long-lived isomers in 190ir

N U C L E A R P H Y S I C S A

ELSEVIER Nuclear Physics A 611 (1996) 68-84

Long-lived isomers in 19°Ir EE. Garrett a'b'l , N. Warr a, H. Baltzer c, S. Boehmsdor f f c, D.G. Burke b,

M. D616ze a, S. Drissi a, j. Gr6ger c, C. Gtinther c, J. Kern a, S.J. Mannanal a, j. Manns c, U. Mtiller c, j._p. Vorlet a, T. Weber c

a Dept. of Physics, University of Fribourg, CH-1700 Fribourg, Switzerland Dept. of Physics and Astronomy, McMaster University, Hamilton, Ont. L8S 4M1, Canada

c Institutfiir Strahlen und Kernphysik, Nussallee 14-16, D-53115 Bonn, Germany

Received 9 July 1996

Abstract

Long-lived isomeric states in I~'Ir have been investigated with the t92Os(p,3n)lg°Ir and t92Os(d,4n)lg°Ir reactions using beams of 18-31 MeV protons and 27.8 MeV deuterons, respectively. A series of measurements, including excitation functions, half lives, e - y coincidences and e e coincidences, was performed. Five new transitions were observed, and the results of e - y and e - e - coincidences indicate that these transitions are fed by the 148.7 keV M4 transition that depopulates the 1 1- isomer. The previous decay scheme is shown to be incorrect, and the results allow the ground state parity and mass excess to be determined.

Keywords: NUCLEAR REACTIONS 192Os(p,3n~), E = 18.6, 20.8, 24.2, 27.2, 31.1 MeV; measured E~,, lr(Ep). Enriched targets, Ge detector. RADIOACTIVITY 19°mlr [from 192Os(p,3n), 192Os(d,4n) ]; measured Ey, 17, le_, e - e - coin., e - y coin., Tl/2; deduced levels, J, ~r, ICC, y-multipolarities. Enriched target, Ge detector,/3-ray spectrometer.

PACS: 21.10.Dr, 21.10.Hw, 21.10.Tg, 23.20.Lv, 23.20.Nx

1. Introduct ion

The nuclear structure of the o d d - o d d nucleus 19°Ir was relatively unknown unti l very

recently, even though it lies next to the stable nuclei 189Os and WlIr. Harmatz and

Handley [1] investigated two isomeric states with the W°Os(p,n)W°Ir reaction ut i l iz ing

12 MeV proton beams. Garrett and Burke [2] have studied W°Ir via s ingle-nucleon

I Present address: Dept. of Physics and Astronomy, University of Kentucky, Lexington, KY 40506, USA.

0375-9474/96/$15.00 Copyright (~) 1996 Published by Elsevier Science B.V. All rights reserved. PII S0375-9474 (96 )0 0 3 3 0 -2

P.E. Garrett et al./Nuclear Physics A 611 (1996) 68-84 69

transfer reactions on targets of 191Ir and 1890S, and found a large number of levels

( ~ 60) below 1 MeV excitation energy, all of which have spin ~< 5 and negative parity. No connection was established between these levels and the isomeric states.

The ground state of 19°Ir is known to decay with a half life of 11.78 days, and has 3 +[402]~- + been assigned [1 ] as having I ~" = 4- , with a possible configuration of - 2

11 + T [615]~. The spin 4 assignment comes from the observation [3] that states in 19°Os of spin 3, 4 and 5 are fed by electron capture from the 19°Ir ground state. The positive

parity assignment for the ground state was inferred from the decay characteristics of the isomeric levels in 19°Ir as follows. The 3.2 h isomeric state in 19°It decays to the 10-

state at 1705.8 keV in ~9°Os via electron capture with a logf t = 4.7. In a recent study,

Sood et al. [4] have found that for allowed-unhindered decays, which have logft values

below 5.2, the proton---,neutron or neutron--,proton transition involves only a spin-flip

of the particle, with all other Nilsson quantum numbers remaining the same. They find no instances where this rule has been broken. For low-lying states in the A _~ 190 mass region, only the 9- [505]~ and ~ - [ 5 0 5 ] ~ orbitals satisfy the spin-flip condition.

The isomeric 10- state in 19°Os has been assigned [1] the 9- [505]~ + ~ [ 6 1 5 ] ~ ,

configuration, which implies that the 3.2 h isomeric level in 19°Ir is the ~ - [ 5 0 5 ] . + 11 ~ T [615]~ configuration. This argument yields the assignment of 11- for the isomeric

level in 19°It. Two transitions, a 3.2 h M4 transition of 148.7 keV and a 1.2 h M3

transition of 26.3 keV, were observed in the conversion electron study of Harmatz and Handley [1]. Since no other decay transitions were observed, the 148.7 keV M4 transition was assigned as feeding the 1.2 h isomer which decays via the 26.3 keV M3 transition. In this way, the difference in angular momentum of the 3.2 h isomer ( 11- ) and the ground state (spin 4) could be explained, and this implied that the parity of the

ground state was positive. In an effort to gain a better understanding of the structure of 19°Ir, and to complement

the single-nucleon transfer data, a series of in-beam 3/-ray measurements, outlined in Section 2, was performed using the 192Os(p,xn3/)193-XIr reaction. After the irradiation

of the ~92Os targets, a measurement of the decay 3/-rays revealed the presence of four

previously unknown transitions, at 36.2, 56.1, 135.4 and 205.2 keV, whose energies

matched very well known lines which had been determined from an excitation thnction measurement as belonging to 19°Ir. This opened up the possibility that either there was

an additional, unknown, long-lived isomer, or that the decay path for one of the two

previously known isomers had to be modified. The present work shows that the latter possibility is realized, and that the assigned parity of the ground state must be changed.

