The 23Na(p, γ)24Mg reaction and the giant dipole resonance in 24Mg

Nuclear Physics All6 (1968) 682.--694; (~) North-Holland Publishing Co., Amsterdam Not to be reproduced by photoprint or microfilm without written permission from the publisher

T H E 23Na(p, ?)Z4Mg R E A C T I O N A N D T H E G I A N T D I P O L E R E S O N A N C E IN 24Mg t R. C. BEARSE and L. MEYER-SCHI]TZMEISTER Argonne National Laboratory, Argonne, Illinois and R. E. SEGEL Argonne National Laboratory, Argonne, Illinois and Northwestern University, Evanston, Illinois Received 7 June 1968 Abstract: The 23Na(p,7)24Mg reaction has been studied over the region of excitation corresponding to the giant dipole resonance. Gamma rays to the ground state and to the first and to the second plus third excited states have been observed. The (p, 7o) cross section is on the average smaller than it is in other doubly even nuclei in this region. The (P, Tx) cross section is considerably larger than the (p, 70) and clearly follows a giant-resonance shape. A part of the giant resonance built upon the second and/or third excited states is also observed. A fluctuation analysis finds the coherence width to be about 75 keV. The angular distributions of both ~'0 and 7~ show the usual small energy variation that has come to be associated with the giant resonance.

E I

I

NUCLEAR REACTIONS 23Na(p,~,), E = 4-12.4 MeV; measured ¢~(E,E~, 0).

1. Introduction

Over the past several years, much i n f o r m a t i o n concerning the mechanism of the giant dipole resonance and its fine structure has been obtained from the study of (P, 7) reactions on light nuclei. Ill particular, a n g u l a r distributions and excitation functions for the ground-state transition 7o have been taken 1-6) in quite fine resolution over the giant dipole resonance in 4He, IzC, 160, 2°Ne, zSSi and 3zS. F o r t2C, Z°Ne and 28Si, the transition to the first excited state has been studied as well z, 3,5). The g a m m a - r a y angular distributions have been found to be almost constant over the entire region of the giant dipole resonance, and this constancy has led to the hypothesis that the giant dipole resonance is d o m i n a t e d by a single configuration v). As part of a c o n t i n u i n g p r o g r a m to test and further develop this picture, we report here a study of the 23Na(p,-/)24Mg reaction. One special feature of the present case is that the shell-model configurations of the g r o u n d states of 23Na a n d 2"~Mg require that the direct-reaction c o m p o n e n t iq the (P, 7o) yield be suppressed. These nuclei lie in the lower part of the 2 s l d shell where Work performed under the auspices of the U. S. Atomic Energy Commission. 682

23Na(p,),)S~'MREACTION g

683

the valence nucleons are expected to spend the majority of their time in ld~ and 2s, orbits. On the other hand, the only way by which the ~+ 23Na ground state can be made directly from the 0 ÷ 24Mg ground state is by the removal of a d~_nucleon - and these constitute but a small part of the 24Mg ground-state wave function. Thus, the

N~3(p,~')Me" Ep'9.3 MeV

i!~ii~j e'9°° I~

[~/I

/ i'

30.5 cm t h i c k

!t~N*

•

25.4 cm diam. 20,3 cm t h i c k

Fig. I. Pulse-height spectra o b t a i n e d in the two N a I ( T I ) crystals f r o m the 23Na(p,7) reaction at Ep =- 9.3 MeV, 0 = 90 ° .

direct-reaction component in the cross section will be suppressed and, since in other cases statistical analysis has shown that the direct-reaction component is the major contributor to the yield, a smaller than usual (average) cross section might be expected for 23Na(p, 70). The compound-nucleus component known to be present in

684

R.C. BEARSE et al.

