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Coupled-channels analysis of 800 MeV polarized proton inelastic scattering from 18O
COUPLED-CHANNELS ANALYSIS OF 800 MeV POLARIZED PROTON INELASTIC SCATTERING C. GLASHAUSSER,
I4 October 1982
PHYSICS LETTERS
Volume 116B, number 4
FROM “0
R. DE SWINIARSKI ‘, K. JONES, S. NANDA
Rutgers University, New Brunswick,
NJ 08903,
USA
F.T. BAKER, M. GRIMM 2, V. PENUMETCHA, A. SCOTT University of Georgia, Athens, GA 30602,
USA
G. ADAMS 3, G. IGO University of California, Los Angeles, CA 90024,
USA
G.W. HOFFMANN University of Texas, Austin, TX 78712,
USA
J. MOSS Los Alamos National Laboratory,
Los Alamos, NM 87545,
USA
W. SWENSON Oregon State University, Cowallis, OR 97330,
USA
and B.E. WOOD 4 University of Oregon, Eugene,
OR 97403,
USA
Received 8 June 1982
Differential cross sections and analyzing powers have been measured for the “0 (T, p’) IsO reaction at 800 MeV. A coupled-channels analysis of the O:, 2f, and 4; data yields good agreement with a rotational model description with a large p4 deformation. The effects of channel coupling are large. The angular distributions of Ay are reproduced well only with a full Thomas spin-orbit deformation approximately equal to the central deformation.
We present here an analysis of new cross-section (da/da) and analyzing-power (A,) data for the excitation of the ground (Of), 1.98 MeV (2f), and 7.12. ’ Permanent address: Universite de Grenoble, 38026 Grenoble Cedex, France. 2 Present address: University of Louisville, Louisville, KY 40208, USA. 3 Present address: Massachusetts Institute of Technology, Cambridge, MA 02139, USA. 4 Present address: Los Alamos National Laboratory, Los Alamos, NM 87545, USA. 0 031-9163/82/0000-0000/$02.75
0 1982 North-Holland
MeV (4;) states in l8 0 by inelastic scattering of 800 MeV protons. The analysis reveals that coupledchannels effects are significant in the determination of shape parameters of the neutron transition density. It confirms, for the first time, the effects of channel coupling on A,, which were recently predicted by Ray [ 1] ; the deformation parameters of the spinorbit and central potentials must be about equal. Finally;the analysis shows that the data for the 4+ state can be described only if a large hexadecapole 215
Volume 116B, number 4
deformation is assumed. This result is expected if the deformed state in I* 0 is like 2oNe. it was suggested previously on the basis of low-energy’proton scattering [2]. The data were taken with the High Resolution Spectrometer (HRS) using the 800 MeV polarized proton beam at the Los Alamos Meson Physics Facility (LAMPF). The experimental arrangements there have been described previously [3]. The polarization of the beam was monitored continuously with a CH, polarimeter; the average polarization of the beam was about 75%. The energy resolution, generally about 200 keV, was limited by the thickness (150 mg/cm2) of the I8 O-ice target [4] which had an I60 contamination of about 4%. Absolute normalization of the data was carried out in a later run with several Be0 targets [5] with varying ratios of ‘*O to 160. These data were compared with previous 12C, 160, and “*Pb elastic scattering data [6-81 to yield an absolute normalization good to +lO%. Angular distributions of the cross section and analyzing power were measured over the angular range from 5” to 17” in the laboratory, generally in steps of 2.0”. The angular acceptance of the HRS was about 1.5”, but the data at each angle were divided into bins of smaller angular width. The angular resolution was about 0.05”, and the error in the position of zero degrees is less than kO.1”. The data for the O+, 2+, and 4+ states are illustrated in figs. 1 and 2 along with theoretical curves described below. Analysis of data for other states will be reported later. Coupled-channels calculations in terms of the rotational model were carried out with the computer code ECIS 79 of Raynal [9]. This program includes relativistic kinematics and allows a search on all parameters based on a x2 fit to both elastic and inelastic data. The rotational model is known to be too simple for IsO where spherical and deformed states coexist in a complicated way [ 10 1. In addition, neutron and proton transition densities are generally different in I8 0 [ 111, whereas only a single deformation parameter is used here. Future analysis of the present data will be concerned with the extraction of information on tehse detailed features of 180. One point of the present paper is to show that such precise information unfortunately cannot be derived from an analysis which omits coupled-channels effects, at least with 800 MeV protons. To show this, 216
14 October 1982
PHYSICS LETTERS
E,=O.O MeV (OfI -with
p2 Paand
P6
----DWEA -.-.-.with
I
I
I’
,’
/’
,’
5
0
/
I
I
E;?.l2MeV(4+) --__ ,I * ‘\ *
\\\
\\ *
/
‘\
*
IO’
/.^.-I-
I
I
IO0
S2 only
.’
