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Observation of resonances in 24Mg at 32–40 MeV excitation energy via the 12C(12C, 8Be)16O reaction
Volume 56B, number 5
PHYSICS LETTERS
26 May 1975
OBSERVATION OF RESONANCES IN 24Mg AT 32-40 MeV EXCITATION ENERGY VIA THE ‘*C(’ *C, “Be)160 REACTION * &A. EBERHARD,
E. MATHIAK, J. STETTMEIER, W. TROMBIK, A. WEIDINGER, L.N. WtiSTEFELD and K.G. BERNHARDT
Sektion Physik, Universitiit Miinchen, D-8046 Garching, Germany and Nuclear Physics Laboratory,
University of Washington, Seattle, WA 98195,
uSA
Received 31 March 1975
Strongly correlated resonances are observed in r2C(12C, 8Be)‘60 excitation functions at Ex(24Mg) = 32.4 and 39.4 MeV for various excited states in 160. At these energies correlated structures are also present in 12C(‘2C,o)20Ne and 1oB(14N,o)20Ne excitation functions as reported recently by other workers.
The puzzling kind of structures observed in excitation functions for heavy-ion scattering and reactions at energies near and above the Coulomb barrier has been of particular interest in heavy-ion research in recent years. Most of the ex erimental work has been concentrated on the ‘*C + “C and 160 + 160 systems, for which elastic and inelastic scattering and a great many reaction channels have been investi ated. In a systematic study of the 12C(12C, 1Be)160 reaction we have found structures in the excitation functions, which are 400 to 1000 keV wide and which are strongly correlated for a number of *Be + 160 exit channels. The most prominent peaks are observed at E(c.m.) = 18.5 and 25.5 MeV corresponding to excitation energies in the compound nucleus 24Mg of E, = 32.4 and 39.4 MeV. Similar structures have been observed in recent months in ‘*C(‘*C. o)*%e [I] and 1oB(14N,a)20Ne [l] and 1oB(i4N,i)%Je [2] excitation functions at the same compound nucleus ener ‘es. The ‘*C( B*C, 8Be)160 excitation functions were measured using our eight-detector array [3] with high detection efficiency for the particle unstable nucleus *Be and a ‘*C beam of 300-800 nA from the Munich MP tandem accelerator. The energy range E(c.m.) = 17.5 - 34.5 MeV was covered in steps of 250 keV (c.m.), and partly in 125 keV (c.m.) steps. The target thickness of 51 &cm* corresponds to an average energy loss of the ‘*C ions in the target of 150 keV. * Work supported in part by U.S. Atomic Energy Commission.
At some energies angular distributions were taken also. A complete presentation of the data and an analysis in terms of statistical fluctuations will be given elsewhere [4]. Here we focus on the observed correlated structure and on a comparison with recent data from other reactions. Excitation functions for ‘*C(‘*C,cr)*‘Ne to several excited states in *%Ie at 0 (lab) = 2’ [5] and at 5’ [6] for E(c.m.) between 17 and 20 MeV have recently been reported. A number of large correlated peaks in ‘*C(l*C, o)*%e excitation functions at 13(lab) = 5’ have been observed [I] at E(c.m.) = 17.9, 18.4, 18.6, 19.0 and 19.4 MeV, each of which is present in a number of different (Y+ *%Je channels; the authors interpret them as actual states in 24Mg which are formed as ‘*C + l*C resonances. The strongest resonance occurs at E(c.m.) = 18.4 MeV in a great number of (Y+ *%Ie exit channels. Inspection of our 12C(12C, *Be)160 excitation func tions reveals that the most prominent structures are observed at E(c.m.) = 18.5 MeV. Since this ener Ne within 100 keV (cm.) of the strong 12C(12C, a) %‘” structure at E(c.m.) = 18.4 MeV obtained by the Argonne group [I] (which, according to ref. [ 11, is well within the uncertainties in the absolute energy calibration of the tandem accelerators involved) and since the present structure also has a width of about 400 keV (cm.), it is concluded that they are due to the same resonance. As an example, fig. 1 shows the to the 6.05 MeV excitation functions correspondin (O+) and 6.13 MeV (3-) states in !6 0; these two 445
Volume 56B, number 5 ,
5 x t O 3.
