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Mutual excitation of 22Ne and 126Te in inelastic scattering
Volume 83B, number 1
PHYSICS LETTERS
23 April 1979
MUTUAL EXCITATION OF 22Ne AND 126Te IN INELASTIC SCATTERING T.P. CLEARY, J.L.C. FORD Jr., E.E. GROSS and D.C. HENSLEY Oak Ridge National Laboratory, Oak Ridge, TN 3 7830, USA and
C.R. BINGHAM and J.A. VRBA University of Tennessee, Knoxville, TN 37916, USA Received 20 December 1978
Angular distribution data for the mutual excitation of the 126Te and 22Ne 2+ states have been measured for the inelastic scattering of 93.5 MeV 22Ne on 126Te. Data for transitions to the ground states, the 126Te 2 + state and the 22Ne 2 + state were also obtained. A coupled-channels analysis of the reaction is found to provide a good description of the general features of the data.
The possibility of achieving simultaneous excitation of both target and projectile in inelastic scattering is an aspect of heavy-ion collisions that might afford us new insight into the shapes of nuclear surfaces. Until now this phenomenon has been observed only in the scattering o f relatively light nuclei, such as 12C on 12C [1 ], at energies which emphasized the nuclear interior rather than the surface region. Indeed, the low energies demanded for surface collisions between light nuclei makes such measurements extremely difficult. In contrast, surface collisions between heavy systems occur at energies which present no such problems. In this letter, we report on the results of a search for mutual excitations in the scattering of 22Ne from 126Te. The presence of strongly collective 2 + states in both o f these nuclei and the energy spacing of the low-lying levels made this an ideal system in which to look for these excitations. The measurements were conducted with a 93.5 MeV 22Ne beam from the Oak Ridge isochronous cyclotron. Angular distributions o f the scattered 22Ne ions were measured by means o f a position-sensitive gas proportional counter located at the focal plane of a Q1D magnetic spectrograph. The target consisted of 225/ag/cm 2 of tellurium, enriched to 98.7% in 126Te, on a 4 0 / l g / c m 2 carbon backing foil. The overall target
thickness limited the energy resolution o f the scattered particles to 160 keV and this was sufficient to separate the transitions of interest. The measured cross-section angular distributions are displayed for elastic scattering on the right side of fig. 1 and in fig. 2 for the transitions to the 0.666 MeV state in 126Te, the 1.275 MeV state in 22Ne and to the mutual excitation of these two states which corresponds to an excitation energy o f 1.941 MeV in the composite system. The only other transition that was observed in this measurement was that leading to the 2.391 MeV 3 - state in 126Te and it was quite weak. The cross sections for exciting the other low-lying states in 126Te should also be extremely small and hence their effects on the extracted yields should be negligible. Normalization of the spectrograph data was established in a separate measurement in which a position-sensitive solid-state detector (PSD) was used to measure cross sections at 15 angles simultaneously in 1° steps. This technique has been described in detail elsewhere [2]. The PSD measurements covered an angular range from 12 to 75 ° in the laboratory system and the data forward of 25 ° were used to establish the absolute cross section normalization by assuming Rutherford scattering at these angles. Since the energy resolution for the PSD data was not sufficient to separate elastic from inelastic scattering, the cross section was integrated over the low51
Volume 83B, number 1
PHYSICS LETTERS
22N e + 126Te
22Ne + |26T~ Ellab = 9 5 . 5 MeV
0++t26Te(2+) + 22Ne(2+ )
i
.~ 0.75,
-
!
I
•
23 April 1979
i
]
i t
ELA B = 95.5
MeV
t
t26Te 2 +
i
1
!
"b T
t0 2 0.0 20
I 40
60 ~c. m,
80
20
40
60
80
Fig. 1. The ratio of the cross section to Rutherford scattering for elastic scattering measured with a spectrograph (right side of the figure) and the elastic plus inelastic scattering to the 126Te and 22Ne 2 + states obtained in the PSD measurement (left side). Lines through data points represent the coupledchannels calculation.