,,‘.
.\
\
\
’
*
/
/
8c.m.
Fig. 1. Cross sections for the 180 ($, p’) 180 reaction at 800 MeV. The solid line is a coupled-channel calculation using the rotational model and the deformation parameters p2, p4, and p6 discussed in the text. The dotted-dashed curve is the same calculation with 04 and 06 set to zero. The dashed curve is the result of a DWBA calculation.
and to discuss the other items mentioned in the introduction, the rotational model is sufficient. Preliminary optical model parameters were obtained by simultaneously fitting elastic scattering do/d0 and AY data using the code CUPID of Comfort [ 121 and searching on all parameters. These parameters were then introduced into ECIS 79 as
I
I
I
I
small percentage of the total reaction cross section at 800 MeV. The best overall agreement was obtained with the rotational model and the optical parameters of table 1 (in the notation of ref. [14]), The final phenomenological deformation parameters, which are reasonably close to the low-energy values, are: f12 = 0.34 + 0.02 (OR = 0.87 fm), p4 = 0.16 * 0.02 (S? = 0.41 fm), and 06 = 0.05 + 0.02 @R = 0.14 fm). The errors on the deformation parameters were estimated from the imperfections in the fits and from the effects of optical model parameter variation. Addition of the f16 deformation significantly improves the fits, but, given the simplicity of the model and the lack of data for a 6+ state, this value is certainly not definitive. The fits corresponding to these parameters are very good, generally much better than those obtained at the lower energy where resonance effects have been noted [ 151. They are illustrated by the solid lines in the figures. The dashed curves in these figures were calculated without channel-coupling effects, i.e., using the distorted wave Born approximation. The dotted-dashed curve was calculated in coupled channels with a & deformation only. It is interesting that the rotational model yields a satisfactory fit to both do/da and A, without changing the radius of the form factor from the value determined by the elastic scattering. Because of the simplicity of the model, we cannot conclude that the neutron and proton transition densities have the same effective radius as the ground state density. But it is important to note that the effect of channel coupling on the 2f predicitions, for example, appears similar to the effect of a change in radius; it displaces the angular position of the diffraction structure. Including a flz deformation in coupled channels shifts the predicted distribution around 1.5” to the left; adding f14 and 06 yields a similar shift. The effects of channelcoupling on the 4+ distributions are even more dramatic. Kelly et al. [16] have recently shown that it
1
I
E,= I .98 MeV (2: 1 r\ k +0.5 H 0 s z 3 -0.5 z
+0.5
0
-0.5
I.. 1 5
14 October 1982
PHYSICS LETTERS
Volume 1168, number 4
IO
-
with &.&and
---.-.-
DWBA with j3, only
15
20
i
BE
8c.m.
