Download pdf - Levels of 188os populated in the 189Os(d, t)188Os reaction and in the decay of 41h 188Ir

Nuclear Physics A245 (1975) AAA. A.50; ~ ) North-Holland Publishing Co., Amsterdam

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L E V E L S O F laaOs P O P U L A T E D IN T H E 189Os(d, t ) laSOs R E A C T I O N A N D IN T H E D E C A Y O F 41h 188Ir

ROGER THOMPSON*, A. IKEDA and R. K. SHELINE

Physics Department, Florida State University, Tallahassee, Florida 32306, USA *t

and

J. C. CUNNANE, S. W. YATES*** and P. J. DALY

Chemistry Department, Purdue University, West Lafayette, Indiana 47907, USA t

Received 19 July 1974

(Revised 4 March 1975)

Abstract: The level structure of 188Os has been investigated by the lS9Os(d, t)~SSOs reaction using a broad range magnetic spectrograph, and the properties of the ~SSOs levels populated in the decay of ~ SSIr have been re-examined. The (d, t) results yield new information about the location of two- neutron excitations in ~SSOs involving the ~[512] orbital. Since the lS9Os ground state contains admixtures of both K = 23- and K = ½ character, cross-section formulae for single-neutron transfer from a target state which is not pure in K are considered, and it is found that rather small K = ½ admixtures in the ~ S9Os ground state give rise to striking interference effects, which are manifested in the experimental (d, t) cross sections into the members of the ~SSOs ground state band. The con- sequences of the mixed character of the target state on the (d, t) population of members of the K ~ = 2 + y-vibration and of higher-lying two-quasiparticle bands are also discussed.

NUCLEAR REACTIONS 1 s 9Os(d ' t) l a SOs , E = 12, 14 MeV; measured a(Et, 0); deduced Q. l SSOs deduced levels, l~nriched target.

RADIOACTIVITY 18SIr [from lSTRe(ct, 3n)]; measured E~, I~, 7y-coin. lSSOs deduced levels, J, n. Enriched target, Ge(Li) detectors.

1. Introduction

The level s t ruc tu re o f t he t r a n s i t i o n a l nuc l eus ~SSOs has been i nves t i ga t ed pr inc i -

pa l ly in s tudies o f t he r a d i o a c t i v e decay o f 17 h lSSRe [-refs. 1 -9 ) ] a n d 41 h 18SIr

[refs. 6, 9, 10)]. In ea r ly ana lyses o f t he decay resul ts , W a r n e r a n d She l ine 6) a n d

Y a m a z a k i 7) c o n c l u d e d tha t t he 188Os level s c h e m e was m o r e cons i s t en t w i th the

B o h r - M o t t e l s o n s y m m e t r i c r o t o r d e s c r i p t i o n ~x) t h a n wi th t he D a v y d o v - F i l l i p o v

* Present address : Nuclear Structure Research Laboratory, The University of Rochester, Rochester, New York 14627.

*t Work supported by the US Atomic Energy Commission and in part by the National Science Founda- tion.

*** Present address: Argonne National Laboratory, Argonne, Illinois 60439. t Work supported by the US Atomic Energy Commission.

444

189Os(d, t)lSaOs 445

asymmetric rotor model 12). Accordingly, low-lying 188Os levels identified in the decay studies were classified as the low-spin members of t h e / C -- 0 + ground state a n d / C -- 2 + y-vibrational bands, and a second 0 + level at 1086 keV was tentatively assigned as the bandhead of a K ~ -- 0 + two-phonon y-band, even though the decay characteristics of the level did not match the theoretical predictions closely 6, 7).

Subsequently, Harmatz and Handley 1 o) and Yamazaki and Sato 9) have performed much more detailed radioactivity measurements and have proposed extensive 18aOs level schemes, which are generally consistent with one another. However the spin- parity assignments proposed for several levels are conflicting.

More recently the 18aOs level structure has been investigated by several other techniques. A thermal neutron capture y-ray study by Barchuk et al. 13)has identified 27 188Os levels below 3312 keV which are populated by primary y-rays from the (0, 1)- capture state and which therefore have probable I ~ values of 0 +, 1 + or 2 +. Three groups 14-16) have studied the reaction 186W(ot, 2ny)laSOs by in-beam y-ray spectroscopy and have located a number of high spin levels not observed in the 1 s aRe and lSaIr decays, including a proposed K ~ = 4 + band based at 1279 keV. The reaction 19°Os(p, t)lSaos, which selectively populates natural parity states, has been studied by two groups 17--19), and new 0 + levels at 1480 and 1705 keV have been identified. Moreover, Sharma and Hintz 19) have advanced additional arguments favouring a description of the K ~ = 0 + and K ~ = 4 + excitations at 1086 and 1279 keV as two-phonon y-vibrations.

