of the 92 keV y-ray. Similarly, the decay time of the 50 keV y-ray is explained by feeding of the 141 keV state from both isomers. The experimental decay time of 7.6 s is reproduced if we adopt twice the number of feedings from the 9.8 s isomer than from the 2.2 s isomer. The decay scheme of "3 Ir is proposed in fig. 6. The relative 8-decay branchings were obtained from the y-ray intensities measured in coincidence with the 92 keV transition . The absolute value of the fl-branch (compared to the a-decay) was derived from the number of a -counts and the 50 keV singles countrate. As indicated in fig . 6, our results can be explained if the feeding of the 141 keV state from the two " 3 Ir isomers proceeds at least via one higher-lying state with intermediate spin. Further y-rays measured in the singles spectra were detected and might be candidates for such transitions. 5.4. THE DECAY OF "4Ir

In the "4 Ir decay, an a-ray with an energy of 5.478 MeV and a half-life of 4 (l) s was known [ref. 14 )]. The half-life of the present work is 4.9 (3) s. Besides the strong a-ray, we observed further a-rays of 5.316 MeV, and 5 .275 MeV with decay times of 5.5 (6) s and isomeric 9 (2) s, respectively . 173I r (111215051) (51214021 .3121402))

(5/214020/215411) (9/2-15141)

2 .20(5)s

9.80 .14s

X .T

169Re 173 Fig. 6. Decay scheme of In The ground-state j9-feeding of 1730s in the decay of the low-spin state is neglected .

W.-D. Schmidt-Oit et al. / Isomerism

65 7

Several y-rays were seen in coincidence with these a-transitions as given in table 1 . We measured 210,190, and 20 keV -y-rays in coincidence with the 5.478 MeV a-rays (see fig. 2). The intensity balance of the latter two y-rays requires El multipolarity of the 20 keV transition. The 210 keV y-ray probably represents the cross-over transition. The small X-ray intensity as given in fig. 2 and table 1, and the minor summing of 5 .478 MeV a -rays and conversion electrons ofthese transitions are suggesting El multipolarity for the 190, and E2 for the 210 keV -y-ray, respectively. With the coincidence gate on the 5 .316 MeV a-rays, these three and additional y-rays were observed, particularly at 160 keV. This transition energy corresponds to the energy difference between the 5.478, and 5.316 MeV a-rays. The 5.275 MeV a-ray with 9 s half-life is followed by three y-rays (see table 1), the 31 and 194 keV transitions are probably cascading, and the 225 keV y-ray may be the cross-over transition. A low multipole order ofthe three transitions is suggested by the weak rhenium KX-ray intensity in the coincidence spectra. The ground-state rotational band of 17406 is known from in-beam experiments [refs. 14,27)] . The 2+ state at 159 keV is confirmed in the a-decay of 178pt [refs . 6,14)] . Our ß-decay data of 174 Ir are given in table 2. The cascading transitions of the ground-state band [ref. 14)] were detected up to spin 8+ . Aside-feeding ofthe 159 keV state via 532 keV y-rays was observed. The decay curves of the -y-rays are shown in fig . 4 and compared with those measured for the a-rays. A systematic increase in the half-life of the lower-lying band members was found. The decay ofthe 342 keV (6+ -> 4+ ) transition corresponds to the 4.9 s half-life of the 5.478 MeV a-rays, while the 276 keV (4+ -> 2+ ), and 159 keV (2+ -> 0-r ) transitions with half-lives of 5.6 s and 6.0 s, respectively, show contributions of the 9 s isomer. The 4.9 s decay is therefore assigned to a high-spin isomer of !74 Ir, and the 9 s decay to an isomer with lower spin . Respecting the E2 conversion [ref. 22)] in the ground-state rotational band, and the side-feeding from the experimental intensities given in table 2, the contributions of the two isomeric decays in the 159, and 276 keV transitions were derived and the observed half-lives of these transitions were perfectly reproduced . The decay scheme of the 174 Ir isomers is proposed in fig. 7. Tentative spin assignments may be deduced from the ß-decay and are discussed in sect. 6. "5 Ir 5.5 . THE DECAY OF

One a-ray with an energy of 5.393 MeV and a half-life of 4.5 (10) s was reported in the decay of 175 Ir [ref. 16)]. Our half-life value T1/2(a) = 7.2 (13) s is definitely 16)] . No -y-rays were seen larger than the one found in the earlier experiment [ref. in coincidence with the 5.393 MeV a-ray. The )9-decay exhibits a prominent 105 .4 keV y-ray in coincidence with osmium KX rays, which was only detected at the lowest 36Ar irradiation energy of 175 MeV . The half-life of this y-ray was measured to be 11 (3) s, still in agreement with the result from the a-decay. Thus, a mean half-life value of 8 (1) s for 175 Ir was derived.

