The 154Eu(t, p) reaction: Some new systematics in the N = 90 spherical-deformed transition region

Nuclear 0

Physics

North-Holland

A413 (1984) 236246 Publishing

Company

THE ls4Eu(t, p) REACTION: Some new systematics in the N = 90 spherical-deformed R. G. LANIER, Nuclear

R. K. SHELINE’

Chemistry

Cl. L. STRUBLE

and L. Cl. MANN

Division, Lawrence Livermore National Livermore, CA 94550, USA

J. A. CIZEWSKI Physics Division,

Los Alamos

National

Laboratory,

+’

Laboratory,

Received

transition region

Los Alamos,

NM 87544, USA

5 July 1983

Abstract: A radioactive target of ‘54E~(8.3 y) has been used to study the ““Eu(t, p)‘s6Eu reaction at an incident energy of 17 MeV. The bandhead and one rotational state of the {n+[413]; vJ$[SOS]},=,_ configuration have been identified in ‘s6Eu The excitation energy of the 3- bandhead is determined to be 448 + 15 keV. The angular distribution of the first excited rotational state is anamolous and may indicate evidence for a strong two-step component in the reaction mechanism. The energy systematics of the Eu-Sm transition region are also investigated. We find that the systematics of h2/2./ suggest that at N = 87 the “‘Eu {x$413]; #[505]},=,_ excited configurations has a significantly more stable deformed structure than the corresponding y [ 5051 one-quasiparticle structure in ‘@Sm.

E

NUCLEAR REACTIONS rs4Eu(t, p), E = 17 MeV, measured Q-value, o(B). ‘s6Eu deduced levels J, rc, Nilsson configuration, anamolous L = 2 angular distribution. 141.143.145.147.149.15L.153.,55.142.L44.,46.,48.15O.15Z.154Sm NUCLEAR STRUCTURE 142.143.144.145.146.147.148.149.L5O.L51.152.153.154.155.156 Eu; analyzed S(2n). moment of inertia,

systematics.

“‘Eu

deduced

shape coexistence.

1. Introduction The availability of thin, isotopically pure targets ‘) of radioactive ‘52Eu(13y) and 154Eu(8.3y) have provided new opportunities to study the spherical-deformed transition along an unbroken sequence of odd-Z Eu isotopes in the range of 87 5 N I- 93. An interesting feature of charged-particle studies 2-7) with these radioactive targets is that strongly deformed two- and three-quasiparticle states associated with the ?[505] neutron orbital can be identified on the spherical side of the N = 90 shape transition region. In this paper we report on studies of the

”

’ Permanent address: Chemistry Department, Florida Present address: Wright Nuclear Structure Laboratory, 236

State University, Yale University,

Tallahassee, FL 32306, USA. New Haven, CT 06511, USA.

R. G. Lanier et al. 1 The ls4Eu(t, p) reaction

237

‘54Eu(t, p) reaction and the two-quasiparticle states populated in ls6Eu. This experiment is the last in a series 2-7) which have systematically investigated the properties of the {7$[413]; v~[SO5]},= 3- configuration in the Eu isotopes. Accordingly, we also take this opportunity to compare in some detail the general systematics of the Sm-Eu transition region and, against this background, point out some particular features of the ?[505] orbital and its role in shape coexistence.

2. Experiment The (t, p) experiments were performed at the Los Alamos National Laboratory (LANL) P-9 Tandem Van de Graaff accelerator facility using a beam of 17 MeV tritons (0.3-0.6 PA). The outgoing protons were momentum-analysed with a quadrupole-triple-dipole (Q3D) magnetic spectrograph and were detected with a 1 m helix detector mounted along the focal plane of the spectrograph. Details of the experimental arrangement “) and the target preparation procedures have been described previously ‘). Unfortunately, the radioactive 154E~ target used for these experiments was rather thin (- 15 pg/cm2) and was mounted on a relatively thick carbon backing (- 70 pg/cm2). As a result the overall reaction yield from the main target material was relatively low, and the spectra obtained show only three peaks. A (t, p) spectrum at 55” is shown in fig. 1. Angular distributions were measured by recording spectra at selected angles

‘“Eu (t p) ‘=Eu QLAE;= 55”

I

400

600

800

Channel

Fig. 1. Proton spectrum from the ‘54Eu(t p) reaction at 55’. The inset shows the data obtained at 60°. The peaks belonging to lS6Eu are’labelled to correspond with the numbering in table 1.