In the present work, the la9Os(d, t)laaOs reaction has been studied in order to gain a clearer insight into the nature of the lSaOs levels. To complement the transfer reaction study, the decay of 41 h 188Ir has been re-examined using high resolution Ge(Li) detectors in an effort to resolve discrepancies between the conclusions of earlier workers.

2. Experimental procedures and results

2.1. THE 1890s(d, t)lSSOs MEASUREMENTS

An isotopically enriched 10-15 //g/cm 2 189Os target deposited on a carbon backing was prepared with the Florida State University isotope separator. This target was bombarded with deuterons from the Florida State University tandem Van de Graaff, and the reaction products were analysed with a 6/5 scale Brown- Buechner magnetic spectrograph 20) and recorded on Kodak NTB nuclear emulsion plates. Measurements were made at angles of 65 ° and 70 ° using 12 MeV incident deuterons and at 45 ° using 14 MeV deuterons. The plates were developed and scanned for triton tracks in 0.5 mm intervals.

In fig. 1, the 45 ° triton spectrum is shown. Least squares fits of the peaks to gaussian distributions were performed and Q-values and relative cross sections expressed in numbers of triton tracks per peak were extracted. The ground sta/e

446 R. THOMPSON et al.

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• 0 9 i !

0.5 1.0 1.5 2 .0

EXCITATION ENERGY (MeV)

Fig. 1. Triton spectrum from the 189Os(d, t)~SSOs reaction•

TABLE 1

Energies and relative cross sections forlevels excited in the 189Os(d,t)lSSOs reaction

Peak Energy') Cross section b) Peak Energy =) Cross section b) no. (keV) 45 ° no. (keV) 45 °

1 0 28(5) 17 1973(3) 473(27) 2 155(1) 1018(32) 18 2015(2) 352(24) 3 479(1) 129(11) 19 2066(6) 129(17) 4 633(1) 119(15) 20 2100(3) 134(17) 5 790(2) 290(17) 21 2208(3) 152(13) 6 967(2) 62(8) 22 2264(3) 328(20) 7 1179(4) c) 21(5) 23 2308(3) 65(15) 8 1296(8) c) 13(4) 24 2353(3) 169(15) 9 1459(2) 225(15) 25 2446(8) 133(20)

10 1516(8) °) 12(6) 26 2503(6) 102(18) !1 1621(1) 366(19) 27 2567(3) 858(31) 12 1690(3) 90(9) 28 2626(6) 186(25) 13 1763(6) 91(15) 29 2666(3) 326(25) 14 1810(3) 291(25) 30 2699(4) 140(18) 15 1847(2) 832(33) 31 2816(4) 218(18) 16 1943(5) 382(25) 32 2938(6) 197(20)

a) Estimated excitation energy uncertainties (in keV) are shown in parentheses. b) The cross section is expressed in the number of triton tracks per peak. The uncertainties in the

number of triton tracks are shown in parentheses. c) These weak levels are not observed in the 65 ° and 70 ° spectra and may not be xSSos levels.

1 8 9 0 s ( d , t ) 1 8 8 0 s 4 4 7

Q-value for the 189Os(d, t ) laaos reaction was determined to be 3354-15 keV in excellent agreement with the value given in the Nuclear Data Tables 2 t). The excitation energies of the levels populated and the relative (d, t) cross sections are summarized in table 1.

In the 45 ° spectrum a F W H M resolution of about 22 keV was obtained. The 65 ° and 70 ° spectra were utilized primarily to verify that the observed levels were populated in a mass 188 nucleus. The ground state of x2C populated in the ~3C(d, t)12C reaction has been identified as an impurity peak in the three spectra and its known Q-value has been used as an energy standard to determine the incident energy of the incoming deuteron. The broad structure in fig. 1 between levels 2 and 3 is the ground state group from the ~2C(d, a)~°B reaction.

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2.2. THE laSIr RADIOACTIVITY MEASUREMENTS

The 188Ir sources were prepared by bombarding XaVRe (99~ enriched) with 34 MeV a-particles from the Argonne 152 cm cyclotron and chemically separating the Ir activities by standard radiochemical procedures. A 43 cm 3 Ge(Li) spectrometer with a resolution of 1.9 keV FWHM at 1332 keV was used in ),-ray singles measurements, and sequential ),-ray spectra were recorded over a period of several days in order to distinguish, on the basis of half-lives, between 188ir ),-rays and those due to other Ir activities in the sources. To illustrate the quality of the data, a portion of one of the 188Ir ),-ray spectra recorded is shown in fig. 2. The energies and relative intensities of the 7-rays assigned to the 18Sir decay are listed in table 2. We note that for many strong transitions above 1400 keV, the y-ray intensities determined in this work are larger by factors of 1.4-1.7 than those of Yamazaki and Sato 9) and consequently the corresponding ~K values given in ref. 9) are probably too high. The transition multipolarities listed in table 2 are those of ref. 9) except in those cases where adoption of the present y-ray intensities led to different conclusions about the probable multipolarities.

