The reactions 152Sm(p, t)150Sm and 154Sm(p, t) 152Sm

Nuclear Physics A159 (1970) 615--624; ~ ) North-Holland Publishing Co., Amsterdam Not to be reproduced by photoprint or microfilm without written permission from the publisher

THE REACTIONS lS2Sm(p, t)lS°Sm and lS4Sm(p, t)lS2Sm WILLIAM McLATCHIE t

Queen's University, Kingston, Canada and Nuclear Physics Laboratory, Oxford and W. DARCEY tt and J. E. KITCHING ttt

Nuclear Physics Laboratory, Oxford Received 29 July 1970 Abstract: The reactions xsz. xS4Sm(p ' t) have been carried out at a bombarding energy of 20.6 MeV. The angular distributions of the outgoing tritons appear characteristic of the angular momentum transfer but for L = 2 exhibit some variety. The L = 0 cross sections from the present work are compared with existing (t, p) cross sections and support the suggestion of shape coexistence in the transitional nuclei tS°Sm and tS2Sm. N U C L E A R REACTIONS ts2. tS,LSm(p' t), E = 20.6 MeV; measured a(Et, 0). 1so. tSZSm deduced levels, L, d, ~. Enriched targets.

I

I

1. Introduction The stable isotopes of samarium (Z = 62) extend from the spherical single closed shell nucleus 144Sm(N = 82) to the permanently deformed l S4Sm(N = 92). The change in coupling scheme indicated by these observations appears to occur rather abruptly between N = 88 and N = 90, inasmuch as 150Sm does not exhibit a rotational spectrum whereas I S2Sm does. The nuclei in this transitional region have been extensively studied and as a result, their gross properties, if not completely understood, have at least been catalogued I, 2). In the present work, the reactions lS2Sm(p, t)lS°Sm and lS4Sm(p, t)tS2Sm have been studied in an attempt to investigate further the effects accompanying the change in the nuclear coupling scheme at N ,~ 90; the work complements the Sm(t, p) reaction studies of Bjerregaard et al. 3) and represents an extension of the earlier low resolution (p, t) work of Maxwell, Reynolds and Hintz 4). Unfortunately, (p, t) reactions have not yet been carried out on a large enough sample of nuclei in this region of the periodic table to establish the extent to which the angular distributions of the outgoing tritons are characteristic of the transferred orbital angular momentum, L. Further, the feature of such two-nucleon transfer reactions which makes them such * Present address: Queen's University, Kingston, Canada. tt Present address: Dept. of Physics, Rutgers - the State University New Brunswick, New Jersey. *tt Present address: Foster Radiation Laboratory, McGill University, Montreal, Canada. 615

616

w.

M c L A T C H I E et al.

a useful sl~ectroscopic tool, viz. the sensitivity of the cross section to pairing correlations in the nuclear wave functions s), makes detailed analysis of the observed cross sections difficult in the absence of microscopic model wave functions for the nuclei under investigation. Nevertheless, the present results permit an examination of the potential utility of such studies of other nuclei in this mass region.

2. Experimental procedure and results Self-supporting metal targets isotopically enriched in i s 2Sin and x54Sm respectively (see table 1) were bombarded with 20.6 MeV protons from the Oxford coupled Van de Graaff generators. The outgoing tritons from the reactions 152Sm(p ' t)l SOSm and 154Sm(p ' t)152Sm were analysed in the Oxford multichannel spectrograph and detectTAnLe 1

Target thickness and isotopic composition (~o) a) Target isotope

~4"Sm

~47Sm

~4SSm

xagSm

tS°Sm

*S2Sm lS4Sm

0.03 0.07

0.17 0.17

0.13 0.13

0.19 0.23

0.16 0.09

~SZSm lSaSm

98.55 0.55

0.76 98.76

Thickness (/~g/cm 2)

Conversion factor b) ( × 1 0 -4 )

310=k40 3104-40

2.41 2.16

*) The target material was obtained from the Electromagnetic Separation Group, AERE, Harwell. The isotopic compositions are those determined by that Group. b) The absolute differential cross sections in mb/sr may be obtained from fig. 3 and fig. 4 by multiplying the arbitrary units by these figures.