Fig. 2. Analyzing powers for the 180 ($, p’) 180 reaction at 800 MeV. The curves are described in the caption for fig. 1. parameters. About 100 partial waves were used and integration of the differential equations was carried out between 0.7 fm and the matching radius of 20 fm. The OT, 2;, and 4; states were coupled, and both rotational and vibrational models were tried. Contrary to low energy results, but in agreement with previous 800 MeV coupled-channel calculations [ 131, the optical parameters were little affected by the channel coupling, presumably because inelastic scattering to low-lying states is a very starting
Table 1 Optical model parameters for I80 (F, p’) at 800 MeV. wd
a0
(fm> -4.46
0.985
0.454
rI
Urn)
70.21
0.0
0.972
(fm)
@feV) (fm)
as0 @ml
(MeV)
if:)
0.573
0.704
0.601
1.909
0.984
aI
vso
%O
0.988
wso
?f
i‘%o (fm) 0.562
217
Volume 116B, number 4
PHYSICS LETTERS
is possible to extract precise information about the neutron transition density for the 2f state in I80 from an analysis of proton inelastic scattering at 135 MeV. In an analysis which neglected coupled-channels effects, it was concluded that the neutron and proton transition densities have significantly different radii. The effects of coupled-channels at 135 MeV are likely to be smaller than at 800 MeV because the cross sections are much smaller. Nevertheless, the present results certainly suggest that these effects must be included before such relatively small differences in shape can be accurately determined. The magnitudes of the deformation parameters determined here are related to the shape of the deformed state which mixes with spherical basis states to form the physical states of ‘* 0. Because this admixture is state-dependent (in the calculation of Morrison et al. [15], for example, the 0: and 2f states have deformed components of about 25% while the 4; state is about 90% deformed), and because the contribution of the spherical components is not explicitly included in the rotational calculations, the deformation parameters determined here do not directly yield the shape of the deformed component. The actual deformation is likely to be considerably larger, since the matrix elements between the deformed components are the largest and these contribute at much less than full strength. In fact, then, the deformed component may well have a f14 deformation even larger than determined here, comparable to the value of about 0.24 observed in 2oNe [141. Some comments on the coupled-channels predictions of A, alone are also appropriate here. Angular shifts similar to those discussed above due to channel coupling were recently predicted by Ray in his calculations for 24Mg [ 11. The present data constitute the first experimental observation of these effects. This does not imply that A, here conveys new shape information not contained in do/dQ Similar shifts do appear in the cross section, as expected from the data-to-data relations between do/da and A, discussed by Amado et al. [17] and confirmed in a recent study of “Zr and g2Zr [ 181. At lower energies, it is often found that the ratio h of spin-orbit and central deformations is much larger than one [ 14,191. The origin of this difference is not always clear, although in the energy region around 150 MeV it is presumably due to the 218
14 October 1982
strong q-dependence of the effective nucleon-nucleon t-matrix [20]. In fitting the present 800 MeV data, we have tried different forms of the spin-orbit geometry as well as different values of the ratio h. Reasonable fits could be achieved only with the full Thomas form of the distorted spin-orbit potential; the best value of h is unity, for both the 2f and 4’; states. Increasing or decreasing h by twenty per cent increased x2 by about 30 per cent. This result is consistent with, though more precise than, previous work at 800 MeV, but it is somewhat surprising since the 800 MeV t-matrix does retain some q-dependence. The present work was supported in part by the National Science Foundation, the Department of Energy, and the Welch Foundation. We want to thank W. Bertozzi for the loan of the Be0 targets. We are grateful to L. Ray for helpful discussions and to J. Kelly for his preprint containing the 135 MeV results for the 2f state in l8 0. Permission of J. Comfort and J. Raynal to use the computer codes CUPID and ECIS 79 is very much appreciated. References 111 L. Ray, Phys. Lett. 102B (1981) 88. PI F.G. Resmini et al., Phys. Lett. 37B (1971) 275. ]31 G.S. Blanpied et al., Phys. Rev. Lett. 39 (1977) 1447. 141 C. Pacheco et al., Los Alamos report LA-8109-MS, unpublished. [51 M. Hynes, Thesis, Massachusetts Institute of Technology (1978), unpublished. 161 G.S. Blanpied et al., Phys. Rev. Cl8 (1978) 1436. [71 G. Adams et al., Phys. Rev. Lett. 43 (1979) 421. 181 L. Ray, G.W. Hoffmann and R.M. Thaler, Phys. Rev. C22 (1980) 1454. PI J. Raynal, private communication. [lOI G.E. Brown, Proc. Intern. Conf. on Nuclear physics (Paris, 1964) p. 129. [Ill S. Iversen et al., Phys. Rev. Lett. 40 (1978) 17. WI J. Comfort, private communication. iI31 L. Ray et al., Phys. Rev. Lett. 40 (1978) 1547. v41 R. de Swiniarski et al., Nucl. Phys. A261 (1976) 111. 1151 I. Morrison et al., Phys. Rev. Cl7 (1978) 1485. 1161 J. Kelly et al., submitted to Phys. Rev. Lett. [17] R.D. Amado, J.A. McNeil and D.A. Sparrow, Phys. Rev. c23 (1981) 2114; J.A. McNeil and D.A. Sparrow, Phys. Rev. C23 (1981) 2124. [18] F. Todd Baker et al., Phys. Rev. Lett. 47 (1981) 1823. [19 ] A. Scott et al., private communication. 1201 W.G. Love and M.L. Franey,Phys. Rev. C24 (1981) 1073.