TABLE 2

Transitions observed in the decay of XSalr

Energy a) Relative 7-ray Placement (keY) intensitya) Multipolarity (keV)

2015 (160) E2 155--, 0 1.8( 3) 1729-,1457

13.4(13) 790--, 478 110.2(88) E2 478--, 155

4.9( 5) 966--* 633 2 . 4 ( 4 )

16.0(15) M1 1843--,1457

155.05(4) 271.56(5) 312.00(4) 322.91(4) 332.62(5) 383.47(8) 385.46(5) 389.94(15) 411.77(20) 413.73(8) 424.71(15) 448.10(8) 453.26(20) 477.99(4) 487.70(6) 491.64(8) 514.77(10) 522.68(10) 534.21(20) 538.06(8) 566.59(8) 586.44(15) 594.06(8)

1.3(3) 0.8, 3) 2.4, 4) 2.3, 4) 5.1, 10) 2.1, 8)

1000 14.4 15) 3.4 6) 8.9, 9) 1.4 4) 0.9~ 4)

13.2~ 15) 14.41 14) 2.41 4) 6.51 8)

E2

E2+M1

E2+MI

2377---1965 2099--,1685 1729---1305 2069--.1620 1086--, 633 633--. 155 966--* 478

1457---, 966 1305--* 790 2252---,1729 2377---1843 1843---,1305 2252--.1685

2215 ~1620

189Os(d ' t)l SSOs 449

TABLE 2 (continued)

Energy a) Relative ~,-ray Placement (keV) intensity ") Multipolarity (keV)

596.41(15) 601.09(20) 623.75(8) 633.02(10) 634.91(15) 641.59(5) 646.14(15) 652.58(15) 663.40(10) 667.43(15) 672.50(5) 695.43(15) 703.38(18) 719.58(15) 730.52(10) 736.56(8) 747.31(15) 752.09(10) 757.21(8) 763.91(15) 776.80(25) 777.93(20) 781.90(20) 794.17(15) 810.60(8) 824.34(8) 829.42(6) 844.99(8) 886.20(8) 895.33(8) 899.90(10) 909.68(15) 931.33(8) 933.95(20) 935.26(20) 939.57(6) 946.98(8) 972.14(20) 979.42(20) 985.08(20) 987.43(6) 999.38(15)

1012.54(8) 1017.63(6) 1052.11(20) 1096.54(6) 1128.33(15) 1132.45(35) 1142.54(10)

1 .8 (4 ) 1 .9 (5 )

17.6(19) 1220 (180) 339 (51) 26.4(27) 2.z(6) 1 .4 (5 ) 4 . 3 ( 7 ) 3 . 2 ( 7 )

97.8(73) 2 . 4 ( 4 ) 1 .7 (6 ) 1.5(5) 5 .2 (8 )

19.5(20) 3.3q 6) 5 . 1 ( 8 )

26.4 25) 2 . 1 ( 5 ) 3.6(12) 6.5(12)

11.8 21) 2.51 5)

11.6 12) 70.1 60)

349 25) 11.8 12) 17.2 17) 9.5 10) 7.4 8) 2.3 6)

17.9 17) 10.1 20) 11.4, 22) 44.8 35)

8.8 9) 1.9 6) 4.0, 8) 4.4 8)

62.6 60) 4.8d 8) 8.2q 12)

72.41 56) 2.31 7)

99.5 80) 4.3q 5) 1.9~ 7)

25.13 50)

E2 633--, 0 E2 790~ 155

M1 +E2 2099~1457

1957~1305

1457-* 790 E1 1462~ 790

M1 MI

MI(+E0)

E2+M1 M1

M1 MI El

MI

MI

M1

M1

M1 + E2

1685--* 966 2416-.1685 2215~1478 2205~1457 2215--1462 2215-,1457 1729-* 966

2099~1305 966--* 155

1457-* 633 1462--, 633 14784 633 2349~1462 1685-, 790 2205-.1305 2215---1305 1086~ 155

1729~ 790 2252---1305

1457-* 478

1620--* 633 1965~ 966

1808~ 790 1685--* 633 1729-*-* 633 2215~1086

1620~ 478


TABLE 2 (continued)

Energy a) Relative 3,-ray Placement (keV) intensity a) Multipolarity (keV)

