This research was sponsored by the National Science Foundation Grant NSF PHY 87-1440

Blann, H.M.

Bonotti, R.

Byrd, R.C.

Foster, C.C.

Grimes, S.M.

Pohl, B.A.

Remington, B.R.

Scobel, W.

Trabandt, M.

English

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PREEQUILIBRIUM NEUTRON EMISSION IN (p, xn) REACTIONS 
WITH 80-160 MeV PROJECTILES* 
W. Scobel and M. Trabandt 
University of Hamburg, West Germany 
H.M. Blann, B.A. Pohl, and B.R. Remington 
Lawrence Livermore National Laboratory, California 94550 
R. Bonotti 
Istituto di Risica Generale Applicate dell ' Universita Milano, Italy 
S.M. Grimes 
Ohio University, Athens, Ohio 45701 
R.C. Byrd and C.C. Foster 
Indiana University Cyclotron Facility, Bloomington, Indiana 4 7405 
The reactions 90Zr,208Pb (p,xn) have been studied in the neutron continuum 
En > 20 MeV and the angular range O0 5 0 < 145O for projectile energies of 80, 120, and 
160 MeV. Results are compared to predictions of phenomenological parameterizations, a 
semiclassical precompound and a quantum statistical multistep model. 
Spectral and angular distributions of continuum neutrons from (p,n) reactions are 
particularly appropriate to study the preequilibrium reaction phase, because a compet- 
ing break up mechanism in the case of complex projectiles as well as inelastic projectile 
scattering that would complicate the interpretation of the data are excluded. In the last 
ten years the (p ,n) equilibration process has been studied quite frequently1 with projectile 
energies up to 45 MeV. 
Several nuclear models consider the equilibration to proceed through increasingly 
complex configurations of single particle excitations. On one hand there is the class of 
semiclassical models that apply the quasifree intranuclear nucleon-nucleon collision picture; 
they yield convincing results for angle integrated nucleon energy spectra,2 but mostly fail 
to reproduce double differential cross sections in the backward h e m i ~ ~ h e r e . ~ j *  For the 
latter, finite size effects, incorrect treatment of higher chance nucleon emission and of the 
balance between single and multiple nucleon emission have been blamed.2j5 
Quantum statistical models have been that succeeded in describing also 
(p,n) and (p,p') data for 0 > 90". The Statistical Multistep ~ o d e l ~  provides excellent 
agreement not only in the energy regime of prevailing Multistep Compound (SMCE)* 
or Direct (sMDE)' emission, but also in the transition region.' The residual interaction 
strength Vo in this model has been brought to consist en^^'^ with direct reaction results. 
The phenomenological model of Kalbach and Mannl1 finally may be considered a derivative 
of the exciton and the Statistical Multistep Model. 
The contribution of preequilibrium emission to the nucleon yield increases with projec- 
tile energy. Therefore, accurate (p,n) data for Ep > 50 MeV that extend with good energy 
resolution and low background to very backward angles would provide a crucial test of 
both serniclassical and quanta1 preequilibrium models. This data, however, is scarcely1' 
available. 
In this report we present first results from a study of the (p,n) continuum for Ep = 80.5, 
120 and 160 MeV in the angular range O0 - 145O for the targets 'OZr and '08pb. The results 
will be compared on an absolute scale to a semiclassical12 as well as a quantum statistical7 
model and with the recent parameterization of ~ a 1 b a c h . l ~  
The time-of-flight (TOF) experiment has been performed at the IUCF. A burst sep- 
aration of 1.8 ps was achieved with the stripper loop.14 The long beam-off time allowed 
neutron time-of-flight spectroscopy with high resolution over a broad dynamic range. The 
magnetic beam swinger15 was operated with deflection angles of 0°, 11°, and 24", respec- 
tively, to cover the angular range from Oo to 144O with 5 fixed detectors; the TOF path 
