States in the N = 87 Nuclei 149Sm and 151Gd Populated by the (d, t) and (3He,a) Reactions

AND

D. G. BURKE McM~s te r Utzi~~c~rsity, Hnmiltot~, Ot~tcrrio

Received November 15, 1974

The (d, t) and ('He, a ) reactions on targets of lSoSm and lS2Gd have been studied using a magnetic spectrograph and photographic emulsions to analyze and detect the reaction prod- ucts. The ('He, a ) spectra were measured at two angles for each target using 24 MeV beams of 'He. The IS0Sm(d, t)149Sm reaction was studied at 13 angles with 12 MeV deuterons. The lSLGd(d, t)IS'Gd spectra were recorded at 4 angles with 15 MeV deuterons. The I values for a number of low spin states were determined from the (d, t) angular distributions. The ratios of ('He, 00 and (d, t) cross sections were used to obtain I values for several other states. It is pos- sible to explain the observed strength in terms of the spherical shell model although there is fragmentation of the spherical states. One striking feature is the similarity in the structures of the two nuclei. In both 149Sm and Is1Gd there are high spin i13/2 states at -0.85 MeV and states at -1.25 MeV.

On a etudie les reactions (d, t) et (3He, a ) sur des cibles de lS0Sm et IS2Gd, en utilisant un spectrographe magnetique et des Cmulsions photographiques pour analyser et detecter les pro- duits de reactions. Les spectres PHe, a ) ont CtC mesures deux angles differents pour chacune des cibles, en utilisant des faisceaux de 3He de 24 MeV. La reaction lSoSm(d, t)149Sm aCte Ctudiee a 13 angles, avec des deuterons de 12 MeV. Les spectres lSZGd(d, t)lSIGd ont etC enregistres pour 4 angles, avec des deuterons de 15 MeV. Les valeurs de I pour un certain nombre d'ttats a spin peu Cleve ont kt6 determinees a partir des distributions angulaires (d, t). Les rapports entre les sections efficaces (3He, a) et (d, t) ont servi pour obtenir les vaieurs de I pour plusieurs autres Ctats. 11 est possible d'expliquer la force de reaction observee dans le cadre du modele a couches spheriques en dCpit de la fragmentation des etats spheriques. Une caracteristique frappante est la similarit6 de structure des deux noyaux. Dans les deux cas, celui de 149Sm et celui de IS1Gd, i l existe des etats a spin eleve situes a -0.85 MeV et des etatsl1, ,,, situks a -1.25 MeV.

[Traduit par le journal] Can. J. Phys.. 53. 1182 (1975)

1. Introduction The nuclei 14'Sm and 15'Gd, with 87 neutrons,

are situated in the region where the deformation changes rapidly with neutron number. Both of these nuclides are believed to be spherical with f,/, ground states. However, in this region one may find both rotational and spherical states as low lying excitations in the nucleus. Therefore the level structure of the N = 87 nuclei may be expected to show some interesting features.

The nucleus 15'Gd has previously been studied by means of (d, t) reactions (Tjam and Elbek 1967), radioactive decay from I5'Tb (Gonsior et a/. 1971), and (a, xny) reactions (Smith et al. 1973; Kleinheinz et al. 1974). I t is interesting to note that a t least two bands have been observed in 151Gd (Smith et al. 1973; Kleinheinz et al. 1974), one which is rotational and one which

appears to be a "decoupled" band as discussed by Stephens et al. (1972).

The 14'Sm nucleus has been investigated by Coulomb excitation (Gal'perin et al. 1970), by the decay of 1 4 ' ~ u (Kownacki et al. 1970), by (d, p), (d, t), and (d, d') reactions (Veje 1967), and by the I4'Nd(a, 3,) reaction (Kownacki et al. 1974). The last of these studies has shown the presence of two rotational bands in this nucleus also.

The aim of the present study is to complement these earlier works by measurements of (3He, a ) spectra and (d, t) angular distributions. The (3He, a ) reaction is considered particularly useful since high spin states are populated strongly in this process. This is important as the rotational bands expected in this region are very likely associated with the i13/, and hill, states.

