Rotational-vibrational band structure in 155Sm

Nuclear Physics A376 (1982) 149- 182 © North-Holland Publishing Company

ROTATIONAL-VIBRATIONAL BAND STRUCTURE IN ' ss Sm I K. SCHRECKENBACH, A. I . NAMENSON ", W. F. DAVIDSON ", T. VON EGIDY ""', H . G. BORNER and J. A. PINSTON ** Institut Laue-Langevin, 38042 Grenoble Cédex, France R. K. SMITHER

Argonne National Laboratory, Argonne IL, USA D. D. WARNER, R. F. CASTEN and M. L. STELTS r*

Brookhaven National Laboratory, Upton, New York 11973, USA D. H. WHITE

Oregon College o( Education, Monmouth, Oregon, USA and W. STOFFL

f**

Physik Department, TU Mönchen, W.-Germany Received 31 July 1981 Abstract : The nuclear structure of is'Sm has been studied via primary and secondary y-rays and by internal conversion electrons following thermal neutron capture. Furthermore average resonance neutron capture spectra for various neutron energy ranges were recorded. A detailed level scheme with a complete set of levels with spin < I has been well established up to 1400 keV excitation energy . The neutron binding energy has been determined to be 5807 .2(3) keV. The scheme is interpreted in a basis of Coriolis and dN = 2 coupled Nilsson states and compared with the less deformed Sm isotopes . A possible additional band coupling via the octupole vibrational mode is discussed. NUCLEAR REACTIONS "°`Sm(n, y), E= thermal, z 100 eV, 2 keV, 24 keV; measured 154 Sm(n, p, e), E = thermal, measured E_ I_ ' 'Sm deduced levels, transitions, multipolarities, J, n. Ge(Li) pair spectrometers, curved crystal spectrometers, magnetic conversion electron spectrometer. enriched targets . Nilsson model with Coriolis and dN = 2 coupling, Y30 coupling via the octupole vibrational mode . ' 'Sm revised (d, p) .

E

EÏ, IÏ.

Research has been performedin part undercontract DE-AC02-76CH00016 with the US Department of Energy. ' Present address : Naval Research Laboratory, Washington DC, USA. " Present address : National Research Council, Ottawa, Canada . "' Present address : Physik Department, TU München, W.-Germany. r* Present address : Centre d'Etudes Nucléaires de Grenoble, CPN, Grenoble, France . *t* Present address : Los Alamos National Laboratory, Los Alamos, USA. 149 March 1982

150

K. Schreckenhach et al. j `Snt

1 . Introduction The nuclear deformation for the Sm isotopes changes rapidly with neutron number . Forthe odd-A nuclei deformation sets in at' S' Sm and is clearly pronounced in' 53 Sm. Both nuclei were extensively studied and described in terms of a refined Nilsson model [refs. ' - 4) and references therein], but it has not yet been possible to follow the systematic behaviour of Nilsson states further out to ' 53 Sm, where the deformation is expected to reach about the maximum value for rare earth nuclei of EZ 0.3. Previous experimental results on' SSSm were very scarce and limited to (d, p)measurements 5 . e) and primary -j-rays after thermal neutron capture'). Thus only a few and uncertain assignments of rotational bands have been proposed S . 6). The nucleus . .. Sm is difficult to investigate by thermal neutron capture reactions because of the small cross section of only 5.5 b for ' S 'Sm, and has not yet been investigated via ß-decay of 155 . In the present work transitions in ' SSSm following neutron capture have been studied in the reactions ' 54Sm(n, /)' 53Sm and '54Sm(ntn, e- )' 53Sm. Thermal primaries, secondary 7-rays and conversion electrons were measured with the pair spectrometer, the GAMS curved crystal spectrometers and the BILL ß-spectrometer, respectively, at the high flux reactor of the Institut Laue-Langevin, Grenoble, taking advantage of the high sensitivity and resolving power of these facilities . Primary /-rays following average resonance neutron capture (ARC) with several mean neutron energies were studied at the filtered-beam facility of the Brookhaven National Laboratory high flux reactor and earlier at the Argonne National Laboratory . These ARC results provided a complete set of states with spin <_ 3 and have formed a basis for the placing of the secondary ,7-rays in the level scheme . An earlier report e) on the status of this' 15SM work had to be partly revised when the ARC results from Brookhaven became available. The present data now allow us to establish a comprehensive level scheme up to 1400 keV excitation energy and to interpret it in the particle-rotor model . During the investigation of the level structure of ' SSSm it became evident that a vibrational degree of freedom, namely the KR = 0 - octupole vibration, may play an important role in the coupling of the positive-parity states . 2. Experiments and results 2.1 . AVERAGE RESONANCE NEUTRON CAPTURE (ARC) MEASUREMENTS

ARC measurements on "'Sm have been performed at the Argonne and Brookhaven National Laboratories with the aim of establishing a complete set of low-spin levels via the systematic intensities of the primary y-rays in this reaction. The basic approach is to average theneutron capture y-spectrum overmany neutron resonances . This averaging process reduces the strong Porter-Thomas fluctuations in the /intensity of primary y-rays normally associated with y-spectra from neutron cap-

K. Schreckenhach et al. / "'Sm CAPTURE STATE (s-wave)

FINAL STATE

CAPTURE STATE (p-wave)

I/2+

I/2 +

EI

MI

EI

MI

1

l \ I/2- 3/2 -

I/2+

3/2 +

I/2-

I/2-

3/2-

3/2I

FINAL STATE

151

lr \l \ I/2 + 3/2 + 5/2 +

mi\mmi

\I

v 1~ \ I/2 - 3/2- 5/2-

Fig. I . Schematic picture of the possible dipole y-ray transitions following s-wave and p-wave neutron capture . The final spins and parities that can be reached by these primary transitions are shown.

ture in single resonances . When the averaging is done over a sufficient number of resonances the primary y-transition intensities are grouped corresponding to their multipolarity and to the population systematics of the capture states 9). In the case of a "4Sm sample, the s-wave neutron capture in the 0+ ground state . Similarly, p-wave will always result in a I+ capture state in the final nucleus '"Sm neutron capture will form either a I - or I - capture state. The dominant y-decays that follow s-wave and p-wave neutron capture are given in fig. 1 . As the neutron energy is increased, the p-wave capture component increases and hence modifies the total y-transition strength to the various final spin and parity groups . For the present investigation of . . .Sm, data have been taken for widely different neutron energies . Thus firm assignments of spin and parity for states up to IR = t were possible. 2.1.1. The Argonne average resonance capture measurement. The Argonne in-pile y-facility at the CP-5 research reactor 9) was used to measure the strength of the primary y-rays following average resonance neutron capture in the 154Sm(n, y)155Sm reaction . The capturing sample consisted of 4 g of 154 Sm02 enriched to 99 .54 % and surrounded by J" of 1°B . It was positioned in the center of a through tube near the core ofthe reactor where the fast neutron flux is relatively high . The t aB shield absorbs virtually all the neutrons with energies below 100 eV. This limited the effective range of neutron energies contributing to the average capture spectrum to the region from 100 eV to 2 keV with most of the strength concentrated below 1 keV. The original assignment of y-transitions to 155Sm was made by comparison of this ARC data with similar ARC data for the final nuclei 1stSm and 153Sm . The strong 155Sm lines show up weakly in the 153Sm data. The weak lines appear only in the `Sm data . Most of the background lines appear in all these ARC data sets. The energies and

K. Schreckenhach et al. ./ "'Sin

152

TABLE I

Results of the average resonance neutron capture to " S Sm Ir(EOIE7)5 E, ')

(keV)

-0 .2 (1) 16 .1 (3) 52 .2 (8) 425 .9 (3) 617 .2 (2) 657 .7 (4) 778 .2 (1) 819 .8 (1) 844 .0 (1) 865 .5 (3) 881 .0 (7) 903 .6 (2) 907 .1(13) 915 .5 (1) (919 .7 (9)) 930 .6 (1) 967 .0 (2) (985 .5 (9)) 1010.3 (7) (1097 .0 (6)) 1106 .6 (2) 1154.8 (4) 1168 .5 (2) (1217 .7 (7)) 1282 .5 (3) 1327 .4 (6) 1335 .8 (7) 1362 .1 (3) 1390.6 (7) 1403 .8(10) 1508 .2 (4) 1424.7 (7) 1474.0 (5) 1478 .0 (9) 1481 .6 (8) 1499 .3 (2) 1503 .1(12) 1524.8 (1) 1531 .9 (9) 1548 .4 (2) 1567 .0 (2) 1570.9 (8) 1584.6 (3) 1600.8 (3) 1614.5 (3)

Argonne experiment 140((30) 48 (8) < 9 < 3 129 (9) 31 (6) 279(22) 410(33) 250(25) `) 38(15) 205(21) < 39 556(33) < 19 211(32) 253(25) < 14 49(12) `) 190(30) < 17 143(36) < 17 115(35)

b)

Brookhaven experiment 2 keV

24 keV

330 (3) 35 (3) 7 (2) < 6 144 (7) 29 (5) 171 (8) 291(10) 302(12) 76(17) 23 (7) 127(10) < 20 322(12) < 25 402(15) 13((10) < 10 21 (6) < 10 127(13) < 20 135(12) < 10 74(10) 36(10) 30(10) 126(13) < 25 < 35 84(12) < 20 152(30) 178(36) 113(40) 186(20) < 30 589(42) 37(14) 240(20) 404(50) 87(27) 187(19) 125(20) 323(35)

324(10) 156(10) 46 (5) 56 (8) 214(12) 104 (9) 158(12) 292(15) 222(16) 231(16) 266(16) 255(25) 44(20) 150(12) 56(20) 274(18) 199(16) 35(14) 127(15) 51(15) 245(20) 109(14) 132(17) 52(16) 213(22) 161(20) 96,(20) 176(20) 70(18) 72(20) 160(24) 66(25) 134(25) 312(35) 140(35) 231(30) 72(30) 221(30) 154(30) 391(50) 352(35) 318(40) 247(30) 205(30) 215(36)

2 keV/24 keV

<

< < < < < <

< < <

<

1 .02 0.22 0.15 0.10 0.67 0.28 1 .08 1 .00 1 .36 0 .33 0 .09 0 .50 0 .45 2 .1 0 .45 1 .50 0 .65 0 .29 0 .17 0 .20 0 .52 0 .18 1 .02 0 .19 0 .35 0 .22 0 .31 0 .72 0 .36 0 .50 0 .53 0 .30 1 .13 0 .57 0 .81 0 .81 0 .42 2 .7 0 .24 0 .61 1 .15 0 .27 0 .76 0 .61 1 .50

Adopted spin `) and parity ) -.l 1+

l- + 1+ .