lengths varied from 11 m for the most backward to 61 m for the most forward detector. 
The neutron detectors consisted of cylindrical cells (30.5 cm diam. x 20.3 cm) filled with 
the liquid scintillator BI501. Sheets of plastic scintillator were placed in front and on top of 
the detectors to discriminate against protons and to suppress cosmic radiation. It was nec- 
essary to perform n-7 pulse shape discrimination; Fig. 1 shows the excellent discrimination 
features of this large detector. Background was measured by placing tubes of appropriate 
diameter filled with water as shadow bars midway between target and detectors; their 
lengths corresponded to flux attenuations of at least 97% for En < 160 MeV. Conversion 
of the TOF results (cf. Fig. 2) into energy spectra was performed with the efficiencies from 
Ref. 16. 
The energy spectra shown in Fig. 3 represents a selection from ' ' ~ r ( ~ , n )  at 
Ep = 120 and 160 MeV including two independent measurements at 0 = 24O. The giant 
and analog resonance structure visible (in full resolution) at O0 is in quantitative agreement 
with recent results.17 The spectra undergo a characteristic transition from a flat energy 
dependence in the forward continuum to an exponential shape at backward angles; their 
slopes, however, exceed the ones expected for equilibrated systems by more than a factor 
of 3 (Table I). Also, there is no indication at low energies En for an increase of the cross 
section with 8. 
W Figure 1. Scatter plot of pulse height vs. I: 
pulse shape signal (Ep = 80 MeV). W 
1 ,  
-PULSE SHAPE ----------t- 
0 200 400 600 800 1000 0 200 400 
NEUTRON TIME-OF-FLIGHT T, (ns) Tn (ns) 
Fi~ure 2. TOF spectra of effect and shadow bar run. 
10-4 10-4 
40 60 80 100 120 40 60 80 100 120 140 160 
En lab ( M e V )  En lab ( M e V )  
Figure 3. Energy spectra for a selection of angles in 1 MeV bins. The g.s. Q-value is 
-6.9 MeV. 
Table I: Excitation energies and temperatures of 'OZr + p 
and the slopes T(6) of the most backward neutron spectra (in 
MeV). 
The dominant reaction mechanism therefore seems to be the SMDE type with only 
small SMCE contributions. If so, the data should agree with the systematics condensed 
into a parameterization of double differential cross sections in the spectral continuum13: 
d2a  
dOd€ 
= aySD exp(a cos 8) + aySC exp(a cos 0) + exp(--a cos 0) 
Here, aySD and aySD fix the absolute cross sections of the multistep direct and compound 
emission, respectively, and have to be taken from a reaction model. The slope parameter 
a(Ep,c) depends not only on the energy E of the ejectile but also on the type and the 
energy Ep of the projectile. Its parameterization has been derived13 from a broad range 
of angular distributions for light (A 5 4) projectiles and ejectiles with energies up to 600 
MeV. Here, Eq. (1) will be applied under the assumption of 100% MSD contribution. 
Fig. 4 demonstrates that our data exhibit a dependence of the shapes on Ep. There- 
fore they rule out the preceding18 parameterization intended for lower (E, < 80 MeV, 
E 5 45 MeV) energies, whereas Eq. (I) works well. For higher projectile energies Ep and 
a given neutron energy the angular distributions turn flatter and thus may indicate the 
importance of higher order steps with increasing inelasticity. 
The dependence of the angular distributions on En is very pronounced. For 
Ep = 160 MeV we observe (cf. Fig. 5 )  at En = 40 MeV a reduction of the cross sec- 
tions from very forward to backward angles by a factor 10, whereas at En = 140 MeV the 
distribution covers already 5 orders of magnitude in the region experimentally accessible. 
The normalized results of Eq. (1) follow this shape transition reasonably well which is 
another indication for the dominance of the MSD mechanism. Also shown in Fig. 5 is the 
prediction of a semiclassical model - in this case the geometry dependent hybrid model 