In Section 2 are presented the results of various experiments for observing both the 3/- rays and the conversion electrons following the isomeric decays. The new decay scheme resulting from the data and possible interpretation of assignments are given in Section 3.

70 P.E. Garrett et al./Nuclear Physics A 611 (1996) 68-84

2. Experimental details and results

Since y-transitions in 19°Ir had never hitherto been observed, an extended set of measurements which included excitation functions, coincidences, and angular distributions

was performed at the Philips variable energy cyclotron of the Paul Scherrer Institute (PSI) at Villigen, Switzerland. As well, conversion electron measurements were per-

formed at the isochronous cyclotron at the Institut ffir Strahlen und Kernphysik (ISKP)

of the University of Bonn, Germany. Details will be given here only for those experi-

ments which helped in the determination of the decay schemes of the isomers. In-beam y-ray measurements and their results will be presented elsewhere [ 5 ].

2.1. y-ray experiments

The use of excitation functions is a very powerful method for determination of the

reaction channel of y-rays which result from fusion reactions. By measuring the intensity

of the y-rays from the 192Os(p,xn) reaction as a function of the beam energy, charac- teristic shapes were observed for the 2n (191Ir), 3n (19°Ir) and 4n (J89Ir) channels, and

isotopic assignments were obtained, as will be shown hereafter. Targets of 192Os, enriched to 99.0% and ,-~7.6 mg/cm 2 thick, were prepared by

centrifuge deposition of metallic samples onto Kapton foil ( ~ 1 /zm thick). The targets were bombarded by proton beams of energy 18.6, 20.8, 24.2, 27.2 and 31.1 MeV at the

PSI. Singles spectra were recorded with the Fribourg anti-Compton spectrometer [6] utilizing a PGT 13 cm 3 Ge detector. The resolution was 0.9 keV full width half maximum

(FWHM) at 100 keV. The spectrometer was placed at an angle of 55 ° with respect to the beam line. In order to obtain a precise energy calibration, lines from radioactive sources of 182Ta and 192Ir were superimposed on the reaction spectra in additional

measurements. The efficiency calibration of the detector was performed using a source

of ~°Ag in addition to the above sources placed at the target position. The low-energy portion of the y-spectrum, obtained at a beam energy of 24.2 MeV, is shown in Fig. la.

The y-transitions observed in-beam which were also observed in the decay spectrum

are labelled with their energies. In Fig. lb the decay spectrum, which was collected immediately after the beam irradiation using the same experimental setup, is shown. As

can be seen in Table 1, there is an excellent energy correspondence between the lines in the two spectra. The excitation functions of these transitions, shown in Fig. 2, indicate that they belong to the (o,3n) channel. The spectra recorded at the various bombarding energies were normalized to the Os K~, X-ray intensities using the procedure described in Ref. [7]. None of the observed y-lines was previously known to belong to 19°Ir. The transitions which had been assigned previously to 19°It, at 26.3 and 148.7 keV, can be

observed in the conversion electron spectrum only since the conversion coefficients for these lines are very large.

In order to investigate the half life of the isomeric level giving rise to the four y- rays, an experiment was conducted where spectra were recorded in 20 rain bins for approximately 4 h following the bombardment of a 192Os target with 24.9 MeV protons.


IO0

5O x

,~ 2O

& 10

,!i 2

IO0

5O

~ 20

~ lO 5

8

40 80 120 160 200

m m / ~ E =242 MeV

b) I

Lr~ decay spectrum

40 80 120 160 200

E (keV)

Fig, 1, Portions of the y-ray spectrum observed (a) during irradiation of the target with 24.2 MeV protons and (b) immediately after the irradiation. Lines which are determined as belonging to Jg~Ir and observed in both spectra are labelled with their energies.

The 616.3 keV transition, which belongs in the decay path of the 10- state in 19°Os fed by electron capture from the 11- isomer in 19011, w a s also observed in the spectra, and it was found that the decay curves of the four y-rays matched that of the 616.3 keV transition. It was also noted that the ratios of the intensities of the four y-rays to that of the 616.3 keV transition was independent of the beam energy used to activate the target.

2.2. Conversion electron experiments

The fact that the half lives were approximately the same, along with constant intensity ratios (independent of the bombarding energy) between the 616.3 keV transition and

Table 1 Energies and intensities of decay y-rays

Energy (keV) /r a

in-beam decay

36.155(15) 36.184(17) 91.4(48) 56.120(37) 56.128(23) 36.4(22)

135.372(13) 135.348(14) 100.0(19) 205.093(60) h 205.206(26) 44.9(20)

a Observed after target irradiation and normalized to 100 for the 135.4 keV transition. h In-beam peak is part of an unresolved doublet with the much stronger 205.8 keV line from Coulomb excitation of 192Os.