other cases 4-6) should also be here and may now be expected to dominate. If it does, a question of particular interest is whether the angular distributions will remain nearly invariant. The techniques used were only slight variations of those previously rcported 2,4, 5,7). Targets of metallic Na were evaparated onto C foils and transferred via vacuum locks to the reaction chamber. The gamma rays were detected by a 25.4 c m x 30.4 cm and a 25.4 cm x 20.3 cm NaI(TI) crystal in conjunction with the pulse-pile-up rejection circuits 4) described previously. The absolute cross section was determined by measuring the yield from a NaBr target whose thickness had been determined by alpha-particle scattering at an energy low enough so that the bromine scattering could be considered as pure Rutherford. The 23Na(p, ~)24Mg reaction has Q = 11.69 MeV, and the first three excited states of 24Mg are at 1.37, 4.12 and 4.23 MeV, respectively. Fig. 1 shows typical spectra obtained from each detector. The yields of~o and ~ were obtained by fitting the upper end of the spectrum by use of a computer program 4) after which the sum of the yields for 2~2+ ~3 was obtained by subtracting out that part of the spectrum due to ~o and ~ and then summing the counts remaining in the appropriate energy interval. It should be noted that although both detectors have the same diameter, the thicker detector has noticeably better resolution, the reason being that the thicker crystal absorbs a higher percentage of the secondary bremsstrahlung produced by the electron-positon pairs. 2. E x c i t a t i o n curves

The excitation functions for 70, ~ and ~2--1-'~3taken at 90 ° to the incident beam in 25 keV steps from 4.0 to 10.5 MeV and in 30 keV steps from 10.5 to 12.4 MeV are shown in figs. 2-4. The Na target had a thickness of 300 #g/cm 2 (13 keV thick to 8 MeV 24t6

Mg 16

i[

'

(MeV)

18

20

22

24

I

I

I

I

ro

-g 4

2

6

8

I0

12

Ep(MeV) Fig. 2. Yield curve at 90 ° o f the ground-state g a m m a ray from the 23Na(p, 7) reaction.

~:~Na(p, ),)l~4Mg REACTION

685

protons). The average (p, "/o) cross section is considerably smaller than it is for the other doubly-even self-conjugate nuclei in this region of the periodic table 4-6), and the over-all giant resonance envelope is barely discernible. There is some evidence of intermediate structure with the strength divided into two groups, e.g. a cluster between Mg I0

,,,

16

18

I

i

24N

'

(MeV)

20

22

24

I

I

t

6

I

I

A

z

6

,

8 Ep(MeV)

I

,

I0

I

12

Fig. 3. Yield curve at 90 ° o f the g a m m a ray to the 1.37 MeV first excited state in ='Mg from the =3Na (P, 7) reaction. Mg 16 I

8

24~

18 i

(MeV) 20 I

24

22 I

6"2 zk

-

6

8

I0

12

Ep(MeV) Fig. 4. Yield curve at 90 ° o f the (unresolved) g a m m a rays feeding the doublet at 4.12 a n d 4.23 MeV in ~4Mg from the ~aNa(P,7) reaction.

686

R.c.

BEARSE e t

al.