lying transitions. These data are plotted on the left side of fig. 1 in terms of Rutherford scattering. The incident energy of 93.5 MeV was selected so that the reaction might proceed at a center-of-mass energy approximately 20 MeV above the Coulomb barrier. At such energies, the interacting nuclei follow trajectories which, while strongly influenced by the Coulomb potential, also sample the nuclear potential at the surface. Strong Coulomb-nuclear interference phenomena are observed in the cross section at angles corresponding to trajectories which pass through this surface region. It has been demonstrated [3] that the analysis of such angular distributions can provide a sensitive measure of both the charge and nuclear matter deformations of the nuclear surface. In the present study, the angle associated with a grazing collision is approximately 65 ° and is marked by the peak in the angular distribution for elastic scattering (fig. 1). The target and projectile selected for the present measurement each possess very collective 2 + excitations at rather low excitation energies. The cross sections for exciting these states are large and, in such cases, there exist strong couplings between the elastic channel and these particular inelastic channels. As is evident in fig. 1, while the elastic scattering cross section fails below the 52
"-2
~c. m.
"x b
10~
~0o 20
40
60
80
~C m.
Fig. 2. Cross section angular distributions for scattering to the
126Te 2 + state, the 22Ne 2 + state and for the mutual excitation of the 126Te and 22Ne 2 + states. The solid lines through data points represent the full coupled-channels calculation while the short- and long-dashed lines represent the contributions from the direct and indirect routes, respectively. Rutherford value forward of the grazing angle, the inclusion of the cross sections for the 2 + transitions accounts for this missing flux [4]. The general shape of the angular distributions for both 2 + transitions (fig. 2) are similar. Coulomb excitation dominates the forward angle cross section and the smooth oscillations in the cross sections that occur in the vicinity of the grazing angle result from the interference between the Coulomb and nuclear scattering amplitudes. In contrast, the mutual excitation of
Volume 83B, number 1
EXCITATION ENERGY (MeV)
PHYSICS LETTERS
dp
1.941
2+
1. 275
2+
JT" 0
0.666
0.0
0+
0
2+
0
2*
0÷ 2ZNe
+
126Te
Fig. 3. The energy level diagram for the composite Z2Ne + 126Te system. Left side indicates the excitations of 22 Ne while on the right are illustrated the excitations of 126Te. The arrows indicate the couplings included in the coupled-channels calcuiation.
these states gives rise to a markedly different angular distribution (lower part of fig. 2). The strong enhancement of the forward angle cross section associated with Coulomb excitation is absent. Indeed, the data exhibit a bell shape centered about the grazing angle and this points to a collision localized at the nuclear surface. The strong couplings that exist between the elastic and inelastic reaction channels necessitate a coupledchannels treatment o f the present reaction. The calculations were performed with the computer code CHORK [5] and included 300 partial waves with integrations carried out to 41.5 fm in steps of 0.08 fm. The initial optical model parameters were those obtained in a previous study o f 20Ne scattering on 208pb at 131 MeV [6]. These parameters were varied in conjunction with the collective parameters describing the 2 + excitations in 22Ne and 126Te in an effort to achieve a best fit to the data in fig. 1 and the angular distributions for the 2 + transitions in fig. 2. From the sensitivity of the forward angle cross section to Coulomb excitation, values were determined for B(E2, 0 + -+ 2+). The nuclear deformation parameters were extracted from the Coulomb matrix elements by assuming certain nuclear structure models and were therefore not free parameters. Since the 126Te 2 + excitation is vibrational in character, simple 3R scaling was used to derive 32N. In the case of the 22Ne 2 + excitation, which is rotational in character, the nuclear moments were determined by means of the "rolling" prescription of Hendrie [7]. These moments describe the deformations on the
23 April 1979
"nuclear surface", that is, the surface which is generated at the center o f the spherical nucleus as it is rolled over the surface of the deformed nucleus. This procedure was carried to all orders in t3 and moments up to 3 N were retained. A deformed potential, based upon these parameters, was used throughout the present calculation. Inclusion of couplings to the higher excited states of the target and projectile, such as the 22Ne 4 + state at 3.356 MeV, were found to have only small effects on the calculation and therefore these couplings were neglected. The final optical model parameters used were V = 33 MeV, r r = 1.291 fm and a r = 0.71 fm for the real part and W = 13 MeV, r i = 1.40 fm and a i = 0.30 fm for the imaginary part of the potential. Since the