Excited states of 188Ir populated by the (α, 3n) reaction

Nuclear Physics A425 (1984) 397-410 @ North-Holland Publishing Company

EXCITED STATES OF 1881rPOPULATED BY THE (a, 3n) REACTION* A. J. KREINER**,

C. ALONSO ARIAS, M. DEBRAY, D. DIGREGORIO

and A. PACHECO

De~artumento de Fisica, Comisidn Nacionai a’e Energia Atdmica, Av. def ~~bertador 8250, 1429 Buenos Aires, Argentina

and J. DAVIDSON**

and M. DAVIDSON**

Departamento de Fisica, Fact&ad de Ciencias Exactas y Naturales, UBA Ciudad Vniversitaria, 1429 Buenos Aires, Argentina

Received 8 February 1984 Abstract: The deexcitation of states in IssIr, formed through the 18’Re(cr,3n)‘881r reaction, was studied using in-beam and off-beam gamma and electron spectroscopy techniques. A new level scheme, exhibiting a rather high level density, is presented. An inte~~tation of the structure of these states is attempted using i~ormation on one-quasiparticle intrinsic states from nei~bo~ng oddproton and odd-neutron nuclei.

E

NUCLEAR REACTION “‘Re(a, 3ny), E = 30-55 MeV; measured E,, I,, u(E,, E,, t), yy-coin, I, ~. “‘Ir deduced levels, J, R, T’,,, ICC. Ge(Li) and refrigerated surfacebarrier detectors.

1. Introduction The information on IssIr available at the present time in the literature is rather scarce I). Only a few low-spin states are known from the electron-capture decay of the ground state of lssPt [refs. ‘*“)I. In addition, a millisecond isomer has been known for some time through the existence of a set of lines decaying with a common half-life 3*4) of 1: 4.1 ms. In the present work the ‘*‘Re(a, 3n) reaction was used to study thoroughly lssIr utilizing in- and off-beam y-ray and conversion electron spectroscopy techniques. As a result of this a new level scheme was obtained which incorporates the known lines and some others in a consistent way. The decay scheme obtained in the present work is rather complex and not typical

* This research was performed under the auspices of the Consejo National de Investigaciones Cientificas y Tknicas of Argentina and the National Science Foundation of the USA, under agreement no. INT-811906. ** Fellows of the CONICET, Argentina. 397 September 1984

398

A. J. Kreiner et al. / “‘ir

of those obtained from (~1,xn) reactions in deformed regions where only a few rotational bands collect the reaction intensity. In this case most of the population is trapped at the T+ N 4.1 ms isomeric state and from there decays through a series of transitions connecting in most cases different intrinsic states. The level density is high resulting from the coupling of low-lying quasiparticle states known in neighboring odd-proton and odd-neutron nuclei.

2. Ex~rimental

procedures and resuits

Enriched targets of rs7Re were bombarded with or-particles provided by the synchrocyclotron of Buenos Aires. Excitation functions of discrete y-lines for the ls7Re(o:9xn) reaction were measured in the E, = 30-55 MeV bombarding energy range, in 5 MeV steps, and the (a, 3n) reaction leading to the residual nucleus l**Ir was found to reach its maximum at about 40 MeV. Fig. 1 shows a y-ray spectrum at that projectile energy measured with an 8% eflicient coaxial Ge(Li) detector. The largest lines are seen to belong to iSSIr. This spectrum contains both prompt and delayed components produced respectively during the impact of the beam and inbetween the beam bursts of the sync~ocyclotron. Table 1 gives the y-ray intensities obtained from singles spectra taken at 40 MeV with both the Ge(Li) and an X-ray counter. The arguments leading to the multipolarities are given later in the paper and are related to (a) conversion-electron measurements, and (b) consistency arguments from a quantitative evaluation of coincidence spectra. In addition, the time distribution of the y-rays was studied in the microsecond range using a gating signal from the modulation circuit of the cyclotron and in the

ENERGY

[ keV]

Fig. 1. Singles y-ray spectrum from the “‘Ir(a,xn) reaction at E, = 40 MeV obtained with a Ge(Li) detector placed at 900 with respect to the beam direction.