1149.77(10) 36.2(70) M1 +E2 1174.59(10) 90.1(91) M1

1209.77(6) 473 (38) MI 1251.64(20) 1 . 8 ( 6 ) 1286.35(20) 2 . 2 ( 8 ) 1295.44(10) 9 . 1 ( 9 ) MI 1302.29(20) 22.2(20) 1304.72(35) 12.6(18) 1307.64(15) 10.2(14) 1322.96(15) 27.5(22) (El) 1331.81(15) 31.9(25) M1 1336.38(15) 9 . 5 ( 9 ) 1349.54(15) 4 .3 (6) . 1414.57(20) 5 . 2 ( 5 ) 1435.42(15) 101 ( 8 ) M1 +E2 1452.28(15) 72.2(61) M1 1457.19(15) 119 (10) E2 1465.24(15) 91.7(90) M1 +E2 1487.01(25) 5 . 1 ( 7 ) 1530.06(15) 15.5(19) 1558.66(15) 58.7(47) E1 1571.57(35) 7.8(11) 1574.48(15) 179 (14) M1 +E2 1618.77(35) 32.6(29) M1 1652.67(15) 21.2(17) M1 1688.04(15) 49.9(40) M1

1715.75(15) 417 1726.87(45) 8.0~ 1774.17(40) 4.11 1782.79(45) 4.6q 1802.18(20) 66.1~ 1807.79(50) 7.9~ 1810.18(40) 23.3~ 1843.04(40) 10.9~ 1887.89(40) 5.5~ 1903.98(40) 9.2~ 1930.65(25) 19.5q 1944.08(20) 267 1957.29(25) 28.7~ 1971.66(45) 11.4~ 2011.39(25) 41.5~ 2040.76(25) 33.21 2049.78(20) 339 2059.65(20) 476 2068.88(45) 3.9q 2096.88(35) 387 2099.06(35) 324

31) El 8) 5) 5)

53) E1 15) 21) E0+M1 +E2 10) 8) 9)

18) 19) E2+M1 26) 11) 37) M1 +E2 30) 24) M 1 + E2 34) E2

6) 46) (E2) 39) (E2)

1305~ 155 1808-0 633, 1965-o 790 1843--* 633 1729--* 478 2252--* 966 2085~ 790 1457-o 155 1305-0 0 1462~ 155 1478--* 155 1965~ 633

2205-0-* 790 2069-0 633 2085-0 633 1457-o 0 1620~ 155 1965--* 478 1685-0 155 2349-o 790 2205~ 633 1729~ 177 2252---, 633 1808-0 155 1843-0 155, 2166-o 478 2349-o 633 2205~ 478 2252-o 478 2416-0--* 633 1957---, 155 1808--, 0 1965-o 155 1843---, 0 2521~ 478

2085-0 155 2099-0 155 1957~ 0

2166~ 155

2205~ 155 2215-o 155 2069~ 0 2252---, 155 2099~ 0

lS9Os(d , t)lSSOs

TABLE 2 (continued)

451

Energy a) Relative y-ray Placement (keY) intensity a) Multipolarity (keV)

2130.92(30) 18.3(22) 2286-- 155 2133.74(45) 6 .5 (8 ) 2144.85(25) 10.9(11) 2300-. 155 2171.37(30) 4 .9 (6 ) 2326-. 155 2192.30(35) 23.6(43) 2193.67(35) 137 (24) (El) 2349-- 155 2214.59(20) 1270 (89) E2 2215--, 0 2219.11(50) 13.0(13) 2374--, 155 2221.99(50) 15.5(15) 2377-- 155 2252.01(25) 23.5(21) 2252 --, 0 2261.28(30) 5 .6 (7 ) 2416-- 155 2286.16(35) 5 .2 (7 ) 2286 --, 0 2299.73(50) 0 . 9 ( 4 ) 2300 --, 0 2305.61(25) 9.3(12) 2461 -- 155 2326.22(25) 5 .8 (7 ) 2326 --, 0 2336.01(30) 3 .1 (6 ) 2491 ~ 155 2347.25(35) 43.6(48) M1 or M2 (2349--, 0) 2365.27(30) 5 .0 (9 ) 2521-. 155 2374.16(35) 2 .1 (5 ) 2374-. 0 2385.71(35) 1 .3(4) 2394.35(25) 11.2(17) 2549--, 155 2406.23(30) 2 .4 (5 ) 2426.88(35) 1 .4(4) 2582-. 155 2460.51(25) 15.6(18) 2461 --, 0 2467.62(40) 0 .5 (3 ) 2622-- 155 2486.83(30) 2 .1 (6 ) 2504.87(25) 8.1(14) 2520.14(50) 0 .8 (4 ) 2521 --, 0 2565.60(50) 0 .8 (4 ) 2581.72(30) 2 .8 (7 ) 2582--, 0 2622.45(25) 5 .5 (8 ) 2622-- 0

a) Uncertainties in the least significant figures are indicated in parentheses.