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LPVHBIDEN AND BURKE: STATES IN THE N = 87 NUCLEI 1183

2. Experimental Details and Results The experiments were performed with 12 and

15 MeV deuterons and 24 MeV 3He beams from the McMaster University Tandem Van de Graaff accelerator. The reaction products were analyzed with an Enge split-pole magnetic spec- trograph and detected with nuclear emulsions. The 15'Gd target was prepared in an isotope separator by direct deposition of the separated isotope on a carbon foil. It is expected that the isotopic enrichment would be 2 99 97,. The ' 50Sm target was made from a sample of Sm203 iso- topically enriched to 99.9 % 150Sm. The oxide was reduced by heating in a vacuum with lan- thanum metal and the samarium was simul- taneously vacuum evaporated onto a carbon foil. The target thicknesses were -30 pg/cmz. The exposures were normalized by using a Si(Li) monitor detector in the target chamber, re- cording elastically scattered particles a t 9 = 30". The cross sections for elastic scattering were determined from the distorted wave Born ap- proximation (DWBA) calculations described below.

Exposures were recorded at angles of 40" and 60" for the L50Sm(3He, reaction and a t angles of 30" and 55" for the 152Gd(3He, a)I5lGd reaction. Spectra from the ISoSm(d, t)149Sm re- action were measured at 13 angles between 6" and 90" using I2 MeV deuterons. The main value of these angular distributions was in deter- mining the I values for some of the low spin states. In particular, the I = 0 transitions have been found to exhibit a characteristic peaking at forward angles, although in favorable cases I values of 1, 2, and 3 can also be distinguished from each other (Burke et al. 1973'1. It was

- 1

realized that the Q values for the I = 0 states in ' Gd were less than - 3 MeV and the DWBA calculations indicated that forward peaking should not occur for such low Q values with 12 MeV deuterons. Hence four L5ZGd(d,t)1 "Gd spectra were measured with 15 MeV deuterons, primarily at small angles to search for I = 0 transitions.

Sample spectra from each of the four reactions studied are shown in Figs. 1 and 2, and (d, t) angular distributions for a number of states in 14'Sm are presented in Fig. 3. Cross sections for one typical angle and excitation energies aver- aged from all angles are given in Tables 1 and 2. The estimated uncertainties for the energy measurements are +_2 keV and + 5 in the (d, t)

and (3He, a ) reactions respectively. The main uncertainties in the cross section measurements result from the normalization procedure. It is expected that the relative cross sections at dif- ferent angles should be accurate within + 15 % but the absolute values may be in error by as much as 25 97,. It has been shown (Burke et al. 1971; L ~ v h ~ i d e n et al. 1971) that the ratio of the (3He, a ) and (d, t) cross sections is also a very useful indicator of I values since this ratio varies by a factor of several hundred as the I value changes from 0 to 6. Figures 4 and 5 show plots of this ratio as a function of excitation energy in Is1Gd and 149Sm. The solid curves result from DWBA calculations, and the data points represent the experimental results of the present work. The numbers associated with the data points are the excitation energies in keV. The 12 MeV 15'Gd(d, t)151Gd cross sections are taken from T j ~ m and Elbek (1967).

The DWBA calculations were done using the computer program DWUCK written by Kunz (1969). The optical model parameters for the (d, t) reactions are the same as those used by Jaskola et al. (1967), and the (3He, a ) cross sections were computed using the same optical model parameters which had been used for 28 MeV 3He on Yb targets (Burke et al. 1971). The normalization factor used for the DWBA calcu- lation was 3.00 for the (d, t) reaction. The value for the (3He, a ) reaction was set at N = 22.6 in order that the observed cross sections for the 712- ground state in I5lGd would yield the same spectroscopic factor as obtained from the (d, t) data.

3. Interpretation of the Results The assignments of spins and parities for levels

in Is1Gd and 149Sm are discussed in Subsects. 3.1 and 3.2 respectively. These interpretations are based on previously available data as well as the (d, t) and (3He, a ) results from the present work. A number of tentative assignments are suggested on the basis of the remarkable simi- larity of the level structures in the two riuclei. In Subsect. 3.3, the measured spectroscopic factors are compared with those predicted by the basic shell model.