=

. , . . . ~

o Z

104

10 3

102

I

r ' l . . . . I . . . . L '

IK. 129.4 keV (p,2n)

\ 135.4 keV \

36 2

300.5 keV (p,4n) "~ . ~

56.1 keV

/" 1

/

/

( 205.1 keV

2O I i [ ~ I i i I

25 30

Ebeam (MeV)

Fig. 2. Excitation functions of some lines observed with the 192Os(p,xnT) reaction. The 129.4 keV transition belongs to 1911r and shows the (p,2n) excitation function, while the (p,4n) excitation function is typified by the 300.5 keV line. The 36.2, 56.1, 135.4 and 205.2 keV transitions have excitation functions which exhibit a (p,3n) channel behaviour.

the four 3.2 h delayed y-rays, strongly suggested that the four new y-rays belonged to

the decay path of the 11- isomer. In order to explore this hypothesis further, conversion

electron experiments were performed at the ISKP Bonn using the double-orange iron-

free spectrometer [ 8]. For these measurements, a target of 1920S enriched to 99.0% with

a thickness of 110 p ,g /cm 2 and supported by a 30 i zg /cm 2 carbon foil was bombarded

with 27.8 MeV deuterons. The electrons from the reaction were momentum analyzed

with the orange spectrometer and detected with a plastic scintillator ( type NE102). The

typical resolution, A p / p , obtained was 0.5%. Portions of the electron spectra collected

after the irradiation of the target are shown in Fig. 3. The spectrum in Fig. 3a was

collected immediately after an irradiation and shows quite strongly the 26 keV M3

transition from the 1.2 h isomer. The spectrum in Fig. 3b again shows the low-energy

region but was collected approximately 6 h after the irradiation so that the 1.2 h isomer

has decayed significantly. As can be seen, the background in the two spectra is the


1800

1400

1000

"5 600 c c

2 © 200

3500 c

2500

1500

500

55 65 75 85 95 105 115

o) >

, , , , ,

~- b)

~ >

> > ~ ~ ~ od m - t

~ g

gz 188 ~92 1 ~ 200 ~

180 200 22O

Current (A)

c)

~z >

~ . _ r ~ 240

Fig. 3. Portions of the conversion electron spectrum observed after irradiation of a 110 ,ug/cm 2 192Os target with 27.8 MeV deuterons. The spectrum shown in (a) was recorded immediately after irradiation, whereas those in (b) and (c) were recorded 6 and 1 h, respectively, after target irradiation. The expanded part in (c) shows the detail of the 187 to 202 A region. The 38.9 and 186.7 keV lines are from transitions in 19°Os, and the 41.9 and 129.4 keV lines are due to transitions in 1911r. The remaining lines are from transitions in 19°lr.

same, and, in fact, is approximately constant for all analyzing currents. The higher

energy portion of the spectrum shown in Fig. 3c was collected approximately 1 h after

the irradiation. Many electron lines are observed, most of which belong to 19°Ir but with a few lines belonging to ~9°Os (186.7 and 38.9 keV) or to t91Ir (41.9 and 129.4 keV).

The presence of these lines, which have well-defined energies, proved useful in that they

provided an energy calibration. Using the peaks due to conversion of the 38.9, 41.9, 56.1 and 129.4 keV transitions as a calibration, the energy of the 26 keV M3 transition,

previously [ 1 ] reported as being 26.3 keV, was found to be 26.1 ± 0.1 keV. The multipolarities of the four new transitions were determined by considering the

values of the conversion coefficients or by examining subshell ratios. For the 56.1 and 135.4 keV transitions, the multipolarities were determined from the L subshell ratios,

listed in Table 2, to be MI and E2, respectively. The multipolarity of the 36.2 keV transition was determined by examining the conversion coefficients for the L lines using

the 56.1 keV LI line as a reference and, from the values listed in Table 2, was found to be El. The case of the 205.2 keV transition proved to be more challenging since the cex

coefficients for E1 and E2 are small and the difference is only a factor of 3. For the L lines the theoretical coefficients differ by a factor of -~ 10 but are still very small in both cases. A careful scan between 179 and 202 keV of the electron energy spectrum, shown in Fig. 4, revealed no real peaks which could be attributed to the 205.2 keV L lines. An upper limit on the value of cec, + aL2 favours an E1 multipolarity. For the K conversion of a 205.2 keV y-ray, the determination of the ceK coefficient would appear to be a less

74 PE. Garrett et al./Nuclear Physics A 611 (1996) 68-84

Table 2 Determination of multipolarities of transitions in 19°lr from electron singles measurements

Transition Quantity Experimental Theoretical a Deduced ( keV ) measured value values multipolarity

36.18 ~rt, l +or/, 2 0.974-0.25 MI 16.8 El 0.62 El E2 190

56.13 az, i/aL2 9.24- 1.3 MI 10.2 MI

El 1.8 E2 0.018

135.35 (or/, 1 + a/, 2 ) /aL 3 1.41 4- 0.09 MI 115 El 4.6 E2 1.5 E2

205.21 OlK 0.07 4- 0.02 M 1 0.68

El 0.053 El E2 0.16

aLj + OtL2 ~< 0.025 M 1 0.1 I El 0.0075 El E2 0.081

a Values calculated using Ref. [9].