(GDH) with the assumption of a pure intranuclear nucleon-nucleon scattering mechanism. 
It reproduces the angle integrated spectrum on an absolute scale within a factor of no 
more than 2, but underestimates the yield at higher angles even more than observedlg for 
lower proton energies. In contrast, the discrepancies at low angles are not of a principal 
n a t ~ r e . ~  
The multistep model of Feshbach et al.' treats the nuclear reaction as a sequential 
process chaining stages with increasing complexity. At each stage n it differs between two 
I 
cn 
\ 2 lo0 
Y 
Figure 4. Angular distribution of 42 MeV wc 
and 62 MeV neutrons for different projec- rn C 
tile energies. Curves are the normalized u \ lo-' 
results of Eq. (1). b N 
-0 
classes of states or reaction amplitudes. The SMDE mechanism proceeds only through 
states with at least one nucleon being unbound, whereas SMCE proceeds only through 
particle bound states. The calculation of the cross section as an incoherent sum of both 
contributions has been developed by Bonetti et al.*-lo by using a Yukawa type potential 
with range ro = 1 fm and depth Vo = 26 f 1 MeV as residual interaction causing the 
n - 1 -+ n transitions. A recent application to (p,n) data for Ep = 25 MeV may be found 
elsewhere.' 
So far we have applied this model to the 80.5 MeV data only. We have used the optical 
model potential of Schwandt et a1.20 for protons and a straight forward modification of it 
for neutrons. The results are given in Fig. 6. The shape of the angular distributions is well 
reproduced in all cases. The calculations demonstrate the importance of the higher order 
steps even for the highest neutron energies. The experimental data is, however, incompat- 
ible with calculations applying Vo = 25 MeV. A reduction of Vo to 20 MeV eliminates this 
deficiency. It seems improbable that this renormalization reflects an incorrect application 
of partial state densities that were derived in the Fermi gas model with equidistant spac- 
ing, because the same value Vo = 20 MeV works for all neutron energies and two target 
nuclei among which one is doubly magic. This agreement is not restricted to the angular 
distributions, but extends to the energy spectra; an example is shown in Fig. 7. 
We propose that the dependence Vo(Ep) reflects the energy dependence of the ef- 
fective neutron-proton interaction in nuclear matter, averaged over the relative energies 
contribution at  Ep = 25 MeV and 80 MeV, respectively. It is well known that the strength 
of the AS = 0, AT = 0 component dominates the effective nucleon-nucleon interaction for 
(MeV) 
40 I 
F i ~ u r e  5. Angular distributions in comparison to the normalized results of Eq. (1) 
(dashed) and the GDH model prediction (dashed-dotted). 
45 
~ a b  angle (deg) 
0 30 60 90 1 2 0  150 
Lob angle (deg 
Figure 6. Comparison of experimental angular distributions and SMDE calculations with 
effective nucleon-nucleon interaction Vo. 
the proton energies under discussion and decreases with increasing Ep. Evidence for such a 
endence has also been foundz1 in a DWBA analysis of 9 0 ~ r ( p , d o , 1 , 2 ) 8 9 ~ r  for Ep = 20- 
MeV; our reduction of Vo by 25% is in almost quantitative agreement with that 
result. The conclusion, however, needs additional support that may come from the (p,n) 
calculations for Ep = 120 MeV and 160 MeV to be performed. 
In summary, the (p,n) data presented provide evidence that the MSD mechanism is 
the dominant one for a broad range of ejectile energies in the continuum part, and that 
the higher step contributions are important even for the highest ejectile energies. The 
SMDEISMCE model7 give an adequate description of (p,n) data if it is assumed than 
the effective nucleon-nucleon interaction Vo is not const ant but decreases with increasing 
projectile energy. 
\ Figure 7. Experimental neutron energy D 