658

W .-D.

Schmidt -Ott et al. / Isomerism 174I r

174()5

Fig. 7. Decay scheme of 1 ' 4 Ir.

In-beam studies of "SOs revealed a 105 keV y-ray depopulating a 7 + [633] excited state to the -i - [512] ground-state [ref. '')]. In our experiment, Os KX rays, 511 keV annihilation radiation and a new 399 keV y-ray were observed in coincidence with the 105 keV y-ray. Only about 15% of the X-ray intensity is due to the electron capture decays. The rest may be explained by conversion of further undetected y-rays. The proposed partial decay scheme of " S Ir is presented in fig. 8. 'scussion

The investigated iridium isotopes are ofdeformed nuclear shape with deformation parameter values in the range 0.15 < .E < 0.2. In this region, the single-particle states 175i r r71

ir

(111215051)

512-,1/2 -15411 1.5(2)s

/

8(1)S \ \22(1)% 504.4

5.925 MeV -50% N W

(1112 -15051)

rv

(9/2 -15141) 167

-92

5/21402 (5/2;1/2 15411 (9/2'1514))

10

171 R

Re

1 712*16331

1 °

- 1054 .

e 17505

Fig. 8. Decay schemes of

171

Ir, and

175

Ir. The ground-state fl-feeding of " SOS is neglected .

W.-D. Schmidt-Oit et al. / lsomerism

65 9

lie close together, and their relative order is unknown in many cases. Nilsson configurations were sometimes assigned by identification of high-spin states of rotational bands. ltior the Z = 77 iridium nuclei, the proton orbitals near the Fermi level are 1-[505], 2 - [514], 2 + [402], 2 + [402], 2 + [400], and 2 - [541] . A large spin difference between these neighbouring configurations is probably responsible for the isomerism we have found. Similarly, isomers were recently discovered in the rhenium isotopes with Z = 75 [ref.'")]. The corresponding proton states at the Fermi level at 2 - [514], z + [402], and In the present work, the a-decay of iridium to rhenium is investigated . The strongest a-decays feeding excited states in the daughter nuclei may usually be identified with favoured a-transitions between states with the same structure. Alphacoincident y-rays and the measurement of their conversion coefficients proved to be important for the level assignments in the rhenium isotopes . Neutron configurations have to be considered for the even-A iridium and for odd-A osmium isotopes . The respective states for the neutron numbers 91 to 97 at the Fermi surface are 2 - [523], 2 + [642], z - [521 ], 2 + [651 ] and ! + [660]. The systematic trends of the single-particle proton states in iridium isotopes known so far are presented in fig. 9, together with the present results. The close-lying states + [402], i + [400], and z- [505] are responsible for the isomerism in the heavier isotopes [ref. 28 )]. For mass numbers A _ 185, the h9/2 1-[541] intruder state is found below these three states, but a marked increase in its excitation energy is indicated in "3 Ir [ref. 26)] . The 2 - member of this band is the ground state of the isotopes

600

It', Z=77

C ar 500

9/27.1/21541)

400

Q

û 100

w

lpi

)k150 ns

112*(®001

'412 .3 ms

5/2 -.1/2-15411

w Z 300 w Z

0 200

A21ns 11/27[5051 1 %

312*[4021 1112'15051 2.2 s

0 3l2*[4021 5/2* [ 4021

,

; ~4.9 s

,,

512*I4021 5/2-J1215411

173 i75 177 179 181

".

; 183 185 187 189 191

MASS NUMBER A

~ 3.8 h

`l~10 6 d 193

195

Fig. 9. Experimental excitation energies of single-particle states in odd-A iridium isotopes . The data for 173-175Ir are from the present work and refs. 26,29), "'Ir from refs. a.6), "9 Ir from refs . 6.11 ), and '8"-'931r from refs. 28,30.31). Isomeric half-lives of the '-; -, ~2 - [505] state are also given .