3+ } - ,3 - d)

-,

}+ , + + +,, + (3

)

}-

1 - 11 i+ , a +

(}-) 1

(1

+

)

}+,

+

j- , j- d) ) }+ a+ 1+

(1

}+, + }t

;` l+ , i + }-, }+, +

1 +,1 +

)+ ., + }-, 1+ 1+

1+

}-, i-

1+, 1 + 1+ , 1 + 1+ , 1 + 1 -, 1 -

K . Schreckenhach et al. /

153

'-"Sm

TABLE l (continued) 17(E01E7)5 E, ')

(keV)

1618 .8 (3) 1658 .7 (3) (1665 .9 (9)) 1671 .2 (3) 1678 .1 (8) 1696 .5 (6) (1708 .2(12)) 1718 .2 (2) 1723 .9 (3) 1733 .6 (7) 1752.0 (2) 1774 .2 (3) D1787 .9 (5) D1804.7 (2) 1821 .4 (6) 1830 .7(10) 1833 .2 (2) 1857 .2 (3) 1864.9 (6) 1875 .7 (2) 1885 .4 (4) 1889.5 (5) 1899 .4(10) 1904.5 (4) 1920 .1(10) 1925 .7 (6) 1929 .1 1954.2 1965 .2 1978 .8 1987 .2

(7) (2) (6) (5) (5)

Argonne experiment

b)

Brookhaven experiment 2 keV

24 keV

2 keV/24 keV

322(35) < 40 < 40 239(25) 49(16) < 40 34(17) 352(75) 228(25) 83(24) 395(35) 180(25) 182(30) 419(26) < 40 < 60 386(45) 316(45) 104(27) 563(40) 307(43) 228(43) < 60 154(30) 67(30) 296(67)

332(43) 343(40) 114(30) 510(45) 159(40) 141(35) 103(25) 352(40) 315(35) 231(35) 233(36) 354(44) 245(38) 361(40) 158(40) 172(70) 214(70) 282(48) 209(42) 423(50) 428(71) 157(64) 116(50) 220(50) 278(45)

0.97 < 0.12 < 0 .35 0 .47 0 .31 < 0.28 0.33 1 .0 0 .72 0.36 1 .7 0 .51 0 .74 1 .16 < 0.25 < 0.35 1 .1 1 .12 0 .50 1 .33 0 .72 1 .45 < 0.52 0.70 0.24

D533(60)

0 .98

500(55) 95(45) 243(45) 203(50)

0.82 1 .25 < 0 .25 0 .72

225(70) 414(50) 119(30) < 60 146(50)

Adopted spin `) and parity }-, + (}+)

1+j+

d)

W) 5t

2 (? t ) }-, }}

+,

+,

1+

+

1+

}t, }-, 3+

t

1 ~ 14 1+

-

~+ l-, 4}-,}} +, + }-, }l+ , 4+ d ) }~, f

1+

} , } f, }+ ,i+

') Excitation (level) energies are deduced from the 2 keV and 24 keV ARC experiments (see text) . Errors in parentheses are given in units of the last digits . D denotes an unresolved doublet . Levels in parentheses are questionable . b) 1. in relative units. Eo denotes the neutron binding energy, E, the primary y-energy to the level ~ . `) See fig . 3 for grouping of y-intensities. °) Parity based on the 1,(2 keV)/1,(24 keV) ratio . `) Obscured by a contamination line.

15 4

K. Schreckenbach et al . / "'Sin

Fig. 2 . Portion of the primary y-rays following 2 keV neutron capture measured in the pair spectrometer mode . The -,i-lines are labeled with the energy (in keV), spin and parity for the -, ,-populated levels (rounded energies from table 4) . + denotes I` = I+, J + , - stands for 1* = }-, J- .

relative intensities are calibrated with the t4 N(n, y) t5 N spectrum t°) by adding a small amount ofnitrogen gas to the He atmosphere that surrounds the in-pile sample . Later when additional levels were established based on the thermal (n, y) data, (see sect. 3), the new level energies were used to identify corresponding primary y-transitions. The results are summarized in table 1, column 2. Several problems remained open in the interpretation of this ARC measurement. Therefore ARC studies were continued at the facilities at Brookhaven . A common interpretation will be given in subsect. 2 .1 .3. 2.1.2. 2 keV and 24 keV ARC measurements at Brookhaven. Primary y-rays following average resonance capture of 2 keV and 24 keV neutron respectively have been measured at the filtered beam facility at the Brookhaven HFBR . A detailed description of this apparatus is given in refs. 11, 12) . The target consisted of 63 g t S4Sm as oxide enriched to 98.7 %. Resulting y-rays were detected by a Ge(Li) pair spectrometer. A resolution of 5.4 keV at 5.8 MeV was achieved. Fig. 2 shows a part of the y-spectrum after capture of 2 keV neutrons. The spectra have been analyzed by a least squares fit program . The non-linearity in energy and the efficiency of the pair spectrometer were calibrated via lines observed in an accompanying measurement of the "Cl(n th, y) reaction 12) . Level energies of excited states populated in this ARCmeasurement were initially determined with this calibration. When the level energies had been determined more accurately by secondary y-rays (sect. 3), the absolute linear scale of the ARC data was readjusted .

K. Schreckenhach et al./ "'Sir

15 5

The resulting level energies are given in table 1 . The errors in energy represent only statistical errors from fitting the centroid of the populating primary y-rays in the ARC measurement and include no systematic errors apart from a lower limit of 100 eV for the energy precision (corresponding to about ' of the resolution). A comparison of the thus obtained level energies with the level energies deduced from secondary y-transitions showed a fluctuation of less than 500 eV for the 24 keV ARC data . For the 2 keV ARC measurement the extracted energies for the I - , J - states agree with those determined from the secondary y-rays within the statistical errors . The primary ;!-rays in 2 keV ARC to j + states showed a systematic shift of 0.6±0 .2 keV towards higher energies in comparison to those based on level energies deduced from secondary y-rays . For +, + levels the same effect, but of a smaller size, was observed . The size of this shift could not be explained by the energy dependence of the s- and p-wave cross sections over the neutron energy distribution, even if a broadening of the incoming neutron distribution due to neutron scattering in the thick oxide target was taken into account. The intensities in table 1 include efficiency corrections and are multiplied by the usual factor (,
K. Schreckenhach et al. / "'Stn

15 6

100

__k-________~-

_I-- T

W

0

200

400

600

800

1000 1200

1400 1600 1800 2000

Eex (keV)

Fig. 3 . A plot of the reduced primary y-intensities I,,(EO I E,) 5 in the ARC measurements versus the energy of the y-populated level . Plot (a) shows the 2 keV neutron energy data, plot (b) the ratio of 1,(24 keV)/ 1,(2 keV). The shadowed regions indicate the empirical intensity fluctuation for each given set of I'°, determined from the adopted I` assignments (see table 4 and subsect. 2.1 .3). }- states are assumed to show ~th of the y-intensity for }' states.

ARC measurement alone. In the Argonne measurement, } and j levels of different parity are also not well separated, although p-wave capture should contribute less than in the 2 keV ARC measurement. This poor separation is believed to result from too few neutron resonances on the average. Due to the stronger p-wave contribution in 24 keV ARC, -}+ and + levels should be relatively more strongly populated in 24 keV ARC than in 2 keV ARC. Therefore the ratio of intensities between the ARC measurements is a sensitive measure in

15 7

K. Schreckenbach et al. j "'Sm

determining the parities of the 1, J levels as can be seen from fig. 3 . In questionable cases like the 866 keV and 1169 keV levels the I,,(2 keV)/I, (24 keV) ratio has been used as a stronger argument than intensities in the 2 keV measurement alone. If it is assumed that the specific structure of the final state, despite a good averaging over many neutron resonances as initial states, causes some fluctuations in the intensities then, in the ratio 1.,(2)/1,(24), this effect may partly be cancelled. Following those arguments and assumptions spin and parity assignments could be made for nearly all y-rays populating levels up to about 1 .5 MeV. Only a few I - states may be missing due to the weak population in ARC. For levels above 1 .5 MeV the corresponding y-spectrum is more complex and hence assignments are often more ambiguous. The adopted spin assignments from our ARC measurements are given in the last column oftable 1 . 2 .2 . PRIMARY y-RAYS FOLLOWING THERMAL NEUTRON CAPTURE

Thepair spectrometer used for measuring primary y-rays following thermal neutron capture viewed the same t 54 Sm target as the GAMS crystal spectrometers. The target characteristics are described in subsect. 2 .3.1 and in ref. 13 ). The y-beam from the target, collimated to 15 x 2 mm 2 , irradiated a 35 x 18 x 11 .5 mm 3 planar Ge(Li) detector which was surrounded by two 6" x 4" NaI detectors. The double-escape i-spectrum from the Ge(Li) detector was recorded with electronics stabilized by a precision pulse generator. The energy and intensity calibration of the spectrum was performed via lines from the reaction 2 'Al(n, y) 2 'Al [ref. 12 )] . The 2 'Al lines were present as contamination lines in the 'SSSm spectrum due to the Al envelope of the target material . With this absolute energy calibration the neutron binding energy Eo = E,+Erc.il of 155 Sm has been determined to be 5807.2(3) keV. The measured primary y-rays of ' S5 Sm are listed in table 2.