Extensive y-y co inc idence measu remen t s were p e r f o r m e d using the 43 cm 3 Ge(Li )

and a 30 cm 3 Ge(Li ) wi th a r eso lu t ion o f 3.0 keV at 1332 keV. A conven t iona l fast-

s low ce inc idence c i rcui t was e m p l o y e d wi th a resolving t ime 2z = 100 ns. The co in-

c idence d a t a ob ta ined were s imilar in qual i ty to those i l lus t ra ted in ou r c o m p a n i o n

p a p e r 25) on the 19°Os level s tructure, and the results are summar ized in tab le 3.

3. The lSSos level scheme

A l t h o u g h the neu t ron t ransfer results p rov ide the mos t s ignif icant nuc lear in- f o r m a t i o n ob ta ined in the p resen t invest igat ion, the I s ass ignments for the 188Os

levels p o p u l a t e d in (d, t) mus t be based on the results o f o the r s tudies s ince / - t r ans fe r

values cou ld no t be de te rmined f rom the t ransfer data . In this sect ion the p roper t i e s


TABLE 3

Summary of the y-y coincidence results

Gating y-ray (keV) Observed coincident y-rays (keV)

155

323

478

633~35

642 672 737 757 825 829 987

1018 1097 1150 1210 1453

312, 323, (385), (448), 478, (488), 511,538, (566), (594), 624, 635, (642), 672, 737, (757), (778),(782), 811,824, 829, (845),(886), (895),(900), (922),(931), 940,(947), (979),987,(994),1018,1028,1038,1075,1097,(1143),1150,1175,1210,(1295),(1303), 1309, 1323, (1332), (1336), 1436, 1453, 1466, 1531, (1539), 1548, 1560, 1575, 1585, (1611),(1620), 1653, 1689, 1717, 1802,(1810), 1931, 1945, 2012, 2042. 2050, 2060, 2098,(2132),(2146),(2171), 2194, 2222,(2263), 2307, 2339,(2368), 2397,(2410). 155, 312,448, 488, 511,(594),(672), 778, 935, (947), 979, 984,(1018), 1143,(1458), 1487,(1560),(1611), 1687, 1727, 1842. 155, 511, (782), 824, 829, (845), 987, 1097, 1175, 1210, (1303), 1332,(1365), 1436, 1453,1620,1717. 155,(624), 672,(752), 824, 829, 845,(886),(895), 940, 987, 1018, 1097, 1175, 1210, (1295), 1332, 1436,1453, 1560,1620, 1717,(1904). 155,(323),(478),(624), 633-5, 824. 155, 317,(323),(624), 635, 886. 155,(323), 478, 635,845, 1323. 155,(323), 478,(488), 633, 824. 155,(323), 385, 478, 633, 642, 757. 155,478,(623), 633,886. 155, 478,(594), 633. 155,635. 155,478,633. 155, 538, 900, 910, 947. 155,478,633. 155,478,633.

y-rays not coincident with the 155 keV y-ray 633, 794, 1365, 1458, 1705, 1842, 1958, 2171, 2214, 2350, 2462, 2481, 2499, 2515

of the levels populated in the ~88Ir decay are discussed together with the results of the earlier radioactivity, (n, y), (~, 2ny) and (p, t) studies of the 188Os level structure.

The 188Ir decay scheme established in the present investigation is shown in fig. 3. In many respects, this scheme is in agreement with that proposed by Yamazaki and Sato 9), but the much higher quality of the y-ray spectra and the more extensive y-y coincidence data obtained in the present study enabled us to settle important questions about the properties of some of the levels below 2 MeV excitation.

Well developed ground state and y-vibrational bands in 18SOs have been established in the (~, 2ny) investigations 14--16), but in the 188Ir decay only the three lowest

members of each of these bands are detectably populated. The 0 ÷ assignment for the 1086 keV level has been confirmed in the (p, t) measurements 19). Earlier radioactivity studies have indicated probable I s values of 2 + or 3 + for the 1305 keV level, but the observation in the present work of a y-ray (easily distinguishable from the more intense 1302 keV line) de-exciting this level to the ground state eliminates the 3 + possibility. The 2 + assignment is also consistent with the observed population t9)

1890s(d, t)1880s 453

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of the level in (p, t). The de-excitation properties of the 1458 and 1621 keV levels strongly indieate U values of 2 + and 3 + respectively, and the observed branching ratios into members of the ground band are more consistent with a K = 2 assignment for these levels than with any other possible K-value; the M 1 transitions from these levels to the 2 + member of the y-band are remarkably intense. Our data are entirely consistent with the earlier assignments of 2- for the 1462 keV level 10) and 0 + for the 1478 keV level 19). Another 0 ÷ state at about 1705 keV has been identified in the (p, t) studies 17-19). Yamazaki and Sato 9) also placed a level at this energy de- exciting by a moderately strong y-ray to the 0 ÷ ground state; however we have carefully examined the structure and intensity of the peak observed at 1703.6 keV in our spectrum and have concluded that it is a single escape peak associated with the intense 2214.6 keV y-radiation. We find no evidence for a y-ray of ,~ 1705 keV in the decay of lSSlr.

