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The nonelastic projectile break-up cross section from particle-gamma coincidence measurements for the 6li + 40ca reaction at 156 mev
Nuclear Physics A448 (1986) 110-122 ONorth-Holland Publishing Company
THE NONELASTIC PROJECTILE BREAK-UP CROSS SECTION FROM PARTICLE-GAMMA COINCIDENCE MEASUREMENTS FOR THE ‘Li + “Ca REACTION AT 156 MeV R. PtANETAa,
H. KLEWE-NEBENIUSb, J. BUSCHMANN, H.J. GILS, H. REBEL and S. ZAGROMSKI
Kernforschungszentrum Karlsruhe, Institut ftir Kernphysik III, POB 3640, D-7500 Karlsruhe, Federal Republic of Germany T. KOZIK’, L. FREINDL and K. GROTOWSKI Institute of Physics, Jagellonian University and Institute of Nuclear Physics, Cracow, Poland
Received 1 December 1983 (Revised 18 July 1985) ‘he cross sections for nonelastic break-up modes have been studied for the 6Li +““Ca reaction at ELi = 156 MeV. Gamma-ray spectra from target-like residual nuclei were measured in coincidence with beam-velocity projectile fragments and a value for the nonelastic break-up cross section u&,“,= 582 i 110 mb has been found. Together with results of inclusive charged-particle measure-
Abstract:
ments we infer for the total break-up cross section u,$ = 930 I 115 mb comprising about 50% of the total reaction cross section. The nonelastic contribution of the break-up reaction appears to be less than predicted by the DWBA break-up theory. This result is directly evident from the differential cross sections by comparing inclusive and exclusive results.
4oCa(6Li, ay), (6Li,3He NUCLEAR REACTIONS 156 MeV; measured coincidence o(e) versus particle
E
y), (6Li, ty), (6Li,dy), (6Li,py), E= and y-ray energy. Nonelastic break-up
processes.
Introduction
1.
The continuum composite corresponding These fragments
part
particles
of inclusive
often exhibits
to the beam
bumps
are
velocity
readily
2). This phenomenon
reactions.
a b c
In fact, by a series
spectra
from
bumps
centered
and dominating
ascribed
to
the
due
to the
light-ion
of
reactions
around
transitional
projectile
cross
Permanent address: Institute of Physics, Jagellonian University, Cracow, Poland. Kemforschungszentrum Karlsruhe, Institut ftir Radiochemie. Present address: Kemforschungszentrum Karlsruhe, Institut ftir Kemphysik. 110
into
its like
of ‘Li-induced
features
of the inclusive
angles ‘).
projectiles
character with
with
the energies
at forward
the
for loosely-bound
phenomena
of measurements3’4)
nuclear
the spectra
break-up
is very pronounced
6Li > which is of special interest reactions, linking typical
nuclear
particle
bell-shaped
of heavy-ion sections
for
R. Pianeta et al. / Projective~~gQ~-u~
111
the emission of light charged particles when bombard~g various targets by 156 MeV 6Li ions, la r ge y’lelds of light particles have been found. While in e.g. the 6Li + 40Ca case the complete fusion exhausts only 3-48 of the total reaction cross section5), the 6Li break-up channels provide much larger cross sections. Some more exclusive studies of the break up of 6*7Li reveal the importance of nonelastic break-up processes, where one of the fragments interacts inelastically with the target. In particular, a break-up fusion-type process, in which one of the break-up fragments is absorbed by the target nucleus, has been shown to play an important role6-*). This nonelastic break-up mode appears to be strongly related to the incomplete-fusion reactions of complex nuclei which have been shown to contribute increasingly with increasing bomb~ding energy. One may suspect that at least part of the cross section attributed to complete fusion in a number of experiments belongs in fact to partial fusion, since the distinction between evaporation residues originating from these two reaction mechanisms is not easy. The DWBA analysis4) of the inclusive charged-particle spectra observed for 6Li + 40Ca reactions at ELi = 156 MeV predicts large contributions from the nonelastic break-up processes. The present work explores the nonelastic break-up modes in more detail by measuring the y-ray spectra from the heavy residual nuclei in coincidence with light ejectiles (in particular, beam-velocity particles) from bombardment of 40Ca by 156 MeV 6Li ions. The measurements aimed at a determination of the (6Li, cuy), (6Li, 3Hey), (6Li, ty), (6Li,dy) and (6Li,py) cont~butions to clarify more details of the mechanism for the emission of light particles in 6Li-nucleus collisions. As far as nonelastic break-up processes are signalled by y-ray emission from target-like reaction products, the results may provide a serious check of theoretical predictions based on the DWBA break-up theory2). 2. Experiments The 156 MeV 6Li beam of the Karlsruhe Isochronous cyclotron was focussed onto a 14 + 0.15 mg/cm* thick Ca target. The light ejectiles were identified by a Si telescope consisting of a 0.15 mm thick AE detector and two 4 mm thick E-detectors (trans~ssion mounted). The telescope was cooled down to -40 “C and placed = 5O, 9* and 20”, respectively. In order to reduce pile-up as well as random at @r, coincidences, the counting rates were limited to about 5000 counts/s by properly chosen tantalum diaphragms corresponding to solid angles of 0.31, 0.90 and 3.98 msr, respectively. The y-rays were detected with a high-purity Ge detector (resolution: 1.9 keV; efficiency; 16% for the 1332 keV y-rays from 6oCo) placed at 90” with respect to the beam direction at a distance of 6 cm from the target. Both detectors were placed in the same reaction plane. Under these conditions, the y-counting rate was of the same order as the particle-counting rate. A Cu-Cd-Pb absorber mounted in front of the y-detector served to reduce the low-energy part of the y-spectra. The energy and absolute efficiency calibration of the y-branch were performed using a set of
112
R. Piuneta et af. / Project& break-up
standard sources. In order to reduce the prompt y-background originating from nuclear reactions induced anywhere in the neighborhood, the iilterior wall of the scattering chamber was covered with 1 cm polycarbonate, and the carefully shielded Faraday cup was placed at a distance of more than 3 m away. Moreover, a hardware window was set on the prompt peak of a time-to-amplitude converter (TAC) which was started by the y-pulses and stopped by the (properly delayed) high-frequency (HF) signals (33 MHz) of the cyclotron. Thereby, nonprompt y-rays were suppressed. The electronic set-up consisted of conventional NIM modules. The four signals AE, E, TAC( E,-AE), and TAC(E,-HF) generated, if coincident within 100 ns, a gate pulse which enabled a multi-ch~nel analyser to record the digitized A E, E, E,, and TAC(E,-AE) amplitudes event by event on magnetic tape. Besides the “prompt” peak the TAC(E,-AE) spectrum showed a series of “delayed” peaks, nearly equal in height and separated by 33 ns, originating from y-rays and ejectiles produced in different bursts of the cyclotron beam (HF = 33 MHz). These delayed peaks served to evaluate the contribution of random coincidences to the prompt peak as explained in the next section. In addition to the coincidence measurements, inclusive particle spectra (without coincidence requirements) were measured under otherwise identical experimental conditions. Target impurities, in particular oxygen and carbon, were controlled by measuring elastic scattering and are estimated to cont~bute less than 2%.