about 4 and 6 MeV proton bombarding energy and a broad weak group that extends from 6.5 to 12 MeV. "Ihe average (P,3'l) cross section is much greater than the (P,3'o) and here the yield curve clearly displays a giant-resonance envelope but there is the same splitting into the two groupings and the same type of fine structure superimposed upon the whole picture. As measured by the ratio of peak to nearby valley, the fine-structure fluctuations are more violent for Vo. This indicates that here, as expected, the compound nucleus is relatively more important. After conversion to the inverse reaction by use of detailed balance, the integrated (Vo, P0) and ()'t, Po) cross sections each account for about 3.3 ~ of the classical dipole sum of 0.06 (NZ/A)MeV. b. [In converting from (p, V) to (7, P), the former cross section is multiplied by the factor I/(2JF+ 1); and (2JF+ 1) is five times as large for (Vl, Po) as for (Vo, Po).] In the other cases that have been studied, the ()'o, Po) strength is much the greater, and why this is normally to be expected is discussed elsewhere 4). The center of strength for the (P,)'1) reaction lies about 1 MeV above the strength for the (p, Vo) reaction. This follows the trend 2,4.5) for the center of the Vl resonance to be above the ~o center by an amount about equal to the excitation energy of the first excited state (1.37 MeV in the present case). The excitation function for (p, V2÷a) given in fig. 4 is dominated by a giant-resonance envelope which peaks at about 11.3 MeV - about 2.1 MeV above the 9.2 MeV peak of the Vl giant resonance. The energy difference between the first and second excited states is 2.8 MeV. Thus again we have the pattern of a giant resonance of approximately constant energy built on various excited states. The peak cross sections in this structure are approximately the same as those of the (p, Vl) excitation function. Because of interference from the intense-low energy background, the intensity of the peak corresponding to "~'2+ 3 could not be determined as accurately as that corresponding to 3% or Vr Thus, much of the apparent structure in the 72÷3 yield curve could be attributable to experimental error; only the gross features can be considered as established. The 23Na(p, ~2)24Mg reaction was previously studied by Gove 8). On the average, the 3'o/Vl ratio found by Gove was considerably larger than was obtained here. We believe the present results to be the more reliable since in the work of Gove, who used smaller NaI(TI) crystals, Vo and "~'1 were scarcely resolved. After allowing for the thickness (100 keY to 10 MeV protons) of the targets in Gove's work, the structures observed in the two experiments are consistent. The giant resonance in 24Mg has also been studied by gamma-ray absorption 9), electron scattering l o) and through the (),, n) reaction ~1). The results of these investigations and those on Vo and ";1 reported here are compared in fig. 5. The present results have been converted to their ('t, Po) inverse by detailed balance and smeared with a 200 keV energy spread. All of the curves show the same gross structure, namely a broad grouping of strengths centered at about 20 MeV and a satellite grouping at about 17 MeV. For the giant resonance built upon the ground state, the situation is similar to that in 28Si, where 5) it was found that different channels exhibited the same

2SNa (p,7)2¢Mg R E A C T I O N

687

intermediate structure. However, in 28Si there was no correlation between the "tl a n d the 7o intermediate structule. O n the other hand, in Z°Ne it was f o u n d 4) that all of the structure in the ~'0 and ~1 yield curves was strongly correlated. It is n o t presently a p p a r e n t what determines the degree of correlation between the structure in the 70 and y~ yield curves. I

I

I

I A

I

I

I

I

[

I

I I I 20 22 ENERGY,MeV

I 24

I

o J w

i

>-

J14

I

I I P__I 16 18 Mg 24 E X C I T A T I O N

Fig. 5. Giant resonance in ~ M g as observed in various reactions. The (7o, Po) and (~'l, Po) curves are

from the present data, converted to the inverse reaction by use of detailed balance and smoothed over 200 keV. The 0', n) data are from ref. 1~),the gamma-ray absorption from ref. 10) and the electron scattering from ref. it).

688

R.C.

BEARSE

et al.

The fine structure was analysed in terms of Ericson fluctuations a2). The correlation function used in this analysis is

R(e)-

AE ~ a(E)-(a)a(E+e)-(a) E z - E 1 ~=~, (a) (~)

(I)

'

where AE is the energy step size. To remove the possible effect of variations in ( a ) , the correlation functions were computed t w i c e - o n c e using the raw data and once using the data after smearing with an energy interval of 1.5 MeV. The two correlation functions were then subtracted, and the resultant was considered to be the correlation function of interest 4). The 70 autocorrelation (fig. 6) showed a mean square deviation R(0) = 0.53 falling to half of this value at about 8 = 75 keV. The fact that the mean ~quare deviation found here is considerably greater than the value 0.07 obtained in the analysis of the 27Ai(p, y) data 5) thus reflects the suppression of the direct-interaction