238

R. G. Lanier et al. / The 154E~(t, p) reaction

between 15” and 65’. These measurements were complicated by the presence of intense particle groups due to reactions in the backing, which necessitated a significant reduction in the beam current for forward angle measurements to allow the gating electronics to process the high count rates. Absolute cross sections were measured by normalizing the counts recorded in the spectrograph to the integrated sum of triton elastic events recorded in a monitor detector mounted at 30” with respect to the incident beam. The normalized peak groups from the spectrograph were then combined with the known spectrograph-monitor solid-angle ratio and the DWBA 9, triton elastic cross section calculated at 30” to yield absolute cross sections. The optical potential used for these and subsequent calculations is given in ref. lo). Experimental and calculated (t, p) angular distributions are compared in fig. 2. To measure the reaction Q-value and excitation energies, we calibrated the spectrograph by recording successive ground-state (t, p) populations from targets of 14’Sm and 156Gd. A (t, p) spectrum from the 154Eu target was taken between the calibration runs and all reactions were recorded with identical spectrograph settings. Since ls4Gd is the primary decay product from the radioactive target, the ground-state (t, p) population from this isotope was present in the spectrum and was also used as one of the calibration points. An independent (t, p) measurement from a 154Gd target showed that none of the peaks associated with the ‘54Eu(t, p) reaction contain any interfering components from the ls4Gd impurity. The 14’Sm thicker (- 200 pg/cm2 targets were considerably and ’ 56Gd calibration versus N 15 pg/cm2) than the 154Eu target. The energy values that we quote (Q and E,,) have been obtained by properly correcting for these target thickness differences. Using tabulated Q-values for the calibration reactions I’), we obtain Q = 5569k 10 keV for the most strongly excited state indicated in fig. 1. The error quoted for the Q-value is an estimate obtained by adding the statistical error (f 3 keV) to the average error (+ 7 keV) in the tabulated Q-values for the calibration reactions.

3. Results and discussion The ground-state spin and parity of ’ 56E~ has been assigned 12) as In = O+ and the ’ s4E~ target has a ground state assigned as I X= 3- which results 4,7) from the two-quasiparticle contiguration (rc4[413] ; ~?[505]>~ = 3-. The lowest energy level observed in our measurements is strongly excited and exhibits an L= 0 angular distribution. Population of the I n = 0 + ’ 56E~ ground state requires L = 3 transfer ; consequently, the strong L= 0 peak is not the 15’jEu ground state. Combining our measured Q-value for this state with the tabulated “) ground-state Q-value (6017 f 11 keV), we determine that the excitation energy of the strong L= 0 group

R. G. Laker et al. / The ‘54E~(t, p) reaction

l”Eu

239

(t, p) 156Eu

44% keV 3100 1

*

I

0

20

I

I 60

I 60

Fig. 2. Angular distributions for the 448 and 524 keV states observed in the ‘54Eu(t, p) reaction. The data points noted as circles are from the original experiment and those noted as “ x ” are from a repeat measurement. The solid lines are DWBA calculations which have been arbitrarily scaled to best fit the data.

is 448 + 15 keV. The large uncertainty is due to the uncertainties in the tabulated Q-values of the calibration reactions as well as to the error in the ground-state Qvalue. The 448 keV state is assigned as the band head of the (7$[413]; *[505]j, = 3configuration. The L = 0 angular distribution establishes I z = 3- for this state and

R. G. Lanier er al. 1 The 154Eu(t, pi reaction

240

TABLE 1 Levels in ‘56E~

Level number

Et,*

I”

0

448

3-

I

524 + 3.0 (675 +4)”

(4-) (?)