TABLE 2 Primary y-rays in ' °°Sm after thermal neutron capture Er (keV)

5807 .10 5790.22 5028 .86 4987 .06 4963 .11 4941 .56 4891 .12 4876 .30

A Er (keV) ')

Ir 11,111)

Al,, (%)

E, (keV) n)

0 .14 0 .17 0 .10 0 .10 0 .23 0 .28 0 .23 0 .29

0 .60 0 .33 5 .1 26 .0 0 .31 0 .55 0 .71 16 .2

7 10 5 5 13 13 13 5

0 .0 16 .9 778 .2 820 .0 844 .0 865 .5 916 .0 930 .8

') An additional systematic error of 0 .2 keV should be assumed . b) Deduced from energy differences relative to Er = 5807 . 10.

15 8

K. Schreckenhach et al. i ' "Sm .j 4 w

(a) °' ó N

Z Z

a3 x

U V) H2 Z

K

D

E

GAMM1 (SECOND ORDER)

w

.~

20

40

n

12

nN

r-

I

I

rv

O

N O rv

1~

w

w w

w

N

m Nl O

N

m

N

N

tD

cm

I

I

I

n

_

_

(b)

K

1 60

1

1

1

80 100 120 140 160 RELATIVE CHANNEL NUMBER

Fig. 4. Portion of the spectrum of secondary ,1-rays after thermal neutron capture measured with a "Sm target at the GAMS curved crystal spectrometer . Two consecutive scans of the same energy region are shown illustrating the burn-up of "'Eu lines. Scan (a) was taken after 1 day, scan (b) after 10 days from the beginning of the neutron irradiation .

2 .3 . SECONDARY TRANSITIONS FOLLOWING THERMAL NEUTRON CAPTURE

2 .3.1 . Secondary y-rays after thermal neutron capture . Measurements of y-rays in the region 50 to 2000 keV following thermal neutron capture have been performed with the curved crystal spectrometers GAMS at the high flux reactor in Grenoble . The experimental arrangements are described in detail in ref. t 3) . The thin, flat targets 13) consisted of about 100 mg t S4Sm oxide, enriched to 99.996 %, embedded in AI foils . Two targets of this type were irradiated . The energy region from 40 to 600 keV was covered by GAMS 1 (5 m focal length), while for the region 200 to 2000 keV the 24 m twin spectrometers GAMS 2/3 were used. Fig . 4 shows part ofthe measured spectra . The relative energy calibration was obtained in the usual way, as described in ref. 13) . The absolute energy scale was then linked to the K, 1 , Sin X-ray energy of 40.118 keV [ref. t 4 )] . A specific problem arose from the dense spectrum of the contamination lines of 156 Eu. 155 Sm decays with a half-life of 22 min to 15sEu, which has a cross section for thermal neutrons of about 400b. In the neutron flux at the in-pile GAMS target of 5 x 1014 n/s . cm2, those 156Eu lines grew with a period of about three days. In most cases the Eu lines could be identified by their time behaviour (see fig . 4). At y-energies above - 1 MeV these contamination lines were assumed to be weak and below the detection limit of the present measurement due to the high level density in this deformed odd-odd nucleus . Further contamination

159

K. Schreckenbach et al. / "'Sm

TABLE 3

Results of the "4Sm(n,h , y) "'Sm and the "Sm(n, h , e - )' 55Sm reactions E,(AE,) ') (keV) 16 .547(10) ') 40 .324 (5)') 53 .033 (2) 59 .753 (1) 74.664 (1) 76.118 (1) 79 .047 (2) 80 .238 (2) 92 .986 (1) 93 .594 (3) 97 .981 (2) 111 .154 (2) 117 .223 (3) 119 .758 (2) 127 .698 (1) 132 .698 (6) 135 .873 (5) 136 .162 (2) 136.521 (3) 137 .947 (3) 144.382(15) 146 .568 (7) 160.603 (1) 167 .650 (5) 191 .156(10) 193 .382 (7) 196 .426 (8) 240 .816(30) 243 .652(10) 272 .250(15) 285 .923 (4) 297.990 (8) 298 .792(34) 302 .888(14) 315 .176(12) 347 .580 (4) 350 .114 (3) 351 .226(13) 351 .795 (7) 366 .909 (4) 375 .093(27) 375 .827 (4) 400 .439(31) 400 .738 (8) 409 .873 (2) 423 .704 (4) 438 .324 (5)

1, AI,h) (/100n) (%) 3 .80 0 .41 3 .13 1 .18 1 .30 0.31 0 .47 0.47 0.170 0.060 0.090 0 .118 0 .097 0 .086 0 .208 0.031 0.046 0.116 0.079 0 .071 0 .043 0 .032 0.280 0 .096 0 .068 0 .088 0 .099 0 .090 0.076 0.084 1 .60 0.125 0 .040 0 .080 0 .110 0 .190 0 .66 0 .108 0 .29 0 .29 0 .04 0 .19 0 .12 0 .15 2 .66 0 .37 0 .26

35 12 6 8 5 12 10 10 16 20 20 10 20 10 6 25 15 12 12 12 20 20 6 15 20 15 18 15 25 20 6 15 30 25 15 15 8 15 10 10 30 15 15 18 6 7 8

aKe"v

Multi larity . ) Placed") in Ls .

3 .3(4) 2 .9(3) < 0 .5 < 0 .5 2 .4(4)

E1 M1 Ml+3 .0(1)%E2 M1+5 .1(7)%E2 Ml+4 .6(7)%E2 M1 (EI) (E1) M1

< 0 .8 < 0 .8 < 0.5 0 .59(5)

El (El) El E2

< 0 .3 0 .54(12)

(E1) E2, M 1

x x x x x (x) x x x x x

x < 0 .2

0 .080(7)

El

M1

< 0 .035 < 0 .01

El or E2 El

< 0 .03 < 0 .03

El (or E2) El (or E2)

0.0053(6) < 0.02

El El or E2

x x x x x x D x D x x

Comments ) = 4, . . ., 9 L 3 /L, < 0.3, M,/L, = 0 .17 L 3 /L, =0 .24(1) L 3 /L, = 0 .31(4) L 3 /L, = 0 .18(3) u

u M 1 (74 .6)subtracted L 3 /L, > 2 .0 u u u u u u

u u u

x x x x D

x x x

u u

K . Schreckenbach et al . /

160

' 55Sm

TABLE 3 (continued)

EÄAE,,)') (keV) 460.922(21) 467 .738(44) 475 .022 (7) 481 .543(13) 483 .411 (5) 492 .772 (7) 496 .459(23) 522 .638(15) 530 .685(15) 533 .273(36) 541 .318(20) 551 .189(12) 564.507 (4) 574.699(30) 577 .080(44) 579 .905(30) 582 .072 (5) 584.511(ll) 600 .993 (4) 605 .381(14) 617 .549 (7) 621 .545(32) 633 .979(41) 641 .885(31) 658 .396 (9) 660 .640(21) 664.868(15) 668 .49 (10) 668 .987(24) 669 .29 (10) 677 .40 (10) 677 .996(28) 683 .883(50) 684.82 (10) 692.142(20) 702 .854(16) 714 .671(16) 721 .064 (9) 725 .123 (7) 729 .637(27) 731 .455(28) 736.217(80) 741 .790(30) 745 .004(12) 750 .496 (9) 754 .459(24) 761 .631(20) 767 .670(60) 769 .050(60) 778 .156 (8)

L, dl; e) (/100n) («Y.) 0 .11 0 .051 0 .32 0 .19 0 .49 0 .34 0 .23 0 .22 0 .48 0 .06 0 .13 0 .24 1 .21 0 .12 0 .10 0 .10 1 .66 0 .31 6 .30 0 .35 2 .27 0 .11 0 .18 0 .69 0 .65 0 .18 0 .54 0 .09 0 .27 0 .09 0 .19 0 .35 0 .23 0 .13 0 .15 0 .38 0 .36 0 .82 1 .72 0.18 0 .18 0.16 0.30 0.66 0 .99 0 .65 0 .69 0 .11 0 .16 2 .39

15 13 10 15 7 8 15 12 8 20 12 10 6 15 18 15 6 10 6 10 6 15 12 8 8 15 8 20 10 20 20 10 16 20 14 8 8 6 6 9 10 13 12 8 6 7 6 15 16 6

xK"

Multipolarity `)

Placed') in I .s.