660

W,-D. Schmidt-Ort et al. / Isomerism

18','83 ."SIr [ref. 28 )] . The increase in energy of the "'Ir [ref. °)], "9Ir [ref.')], and + [402], 2 + [400], and 2®[505] states should reverse with decreasing mass number, "2_ " 4 Ir. This is in fact observed for the according to the observation of isomers in 'g',"9'"'Ir [ref. 6 )] . z + [402] state, which continuously decreases in the isotopes '"Ir decay. The 2.2 s high-spin isomer is tentatively assigned to the 2- [505] configuration . This assignment is strongly supported by the a-decay to the 136 keV state in "Re, which can be interpreted as an z - excited state (see fig. 6) . This assignment is suggested by the measured M1(E2) multipolarity of the a-coincident 169 136 keV transition feeding the proposed 2®[514] ground state of Re [ref. ' 5 )] . A short-lived component was also seen in the ß -decay of "3 Ir, indicating a feeding of " 3 0s states from the '- isomer. It may proceed via the rotational band built on the ~-[523] configuration. The observed 92 keV transition was reported to depopulate the 2 member to the ~- band head, the 2- and 2- members lie at 220 and 388 keV [ref. '` 0)] . In our coincidence measurements with 92 keV y-rays, we traced the feeding of the 2 - and probably of the 1' - state. A likely interpretation of the "3 Ir low-spin isomer is given by the 2+ [402] or the +[402] configuration (see fig. 9). This assumption is supported by the in-beam study of "3Ir [ref. 26)], where the ~+ [402] and 2 + [400] states were presumed at several hundreds of keV below the 2 - , z - [541] state. The 2- and 2- members of this band are expected at higher excitation energies . We have calculated the positions of the 26)] at -340 and -390 keV, respectively. 4-, and ~- states ofthe decoupled band [ref. 29) Another in-beam work finds band heads of probable 2+ [402] assignment in "5 Ir and "'Ir at 53, and 181 keV, respectively . This low-lying level may, therefore, become the ground state in "3 In A favoured a -decay would only proceed between the two 5} states, cf. fig. 6. An isomer pair with still unknown level position was recently investigated in the a-daughter "Re [ref. '5 )] . For the position of the isomeric states in "3 Ir we may derive the excitation energy of the 2 state to be 370 - X keV, where X denotes the unknown level energy in ' 69Re. The ground state of '73Os was proposed to be 2- [523] with a developed band structure [ref. 2° )] . The 141 keV state was assigned to 2+ [624], which state, however, is expected at higher excitation energy frGm the Nilsson diagram . The lifetime of this 141 keV state of several microseconds [ref. 2° )] to some degree might be caused by the change of the K-quantum number, AK = 2, in the depopulating 50 keV El transition. A lifetime was also suggested by the decoupling of the 50 keV transition from the .8-decay in our data (see sect. 5.3 . ). A strong decay branch of the 9 .8 s isomer of "3 Ir proceeds via this 141 keV state, indicated by the 7 .6 s half-life of the 50 keV transition. We have to assume that this is mediated by available low-spin band members of the 2 + [660], i + [651 ], 2- [521 ], or z + [642] configurations . Further y-rays besides those in the 2 - [523] band were observed in coincidence with the 92 keV transition (see table 2) and are, therefore, bypassing the 141 keV lever . In addition, y-rays assigned to the decay of 173 Ir with