ed in Ilford K2 emulsions. The overall energy resolution was 20-25 keV (FWHM) and the excitation energy of states corresponding to strong triton groups were determined to within + 10 keV. An energy calibration of the spectrogrpah at the high field (14.8 kG) used in the (p, t) experiments was determined from a study of the Z°9Bi(d, t)2°aBi reaction at the same field setting using the energy assignments of Alford et al. 6) for states in 2°SBi. Target thicknesses were determined from elastic scattering of 8.0 and 9.0 MeV deuterons. The emulsions were scanned in 0.25 mm strips and triton angular distribution were obtained in 7.5 ° steps from 0L = 7.5 ° to OL = 75 °. The experimental triton spectra at 0L = 22.5 ° are shown for ~52Sm(p, t)ln°Sm and lS4Sm(p, t)152Sm in figs. 1 and 2 respectively. The corresponding energy level assignments are listed in tables 2 and 3 together with the measured absolute differential cross sections at 0L = 22.5 ° and the results of previous studies of these nuclei. Below an excitation of ~ 2.5 MeV, the present experiments excited twenty-two states in lS°Sm and nineteen states in 152Sm; tables 2 and 3 indicate that the energies determined in this work are in reasonable accord with existing data although for 15ZSm'

Is2, tS4sm(p ' t)

617

152Sm(p,t)15°Sm

O|a b----.2 2 , 5 3550,uC

JO00

°

0

-100 17

1

E

O

N I,M a.

i~li t ii ~

6

5

......... J!.". .....

-10

U

<

6~.0

• 62.0 t

6 ~0

I 64"0

.

i 65.0

i . 66.0

Radius of Curvature [cm.) Fig. 1. Triton spectrum from xS~Sm(p, t)xS°Sm. The triton groups are labelled according to the convention of table 2.

154

• .152 Sm(p,t) Sm

0

lO0O

Osob=2 2 . 5 ° 4000,uC

9

i i ~" "1 ': 17"nJJ'~l:/f

T, 4 i 1i

1

.lOO

2

~! :iLJi~i~ji_!l_j ................ i,,.... J~

40 ..o

6'1"0

"

72"0

63"0'

"'~'0

"65:0"

-

66"0

Radius of Curvature(cm.) Fig. 2. Triton spectrum from sS'Sm(p, t)xS2Sm. The triton groups are labelled according to the convention of table 3.

618

W. MeLATCHIEet aL

o t h e r d a t a , d o n o t e x i s t a b o v e 2.0 M e V . S e v e r a l p r e v i o u s l y r e p o r t e d levels 7 - 9 ) w e r e n o t o b s e r v e d i n t h e (p, t) r e a c t i o n e i t h e r b e c a u s e t h e r e s o l u t i o n w a s i n s u f f i c i e n t t o TABLE 2 Levels excited in the reaction *S2Sm(p, t)*S°Sm This work number

energy (keV)

Previous work do" ~-~m(22.5")

energy

~b/sr)

(keV) 0 334 740 773 1046 1071 1165 1194 1256 1278 1357 1417 1449 1504 1642 1684 1759 1794 1819 1833 1948 1970 1979 2024 2043 2062 2095 2108

0÷ 2+ 0÷ 4+ 2+ 31-, 1 + 2+ 0+ (6 + ) 32+ 4+ 3+ 4+ 32-, 32+ 4+ 2+,5 + 3-, 54+ 3-,44+ 3+,4 + (3 + ) 5+

2121

(3-) 4+ 4+ 3 +, 4 + 33, 4 + 3-, 43 + or 4 + 3+ 3+

0 1 2 3 4 5 6 7 8

0 339=1=10 7474-10 786-t-15 1048-1-10 1084-4-10 11594-10 1206-1-10 12614-10

190 50 170 < 10 <10 20 10 40 90

9 10 11

13604-10 14104-10 14404-10

<10 15 <10

12

17614-10

10

13 14 15 16

18094-10 18414-10 19424-10 19734-10

10 <10 < 10 < 10

17

2019-1-10

30

18 19

21744-10 22204-10

< 10 < 10

20

22854-10

15

21

23614-10

20

22

2451-t-10

15

2194 2223 2260 2280 2290 2328 2360 2370 2453

jn

3-,5-

s e p a r a t e close-lying states or because o f the selection rule w h i c h allows only the excitat i o n o f n a t u r a l p a r i t y states.

ts2. ,S,Sm(p ' t)

619

TABLE 3 Levels excited in the reaction *54Sm(p, t)*52Sm This work number

0

Previous work

energy

d~ ~--~(22.5°)

energy

(keV)