9000

7OOO c c

2

5000

©

3000

1000

J ~o0s

> ~ o~

234 238 242 246 Current (A)

Fig. 4. Portion of the electron spectrum collected immediately after an irradiation. The 186.7 and 207.9 keV lines are due to transitions in 19°Os. The expanded portion of the spectrum shown in the inset has the positions labelled where the peaks due to 205.2 L conversions should be located.

P.E. Garrett et aL /Nuclear Physics A 611 (1996) 68-84 75

55O

450

550

25O

150

5O

~ 35° l 8 25o I

I

20 60 100 140 180 220

e (148.7 L3)- T o)

x-rays

b) e-(56.1 L1)- T

X ruys

2 ×-toys C)

e-(135.4 L2)- T

20 60 100 140 180 220

E: 7 (keV)

Fig. 5. Results of the e-y-coincidences. The T-spectrum in (a) is in coincidence with the 148.7 keV L3 line, that in (b) with the 56.1 keV LI line and that in (c) with the 135.4 keV L2 line. The peaks due to 19°Os in (c) are due to coincidences with the 196.5 keV K line which falls at the same position as the 135.4 keV L2 line in the electron spectra.

sensitive means to determine the multipolarity. However, as is shown in the expanded

portion of Fig. 3c, the 205.2 keV K conversion yields a small peak which can be fitted

with some reliability. The extracted value of aK also favours an E1 multipolarity.

In order to establish the decay scheme, and to determine how the four y-rays belong

in the decay path o f the 11- isomer, e -T-coincidences were performed. The y-rays

were detected with an Intertechnique LEPS detector which had a resolution of 0.9 keV

( F W H M ) at 135 keV. The results of the e -y -co inc idences are shown in Fig. 5. The

time window for accepted events was ~ 500 ns due to the very poor time resolution

of the LEPS detector. The coincidences between the 148.7 keV L3 line and the T-

rays demonstrates clearly (Fig. 5a) that the 56.1, 135.4 and 205.2 keV lines belong

in the decay path of the 11- isomer. The lines in 19°Os in Fig. 5c are present due to

coincidences with electrons from 196.5 keV K conversion which have the same energy

as the 135.4 keV L2 line. It is evident from Fig. 5 that there are two branches under

the 148.7 keV transition, one which contains the 205.2 keV line, and the other the 56.1

and 135.4 keV lines. From the knowledge of the T-intensities and multipolarities, the

branching ratio is determined to be 83.5 + 0 . 7 % for the 56.1 keV branch and 16.5 4-0.7%

for the 205.2 keV branch (assuming an E1 multipolarity for the 205.2 keV transit ion).

Of interest is the absence of the 36.2 keV y-ray in the spectra of Fig. 5, which implies


-10 0 5OO

200- i 100 50 20 10

E E 5 2 © 2

100

50 o o

2O

I0

2

-10

10 20 ,30 0 10 20 30

e-(38,9 L2)-e-(186,7 K)

"prompt"

tl/2~400 ps

llll lllh

. . . . NI, 0 10 20 30

b) e-(148.7 L3)-e-(56.1 LI) I

, tv2=3.7±0.2 ns q

I . . . . I l l 0 10 20 30

200 100 50

20 10 5

2

50

20 10 5

2

Fig. 6. e - e - time spectra. In (a) the time spectrum for coincidences between the 38.9 keV L2 line and the 186.7 keV K in m°Os is shown. The time resolution obtained was "~ 3 ns (FWHM) with a "decay" of approximately 400 ps. The time spectra for coincidences between the 148.7 keV L 3 line and the 56.1 keV L!

line (b), and the 56.1 keV Li line and the 135.4 keV L2 line (c) yield the same half lives of 3.7 ns. The 148.7 keV L3-135.4 keV L2 coincidence time spectrum (d) shows the typical behaviour of buildup followed by decay.

that either it does not belong to the cascade, or the level issuing this transition has

a half life greater than ,~ 2 /xs. The fact that the decay curve of the 36.2 keV T-ray

matched those of the other transitions from the 11 - isomer suggests the latter possibility.

Intensity considerations also favour the latter possibility, and it was determined that the

36.2 keV transition belonged to the branch containing the 56.1 keV T-ray, and not to

the branch that contained the 205.2 keV T-ray.

Lifet imes of the transitions were measured using the two orange spectrometers in co-

incidence, and the results are shown in Fig. 6. In Fig. 6a, the lifetime of the 186.7 keV

K line was measured with respect to the 38.9 keV L2 line of ]9°Os. As the previously

measured [ 10] l ifetime of the 186.7 keV transition, 363 + 6 ps, is less than the instru-

mental response of the system, ,-~ 400 ps, Fig. 6a is labelled as the "prompt" curve. The

data show clearly that there are two different levels which both have half lives of 3.7 ns,

within experimental uncertainties. Fig. 6b shows that the level between the 148.7 and

56.1 keV transitions has Tl/2 = 3.7 i 0.2 ns, and Fig. 6c shows that the one between

the 56.1 and 135.4 keV transitions has T1/2 = 3.7 i 0.1 ns. The time dependence shown

in Fig. 6d for the 148.7 and 135.4 keV transitions shows the buildup followed by decay

that confirms the existence of these two new isomeric levels. These results allowed the

unambiguous determination of the order of the 56.1 and 135.4 keV transitions in the

decay scheme.