< 0 .01

EI or E2

< 0 .09

El or E2

< 0.04

El

x x

0 .0139(14)

Ml

x

0 .0090(8)

M1+50(10)°,óE2

x x x

< 0 .003

El

x

Comments')

u u u u u

u u

x x x x x x

0 .009(2)

< 0 .005

0 .0048(6)

Ml

El or E2

M1 + 45(15)%E2

x

x x x x x

u

K. Schreckenbach et al .,, l55 Sm

161

TABLE 3 (continued)

E,(AE.)') (keV) 779 .131(24) 791 .083 (7) 798 .15 (10) 802 .56 (10) 804 .758(19) 807 .555(40) 812 .819(17) 815 .145(23) 819 .880 (5) 823 .630(50) 827 .61 (10) 829 .150(40) 830 .990(50) 831 .947(38) 844 .108(12) 852 .33 (10) 853 .805(23) 855 .52 (10) 856 .672(49) 860 .61 (10) 864 .50 (12) 865 .843 (9) 868 .55 (10) 877 .600(15) 880 .64 (15) 882 .163(50) 882 .977(90) 883 .263(39) 884 .76 (10) 886 .927(18) 890 .528(68) 891 .552(58) 892 .80 (15) 898 .548(88) 901 .91 (10) 902 .447(23) 902 .946(67) 904 .723(23) 906 .99 (15) 909 .359(61) 915 .490(20) 929 .24 (14) 930 .654(ll) 935 .55 (13) 936 .77 (20) 940 .94 (12) 957 .43 (15) 959 .74 (15) 968 .090(20) 982 .142(40)

1,. dl;b) (/100n) (°,"ó) 0.62 2.38 0.19 0.07 1 .32 0.37 1 .00 0 .83 7 .90 0 .39 0 .17 0 .45 0 .54 0 .55 2 .11 0 .29 0 .58 0 .12 0 .35 0 .23 0 .15 2 .24 0 .25 3 .90 0 .22 0 .66 0 .75 0 .75 0 .17 0 .87 0 .48 0 .36 0 .18 0 .18 0 .40 0 .92 0 .66 0 .66 0 .11 0 .36 4 .00 0 .17 2 .15 0 .19 0 .13 0 .33 0 .25 0 .15 1 .96 1 .30

9 6 20 30 6 20 10 10

6

14 20 12 12 12 6 15 9 20 11 15 20 6 20

6

20 11 10 10 20 8 10 12 20 15 20 8 13 10 20 11 6 15 6 25 25 15 15 20 7 8

aK°

Multipolarity `) Placed") in I .s . M1

x x

< 0 .004

El or E2

x

< 0 .004

El or E2

x

E2+30(15) "  M1

x

0 .0058(9)

0 .0039(3)

D x 0 .0040(15)

MI, E2

x

0 .0033(3)

u u

u

x x

0 .00125(25)

Comments `)

El

x

E2 + 25(14)%M 1

x x

u

u

x x

0 .0024(2)

E2

x x x

0 .0039(4)

M1

x

< 0 .002

E1 (or E2)

x

u u

16 2

K. Schreckenhach el al. / "'Sm TABLE 3 (continued)

E,(AE) ')

(keV)

983 .635(25) 1010 .988(35) 1012 .915(34) 1014 .37 (10) 1026 .540(90) 1026 .697(78) 1034.708(92) 1053 .598(22) 1075 .51 (20) 1090 .20 (20) 1106 .640(24) 1125 .76 (20) 1152 .16 (20) 1184.21 (13) 1187 .09 (17) 1192.46 (17) 1285 .10 (21) 1309 .14 (18) 1327 .84 (14) 1345 .78 (11) 1361 .60 (20) 1406 .12 (21) 1410 .22 (19) 1525 .02 (17) 1619 .59 (13) 1625 .13 (13) 1928 .176(46) 1950 .843(33) 1958 .754(45)

1,

AIrb)

(/100n) (%) 1 .25 0.75 0.62 0.68 0.38 0.38 0 .16 0 .81 0 .11 0.14 1 .64 0 .20 0 .13 0 .49 0 .34 0 .27 0 .51 0 .41 0 .35 0 .72 0 .37 0 .68 0 .70 0 .06 1 .73 1 .20 0 .64 0 .95 0 .64

7 8 9 7 20 10 12 9 25 20 6 20 20 10 15 15 10 9 10 10 12 10 10 10 9 10 15 15 20

aKp

Multipolarity `) Placed') in l .s.

Comments `)

x

x x x x x

x x x

`) Errors in parentheses for last digits. Error for absolute calibration to Sm X-rays not included . Error of absolute calibration to the 104 keV line in . ..Sm ß-decay not included. `) Multipolarity deduced from experimental K-conversion coefficient (1p if not otherwise quoted under comments . Uncertainties for multipolarity admixtures are given in parentheses in units of the last digits . d) x : placed in level scheme ; D: doubly placed . `) Comment u indicates that the time dependence of the intensity does not clearly exclude an assignment to "'Eu (see subsect. 2.3). L3/L, are the measured subshell ratios of ICCs and are only given if important for the multipolarity determination. ~) Energy and intensity from the Si(Li) detector measurement . ~) Energy and intensity taken from the BILL measurements of the L, line . b)

lines originated from the "'Sin. and 156 Eu ß-decays and could be identified with the information of refs. ts. t6) respectively . The results are listed in table 3 . For the intensity calibration the results from the internal conversion electron measurements have been included . This procedure is described in subsect. 2.3.3 . The comment "u" in table 3 means that this line could not be assigned unambiguously to ' 55Sm. In addition to the GAMS measurements low-energy y-rays following thermal

K. Schrec"kenhach et al. / "'Sm

16 3

neutron capture were studied with a Si(Li) detector at the Munich FRM reactor. The essential result was the detection of a 16.547(10) keV transition with an intensity of 3.8(13), calibrated to L, = 3.13 per 100 captured neutrons for the 53.0 keV line (see table 3). In our final level scheme (fig . 6 and sect . 3) the 16.5 keV line has to depopulate a total intensity I, of 24 per 100 captured neutrons. Thus its total conversion coefficient a, = I~lI ï = I,lI,-1 is limited to a range from 4 to 9. The theoretical values for a, at this energy are 7.3, 55 and 7000 for the multipolarities El, Ml and E2 respectively "). Consequently the multipolarity of the 16.5 keV transition is determined as El . 2 .3.2. Conversion electrons after thermal neutron capture . The internal conversion electrons (ICE) after thermal neutron capture were studied with the ß-spectrometer BILL ") at the high flux reactor at Grenoble. Two different targets were prepared to cover the energy region from 17 keV to 1100 keV. For the region from 17 to 500 keV 110 pg/cm2 of 154 Sm, enriched to 99 .996 %, were evaporated onto an area of 3 x 10 cm z on a 200 pg/cm2 Al foil. A thick target of 3 mg/cm2 was prepared by sedimentation ofthe oxide onto a 1 Mg/CM 2 Al foil and used for the energy region 200-1100 keV. The resolution was about dp/p = 6 x 10 -4. The regions were scanned twice for each target in order to identify the 116 Eu lines, which grow in time as in the GAMS measurement . Fig. 5 illustrates a part of the measured ICE spectrum . The electron lines were fitted and calibrated to the GAMS y-energies, in the way described in ref. l e) . The intensity calibration and multipolarity determination will be explained in subsect . 2.3.3 . 2 .3.3. Intensity and multipolarity determination for secondary transitions . The relative efficiency curve of GAMS and BILL are well known at higher energies 13 .1 e) For GAMS the absorption in the edge-faced flat target makes intensity determination difficult at energies below about 150 keV. The BILL efficiency calibration becomes uncertain below about 50 keV due to electron scattering within the target . A preliminary calibration for GAMS and BILL was performed with the known y-intensities and multipolarities of transitions in the " 5 Sm ß-decay's) . The final calibration was undertaken in order to link the GAMS and BILL intensities in an optimum way. As a first step the multipolarities of the 53, 59.8, 74.7 and 127.7 keV transitions were determined directly from BILL using the L-subshell ratios 19) . The theoretical internal conversion coefficients (ICC) of ref. 17) were used. Then the K and L; lines of the 74 .7 keV and 127.7 keV transitions and in addition the strong 104 and 145 keV El transitions in the ß-decay of 1 "Sm were used to obtain a low-energy efficiency curve for BILL independent of the y-intensities. For this purpose the efficiency curve has been varied until the intensity ratios of the experimental L; to K internal conversion electron lines for each transition agreed smoothly with the theoretical ratio for the given multipolarities . This method works well here since the transitions used for this calibration are close together in comparison to the energy difference between L- and K-lines of about 40 keV. Using now these low-energy calibration lines of known multipolarity the GAMS intensities could be calibrated

3 .00 500

3 .24

3 .48

3 .72

600

700

800

900

1000

I

53.033 Sm

y

1100

~_7 E 0

59 .753 Sm L1 L2 L3 cô ~5 5

E N

1200

.

Lnw :r 51 có

oo~

w

.

.

1300 CHANNEL

1

r

104 .338 Sm decay I

-

Fig . 5. Portion of the internal conversion electron spectrum for the reaction '" °Sm(n  e - )' 55 Sm as measured with the ß-spectrometer BILL . The lines are labeled by the corresponding y-energy, the converted atomic shell and by Sm, Eu or Sm decay for lines in' "Sm, in '"Eu and in the' SS Sm #-decay to 15 5 Eu, respectively . ' S 'Eu lines could be distinguished from 115Sm lines by their burn-up and from their differences in atomic binding energy with respect to the y-energy .