3.1. The Nucleus l S 1 Gd The (d, t) results obtained with 15 MeV deu-

terons in the present work are consistent with the earlier measurements of T j ~ m and Elbek

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1184 CAN. J. PHYS. VOL. 53, 1975

E 1 1 ' 1 1 1 1 ' 1 1 I 1 1 1 1 1 1 1 I I 3 - - -

1 5 2 ~ d ( 3He ,d ) I5'C5d - E = 24 MeV

L ' 5 2 ~ d ( d,t)I5'Gd

1050 E = 15 MeV 395 8 = 45O

587

EXCITATION ENERGY IN keV I FIG. I . The 15'Gd(d, t)"'Gd spectrum at 45" and the 152Gd(3He, a)lS1Gd spectrum at 30". The numbers associated

with the peaks are the excitation energies in keV.

(1967). However, the availability of data at more angles in the present work permits assignments of 1 values to be made for several levels. In par- ticular, the forward peaking of the cross sections for the 1050 keV and 1192 keV levels in the 15 MeV experiments indicates that these are 1 = 0 states. The 1 values presented in Table 1 have been deduced from these results and from the ratios of (3He, a) and (d, t) cross sections.

As stated earlier, the previous assignments of I" = 712- for the ground state has been used in the normalization of the (3He, or) DWBA cal-

culations. Hence the data point for this level has been made to fall exactly on the 1 = 3 line in Fig. 4.

The ratio of (3He, ci) and (d, t) cross sections for the 380 keV level is consistent with an 1 value of five. The shell model predicts the h g l 2 state close to the f,,, ground state in this mass region, while the h , configuration should be a deep hole state. Thus a reasonable assignment for the 380 keV level is In = 912-.

One level listed in Table 1, for which the present interpretation differs from previous

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LOVHOIDEN A N D BURKE: STATES IN THE N = 87 NUCLEI 1185

I5Osrn (d,t) IL9srn

1195 E = 12 MeV 8 - 60°

EXCITATION ENERGY IN keV

FIG. 2. The 150Sm(d, t)149Sm spectrum at 60" and the 150Sm(3He, ~ ) ' ~ ~ S r n spectrum at 60'. The numbers associated with the peaks are the excitation energies in keV.

works, is the 587 keV state. Gonsior et al. (1971) reported a state with I n = 112- at 575.3 keV and another with I" = 512- a t 587.3 keV. The strong peak in the (d, t) spectrum occurs a t 587 keV and hence corresponds to the latter of these two states. However, the (3He, a ) cross section to this level is very small and the ratio of (3He, a ) and (d, t) cross sections (seen as an upper limit in Fig. 4) precludes an I = 3 assign- ment. Furthermore the 15 MeV (d, t) cross sec- tions at 6", 12", 15", and 45" are 84, 140, 152,

and 142 pb/sr respectively, which are consistent only with an I value of one. The only assignment consistent with the present work and the pre- vious measurements is I n = 312-.

The ratio of (3He, a ) and (d, t) cross sections shows that the 854 keV level is populated via an I = 6 transfer. This level is thus assigned I" = 13/2+ since the only I = 6 shell model state ex- pected in this region is i,,,,.

The levels at 426 keV, 620 keV, and 671 keV all appear to be populated by I = 3 transfers. I t

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1186 CAN. J. PHYS. VOL. 53, 1975

SCATTERING ANGLE IN DEGREES

FIG. 3. Angular distributions for the lSOSm(d, t)I4=Sm reaction. The solid curves result from DWBA calculations, and the points represent experimental data. Error bars correspond to statistical uncertainties only.

is possible that these states are similar to the 277 keV, 559 keV, and 636 keV levels in 14'Sm, which have In = 712-, 512-, and 712- respec- tively.