I
PHYSICS LETTERS J
I
i
I
L
=zC ('2C, eB%.,.)t60* E'~=6.1 MeV (0+,3")
I0~
5 x 102!
to~ , 5 x i0=': z 5xlO= F-W o~
I0=
0 5xlOtl nO h
7,
10" 5 x I0 z:
I0"
I0; :
~
*
5 x t O i"
E ,2 c (c.m.)[MeV] Fig. 1. Yield functions for the reaction 12C( 12C, 8Be~ s ) 16O
exciting the 6.05 MeV (0~) and 6.13 MeV (3) states in 260. These two states are not resolvedin the experiment. The cross sections are accurate to ± 10%. states are not resolved in the experiment. The peak at 18.5 MeV is dearly seen at all angles investigated. We would like to note that the presence of this resonance is not only observed in the 8Be + 160*(6.1 MeV) channel shown in fig. 1, but is also found in the excitation functions corresponding to the ground state and the 6.92 MeV state in 16O. Higher -eaaergy states in 160 are not excited at this energy. At the other energies ofE(c.m.) = 17.9, 18.6, 19.0 and 19.4 MeV at which, in addition to the one at 18.4 MeV, resonances in the 12C(12C, a)20Ne excita446
26 May 1975
tion functions were reported by the Argonne group [1] we observe (within a consistent energy shift of ~< 100 keV) correlated structure in most of our 12C(12C, 8Be)160 excitation functions; however, it is less pronounced than that at 18.5 MeV and in some cases we see only a slight indication of it. Furthermore the observation of these structures is more or less limited to one or two excit channels; a behavior, which was also found by the Argonne group [1] for 12C(12C, a)20Ne. The structures at E(c.m.) = 17.9, 18.6 and 19.0 MeV, e.g., are not observed in the 8Be + 160*(6.1 MeV) channel as seen 8in fig'l 1;6their presence, however, is indicated in the Be + O(g.s.) and/or 8Be + 160*(6.9 MeV) channel. In further agreement with the Argonne results we find the most pronounced structure at 18.5 MeV and the least pronounced one at 17.9 MeV. We now would like to discuss the observation of another correlated structure at E(c.m.) = 25.5 MeV in our data and a possible connection to "unexl~ected, intermediate structure at high level densities in~4Mz" reported by Marquardt et al. [2] for 10B(14N, a)201~e. Whereas correlations between the reactions 12C(12C, 8Be)160 and 12C(12C, a)20Ne, as discussed above, could possibly be restricted to the particular 12C + 12C entrance channel, any correlation between 12C(12C, 8Be)160 and I°B(14N, a)20Ne, as discussed in the following, would only be reflected in that part of the reaction which proceeds through the formation of the same coml~ound nucleus 24Mg. For the 12C( lz C, 8Be) 16O reaction in this energy range the "a-like" states in 160 at 6.1, 6.9, 10.4, 14.7 and 16.3 MeV are strongly excited. Significantly weaker excited in the 4 + state at 11.1 MeV, which is not "a-like". Consequently the cross section for this state from a direct transfer process is expected to be smaller; thus the fraction of a possible compound reaction component in the cross section should be larger for this state than for the others. The average cross section for this state is found to be about five times smaller than for the other 4 + state at 10.4 MeV; the cross section, however, is still of the order of 100 /~b/sr. With respect to intermediate structure effects, the (12C, 8Be) reaction exciting the 11.1 MeV state is therefore believed to be most suitable for a comparison with the I°B(14N, a)2°Ne reaction. The experimental (12C, 8Be) excitation functions for this state are shown in fig. 2; the dearly corre-
Volume 56B, number 5
PHYSICS LETTERS ,
,2C(12C,8Beg.s.) 160" E ' : II. I MeV (4 +)
I
I
i
1021
,
I
26 May 1975
,
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20 o
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I
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,.,- ', /\ '/' ;/ ~'~ jo:.,
I,,-
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l l i ~ Eizc (c.m.) =18.5 MeV
IO-
ro~
I
m c ('a c , SBe ~l.S.I~'s0 g.S. - - P 2 tz(COS8),---PZ i.(COS8) ]~Doto
A
i
1.0~
~ iOS~
40 °
v) t~ 0 ,~ i0~. ul
I00-:
\ \
I
-
E,2 (c.m.)=25.5 MeV
',!\/
bCl I0
107
i'l iO2-
I0 t
LO=
2'0
;~5
30
'
lOi-
20
25
30
EIZc (c.m.) [MeV] Fig. 2. Same as in fig. 1 but for the 11.4 MeV (4+) state in
lated structure at E(c.m.) = 25°5 MeV is indicated by the sohd line. This energy corresponds to E x = 39.4 MeV excitation in 24Mg and it is interesting to note that the only, correlated structures observed in the 10B(14N, c020Ne excitation functions between E x = 34.3 and 43.4 MeV are found at E x = 38.0, 38.7 and 39.4 MeV [2]. Whereas the structure at 39.4 MeV is observed in both reactions, those at 38.0 and 38.7 MeV are not seen in the excitation functions in fig. 2. The broadness of the E(c.m.) = 25.5 MeV structure on the low-energy side of the peaks in fig. 2 at some angles could be a slight indication of their possible presence. The inspection o f all other excitation functions measured does not yield any further evidence for the presence of these states in any of the various 8Be + 1°O exit channels. The likely absence of these resonances in our data could be an indication that these states have odd spin and/or negative parity, and
0./
6
II, ,
Il
,b
~o
/ /' II,
I
iIl
~o ~o ~o #c... (deg)
~o
~o
Fig. 3. Angular distributions for the 12C(12C, 8 Beg.s.) 16Og.s.