A. J. Kreiner

et al. / “‘Ir

399

TABLE 1 y-ray energies

Era)

and intensities, total transition intensities, adopted multipolarities and total in the ‘s’Re(a, 3n) reaction at E, = 40 MeV coefficients for tssIr transitions

1;

I 101

conversion

Multipolarity

@?’d

El Ml+E2 Ml+E2 Ml El Ml E2+Ml

1.63 14.8 ‘) 26.2 ‘) 6.14 0.243 11.4 6.08 ‘)

1

(kW 33.0 42.0 54.8 56.2 66.2 81.6 96.8 97.2 114.6 137.9 142.9 149.9 156.2 166.2 188.0 202.2 215.4 “) ‘) c) d,

26+8 84+8 136+_9 40+3 778 f 47 60+4 28F14b) 158+ 10 130+_24 400&25 961&49 1736+89 898 k48 215*19 125k13 190+_ 16 832 ?r 46 480 +25

1.8 *0.6 36 +3 100 *7 7.7kO.6 26.0+2 19.9& 1.3 5.3+2.6 29 k5 58&4 60 +3 54 +3 48 k3 11 *1 8.5+0.9 5.5 kO.5 23.9+ 1.3 16.5,0.9(22.4*

1.2)

Ml Ml E2 El E2 E2 Ml El El E2(Ml)

7.19 4.35 1.33 0.162 0.97 0.9 1.52 0.08 0.067 0.273(0.73)

0.1 ,< AE, < 0.2 keV. From branching given in ref. ‘)_ From ref. 6). Theoretical values from ref. ‘).

millisecond range with the aid of a mechanical chopper. All iasIr lines shown in fig. 1 (and also all lower energy lines detected with the X-ray counter) were found to have both significant delayed, and to a lesser extent prompt, components. This means that most of the intensity for the 3n channel goes through an already known Tt = 4.1 kO.3 ns (weighted average of the present work) isomeric state ‘) but also that none of the observed lines depopulates directly the isomer suggesting its decay by a highly converted low-energy transition. No other half-lives were detected in the time ranges studied here. Extensive yy- and yX-coincidence measurements were performed in order to establish a decay scheme. Fig. 2 shows several coincidence spectra obtained by gating on an X-ray detector. Fig. 3 shows a spectrum taken with an X-ray counter in coincidence with the 142.9 keV line. The yX coincidences were essential in disentangling the coincidence relations involving some lowrenergy lines. Tables 2 and 3 give a quantitative evaluation of the coincidence spectra. No clear band structures are visible, a factor which precluded the construction of a level scheme until further information was obtained on transition multipolarities. In fact, for heavy nuclei and low transition energies the y-ray spectra are heavily “distorted”

COUNTS PER CHANNEL

--114.6

k:

A. J. Kreiner et at. / ‘sslr

--l__----

6-9s P%

SIN II03

402

A. J. Kreiner et al. 1 ‘881r

A. J. Kreiner

403

et al. 1 “‘Ir

TABLE 3 Total

transition coincidence

intensities for several spectra gated on an X-ray counter (corresponding

to

fig. 2)

E, (keV) Gate energy

97”)

114.6

137.9

142.9

149.9

440) 3~2) 78(6) 29(l) 21(6) 256(17)

64( 10)

166.2

188.0

202.2

215.4

(keV) 33.0 42.0 54.8 66.2 97”) 114.6 137.9 142.9 149.9 188.0 202.2

Wb) 66(20) ‘) 19(6)

39(16)

65(9) 28(5) 76(7) 34(2) 80(9) 184(12)

4(x10) 77(23) 270) 24(9)

30(;

46(2)

30(5) 138(16) 56( 10)

35(;

103(7)

LL

54(5w)

35(2) 62(10) 120(10)

w

49(4) 97(6)

W

W

W

18;) 61(10)

187 Re (~(45MeV),x~ ‘188

A 743(Mlh75_O(EZl

*Lw (Ml)

f

If 0

18805 0

Ir

‘870s

187

_

A

A

L M 1100 (Ml)

1379 fE2)

L M

l rl

F

L 1429 (El)

z g

*I

10

L M 149.9 (E2CMl)I rnI

Ai

K

L 155.0(E2) 01 186-Z (E3) L M

5x10’

2154 (E2)

K

10

t

t

100

200

FLANNEL Fig. 4. Delayed

electron

spectrum

measured

I

I

300

NU~BEff

with a refrigerated

9(4) W

and affect the last digit.

I

37(g) 7(l)

33(2)

34(3)

M 64.6(M1)+653CEZf

.\972

lw)

llS(12)

90(14) 116(12)

“) 96.8 and 97.2 keV lines unresolved. b, w indicates the presence of a weak line. ‘f Errors are given after the value in parentheses

ai

3217)

50(2)

55(5) 86(16) 106(12)

172(40)

W

19(2) 76(12)

Si surface-barrier

detector.