2

“) An absolute error of + 15 keV must be added to ail energies because of uncertainties in the tabulated Q-values used for calibration and in the tabulated ‘5aEu ground-state Q-value. b, Weak excitation observed only at two angles.

the configuration assignment is made on the basis of the mechanism of twonucleon transfer: The lowest energy L,= 0 state would involve the transfer of spin zero correlated neutron pairs to the target core leaving the unpaired valence nucleons undisturbed. One other state at 524 keV is also clearly observed in these studies. The level is very probably the 4rotational state associated with the (7$413]; v9[505]}, = 3- band. Th is assignment rests primarily on the known rotational systematics for this band in 152E~ and 154E~ [refs. “*“)I. A summary of the data is presented in table 1. The assignment of a spin-parity to the 524 keV state from the angular distribution data is unfortunately somewhat ambiguous. Because the target has nonzero spin (3-), some combination of an L= 2, 4, and 6 mixture should generate a reasonable fit to the data for a 4- state, if the reaction proceeds primarily by a single-step mechanism. In the bottom of fig. 2, the angular distribution for the 524 keV state is plotted and compared with the calculation for pure L= 2 transfer. The correspondence between the data and the calculation are reasonable except at 60° and 65“, where the experimental points are signi~~antly higher than the calculated values. The addition of arbitrary components of L= 4 and L= 6 transfer improves the fit at forward angles but fails to remove the discrepancy at the backward-angle points. The 60” and 65” points are sufficiently anomalous to warrant further comment. The inset in fig. 1 allows a comparison of the raw data at 55Oand 60° for the L= 0 and L = 2 excitations and emphasizes the anomaly. Specifically, although the experimental L = 0 cross section shows a small change between 59 and 60”, the L = 2 intensity jumps by a factor - 4. The DWBA calculations predict L = 2 cross sections at 60” and 65O to be - 257; smaller than and approximated equal to, respectively, the L = 2 cross section at 55O. We note that the experimental and calculated L = 0 angular distributions agree reasonably well, and this suggests that

R. G. Lanier et al. / The ls4Eu(t, p) reaction

241

the mechanical aspects of the calculation are not in error. We have also reviewed other possible sources of error and can identify none. To confirm that there were no problems with the measurements, we repeated part of the experiment - 20 months after the original data were taken. We were forced to use the original 154E~ target and, unfortunately, the target ruptured during the initial irradiation and so the experiments had to be done with a fraction (- 500,;) of the original target (- 15 pg/cm’). Nevertheless, we were able to remeasure the (t, p) spectra at 55” and 60” but with somewhat poorer statistics than the original data. These new data are also plotted in fig. 2 and reasonably confirm our original results. The difficulty of fitting experimental angular distributions of 2+ rotational states populated in two-nucleon transfer from even-even targets is well known 13*14). Reasonable fits, however, can be obtained if one includes a two-step component in the reaction mechanism as has been done by Ascuitto et al. 15). Possibly, the inclusion of these effects may explain the anomalous angular distribution observed for the 524 keV state in our measurements.

4. Systematics of the Sm-Eu transition region In 1956 Morinaga 16) suggested that different nuclear shapes could exist in the same nucleus ; more specifically, he suggested that the ground state of doubly magic 160 would be spherical, whereas the excited O+ state at 6.05 MeV was the bandhead of a deformed rotational band. Since that time it has become clear that Morinaga’s initial speculation was correct. A number of different authors 17-21) have suggested that even-even nuclei in the vicinity of N = 90 have both spherical and deformed states. Shape coexistence has also been suggested in a number of odd-A nuclei 22-26 ) in this region. In particular, it is possible to follow the deformed y[SOS] rotational band over a sequence of Sm, Gd and Dy nuclei which have an even number of protons and which span the transition region between spherical and deformed nuclei. This fact is particularly apparent in the N = 87 nuclei ‘49Sm, lslGd and ls3Dy where most of the levels have at most only a small deformation 23*24). In the odd-A Eu nuclei it is similarly possible to follow the systematics of the deformed 3[413] band 22,25,27) from the strongly deformed region in ls3, lssEu down to “‘Eu where the lowest lying levels clearly do not represent deformed configurations. Finally, various measurements 2-7) of charged particle reactions on radioactive targets of “‘Eu and 154E~ have identified rotational states associated with the {n$[413] ; *[505]}, = 3- configuration in the odd-odd Eu isotopes between 87 $ N 5 93. These data demonstrate that coexistence of deformed and spherical states also exist in odd-proton nuclei and that this coexistence is associated with the q[505] neutron orbital. In figs. 3 and 4 we present the systematics for the two-neutron separation