3. Evaluation of the cross sections and experimentai results From the list-mode data, y-ray spectra correlated with the emission of either LY,
1 p articles have been generated. Due to the insufficient thickness of the 3HeorZ= E-detector of the particle telescope it was not possible to distinguish the 2 = 1 particles (p, d and t). Fig. la displays a y-ray spectrum measured in coincidence with at-particles emitted at &, = 5”, fig. lb shows a y-ray spectrum in coincidence with 3He emission for 8,, = 9”. To correct for random coincidence the spectra were obtained as the difference of the y-ray spectrum correlated with the “true plus random” TAC( ET-A E) peak and that correlated with the “random” TAC( E,-A E) peak. Both peaks are well visible in the TAC spectrum. After the energy calibration and the determination of the peak intensities, all visible y-lines could be assigned to known nuclear transitions9’5) given in standard compilationsl’). In the coincidence spectra altogether 20 y-ray lines were found and identified. Comparing with the inclusive y-ray spectra measured in ref. 5, we notice that the coincident y-ray lines are the most intensive lines seen in the inclusive spectra. Their corresponding intensity exhausts - 65-758 of the total inclusive y-ray intensity. In the spectrum shown in fig. la, 15 y-ray lines are observed related to reaction products with 17 < Z I 20 (in addition to 27A1and 76Ge lines). Statistical-model ~~culations5}
/
c
-I
6. 8
:;C,, 271512d
COUNTS
R. Pfaneta et al. / Projectile break-up
114
160.
%a
(6Li,3Heyb
eLob=
0
1000
1600
1mo CHANNEL
Fig. lb.
y-ray spectrum
with coincident
90
1600
2m
NUMBER
emission of 3 He particles MeV 6Li ions.
from the bombardment
of 40Ca by 156
error of the correction for the random coincidences. The absolute cross sections may additionally be affected by some systematic errors which are considered later on. Fig. 2 compares the inclusive cx-particle spectra at various emission angles with the corresponding spectra measured in coincidence with all observed y-ray peaks, corrected for the detection efficiency of the y-ray detector. Contributions by the y-ray continua adjacent to the lines have been subtracted. At least two different components can be identified in the inclusive and exclusive particle spectra: (i) the bump centered around the beam velocity energy and ascribed to a-particles from the and pre~uilib~um emission projectile break-up, and (ii) an evaporation component 3,4) (including in this term particles from rather direct knock-out contributions). Table 2 compiles the inciusive and exclusive cross sections for coincident light-particle emission integrated over different energy ranges of the particle spectra.
R. Planeta et al. / Projectile break-up
115
TABLE 1 Coincidence cross sections for different product nuclei d omin/ds2 [mb/sr] a E = 18-160 MeV
%c @Ca “K ‘9K ‘SK =Ar “Ar ‘6Ar ‘%I 36Cl 35Cl ‘5s
50
9”
31 + 31 182+54 31* 31 172+53
6 f 10 147+42 6 + 10 116+35 46+ 19 74f26 42k 18 62i 23 11 f 10
136 + 44 35 * 21 93 * 34 61+45 39 + 26 16 k 16
In order nonelastic librium”
16 + 19 56 + 24 15 + 15
E = 65-160 MeV
200
lo+4 11*4 4+2 10+4 8k3 13 k4 l&l 2+2 9+4
p+d+t
)He
50
90
29 f 29 178+52 29 f 29 172+53
4+10 147i_42 4* 10 110+33 37+16 63+22 31*14 45+18 15 + 14 10*17 48 i 21 12+12
121+40 25 + 18 76 k 30 54 f 40 39 + 26 21* 21
E = lo-160 MeV
20”
9+3 9+3 2kl 7k2 2+1 4+2 212 2k 2 1*1
5”
9”
3+5 11&7 3i5 4*7
10 + 10 14 k 8 lo*10 22 i 9
16&10
26210
19+10
11+7 lk3 12* 7
2k3
5i5
E = lo-160 MeV
20”
lkl 3il l&l 3+1 l+l 1 kl 2+1
5”
9”
200
4+5 33+17 4*5 35i18
1+5 71+24 1*5 83+27 30* 15 64+23 14+12 54i21 19 + 15 26k15 37 + 19 7+7
lot10 914 10 f 10 21 k6 11*4 26 f 8 17 k 6 18 + 6 5+5 7+4 7+4
50*21 29+15 36k19 28 k 23 23+15 11* 11
to extract cross sections which are representative for the inclusive and break up components, the “background” from evaporation and “preequiemission has been approximated by a linear tail below a symmetric
gaussian-shaped break-up bump [see ref. 3)]. Figs. 3 and 4 display the angular dependence of the inclusive and exclusive cross sections. In addition to the a-particle yield of the break-up component, the total yield from all components in the spectra is shown in fig. 3. In the Z = 1 case, however, it is not possible to disentangle the break-up and the high-energy tail of preequilibrium processes. The nonelastic break-up contributions found by ((Y,y) coincidence measurements comprise (20 k lo)!%, (54 &-28)%, and (33 + 20)% of the break-up contribution seen in a-particle spectra at 5”, 9” and 20”, respectively. A similar situation is found for the Z = 1 particles. The yield of (Y, 3Heand Z= 1 p articles at very small angles may generally be affected by target contaminations (C and 0). However, 0 and C contaminations of the target are less than 2% (target weight) and the 6Li break-up cross section for C and 0 is significantly smaller than that for Ca at the same angle of observation3). For the exclusive cross section, the coincidence with y-lines from 40Ca-like reaction products suppresses the influence of contaminations of light nuclei. In order to estimate the angle-integrated cross sections we assumed an exponential angular dependence3) fitted to the three values available by the measurements. In the case of the nonelastic a-particle contribution we introduced an alternative parametrization (see fig. 3) in order to account for the smaller ratio of exclusive to inclusive cross sections at very forward angles. But the values for the integrated cross
116
R. Planeta et al. / Projectile break-up
2.5
sections (given in table 3, in figs. 3 and 4 for all eases> are not significantly affected by the particular choice of the p~ame~at~on_
4. Discussion The analyses 4> of the inclusive charged-particle spectra on the basis of the DWBA break-up theory ‘*14) attribute the major part (- 80%) of the inclusive break-up component to the nonelastic break-up mode. Even admitting large statistical uncettainties of the results of the coincidence measurements, the spectra shown in fig. 2