Based on the (3He, a) and (d, t) cross section ratio and the gamma-ray measurements of Gon- sior et al. (1971), the weakly populated state at 913 keV is I = 5. This level appears to be similar to the 665 keV state in 14'Sm which is also 1 = 5 and which has a larger spectroscopic strength in the (d, p) reaction than in (d, t), thereby indicating a particle nature for the state. Thus the 913 keV level is tentatively interpreted as a component of the hg12 state.

The present results are consistent with the pre- viously suggested assignment of d3/, for the 981 keV level (Tjsm and Elbek 1967) and, as

mentioned above, confirm their suggestion of s , ,, for the 1050 keV state. The only other 1=0 strength located by the (d, t) angular distribu- tions was to the relatively weakly populated state at 1192 keV. This level had been previously assigned In = 312' by Gonsior et al. (1971).

The level at 1209 keV has a ratio of (3He, a) and (d, t) cross sections which indicates 1 = 5. A rotational band associated with this level has been reported (Smith et al. 1973; Kleinheinz et al. 1974) and the bandhead has been interpreted as the 1112- [505] Nilsson state.

3.2. The Nucleus 14'srn The ground state and 22 keV level in 14'Sm

were previously assigned as 712- and 512- re- spectively. The ground state is populated much

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LBVHBIDEN AND BURKE: STATES IN THE N = 87 NUCLEI

TABLE 1. Levels in '"Gd populated in the (d, t) and (3He, a) reactions

do/dQ (pb/sr) Energy (keV) 1 Value Assignment SIJ

(d, t) (3He, a) Present (d, t) (3He, a) 45" 30" experiment Previous Adopt. (d, t)' (3He, a) Adopt.

0 0 442 34 1 = 3 712- " 712- 1.89 1.89 1.89 109 111 11 - 3 1 < 4 512- " 512- 0.036 -0.05 0.04 380 375 z 3 0 50 1 = 5 912 - 0.75 1.29 1.02 395 216 < 3 1 < 3 312- a 312- 0.24 - 0.24 426 429 35 3 1 < 3 (712-1 0.16 0.17 0.16 587 142 < 3 I = 1 512- " 312- 0.21 - 0.21 620 39 < 3 1 ~ 3 (512-1 0.22 - 0.22 671 667 83 8 1 = 3(2) W - 1 0.48 0.42 0.45 706 12 810 23 < 3 1 < 4 838 63 < 3 1 < 3 312- a 312- 0.13 0.13 854 850 10 91 1 = 6 13/2+ 1.27 1.92 1.92 913 914 6 11 1 = 5 , 6 K = - l a l = 5 0.22 0.23 0.23 981 981 286 12 1 = 2 (d3/z)b 3/2+ 1.49 1.51 1.50

1050 1049 417 5 1 = 0 ( s l , ~ ) ~ 1/2+ 1.12 1.57 1.12 1086 32 1159 1161 47 9 l = 3 , 4 K = 1' 1 = 4 1.24 0.83 1.04 1192 76 < 3 1 = 0 3/2+ " 1/2+ 0.21 0.21 1209 1209 3 6 94 1 = 5 1112- 2.16 2.24 2.20

1261 8 K = -1' 1364 1351 48 - 3 1478 1488 28 11

1937 23

nAssignment by Gonsior er a/. (1971). bAssigned by 4 a m and Elbek (1967). <In order to get a more consistent treatment of 149Sm and '"Gd the spectroscopic factors for the "'Gd(d,t)'"Gd are calculated using the

12 MeV cross sections measured by Tjam and Elbek (1967).

more strongly than the 22 keV level in the pres- ent experiments. The ratio of (3He, a ) and (d, t) cross sections and the (d, t) angular distribution for the doublet are both consistent with ! = 3 transfers.

In the (d, t) spectrum (Fig. 2 and Table 2) there are two levels at 277 keV and 285 keV which are not clearly resolved from each other. The first of these has been assigned In = 712- by Kownacki et al. (1970). The (3He, a ) spec- trum (Fig. 2 and Table 2) contains a very strong peak corresponding to an energy of 282 keV and it is much stronger, relative to the (d, t) cross section, than would be expected for an 1 = 3 transfer. The ratio of (3He, a ) and (d, t) cross sections indicates 1 % 5 for the 285 keV level, when the expected 1 = 3 strength for the 712- level at 277 keV is subtracted from the composite (3He, a ) peak. Thus it is likely that the 285 keV level in 14'Sm has the same con- figuration as the 380 keV level in 151Gd, which was labelled /I,,,. Kownacki et al. (1974) have recently reported an In = 912- interpretation

for the 285.5 keV level, and have observed a rotational band based on it.