0.4 0.6

'

'

'

l

~

'

~

'

I

'

r,

~. o.z

0

I000

500

E (keY] Fig. 6. Auto¢orrelation function of~0.

component in 23Na(p, Yo). Ericson t2) has shown that

R(o)

=

1 (1 _y2),

(2)

where N is the number of degrees of freedom. It has previously been shown 5) that for ~'o at 90 °, N must be between 1 and 2 (N need not be an integer) with the nearly constant angular distributions implying that N ~ 1. For N = 1, eq. (2) yields YD = 70 per cent. This is most likely an overestimate; a 50 ~ direct-interaction component is likely to be closer to the correct value. The Yo autocorrelation indicates that about half of the Yo yield comes through the compound nucleus. From this it follows that approximately 1.5 ~ of the ?o giant resonance leads to ground-state protons that have been emitted by the 2*Mg compound nucleus. The decay of these long-lived states will depend principally on extranuclear factors, chiefly barrier penetrabilities. In order to get an estimate of the total fraction of the dipole resonance that proceeds through the compound nucleus, one can make the approximation that all proton channels with Ep > 3 MeV and all energetically allowed neutron channels are to be considered as open. If the 1.5 ~ of the dipole sum

=SNa(p, 7)S4Mg

689

REACTION

found for the strength of the compound-nucleus contribution to the ground-state proton is taken as representing the compound-nucleus contribution in each open channel, it follows that about 25 ~ of the giant dipole resonance in 2 4 M g proceeds through the long-lived states of the compound nucleus. The e for which R(e) = ½R(0) was shown by Ericson 12) to be just equal to the width F of the levels of the compound nucleus. In the present case, the width is expected to be near to the mean between the F = 70 keV extracted from the 27Al(p,7) data 5) and the F = 180 keV from the 1917(p, 7) data 4) but the F = 75 keV found here is somewhat less than this. That this discrepancy is due to the strong sharp resonances in the 5-6 MeV region is cvident from the fact that F = 100 keV was obtained when only the data above 6.5 MeV were used in the analysis. An analysis by Temmer 13) of 23Na(p, ct) data 14) over the range from 8.0 to 12.1 MeV yielded /- = i15 keV, which is in satisfactory agreement with the present results. The 7o autocorrelation does not seem to follow the expected Lorentzian shape but rather shows some evidence for a broader component in the yield curve. This perhaps indicates the presence of intermediate structure. 0.6

,

,

,

,

I

'

I

0.4

r,

r--l,,-,rl,,

~-~

0.4

0.2 o.~

u A -0.~ 0

500

I

I

[

f

I

I

I

I

I

I

,

l

IO0O

500

Iooo

e(keV)

~(keV)

Fig. 7. Autocorrelation function of 71.

Fig. 8. Cross-correlation function between Y0 and ~'l.

Two widths are most apparent in the 7x autocorrelation (fig. 7). Here the mean square deviation is 0.12 with components of F = 75 keV and F = 450 keV making approximately equal contributions. Again, when only the upper part of the region is considered, a somewhat larger width is obtained for the fine-structure component. The width for the intermediate structure is similar to that found in the 27Ai(p, 7) data 5). For the cross correlation between 7o and Yt (fig. 8), the fractional correlation was found to be C'(0) = Cror,(0) = 0.33,

~/n,o(O)&,(O)

which is somewhat greater than the cross correlation found 5) for 2 7 A l ( p , 7) but significantly less than that found 4) for 19F(p, 7)- The cross-correlation function computed here for the 2 3Na(p ' 7) data indicates that about half of the cross correlation is in the fine-structure component.

690

R.c. BEARSEet al.