R. Planeta et al. / Projectile break-up
117
TABLE2
Inclusive and exclusive cross sections in mb/sr for emission of light particles in the ‘Li + ““Ca reaction at different emission angles
Ejectile
a inclwive
a exclusive
fiew integration
0 lab 5”
go
20”
18-160 MeV total break-up (background subtracted)
5552 & 1110
1609 I 322
316 f 63
4963 rt 1092
1245 + 273
81 f 18
18-160 MeV nonelastic break-up (background subtracted)
1054 _t 279
791* 212
87 + 25
969 + 287
678 + 199
27 f 20
725 & 145
266 + 53
66k 13
153 f 38
19i7
3He inclusive
lo-160 MeV
3He exclusive
lo-160 MeV
p+d+t inclusive
lo-160 MeV
5435 + 1087
2798 + 560
1184 k 237
p+d+t exclusive
lo-160 MeV
370 f 121
550 rt 153
212 + 53
89+33
to contradict the theoretical expectations as far as nonelastic contributions are signalled by y-ray transitions induced by nonelastic interactions of one of the fragments with the target nucleus. The discrepancy is especially drastic for the 3He appear
and t inclusive spectra although it is also noticed for the a-particles. We have to look for effects which might have reduced the yield of coincident y-rays. The analysis is based on the simplifying assumption of an isotropic y-ray emission. There are several indications li) support~g this assumption. The y-deexcitation occurs after evaporation of several light particles, removing most of the initial angular momentum in several independent steps and resulting in relatively large attenuation factors for the spin orientation5,9). In addition, effects of dipole and quadrupole transitions cancel mutually when summing up cross sections of y-lines with different multipolarities. In addition to the angular correlation effects, side-feeding parallel to the y-ray transitions in the decaying nuclei could systematically falsify the value of the
118
R. Pianeta et al. / Projectile break-up
Alpha - particle integrated Inclusive
component
from 65 - 160 MeV
break up part
+ without o with background subtraction Exclusive
break up part
x without l with background subtraction
loo-
Parametrization /[%].9”,, 10
1
I I
I
I
0
5
10
15
Fig. 3. Angular
distribution
20
= lL11.2 e-328,2,b
\ %_ab
of the inclusive and exclusive cross sections the 6 Li + 4o Ca reaction at 156 MeV.
[degl
for the emission
of cr-particles
in
nonelastic break-up cross section. Results of inclusive y-ray spectroscopy experiments5,9) in the nuclidic region around Ca suggest that the side-feeding correction may be between zero and 30% depending on the entrance reaction channel and consequently on the entry region. Even with these systematic uncertainties inherent to the applied method the differences between experimental and theoretical values of the nonelastic break-up cross sections seem to be significant. Table 3 compiles the integrated cross sections experimentally known for various reaction modes in the 6Li + 40Ca interaction at 156 MeV. The elastic break-up cross sections are just the difference between the inclusive break-up cross sections and the nonelastic break up cross sections in the a-particle and 3He channels, anticipating that the elastic break-up primarily proceeds by an (Y+ d and a t + 3He break-up, respectively, and that the elastic break-up path 6Li + (Y+ p + n is already included in the observed a-particle yield. Summing up elastic and nonelastic break-up cross
R. Pianeta et al. / Projectile break-up
119
Z = 1 and 3He components -+--lp+d+tlincl.
;]
-0
3He
inch.
3He excl.
1000
100
\ \o
\
\
La=
f 10
T
Fig. 4. Angular
1
I
I
I
5
10
15
20
distribution
= 107 mb
31mb
%,b[deg]
of the inclusive and exclusive cross sections for the emission ‘He ejectiles in the 6Li +@Ca reaction at 156 MeV.
of Z = 1 and
we find that the total break-up cross section updtu.= 930 k 115 mbt comprises - 50% of the total reaction cross section. Using the measured inclusive cross sections4) [tabulated in ref. 16)] for the 6Li + 40Ca reaction at 156 MeV we are able sections
to consider the contributions (“background”) in the charged-particle spectra which are presumably not related with the emission of beam-velocity particles. In addition to the equilibrium and preequilibrium emission an additional broad bump at lower energies
has been observed
in the triton
spectra4).
The contribution
of this compo-
nent is included in the value of the inclusive break-up cross section a(t*) = 280 + 56 mb adopted from ref.4) (see table 3). Only - 80 mb ( f 25%) are attributed to the spectator break-up component, while the rest is ascribed to the other components. Summing up these “background” contributions and accounting for a neutron part equal to the proton part, we get uback = 1200 mb, which is a production cross section. In order to get the reaction cross section from light “background” ejectiles, we have to divide this value by the particle multiplicity factor (M). For a-particles we + This result includes the triton channel equal to the 3He channel as suggested by an improved analysis of the inclusive triton spectra. This is additionally supported by some exploratory measurements of eLi, t) coincidences using a A E-E particle telescope with a 15 mm thick high-purity Ge detector 15)
R. Planeta et al. / Projectile break-up
120
TABLE3 Integrated experimental cross sections for the 6Li +40Ca reaction at 156 MeV Reaction channel
Cross section [mb]
Ref.
total reaction cross section
1980 + 200
12 1
complete fusion
67 i 20
*)
320 f 64 360 + 72 280 + 56
4,
80 f 16 440-188 453 * 90
this work
3Hea)
346 + 138 140 + 56 31+ 12
this work this work this work
cross sections for non-break-up components in the ejectile spectrab): P d t ‘He 0
167 + 100 320 + 70 2OOf60 27 t 6 327 + 72
4,
inclusive break-up cross sections for emission of charged-projectile fragments: P d t”) 3He a
nonelastic break-up cross sections with emission of charged ejectiles: p+d+t*) a
quasi-symmetric binary splitting of the compound system
5.6 + 1.4
1;
:;
13
)
“) Without “background” subtraction. b, Energy integration was done starting at 10 MeV for 2 = 1 particles, and 18 MeV for 2 = 2 particles, respectively.