W.-D. Schmidt-Ott et al. / Isomerism

66 1

energies of 148, 285, and 295 keV were traced in the singles spectra. They exhibit longer half-lives and may consequently be related to the decay of low-spin states. t't.t'slr decays. In the "'Ir decay we found one a-ray with coincident X- and probably 92 keV y-rays, the intensities being in accord with a presumable Ml (E2) transition in the '67 Re daughter. Considering the Nilsson states which are close to the Fermi level in the rhenium isotopes [ref. 's )], the M1(E2) transition is likely to connect the 2-[505] and the 2-[514] states in analogy to the decay of the high-spin state of "3 Ir. So far, the low-spin isomer in "'Ir was not found and the position of the li- state is not known. The most likely spin assignment for the "5 Ir ground state is i -, !-[541 ] [ref. 29)] . For the a-daughter nucleus of "5 Ir, namely "' Re, some experimental information is known. The Nilsson configuration 2 -[514] was assigned to the ground state of "'Re on the basis of its decay into excited states of 171W [ref. ')]. In-beam studies of "' Re propose the z +[402] and the z -, 2 -[541 ] state below 200 keV, with the 2+ state at 42 keV above the 2 - state [ref. 's )]. This offers the possibility of favoured a-decay of a z - , 2 -[541 ] .75Ir ground state to the same structure in "' Re. The non-observation of a-coincident y-rays in our experiment may be explained by a lifetime of the 2 - state due to the 4K = 4 change of the respective transition to the "' Re ground state. A very tentative decay scheme of "5 Ir is proposed in fig. 8. In-beam studies of " S OS, the ß -decay daughter of "5 Ir, revealed a 2 -[512] ground state and a 105 .7 keV level with mainly the i+ [633] assignment [ref. 2' )]. A strong -/-ray with this energy was also traced in our ,8-decay measurement of "5 Ir. Further unobserved transitions, probably of low energy, account for the measured KX-ray intensity. The coincident 399 keV y-ray was not seen in the in-beam work. We may tentatively assign this transition to an inter-band transition from the '[523] state, which is proposed to be close to the 2-[512] ground state [ref. 2' )]. "2Ir decay. A strong feeding of the 6+ and 8+ members of the ground-state rotational band of '720S was observed in the 8-decay of the 2.0 s-isomer of "2Ir. Assuming a spin 7+ with a large K-quantum number, the K-forbiddeness in the ß-decay to the K = 0 ground-state band has to be respected. One may assume that the feeding of the 6+ and 8+ states is mediated by higher-lying excited states with K > 0. Many examples of similar decays of odd-odd high-spin nuclei are known [ref. 28 )]. Transitions from higher-lying states may be expected on grounds of the 24)]. Qß value of -10 MeV [ref. The spin 7+ could be explained by the available Nilsson states Iff - [505] and '68 Re daughter could v2 -[521]. The favoured a-decay to the 162 keV state in the be interpreted as a transition to the same configuration, which depopulates to a 6+ (ire -[514], v2 -[521]) ground state. This spin (6+ ) would be in accordance with the measured M1 (E2) multipolarity of the 162 keV transition and also with the 8-decay of '68 Re feeding mainly the 6+ state in '68W [ref. ' 5 )]. For the 4 .4 s isomer of "2Ir, the spin (3+ ) may be conjectured from the feeding of 2+ and 4+ states in the Iß-decay . Direct .ß-feeding is likely, but transitions from

66 2

W.-D. Srhmidt-O11 et al. / Isomerism

higher-lying states cannot be excluded, though they were not observed. According to the coupling rules ?-), a Or -~. - [505], v -[523]) configuration comes into question . The favoured a-decay of this state may feed a level with similar configuration in "`'Re, which is depopulated via the three a-coincident y-rays. Since only one S), isomeric state was found in '6"Re' we tentatively assume that these transitions are feeding the (6') state. "4 Ir decay . The ß-decay of the two "4Ir isomers can be explained in correspondence with the decay of 172 Ir. A high-spin state with the probable spin 7' and a half-life of 4.9 s feeds the 6+, and 8+ members of the "'Os ground-state rotational band. Again, high-lying states with K > 0 may be involved, which is expected with the high Qp value of -9 MeV [ref. '`')]. The .8-decay of the 9 s isomer with probable spin 3 + feeds the 2+ , and 4} band members. The same Nilsson configurations as in "4 the decay of "`Ir may be assumed for the Ir isomers . The a-decay of the 7+ isomer is probably a favoured transition to a similar 7+ configuration in " " Re, with a weaker branch to a higher-lying state. The coincident y-rays feed a further state in ""Re and are assumed to end in the (5+ ) ground state. The relative order of the 190 - 20 keV cascade is not known. Since the 20 keV transition has El multipolarity (see sect. 5.4.) the intermediate level is tentatively assigned to a 6- state. Consequently, the 190 and 210 keV transitions have E1 and E2 multipolarity, respectively, in accordance with the conversion estimate, cf. sect. 5.4 . The a-decay of the 9 s isomer could proceed to a (3+ ) excited state in "°Re which is deexcited through the 194-31 keV cascade of unknown order and the 225 keV cross-over transition . We assume that these transitions feed the (5+ ) ground-state. If so, a spin of 4 is reasonable for the intermediate level. The calculated a-decay branchings of the iridium isotopes are included in figs. 5-8. Although no ground-state fß-feeding was respected in the ";Ir low-spin and 175 Ir decay, the expected decrease of the a-decay probability from "' Ir to "5 Ir was found. Our a-branching ratios of the main a-decays of 1721r, '"Ir, and "4 Ir of 23%, 12%, and 2.2%, respectively, can be compared with the earlier results -3%, 2% and 0.5% derived from parent-daughter relationships 33) and with the calculated a-branchings of "3 Ir and "4 Ir of 32% and 5 .5% [ref. ;;)]. From the a-ray energies and the coincident KX- and -y-radiation we derived Q, values of the "' -"5 Ir decays. For "5 Ir, we assumed that the a-decay populates the -, ' -[541] state of "' Re at < 160 keV [ref. '')]. The results are summarized in table 3. A general trend of decreasing Q, values is observed when proceeding towards the line of stability, however, the occurrence of the isomer pairs causes anomalies. In table 3 our experimental Q, values are compared with those derived from mass predictions '4) . The energy distances of the isomer pairs in "` -"4 Ir were estimated as follows. In the case of "'Ir, we assume ; but did not measure, that the a-coincident y-rays of 90, 122, and 136 keV are cascading. If so, the 5.510 MeV a-decay proceeds to a