The P values shown in fig. 3 for the 1730, 1808 and 1843 keV levels are in agreement with those proposed by Yamazaki and Sato 9) and they are based on similar arguments; several additional weak transitions have been placed which strengthen the assignments. We find no evidence in the present decay study for the levels at 1519 and 1936 keV proposed in ref. 9), but the existence of a new level at 1685 keV, with I = 2 or 3, is strongly indicated by our data. For the 1958 keV level, P = 1- or 2- has been proposed 9); however the upper limit on ~K given in ref. 9) for the critical 1802 keV transition is not consistent with the experimental data quoted. Moreover, Barchuk et al. 13) have identified an 188Os level at 1959 keV which is populated directly from the (0, 1)- capture state and which presumably has positive parity. For these reasons, the parity of the 1958 keV level seen in the 188Ir decay appears to be open to question. We place the next highest level at 1964.8 keV (rather than 1967.4 keV, ref. 9)) and propose a n / " = 2 + assignment.

On the basis of energy-sum relationships and the y-y coincidence results, we propose 22 additional levels at energies greater than 2 MeV and suggest probable spins and parities. At present, these levels seem of limited interest, since in this region the density of low-spin 188Os levels is so high that it is extremely difficult to identify firmly levels seen in the decay with those populated in the (d, t), (p, t) and (n, y) reactions. The EC branching into the 188Os levels was deduced from transition intensity imbalances, and approximate log f t values for those levels populated directly in > 3 % of the 188Ir decays were calculated using a value of QEC = 2780 keV. However a satisfactory understanding of the composition of the 18air(2)- ground state is lacking and so the nuclear structure implications of these log f t values remain obscure.

4. Discussion

The (d, t) cross section for the population of a state I K in a final even nucleus from an odd-N target with the ground state IoK o is normally determined from the Satchler

formula 22),

1 a9Os(d, t)lSSOs 455

da(O) _ g2 ~ [(jlo (K - Ko)KolIK)Cjt P]2~bt(~b). (1) do~ j.

Here j and l are the total and orbital angular momenta respectively of the transferred neutron, and g2 is equal to 2 when two neutron in an identical orbital combine to K = 0 and is otherwise unity. The pairing probability amplitude P is equal tO U for the ground band population and to Votherwise, and Cjt is the coefficent of expansion of the Nilsson orbital. The single DWBA cross sections ~b~ were calculated in the present work with the computer code D W U C K 23), using the optical model param- eters of ref. 24).

As we have pointed out previously 25), eq. (1) is strictly applicable only when the target state is pure in K. Such is not the case in the ~ 89Os(d, 0188Os reaction. While the 189Os ground state configuration is predominantly 3-23-[512-1, a significant Coriolis admixture of 3-½1510] character must also contribute to its composition. As will be seen in following subsections, such an admixture can sharply alter the relative transfer cross sections for certain bands in the (d, t) reaction.

4.1. THE 18aOs GROUND STATE BAND

The ground band of a deformed even nucleus consists of a superposition of two- quasiparticle states with components in the same intrinsic orbital but with One in a time reversed configuration. When the target state is not pure in K, the (d, t) population of the ground band of the even nucleus proceeds by transfer from more than one intrinsic state and the (d, t) cross section is given by 26)

da(0) - 2 ~ [ ~ ( - 1) K'- ½ai(j lo- K, gillO)C~, U,]2~bt(0), (2)

d o j.i i

where i is a label for the Nilsson state of pure K~ admixed in the target state. The Cj, and the pairing factor U~ refer to the orbital of the transferred neutron, and.the a~ are the amplitudes of the pure Nilsson state in the mixed target wave function. The ( - 1 ) r'-½ phase factor arises from the destruction of the single quasiparticle in the target nucleus and in a case such as we are considering it can give rise to sizable interference effects. In contrast, when the target ground state configuration is pure, the phase factor appears only in squared form and is therefore of no consequence.