can roughly estimate it by comparing the inclusive and exclusive ‘“background” spectra. We obtain (M), = 2.8, which may be different in the other reaction channels. However, this un~rt~ty does not affect the calculation of the total reaction cross section. As far as the emission of irbackground” particles follows complete fusion and nonelastic break-up processes, the background contribution is contained in the
R. Planeta et al. / Projectiie break-up
121
sum of the complete fusion (u = 67 mb) and nonelastic break-up (ut;z = 582 mb) cross sections. Summin g up all known contributions (table 3) to the total reaction cross section, we exhaust only 50% of the total reaction cross section crR= 1980 mb. Therefore, there is room left for contributions of other reaction channels, e.g. from pick up reactions, as suggestedis) by the predictions of the Wilczynski “sum rule” model 17), in addition to continuum excitation by (6Li, 6Li’). 5. Con~~ing
remarks
In the present experiment we have measured the contribution to the projectile break-up associated with y-ray emission from nonelastic interactions of the nonspectator fragments with the target. For the case studied of the 6Li -t4Ca reaction at E,, = 156 MeV the contributions comprise a summed-up nonelastic break-up cross section of (I = 582 + 110 mb which should be compared with the value u = 930 f 115 mb of the total break-up cross section (as deduced from the data available for the case considered). The differential cross sections for the nonelastic break-up are found to be si~ficantly smaller than the values predicted4) by the DWBA break-up theory. This theory is essentially based on a spectator mechanism and ignores the sequential break-up mode which might considerably contribute to the elastic break-up modei8*i9), especially at forward angles. The trend of the exclusive break-up component seen in fig. 3 seems to indicate an increasing elastic break-up contribution at 19,~~ = 5” (i.e. below the grazing angle), where in fact the Coulomb interaction becomes important. It is the matter of more detailed particle-particle coincidence experiments to determine the elastic break-up cross section more directly and to look for processes in which spectator fragments are emitted in coincidence with knock-out particles, e.g. from nonelastic interactions of the nonspectator fragments. Such processes contribute to the background of light particles and are not necessarily accompanied by y-ray emission”) though being a nonelastic break-up process. We thank Prof. Dr. G. Schatz for his continuous interest and encouragement in this work. We are grateful to Dr. B. Neumann and Dr. S. Micek for substantial help and clarifying discussions, and Ing. M. Adamczyk and Dipl. Phys. H. Jelitto for technical assistance in the experiments. Some of the authors (R.P., T.K., L.F., K.G.) wish to express their gratitude for the kind hospitality in Institut fur Kemphysik of Kemforschungszentrum Karlsruhe. This work was supported by the Polish MNSWiT (grant no. MR.5.5) and the International Bureau of the Kernforschungszentrum Karlsruhe. References 1) A. Budzanowski, Proc. 3rd Adriatic Europhys. Conf. on dynamics of heavy ion collisions, Hvar, Yugoslavia, May 2%31,198X
122
R. Planeta et al. / Projectile break-up
2) G. Baur, F. R8se1, D. Trautmann and R. Shyam, Phys. Reports 111 (1984) 335 3) B. Neumann, H. Rebel, J. Buschmann, H.J. Gils, H. Klewe-Nebenius and S. Zagromski, Z. Phys. A296 (1980) 113 4) B. Neumann, H. Rebel, H.J. Gils, R. Planeta, J. Buschmann, H. Klewe-Nebenius, S. Zagromski, R. Shyam and H. Machner, Nucl. Phys. A382 (1982) 296 5) J. Brzychczyk, L. Freindl, K. Grotowski, Z. Majka, S. Micek, R. PIaneta, M. Albihska, J. Buschmann, H. Klewe-Nebenius, H. J. Gils, H. Rebel and S. Zagromski, Nucl. Phys. A417 (1984) 174 6) B. Neumann, J. Buschmann, H. Klewe-Nebenius, H. Rebel and H.J. Gils, Nucl. Phys. A329 (1979) 259 7) C.M. Castaneda, H.A. Smith, P.P. Singh and H. Karwowski, Phys. Rev. C21 (1980) 179 8) H. Utsunomiya, S. Kubono, M.H. Tanaka, M. Sugitani, M. Morita, T. Nomura and Y. Hamajima, Phys. Rev. C28 (1983) 1975 9) K. Grotowski, P. Belery, Th. Delbar, Y. el Masri, Gh. Gregoire, R. Janssens, J. Vervier, G. Paic, M. Albihska, J. Albihski, S. Kopta, T. Kozik and R. Planeta, Phys. Rev. C23 (1981) 2513 M.M. Albihska, Ph.D. thesis, Jagelloman University, Cracow (1981) 10) C.M. Lederer and V.S. Shirley, ed., Tables of isotopes, 7th ed. (Wiley, New York, 1978) 11) R.A. Dayras, R.G. Stokstad, D.C. Hensley, M.L. Halbert, D.G. Sarantites, L. Westerberg and J.H. Barker, Phys. Rev. C22 (1980) 1485 12) J. Cook, H.J. Gils, H. Rebel, H. Klewe-Nebenius and Z. Majka, Nucl. Phys. A388 (1982) 173 13) K. Grotowski, Z. Majka, R. PIaneta, M. Szczodrak, Y. Ghan, G. Guarino, L.G. Moretto, D.J. Morrisey, L.G. Sobotka, R.G. Stokstadt, I. Tserruya, S. Wald and G.J. Wozniak, Phys. Rev. C30 (1984) 1214 14) G. Baur, R. Shyam, F. R&e1 and D. Trautmann, Helv. Phys. Acta 53 (1980) 503 15) R. Planeta, H. Klewe-Nebemus, B. Neumann, J. Buschmann, H.J. Gils, H. Rebel, S. Zagromski, L. Freindl and K. Grotowski, KfK 3642 (1983); R. Planeta and B. Neumann, unpublished results (1982) 16) K. FeiBt, B. Neumann, J. Buschmann, H.J. Gils and H. Rebel, Energiespektren und Produktionswirkungsquerschnitte ftir leichte geladene Teilchen in 40Ca(6Li,x)-Kernreaktionen bei Eri = 156 MeV; unpublished results, Kemforschungszentrum Karlsruhe (1981) 17) J. Wilczyhski, K. Siwek-Wilnyhska, J. van Driel, S. Gonggrijp, D.C.J.M. Hagemann, R.V.F. Janssens, J. Lukasiak and R.H. Siemssen, Phys. Lett. 45 (1980) 606 18) J. Kleinfeller, J. Bisplinghoff, J. Ernst, T. Mayer-Kuckuk, G. Baur, B. Hoffmann, R. Shyam, F. R&e1 and D. Trautmann, Nucl. Phys. A370 (1981) 205 19) J. van Dreil, S. Gonggrijp, R.V.F. Janssens, R.H. Siemssen, K. Siwek-Wilczyhska and J. Wilczyhski, Phys. Lett. 98B (1981) 351 20) H.R. Schmidt, K.T. Kniipfle, G. Seegert, P. Grabmayr and G.J. Wagner, Proc. Workshop on coincident particle emission from continuum states in nuclei, Bad Honnef, June, 4-7, 1984, ed. H. Machner and P. Jahn (World Scientific, Singapore, 1984)
THE NONELASTIC PROJECTILE BREAK-UP CROSS SECTION FROM PARTICLE-GAMMA COINCIDENCE MEASUREMENTS FOR THE ‘Li + “Ca REACTION AT 156 MeV R. PtANETAa,
H. KLEWE-NEBENIUSb, J. BUSCHMANN, H.J. GILS, H. REBEL and S. ZAGROMSKI
Kernforschungszentrum Karlsruhe, Institut ftir Kernphysik III, POB 3640, D-7500 Karlsruhe, Federal Republic of Germany T. KOZIK’, L. FREINDL and K. GROTOWSKI Institute of Physics, Jagellonian University and Institute of Nuclear Physics, Cracow, Poland
Received 1 December 1983 (Revised 18 July 1985) ‘he cross sections for nonelastic break-up modes have been studied for the 6Li +““Ca reaction at ELi = 156 MeV. Gamma-ray spectra from target-like residual nuclei were measured in coincidence with beam-velocity projectile fragments and a value for the nonelastic break-up cross section u&,“,= 582 i 110 mb has been found. Together with results of inclusive charged-particle measure-
Abstract:
ments we infer for the total break-up cross section u,$ = 930 I 115 mb comprising about 50% of the total reaction cross section. The nonelastic contribution of the break-up reaction appears to be less than predicted by the DWBA break-up theory. This result is directly evident from the differential cross sections by comparing inclusive and exclusive results.