(/zb/sr)

(keV)

0

jn

510

0

0+

I 2 3

1224-10 3704-10 6944-10

34 37 100

4

8194-10

40

2+ 4+ 0+ 6+ 2+ 14+

5

10454-10

< I0

122 366 686 705 812 966 1020 1041

6

10994-10

45

7

15974-10

< I0

8

16744-10

< 10

9 10 11 12 13 14 15 16 17 18 19

17794-10 1917-4-10 19734-10 20314-10 2100±10 2140:h10 22104-10 22644-10 2347+10 24254-10 26004-10

125 53 < 10 10 10 19 < 10 56 12 10 12

1088

32+

1091 1222 1372 1510 1578

0+ 54+ 13-

1615 1726 1765

53-, 4-

3. D i s c u s s i o n

3.1. GENERAL REMARKS ON THE ANGULAR DISTRIBUTIONS T h e a n g u l a r d i s t r i b u t i o n s c o r r e s p o n d i n g to the strongest t r i t o n g r o u p s seen in the *52Sm(p, t)*S°Sm a n d *5*Sm(p, t)xS2Sm reactions are shown in figs. 3 a n d 4 respectively. O f these, the states b e l o w 1.5 M e V o f excitation have been previously assigned definite J~ values a n d m a y therefore be used to examine the c o r r e s p o n d e n c e between o b s e r v e d a n g u l a r d i s t r i b u t i o n s a n d transferred o r b i t a l a n g u l a r m o m e n t u m , L. T h e 0 + states (15°Sm: 0, 747, 1261 keV; lS2Sm: 0, 694 keV) exhibit identical a n g u l a r d i s t r i b u t i o n s a n d are m a r k e d l y different f r o m the others. T h e 2 + states (*S°Sm: 339, 1048, 1206, 1410 keV; ' 5 2 S m : 122, 819, 1099 keV) are also readily distinguished

w . McLATCHIE et aL

620

although the distributions show some variety, particularly at forward angles; here the state observed at 1099 keV in I S2Sm has been assumed to correspond to the known 2 + state at 1088 keV, classified as a ~-vibrational state 10). Only the lowest known 3 state in each nucleus was excited and although the 1084 keV state in ~5°Sm has a distribution different from the L = 0 and L = 2 cases, the 1045 keV state in 152Sm

=ooli'

i l i l

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1000

.,=oil

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#12"1761keY

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log

i 37s

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![\/:\..,

15 30 456075

1530 45 60 75

750 . 375 -

I2+l

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i/'\ ,

.,

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l

5oot

#,

250

", iv;,..,

i

,o.,,,oi.v

"63e~6o"~

It

! ,\: "i 1530456075 e .... (t),~(;.,=s)

Fig. 3. Angular distributions of triton groups corresponding to excited states in lS°Sm. The groups are labelled as in table 2. The curves have no theoretical significance. The arbitrary units correspond to observed counts and may be converted to mb/sr on multiplication by the conversion factor in table 1. was not observed with sutl~cient intensity to extract a meaningful distribution. As only one 4 + state was observed (370 keV in 152Sm), it is not possible to determine whether the distribution is characteristic of an L = 4 transfer. Thus the (p, t) reaction on these nuclei would appear to allow rather unambiguous identification of 0 + states; in contrast the angular distributions to the 2 + states show a variety of shapes but appear to fall into two main categories, viz. [15°Sm: 339, 1048 keV; 1S2Sm: 122 keV] and [is°Sin: 1206, 1410 keV; lS2Sm: 819, 1099 keV]. On this basis, the following assignments are proposed: in ~52Sm: L --- 2 transfer to the states at 1779, 2264 keV; in xS°Sm: L = 0 and L - 2 transfer to the 1761 and 2019 keV states respectively. As these last two states lie near the energy gap in a region of high-level density, it is not clear whether these assignments contradict the previous proposals 7-9) of 2-, 3 - for a state at 1759 keV and 4 + for a state at 2024 keV. It may be significant that the above categories are also distinguished by the fact

lSz, lS4sm(p' t)

621

that in the first group, each transition carries > 20 ~o of the ground state cross section in the same nucleus, whereas the members of the second group (which includes the new assignments) are excited less strongly than this relative figure. The origin of these effects is not clear. It has been shown 1l) that the relative cross sections for two-nucleon transfer reactions are sensitive to the assumed deformation parameters of the micro1500C