I4 October 1982
PHYSICS LETTERS
Volume 116B, number 4
FROM “0
R. DE SWINIARSKI ‘, K. JONES, S. NANDA
Rutgers University, New Brunswick,
NJ 08903,
USA
F.T. BAKER, M. GRIMM 2, V. PENUMETCHA, A. SCOTT University of Georgia, Athens, GA 30602,
USA
G. ADAMS 3, G. IGO University of California, Los Angeles, CA 90024,
USA
G.W. HOFFMANN University of Texas, Austin, TX 78712,
USA
J. MOSS Los Alamos National Laboratory,
Los Alamos, NM 87545,
USA
W. SWENSON Oregon State University, Cowallis, OR 97330,
USA
and B.E. WOOD 4 University of Oregon, Eugene,
OR 97403,
USA
Received 8 June 1982
Differential cross sections and analyzing powers have been measured for the “0 (T, p’) IsO reaction at 800 MeV. A coupled-channels analysis of the O:, 2f, and 4; data yields good agreement with a rotational model description with a large p4 deformation. The effects of channel coupling are large. The angular distributions of Ay are reproduced well only with a full Thomas spin-orbit deformation approximately equal to the central deformation.
We present here an analysis of new cross-section (da/da) and analyzing-power (A,) data for the excitation of the ground (Of), 1.98 MeV (2f), and 7.12. ’ Permanent address: Universite de Grenoble, 38026 Grenoble Cedex, France. 2 Present address: University of Louisville, Louisville, KY 40208, USA. 3 Present address: Massachusetts Institute of Technology, Cambridge, MA 02139, USA. 4 Present address: Los Alamos National Laboratory, Los Alamos, NM 87545, USA. 0 031-9163/82/0000-0000/$02.75
0 1982 North-Holland
MeV (4;) states in l8 0 by inelastic scattering of 800 MeV protons. The analysis reveals that coupledchannels effects are significant in the determination of shape parameters of the neutron transition density. It confirms, for the first time, the effects of channel coupling on A,, which were recently predicted by Ray [ 1] ; the deformation parameters of the spinorbit and central potentials must be about equal. Finally;the analysis shows that the data for the 4+ state can be described only if a large hexadecapole 215
Volume 116B, number 4
deformation is assumed. This result is expected if the deformed state in I* 0 is like 2oNe. it was suggested previously on the basis of low-energy’proton scattering [2]. The data were taken with the High Resolution Spectrometer (HRS) using the 800 MeV polarized proton beam at the Los Alamos Meson Physics Facility (LAMPF). The experimental arrangements there have been described previously [3]. The polarization of the beam was monitored continuously with a CH, polarimeter; the average polarization of the beam was about 75%. The energy resolution, generally about 200 keV, was limited by the thickness (150 mg/cm2) of the I8 O-ice target [4] which had an I60 contamination of about 4%. Absolute normalization of the data was carried out in a later run with several Be0 targets [5] with varying ratios of ‘*O to 160. These data were compared with previous 12C, 160, and “*Pb elastic scattering data [6-81 to yield an absolute normalization good to +lO%. Angular distributions of the cross section and analyzing power were measured over the angular range from 5” to 17” in the laboratory, generally in steps of 2.0”. The angular acceptance of the HRS was about 1.5”, but the data at each angle were divided into bins of smaller angular width. The angular resolution was about 0.05”, and the error in the position of zero degrees is less than kO.1”. The data for the O+, 2+, and 4+ states are illustrated in figs. 1 and 2 along with theoretical curves described below. Analysis of data for other states will be reported later. Coupled-channels calculations in terms of the rotational model were carried out with the computer code ECIS 79 of Raynal [9]. This program includes relativistic kinematics and allows a search on all parameters based on a x2 fit to both elastic and inelastic data. The rotational model is known to be too simple for IsO where spherical and deformed states coexist in a complicated way [ 10 1. In addition, neutron and proton transition densities are generally different in I8 0 [ 111, whereas only a single deformation parameter is used here. Future analysis of the present data will be concerned with the extraction of information on tehse detailed features of 180. One point of the present paper is to show that such precise information unfortunately cannot be derived from an analysis which omits coupled-channels effects, at least with 800 MeV protons. To show this, 216
14 October 1982
PHYSICS LETTERS
E,=O.O MeV (OfI -with
p2 Paand
P6
----DWEA -.-.-.with
I
I
I’
,’
/’
,’
5
0
/
I
I
E;?.l2MeV(4+) --__ ,I * ‘\ *
\\\
\\ *
/
‘\
*
IO’
/.^.-I-
I
I
IO0
S2 only
.’