In order to check that the intermediate structure found is of physical significance and not the result of some quirk in the analysis, the Z3Na(p ' ~) data reported by Warsh et al. 14) were run through the same analysis. Only the fine-structure component was present in the resultant autocorrelation. This result supports the conclusion that the intermediate width that appears in the 23Na(p ' 7) autocorrelations represents a real physical process. It has previously been noted v) that the intermediate width that sometimes appears in radiative-capture studies of the giant resonance probably reflects the presence of the doorway-state structure proposed by Rodberg, Kerman and Young t s). The sum total of detailed nuclear reaction data indicates that typically the doorway states in a compound nucleus overlap and thus produce a complicated pattern in which the characteristics of the doorway states are difficult to discern. If, however, there is a reaction in which the incident partial waves that can contribute are severely restricted, then only a few doorway states will be involved and they may be prominent. Since the giant dipole resonance has been shown to be dominated by a single configuration 7), only the few doorway states that have a strong overlap with this configuration will be excited. Another place where intermediate structure has been observed 16) is in the scattering of neutrons whose energy is about 1 MeV. The low energy assures that only the waves of very low angular m o m e n t u m will participate; and this, in turn, allows only a small fraction of the doorway states in the energy region to participate. Even in a case in which a number of partial waves are contributing, if one can be singled out this has the effect of observing only a small number of the many intermediate-structure states that may be present. This technique has led to the observation of intermediate structure in alpha-particle scattering 17) where the behavior of the various partial waves was determined by a phase-shift analysis. Finally, we note that where many partial waves can take part, as in the extensive (p, ~) studies that have been made in lighter nuclei 13), there is no evidence of intermediate structure.

3. Angular distributions Angular distributions of To and 71 were taken at 100 keV intervals between 5.0 and 6.7 MeV, 7.1 and 7.9 MeV and 9.0 and 10.7 MeV. At each energy, one crystal was used at 30 °, 90 ° and 149 ° and the other at 60 °, 90 ° and 120 ° . In order to check for systematic error at several energies, data were taken at nine angles with each crystal. The angular distributions taken between 9.0 and 10.7 MeV are shown in fig. 9. As in the other cases that have been studied 2,4-6), for both ?o and 71 the shapes of the distributions change only slightly as the bombarding energy is varied. Figs. 10 and 11 show the coefficients of the Lcgendre polynomial expansion of the angular distributions as functions of energy. Even though many of the 7o angular distributions do appear to show a positive a 4 term in the 5-6.7 MeV range, the more complete data (i.e. those taken at nine angles with each crystal) do not show this term - except perhaps at 6.5 MeV where the ?o yield is especially small. It is therefore concluded that

691

~3Na(p. 7)t~Mg R£ACTION

a4 is usually near zero, and that the observed deviations are attributable to experimental error; the error can be expected to decrease with decreasing polynomial order. Since in the giant-resonance region the quadrupole intensity should be very small c o m p a r e d to the dipole, a negligibly small a,t is expected. Indeed, for )'l, one sees that a4 is close to zero throughout. In general, the coefficients of the ~ angular distributions vary less with energy than those for ~o. No

23

2q

(p,T)

Mg

r, 1 1 1 1 1

M e V l- ' I 'r 10.7

, r ~.

L_

no.6

t

[- ~

L

10.4 -

-I

'

~

-~

,0.3 L ~ ~ '

I

Io.2 - ~ - " - ~

-

,°.,

IO.O ~- ~

.2

9.9

,,--"--'~"~ -1

9.8

,,--*---*~ ~

~.~ - ~ _ !

~.~ . - . - ~ ! 9.4

9.2 9.1 9.0 60'

120" 0

I

I

I

60"

I

I/

120" 0

Fig. 9. Some angular distributions ofT0 and 7~.

For the (p, Yo) reaction, a~ is usually negative; its average value is - 0 . 0 6 . The experimental errors are small enough so that the deviation f r o m zero can be considered as significant. Thus, for 70 the yield backward of 90 ° is, on the average, greater than

I

::L

J

I,,

o-

g

4I

I

I

I

I

I

No23( p, TO) Mg z4 •

•

I I

2

I

•

o• •

I

W(0) =AO [I + ~ ° . P . (C0S O)]

I

I

II

!

l

i

•

I !