A. 1. Kreiner et 41. / ‘**Ir

404

by the strongly competing internal conversion decay. Thus, the complementary information has to be obtained from conversion electron measurements. For this purpose a refrigerated Si surface-barrier detector (1700 m thick and active area of 50 mm*) was placed through a telescopic system into the reaction chamber. Electron and y-ray spectra were recorded simultaneously between the beam bursts in order to eliminate prompt contributions from other reaction channels. Fig. 4 shows such an electron spectrum. The strong 186.2 keV E3 isomeric transition from i”Ir [refs. “,“)I was used for normalization. This spectrum is not of good quality, the limitation to the resolution stemming from target thickness effects (the detector itself was rather good having a resolution of about 3 keV at 970 keV). The target material was ‘*?Re in form of metal grains and enriched to 99.2 & Rhenium is difficult to handle because of its very high melting point. Here the metal grains were reduced mechanically in size as much as possible an glued onto a mylar backing. In spite of the poor resolution and due to the very large differences in internal conversion for El, Ml and E2 multipolarities this measurement turned out to be crucial for the construction of the level scheme. Table 4 gives the results for

TABLE 4

L-shell conversion coefficients measured in the ‘*‘Re(c(, 3n) reaction at 45 MeV (corresponding to fig. 4)

97

0.6

+0.3

114.6

0.7

*0.3

137.9

0.7

40.2

142.9

0.03 +0.02

149.9

0.4

202.2

0.020+0.015

215.4

to.1

0.14 fO.08

0.94 3.39 0.065 0.59 1.58 0.04 0.34 0.69 0.025 0.32 0.56 0.023 0.27 0.46 0.02 0.12 0.12 0,009 0.10 0.10 0.008

The three numbers in the column of a? are for Ml, E2 and El transitions. “) From ref. “).

A. J. Kreiner et al. 1 ‘ssfr

405

the L-shell conversion coefficients for some of the lssIr lines. Theoretical were interpolated from the tables of Rose1 et al. “).

values

3. The level scheme As mentioned in the introduction only a few low-spin low-lying energy levels are firmly established from radioactive decay studies of “‘Pt [ref. “)I. The only contact between the published ‘) level scheme and the one presented in this work, shown in fig. 5, is the common population of the two first excited states. The ground state (gs.) is reported ‘) as having 1” = (Z)- and the first excited state at 54.8 keV as I* = (i,2)- and decays to the g.s. through a transition of known ‘) multipolarity; in this work this transition is in fact the strongest one (see table 1). The second excited state is at 96.8 keV, having I” = (1,2).- and decays through two transitions, of 96.8 and 42.0 keV, to the g.s. and first excited state respectively. Apparently the highest spin populated in the ls8Pt decay is 2 and hence we shall suppose that the new levels seen in this work have at least I = 3. The rest of the scheme is completely new. The main arguments which lead to this scheme come from a careful quantitative evaluation of singles and especially coincidence spectra

.923.7 v15.7 676.0

215.4 202.9

(8.9)‘ w -

(5)’

I

354.3

I_

-42.0

Fig. 5. Proposed level scheme for lssIr.

922

211.4

96.6 54.8 0.

406

A. J. Kreiner et al. / L881r

interpreted in the light of the multipolarity information from the internal conversion measurement. In particular, the recognition that the strongest line in the y-ray spectrum, namely the 142.9 keV transition, is El was essential to disentangle the level scheme. We shall discuss some of the arguments leading to the level scheme. The line at ‘v 97 keV is double since a component of it is in coincidence with the 54.8 keV transition (see fig. 2 and table 2) while it is not with the 42.0 keV line. The main part of the ir 97 keV doublet belongs to this new transition (of 97.2 keV) and has Ml character (table 4). This situation defines a new negative-parity level at 152.0 keV and its spin has been chosen as I = (3) since it is not observed in the 18*Pt decay while it decays through Ml to a level of maximum probable spin 2. The strong coincidence (fig. 2) between the 97.2 and the 202.2 keV (of El nature, table 4) lines suggests a positive-parity level of spin 4 at 354.3 keV since a lower spin value would imply transitions to the lower levels of spin 2. This level is confirmed by two other y-ray sequences: (a) the 114.6 (Ml) and 142.9 (El) keV, and (b) the 166.2 and 188.0 keV transitions, defining two other new negative-parity levels of I = (3) at 211.4 and 166.2 keV respectively. The level at 211.4 keV is further corroborated by the presence of the 156.2 keV line. The next strong transition is the 137.9 keV line (most likely E2, see table 4) which defines a positive-parity level at 492.2 keV of I = (6). This line is actually the crossover of two other 81.6 and 56.2 keV transitions which are clearly seen in coincidence with the 142.9 keV line (fig. 3) while they are not with the 137.9 keV line itself. Possibly the three levels at 354.3, 435.9 and 492.2 keV are members of a distorted rotational band. The following 149.9 keV line, most likely of E2 character, defines an I” = (8)’ level at 642.1 keV. From here on a new parity change occurs. The 66.2 keV line shows a very large y-ray intensity in singles and coincidence spectra which is only compatible with El multipolarity. A similar argument applies to the 33.0 keV line which would be unacceptably large (in coincidence spectra) if its multipolarity is different from El. The two new levels at 675.1 and 708.3 keV are most likely constrained to I” = (8,9)-. The highest lying transition is the 215.4 keV line which is either E2 or Ml (table 4). The level scheme is consistent, in a quantitative way, with the coincidence information given in tables 2 and 3. It must be said that while we consider the level scheme up to the 642.1 keV state as firmly established the degree of completeness is somewhat lesser from there on. the out-of-beam intensity balance (measured between the In particular, synchrocyclotron beam bursts) coming from the 4.1 ms isomeric state is very good below this level while some intensity is missing from above indicating that, at least, there are some unobserved transitions. Especially, as already mentioned in sect. 2, the isomeric transition itself remains unknown. It should be mentioned that there has been a singte and relatively recent