,,‘.
.\
\
\
’
*
/
/
8c.m.
Fig. 1. Cross sections for the 180 ($, p’) 180 reaction at 800 MeV. The solid line is a coupled-channel calculation using the rotational model and the deformation parameters p2, p4, and p6 discussed in the text. The dotted-dashed curve is the same calculation with 04 and 06 set to zero. The dashed curve is the result of a DWBA calculation.
and to discuss the other items mentioned in the introduction, the rotational model is sufficient. Preliminary optical model parameters were obtained by simultaneously fitting elastic scattering do/d0 and AY data using the code CUPID of Comfort [ 121 and searching on all parameters. These parameters were then introduced into ECIS 79 as
I
I
I
I
small percentage of the total reaction cross section at 800 MeV. The best overall agreement was obtained with the rotational model and the optical parameters of table 1 (in the notation of ref. [14]), The final phenomenological deformation parameters, which are reasonably close to the low-energy values, are: f12 = 0.34 + 0.02 (OR = 0.87 fm), p4 = 0.16 * 0.02 (S? = 0.41 fm), and 06 = 0.05 + 0.02 @R = 0.14 fm). The errors on the deformation parameters were estimated from the imperfections in the fits and from the effects of optical model parameter variation. Addition of the f16 deformation significantly improves the fits, but, given the simplicity of the model and the lack of data for a 6+ state, this value is certainly not definitive. The fits corresponding to these parameters are very good, generally much better than those obtained at the lower energy where resonance effects have been noted [ 151. They are illustrated by the solid lines in the figures. The dashed curves in these figures were calculated without channel-coupling effects, i.e., using the distorted wave Born approximation. The dotted-dashed curve was calculated in coupled channels with a & deformation only. It is interesting that the rotational model yields a satisfactory fit to both do/da and A, without changing the radius of the form factor from the value determined by the elastic scattering. Because of the simplicity of the model, we cannot conclude that the neutron and proton transition densities have the same effective radius as the ground state density. But it is important to note that the effect of channel coupling on the 2f predicitions, for example, appears similar to the effect of a change in radius; it displaces the angular position of the diffraction structure. Including a flz deformation in coupled channels shifts the predicted distribution around 1.5” to the left; adding f14 and 06 yields a similar shift. The effects of channelcoupling on the 4+ distributions are even more dramatic. Kelly et al. [16] have recently shown that it
1
I
E,= I .98 MeV (2: 1 r\ k +0.5 H 0 s z 3 -0.5 z
+0.5
0
-0.5
I.. 1 5
14 October 1982
PHYSICS LETTERS
Volume 1168, number 4
IO
-
with &.&and
---.-.-
DWBA with j3, only
15
20
i
BE
8c.m.