In order to determine a precise value of the half life of the 11- isomer and to

PE. Garrett et al./Nuclear Physics A 611 (1996) 68-84 77

0

1 0 0 0 0

1 0 0 0

1 0 0

10

~ i ) h ~

0 1 0 0 2 0 0 3 0 0 4 0 0 5 0 0 6 0 0

T i m e ( m i n )

Fig. 7. Decay curves of the 56.1 keV L1 and 26.1 keV L 3 lines. The data points were collected in 12 min bins starting immediately after a target irradiation. The dashed line shows the 3.1 h component which would result in the 26.1 keV transition decay curve if it were fed by the 205.2 keV transition (the weakest of the lines following the decay of the 3.1 h isomer). As can be seen there is no indication of a 3.1 h component in the decay of the 26.1 keV transition.

investigate the possibi l i ty that one of its branches feeds the 1.2 h isomer, a decay

measurement was performed using the orange spectrometer. The magnetic field of the

spectrometer was consecutively and repeatedly altered from the 26.1 keV L3 peak

and its adjacent background to the 56.1 keV Li peak and its adjacent background. A

measurement cycle consisted of 3 scanning cycles, where for each scan, the magnetic

field was incremented to the values needed to count the various peaks or backgrounds

each for 30 s, then decreased. Therefore, each measurement cycle was 12 min in duration,

and 39 cycles were performed. The resulting decay curves are shown in Fig. 7. Weighted

least-squares fits to the data yield half lives of 3.087 + 0.012 h for the 11- isomer and

1.120 4-0 .003 h for the 26.1 keV transition. These values are much more precise than

those previously established [10]. The dashed curve shows the 3.1 h component of

the 26.1 keV decay which would result if it were fed by the 205.2 keV transition, the

weakest of the transitions observed following the decay of the 11 - isomer. There is no

evidence that the decay curve of the 26.1 keV line has a 3.1 h component, and it is

concluded that the 3.1 h isomer and the 1. I h isomer are independent of each other.

The fact that the 26.1 keV transition does not belong in the decay path of the 3.1 h

isomer, and also that the energy sums of the respective branches under the 3.1 h isomer

were not equal, implied that there were other very low-energy transitions. One possibi l i ty

was that there was a 22.5 keV transition fed by the 205.2 keV 3~-ray (or vice versa).

Rather than search for the 22.5 keV L conversion lines at ,-~ 10 keV where the efficiency

of the spectrometer is very low and where Auger transitions would be expected, it was

decided that a search for the M conversion lines would prove more fruitful. Shown in

Fig. 8a is a scan around the 20 keV region approximately 10 h after bombardment of a 192Os target 83 /~g/cm 2 thick a on 32 /zg /cm 2 thick carbon foil. As can be seen, a


Current (A) 68 70 72 74

22.5 keY %

51000 ~ ~ ~ ~ a)

30000

29000

c E o 28000 ¢- o

:- 55 e - (22 .5 M) - "y b)

o (.9 45

8 35

X-rays

50 150 250 350

E (keV)

Fig. 8. Portion of the electron spectrum in the vicinity of 20 keV showing the small peak due to M conversion of a 22.5 keV transition. The lower portion of the figure shows y-coincidences with the peak in (a). The lines due to 19°Os are coincidences with electrons in the "tail" of the 38.9 keV L peak (see Fig. 3).

small peak is observed at the exact position expected for 22.5 keV M-conversion lines. A search through the literature reveals that there are no other possible isomeric lines which would be produced in reasonable quantity in the reaction that would give rise to this peak. It should also be noted that the peak is consistent with an E2 multipolarity for the 22.5 keV transition. If the 205.2 keV transition had a multipolarity of E2 instead of E1 (a possibility given the uncertainty on aK and aLl Jr-aL2), the 22.5 keV transition would be El in nature. However, the Ml, M2 and M3 conversion coefficients are approximately equal for an El multipolarity, and thus the peak shown in Fig. 8a would be expected to be much broader. For an E2 multipolarity, on the other hand, the M2 and M3 coefficients are much greater than the others, with M3 the largest. This is consistent with the observed peak shape. Conclusive evidence that the aforementioned peak belongs to 19°Ir comes from e-y-coincidences shown in Fig. 8b. A small peak in

PE. Garrett et aL/Nuclear Physics A 611 (1996) 68-84 79

the y-ray spectrum at 205 keV is observed to be in coincidence with the 22.5 keV M

lines. Other peaks observed in the y-ray spectrum are due to ~9°Os which arise from the

tail of the peak due to the 38.9 keV L electrons. Using the intensity of the 616.3 keV transition, which results solely from electron

capture from the 11- state in J9°Ir, observed in the y-ray experiments (see Subsec- tion 2.1), the electron capture branching ratio is determined to be 91.4 :t: 0.2%. This is

in reasonable agreement with the value of 94% from the earlier study [ 1 ]. The present value assumes the above determined multipolarities for transitions in 19°Ir, and uses

values for conversion coefficients from Ref. [9].