present reaction is predominantly sensitive to the nuclear surface, there is a degeneracy in the depth and radius parameters and hence there are actually only four independent optical model parameters. For the Coulomb part of the potential, a uniform sharp-edged charge distribution was assumed and the radius parameter was taken to be 1.25 fm. The various channel couplings that were included in the calculation are indicated by the arrows in fig. 3. The double arrows signify that the couplings were taken to all orders while the inclusions o f reorientation matrix elements are represented by circular arrows. All the couplings indicated in the figure were found to be important. It should be noted that the direct coupling of the ground states to the mutual excitation (0.0 to 1.941 MeV transition) and the coupling of the 126Te 2 + to 22Ne 2 + state (0.666 to 1.275 MeV transition) require a q u a d r u p o l e - q u a d r u p o l e interaction. In fig. 2 are compared the contributions of this directly coupled route and that of the two-step route to the cross section for the mutual excitation. The results of the full calculation are shown by the solid curves in figs. 1 and 2. The calculated results are in reasonable agreement with the data, both in absolute magnitude and general shape. Since only 300 partial waves were included in the calculation, the calculated angular distributions are unreliable forward of 30 °. The B(E2; 0 + ~ 2 +) values were determined to be 0.020 e2b 2 for 22Ne and 0.482 e2b 2 for 126Te. These results are consistent with the values obtained in other measurements [8,9]. Based upon the reorientationmatrix elements that were needed to fit the back angle data, the static quadrupole moments, Q2, were found for the 22Ne 2 + state to be - 1 . 3 0 IQrot I or - 0 . 1 7 eb and for the 126Te 2 + state to be - 0 . 9 4 e b . 53
Volume 83B, number 1
PHYSICS LETTERS
While the value o f Q2 determined for the 22Ne 2 + state is in general agreement with previous measurements [8], the value extracted for the 126Te 2 + state is at least a factor o f three larger [9]. The large reorientation matrix element required to fit the 126Te data could be compensating for certain failures in the nuclear structure models assumed. Additional evidence in support of this contention is provided by the deviations between calculation and data that occur for b o t h the 126Te and 22Ne 2 + transitions in the region of C o u l o m b - n u c l e a r interference (55 ° to 70°). Furthermore, the large dip that appears in the angular distribution for the mutual excitation calculation, again associated with C o u l o m b - n u c l e a r interference, is not reflected in the data. Nuclear excitation plays a much larger role compared to Coulomb excitation in the case o f mutual excitation than it does for the individual 2+ transitions. This greater competition from the nuclear part o f the form factor vis-fi-vis the Coulomb part leads to the large interference effects backward o f 55 ° . The present results, therefore, suggest that form factors based upon parameters derived by means of the rolling and scaling models or upon the sharp-edged model of the charge distributions do not provide an adequate description of the interaction between such highly collective nuclei as 126Te and 22Ne. In the light of these results it would be desirable to perform these calculations with more realistic form factors, such as those based upon the double folding model [10]. The more realistic form factors may provide additional insight into the structure o f the nuclear surface. In summary, we have presented the first data on the
54
23 April 1979
mutual excitation o f target and projectile measured in inelastic scattering at an energy which emphasizes collisions at the nuclear surface. The generally good fits to the data that are obtained indicate that the coupled-channels reaction formalism provides an adequate framework in which to analyze the reaction, while the detailed differences between calculation and data, particularly in the case o f the mutual excitation, point to a need for a more sophisticated form factor to describe the interaction. This work was supported by the U.S. Department of Energy under contract with Union Carbide Corporation.
References [1 ] G.T. Garvey, A.M. Smith and J.C. Hiebert, Phys. Rev. 130 (1963) 2397. [2] J.B. Ball et al., Nucl. Phys. A252 (1975) 208. [3] D.L. Hillis et al., Phys. Rev. C16 (1977) 1467. [4] C.E. Thorn et al., Phys. Rev. Lett. 38 (1977) 384. [5] P.D. Kunz and L.D. Rickertsen, Computer Code CHORK, unpublished. [6] E.E. Gross, T.P. Cleary, J.L.C. Ford, D.C. Hensley and K.S. Toth, Phys. Rev. C17 (1978) 1665. [7] D.L. Hendrie, Phys. Rev. Lett. 31 (1973) 478. [8] D.K. Olsen, A.R. Barnett, S.F. Biagi, N.H. Merrill and W.R. Phillips, Nucl. Phys. A220 (1974) 541. [9] T.V. Ragland, R.J. Mitchell and R.P. Scharenberg, Nucl. Phys. A250 (1975) 333; A.M. Kleinfeld, G. Mgggi and D. Werdecker, Nucl. Phys. A248 (1975) 342. [10] P.J. Moffa, C.B. Dover and J.P. Vary, Phys. Rev. C16 (1977) 1857.