O U

0 3 .96

4 .20

3

i<

n

K. Schreckenbach et al. / "'Sm

165

to the efficiency-corrected BILL data via the theoretical ICCs. Extrapolation to higher energies, where the relative efficiency as a function of energy is well known for BILL and GAMS, determined the multipolarity of both the 409.9 and 865 .8 keV transitions as El . They were included in the link of GAMS to BILL in order to give a better adjustment of their relative intensities. The absolute intensity scale has been calibrated to the 104 keV El transition in the ß-decay of 'Sm. An absolute intensity of 72 % per ß-decay has been derived from the population balance ofthe 104 keV level as given in ref. 15) and from the theoretical aK = 0.26 for this E1 transiton l'). The results of this calibration are given in table 3. The listed errors in the y-intensities include no contribution due to the absolute intensity calibration. The experimental ICCs in column 4 are those for the Kconversion lines. Measured L-conversion ratios are listed under comments if they were important for the multipolarity determination. The errors in the experimental ICCs include errors from the relative adjustment of BILL and GAMS . The theoretical ICCs of ref. ") have been used to derive multipolarities from measured ICCs . All measured ICCs could be explained by E 1, M1, E2 or M 1 + E2 multipolarities and no EO admixture was detected . 3. Level scheme of 1s5Sm Previous level schemes of S'Sm were based on (d, p) measurements 5, e) and primary y-rays following thermal neutron capture'). With that information only a few levels could be established and the spin and parity assignments were based on expected Nilsson configurations. In the present work most of the levels and their IR assignments were established model independently. The level scheme of 1 "Sm up to 1400 keV resulting from the present work is shown in fig. 6. Although this study includes information about levels above 1400 keV (see table 1), a reasonable extension of the level scheme in this region is difficult with the present data. In particular a sensitive measurement of neutron capture y-rays with EY > 1 MeV is missing, since the efficiency of the curved crystal spectrometer GAMS 2/3 drops rapidly above 700 keV with increasing energy . The construction of the level scheme was initially based on the ARC results which provided a complete set of states with spins <_ 1. Unique spin assignments could be made by combining the information from 2 keV and 24 keV average resonance neutron capture for I - and jr+ states . No distinction could be made by this method alone between spin I and spin 1. The detected levels were then reproduced by combinations of secondary y-rays measured with GAMS . Multiple placings of y-rays were limited because of the precise energies and the measured multipolarities . Additional levels were found by the Ritz combination principle where is general three precisely known y-rays (error z 20 eV) were used as a minimum criterion to define a level not observed in ARC or in the (d, p) reaction . Since this class of levels is not detected in the ARC measurements, their spin must be >_ 1, or I- in some cases.

K. Schreckenhach et al. % ' "Sni

166

TABLE 4

Summary of information on levels in ' "Sm below 1400 keV deduced from the present work (columns 3-5) and from (d, p) 5- a ) Ex (keV) ') (adopted) 0 16 .547 53 .034 76 .299 127 .698 152 .417 220 .684

(2) (1) (2) (1) (3) (2)

426 .418 (4) 500 .000 (4) 617 .543 (3) 658 .387 (4) (736.930(10)) 778 .146 (3) 819.880 (5) 821 .304(11) 844.113 (5) 865 .848 (7) 882.181(16) 903 .466 (5) 906.836(17) 915 .525(20) 930 .642 (7) 962 .420(l8) 968 .090(20) (984 .450(50)) 1010 .924(25) 1106 .668(10) 1154 .430(78) 1168 .743 (9) 1282 .435 (6) 1327 .525(12) 1362 .131(12)

/` limitations

/' b)

(adopted) 1}+ 17+ 2 7-

Z+ > 2.

~}21

>_ ;

}

+ t

1j -, j 3 3~+

3+ + . 3+

}-

-, i t )+, }+

Z } Z }

+

a+ 3+ 1-

Z } }+, ,)+ }+ 4+

ARC 1-, 3+ Z 2

z = ? Z 2t 1

secondary `) transitions +, s - <-_ }

7 2

1+ Z} >_-

1 1

`, 3', }" ;-, i - , ~-

l-

>

27 37

7+ ', 1+, 2

2> 7 2 Z

Sum over") depopulation

i

1 -, 3 -, 3 , k> } 3)+, - 13 3+

, 2-

9 .8 3 .8

+

3

~ =, ? f , 3 - , ~-, s 72

+, )*,

?f,

33 -+ 1, ~}+

3 , >_ }

'-', i' - ', ( < i,

}+, + + } Z Z } ) +.1 +

1+ 1 - 14 >__ } }+, }+

)+ 1+, }+

~

(3+),

3 - 13 1, 11 +, s

-

3 .4 0 .56

+, 27,

+ 1+ , 1 + 3

1 .5 0 .80

-,

,}

-

_ )+,

~+

*

,

)

}t

t

7, 1 3 =, 3', 3 +

< }+, }~, *, ) 7 a± , i=, 2

0 .72 5 .2 7 .9 2 .0 4 .7 3 .2 1 .8 2 .7 1 .3 4 .1 6.1 1 .4, (0 .76) 2 .0 0.64 1 .8 2.7 0.46 0.45 1 .5 1 .1 1 .5

(d, p) `) E, (keV)

l

Comments

0 25 51 80

0, 1,2

151 227 338 364 426 499 601 617

3,4

') ')

Z 6 2,3 2,3

5)

716 748 786 824

0, 1,2 0,1,2

874

2, 3, 4

909

2,3

937 963

0, 1,2 >-- 3

999 1018 1043 1076 1107

z3

b)

')

j) ') t)

') )

m

")

> 3

1163 1180 1339 1362

') Errors are given in units of the last digit . The levels at 1335 .8(7) keV, It and 1217 .7(7) keV, Q - ) detected in the ARC measurements by weak lines are not included in this table, since they are not well reproduced by secondary y-rays . b) Adopted spin and parity after combining all available experimental information .

K. Schreckenbach et al. / "'Srn

167

Finally, level energies with errors were determined by a least squares fit procedure to the transitions between them . Only transitions with a deviation of less than twice the energy error have been assumed to be well placed between the corresponding levels . Doubly placed transitions were excluded for the calculation of the level error. Table 4 summarizes the detected levels with arguments for the spin and parity assignments. Thus only a few additional remarks will be given in the following to illustrate the arguments. In many cases the placements and multipolarities of secondary y-rays could be used to distinguish between spin j and spin 1. For instance, the 618 keV level definitely has IR = J' : ARC predicts J+, J+, but the decay by the 564.5 keV El transition to the 53 keV, - level, as well as the M1 component in the 600.99 keV transition to the 17 keV, + state, excludes }+ . Similar arguments were used for the spins of the 778, 844, 866, 931, 1107 and 1362 keV levels . Parity assignments from the ARC measurements were uncertain for the 778 keV and 1169 keV levels although IR = I - , j - were favored due to the 1,,(2 keV)/1,. (24 keV) ratio (sect. 2). The multipolarities in the decay mode of the 778 keV level determine its spin and parity definitely as I- . As for the 1169 keV level only a modeldependent argument supports the ARC results. All I+ states should be detected in the ARC measurement. Assuming that the J +, 1106 keV and J+, 1154 keV levels are members ofthe same band, no additional positive-parity state remains which could be associated with a K = J + or j+ band head at 1169 keV. In conclusion the absence of a close-lying I+ level in the ARC data and the decay mode favor strongly I* _ for the 1169 keV level .

TABLE

4 (continued)

`) The placement of transitions can be read from fig. 6. Transitions with unknown multipolarities are assumed to be of El, M1 or E2 type . Transitions which are doubly placed, uncertain in placement or in assignment to "Sin were not used to obtain the I` limitations. +, < } means positive parity and spin < }. I < 1~ assumed. ") Determines the population of a state in (n,h, -,,) if all depopulating lines are detected . Values given per 100 captured neutrons including internal conversion . `) Energies are taken from ref. '), where energy uncertainties of about 5 keV are quoted. A comment is given if energies in ref. e) disagree by more than 5 keV with those values . The 1-values are adopted from those given in ref. 5) and from the (d, p) angular distributions measured by ref. °). c) Population systematics favors the adopted spin . ") (d, p) energy in ref. °) is 358 keV. ") 1 = 2, 3 in (d, p) limits I* to j + , i * , i -, among which ARC allows only 3 - . ') Level uncertain, since two of the transitions defining the level are doubly placed in the scheme and since the (d, p) energy fits badly. Population favors spin 1 . J) 824 keV, (d, p) level cannot correspond to the 821 .3(4 keV level since 1 = 0, 1, 2. `) (d, p) energy in ref. e) is 868 keV. ') (d, p) energy in ref. b) is 930 keV. m) The 963 keV (d, p) level cannot correspond to the 968 keV state in (n, y) due to 1 Z 3. Placement of the 80 .2 keV (El) transition with L, = 0.47 is uncertain. °) Level uncertain.

I!!