•

•

•

leo

•eoO•eeoe•••

•

•

o.s"

0 -0.5

.

:.e

o mole•

o.5'o

•

loee

~eoe6oeooeioeo

ex

•

el

•

•

e;ow-

• •

• I

;

e°eo

°e.e

:-e

• °

• • ° •

. g

•

•

•

•

-

•

oe

Oll°.°

•

-0.5

lx.,;

0

e ° v"

•.

el x

°ix • x

eo

•

. Ol! •

• .

ol

• . •

•

•

• .

•

• •

-0.5I 9leo°lille

_• • 4.0

e

•

•

xe

•

•

90

oe

oal

•

ell

•

•

•

e

• el

•

I

I

I

I

t

I

5.0

6.0

7.0

8.0

9.0

I 0.0

•

.

eel

*

I

I 1.0

Ep(MeV)

Fig. 10. Coefficients obtained in fitting series of Legendre polynomials to the angular distributions for =SNa(P,70)~;Mg. The data were taken with 100 keV resolution. l

I

I

I

Z3 NO

t

I

(p,)'U)

Mg

w(0):.o ['* ~ °.~. (~o~ 0)] 8O

;

244("

.

oe

•

• o

•

•

A

.~

eo

40

• •

•

•

•

•

•

•

ll;

~

IA;

°o,

ml °

•

ooe

•

OO Ot Oo

0 0.5

o-

0

t~t41°o°oi=m;l::

O~lix

x ~o

._e

oeoi

i1.

go

oOoooo.

o°

w"

e•

051 o

-0.5

~"

-o.5[4.0

.i,.~ •

•

Ig x l m ..

I 5.0

•

-

• -;;-

•

= -

oo

w;

•

•

t • e ° • °.6.• •

i 6.0

•

oe

-0.5

~

• m. °o o.

ooo

oil

I 7.0

•

46o

eli

o• o •

all

•

•

I 8.0 El) I M e V l

9

-

I 9.0

• •

OoeO ;

•go

--

_.

--

_.

. . . .

--e•-•-

I I0.0

t

•

•

-T

•

|

I II.O

7

Fig. 11. Coefficients obtained in fitting series of Legendre polynomials to the angular distributions for ffi3Na(p,)q)"*Mg. The data were taken with 100 keV resolution.

23Na (p, 7)24Mg RFACTION

693

that forward. In all other cases ~-6), including that of 7~ here, a so-called forward peaking has been found (i.e. yield forward of 90 ° is the greater). Since the El-E2 interference does not contribute to a 2, and since the E2 intensity can be assumed to be negligibly small compared with the El, az can be calculated on the basis that the radiation is pure El. For the simpler case of~o, for which only 1 states can contribute to the giant resonance, a 2 is determined solely by the mixture of partial waves in the incident-proton channel. This mixture is not uniquely determined by the measurement of a2, however, since there are three possible incoming waves, and therefore two relative amplitudes - and these cannot be fixed by the measurement of one number. Fixing a2 does, however, lead to the requirement that the amplitudes for the partial waves lie on certain lines 2) and, in the present case, the measured average value ~2 = 0.33 for ~o requires that 30-100 ~ of the capture is p-wave. We note that Z7Al(p, "~'o)appeared to proceed mainly by p-wave capture 5); for ~9F(p, )%) the z + target rendered p-wave the only possible capture mode.