3. Discussion and interpretations

3.1. The level s cheme

As was discussed in the introduction, the I ~ value of the 3.1 h isomer is known

to be 11-. This state decays via an M4 transition to a 7 + state. It was at this point

that Harmatz and Handley [ 1 ], based on the data which were available at the time, proposed that the 26 keV M3 transition proceeds from this 7 + state to the ground state (spin 4), this being the only way to explain the angular momentum difference. The results presented in the previous section, and consideration of the results [2] of the

single-nucleon transfer reactions, discussed below, lead to the decay scheme presented

in Fig. 9. The previous arguments [ 1 ] regarding the parity of the ground state are thus

invalid. In the (d,t) study [2] of 19°Ir, the lowest energy state populated was determined to

be 4 - . This assignment came from the strong similarity (see Ref. [2]) with the level at 66 keV in 192Ir. In fact, the three lowest levels populated in 192Ir with the (d,t)

reaction [11] are analogous to the three lowest levels populated in the 191Ir(d,t)19°Ir

reaction. A 1- level was assigned [2] at 25.9 + 0.1 keV relative to the 4 - state in 19°It. No levels were observed to lie between the 4 - state and the 1- state. A question then arises as to what are the possible decay paths for the l - state. The 193Ir(d,t)192Ir

experiment populated all known negative-parity states with spin ~ 4 below 200 keV, and the J91Ir(d,t) reaction would populate all the corresponding levels in 19°Ir. Therefore,

it is likely there are no other negative-parity levels with spin ~< 4 in the vicinity of the 4 - state in 19°Ir that have not been detected (assuming, of course, that there are no unresolved doublets). In 192Ir, the ground state was assigned as having I '~ = 4 +. If

a 4 ~ state forms the ground state in 19°It, the 1- level could decay to it via an E3 transition, as is observed in 192Ir, with a half life on the order of minutes. However, no such transition has been observed. The very good correspondence between the energy spacing of the 4 - and 1- levels in the (d,t) study (25.9 + 0.1 keV) and the energy of the M3 transition (26.1 4-0.1 keV) suggests that the 1.1 h isomer be identified with the 1- state, and that the 4 - level be identified as the ground state.

The 3.1 h 11- isomer, which decays via an M4 transition, feeds a 7 + level. From the


3.O9 h 376.4 ~ 11

148.7 M4

3.7 ns 7 + 227.66

F 56.13 M1

s 6+ 205.21 E1

171.53

m I 1135.35 E2

>2 ,u.s 36.18 ~ 4 +

6- 0 5 22.45 E2 4

112 h 26.0 ~ 1

26.1 M3

19Olr

Fig. 9. Level scheme associated with the decay of the long-lived isomers in 19°Ir as deduced in the present work. The relative intensities of the electrons (light portions of the arrows) were calculated using the tables of Hager and Seltzer 191.

results of the previous section, the 7 + level decays via two branches, a 205.2 keV E1

transition and a 56.1 keV M1 transition. I f the 56.1 keV transition populated a spin 7

or 8 state, as would be allowed by angular momentum coupling, the 11 - isomer would

also populate those states via a 204.8 keV M4 or E3 transition, respectively. Since this

is not the case, the 56.1 keV M1 transition must populate a 6 + state. Both the 7 + and

6 + states must form band heads. I f the 7 + level were a member of a band based on

the 6 + state, the rotational parameter would be 4 keV, much smaller than the values

observed in the odd-A neighbours. A similar argument rules out the possibil i ty that the

6 + level is a member of a K ~" = 4 + band. If the 6 + state were a member of a K ~" = 5 +

band, one would expect the 135.4 keV transition to be of M1 character, rather than E2.

Furthermore, there are no K ~" = 5 + bands predicted to lie close to the ground state.

The decay of the 6 + band head proceeds via the 135.4 keV E2 transition, and so it

could feed levels with I 'r = 4+-8 +. The spin range can immediately be narrowed to

4+ -6 + using arguments on the isomer decay stated above. Further restrictions on the

spin can be placed by considering how such a state could decay. Since the 36.2 keV E1

transition must lie below the 135.4 keV line, and assuming no unobserved transition lies

in the decay path, this can change the spin by at most 1 unit. Therefore, the 36.2 keV

El transition feeding the 4 - ground state can depopulate a 4 + or 5 + level. The spin

of 4 + is favoured since it is expected to be the lowest-lying positive-parity state, as it

forms the ground state in 192Ir. The 205.2 keV transition is assigned as depopulating the

7 + state, feeding a 6 - state (the spin of 6 is chosen due to the fact that it is not fed

directly by the 11- isomer) , which decays by the 22.5 keV transition to the 4 - ground

PE. Garrett et al./Nuclear Physics A 611 (1996) 68-84

Table 3 log f t values to states in 19°Os from the 19°Ir ground state

81

Excitation Configuration Transition log f t energy (keV) 1, K ~ Allowed (A) / I st Forbidden (F)

955.2 4.2 + F 8.7(4) 1163.1 4,4 + F 8.2(3) 1386.9 3, 3- A 8.6(5) 1446.0 5, 4 + F 8.7(4) 1583.8 4, 3- A 7.5(2) 1681.6 5- A 6.6([) 1708.3 ( 3 +, 4 + ) F 8.9 ( 2 ) 1872.1 (5,3-) A 6.8(2) 1903.3 ( 3 +, 4- ) 7.4(2)

state. The other scenario, that the 22.5 keV transition depopulates the 7 + state feeding

a 5 + state, is rejected by considering the possible decay paths. A 5 + state at 205.2 keV

would be expected to decay not only to the 4 - ground state, but also via a 169 keV

M 1 / E 2 transition to the 4 + state at 36.2 keV. As no 169 keV transition has been found,

the 205.2 keV y-ray is assigned as depopulating the 7 + level.