The 350 keV state was previously assigned In = 712- by Kownacki et a!. (1970). However, the ratio of (3He, a ) and (d, t) cross sections is consistent with an 1 value or' 0 or 1 and the (d, t) angular distribution indicates ! = 1 for this state. A reasonable guess is that this state may be similar to the In = 312- state at 394 keV in 151Gd.

The 529 keV level also has a (d, t) angular dis- tribution which indicates an 1 = 1 transfer. This is consistent with the assignment of negative parity by Veje (1967). Since this level appears to be similar in character to the 587 keV In = 312- level in 151Gd, the most likely assignment is In = 312-.

The cross section ratio and (d, t) angular dis- tribution for the 559 keV level are both consis- tent with 1 = 3 and thus are in accord with the In = 512- assignment of Gadsby et al. (1973).

The Coulomb excitation experiments of Gal'perin et al. (1970) limited the 636 keV level

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CAN. J . PHYS. VOL. 53, 1975

TABLE 2. Levels in 149Sm populated in the (d, t) and (3He, a) reactions

do/dR (pb/sr) Energy (keV) - 1 Value Assignments SIJ - (d, t) (3He, a) Present (d, t) (3He, a) 90" 40" experiment Previous Adopt. (d, t) (3He, a) Adopt.

'Kownacki el 01. (1970). *Veje (1967). CGadsby el 01. (1973). 'Kownacki el 01. (1974).

to I" values ranging from 312- to 1112-. The present (d, t) angular distribution shows that this state is populated by an I = 3 transfer and thus I" = 512- or 712-. In the l5'Sm(p, t)14'Srn study of Gadsby et al. (1973), this state was populated by an angular momentum transfer of I # 0 from the I" = 512- ground state of l5'Sm. As (p, t) reactions connecting states of the same spin in this mass region usually have a dominant I = 0 component the most likely assignment for the 636 keV level is I" = 712-.

The 697 keV and 710 keV states were both tentatively assigned positive parity by Veje (1967). Both levels have (d, t) angular distribu- tions consistent with either I = 1 or 1 = 2 transfers and could thus be components of the P3/2, d3/2, or d5/2 states. However, these two levels are populated very strongly in the (d, p) reaction (Veje 1967), thus making p3/, the most likely assignment as it is expected to be a par- ticle state in 14'Srn, whereas d3I2 and d5/2 are expected to be hole states.

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LOVHOIDEN AND BURKE: STATES IN T H E N = 87 NUCLEI 1189

Q( d,t ) IN MeV

FIG. 4. Ratios of the (3He, a) cross sections at 30" to the (d, t) cross sections at 90' for 15'Gd. The 12 MeV (d, t) cross sections are taken from Tj0m and Elbek (1967). The solid curves result from a DWBA calcula- tion, and the points represent experimental ratios for the levels indicated. Points with arrows indicate upper limits, and the error bars correspond to the statistical uncertainties only.

The level a t 878 keV is populated strongly in the (3He, a ) reaction and the cross section ratio corresponds to 1 = 6. Veje (1967) has also as- signed this state to have positive parity. As the only I = 6 strength expected in this mass region is due to the i , 312 state it is reasonable to assume that the 878 keV level is a component of the i13/, state. Kownacki et 01. (1974) have also recently interpreted this level as I" = 1312' and have observed members of a rotational band based on it.

The level a t 1049 keV has a ('He, a ) to (d, t) cross section ratio showing I = 2 or 1, and the (d, t) angular distribution is consistent with an 1 = 2 transfer to this level. Since this level is strongly populated in the (d, t) reaction and not at all in the (d, p) reaction (Veje 1967), it is likely a component of the d3/, state, which was found at an excitation - energy of 981 keV in ' 'Gd. The parity of the 1049 keV level was pre- viously measured to be negative (Veje 1967), but this assignment may be somewhat doubtful, since it is based on cross sections measured a t only two angles.