reaction on resonance. The solid curve shows the square of the l = 12 Legendre polynomial, the dashed one corresponds t o / = 14. thus would not be populated in a reaction induced with identical particles such as 12C(12C, 8Be)160 studied here. The resonance at E x = 39.4 MeV, on the other hand, is seen in all 8Be + 160 exit channels, although generally not as pronounced as in fig. 2 for the 8Be + 160*(11.4 MeV) channel. The observation of this resonance in reactions with different entrance and exit channels leads to its interpretation as being an intermediate resonance in the highly excited compound nucleus 24Mg. This also would readily explain that this resonance is more pronounced in the 8Be + 160*(11.4 MeV) channel than in other channels which have a larger direct reaction component. For the ground-state transition 12C(12C, 8Beg.s.)16Og.s., where only spin-zero particles are involved, the angular distributions measured on re447
Volume 56B, number 5
PHYSICS LETTERS
sonance should be dominated by the square of the Legendre polynomial p2(cos 0), where J is the spin of the resonance. Since the particles in the entrance channel are identical, the spins are restricted to even values and the parity is positive. In fig. 3 angular distributions at E(c.m.) = 18.5 and 25.5 MeV are shown along with p2(cos 0) for J = 12 and 14. As can be seen the angular distribution at E(c.m.) = 18.5 MeV is dominated by the 1 = 12 partial wave, whereas the one at 25.5 MeV is dominated by l = 14. The grazing /-values for 12C + 12C at E(c.m.) = 18.5 and 25.5 MeV are also l = 12 and l = 14, respectively. Thus certain criticism should be taken in assigning spins o f J = 12 and 14 to the resonances at E(c.m.) = 18.5 and 25.5 MeV. Further experimental work on this question is in progress. Finally, we like to point out that in agreement with the results of Stokstad [7] and Shapira et al. [5] for the 12C(12C, a)20Ne reaction we find from a statistical model analysis [4] that most of the structures in our excitation functions are likely to be Ericson type fluctuations. At a few energies, however, the persistence of the structure for various angles (separated by far more than the statistical coherence angle) and for different exit channels along with the close
448
26 May 1975
correlation to similar structures in 12C(12C, a)20Ne and 10B(14N, ot)20Ne excitation functions of other workers seems to be clearly beyond the scope of statistical cross-section fluctuations. The origin and nature of the observed resonances in the highly excited compound nucleus 24Mg (or possibly in the 12C + 12C system for the one at E(c.m.) = 18.5 MeV) remains to be an open question at present and a challenge for future work°
References [1] H.T. Fortune, L.R. Greenwood, R.E. Segel and J.R. Erskine, Argonne National Laboratory, preprint, 1975. [2] N. Marquardt, R. Voiders, C. Cardinal and J. L'Ecuyer, Phys. Rev. Lett. 33 (1974) 1389. [3] J.G. Cramer et al., Nucl. Instr. 111 (1973) 425. [4] K.A. Eberharfl et al., to be published. [5] D. Shapiro, R.G. Stokstad and D.A. Bromley, Phys. Rev. C10 (1974) 1063. [6] L.R. Greenwood, H.T. Fortune, R.E. Segel and J.R. Erskine, Phys. Rev. C10 (1974) 1211. [7 ] R.G. Stokstad, in Reaction between complex nuclei (North-Holland Publ. Co., Amsterdam, 1974) Vol. II, p.327.