3.2. Consequences o f the I~=4 - ground state

The fact that the ground state is 4 - and was populated in the (d,t) reaction [2]

allows the extraction of the mass excess of m°Ir. Using the (d,t) Q-value to the 4 - state

of - 1 7 6 9 . 3 i 0 . 4 keV, the mass excess is calculated to be - 3 6 . 7 5 4 4-0.003 MeV. This is

consistent with, but considerably more precise than, the value of - 3 6 . 7 1 0 + 0.200 MeV

published in the most recent mass tables [ 12]. This value can be used to calculate the

electron capture Q-value, Qac = 2.030 + 0.004 MeV. Since the log jet values for electron

capture have a dependence on the QEC value, especially for high excitation energies

where QEC - Eex is small, these have been recomputed for the ground state decay and

are given in Table 3. The l o g f values were obtained from Gove and Martin [ 13].

The electron capture from the 4 - ground state proceeds via allowed transitions to

negative-parity spin 3, 4 and 5 states, and first forbidden transitions to positive-parity

spin 3, 4 and 5 states in 19°Os. Hence, what were previously believed to be first

forbidden transitions are now allowed and vice versa. Three allowed transitions proceed

to the K '~ -- 3 - octupole band head (at 1387 keV), the 4 - rotational band member (at

1584 keV) and the 5 - member (at 1872 keV), with l o g f t values of 8.6, 7.5 and 6.8,

respectively. As was shown in Ref. [ 14], and the results of which are still valid, the

ratios of the f t values agree nicely with the ratios of the square of the Clebsch-Gordon

coefficients for the transitions. While there is not a great difference in the log f t values

of al lowed and forbidden transitions, it is satisfying that the average log f t value for the

allowed transitions (7 .3) , is now less than that for the first forbidden transitions (8.6) .

The measured value [ 14] of the magnetic moment of the 19°Ir ground state is ]#g.s.[ --

0.04 ± 0.01/xN. The possible Nilsson configurations having K ~ = 4 - which may be

8 2 P.E. Garrett et al./Nuclear Physics A 611 (1996) 68-84

expected in 19°Ir are the - ½ + [ 4 0 0 ] ~ + 9 - [505 ]~ , 3+[402],~ + 5 - [ 5 0 3 ] ~ and the 1 + 7 - 0 6 free [400]~, + [503]~ configurations. Using gR = Z/A, g.~ = . gs and Nilsson wave

functions calculated with e2 = 0.15 and e4 = 0.05, the above configurations are predicted to have values of /zl = --0.09(--0.34)/ZN, 1.63(1.45)/zN and 0.75(1.00)/xN, respectively, where the values in parentheses are computed with gs = g~ree. The only configura-

9 - tion which comes close to the measured moment is - ½ + [400],~ + ~ [ 505 ] ~. However, this configuration alone would not explain the single-neutron transfer results of Ref. [2].

The level which is now assigned as the ground state was populated with an l = 3 tran-

sition with a spectroscopic strength of $3 = 0.100 ! 0.008. This implies that the wave

function must have a component on the order of 18% of a 5 + [ 4 0 2 ] ~ ® (v f5~2 o r f 7 / 2 )

3+[402]~ r + 5- 1+[400]~ + 9-[505],, and ~ [503]v configuration. A mixture of the - 2 configurations could simultaneously explain the single-neutron transfer results and the

/ f ree magnetic moment, provided t h a t g,free > gs > t).Ogs . Similar large mixings of two-

quasiparticle configurations have been invoked for the 4 + ground state of 192Ir [ 16] and

a satisfactory theoretical explanation for both these cases remains to be found.

3.3. Interpretations

3.3.1. The negative-parity levels As was shown above there exists no satisfactory interpretation for the 4 - state ground

state. There are, however, many other degrees of freedom that cannot be taken properly

into account at the present time, such as the effects of y-softness, and it is possible

that the 4 - state is strongly influenced by these. The 6 - state at 22.5 keV may also fall into this category. The 3+ [402]~-+ 9 - [ 5 0 5 ] ~ configuration is expected to lie at

low excitation energies, but has been tentatively assigned at 441.7 keV in the (d,t) study [2]. No other 6 - states are expected at low energies, and thus the nature of this

level remains an enigma. 3+[402]~r- The 1 - state at 26 keV was discussed previously [2], and shown to be the

1 - [ 510] configuration. Using the theoretical conversion coefficient computed from the 2 v

tables in Ref. [9] , the 26 keV M3 transition has a B(M3) value of 1.9 × 10 -3 W.u. Other

M3 transitions observed in this mass region [ 15] have B(M3) values of ,-~ 30 × 10 -3 W.u., and thus the 26 keV transition is hindered, either via f-forbiddenness (where

the angular momentum rules are satisfied but the wave functions do not overlap) or ~ - forbiddenness. The 11- isomer, now assigned at 376.4 keV, was previously assigned [ 1 ] as the - ~ - [ 5 0 5 ] ~ - + -~+[615]~ configuration. The B(M4) value for the 148.7 keV

transition is 2.6 W.u., and thus is unhindered.