The three lowest members of the 188Os ground band are seen in the (d, t) reaction and the 2 + band member is the most strongly populated state below 3 MeV. The calculated (d, t) cross sections into the 0 +, 2 + and 4 + states are quite sensitive to the magnitude of the 3-½[510] admixture in the 189Os ground state. This is illustrated in fig. 4 which shows the calculated cross section ratios for various assumed admixtures of K ~ ½ character in the 189Os ground state. For zero admixture, i.e. a pure ~-~[512] ground state, the 2+/0 + and 4+/0 + cross section ratios are much lower than

456 R. T H O M P S O N et al.

IOC

O . L I . ( ~ ~ ,

. . . . ~ . . . . ~lJ' . . . . . . i . . . . . . . . . i . . . .

RATIOS / ~ CROSS --

SECTIONS - 400

,oo

, I ~ , L , , L , +I.0 -lid -0.5 0 +Q,5 +I;3 -0.5 " 0 015 ~ ' .... ~ ' ' ' ' ' '' "~J Y' ' ' '0

AMPLITUDE OF [5 lOt ]

Fig. 4. The (d, t) cross sections into members of the ~SSOs ground state band calculated from eq. (2) for various admixed [5101"] amplitudes in the lS9Os ground state; the calculated cross section ratios are

shown in the left portion and compared with the experimental ratios.

those experimentally observed, but it is clear that a positive amplitude of the 3-½1510] admixture drastically increases these ratios.

Since the (unperturbed) 31512] bandhead lies below the ½1510] band in 189Os, the amplitude of the two components in the ground state must have the same sign. A 10 % admixture of 3-½1510] has been determined 27) for the ground state of IssW, a nucleus which closely resembles 189Os in low energy level ordering and spacing. Accordingly, we have adopted the amplitudes a51ot = +0.30 and a512~ = +0.95 as the best available estimate of the composition of the ~S9Os ground state. Table 4 shows that the cross sections calculated from eq. (2) using these amplitudes are in much better agreement with the experimental data than the cross sections calculated from eq. (I) for a pure 3-31512] target state. A similar result was obtained for the population of the ~9°Os ground band in the (d, p) reaction on an ~89Os target 25). It

TABLE 4

A comparison of calculated and experimental relative (d, t) cross sections for members o f the lSSOs ground band

Calculated cross sections Experimental

Spin -- - from eq. (1) a) from eq. (2) b) cross sections

0 99 23 28 2 232 343 1018 4 34 21 129 6 1.4 0.4

a) Assuming a pure ~-31512] lS9Os ground state. b) Assuming the admixed amplitudes as12~ = +0.95, aslo~ = +0.30 for the lS9Os ground state.

1S9Os(d, t)lSSOs 457

is obvious that the details of the composit ion of the odd-A target state greatly affect the spectroscopic factors as a result of the coherence effects.

4.2. THE lSSOs y-VIBRATIONAL BAND

Vibrational states are populated in single neutron transfer through two-neutron components involving the ground state orbital. Just as in the case of the ground band population, this process is more complicated when the target state is not pure in K. The (510T, 51'2,[)K ~ = 2 + configuration is one of the principal components of the lS8Os y-band which can be strongly populated in the 189Os(d, t)188Os reaction. Not

only can this configuration be populated by transfer o f a 510 T neutron, but since there is a 5101" admixture in the target ground state, it can also be populated by transfer of a 512~ neutron. These two transfer modes can interfere and the (d, t) cross section for the (510T, 512J,)K ~ = 2 + configuration will be given by the equation 26)

da(0) _ ~ [ ~ ( - 1)x, + ~ak(j~Ki(2--K,)II2)C~t V/]2(~/(0), (3) dco i,1

i ~ k

where i labels the state of the transferred neutron and k the corresponding intrinsic state of projection 2 - K~.

In table 5, the experimental data for the ~S8Os y-band are compared with the (d, t) cross sections for the (5101", 5125) /C = 2 + configuration calculated f rom eq. (1) for a pure 3-31512] target and from eq. (3) for the adopted admixed amplitudes a51ot = +0.30 and a512, = +0.95. While the occurrence of strong interference effects is again obvious, the agreement between the calculated cross sections and the experimental data is poorer than for the :SSOs ground band. This is not really surprising since other two-quasiparticle configurations such as (503T, 512]) K" = 2 ÷,

TABLE 5

A comparison of calculated and experimental relative (d, t) cross sections for members of the 18SOs y-band

Calculated cross sections Experimental Spin from eq. (1) a) from eq. (3) b) cross sections

2 284 194 119 3 320 409 290 4 44 18 62 5 8 11 21 6 1.6 0.5

") Calculated for the (510T, 5125) K ~ = 2 + configuration assuming a pure 23-~21512] xSgOs ground state. b) Calculated for the (510T, 512+) K" = 2 + configuration assuming the admixed amplitudes a5~2~ =

+0.95, a51ot = +0.30 for the lS9Os ground state.