The 1195 keV level is very strongly populated in the (d, t) reaction. The (d, t) angular distri- bution indicates I = 0 for this level which is a major component of the sl12 shell model state.

The level a t 131 1 keV has high spin, as can be deduced from the ratio of (3He, a ) and (d, t) cross sections. I t may actually be a closely spaced doublet (Veje 1967) in which case the point in Fig. 5 represents a lower limit for the cross section ratio of the highest spin member of the doublet. This level has properties similar to the 1112- [505] state identified in 15'Gd, and it is thus likely that the 131 1 keV level is due to this Nilsson orbital.

3.3. Spectroscopic Factors The theoretical differential cross section for a

direct single particle pickup reaction on an even- even target is given by

where N is taken to be 3.0 and 22.6 for the (d, t) and (3He, a ) reactions respectively.

In a shell model basis we may consider the target nuclei 150Sm and 15,Gd to be systems with 6 active neutrons outside the 'inert' N = 82 core. We expect the orbits ST/,, h,/,, i , 312, p3,,, f5/,, and possibly p l 1 2 to correspond to the lowest lying particle states in 14,Srn and lS1Gd. With the pickup reactions we would expect to observe an appreciable population of these states even though they may be predominantly particle states. However, the hole states such as d3/,, sl12, h l l12, d5/,, and g7/, in the N = 50-82 neutron shell would be expected to have large spectroscopic factors in the pickup reactions.

By summing the strengths CS,/ obtained from the (d, t) process for all states originating in the N = 82-126 shell, one gets the average number of particles in the target nucleus outside N = 82. Theoretically, this number is 6 for lS2Gd and lS0sm. In Table 3 are listed spectroscopic factors Sf, for states in 14,Srn and 15'Gd for which the assignments are fairly certain. It is interesting to note the similarity of the spectroscopic factors for the two nuclei, which gives some further con- fidence in the interpretation of the observed states. Furthermore, the sums CSlj for all states originating in the N = 82-126 shell are 5.95 and 6.51 for 14,Srn and 15'Gd respectively, in good agreement with the expected value of 6.

The manner in which the 6 valence neutrons are distributed amongst the various shell model states outside the N = 82 closed shell can be

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CAN. J. PHYS. VOL. 53, 1975

I I

FIG. 5. Ratios of the ("e, a) cross sections at 60D to the (d, t) cross sections at 90' for 14'Srn. See caption to Fig. 4.

TABLE 3. Spectroscopic information for states populated in 149Srn and I5'Gd

Excitation energy (E) (keV) SIJ ZSl~l(2j + 1) E (keV) EJ (keV)

compared with the predictions of pairing theory. The 'fullness factor,' vJ2, which is the probability that a particular magnetic substate is occupied by a pair of neutrons, should be determined by the energy of the state relative to the Fermi sur- face. Thus, from the experimental excitation energies it is possible to estimate the single par- ticle energies, relative to the Fermi surface, and hence to obtain predictions for the VjZ factors.

Since the number of particles in a particular shell model state, I j , should be (2j + 1 ) ~ ; it is seen that the quantities V; should be the same as the CS,j/(2j + 1) values, to a first approxima- tion.

According to pairing theory, the fullness factor is

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LQVHQIDEN AND BURKE: STATES IN THE N - 87 NUCLEI 1191

TABLE 4. Pairing factors for the N = 82 -, 126 neutron states in 14'Sm and 15'Gd

where E~ and Ej are the single particle energy and the quasi-particle energy respectively for the state, and h is the Fermi energy for the system. The quasi-particle energy is