PHYSICS LETTERS
23 April 1979
MUTUAL EXCITATION OF 22Ne AND 126Te IN INELASTIC SCATTERING T.P. CLEARY, J.L.C. FORD Jr., E.E. GROSS and D.C. HENSLEY Oak Ridge National Laboratory, Oak Ridge, TN 3 7830, USA and
C.R. BINGHAM and J.A. VRBA University of Tennessee, Knoxville, TN 37916, USA Received 20 December 1978
Angular distribution data for the mutual excitation of the 126Te and 22Ne 2+ states have been measured for the inelastic scattering of 93.5 MeV 22Ne on 126Te. Data for transitions to the ground states, the 126Te 2 + state and the 22Ne 2 + state were also obtained. A coupled-channels analysis of the reaction is found to provide a good description of the general features of the data.
The possibility of achieving simultaneous excitation of both target and projectile in inelastic scattering is an aspect of heavy-ion collisions that might afford us new insight into the shapes of nuclear surfaces. Until now this phenomenon has been observed only in the scattering o f relatively light nuclei, such as 12C on 12C [1 ], at energies which emphasized the nuclear interior rather than the surface region. Indeed, the low energies demanded for surface collisions between light nuclei makes such measurements extremely difficult. In contrast, surface collisions between heavy systems occur at energies which present no such problems. In this letter, we report on the results of a search for mutual excitations in the scattering of 22Ne from 126Te. The presence of strongly collective 2 + states in both o f these nuclei and the energy spacing of the low-lying levels made this an ideal system in which to look for these excitations. The measurements were conducted with a 93.5 MeV 22Ne beam from the Oak Ridge isochronous cyclotron. Angular distributions o f the scattered 22Ne ions were measured by means o f a position-sensitive gas proportional counter located at the focal plane of a Q1D magnetic spectrograph. The target consisted of 225/ag/cm 2 of tellurium, enriched to 98.7% in 126Te, on a 4 0 / l g / c m 2 carbon backing foil. The overall target
thickness limited the energy resolution o f the scattered particles to 160 keV and this was sufficient to separate the transitions of interest. The measured cross-section angular distributions are displayed for elastic scattering on the right side of fig. 1 and in fig. 2 for the transitions to the 0.666 MeV state in 126Te, the 1.275 MeV state in 22Ne and to the mutual excitation of these two states which corresponds to an excitation energy o f 1.941 MeV in the composite system. The only other transition that was observed in this measurement was that leading to the 2.391 MeV 3 - state in 126Te and it was quite weak. The cross sections for exciting the other low-lying states in 126Te should also be extremely small and hence their effects on the extracted yields should be negligible. Normalization of the spectrograph data was established in a separate measurement in which a position-sensitive solid-state detector (PSD) was used to measure cross sections at 15 angles simultaneously in 1° steps. This technique has been described in detail elsewhere [2]. The PSD measurements covered an angular range from 12 to 75 ° in the laboratory system and the data forward of 25 ° were used to establish the absolute cross section normalization by assuming Rutherford scattering at these angles. Since the energy resolution for the PSD data was not sufficient to separate elastic from inelastic scattering, the cross section was integrated over the low51
Volume 83B, number 1
PHYSICS LETTERS
22N e + 126Te
22Ne + |26T~ Ellab = 9 5 . 5 MeV
0++t26Te(2+) + 22Ne(2+ )
i
.~ 0.75,
-
!
I
•
23 April 1979
i
]
i t
ELA B = 95.5
MeV
t
t26Te 2 +
i
1
!
"b T
t0 2 0.0 20
I 40
60 ~c. m,
80
20
40
60
80
Fig. 1. The ratio of the cross section to Rutherford scattering for elastic scattering measured with a spectrograph (right side of the figure) and the elastic plus inelastic scattering to the 126Te and 22Ne 2 + states obtained in the PSD measurement (left side). Lines through data points represent the coupledchannels calculation.
lying transitions. These data are plotted on the left side of fig. 1 in terms of Rutherford scattering. The incident energy of 93.5 MeV was selected so that the reaction might proceed at a center-of-mass energy approximately 20 MeV above the Coulomb barrier. At such energies, the interacting nuclei follow trajectories which, while strongly influenced by the Coulomb potential, also sample the nuclear potential at the surface. Strong Coulomb-nuclear interference phenomena are observed in the cross section at angles corresponding to trajectories which pass through this surface region. It has been demonstrated [3] that the analysis of such angular distributions can provide a sensitive measure of both the charge and nuclear matter deformations of the nuclear surface. In the present study, the angle associated with a grazing collision is approximately 65 ° and is marked by the peak in the angular distribution for elastic scattering (fig. 1). The target and projectile selected for the present measurement each possess very collective 2 + excitations at rather low excitation energies. The cross sections for exciting these states are large and, in such cases, there exist strong couplings between the elastic channel and these particular inelastic channels. As is evident in fig. 1, while the elastic scattering cross section fails below the 52
"-2
~c. m.