. = 0 - ground;state

//

7500 Z 0

(n (n

5000 1 1 1 - 122 keV 2

750

# 2 - 370 keV "~ 4

25001-! / /,-t\

, tiJi\r"

0

5000

0

:~3" 694 keY * o

t/\.,.,. O

1000~ # 4 - 819 k~

/\

2

oQ:

I

I I

~ 1

#5-1045 keV

,0°t!

i

(J .J

U,l n-

~

50=

2500

mo

~ t t

c "~ ( I 'r" lOOC

~

ol

I ~ } I !

. 1099 keV

I'"'\,._.,\ /!

i

l

=

I

i

15 3045 60 75

I

I I

I

I

#16- 2264 keY

1500~

~u 50Q

o

t II

OI

I

I

15oo "/ i\

14

75O

\....% I

I

I

15 30 45 60 75

1

1

1

I

I

15 3 0 4 5 6 0 75

0 .... (DEGREES) Fig. 4. Angular distributions o f triton groups corresponding to excited states in xS2Sm. The groups are labelled as in table 3. See caption o f fig. 3.

scopic description, while Glendenning has suggested 12) that relative cross sections may also be sensitive to competing inelastic processes. Both these effects would be expected in the region under study since it is one of changing deformation and strong collective excitations; whether these effects can manifest themselves in L = 2 angular distributions has not yet been established. It would appear that two-neutron transfer reactions show considerable promise for the investigation of nuclei in the rare-earth region, although their potential for assigning spins and parities must await further data, particularly for the higher-L transfers. 3.2. S T R U C T U R E C O N S I D E R A T I O N S

The (t, p) reaction studies across the Sm isotopes by Bjerregaard et aL 3) provided interesting information on the processes which accompany the onset of nuclear

622

w. McLATCHIEet al.

deformation at N ,,~ 90. In particular, these experiments revealed the presence of a 0 + state in i S2Sm at 1091 keV, excited with comparable strength to the 152Sm ground state yet unobserved in either other reactions or decay studies. The apparent strong overlap of this state with the 15°Sin ground state and the failure of other reactions to excite it, combined with an interpretation of binding energy systematics 13)

0 ÷ States Below the"Energy Gap" LEGEND

(t,p)

(p,t)

4

~5

I00

,~eSm N=86

43

25

40

3a

~SOSm N=88

100

~S2Sm N=90

53

53

~S¢Sm N=92

~SeSm N=94

Fig. 5. The L = 0 c r o s s s e c t i o n s for J n = 0 + excited below the energy gap in (t, p) and (p, t) reactions o n t h e samarium isotopes. In this mass region the energy gap for neutrons 2A. is typically 2 MeV, see ref. 13). The cross sections for ~48Sm(t, p ) ' S ° S m and tS"Sm(p, t)152Sm have each been expressed as 100 units. All other c r o s s s e c t i o n s are expressed in this system o f units. For each state the (t, p) c r o s s s e c t i o n is shown on the left and the (p, t) cross section on the right. The (t, p) data are from ref. 3).

suggested that this 1091 keV 0 + state was a spherical state coexisting in the permanently deformed nucleus, 152Sm" Group 6 in fig. 2 corresponds to the region of excitation energy occupied by the 0 + state in question and also by the 1088 keV y-vibrational state with J~ = 2 +. The corresponding angular distribution in fig. 4 is clearly L = 2 and hence this 0 + state is not strongly excited in the (p, t) reaction; in fact an upper limit of 3 ~o of the ground state cross section, based on the L = 0 and L = 2 angular distributions observed in this reaction, may be set for the excitation of the 0 + member of the doublet. This result supports the description of the state as an example of shape coexistence inasmuch as its parentage in the deformed 154Sm ground state is extremely small. A further case of shape coexistence has been suggested in 150Sin from the (t, p) studies and from earlier (p, t) work by Maxwell, Reynolds and Hintz 4). The 0 ÷ state at 1261 keV in 15°Sm was not observed in the (t, p) reaction but is strongly excited in the lS2Sm(p, t)lS°Sm reaction, see fig. 3. The poor overlap with 14aSm as evidenced in the (t, p) reaction and the good overlap with 1S2Sm as seen in the

xs2. t S,tsm(p' t)