4oCa(6Li, ay), (6Li,3He NUCLEAR REACTIONS 156 MeV; measured coincidence o(e) versus particle
E
y), (6Li, ty), (6Li,dy), (6Li,py), E= and y-ray energy. Nonelastic break-up
processes.
Introduction
1.
The continuum composite corresponding These fragments
part
particles
of inclusive
often exhibits
to the beam
bumps
are
velocity
readily
2). This phenomenon
reactions.
a b c
In fact, by a series
spectra
from
bumps
centered
and dominating
ascribed
to
the
due
to the
light-ion
of
reactions
around
transitional
projectile
cross
Permanent address: Institute of Physics, Jagellonian University, Cracow, Poland. Kemforschungszentrum Karlsruhe, Institut ftir Radiochemie. Present address: Kemforschungszentrum Karlsruhe, Institut ftir Kemphysik. 110
into
its like
of ‘Li-induced
features
of the inclusive
angles ‘).
projectiles
character with
with
the energies
at forward
the
for loosely-bound
phenomena
of measurements3’4)
nuclear
the spectra
break-up
is very pronounced
6Li > which is of special interest reactions, linking typical
nuclear
particle
bell-shaped
of heavy-ion sections
for
R. Pianeta et al. / Projective~~gQ~-u~
111
the emission of light charged particles when bombard~g various targets by 156 MeV 6Li ions, la r ge y’lelds of light particles have been found. While in e.g. the 6Li + 40Ca case the complete fusion exhausts only 3-48 of the total reaction cross section5), the 6Li break-up channels provide much larger cross sections. Some more exclusive studies of the break up of 6*7Li reveal the importance of nonelastic break-up processes, where one of the fragments interacts inelastically with the target. In particular, a break-up fusion-type process, in which one of the break-up fragments is absorbed by the target nucleus, has been shown to play an important role6-*). This nonelastic break-up mode appears to be strongly related to the incomplete-fusion reactions of complex nuclei which have been shown to contribute increasingly with increasing bomb~ding energy. One may suspect that at least part of the cross section attributed to complete fusion in a number of experiments belongs in fact to partial fusion, since the distinction between evaporation residues originating from these two reaction mechanisms is not easy. The DWBA analysis4) of the inclusive charged-particle spectra observed for 6Li + 40Ca reactions at ELi = 156 MeV predicts large contributions from the nonelastic break-up processes. The present work explores the nonelastic break-up modes in more detail by measuring the y-ray spectra from the heavy residual nuclei in coincidence with light ejectiles (in particular, beam-velocity particles) from bombardment of 40Ca by 156 MeV 6Li ions. The measurements aimed at a determination of the (6Li, cuy), (6Li, 3Hey), (6Li, ty), (6Li,dy) and (6Li,py) cont~butions to clarify more details of the mechanism for the emission of light particles in 6Li-nucleus collisions. As far as nonelastic break-up processes are signalled by y-ray emission from target-like reaction products, the results may provide a serious check of theoretical predictions based on the DWBA break-up theory2). 2. Experiments The 156 MeV 6Li beam of the Karlsruhe Isochronous cyclotron was focussed onto a 14 + 0.15 mg/cm* thick Ca target. The light ejectiles were identified by a Si telescope consisting of a 0.15 mm thick AE detector and two 4 mm thick E-detectors (trans~ssion mounted). The telescope was cooled down to -40 “C and placed = 5O, 9* and 20”, respectively. In order to reduce pile-up as well as random at @r, coincidences, the counting rates were limited to about 5000 counts/s by properly chosen tantalum diaphragms corresponding to solid angles of 0.31, 0.90 and 3.98 msr, respectively. The y-rays were detected with a high-purity Ge detector (resolution: 1.9 keV; efficiency; 16% for the 1332 keV y-rays from 6oCo) placed at 90” with respect to the beam direction at a distance of 6 cm from the target. Both detectors were placed in the same reaction plane. Under these conditions, the y-counting rate was of the same order as the particle-counting rate. A Cu-Cd-Pb absorber mounted in front of the y-detector served to reduce the low-energy part of the y-spectra. The energy and absolute efficiency calibration of the y-branch were performed using a set of
112
R. Piuneta et af. / Project& break-up
standard sources. In order to reduce the prompt y-background originating from nuclear reactions induced anywhere in the neighborhood, the iilterior wall of the scattering chamber was covered with 1 cm polycarbonate, and the carefully shielded Faraday cup was placed at a distance of more than 3 m away. Moreover, a hardware window was set on the prompt peak of a time-to-amplitude converter (TAC) which was started by the y-pulses and stopped by the (properly delayed) high-frequency (HF) signals (33 MHz) of the cyclotron. Thereby, nonprompt y-rays were suppressed. The electronic set-up consisted of conventional NIM modules. The four signals AE, E, TAC( E,-AE), and TAC(E,-HF) generated, if coincident within 100 ns, a gate pulse which enabled a multi-ch~nel analyser to record the digitized A E, E, E,, and TAC(E,-AE) amplitudes event by event on magnetic tape. Besides the “prompt” peak the TAC(E,-AE) spectrum showed a series of “delayed” peaks, nearly equal in height and separated by 33 ns, originating from y-rays and ejectiles produced in different bursts of the cyclotron beam (HF = 33 MHz). These delayed peaks served to evaluate the contribution of random coincidences to the prompt peak as explained in the next section. In addition to the coincidence measurements, inclusive particle spectra (without coincidence requirements) were measured under otherwise identical experimental conditions. Target impurities, in particular oxygen and carbon, were controlled by measuring elastic scattering and are estimated to cont~bute less than 2%.