"x b
10~
~0o 20
40
60
80
~C m.
Fig. 2. Cross section angular distributions for scattering to the
126Te 2 + state, the 22Ne 2 + state and for the mutual excitation of the 126Te and 22Ne 2 + states. The solid lines through data points represent the full coupled-channels calculation while the short- and long-dashed lines represent the contributions from the direct and indirect routes, respectively. Rutherford value forward of the grazing angle, the inclusion of the cross sections for the 2 + transitions accounts for this missing flux [4]. The general shape of the angular distributions for both 2 + transitions (fig. 2) are similar. Coulomb excitation dominates the forward angle cross section and the smooth oscillations in the cross sections that occur in the vicinity of the grazing angle result from the interference between the Coulomb and nuclear scattering amplitudes. In contrast, the mutual excitation of
Volume 83B, number 1
EXCITATION ENERGY (MeV)
PHYSICS LETTERS
dp
1.941
2+
1. 275
2+
JT" 0
0.666
0.0
0+
0
2+
0
2*
0÷ 2ZNe
+
126Te
Fig. 3. The energy level diagram for the composite Z2Ne + 126Te system. Left side indicates the excitations of 22 Ne while on the right are illustrated the excitations of 126Te. The arrows indicate the couplings included in the coupled-channels calcuiation.
these states gives rise to a markedly different angular distribution (lower part of fig. 2). The strong enhancement of the forward angle cross section associated with Coulomb excitation is absent. Indeed, the data exhibit a bell shape centered about the grazing angle and this points to a collision localized at the nuclear surface. The strong couplings that exist between the elastic and inelastic reaction channels necessitate a coupledchannels treatment o f the present reaction. The calculations were performed with the computer code CHORK [5] and included 300 partial waves with integrations carried out to 41.5 fm in steps of 0.08 fm. The initial optical model parameters were those obtained in a previous study o f 20Ne scattering on 208pb at 131 MeV [6]. These parameters were varied in conjunction with the collective parameters describing the 2 + excitations in 22Ne and 126Te in an effort to achieve a best fit to the data in fig. 1 and the angular distributions for the 2 + transitions in fig. 2. From the sensitivity of the forward angle cross section to Coulomb excitation, values were determined for B(E2, 0 + -+ 2+). The nuclear deformation parameters were extracted from the Coulomb matrix elements by assuming certain nuclear structure models and were therefore not free parameters. Since the 126Te 2 + excitation is vibrational in character, simple 3R scaling was used to derive 32N. In the case of the 22Ne 2 + excitation, which is rotational in character, the nuclear moments were determined by means of the "rolling" prescription of Hendrie [7]. These moments describe the deformations on the
23 April 1979
"nuclear surface", that is, the surface which is generated at the center o f the spherical nucleus as it is rolled over the surface of the deformed nucleus. This procedure was carried to all orders in t3 and moments up to 3 N were retained. A deformed potential, based upon these parameters, was used throughout the present calculation. Inclusion of couplings to the higher excited states of the target and projectile, such as the 22Ne 4 + state at 3.356 MeV, were found to have only small effects on the calculation and therefore these couplings were neglected. The final optical model parameters used were V = 33 MeV, r r = 1.291 fm and a r = 0.71 fm for the real part and W = 13 MeV, r i = 1.40 fm and a i = 0.30 fm for the imaginary part of the potential. Since the present reaction is predominantly sensitive to the nuclear surface, there is a degeneracy in the depth and radius parameters and hence there are actually only four independent optical model parameters. For the Coulomb part of the potential, a uniform sharp-edged charge distribution was assumed and the radius parameter was taken to be 1.25 fm. The various channel couplings that were included in the calculation are indicated by the arrows in fig. 3. The double arrows signify that the couplings were taken to all orders while the inclusions o f reorientation matrix elements are represented by circular arrows. All the couplings indicated in the figure were found to be important. It