623

(p, t) reaction suggest that this state may well be a case of a deformed 0 + state coexisting a in "spherical" nucleus. The 0 + states excited in the transitional Sm isotopes by two-nucleon transfer reactions are shown schematically in fig. 5. The (p, t) cross sections from the present work and the (t, p) cross sections of ref. 3) have been used. The indicated values are relative figures derived by attributing a value of 100 to the ground state transition in both the 14SSm(t ' p)l 50Sm and i S,Sm(p, t) t 52Sm reactions; all other values are relative to these two, chosen since they are transitions in which there are no gross changes in the nuclear COUl:ling scheme. The most striking feature of fig. 5 is the strong analogy between the present (p, t) results and the l*8.15°Sm(t, p) data. For the two reactions which connect nuclei of similar shapes viz. 14SSm(t,p)XS°Sm and 15aSm(p, t)lS2Sm, 8 0 ~ of the L = 0 strength lies in the ground state transition with the remainder appearing in the collective first excited 0 + state; in each case the second excited 0 + state is not populated. On the other hand, the reactions which connect nuclei with different coupling schemes viz. 15°Sm(t, p)t52Sm and ~52Sm(p, t)aS°Sm, appear to excite the ground state, the collective first excited 0 + state and the shape coexisting 0 + state with equal intensity. The reaction 154Sm(t, p)156Sm appears to indicate a return to the more normal condition of two-nucleon transfer reactions, a concentration of the L = 0 strength in the ground state transition, as the new coupling scheme becomes stabilised. The appearance of significant L = 0 strength to the coexisting 0 + states is a consequence of the changing coupling scheme while the excitation of both the ground states and the collective 0 ÷ states in 15°Sm(t, p)152Sm and 152Sin(p, t)lS°Sm is probably due in large part to zero-point oscillations in the target ground states. Such oscillations, predicted by the calculations of Kumar and Baranger 14) allow pair addition to 5°Sm and pair subtraction from x52Sm to connect with states of any shape. On the other hand, 14SSm and 154Sm have shapes which in their respective coupling schemes are sufficiently stable to limit two-neutron transfer reactions to the excitation of only those states with similar shapes. In summary, the present work supports the suggested occurrence of shape coexisting 0 ÷ states, although detailed information on their decay modes would be of value. Further, the microscopic character of the ground states and 0 + collective states would appear to be sufficiently complex to eliminate any description of them in terms of simple models. Two of us (W. McL. and J.E.K.) acknowledge the financial support of N.R.C. (Canada) and the hospitality extended to us at the Nuclear Physics Laboratory, Oxford. References 1) C. M. Lederer, J. M. Hollander and I. Perlman, Table of isotopes, sixth ed. (Wiley, New York, 1968) 2) Nucl. Data 132 (1967) numbers l, 4

624 3~ 4) 5) 6) 7) 8) 9) 10) 11) 12)

w. Mc.LATCHIEet al.

J. H. Bierregaard, O. Hansen, O. Nathan and S. Hinds, Nucl. Phys. 86 (1966) 145 J. R. Maxwell, O. M. Reynolds and N. M. Hintz, Phys. Rev. 151 (1966) 1000 I. S. Towner and J. C. Hardy, Adv. in Phys. 18 (1969) 401 W. P. Alford, J. P. Schiffer and J. J. Schwartz, Phys. Rev. Lett. 21 (1968) 156 R. K. Smither and D. J. Buss, Bull. Am. Phys. Soc. 15 (1970) 86 J. Barrette, M. Barrette, S. Monaro, S. Santhanam and S. Markiza) Can. J. Phys. 48 (1970) 1161 E. Veje, B. EIbek, B. Herskind and M. C. Olesen, Nucl. Phys. A109 (1968) 489 G. C. Seaman, J. S. Greenberg, D. A. Bromley and F. K. McGowan, Phys. Rev. 149 (1966) 925 R. A. Broglia, C. Riedel and T. Udagawa, Nucl. Phys. A135 (1969) 561 N. K. Glendenning, Prec. Int. Conf. on properties of nuclear states, (Les Presses de l'Universit~ de Montrdal, 1969) p. 245 13) J. D. MacdougaU, W. McLatchie, S. Whineray and H. E. Duckworth, Nucl. Phys. A145 (1970) 223 14) K. Kumar and M. Baranger, Nucl. Phys. A l l 0 (1968) 529