Mom

155 3m

a IIIVIIII~IIIIIIII lima !!

a"

3/2" 5/2" 5/2-7R+ , 1/2-,3/2 5/2- .7/2 "3/21/2- .3/25/21/2" .3/2 " 5/2" 3/23/25/2 112 .3/23/27/2--

Fig. 6. Level scheme of 1 'Sm deduced in the present work. Dashed levels are not well established. The thickness of the arrows is proportional to the total transition probability I, = /.,(1 +a,), while the filled part of each arrow gives the portion of the ~,,-intensity . For the transitions, the energy in keV, the ;intensity per 100 captured neutrons is given and where measured the multipolarityor multipolarity limitations.The label " denotes adoubly placed transition

N

T

m

m N """""""""""""

1106 .668 1010 .824 - 884 .450 868 .080 962.420 930.642 815.525 906.836 803.466 882.181 865.848 e 44 .113 821 .304 819. 880 778.146 - 736.930

K. Schreckenbach et al. / 'ssSn:

169

Primarily on the basis of the ARC measurement the level scheme should include all low-spin states up to 1400 keV. Therefore the sum over the ground-state population is expected to be close to 100 %. In order to avoid the uncertainty in the intensity ofthe 16.5 keV transition one may take the sum over the population of the 16.5 keV level and the ground-state population without the 16.5 keV level. Our level scheme gives 24 %+67 % = 91 % for this sum, which is a reasonable value for a rather complete level scheme . A further study was made of the population systematics of all detected levels (table 4). As expected, the population (= sum over depopulating lines) decreases with increasing excitation energy and increasing spin of the level. Within rotational bands those systematics show up even more clearly ' o). In some cases the placement of a transition can be checked by the population systematics and the population balance of the involved levels . In the final level scheme only five weak transitions are placed twice. They are marked by "D" in table 3 and by * in fig. 6. 4. Rotational bands in "SSm Previously only a small number of band assignments in terms of the Nilsson diagram have been made for 's'Sm based only on the (d, p) results s . 6). The present investigation of the rotational band structure is based on the model independent 5/2 1328 L;3/2]2$2 1l2[660]

5/2`1011 1/2*3Z=

mY 712* 737

5,'2882 jZ31 `903 3n 1,`2{[400] 312[402]

5/21154 3121107 3~T521]"0

5/2-521 3~~ (11/2JU16] 3/

(11/211163] (912110_'2J

L

(7/2koll~ 5/ 962 5%7/2' 51T 907 31/21530] 1i21521]

(9!2)01 T2 500 (11/2T338 9/27221

5 ;2 _426 5/22523]

155

Sm

7/2

5/2 53 3/20 ~V 2T521]

Fig . 7 . Rotational bands in issSm with the main component of the Nilsson configurations . Spin and parities are given as determined in the present work. The assumed band member spin is underlined, if I` of the corresponding level is not uniquely known . Levels detected only in (d, p) are tentatively assigned to rotational bands and denoted by their energies in square brackets and their assumed I` in parentheses .

170

K. Schreckenbach et al. /

"5Sm

level scheme developed in sect . 3, on the (d, p) results and on systematics of rotational bands in nuclei close to is5Sm, particularly 113 SM [ref. 4)] 'S'Gd[refs . 21 . 22 )] and "9Dy[ref. 23)]. The rotational bands found in the present work are shown in fig. 7 and will be discussed in the following sections. In sect. 5 band mixing calculations will be presented and the (d, p) intensities reinterpreted to confirm the proposed band structure. 4.1 . IDENTIFICATION OF BANDS WITH NEGATIVE PARITY

According to the Nilsson diagram (deformation E 2 - 0.3) and the neighbouring nuclei, the following negative single-particle states with K --< j can be expected for neutron number 93,atlowenergies ( ;~ 1 MeV) :I-[521],I-[530],}-[510],J-[521], J - [532] and J - [523]. Additional levels may occur from the coupling of singleparticle states to vibrational modes where particularly the KR = 0 - (octupole vibration) and KR = 0+ modes may contribute at energies around 1 MeV, since their energies in the neighbouring even-even nucleus "4Sm are at 922 keV and 1100 keV respectively 24) . j - [521] ground-state band : Levels at 0, 53, 128, 221 and 338 keV. The I - , and I - members of this band have already been assigned in both (d, p) studies, the I - member only tentatively in ref. 5) . Our work confirms those levels with spin assignments. A rotational parameter of A = ü2l2J = 10.7 keV, defined by E(1) _ AI(I+ 1), fits the first 4 members of this regular band well and predicts the 4 member to be at 339 keV. A weak (d, p) level at 338 keV corresponds to this energy . J - [523] band: Levels at 426, 500, 601 and 716 keV. Members of this band have been assigned differently in the two (d, p) studies. Due to our spin assignments the 426 keV level is the I - band head as proposed by ref. S) . With A = 10.5 keV a regular band up to -'z1- can be constructed, where the 3 - band and 'I' - members were only detected as weak lines in (d, p) studies. I - [521] band : Levels at 820, 844, 907, 962, 1076 and 1163 keV. The I - band head at 820 keV has been identified unambiguously in both (d, p) studies, since theoretically it should have the largest cross section in the (d, p) reaction . The further assignments of band members in the (d, p) studies are in contradiction to our level scheme . In particular the 868 keV level, assigned in (d, p) as the j - memberhas no corresponding j - level in our level scheme known to be complete for spins I and J. The only close I level is at 844 keV and is therefore assigned to the } - [521] band. The J- member should have a strong (d, p) cross section. The 907 keV I - level has such a feature although the (d, p) intensity may be shared with a nearby 903 keV level . These three band members determine the rotational parameter as A = 10.4 keV and the decoupling parameter as a = -0 .22. Using these parameters the 1 - , 3 - and - members are predicted at 964, 1078 and 1166 keV respectively . Corresponding levels are seen in (d, p) at 963, 1076 and 1163 keV respectively . The (d, p) level at 963 keV is reproduced in the present scheme by a I-, It state at 962 keV, the 1163 keV (d, p) level could also correspond to the J- , 1169 keV state. The strong E2 transition from the 820 keV

K. Schreckenbach el al . / "'Sm

17 1

level to the ground state may be due to components of the y-vibration {j- - [521] - 2+ } in this band as also indicated by the missing (d, p) strength 5). This admixture is favored by the corresponding E2 selection rules for the asymptotic quantum numbers [N, n., A] : AN = An- = 0 . AA = 2 [ref. 2s)] . - [530] band: Levels at 916, 931, 984 and 1018 keV. Despite being a hole state the member of the J - [530] band should be rather strongly populated in (d, p). The only close }-, J - levels below 1500 keV are at 916 and 931 keV, where the 931 keV level definitely has IR = J - and is strongly populated in (d, p). Consequently we assign the 916 keV level as the I - band head and the 931 keV level as the I - member ofthe I- [530] configuration. The j - member at 984 keV and J- member at 1018 keV are assigned only tentatively. The assignment for the 984 keV level is supported by the decay mode of this level . The 1018 keV level, only seen in (d, p), fits in energy with the parameter A = 7 .86 keV and a = -0.36 deduced from the first three band members, but it could also correspond to the J- - , 1011 keV level. An alternative assignment ofthis band as the j - [510] configuration is excluded by the weak (d, p) intensity to the IR = I - band head . I - [532] band : Levels at 778 and 821 keV. The only remaining - levels not yet assigned below 1500 keV are at 778 and 1169 keV. The band head of the J - [532] state in nuclei with the same neutron number such as 157 Gd and "9Dy is at 700 keV and 627 keV respectively . Therefore we propose the 778 keV level in ' SSSm as the band head of the j-[532] state. A level at 821 keV is tentatively assigned as a further band member. The decay mode of the I- member, dominated by El transitions to the J+[642] band, is quite different from that of the 778 keV band head . This feature may be caused by Coriolis mixing components from the J - [523] band, which can only be present for spins >_ 1. The members of this J- [523] band also decay by El transitions to the J+ [642] band. No reasonable assignments could be found for the remaining negative-parity levels of J -, 1169 keV and Q- ), 1218 keV, where the latter is not well established . 4 .2 . IDENTIFICATION OF THE MAIN COMPONENTS IN THE POSITIVE-PARITY STATES

A complex system of strongly mixed positive-parity bands is expected at low energies . Inspecting the Nilsson diagram and neighbouring nuclei the following bands may occur below about 1500 keV : }+[660], J + [651], J+ [642] and J+ [633] from the i .. shell ; }+ [400] and J+ [402] from the st and dt shell respectively . The bands from the N = 6, i,~ shell mix strongly, because of the Coriolis force, with each other. Furthermore they may interact through AN = 2 mixing with the N = 4 bands which cross the N = 6 configurations in the Nilsson scheme for a deformation parameter of s 0.3. The 0- octupole vibration built on the ground-state band may create another I+ band as will be discussed below. Due to the strong mixing of all these configurations only their main components are identified in this section. In subsect . 5.2 a full mixing calculation will be presented.

17 2

K. Schreckenbach et al. i "'Sin

1+[642] band : Levels at 17, 76,152 and -'2 + , 364 keV. The l+ member has already been identified in the (d, p) studies. The I + , I+ and 3+ members are well established in the present study. With a rotational parameter of A = 8.51 keV the '7'-+ and 4+ levels are predicted to be at 246 and 357 keV respectively . The 4+ member at 357 keV should have a relatively large (d, p) cross section . A (d, p) level at 364 keV may be assigned to that state. J + [651] band : Levels at 618, 658 and 737 keV. KR = + bands are expected in this energy region from the J+[402] and J + [651] states . The J+ , 618 keV level is too weakly populated in (d, p) to have a dominant component of the J + [402] configuration . Consequently the J + [651] configuration is proposed as a major component for this level with further band members at 658 and 737 keV . This assignment is supported by the strong M1+E2 transitions to the 1 + [642] band, where the E2 component arises from the expected Coriolis mixing between the J+[651] and [642] states . The (d, p) intensity to the 618 keV level indicates some 1+[402] component in this state as predicted from AN = 2 mixing . The spacing of the + [651] band is rather irregular with a rotational parameter of about 8.2 keV . J + [402] band : Levels at 866 and 882 keV. The J+[402] band is expected to be quite close to the J + [651] band, particularly since the (d, p) cross section of the J+, 618 keV level could hardly be explained by any other effect besides AN = 2 mixing between those configurations . The J+, 866 keV level is proposed as the band head of the J+[402] state. The corresponding (d, p) intensity favors this assignment . The only I + state which can belong to this band is the J+, 882 keV level. As reflected in this small energy spacing, the band is highly irregular, indicating strong mixing with other configurations. The decay of this band via El transitions to the ground state will be discussed in connection with the octupole vibrations built on the groundstate band (subsect . 5.2). }+ [400] band : Levels at 903,968 and 1011 keV. Two levels of spin I + or J + and only one I + level occur around 950 keV. Thus these levels must form a Kx = ]:+ band as a + consequence of the completeness of the Ix = I , J+ and I + levels in the present study (ARC results) z e) . The ]; + [400] and J+ [660] bands are expected at about this excitation energy . The J + [660] is excluded since the theoretical decoupling parameter of this configuration is +6 .0 . Hence we propose } + [400] for those three levels . The deduced band parameters are A = 15 .0 keV and a = + 0.53 . The large A-parameter may be caused by mixing with the J+[402] band . I + [660] : Levels at 1282 keV, }+ and 1328 keV, I + . For a decoupling parameter of +6 .0 as expected for the ]: + [660] configuration the I+ member would be below the I+ band head and the I+ member about 250 keV higher than the I + . Coriolis mixing may change slightly this picture. In order to explain the small spacing of the J+[402] band Q+ band head at 866 keV, J+ at 882 keV) a strong interaction with a J + state higher in energy should occur without strong interaction between the corresponding J + levels of those bands. The I+[660] band has exactly this effect since the J+ level is expected to be well above the J + level. The strong interaction between the J+ [402]