PHYSICS LETTERS
26 May 1975
OBSERVATION OF RESONANCES IN 24Mg AT 32-40 MeV EXCITATION ENERGY VIA THE ‘*C(’ *C, “Be)160 REACTION * &A. EBERHARD,
E. MATHIAK, J. STETTMEIER, W. TROMBIK, A. WEIDINGER, L.N. WtiSTEFELD and K.G. BERNHARDT
Sektion Physik, Universitiit Miinchen, D-8046 Garching, Germany and Nuclear Physics Laboratory,
University of Washington, Seattle, WA 98195,
uSA
Received 31 March 1975
Strongly correlated resonances are observed in r2C(12C, 8Be)‘60 excitation functions at Ex(24Mg) = 32.4 and 39.4 MeV for various excited states in 160. At these energies correlated structures are also present in 12C(‘2C,o)20Ne and 1oB(14N,o)20Ne excitation functions as reported recently by other workers.
The puzzling kind of structures observed in excitation functions for heavy-ion scattering and reactions at energies near and above the Coulomb barrier has been of particular interest in heavy-ion research in recent years. Most of the ex erimental work has been concentrated on the ‘*C + “C and 160 + 160 systems, for which elastic and inelastic scattering and a great many reaction channels have been investi ated. In a systematic study of the 12C(12C, 1Be)160 reaction we have found structures in the excitation functions, which are 400 to 1000 keV wide and which are strongly correlated for a number of *Be + 160 exit channels. The most prominent peaks are observed at E(c.m.) = 18.5 and 25.5 MeV corresponding to excitation energies in the compound nucleus 24Mg of E, = 32.4 and 39.4 MeV. Similar structures have been observed in recent months in ‘*C(‘*C. o)*%e [I] and 1oB(14N,a)20Ne [l] and 1oB(i4N,i)%Je [2] excitation functions at the same compound nucleus ener ‘es. The ‘*C( B*C, 8Be)160 excitation functions were measured using our eight-detector array [3] with high detection efficiency for the particle unstable nucleus *Be and a ‘*C beam of 300-800 nA from the Munich MP tandem accelerator. The energy range E(c.m.) = 17.5 - 34.5 MeV was covered in steps of 250 keV (c.m.), and partly in 125 keV (c.m.) steps. The target thickness of 51 &cm* corresponds to an average energy loss of the ‘*C ions in the target of 150 keV. * Work supported in part by U.S. Atomic Energy Commission.
At some energies angular distributions were taken also. A complete presentation of the data and an analysis in terms of statistical fluctuations will be given elsewhere [4]. Here we focus on the observed correlated structure and on a comparison with recent data from other reactions. Excitation functions for ‘*C(‘*C,cr)*‘Ne to several excited states in *%Ie at 0 (lab) = 2’ [5] and at 5’ [6] for E(c.m.) between 17 and 20 MeV have recently been reported. A number of large correlated peaks in ‘*C(l*C, o)*%e excitation functions at 13(lab) = 5’ have been observed [I] at E(c.m.) = 17.9, 18.4, 18.6, 19.0 and 19.4 MeV, each of which is present in a number of different (Y+ *%Je channels; the authors interpret them as actual states in 24Mg which are formed as ‘*C + l*C resonances. The strongest resonance occurs at E(c.m.) = 18.4 MeV in a great number of (Y+ *%Ie exit channels. Inspection of our 12C(12C, *Be)160 excitation func tions reveals that the most prominent structures are observed at E(c.m.) = 18.5 MeV. Since this ener Ne within 100 keV (cm.) of the strong 12C(12C, a) %‘” structure at E(c.m.) = 18.4 MeV obtained by the Argonne group [I] (which, according to ref. [ 11, is well within the uncertainties in the absolute energy calibration of the tandem accelerators involved) and since the present structure also has a width of about 400 keV (cm.), it is concluded that they are due to the same resonance. As an example, fig. 1 shows the to the 6.05 MeV excitation functions correspondin (O+) and 6.13 MeV (3-) states in !6 0; these two 445
Volume 56B, number 5 ,
5 x t O 3.