[31 E~ = J(E~ - + A'

where A is a measure of the pairing energy. For the ground state, cj - h is usually appreciably smaller than A so its quasi-particle energy is z A. Thus, for any other state, the quasi-particle energy would be approximately the excitation energy plus A. In actual fact, however, the ob- served states are fragmented, so an effective value for the quasi-particle energy can be obtained from the expression

where E is the weighted average excitation energy

Here the sums range over all the observed states with the particular 1 and j, and the E values are the experimentally observed excitation energies. The quasi-particle energies deduced in this way are listed in Table 3 for both 149Sm and 15'Gd. These values are then used to estimate the full- ness factors, Vj2, using the expression

[6] V; = 1/2[1 f J-]

which is obtained from [2] and [3]. The pairing energy parameter A can be found from the neutron separation energies

These quantities were evaluated from the neutron separation energies given by Meredith and Barber (1 972) to be 1 125 keV and 1 1 1 1 keV for 150Sm and 15'Gd respectively.

In Table 4 the deduced values for V; are compared with the corresponding observed

spectroscopic factors CSll/(2j + 1) for each value of 1 and j. It is seen that very good agree- ment is obtained, except for the f5/, levels. For this case, the observed strength is less than ex- pected, and the missing strength may be present in some of the unassigned levels at higher exci- tation energies. For each of the other shell model states, the spectroscopic factors are pre- dicted very well by pairing theory.

For the states in Table 4 where there is agree- ment between V; and CSlj/(2j + I), values for the single particle energies, relative to the Fermi surface, can be obtained from the experimental data by means of [2] rewritten in the form

Using the empirical quantities CSlj/(2j + 1) (for the v;) and the Ej = E + A values from Table 3 in this equation, the single particle energies shown in Table 5 are obtained. Again it is seen that the results from the two nuclides have a very strong similarity.

The CSlj values presented in Table 3 show that the shell model states below the N = 82 gap are severely fragmented. For example the d3/, state, which has a theoretical CSlj value of 4, was found to have only 26 % of its expected strength in 149Sm and 37 % in 15'Gd. The ob- served sl12 strengths are 74 % and 66 % of the theoretical values in 149Sm and 15'Gd respec- tively. It is interesting to note that the observed intensities for the d3/, and sl12 states in 151Gd are 100 % and 120 % respectively of those measured for the 3/2+ [402] and 1/2+ [400] bandheads in lS3Gd (Lravhraiden and Burke

TABLE 5. Single particle energies (in keV) for the N = 82-126 neutron states relative to the Fermi

surface h

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1192 CAN. J. PHYS.

1973). Also the spectroscopic factors for the 1112- states correspond within experimental limits to the expected value for the 1112- [505] Nilsson state but not with that for a pure h , hole state. In view of the fact that rotational bands based on several of the high spin states have been found, one wonders whether the In = 3/2+ and 1/2+ states, which are observed to have fractions of the shell model d3/, and sll, strengths similar to those predicted by the Nilsson model for the 312' [402] and 1/2+ [400] orbitals, may not also be deformed states. It would be interesting to search for rotational bands based on these levels. It would also be worthwhile to search for a band based on the In = 1112-, 131 1 keV state in 14'Sm, and to compare its properties with the corresponding 1209 keV state in l 5 'Gd.

Whereas the excited states discussed above show some deformed characteristics, the ground states can still be explained in terms of a spher- ical representation. The observed strengths to the 712- ground states in 14'Sm and Is1Gd are about 24- times larger than for the strongest I = 3 transition in the (d, t) reaction leading to 'deformed' states in Is3Gd (Lsvhsiden and Burke 1973). Also, no rotational bands have been found based on the ground states. It is thus likely that these N = 87 nuclei have relatively spherical ground states but the potential well shape is soft enough that a deformed configuration is energetically favorable at relatively small ex- citation energies.

Acknowledgments

The authors are very thankful to John Hui- zenga of the University of Rochester Nuclear Structure Research Laboratory for the loan of

VOL. 53, 1975

the ' 52Gd target. Financial support for this work was provided by the National Research Council of Canada and is gratefully acknowledged. One of us (G.L.) wants to thank Meltzers Hsyskole- fond for a travel grant.

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KOWNACKI, J . , LUDZIEJEWSKI, J . , MOSZYNSKI, M., and STRYCZNIWICZ, K. 1970. Acta Phys. Pol. B l , 173.

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