24:

R. G. Laker

et al. / The ls4Eu(t,

p) reaction

18

12) 79

81

83

85

87

89

91

93

Neutron number(N)

141

I

I

I

I

I

I

I

143

145

147

149

151

153

155

Mass number (A)

Fig. 3. Two-neutron separation energies plotted versus N and A for a number of odd- and even-A Sm isotopes ‘1.12.24). Note the break in the systematics for ground-state bands in the region N _ 86-90 and that there is no corresponding break in the systematics (dashed line) for the ?[505] deformed band.

energies, S(2n), as a function of neutron number t. Fig. 3 shows these systematics for the Sm isotopes, and fig. 4 shows them for the Eu isotopes. If we consider the simplest version of the liquid-drop mass formula, then we would expect a nearly linear change in the two-neutron binding energy as a function of neutron number. This result is expected because of the symmetry-energy potential term. Our plots of the ground-state separation energies show two discontinuities: one at N - 82 and the other at N - 88. The first is due to a major shell gap; the second is due to the ’ For purposes of the plots in figs. 3 and S(2n) = - M(A +2, Z)+M(A, Z)+ M(2n). In values of A and N given in the expression. The except for a redefinition of arguments. Values always calculated with respect to ground states

4, the two-neutron separation energy is given by these figures, therefore, S(2n) is plotted for the smaller definition of S(2n) is identical with that given in ref. rt) of S(2n) plotted for excited states in A(or A+2) are in A f2 (or ,4).

R. G. Lanier et al. / The ‘54Eu(t, p) reaction I

I

I

I

I

I

243 I

I

0 20

Even @I and odd (X) - A Eu ground states

X

Even(O)-AEuK=3bands

t

I -i

WI-

0

E E. T N v)

16

14 :

12,

’ 79

I 81

I 83

I 85

I 87

I 89

I 91

I 93

Neutron number (N)

I 142

I 144

I

I

I

I

I

I

146

148

150

152

154

156

Mass number (A)

Fig. 4. Two-neutron separation energies plotted versus N and A for a number of odd- and even-A Eu isotopes 7.11*12).Note the break in the systematics for ground-state bands in the region N - 86-90 and that there is no corresponding break in the systematics(dashed line) for the K = 3- states. The K = 3- states have the configuration {x$[413]; &&SOS]} which forms the ground states of ‘52*‘54E~ and the excited states in 150*156Eu.The unusual similarity of these data with those shown in fig. 3 emphasizes the important role that blocking of the 9[505] orbital has on maintaining stable deformed structures.

onset of deformation. The interesting feature in both plots is that configurations involving a neutron quasiparticle in the y[SOS] orbital do not exhibit a discontinuity at N m 88. From this we assume that all such configurations are deformed and that nuclei such as 149Sm and “‘Eu exhibit shape coexistence. The y[SOS] orbital is important in determining the shape of a nucleus because it is very steeply upsloping. Neutrons in this orbital resist deformation since their contribution to the total binding energy increases rapidly as the deformation increases. With one quasiparticle in this hole-like orbital, the orbital is only half occupied and the resistance to deformation is significantly reduced compared to the case of full occupation. This idea has been put in more rigorous form by