W.-D. Schmidt-Oit et aL / 1somerism

66 3

TABLE 3

71 7si r derived from the measurement -1

Qs, values of of a-ray energies and a -delayed y-rays are compared with the values from the mass prediction [ref. 1

-'4)]

Q, [ MeV]

Isotope

171

Ir Ir 172m Ir 173m Ir 174g Ir 174m Ir 175 Ir 172g

This work 6 .159(5) 5 .989`') 6 .129 ') 5 .945(5) 5 .624 (10) 5 .817(5) 5 .679 ")

[ref.

24)]

6.07 5 .97 5 .95 5 .61 5 .53

")

Assumptions used for the Q-value estimate are given in the text. Assuming a limit of -160 keV for the excitation energy of the ^, ; [541 ] state in the daughter 1 ' Re [ref. ")] .

h)

-

7

348 keV excited state in '6"Re (see fig. 5) . If further the decays of both "`Ir isomers are ending in the same '6"Re state (see above), we find the (7+ ) isomer at 140 (11) keV above the (3 + ) ground state in "'Ir. The energy distance of the isomer pair in ' ;Ir can only be restricted to less than 370 keV. Without restrictive assumptions, the level distance of the 174 Ir isomers is derived from the experimental data if both decays end in the ""Re ground-state . The (7 +) state is the isomer at an excitation energy of 193 (11) keV above the (3+) ground state. No isomer pairs were found in "' Ir and 175 In In "' Ir, a low-spin isomer may be expected in analogy with "; Ir, and a search for this isomer is intended . The isomerism of the (high-lying) '-,' -[505] state in 175 Ir seems to be prevented by the occurrence of the rotational states of the ! -[541], and ~+ [402] bands at lower excitation energy, as it is the case in the heavier iridium isotopes . The authors would like to thank the target laboratory of the GSI Darmstadt for their help in preparing the praseodymium targets. References t

F . Runte, F . Meissner, V . Freystein, T. Hild, H . Salewski, W.-D. Schmidt-Ott and R. Michaelsen, Z . Phys . A328 (1987) 373 2) T. Hild, W.-D . Schmidt-Ott, V. Freystein, F . Meissner, E . Runte, H . Salewski and R . Michaelsen, Nucl . Phys . A492 (1989) 237 3) F. Meissner, W .-D . Schmidt-Ott, V . Freystein, T. Hild, E . Runte, H . Salewski and R . Michaelsen, Z . Phys . A332 (1989) 153 4) U . Bosch, P. Koschel, W .-D . Schmidt-Ott, V . Freystein, T. Hild, F. Meissner, H . Salewski, U . Ellmers and R . Michaelsen, Z. Phys. A336 (1990) 359