should be noted that the direct coupling of the ground states to the mutual excitation (0.0 to 1.941 MeV transition) and the coupling of the 126Te 2 + to 22Ne 2 + state (0.666 to 1.275 MeV transition) require a q u a d r u p o l e - q u a d r u p o l e interaction. In fig. 2 are compared the contributions of this directly coupled route and that of the two-step route to the cross section for the mutual excitation. The results of the full calculation are shown by the solid curves in figs. 1 and 2. The calculated results are in reasonable agreement with the data, both in absolute magnitude and general shape. Since only 300 partial waves were included in the calculation, the calculated angular distributions are unreliable forward of 30 °. The B(E2; 0 + ~ 2 +) values were determined to be 0.020 e2b 2 for 22Ne and 0.482 e2b 2 for 126Te. These results are consistent with the values obtained in other measurements [8,9]. Based upon the reorientationmatrix elements that were needed to fit the back angle data, the static quadrupole moments, Q2, were found for the 22Ne 2 + state to be - 1 . 3 0 IQrot I or - 0 . 1 7 eb and for the 126Te 2 + state to be - 0 . 9 4 e b . 53
Volume 83B, number 1
PHYSICS LETTERS
While the value o f Q2 determined for the 22Ne 2 + state is in general agreement with previous measurements [8], the value extracted for the 126Te 2 + state is at least a factor o f three larger [9]. The large reorientation matrix element required to fit the 126Te data could be compensating for certain failures in the nuclear structure models assumed. Additional evidence in support of this contention is provided by the deviations between calculation and data that occur for b o t h the 126Te and 22Ne 2 + transitions in the region of C o u l o m b - n u c l e a r interference (55 ° to 70°). Furthermore, the large dip that appears in the angular distribution for the mutual excitation calculation, again associated with C o u l o m b - n u c l e a r interference, is not reflected in the data. Nuclear excitation plays a much larger role compared to Coulomb excitation in the case o f mutual excitation than it does for the individual 2+ transitions. This greater competition from the nuclear part o f the form factor vis-fi-vis the Coulomb part leads to the large interference effects backward o f 55 ° . The present results, therefore, suggest that form factors based upon parameters derived by means of the rolling and scaling models or upon the sharp-edged model of the charge distributions do not provide an adequate description of the interaction between such highly collective nuclei as 126Te and 22Ne. In the light of these results it would be desirable to perform these calculations with more realistic form factors, such as those based upon the double folding model [10]. The more realistic form factors may provide additional insight into the structure o f the nuclear surface. In summary, we have presented the first data on the
54
23 April 1979
mutual excitation o f target and projectile measured in inelastic scattering at an energy which emphasizes collisions at the nuclear surface. The generally good fits to the data that are obtained indicate that the coupled-channels reaction formalism provides an adequate framework in which to analyze the reaction, while the detailed differences between calculation and data, particularly in the case o f the mutual excitation, point to a need for a more sophisticated form factor to describe the interaction. This work was supported by the U.S. Department of Energy under contract with Union Carbide Corporation.
References [1 ] G.T. Garvey, A.M. Smith and J.C. Hiebert, Phys. Rev. 130 (1963) 2397. [2] J.B. Ball et al., Nucl. Phys. A252 (1975) 208. [3] D.L. Hillis et al., Phys. Rev. C16 (1977) 1467. [4] C.E. Thorn et al., Phys. Rev. Lett. 38 (1977) 384. [5] P.D. Kunz and L.D. Rickertsen, Computer Code CHORK, unpublished. [6] E.E. Gross, T.P. Cleary, J.L.C. Ford, D.C. Hensley and K.S. Toth, Phys. Rev. C17 (1978) 1665. [7] D.L. Hendrie, Phys. Rev. Lett. 31 (1973) 478. [8] D.K. Olsen, A.R. Barnett, S.F. Biagi, N.H. Merrill and W.R. Phillips, Nucl. Phys. A220 (1974) 541. [9] T.V. Ragland, R.J. Mitchell and R.P. Scharenberg, Nucl. Phys. A250 (1975) 333; A.M. Kleinfeld, G. Mgggi and D. Werdecker, Nucl. Phys. A248 (1975) 342. [10] P.J. Moffa, C.B. Dover and J.P. Vary, Phys. Rev. C16 (1977) 1857.