K. Schreckenbach et al . / 's'Sni

17 3

band and the 1+ [660] band could be caused by a relatively strong J'[651] component in the J+[402] state as expected from AN = 2 mixing . In conclusion, the spacing of the 866-882 keV band may be explained by a }+ [660] band not too much higher in energy . The most likely candidates below 1400 keV are the J+, J+, 1282 keV and J+, 1328 keV levels . The above-mentioned mixing moves the I+ state above the I+ state. The strong 664.9 keV transition from the 1282 keV level to the 618 keV band head of the J+ [651] configuration supports our assignment . The mixing calculation in subsect. 4.2 .2 will further confirm this assignment . The J + member is expected at about 1500 keV and has not been identified . {1-[521]+0-} band : Levels at 1107 and 1154 keV. No more single-particle states with KR = I+ are expected around 1 MeV. The decay mode ofthe 1107 keV and 1154 keV levels and the small (d, p) cross section indicates a main component of the octupole vibration built on the ground-state band. The weak (d, p) cross section may be due to some single-particle admixture or may belong to a different level such as the I - state of the I - [530] band . The E1 branching ratios to the ground state (table 5) obey the Alaga rule for K; = Kf = j and support our assignment . The splitting of this configuration among single-particle states will be discussed in subsect. 5.2. 5. Discussion of the band structure in f "Sm 5 .1 . CORIOLIS-MIXING CALCULATION FOR THE NEGATIVE-PARITY STATES

A conventional Coriolis-mixing calculation has been performed for the negativeparity states in order to compare our assignments with the experimental (d, p) intensities. The coupling matrix elements were taken from a Nilsson-model calculation [code CJ z')] with the parameters y = 0.23, K = 0.05,1-. 2 = 0.3 and f:4 = -0.05 . A common reduction factor of 0.8 for the coupling strength has been assumed . The p- and x-values for the Nilsson potential are based on the systematics of Rekstad and Lovhoiden ze), which demonstrate that previously accepted values of U and K do not reproduce the correct sequence of spherical states in the N = 82 spherical limit. It should be noted that the effect of this change at an e Z value of 0.3 employed here is small compared to that found for the transitional N = 89, 91 nuclei . The rotational parameter A was chosen as 10.4 keV so as to reproduce the experimental level spacing. This value is different from that of the neighbouring even-even isotopes and also from that used for the positive-parity states (see subsect. 5.2). The actual mixing calculation was performed with the code COMIX z9) . In fig. 8 the calculated and observed (d, p)cross sections are compared . The spectroC Uj)Z, scopic factors S;t for these calculated values were deduced from S; t = where a t denote the mixing amplitude of a Nilsson orbit resulting from the Coriolismixing calculation, C;;1 are the Nilsson coefficients (again from code CJ) and Ut the occupation amplitudes . The agreement is in general good. The most significant effect

(I jai ."

174

K. Schreckenbach et al. l ""Sr? :

z

0 FU W N

Fig . 8 . Comparison of calculated and measured (d, p) intensities for the negative-parity states . The (d, p) data are from ref.') . A small correction was made to give Q-reduced values corresponding to Q = 3 .6 MeV . States are labeled with twice the spin values . Circles represent very weak (d, p) cross sections .

of the Coriolis coupling is the transfer of strength between the J - states at 844 and 931 keV . If the orbits were pure, most of this strength would lie in the lower state, contrary to the experimental result . The lack of (d, p) strength for the }-, 820 keV state may be caused by an admixture of the y-vibration built on the ground state. The systematic behaviour of this strength over various nuclei is discussed in refs. 21,22) and explained by this admixture of the {4-[521]-2+} configuration. 5 .2 . CORIOLIS, AN = 2 AND Y3o COUPLING FOR THE POSITIVE-PARITY BANDS

The positive-parity states mix via Coriolis interaction, which is particularly strong between states from the it3n shell, and also by AN = 2 coupling. In addition the coupling between the J + [651] and the {I-[521]+0-} configuration may be important since those states are connected by a strong Y3o matrix element (orbits differ by An z = 3, AA = 0) 3°). Those coupling mechanisms should give an explanation for the various irregular level spacings in the level scheme such as the close J+, I+ members of the I+[402] band, the large spacing of the I+[400] band and the I+ state above the jr+ state for the 1+[660] band . In order to investigate these main features of the experimental level spacing without too many free parameters we tried to get an independent estimate for all coupling matrix elements, the unperturbed rotational parameter A and the decoupling parameters a. Finally only the energies of the unperturbed band heads were adjusted to the experimental level scheme. The matrix elements for the Coriolis coupling as well as the Nilsson coefficients Cat

K. Schreckenbach et al . /' -"Sm

17 5

have been calculated similarly to the negative-parity states with the code CJ for a deformation of e2 = 0.30 and E4 = -0.05. In the actual mixing calculation, again a common reduction factor of 0.8 was applied to those values . For the band parameter A = ü212Ja value of 13 .4 keV has been taken for all bands, which corresponds to the mean of the A-values in the ground-state bands of 154Sm . The decoupling parameters a for the K" _ + bands have again been and "'Sm calculated with the program CJ. The size of the matrix element in the AN = 2 coupling between the +[651] and J + [402] configurations could be estimated from the observed (d, p) cross sections in the corresponding I+ band heads assuming only two-band mixing s') . The experimental (d, p) cross sections, dominated by the 1+ [402] configuration, are 17 and 36 (in relative units) for the j+, 618 keV and 1+, 866 keV levels respectively 5 ). Thus, including pairing factors with a pairing gap of 1 MeV, the I - [402] component is 26 % in the 618 keV level and 74 % in the 866 keV level. With an experimental energy spacing of 866-618 = 248 keV the coupling matrix element must be 110 keV in order to produce those mixing components . The AN = 2 coupling matrix element between the 1 + [660] and J+[400] could not be derived directly from our study of (d, p) cross sections . It has been taken as 100 keV, which is a typical value for well-deformed nuclei in this region s1) . A striking feature in the 155Sm level scheme is the large number of strong El transitions. In 154Sm there is a 1 - , 922 keV level of the K" = 0- octupole vibration with a half-life of 0.026 ps [ref. 24)], i .e. close to the Weisskopf single-particle estimate for M1 transitions (0 .03 ps) of the same energy . Hence, in 155Sm, El transitions originating from the deexcitation of this octupole vibration and M1 single-particle transitions should compete with a similar transition probability. The K` = 0 - octupole vibration in `Sm built on the J - [521] ground-state configuration is expected at about 1 MeV, analogously to 154 Sm. The configuration {1-[521]+0-} may mix with the J + [651] state because of the strong Y3o coupling matrix element. In subsect. 4.2 we proposed the I+,1107 keV level as the band head of the { - [521 ] +0 - } configuration. Due to Y3o mixing, components of this configuration may be expected in bands which contain a significant } + [651] amplitude. Such bands are in particular the I+[651] band at 618 keV itself and the +[402] band at 866 keV. This may yield an explanation, although only qualitatively at present, for the observed strong El decay branches from these bands to the ground-state band. All those El branches obey the Alaga rule for transitions between bands with K; = Kf = I (table 5). El transitions between the corresponding Nilsson configurations themselves are unfavoured due to selection rules in the assymptotic Nilsson quantum numbers 25). The relative splitting of the {1--[521]+0-} configuration between the 618 keV and 1107 keV levels may be estimated from decay branching ratios to the ground state and the J+[642] band . This estimate contains uncertainties, particularly since the E2 admixture ofthe 1090 keV transition is not known a priori and since only the mixing of two levels is considered . Assuming that only the J+ [651] and {j-[521]+0-} configurations govern the M1 and El transitions, respectively, the problem can be

K. Schreckenbach et ai. i

17 6

"I SM

Fig. 9 . M 1-E 1 branching in the decay of the 617.5 and 1106 .7 keV level . See text for the estimate of the M 1 branch in the 1090.20 keV transition . .

simplified by considering the following wave functions with amplitudes a and P (see fig. 9) ~ (618 keV) = a1651> +P1521 +0 - ), ~ (1107 keV) = P 1 651)-al521+0->, with a2 + P 2 = 1 . The branching ratios R; = r'l(M l)/n' 1 (E l) = B(Ml)/B(E1) from the level i can be written as ORbI e

a
P(El)

"V

/

io7

P P(E1) ,

where and denote the matrix elements for transitions to the J + [642] and J- [521] band heads respectively . The reduced transition probabilities B(.1) are proportional to <),) 2. Neglecting the pairing factor P leads to R618/R,107 = a4 /P 4 . The 601 keV transition, from the618 keV level to the 17 keV level, has an experimental mixing ratio of 6' = I,,(E2)/I,,(M 1) = 1 .0 (see table 3). The M 1 part could be attributed to a single-particle transition from the J+[651] to the j+[642] configuration, while the E2 part may be due to the Coriolis mixing of these two states, i .e ., an intraband type oftransition to the [651 ] component in the 17 keV level. Based on the same assumptions, the J + [651 ] component in the 1107 level should cause the same reduced mixing ratio B(E2)/B(M 1) for the 1090 keV transition to the 17 keV level as measured for the 601 keV transition. The corresponding 6' is proportional to EY2 and thus gives an estimate for the 1090 keV transition of 62 = 1 .0(1090/610) 2 = 3.3. With the result- .