664

W.-D. Schmidt-Ott et al. / Isomerism

5) F. Meissner, W.-D . Schmidt-Ott, V. Freystein, T. Hild, E. Runte, H . Salewski and R. Michaelsen, Z. Phys. A337 (1990) 45 6) H. Salewski, F. Meissner, W.-D. Schmidt-Ott, U. Bosch-Wicke, P. Koschel, V. Kunze and R. Michaelsen, Z. Phys. A (to be published) 7) F. Meissner, W.D. Schmidt-Ott, K. Becker, U. Bosch-Wicke, U. Ellmers, H. Salewski and R. Michaelsen, Z. Phys. A339 (1991) 315 8) U. Bosch-Wicke, W.-D. Schmidt-Ott, F. Meissner, H. Salewski and R. Michaelsen, a . Phys. A341 (1992) 245 9) P. Koschel, PhD Thesis, Göttingen 1990 10) P. Koschel, H. Salewski, W.D. Schmidt-Ott, U. Bosch-Wicke, V. Kunze, F. Meissner and R . Michaelsen, Hahn-Messner-Institut annual report HMI-B490 (1990) p. 195 11) H. Salewski, P. Koschel, U. Bosch-Wicke, F. Meissner, W.-D. Schmidt-Ott and R. Michaelsen, Verhandl . DPG (VI) 26 (1991) 566 12) W. Gongquing, Nucl. Data Sheets 51 (1987) 577 13) V.S. Shirley, Nucl. Data Sheets 54 (1988) 589 14) E. Browne, Nucl. Data Sheets 62 (1991) 1 15) F. Meissner, H. Salewski, W.-D. Schmidt-Ott, U. Bosch-Wicke and R. Michaelsen, Z. Phys. A in press (1992) 16) M.M. Minor, Nucl. Data Sheets 18 (1976) 331 17) V.S. Shirley, Nucl. Data Sheets 43 (1984) 127 18) R.A. Bark, G.D. Dracoulis, A.E. Stuchbery, A.P. Byrne, A.M. Baxter, F. Ries and P.K. Weng, Nucl. Phys. A501 (1989) 157 19) S. Juutinen, P. Ahonen, J. Hattula, R. Julin, A. Lampinen and A. Pakkanen, University of Jyväskylä (JVFL) annual report 1988 (1989) p. 81 20) R.A. Bark, G.D. Dracoulis and E.A. Stuchbery, Nucl. Phys. A514 (1990) 503 21) U.J. Schrewe, W.-D . Schmidt-Ott, R.-D . v. Dincklage, E. Georg, P. Lemmertz, H. Jungclas and D. Hirdes, Z. Phys. A288 (1978) 189 22) F. Rbsel, H.M. Fries, K. Alder and H.C. Pauli, At. Data Nucl. Data Tables 21 (1978) 293 23) J.C. Wells, N .R. Johnson., C. Baktash, 1 .Y. Lee, F.K. McGowan, M.A. Riley, A. Virtanen and J. Dudek, Phys. Rev. C40 (1989) 725 24) A.H. Wapstra and G. Audi, At. Data Nucl. Data Tables 39 (1988) 281 25) E. Hagberg, P.G. Hansen, P. Hornsh0j, B. Jonson, S. Mattson and P. Tidemand-Petersson, Nucl. Phys. A318 (1979) 29 26) S. Juutinen, P. Ahonen, J. Hattula, R. Julin, A. Pakkanen, A. Virtanen, J. Simpson, R. Chapman, D. Clarke, F. Khazaie, J. Lisle and J.N. Mo, Nucl. Phys. A526 (1991) 346 27) B. Fabricius, G.D. Dracoulis, R.A. Bark, A.E. Stuchbery, T. Kibèdi and A.M. Baxter, Nucl. Phys. A511 (1990) 345 28) C.M. Lederer and V.S. Shirley (ed .), Table of isotopes, 7th ed. (Wiley, New York, 1978) 29) G.D. Dracoulis, B. Fabricius, T. Kibèdi, A.M. Baxter, A.P. Byrne, K.P. Lieb and A.E. Stuchbery, Nucl. Phys . A534 (1991) 03 30) G. Schlick, A. Knipper, C. Richard-Serre, V. Berg, A. Zerronki, J. Genevey-Rivier and ISOLDE collaboration, in Future directions in studies of nuclei far from stability, Nashville (Tennessee), 1980, ed. J.H. Hamilton, E.H. Spejewski, C.R. Bingham and E.F. Zganjar (North-Holland, Amsterdam, 1980) p. 127 31) W. Nazarewicz, M.A. Riley and J.D. Garrett, Nucl. Phys. A512 (1990) 61 32) C.J. Gallagher Jr and S.A. Moszkowski, Phys. Rev. 111 (1958) 1282 33) J.G. Keller, K.-H . Schmidt, F.P. Hessberger, G. Münzenberg, W. Reisdorf, H .-G. Clerc and C.C. Salm, Nucl. Phys. A452 (1986) 173