A. J. Kreiner et al. 1 ’ “Ir

attempt variance

6, to construct

a level scheme

with the one presented

of “‘Ir

407

which

is, however,

at complete

here.

4. Discussion The rather irregular ordering of the levels and the successive changes of parity indicate a relatively dense sequence of intrinsic states. A basis for an, at least qualitative, understanding of the l*‘Ir structure can be gained from the knowledge on intrinsic proton and neutron states in neighboring Ir and OS nuclei. Since ls81r has 111 neutrons we shall take its isotone 1870s [ref. ‘)I to obtain the intervening intrinsic neutron states. Up to 2: 300 keV they are the f-[510], $-[512], 5-[503] and y’ [615] Nilsson orbits 7). As far as the protons are concerned there are two possible choices, namely ls71r(N = 110) and “‘Ir(N = 112) [refs. 3,4)]. The dominant low-lying intrinsic states are, on one hand, the positive-parity $+[402] and ++[400] Nilsson orbits and, on the other hand, the strongly Coriolis distorted systems of levels based on the high-j h, and h, orbitals. On account of the fact that Ir nuclei in this mass region have prolate-like shapes ‘) (y < 30”) the h, particle-like bands have decoupled character while the h, hole-like bands have “strongly coupled’ (dl = 1) character. While the positive-parity proton orbits, and to a lesser extent the h, orbital, remain remarkably constant in excitation energy as a function of neutron number the h4 excitation shows a marked decrease from N = 112 to N = 110 [eventually this excitation becomes the ground state in lighter Ir nuclei ‘)I. To obtain the excitation energy representative for 1881r (N = 111) we shall adopt, as a first-order procedure, the average from the N = 110 and 112 values. Table 5 gives the relevant figures for the excitation energies of h; and h, orbitals. As can be seen these two states cross each other lying only 28 keV apart for I = 111. When these proton excitations combine with neutron orbitals in the doubly odd nucleus it is likely that the relative ordering will be altered to some extent due, in particular, to the proton-neutron (pn) interaction.

TABLE 5 Excitation energies (in keV) with respect

to the ground states for the h, ,,z and h,,, orbitals and linear interpolation for ‘**Ir (N = 111) Neutron

number

Orbital N= nh ,112 “h,,,

110

434 186

N=

112

372 563

N = 111 403 375

in 1s7.1891r

A. J. Kreiner et al. / “‘Ir

408

It is worth pointing out that a situation in which the h, lies below the h, excitation is more likely to produce isomerism (we shall come back to this point shortly). We shall now construct a zero-order approximation to the excitation spectrum of 18sXrjust by combining the neutron states of ls70s with the interpolated proton states. For intervening states of good angular momentum projection quantum number D we may label the band-head states in rssIr with Kg = lQ,iQ,[. The situation is somewhat more involved for those states in is81r in which the decoupled h, proton is participating. The decoupled band-head state I” = z- has as its largest component the $-[541] orbital which is, however, strongly admixed with the other larger-Q (a = $ s,...) orbitals of h, parentage. This state is furthermore characterized by a fairly large value for the alignment which is the average of the particle’s angular momentum along the rotation axis 9, being of the order ofj, = p. The coupling of such a proton state with a neutron having a good Q, value leads to a situation called semi-decoupling l”,ll). The band-head spin value can be approximately obtained in this case through the prescription ‘O,‘l) Z(Z+l) “v j&,+l)+C$ on account of the fact that both components lie perpendicular to each other. Table 6 and fig. 6 give the results of this zero-order procedure for issIr. Most of the states are labeled by the two Kg = IS, + S2,1 values. This degeneracy will in general be removed by the pn interaction. States labeled by a spin value given in parentheses correspond to semi-decoupled situations, The low-lying low-spin negative-parity states in ‘**Ir are obtained from the combination of the $‘[402] and )+[400] proton3S4) and the $-[SlO], $-[512]