3.3.2. The positive-parity levels The 4 + state at 36 keV should have a configuration similar to the ground state in

3+ [402]~- + ~ + 192Ir, which has been assigned [ 16] as a mixture of the - 2 [615]v and

I I - - "rr 3 - - T [505] - [512]~ configurations. Since the t89Os ground state is the 3-[512]~,

orbital, the ~ - [ 505] ~ - 3 - [512] ~ configuration should have been populated in the

PE. Garrett et al./Nuclear Physics A 611 (1996) 68-84 83

189Os(a,t) 19°Ir reaction with an 1 = 5 transition. In the single-proton transfer study [2] ,

a large amount of l = 5 strength was located at 465 keV, and this was tentatively - - 3 - attributed to the K ~ = 4 +, ~ - [ 5 0 5 ] ~ ~ [512]~ configuration. An examination of

the (a , t ) spectrum at 50 °, published in Ref. [2] , indicates that at an energy of 36 keV

an upper limit of ,-~ I # b / s r can be established. This corresponds to an upper limit of 3 - 4% for the admixture of the ~ - [ 5 0 5 ] r r - 2 [512] , configuration in the 36 keV

4 + wave function, and thus the tentative assignment of the -3+[402]~r + ~ + [615]~

configuration is made. This is much different than the ground state in 192Ir, where, in 3 - order to explain the magnetic moment, the admixture of the ~ - [ 5 0 5 ] ~ - ~ [512]~

configuration needed is ,-~ 50%. While the negative-parity states showed [2,11] a great

similarity in their structures between 19°Ir and 192Ir, the 4 + states appear to be remarkably

different. This difference is also reflected in the upper limit of the B(E1) value for the 4 + to 4 - transition, which must be less than 10 -6 W.u., whereas in 192Ir the 4 - to 4 +

transition (66.8 keV) has a B(E1) value of 2.8 × 10 -5 W.u. (Ref. [ 16]).

The 7 + level at 227.7 keV could have the configuration 3 + [ 402 ] ~r + ~ + [ 615 ],,. This

would be populated by an l = 6 transition in the (d,t) reaction, but unfortunately it

would be masked by some rather strong 1 = l and 3 transitions. The other possible

candidate forming a 7 + band head is the ~ - [ 5 0 5 ] ~ r + 3 - [ 5 1 2 ] ~ configuration, but

this should have been detected in the single-proton transfer reactions. As it was not, the

3+2 [402]~ + ~ + [ 6 1 5 ] ~ configuration is favoured. It should be noted that both of the

above configurations would be consistent with an unhindered M4 transition from the

11 - isomer.

There are two candidates for the 6 + level at 171.5 keV, as both the - ~ - [ 5 0 5 ] ~ - +

~ - [510] ~ J and ½ + [ 400] ~ + ~ + [ 615 ] ~ configurations are predicted to lie at low energies.

Neither configuration would be populated strongly in the (d,t) or the (o~,t) reactions.

The 135.4 keV E2 transition which depopulates this level has a B(E2) value of 22 W.u.;

this large value implies that there must be a large overlap of the wave functions of the

initial and final states. As the 4 + level is favoured to have the - 3 + [ 4 0 2 ] ~ . + ~ + [ 6 1 5 ] ~ 1 + ~ + configuration, the 6 + state must have the i [400]~-+ [615] , configuration. It

should be noted that a similarly large B(E2) value has been observed [ 17] for the 1 ~- l+ l+ U r 3+ +[402]~ inl91Ir, = ~ , ~ [400]~- --~ = ~ , 3 transition and has been attributed to

t +[400]~ wave function. an admixture of the K - 2 y-vibration in the

4. Conclusions

Five new transitions were observed to belong to 19°Ir, and the results of e - y - and

e e -coincidences, along with decay measurements and lifetimes of levels, established

a new decay scheme for the long-lived isomers. The new transitions are shown to belong

to the decay path of the 1 l - isomer, while the 1.1 h isomer, previously thought to be

a 7 + state, is identified with a 1- state lound in the single-nucleon transfer study. The ground state is identified as the 4 - state observed in the (d,t) reaction, and this


allowed a new precise value of the mass excess to be determined for 19°Ir, lowering the uncertainty from -t-200 to -4-3 keV. The establishment of the decay scheme of the 11 - isomer will provide a crucial link between the results of in-beam y-spectroscopy and charged-particle spectroscopy for 19°Ir.

Acknowledgements

The authors would like to thank Dr. H.J. Maier from LMU Mtinchen and J.R Richaud for producing the high-quality 192Os targets, and the cyclotron crews at the PSI and the University of Bonn. Financial support was provided by the Natural Sciences and Engineering Research Council of Canada, by the Swiss National Science Foundation, by the Deutsche Forschungsgemeinschaft, and by the Paul Scherrer Institute, Villigen, Switzerland.

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