R. G. Laker

244

et al. / The ‘54E~(t, p) reaction

Smith 28) who used microscopic Hartree-Bogoliubov calculations to study shape stability in (N = 87) 15’Gd. These calculations yield both a stable and a strongly deformed nuclear structure whenever the ?[505] orbital is blocked. The stabilizing role of the ?[505] orbital is emphasized by the plot of h2/2.a versus neutron number in fig. 5. From the data we note that a rotational spacing persists deeper into the spherical region for both the odd-A and odd-odd systems which involve states associated with this orbital. At N = 87, however, we observe that the y[505] band in Sm, although still reasonably deformed, is beginning to show a strong departure from a smooth linear trace. We further note that this effect is not observed for the 3- states in the odd-odd Eu nuclei; the plot is virtually linear and rather flat in the range 87 s N s 93. Such behavior for twoquasiparticle 3- states is somewhat unexpected because the j[413] orbital, which forms the proton component of these states, should provide little or no assistance to preserving deformation. This is suggested both by the systematics noted in fig. 5 for the lone orbital in the odd-A Eu nuclei and by the relatively flat deformation dependence of this orbital in a Nilsson diagram. Within the framework of the strong-coupling model, the only additional deformation stability achieved by the

f?/Z#values

50 > P s c! %

for

0 GS bands of even-A Sm + 5/2 + 14131 bands of odd-A 0

40

Eu

1 l/2 - [5051 bands of odd-A Sm

A 3- bands of even-A Eu

_t

Neutron number (N)

Fig. 5. Inverse moments of inertia for a variety of different bands in Sm and Eu nuclei. Note that with decreasing neutron number the deformation of the Of ground-state bands of the even-even nuclei decreases dramatically first, followed by the 3[413] and y[SOS] bands of the odd-A Eu and Sm. respectively. The 3- bands of the odd-odd Eu nuclei show little tendency to decrease their deformation down to neutron number 87. The data used in constructing this figure are from refs. 2-7~12~25-2’) and this article.

R. G. Laker et al. / The “‘Eu(t,

p) reaction

245

odd-odd Eu nuclei would be due to the residual interaction between the uncoupled neutron and proton. We have tested this possibility by evaluating the diagonal matrix elements for the residual interaction potential given in ref. 29) as a function of deformation and assuming the configuration (7c$[413]; v~[505]}, = 3-. This term increases the binding energy as a function of increasing deformation but only by - 30 keV between j3 = 0 and p = 0.35. Although the trend is correct, the magnitude of this effect is much too small to explain the behavior exhibited in fig. J.

5. Summary and conclusions We have studied the levels populated in 15‘jEu by a (t, p) reaction on a radioactive target of ls4Eu(8.3 y). Rotational states associated with the {rc3[413]; v~[505]}, = 3- contiguration have been tentatively assigned up to I = 4-. The angular distribution data for expected L= 2 transfer to the first rotational state in 156E~ is anomalous and probably indicates that population proceeds through a two-step mechanism. The experimental data from the present study have been combined with our previously published 2 - ‘) results and, taken together, the data form a systematic body of information on the odd-Z Eu isotopes in the range 87 s N 5 93. We have compared the energy systematics of these nuclei, particularly those involving states associated with the y[505] neutron orbital, with those of the Sm nuclei across the N = 90 transition region. We note from this comparison that two-quasiparticle states involving the 9[505] orbital in the Eu isotopes show approximately the same deformation stability as do the one-quasiparticle states in Sm which also involve this orbital. These states maintain a well deformed structure down to N = 87. Finally, and perhaps of most interest, we find that the systematics of h2/2.f (fig. 5) suggest that at N = 87 the ’ “Eu two-quasiparticle q[505] structure appears more stable than the corresponding one-quasiparticle structure in 149Sm. This is interesting because there is no evidence to suggest that the 3[413] proton component of the ’ 50Eu two-quasiparticle state will provide any deformation stabilization effect to the system. We wish to thank the staffs of the LLNL isotope separator facility and of the the LANL Tandem Van de Graaff facility for their important contributions to the (t, p) experiments. We also thank Dr. E. R. Flynn for assistance with the operation of the spectrograph. This work was performed under the auspices of the US Department of Energy by the Lawrence Livermore National Laboratory under contract number W-7405ENG-48, and under contract PHY79-08395 between the National Science Foundation and the Florida State University.

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R. G. Lanier et al. / The ‘54Eu(t, p) reuction

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