spectra compared to SMDE calculations .! "' 
with Vo = 20 MeV. 
20 40 60 80 
N e u t r o n  e n e r g y  En [ M e V  I 
*Supported by the BMFT (Vorhaben 06 HH 175) and by the DOE under 
contract No. W-7405-Eng-48. 
1. E. Mordhorst, et al., Phys. Rev. C34,103 (1986) and references therein. 
2. J. Bisplinghoff, Phys. Rev. C 33, 1569 (1986). 
3. M. Blann, et al., Phys. Rev. C 30, 1493 (1984). 
4. H. Gruppelaar, et al., Riv. Nuov. Cim. 9, 1 (1986). 
5. K. Sato, Phys. Rev. C32, 647 (1985). 
6. T. Tamura, et al., Phys. Rev. C 26, 379 (1982). 
7. H. Feshbach, et al., Ann. Phys. (N.Y.) 125, 429 (1980). 
8. R. Bonetti, et al., Phys. Rev. C 27, 1003 (1983). 
I 9. R. Bonetti, et al., Phys. Rev. C 24, 71 (1981). 
10. R. Bonetti, et al., Phys. Rev. C 28, 980 (1983) 
11. A.M. Kalend, et al., Phys. Rev. C 28, 105 (1983). 
12. M. Blann, et al., Phys. Rev. C 28, 1475 (1983). 
13. C. Kalbach, Report LA-UR-87-4139 (1987). 
14. D.L. Friesel, Nucl. Instr. and Meth. B10/11, 864 (1985). 
15. C.D. Goodman, et al., IEEE Trans. Nucl. 26, 2248 (1979). 
16. R.A. Cecil, et al., Nucl. Instr. and Meth. 161, 439 (1979). 
17. T. Taddeucci, et al., Phys. Rev. C 33,746 (1986). 
I 18. C. Kalbach, et al., Phys. Rev. C 23, 122 (1981). 
19. Y. Holler, et al., Nucl. Phys. A442, 79 (1985). 
20. P. Schwandt, et al., Phys. Rev. C 26, 55 (1982). 
21. S. Kosugi, et al., Phys. Lett. 127B, 389 (1983). 
A STUDY OF THE (3He,t) CHARGE-EXCHANGE REACTION AT E(3~e)=200 MeV 
J. Janecke, F.D. Becchetti, V. Cianciolo, A. Nadasen, D. Roberts 
University of Michigan, Ann Arbor, Michigan 4 8128 
G.P.A. Berg, R. Sawafta, E.J. Stephenson 
Indiana University Cyclotron Facility, Bloomington, Indiana 4 74 05 
M.N. Harakeh 
Vrije Universit ei t ,  Amsterdam, The Netherlands 
S.Y. van der Werf 
Kernfysisch Versneller Instituut, Groningen, The Netherlands 
The (3He,t) charge-exchange reaction has been studied at a bombarding energy of 
200 MeV on 12 targets ranging from 12C to 2 4 4 P ~  including 'OZr, 120Sn, 2 0 8 ~ b  and five 
radioactive actinide targets. The objective of this work was to establish the feasibility of 
taking data at zero degrees with the K600 spectrometer and to determine the sensitivity 
of this charge-exchange reaction to spinflip and non-spinflip transitions. The results could 
then be compared to the (3He,t) charge-exchange reaction at higher bombarding energies1 
and to (p,n) charge exchange2. The selectivity to non-spinflip L=O transitions is of partic- 
ular interest for the observation of isobaric analog states (IAS) and certain isovector giant 
resonances. In a particular application, properties of IAS were measured with emphasis 
on heavy nuclei. 
The experiment was carried out with the K6OO magnetic spectrometer at zero degrees. 
The 3He++ beam entered the spectrograph and hit a beam stop inside the first dipole 
magnet. The beam stop was electrically insulated so that the beam current could be 
read directly from it. Background in the focal plane detector due to particles originating 
from the beam stop and from beam halo was low when proper precautions were taken. 
Singly-ionized 3 ~ e +  particles produced in the target with an intensity on the order of 
lo-* compared to the incident beam reached the focal plane where they increased the 
event rate by approximately 30% unless eliminated with an 'absorber in front of the focal 
plane detector. Spectra were measured mostly in singles mode, but preliminary coincidence 
spectra were also obtained in a few instances. In this case, decay protons from unbound 
states were detected in the scattering chamber at backward angles with two large-area 
Li-drifted Si detectors. 


Preequilibrium Neutron Emission in (p, xn) Reactions with 80-160 MeV Projectiles

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Preequilibrium Neutron Emission in (p, xn) Reactions with 80-160 MeV Projectiles

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