4. Conclusions

A major result of the present study is that the most striking feature of the (p, y) reaction in the giant-resonance region, the near constancy ot the gamma-ray angular distributions, is preserved even when the direct-reaction component is suppressed. The conjecture 7) that the giant resonance is dominated by a single configuration is thus strengthened. The (Yo) giant resonance in 24Mg has been described in terms of rotational states [refs. a. is)]. States with angular momentum J = 1 - can be members of either K = 0 or K = 1 bands. The calculations of Bassichis and Scheck ,8) predict that the K = 0 states will, on the average, lie at lower energies and will be primarily of configuration (s, d ) - ' (p, f). The K = 1 states are expected to primarily represent promotion of an inner nucleon to the valence shell, i.e. to be ( p ) - l ( s d ) . In this picture, then, the (p, "F) reaction would be expected to proceed mainly through the K = 0 states and, indeed, a case can be made s) for there being some qualitative agreement between the observed and the predicted structure. However, the near constancy of the angular distributions do not seem to follow naturally from this theoretical picture. Furthermore, the 22.14 MeV state predicted to contain over half of the dipole strength is missing in both the electron-scattering data ~o) and the (y, n) data 1~); its absence must be taken as strong evidence that the theory is inadequate. In summary, then, the 23Na(p, 7) study reported here buttresses the picture developed in previously reported 1-7) radiative-capture studies. The small integrated cross section and large mean square deviation for Yo are attributable to a suppression of the direct-interaction component which, in other cases, plays the dominant role. This inhibition of the direct-interaction component is expected on the basis of the shell model. Nevertheless, even when the direct-reaction component is suppressed, a single configuration appears to dominate the giant resonance, and it still remains un-

694

R.C. BEARSEet al.

c l e a r h o w t h e s e o b s e r v a t i o n s c a n b e fitted i n t o t h e a c c e p t e d a n d o t h e r w i s e s a t i s f a c t o r y particle-hole picture of the giant dipole resonance.

References 1) 2) 3) 4) 5) 6) 7) 8) 9) 10) 11) 12) 13) 14) 15) 16) 17) 18)

D. S. Gemmell and G. A. Jones, Nucl. Phys. 33 (1962) 102 R. G. Alias, S. S. Hanna, L. Meyer-Schiitzmeister and R. E. Segel, Nucl. Phys. 58 (1964) 122 N. W. Tanner, G. C. Thomas and E. D. Earle, Nucl. Phys. 52 (1964) 29 R. E. Segel, Z. Vager, L. Meyer-Schiitzmeister, P. P. Singh and R. G. Allas, Nucl. Phys. 93 (1967) 31 P. P. Singh, R. E. Segel, L. Meyer-Schiitzmeister, S. S. Hanna and R. G. Alias, Nucl. Phys, 65 (1965) 577 G. Dearnaley, D. S. Gemmell, B. W. Hooton and G. A. Jones, Nucl. Phys. 64 0965) 177 R. G. Allas, S. S. Hanna, L. Meyer-SchiJtzmeister, R. E. Segel, P. P. Singh and Z. Vager, Phys. Rev. Lett. 13 (1964) 628 H. E. Gove, Nucl. Phys. 49 (1963) 279 J. M. Wyckoff, B. Ziegler, H. W. Koch and R. Uhlig, Phys. Rev. 137 0965) B576 O. Titze, E. Spamer and A. Goldman, Phys. Lett. 24B (1967) 169 S. C. Fultz, J. T. Caldwell, B. L. Berman, R. R. Harvey and M. Kelly, UCRL-14360 T. Ericson, Ann. of Phys. 23 (1963) 390 G. M. Temmer, Phys. Rev. Lett. 12 (1964) 330 K. L. Warsh, G. M. Temmer and H. R. Blieden, Nucl. Phys. 46 (1963) 45 A. K. Kerman, L. A. Rodberg and J. E. Young, Phys. Rev. Lett. 11 (1963) 422 A. J. Elwyn, J. E. Monahan, R. O. Lane and A. Langsdorf, Jr., Nucl. Phys. 59 (1964) 113 P. P. Singh, B. A. Watson, J. J. Kroepfl and T. P. Marvin, Phys. Rev. Lett. 17 (1966) 968 W. H. Bassichis and F. Scheck, Phys. Rev. 145 (1966) 771