458 R . T H O M P S O N et al.

(510T, 512~) K ~ = 2 ÷ and (514~, 512~) K ~ = 2 ÷ are probable components of the 18 SOs y-vibration and they can also be populated in the 189Os(d, t)la8Os reaction.

4.3. T H E S E C O N D K ~ = 2 + B A N D

The lowest states above the y-band which are strongly populated in the (d, t) reaction are those at 1459 and 1621 keV. Almost certainly these are identical with the 1457 and 1621 keV levels, with p robable /~K values of 2+2 and 3+2, respectively, located in the 1 s 8it decay. The lowest pure two-quasi particle state expected in 18 SOs is the (510T, 512~)K ~ = 2 +. As discussed in the previous subsection, this configuration probably contributes a large portion of the (d, t) strength of the y-band, but it should also be observed as one of the lowest populated bands above the pairing gap. The situation in ~aaOs may be similar to that observed in ~a4W, where the strength of the (510T, 512~) K ~ = 2 + configuration was found to be distributed roughly equally between the y-vibration and a higher-lying K ~ = 2 + band with a composition not unlike that of the y-vibration2S). In support of such an interpretation, we note that the ratio of the (d, t) cross sections into the 2 + and 3 + states is in reasonably good agreement with that calculated from eq. (3) for the (510T, 512~)

~/~ = 2 + configuration. The 4 + band member, which is predicted to be populated with a moderately large cross section, has not been located, but the corresponding triton group could well be an unresolved component of the large peak at 1808 keV.

4.4. T H E S E C O N D /C ~ = 0 ÷ B A N D

It seems likely that the 0 + and 2 + states in x88Os at 1086 and 1305 keV are associated, and are analogous to the 0 + and 2 + states located at 912 and 1115 keV in 19°Os. The nature of these low-lying /C = 0 + bands is not yet clear. Energy systematics suggest that they might b e / C = 0 + two-phonon bands 7. lo), but there are problems with this interpretation. As noted by Yamazaki 7), the observed branching ratio from the 1086 keV 0 + state to the 2~- and 2~- levels differs by more than a factor of ten from the value predicted for a two-phonon bandhead. Moreover, we have found that the 1115 keV 2 + level in X9°Os is populated rather strongly in (d, p) implying that this level contains a sizable admixture of two-neutron character 25). In view of this, it is somewhat surprising that the 1305 keV 2 + state in ~88Os is not detectably populated in the (d, t) reaction.

Very recent investigations z9.3o) of lighter doubly even Os nuclei, in which the one-phonon y-bands occur at much higher energies, have located low-lying excited 0 + states, which probably correspond to the 1086 and 912 keV states in tSSOs and ~9°Os. If so, the energy systematics argument for a / ~ = 0 + two-phonon interpretation loses most of its remaining force. At present, these levels appear to be described best in the pairing-plus-quadrupole treatment of Kumar and Baranger 31)

18 9Os(d ' t) 1 s SOs 459

bu t it wou ld be mos t in teres t ing to gain a deeper insight in to the na tu re o f the nuc lear

exc i ta t ions involved.

4.5. HIGHER EXCITED STATES

The 1 ÷ level a t 1843 keV de-exci tes by an intense M1 t r ans i t ion to the 2~ + level, and

it has been sugges ted 9) tha t it migh t be the b a n d h e a d o f the (510t", 512~) K ~ = 1 +

conf igura t ion . The presen t results tend to suppo r t this i n t e rp re t a t ion since the 1847

keV level s t rongly p o p u l a t e d in (d, t) is p r o b a b l y ident ical wi th the 1843 keV level seen

in the 1SSir decay , and a very large t ransfer cross sect ion is p red ic ted for the 1 + m e m b e r

o f the (510T, 512~) K ~ = 1 ÷ band . There is no recognizab le b a n d s t ructure assoc ia ted

with this level. In x s4W, the s t ructure o f the ro t a t i ona l b a n d bui l t on the same K ~ = 1 ÷

t w o - n e u t r o n state was f o u n d to be s t rongly d i s to r t ed by mix ing with c lose- lying low-

K bands 2s,32). P r e sumab ly the b a n d is s imilar ly d i s to r t ed in lSSOs, bu t we have no

r easonab le basis for choos ing be tween the several s t rongly p o p u l a t e d levels in the

1900-2100 keV region as p r o b a b l e m e m b e r s o f this p a r t i c u l a r / C -- 1 + broad.

W e t h a n k R. L e o n a r d for his efforts in p repa r ing the lS9Os t a rge t and M. Ose lka

for a r r ang ing the A r g o n n e cyc lo t ron i r rad ia t ion . W e pa r t i cu la r ly apprec i a t e m a n y

s t imula t ing and frui tful d iscuss ions wi th Dr. P. K le inhe inz concern ing this work .

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