TABLE6 Zero&order

approximation to the spectrum of *s’Ir

1ZQ*[Nn,A] = $+[402] E, = 0

f’[400] 106.5

$-[541] = 37.5

~-[505] = 403

0-, 1106.5 1-, 2116.3 3-, 4207.2 5+, 6+ 363.9

w+ 1375 (5)+ = 385 (6)+ 2 476 (7)_ L 632

5+, 6+ -403 4+, 7+ z 413 2+, 9+ 5 504 o-, 111660

(kev) $-[SO] 0

t-,2-

f-[512]

o-P3-

9.8 ;-[503] loo.7 q+[615] 251.4

9.8 2-, 5100.7 4+, 7+ 257.4

Each entry in the table gives the two possible values of KS, KS = jQzn,+O,iand the excitation energy of the state (see text).

409

s

1 ii

all-

(6601

t7>-

(632)

2,9*

wd*

(5041 (476)

w

257.4

SC

2072

600

500

400

300

200

100

0.

Fig. 6. Theoretical zeroth-order

level scheme (see text).

and $-[SO31neutron ‘) Nilsson orbitals. it is rewarding that the energy band covered by these states in the theoretical spectrum (E < 207 keV) coincides with the results obtained experimentally (E < 211 keV). The first change to positive parity in l**Ir may be obtained in two different ways: either by promoting the neutron into the i, parentage -12r_‘[615]orbital and

410

A. J. Kreiner et al. / “‘Ir

leaving the proton in the low-lying positive-parity orbitals, or by promoting the proton into the h, or h, &I’-[505]) p arentage states while leaving the neutron unaffected. Also here the energy band and possible spin values compare reasonably well with the experimental results. The next change of parity, finally, may be correlated with the simultaneous promotion of the odd neutron into the ?‘[615] orbital and of the odd proton into the h; and h, based states. In particular, a natural candidate emerges for explaining the structure of the isomeric state, namely ?~+[615] @ 7?~-[505]],,+. In fact there is such an isomer

known

in 19’Ir [ref. ‘)I.

5. Conclusions A careful spectroscopic study of 1881r has yielded a new level scheme which fits, without attempting a detailed comparison, in a natural way into the existing systematics of neutron and proton quasiparticle states known in this region of the nuclidic chart. The pn interaction, known to be relatively weak in deformed nuclei, does not, in fact, seem to invalidate, at least at a qualitative level, a zero-order description in which the states of the doubly odd nucleus are described as the juxtaposition of neutron and proton quasiparticle states.

References I) C. M. Lederer, and V. S. Shirley, ed., Table of isotopes, 7th ed. (Wiley, New York, 1978) 2) Y. A. Ellis, K. S. Toth and H. K. Carter, Phys. Rev. Cl8 (1978) 2713, and references therein 3) S. Andre, J. Boutet, J. Rivier, J. Treherne, J. Jastrzebski, J. Lukasiak, Z. Zujkowski and C. Selville-Schuck, Nucl. Phys. A243 (1975) 229 4) P. Kemnitz, L. Funke, H. Sodan, E. Will and G. Winter, Nucl. Phys. A245 (1975) 221 5) F. Rose], H. M. Fries, K. Alder and H. C. Pauli, At. Nucl. Data Tables 21 (1978) 347 6) B. Singh and D. A. Viggars, Nucl. Data Sheets 33 (1981) 275 [from I. A. Romanii and Yu. N. Rakivnenko, Proc. 31st Ann. Conf. on nuclear spectroscopy and structure of atomic nuclei] 7) M. Sodan, W. D. Fromm, L. Funke, K. H. Kaun, P. Kemnitz, E. Will, G. Winter and J. Berzins, Nucl. Phys. A237 (1975) 333 8) J. Meyer-Ter-Vehn, Nucl. Phys. A249 (1975) 141 9) R. Bengtsson and S. Frauendorf, Nucl. Phys. A314 (1979) 27; A327 (1979) 139 10) A. J. Kreiner, M. Fenzl, S. Lunardi and M. A. J. Mariscotti, Nucl. Phys. A282 (1977) 243 11) A. J. Kreiner, Z. Phys. AZSS (1978) 373