177

K. Schreckenbach et al. / "Sm

âf2+ 1500

512 + 1328 112+ -1282

[660]1

512 + - 1154 312 1' - 1107 1000

512+ 312+

1011 968

~f2+

882 866

312 +

W Z W

[521]1+0 [40 0] 1

][40214 [651]1

(712+) -_______ 737

Z

512 * 3r2 +

H Q

658 618

X 500 W

1312 + -364

0

L

912 +

152

712+

76 17

512+

experiment

[64 2] 1

mixed

unperturbed

Fig . 10 . Illustration of the result of the mixing calculation for positive-parity states in ' "Sm . The unperturbed band head energies were varied to get an optimum energy fit of the mixed states to the experimental levels . The choice of Coriolis, AN = 2 and Y30 coupling matrix elements are described in subsect . 5.2 of the text .

ing values R6,8 = 1 .4(3), R,to7 = 0 .020(6) and thus R6,8/R,to7 = 70(25), ßZ is obtained as 0.11(2) and leads to a Y30 coupling matrix element of 150 f 30 keV. The final mixing calculation has been performed with the code CORCUp 32). This code allows the introduction of spin-independent coupling matrix elements originating from dN = 2 and Y3o mixing . Fig. 10 illustrates the result of the mixing calculation which includes Coriolis, AN = 2 and Y3o coupling as discussed above.

178

K. Schreckenhach et al. l

1 "Sin

TABLE 5

El branching ratios to the ground-state band J - [521] in '"Sm Initial level (keV)

E,

Ground-state band

1,(1)/1,(2)

li-

if(1)

4+

}-

g

-

}-

865 882

}+ }+

}}-

1106

4+

}}-

617 658

}+

~) 1,(1)lIÏ(2) _

If(2)

1-

Alaga rule °)

0.51

0.52

0 .75 1 .24

0 .54 1 .37

0.55

0.45

0.80 1 .39

0 .68 1 .44

-

i-

<1IKII1_r(1)Kr_i 2 (E,( 1 )1 ; .

2 (E.,(2 J)

experiment

0.58

0.49

. K,--Kr=}

TABLE 6

Results of the mixing calculation for positive-parity states in 's'Sm E,

exp 17 76

(keV) calculated 17

152

79 156

[364]

264 366

618

609

658 (737)

654

866 882 903

778 849 998

968

903 969

1011 1107

1005 1113

1154 1282

1159

1327

1282 1314 1577

Ix

1+ Z+ ?+ _1~ + T+

1+ 3 + 3+ 1+ 3+

Calculated amplitudes of Nilsson orbits

j + [642]

1 + [402]

}+[400]

13 - [5211+0 - )

0 .17 0 .25

-0 .05

0

-0 .02

0 .03

0 0.01

0 .37

-0 .09

0 .01

-0 .03 -0 .04

0 .04

0.92

-0 .06 -0 .08

0.86

0 .45 0 .81

-0 .11 -0 .51

-0 .17

0 .78

-0 .25

0 .76 0 .42

-0 .03

0 .30

0.98 0.96 0.93

}+

a+

J + [651]

0.33

J+[660]

-0 .05

0 .10 0 .07

0 .02 0 .01

-0 .05 -0 .28

0 .21 0 .11

-0 .46

0 .06

-0 .25

0 .30

-0 .51 0 .84

0 .02 0 .15

-0 .27 -0 .31

0 .19 0 .07

0 .84

0 .34

-0 .21

0 .23

0 .12

0 .28 0.16

-0 .07

-0 .11

0 .96 0 .97

}+

0 .03

-0 .22 0 .39

-0 .26 0 .15

0 .84 -0 .08

0.31 0.90

0.29 0.08

}+

-0 .03

0 .26

0.15

-0 .31

0 .84

0 .33

-0 .28 -0.29

-0 .30

-0 .16

-0 .04

0 .96 0 .82 0 .98

3+ }+

3+

a+

0 .04

-0.40 -0 .14

-0.03 0.01

Although the free variables have been limited to the band heads of the unperturbed bands the resulting energies agree in general very well with the experimental level scheme . The large level spacing of the J + [400] band is well reproduced. The I+ member of the J + [660] band is calculated to be above the I+ member and close to

K. Schreckenhuch et al . í "'Sm

179

the experimental level. The small spacing between the J- + and I + members of the J+ [402] band is only qualitatively reproduced by the calculation. The calculated J+ member of the I+[660] configuration is at 1576 keV. There is a }+, j + level at 1570 keV according to the ARC measurement, but no conclusive assignment can be made at such a high excitation energy . In table 6 the amplitudes for several strongly mixed states are given as they result from the present mixing calculations . The d N = 2 and Y30 coupling matrix elements, which were estimated from a two-level mixing, reproduces in the full mixing calculation the experimental values still reasonably well . The predicted j+[402] components (= square of these amplitudes in table 6) for the 618 and 866 keV levels are 26 and 71 %, respectively, in comparison to the values of 26 % and 74 % deduced initially from the (d, p) intensities . For the Y3o coupling the 866 keV level disturbs the estimate derived from the mixing of the 618 and 1107 keV levels . But there is still a reasonable agreement between the predicted M1/El branching R618/R1 ,07 = 45 (= corresponding ratios ofsquares of amplitudes of the J+[402] and {1 - [521 ] + 0 - 1 configurations respectively as given in table 6) and the experimental branching of 70±25 . The calculated components of the {j-[521]+0-} configuration in the j+[651] and the J+[402] band are quite similar in size and may be the cause for the strong El transitions from the J + [402] band to the ground-state band (see also table 5).

200

1

---T -

---1

-

I THEORY

100

z0 U W U)

u)

3

w

o 1

3

200

3 EXR

U

â v

100

91

0 600

800

1000

1200

EXCITATION ENERGY (keV)

Fig . 11 . Comparison of calculated and measured (d, p) intensities for positive-parity states, displayed as in fig . 8 .

K. Schreckenhach et al. / "'Sm

180

Fig. 11 compares the calculated and experimental (d, p) cross sections for the positive-parity states . The calculated values were deduced in the same way as described for the negative-parity states in subsect. 5.1 . Again the agreement is good and thus confirms the proposed assignments. 5 .3 . SYSTEMATICS OF NILSSON ORBITS IN THE Sm ISOTOPES

In fig. 12 the experimental band heads of Nilsson configurations are shown for 151, 153. '"Sm. The 151 . 153Sm data were compiled from refs. 1, Z, 4). Hole states are plotted with negative energies. The dominant components of configurations in 1000

w rn W

500

EmjkeV) ImSm

",Sm

U F 0

tn -500 W

Q H tn w-1000

Fig. 12 . Systematics of experimental band head energies in Sm isotopes . States with the same main 1,2,4) component of a Nilsson configuration are connected. The data for 151 " 1 "Sm are taken from refs . . Hole states are plotted with negative energies.

the band heads are followed by connecting lines through the isotopes . In view of the generally strong mixing this figure illustrates more the trends than the exact energies of those orbits . There is a striking difference between the positive-parity bands from the i,? shell and the negative-parity bands. Some of the latter are rising in energy with increasing neutron number in contrast to those positive-parity states . For a constant deformation, Nilsson states should obviously drop with neutron number in the representation of fig. 12. A change in deformation, as from 151Sm to 155 Sm, can alter this picture

K. Schreckenbach et al . /

"'Sm

18 1

drastically. In the Nilsson diagram, the relevant states from the i,4 shell are most strongly down-sloping with increasing deformation for neutron numbers around 90 . Hence when increasing the deformation they are preferentially filled with neutrons. Thus the I - [521] state remains close to the Fermi level, which even drops slightly with the neutron number. The J- [521] state becomes more distant from the J - [521] state with increasing deformation, which is reflected by a rising energy for this particle state in going from . . .Sm to t 55 Sm . The j+[402] and 1+[400] states, upsloping with increasing deformation, remain more stable in energy over these Sm isotopes than the observed states from the i,4 shell. 6. Conclusion An essentially complete set of levels with I < j and E,, < 1400 keV has been established for 'SSSm and the underlying Nilsson band structure can be well understood. The proposed assignment of an octupole configuration at 1107 keV, built on the ground state I - [521], offers an intriguing explanation for some of the observed anomalous E1 decays. Calculations 30) for this region indicate that small admixtures from other octupole configurations may be expected in some of the assigned bands and may be responsible for further strong E1 branches observed in the present study of '"Sm. It is hoped that a more complete understanding of these various ratios may be possible with the type of approach described above. A similar study of El branches in nuclei such as ts 'Gd, which have a very similar band structure, but where the octupole vibration is expected to play a minor role [see also discussions of the (d, d') results in ref. a2)] would be helpful for the assignment of El transitions to octupole vibrational modes in t55 Sm. The authors are grateful to G. Blanc, G. Schmid and the ILL reactor staff for technical assistance . One ofus (K.S.) acknowledges the hospitality and help provided during his stay at the BNL. Two of us (R .K.S., A.I.N .) wish to thank the ILL for the support obtained during their stay at this institute.

1) 2) 3) 4) 5) 6) 7) 8)

References

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