I
PHYSICS LETTERS J
I
i
I
L
=zC ('2C, eB%.,.)t60* E'~=6.1 MeV (0+,3")
I0~
5 x 102!
to~ , 5 x i0=': z 5xlO= F-W o~
I0=
0 5xlOtl nO h
7,
10" 5 x I0 z:
I0"
I0; :
~
*
5 x t O i"
E ,2 c (c.m.)[MeV] Fig. 1. Yield functions for the reaction 12C( 12C, 8Be~ s ) 16O
exciting the 6.05 MeV (0~) and 6.13 MeV (3) states in 260. These two states are not resolvedin the experiment. The cross sections are accurate to ± 10%. states are not resolved in the experiment. The peak at 18.5 MeV is dearly seen at all angles investigated. We would like to note that the presence of this resonance is not only observed in the 8Be + 160*(6.1 MeV) channel shown in fig. 1, but is also found in the excitation functions corresponding to the ground state and the 6.92 MeV state in 16O. Higher -eaaergy states in 160 are not excited at this energy. At the other energies ofE(c.m.) = 17.9, 18.6, 19.0 and 19.4 MeV at which, in addition to the one at 18.4 MeV, resonances in the 12C(12C, a)20Ne excita446
26 May 1975
tion functions were reported by the Argonne group [1] we observe (within a consistent energy shift of ~< 100 keV) correlated structure in most of our 12C(12C, 8Be)160 excitation functions; however, it is less pronounced than that at 18.5 MeV and in some cases we see only a slight indication of it. Furthermore the observation of these structures is more or less limited to one or two excit channels; a behavior, which was also found by the Argonne group [1] for 12C(12C, a)20Ne. The structures at E(c.m.) = 17.9, 18.6 and 19.0 MeV, e.g., are not observed in the 8Be + 160*(6.1 MeV) channel as seen 8in fig'l 1;6their presence, however, is indicated in the Be + O(g.s.) and/or 8Be + 160*(6.9 MeV) channel. In further agreement with the Argonne results we find the most pronounced structure at 18.5 MeV and the least pronounced one at 17.9 MeV. We now would like to discuss the observation of another correlated structure at E(c.m.) = 25.5 MeV in our data and a possible connection to "unexl~ected, intermediate structure at high level densities in~4Mz" reported by Marquardt et al. [2] for 10B(14N, a)201~e. Whereas correlations between the reactions 12C(12C, 8Be)160 and 12C(12C, a)20Ne, as discussed above, could possibly be restricted to the particular 12C + 12C entrance channel, any correlation between 12C(12C, 8Be)160 and I°B(14N, a)20Ne, as discussed in the following, would only be reflected in that part of the reaction which proceeds through the formation of the same coml~ound nucleus 24Mg. For the 12C( lz C, 8Be) 16O reaction in this energy range the "a-like" states in 160 at 6.1, 6.9, 10.4, 14.7 and 16.3 MeV are strongly excited. Significantly weaker excited in the 4 + state at 11.1 MeV, which is not "a-like". Consequently the cross section for this state from a direct transfer process is expected to be smaller; thus the fraction of a possible compound reaction component in the cross section should be larger for this state than for the others. The average cross section for this state is found to be about five times smaller than for the other 4 + state at 10.4 MeV; the cross section, however, is still of the order of 100 /~b/sr. With respect to intermediate structure effects, the (12C, 8Be) reaction exciting the 11.1 MeV state is therefore believed to be most suitable for a comparison with the I°B(14N, a)2°Ne reaction. The experimental (12C, 8Be) excitation functions for this state are shown in fig. 2; the dearly corre-
Volume 56B, number 5
PHYSICS LETTERS ,
,2C(12C,8Beg.s.) 160" E ' : II. I MeV (4 +)
I
I
i
1021
,
I
26 May 1975
,
30*
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.~ 102. Z 0 I-(.3 LU IO~
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,.,- ', /\ '/' ;/ ~'~ jo:.,
I,,-
I
l l i ~ Eizc (c.m.) =18.5 MeV
IO-
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I
m c ('a c , SBe ~l.S.I~'s0 g.S. - - P 2 tz(COS8),---PZ i.(COS8) ]~Doto
A
i
1.0~
~ iOS~
40 °
v) t~ 0 ,~ i0~. ul
I00-:
\ \
I
-
E,2 (c.m.)=25.5 MeV
',!\/
bCl I0
107
i'l iO2-
I0 t
LO=
2'0
;~5
30
'
lOi-
20
25
30
EIZc (c.m.) [MeV] Fig. 2. Same as in fig. 1 but for the 11.4 MeV (4+) state in
lated structure at E(c.m.) = 25°5 MeV is indicated by the sohd line. This energy corresponds to E x = 39.4 MeV excitation in 24Mg and it is interesting to note that the only, correlated structures observed in the 10B(14N, c020Ne excitation functions between E x = 34.3 and 43.4 MeV are found at E x = 38.0, 38.7 and 39.4 MeV [2]. Whereas the structure at 39.4 MeV is observed in both reactions, those at 38.0 and 38.7 MeV are not seen in the excitation functions in fig. 2. The broadness of the E(c.m.) = 25.5 MeV structure on the low-energy side of the peaks in fig. 2 at some angles could be a slight indication of their possible presence. The inspection o f all other excitation functions measured does not yield any further evidence for the presence of these states in any of the various 8Be + 1°O exit channels. The likely absence of these resonances in our data could be an indication that these states have odd spin and/or negative parity, and
0./
6
II, ,
Il
,b
~o
/ /' II,
I
iIl
~o ~o ~o #c... (deg)
~o
~o
Fig. 3. Angular distributions for the 12C(12C, 8 Beg.s.) 16Og.s.