3. Evaluation of the cross sections and experimentai results From the list-mode data, y-ray spectra correlated with the emission of either LY,
1 p articles have been generated. Due to the insufficient thickness of the 3HeorZ= E-detector of the particle telescope it was not possible to distinguish the 2 = 1 particles (p, d and t). Fig. la displays a y-ray spectrum measured in coincidence with at-particles emitted at &, = 5”, fig. lb shows a y-ray spectrum in coincidence with 3He emission for 8,, = 9”. To correct for random coincidence the spectra were obtained as the difference of the y-ray spectrum correlated with the “true plus random” TAC( ET-A E) peak and that correlated with the “random” TAC( E,-A E) peak. Both peaks are well visible in the TAC spectrum. After the energy calibration and the determination of the peak intensities, all visible y-lines could be assigned to known nuclear transitions9’5) given in standard compilationsl’). In the coincidence spectra altogether 20 y-ray lines were found and identified. Comparing with the inclusive y-ray spectra measured in ref. 5, we notice that the coincident y-ray lines are the most intensive lines seen in the inclusive spectra. Their corresponding intensity exhausts - 65-758 of the total inclusive y-ray intensity. In the spectrum shown in fig. la, 15 y-ray lines are observed related to reaction products with 17 < Z I 20 (in addition to 27A1and 76Ge lines). Statistical-model ~~culations5}
/
c
-I
6. 8
:;C,, 271512d
COUNTS
R. Pfaneta et al. / Projectile break-up
114
160.
%a
(6Li,3Heyb
eLob=
0
1000
1600
1mo CHANNEL
Fig. lb.
y-ray spectrum
with coincident
90
1600
2m
NUMBER
emission of 3 He particles MeV 6Li ions.
from the bombardment
of 40Ca by 156
error of the correction for the random coincidences. The absolute cross sections may additionally be affected by some systematic errors which are considered later on. Fig. 2 compares the inclusive cx-particle spectra at various emission angles with the corresponding spectra measured in coincidence with all observed y-ray peaks, corrected for the detection efficiency of the y-ray detector. Contributions by the y-ray continua adjacent to the lines have been subtracted. At least two different components can be identified in the inclusive and exclusive particle spectra: (i) the bump centered around the beam velocity energy and ascribed to a-particles from the and pre~uilib~um emission projectile break-up, and (ii) an evaporation component 3,4) (including in this term particles from rather direct knock-out contributions). Table 2 compiles the inciusive and exclusive cross sections for coincident light-particle emission integrated over different energy ranges of the particle spectra.
R. Planeta et al. / Projectile break-up
115
TABLE 1 Coincidence cross sections for different product nuclei d omin/ds2 [mb/sr] a E = 18-160 MeV
%c @Ca “K ‘9K ‘SK =Ar “Ar ‘6Ar ‘%I 36Cl 35Cl ‘5s
50
9”
31 + 31 182+54 31* 31 172+53
6 f 10 147+42 6 + 10 116+35 46+ 19 74f26 42k 18 62i 23 11 f 10
136 + 44 35 * 21 93 * 34 61+45 39 + 26 16 k 16
In order nonelastic librium”
16 + 19 56 + 24 15 + 15
E = 65-160 MeV
200
lo+4 11*4 4+2 10+4 8k3 13 k4 l&l 2+2 9+4
p+d+t
)He
50
90
29 f 29 178+52 29 f 29 172+53
4+10 147i_42 4* 10 110+33 37+16 63+22 31*14 45+18 15 + 14 10*17 48 i 21 12+12
121+40 25 + 18 76 k 30 54 f 40 39 + 26 21* 21
E = lo-160 MeV
20”
9+3 9+3 2kl 7k2 2+1 4+2 212 2k 2 1*1
5”
9”
3+5 11&7 3i5 4*7
10 + 10 14 k 8 lo*10 22 i 9
16&10
26210
19+10
11+7 lk3 12* 7
2k3
5i5
E = lo-160 MeV
20”
lkl 3il l&l 3+1 l+l 1 kl 2+1
5”
9”
200
4+5 33+17 4*5 35i18
1+5 71+24 1*5 83+27 30* 15 64+23 14+12 54i21 19 + 15 26k15 37 + 19 7+7
lot10 914 10 f 10 21 k6 11*4 26 f 8 17 k 6 18 + 6 5+5 7+4 7+4
50*21 29+15 36k19 28 k 23 23+15 11* 11
to extract cross sections which are representative for the inclusive and break up components, the “background” from evaporation and “preequiemission has been approximated by a linear tail below a symmetric
gaussian-shaped break-up bump [see ref. 3)]. Figs. 3 and 4 display the angular dependence of the inclusive and exclusive cross sections. In addition to the a-particle yield of the break-up component, the total yield from all components in the spectra is shown in fig. 3. In the Z = 1 case, however, it is not possible to disentangle the break-up and the high-energy tail of preequilibrium processes. The nonelastic break-up contributions found by ((Y,y) coincidence measurements comprise (20 k lo)!%, (54 &-28)%, and (33 + 20)% of the break-up contribution seen in a-particle spectra at 5”, 9” and 20”, respectively. A similar situation is found for the Z = 1 particles. The yield of (Y, 3Heand Z= 1 p articles at very small angles may generally be affected by target contaminations (C and 0). However, 0 and C contaminations of the target are less than 2% (target weight) and the 6Li break-up cross section for C and 0 is significantly smaller than that for Ca at the same angle of observation3). For the exclusive cross section, the coincidence with y-lines from 40Ca-like reaction products suppresses the influence of contaminations of light nuclei. In order to estimate the angle-integrated cross sections we assumed an exponential angular dependence3) fitted to the three values available by the measurements. In the case of the nonelastic a-particle contribution we introduced an alternative parametrization (see fig. 3) in order to account for the smaller ratio of exclusive to inclusive cross sections at very forward angles. But the values for the integrated cross
116
R. Planeta et al. / Projectile break-up
2.5
sections (given in table 3, in figs. 3 and 4 for all eases> are not significantly affected by the particular choice of the p~ame~at~on_
4. Discussion The analyses 4> of the inclusive charged-particle spectra on the basis of the DWBA break-up theory ‘*14) attribute the major part (- 80%) of the inclusive break-up component to the nonelastic break-up mode. Even admitting large statistical uncettainties of the results of the coincidence measurements, the spectra shown in fig. 2