Fig. 2. Analyzing powers for the 180 ($, p’) 180 reaction at 800 MeV. The curves are described in the caption for fig. 1. parameters. About 100 partial waves were used and integration of the differential equations was carried out between 0.7 fm and the matching radius of 20 fm. The OT, 2;, and 4; states were coupled, and both rotational and vibrational models were tried. Contrary to low energy results, but in agreement with previous 800 MeV coupled-channel calculations [ 131, the optical parameters were little affected by the channel coupling, presumably because inelastic scattering to low-lying states is a very starting
Table 1 Optical model parameters for I80 (F, p’) at 800 MeV. wd
a0
(fm> -4.46
0.985
0.454
rI
Urn)
70.21
0.0
0.972
(fm)
@feV) (fm)
as0 @ml
(MeV)
if:)
0.573
0.704
0.601
1.909
0.984
aI
vso
%O
0.988
wso
?f
i‘%o (fm) 0.562
217
Volume 116B, number 4
PHYSICS LETTERS
is possible to extract precise information about the neutron transition density for the 2f state in I80 from an analysis of proton inelastic scattering at 135 MeV. In an analysis which neglected coupled-channels effects, it was concluded that the neutron and proton transition densities have significantly different radii. The effects of coupled-channels at 135 MeV are likely to be smaller than at 800 MeV because the cross sections are much smaller. Nevertheless, the present results certainly suggest that these effects must be included before such relatively small differences in shape can be accurately determined. The magnitudes of the deformation parameters determined here are related to the shape of the deformed state which mixes with spherical basis states to form the physical states of ‘* 0. Because this admixture is state-dependent (in the calculation of Morrison et al. [15], for example, the 0: and 2f states have deformed components of about 25% while the 4; state is about 90% deformed), and because the contribution of the spherical components is not explicitly included in the rotational calculations, the deformation parameters determined here do not directly yield the shape of the deformed component. The actual deformation is likely to be considerably larger, since the matrix elements between the deformed components are the largest and these contribute at much less than full strength. In fact, then, the deformed component may well have a f14 deformation even larger than determined here, comparable to the value of about 0.24 observed in 2oNe [141. Some comments on the coupled-channels predictions of A, alone are also appropriate here. Angular shifts similar to those discussed above due to channel coupling were recently predicted by Ray in his calculations for 24Mg [ 11. The present data constitute the first experimental observation of these effects. This does not imply that A, here conveys new shape information not contained in do/dQ Similar shifts do appear in the cross section, as expected from the data-to-data relations between do/da and A, discussed by Amado et al. [17] and confirmed in a recent study of “Zr and g2Zr [ 181. At lower energies, it is often found that the ratio h of spin-orbit and central deformations is much larger than one [ 14,191. The origin of this difference is not always clear, although in the energy region around 150 MeV it is presumably due to the 218
14 October 1982
strong q-dependence of the effective nucleon-nucleon t-matrix [20]. In fitting the present 800 MeV data, we have tried different forms of the spin-orbit geometry as well as different values of the ratio h. Reasonable fits could be achieved only with the full Thomas form of the distorted spin-orbit potential; the best value of h is unity, for both the 2f and 4’; states. Increasing or decreasing h by twenty per cent increased x2 by about 30 per cent. This result is consistent with, though more precise than, previous work at 800 MeV, but it is somewhat surprising since the 800 MeV t-matrix does retain some q-dependence. The present work was supported in part by the National Science Foundation, the Department of Energy, and the Welch Foundation. We want to thank W. Bertozzi for the loan of the Be0 targets. We are grateful to L. Ray for helpful discussions and to J. Kelly for his preprint containing the 135 MeV results for the 2f state in l8 0. Permission of J. Comfort and J. Raynal to use the computer codes CUPID and ECIS 79 is very much appreciated. References 111 L. Ray, Phys. Lett. 102B (1981) 88. PI F.G. Resmini et al., Phys. Lett. 37B (1971) 275. ]31 G.S. Blanpied et al., Phys. Rev. Lett. 39 (1977) 1447. 141 C. Pacheco et al., Los Alamos report LA-8109-MS, unpublished. [51 M. Hynes, Thesis, Massachusetts Institute of Technology (1978), unpublished. 161 G.S. Blanpied et al., Phys. Rev. Cl8 (1978) 1436. [71 G. Adams et al., Phys. Rev. Lett. 43 (1979) 421. 181 L. Ray, G.W. Hoffmann and R.M. Thaler, Phys. Rev. C22 (1980) 1454. PI J. Raynal, private communication. [lOI G.E. Brown, Proc. Intern. Conf. on Nuclear physics (Paris, 1964) p. 129. [Ill S. Iversen et al., Phys. Rev. Lett. 40 (1978) 17. WI J. Comfort, private communication. iI31 L. Ray et al., Phys. Rev. Lett. 40 (1978) 1547. v41 R. de Swiniarski et al., Nucl. Phys. A261 (1976) 111. 1151 I. Morrison et al., Phys. Rev. Cl7 (1978) 1485. 1161 J. Kelly et al., submitted to Phys. Rev. Lett. [17] R.D. Amado, J.A. McNeil and D.A. Sparrow, Phys. Rev. c23 (1981) 2114; J.A. McNeil and D.A. Sparrow, Phys. Rev. C23 (1981) 2124. [18] F. Todd Baker et al., Phys. Rev. Lett. 47 (1981) 1823. [19 ] A. Scott et al., private communication. 1201 W.G. Love and M.L. Franey,Phys. Rev. C24 (1981) 1073.