reaction on resonance. The solid curve shows the square of the l = 12 Legendre polynomial, the dashed one corresponds t o / = 14. thus would not be populated in a reaction induced with identical particles such as 12C(12C, 8Be)160 studied here. The resonance at E x = 39.4 MeV, on the other hand, is seen in all 8Be + 160 exit channels, although generally not as pronounced as in fig. 2 for the 8Be + 160*(11.4 MeV) channel. The observation of this resonance in reactions with different entrance and exit channels leads to its interpretation as being an intermediate resonance in the highly excited compound nucleus 24Mg. This also would readily explain that this resonance is more pronounced in the 8Be + 160*(11.4 MeV) channel than in other channels which have a larger direct reaction component. For the ground-state transition 12C(12C, 8Beg.s.)16Og.s., where only spin-zero particles are involved, the angular distributions measured on re447
Volume 56B, number 5
PHYSICS LETTERS
sonance should be dominated by the square of the Legendre polynomial p2(cos 0), where J is the spin of the resonance. Since the particles in the entrance channel are identical, the spins are restricted to even values and the parity is positive. In fig. 3 angular distributions at E(c.m.) = 18.5 and 25.5 MeV are shown along with p2(cos 0) for J = 12 and 14. As can be seen the angular distribution at E(c.m.) = 18.5 MeV is dominated by the 1 = 12 partial wave, whereas the one at 25.5 MeV is dominated by l = 14. The grazing /-values for 12C + 12C at E(c.m.) = 18.5 and 25.5 MeV are also l = 12 and l = 14, respectively. Thus certain criticism should be taken in assigning spins o f J = 12 and 14 to the resonances at E(c.m.) = 18.5 and 25.5 MeV. Further experimental work on this question is in progress. Finally, we like to point out that in agreement with the results of Stokstad [7] and Shapira et al. [5] for the 12C(12C, a)20Ne reaction we find from a statistical model analysis [4] that most of the structures in our excitation functions are likely to be Ericson type fluctuations. At a few energies, however, the persistence of the structure for various angles (separated by far more than the statistical coherence angle) and for different exit channels along with the close
448
26 May 1975
correlation to similar structures in 12C(12C, a)20Ne and 10B(14N, ot)20Ne excitation functions of other workers seems to be clearly beyond the scope of statistical cross-section fluctuations. The origin and nature of the observed resonances in the highly excited compound nucleus 24Mg (or possibly in the 12C + 12C system for the one at E(c.m.) = 18.5 MeV) remains to be an open question at present and a challenge for future work°
References [1] H.T. Fortune, L.R. Greenwood, R.E. Segel and J.R. Erskine, Argonne National Laboratory, preprint, 1975. [2] N. Marquardt, R. Voiders, C. Cardinal and J. L'Ecuyer, Phys. Rev. Lett. 33 (1974) 1389. [3] J.G. Cramer et al., Nucl. Instr. 111 (1973) 425. [4] K.A. Eberharfl et al., to be published. [5] D. Shapiro, R.G. Stokstad and D.A. Bromley, Phys. Rev. C10 (1974) 1063. [6] L.R. Greenwood, H.T. Fortune, R.E. Segel and J.R. Erskine, Phys. Rev. C10 (1974) 1211. [7 ] R.G. Stokstad, in Reaction between complex nuclei (North-Holland Publ. Co., Amsterdam, 1974) Vol. II, p.327.