R. Planeta et al. / Projectile break-up
117
TABLE2
Inclusive and exclusive cross sections in mb/sr for emission of light particles in the ‘Li + ““Ca reaction at different emission angles
Ejectile
a inclwive
a exclusive
fiew integration
0 lab 5”
go
20”
18-160 MeV total break-up (background subtracted)
5552 & 1110
1609 I 322
316 f 63
4963 rt 1092
1245 + 273
81 f 18
18-160 MeV nonelastic break-up (background subtracted)
1054 _t 279
791* 212
87 + 25
969 + 287
678 + 199
27 f 20
725 & 145
266 + 53
66k 13
153 f 38
19i7
3He inclusive
lo-160 MeV
3He exclusive
lo-160 MeV
p+d+t inclusive
lo-160 MeV
5435 + 1087
2798 + 560
1184 k 237
p+d+t exclusive
lo-160 MeV
370 f 121
550 rt 153
212 + 53
89+33
to contradict the theoretical expectations as far as nonelastic contributions are signalled by y-ray transitions induced by nonelastic interactions of one of the fragments with the target nucleus. The discrepancy is especially drastic for the 3He appear
and t inclusive spectra although it is also noticed for the a-particles. We have to look for effects which might have reduced the yield of coincident y-rays. The analysis is based on the simplifying assumption of an isotropic y-ray emission. There are several indications li) support~g this assumption. The y-deexcitation occurs after evaporation of several light particles, removing most of the initial angular momentum in several independent steps and resulting in relatively large attenuation factors for the spin orientation5,9). In addition, effects of dipole and quadrupole transitions cancel mutually when summing up cross sections of y-lines with different multipolarities. In addition to the angular correlation effects, side-feeding parallel to the y-ray transitions in the decaying nuclei could systematically falsify the value of the
118
R. Pianeta et al. / Projectile break-up
Alpha - particle integrated Inclusive
component
from 65 - 160 MeV
break up part
+ without o with background subtraction Exclusive
break up part
x without l with background subtraction
loo-
Parametrization /[%].9”,, 10
1
I I
I
I
0
5
10
15
Fig. 3. Angular
distribution
20
= lL11.2 e-328,2,b
\ %_ab
of the inclusive and exclusive cross sections the 6 Li + 4o Ca reaction at 156 MeV.
[degl
for the emission
of cr-particles
in
nonelastic break-up cross section. Results of inclusive y-ray spectroscopy experiments5,9) in the nuclidic region around Ca suggest that the side-feeding correction may be between zero and 30% depending on the entrance reaction channel and consequently on the entry region. Even with these systematic uncertainties inherent to the applied method the differences between experimental and theoretical values of the nonelastic break-up cross sections seem to be significant. Table 3 compiles the integrated cross sections experimentally known for various reaction modes in the 6Li + 40Ca interaction at 156 MeV. The elastic break-up cross sections are just the difference between the inclusive break-up cross sections and the nonelastic break up cross sections in the a-particle and 3He channels, anticipating that the elastic break-up primarily proceeds by an (Y+ d and a t + 3He break-up, respectively, and that the elastic break-up path 6Li + (Y+ p + n is already included in the observed a-particle yield. Summing up elastic and nonelastic break-up cross
R. Pianeta et al. / Projectile break-up
119
Z = 1 and 3He components -+--lp+d+tlincl.
;]
-0
3He
inch.
3He excl.
1000
100
\ \o
\
\
La=
f 10
T
Fig. 4. Angular
1
I
I
I
5
10
15
20
distribution
= 107 mb
31mb
%,b[deg]
of the inclusive and exclusive cross sections for the emission ‘He ejectiles in the 6Li +@Ca reaction at 156 MeV.
of Z = 1 and
we find that the total break-up cross section updtu.= 930 k 115 mbt comprises - 50% of the total reaction cross section. Using the measured inclusive cross sections4) [tabulated in ref. 16)] for the 6Li + 40Ca reaction at 156 MeV we are able sections
to consider the contributions (“background”) in the charged-particle spectra which are presumably not related with the emission of beam-velocity particles. In addition to the equilibrium and preequilibrium emission an additional broad bump at lower energies
has been observed
in the triton
spectra4).
The contribution
of this compo-
nent is included in the value of the inclusive break-up cross section a(t*) = 280 + 56 mb adopted from ref.4) (see table 3). Only - 80 mb ( f 25%) are attributed to the spectator break-up component, while the rest is ascribed to the other components. Summing up these “background” contributions and accounting for a neutron part equal to the proton part, we get uback = 1200 mb, which is a production cross section. In order to get the reaction cross section from light “background” ejectiles, we have to divide this value by the particle multiplicity factor (M). For a-particles we + This result includes the triton channel equal to the 3He channel as suggested by an improved analysis of the inclusive triton spectra. This is additionally supported by some exploratory measurements of eLi, t) coincidences using a A E-E particle telescope with a 15 mm thick high-purity Ge detector 15)
R. Planeta et al. / Projectile break-up
120
TABLE3 Integrated experimental cross sections for the 6Li +40Ca reaction at 156 MeV Reaction channel
Cross section [mb]
Ref.
total reaction cross section
1980 + 200
12 1
complete fusion
67 i 20
*)
320 f 64 360 + 72 280 + 56
4,
80 f 16 440-188 453 * 90
this work
3Hea)
346 + 138 140 + 56 31+ 12
this work this work this work
cross sections for non-break-up components in the ejectile spectrab): P d t ‘He 0
167 + 100 320 + 70 2OOf60 27 t 6 327 + 72
4,
inclusive break-up cross sections for emission of charged-projectile fragments: P d t”) 3He a
nonelastic break-up cross sections with emission of charged ejectiles: p+d+t*) a
quasi-symmetric binary splitting of the compound system
5.6 + 1.4
1;
:;
13
)
“) Without “background” subtraction. b, Energy integration was done starting at 10 MeV for 2 = 1 particles, and 18 MeV for 2 = 2 particles, respectively.
can roughly estimate it by comparing the inclusive and exclusive ‘“background” spectra. We obtain (M), = 2.8, which may be different in the other reaction channels. However, this un~rt~ty does not affect the calculation of the total reaction cross section. As far as the emission of irbackground” particles follows complete fusion and nonelastic break-up processes, the background contribution is contained in the
R. Planeta et al. / Projectiie break-up
121
sum of the complete fusion (u = 67 mb) and nonelastic break-up (ut;z = 582 mb) cross sections. Summin g up all known contributions (table 3) to the total reaction cross section, we exhaust only 50% of the total reaction cross section crR= 1980 mb. Therefore, there is room left for contributions of other reaction channels, e.g. from pick up reactions, as suggestedis) by the predictions of the Wilczynski “sum rule” model 17), in addition to continuum excitation by (6Li, 6Li’). 5. Con~~ing
remarks
In the present experiment we have measured the contribution to the projectile break-up associated with y-ray emission from nonelastic interactions of the nonspectator fragments with the target. For the case studied of the 6Li -t4Ca reaction at E,, = 156 MeV the contributions comprise a summed-up nonelastic break-up cross section of (I = 582 + 110 mb which should be compared with the value u = 930 f 115 mb of the total break-up cross section (as deduced from the data available for the case considered). The differential cross sections for the nonelastic break-up are found to be si~ficantly smaller than the values predicted4) by the DWBA break-up theory. This theory is essentially based on a spectator mechanism and ignores the sequential break-up mode which might considerably contribute to the elastic break-up modei8*i9), especially at forward angles. The trend of the exclusive break-up component seen in fig. 3 seems to indicate an increasing elastic break-up contribution at 19,~~ = 5” (i.e. below the grazing angle), where in fact the Coulomb interaction becomes important. It is the matter of more detailed particle-particle coincidence experiments to determine the elastic break-up cross section more directly and to look for processes in which spectator fragments are emitted in coincidence with knock-out particles, e.g. from nonelastic interactions of the nonspectator fragments. Such processes contribute to the background of light particles and are not necessarily accompanied by y-ray emission”) though being a nonelastic break-up process. We thank Prof. Dr. G. Schatz for his continuous interest and encouragement in this work. We are grateful to Dr. B. Neumann and Dr. S. Micek for substantial help and clarifying discussions, and Ing. M. Adamczyk and Dipl. Phys. H. Jelitto for technical assistance in the experiments. Some of the authors (R.P., T.K., L.F., K.G.) wish to express their gratitude for the kind hospitality in Institut fur Kemphysik of Kemforschungszentrum Karlsruhe. This work was supported by the Polish MNSWiT (grant no. MR.5.5) and the International Bureau of the Kernforschungszentrum Karlsruhe. References 1) A. Budzanowski, Proc. 3rd Adriatic Europhys. Conf. on dynamics of heavy ion collisions, Hvar, Yugoslavia, May 2%31,198X
122
R. Planeta et al. / Projectile break-up
2) G. Baur, F. R8se1, D. Trautmann and R. Shyam, Phys. Reports 111 (1984) 335 3) B. Neumann, H. Rebel, J. Buschmann, H.J. Gils, H. Klewe-Nebenius and S. Zagromski, Z. Phys. A296 (1980) 113 4) B. Neumann, H. Rebel, H.J. Gils, R. Planeta, J. Buschmann, H. Klewe-Nebenius, S. Zagromski, R. Shyam and H. Machner, Nucl. Phys. A382 (1982) 296 5) J. Brzychczyk, L. Freindl, K. Grotowski, Z. Majka, S. Micek, R. PIaneta, M. Albihska, J. Buschmann, H. Klewe-Nebenius, H. J. Gils, H. Rebel and S. Zagromski, Nucl. Phys. A417 (1984) 174 6) B. Neumann, J. Buschmann, H. Klewe-Nebenius, H. Rebel and H.J. Gils, Nucl. Phys. A329 (1979) 259 7) C.M. Castaneda, H.A. Smith, P.P. Singh and H. Karwowski, Phys. Rev. C21 (1980) 179 8) H. Utsunomiya, S. Kubono, M.H. Tanaka, M. Sugitani, M. Morita, T. Nomura and Y. Hamajima, Phys. Rev. C28 (1983) 1975 9) K. Grotowski, P. Belery, Th. Delbar, Y. el Masri, Gh. Gregoire, R. Janssens, J. Vervier, G. Paic, M. Albihska, J. Albihski, S. Kopta, T. Kozik and R. Planeta, Phys. Rev. C23 (1981) 2513 M.M. Albihska, Ph.D. thesis, Jagelloman University, Cracow (1981) 10) C.M. Lederer and V.S. Shirley, ed., Tables of isotopes, 7th ed. (Wiley, New York, 1978) 11) R.A. Dayras, R.G. Stokstad, D.C. Hensley, M.L. Halbert, D.G. Sarantites, L. Westerberg and J.H. Barker, Phys. Rev. C22 (1980) 1485 12) J. Cook, H.J. Gils, H. Rebel, H. Klewe-Nebenius and Z. Majka, Nucl. Phys. A388 (1982) 173 13) K. Grotowski, Z. Majka, R. PIaneta, M. Szczodrak, Y. Ghan, G. Guarino, L.G. Moretto, D.J. Morrisey, L.G. Sobotka, R.G. Stokstadt, I. Tserruya, S. Wald and G.J. Wozniak, Phys. Rev. C30 (1984) 1214 14) G. Baur, R. Shyam, F. R&e1 and D. Trautmann, Helv. Phys. Acta 53 (1980) 503 15) R. Planeta, H. Klewe-Nebemus, B. Neumann, J. Buschmann, H.J. Gils, H. Rebel, S. Zagromski, L. Freindl and K. Grotowski, KfK 3642 (1983); R. Planeta and B. Neumann, unpublished results (1982) 16) K. FeiBt, B. Neumann, J. Buschmann, H.J. Gils and H. Rebel, Energiespektren und Produktionswirkungsquerschnitte ftir leichte geladene Teilchen in 40Ca(6Li,x)-Kernreaktionen bei Eri = 156 MeV; unpublished results, Kemforschungszentrum Karlsruhe (1981) 17) J. Wilczyhski, K. Siwek-Wilnyhska, J. van Driel, S. Gonggrijp, D.C.J.M. Hagemann, R.V.F. Janssens, J. Lukasiak and R.H. Siemssen, Phys. Lett. 45 (1980) 606 18) J. Kleinfeller, J. Bisplinghoff, J. Ernst, T. Mayer-Kuckuk, G. Baur, B. Hoffmann, R. Shyam, F. R&e1 and D. Trautmann, Nucl. Phys. A370 (1981) 205 19) J. van Dreil, S. Gonggrijp, R.V.F. Janssens, R.H. Siemssen, K. Siwek-Wilczyhska and J. Wilczyhski, Phys. Lett. 98B (1981) 351 20) H.R. Schmidt, K.T. Kniipfle, G. Seegert, P. Grabmayr and G.J. Wagner, Proc. Workshop on coincident particle emission from continuum states in nuclei, Bad Honnef, June, 4-7, 1984, ed. H. Machner and P. Jahn (World Scientific, Singapore, 1984)