This research was sponsored by the National Science Foundation Grant NSF PHY-931478

Carlin, N.

de Souza, R.T.

Fields, D.E.

Gelbke, C.K.

Gong, W.G.

Kwiatkowski, K.

Lynch, W.G.

Morley, K.B.

Renshaw, E.

Tsang, M.B.

Viola, V.E.

Wile, J.L.

Xu, H.M.

Yennello, S.J.

English

IUScholarWorks

with target nucleons, the null1 value of the analyzing power indicates t,hat formahion of 
complex fragments does not proceed via a simple direct reaction mechanism. 
1. S.J. Yennello, et al., Phys. Rev. C 42, 79 (1990). 
2. R.E.L. Green, et al., Phys. Rev. C 22, 1594 (1980). 
3. R.E.L. Green, et al., Phys. Rev. C 25, 828 (1982). 
IMF EMISSION IN THE 14N f natAg, Au REACTIONS 
AT E/A = 60-100 MeV 
J.L. Wile, D.E. Fields, K. Kwiatkowski, K.B. Morley, E. Renshaw, 
V.E. Viola, and S.J. Yennello 
Indiana University Cyclotron Facility, Bloomington, Indiana 47405 
R.T. de Souza, C.K. Gelbke, W.G. Lynch, M.B. Tsang, W.G. Gong, 
H.M. Xu, and N. Carlin 
National Superconducting Cyclotron Laboratory, Michigan State University, 
East Lansing, Michigan 48824 
In a recent study1y2 of IMF emission in the 14N + natAg, Au systems at bombarding 
energies of 20 to 50 MeV per nucleon, three sources of intermediate mass fragment (IMF) 
production were reported. The most important emission source at low bombarding ener- 
gies produced IMFs with kinetic energies neax the Coulomb repulsion energy and angular 
distributions that were relatively isotropic. The energy and angular distributions of these 
IMFs were well described by the model of M ~ r e t t o ; ~  thus, this source was identified with 
emission from a fully equilibrated compound nucleus. In addition to this "equilibrium" 
source, two other sources were observed. The first of these was associated with projectile 
fragmentation-like events and was analyzed using the empirical model of Kiss, et The 
remaining source of IMF production grew in importance as the borriha.rding energy was 
increased. The IMFs produced by this "non-equilibrium" source are highly energetic and 
strongly forward-focused, and they constitute a significant part of the total cross section 
well- beyond the grazing angle. These characteristics tend to indicate that the emission 
takes place on a relatively short time scale. However, polarization studies5 imply that 
these IMFs retain no memory of the initial beam pc~larization, therefore, the projectile 
must undergo several collisions with target nucleons before the IMF emission takes place. 
In order to shed more light on the underlying mechanisms of IMF emission, as well as 
to investigate the possible emergence of mu1tifra.gmentation as a, source for IMF production, 
the systema.tics of the previous 14N + natAg, Au studies were extended to 100 MeV per 
nucleon. The experiment took place at the MSUNSCL K1200 cyclotron, which delivered 
14N beams of 60, 80, and 100 MeV per nucleon. High-purity, self-supporting targets of 
natAg and natAu were surrounded by seven IMF detector telescopes at 20, 50, 70, 90, 
120, 140, and 160 degrees in the laboratory. The detector telescopes consisted of a CF4 
gas-ionization chamber followed by a combination of silicon strip detectors, lithium-drifted 
silicon detectors, and cesium-iodide crystals viewed by silicon PIN diodes. The 20 degree 
detector was positioned on a moveable arm and also sampled data at 35 degrees in the 
1abora)tory. Inclusive data were recorded for both taxgets, and the carbon contamination 
of the targets was found to be negligible by comparison of the silver and gold data to shqrt 
runs with a carbon target. 
An example of the energy spectra of boron fragments from the reaction 14N + natAg at 
Elab = 80 MeV per ~iucleon is shown for all detection angles in Fig. 1. Examination of the 
figure reveals the same IMF characteristics as reported previously, i.e., high kinetic energy 
fragments whose cross sections drop sharply as the labora,tory angle increases, and low ki- 
netic energy fragments at backward angles whose angular distributions are rather isotropic. 
The daka were separated into equilibrium (dotted curve), non-equilibrium (dashed curve), 
and projectile fragmentation (dot-dashed curve) sources by utilizing a sim~iltaneous, three- 
source fitting procedure.2 The total fit (solid curve) does an excellent job of describing the 
energy spectra at all angles. One interesting result from this fit is that at 100 MeV per 
nucleon, the temperature parameter of the equilibrium source, averaged over all elements 
1 4 ~  + D a l ~ g  E/A = 80 MeV 
Figure 1. Energy dis- 
tributions of boron frag- 
ments from the reaction 
14N + natAg, at E/A = 
80 MeV. The solid curves 
illustrate the results of a 
simultaneous fit which as- 
sumes IMF emission from 
equilibrium (dotted) , non- 
equilibrium (dashed), and 
projectile fragment ation 
(dot-dashed) sources. The 
angles indicated are in the 
laboratory. 
detected, was (7.1 + 0.8 MeV). If this pa-rameter is interpreted as the actual temperature 
of the compound nucleus, then the da.t a indica#te that nuclei can t hermalize significantly 
more excitation energy than predicted,' at least for this subset of the reaction cross section. 
The fit discussed above allows an identification of the three IMF sources, and a deter- 
mination of the IMF production cross sections from each of these sources. Fig. 2 illustrates 
the behavior of the equilibrium (Fig. 2a) and non-equilibrium (Fig. 2b) emission cross sec- 
tions for even -Z elements as a function of bombarding energy over the entire range spanned 
1 4 ~  + " " A ~  
1 1 1 1 1 1 1 1 1 1 1 ' 1 1 1 1 1 r 1 ( m 1 1 L 1 3 q 8 n  
a. equilibrium 
Figure 2.  IMF production 
cross sections for even-Z frag- 
ments from equilibrium (a), 
non-equilibrium (b) , and both 
(c) sources as a function of bom- 
barding energy. Only even-Z 
elements are represented. The 
solid curves are meant to guide 
the eye. 
E/A (MeV per nucleon) 
by both this experiment and the one of Ref. 2. Although the data indicate that the 
evolution of the IMF production cross sections is rather smooth over the energy range 
studied, a, few points are noted. First, the equilibriuln emission cross sections tend to 
increase more rapidly for the heavier elements ( Z  > 10) than for the lighter ones between 
60 and 80 MeV per nucleon. In the 80 to 100 MeV per nucleon range, however, the cross 
sections for all elements saturate. Second, while this trend is quite clear for the equilibrium 
source, the cross sections for IMFs produced by the nun-equilibrium source do not behave 
in a similar manner. For all elements, the non-equilibrium IMF production cross sections 
seem to saturate over the 60 to 100 MeV per nucleon range. The total summed cross 
sections (Fig. 2c) exhibit trends that essentially mirror those of the equilibrium cross 
sections. 
The emission cross sections for all ele~nents were fit as a function of atomic number 
with a simple power law parameterization, ( ~ ( Z ) W Z - ~  and the results of this fit are shown 
in Fig. 3. The figure shows that for the equilibrium (Fig. 3a) and summed (Fig. 3c) IMF 
production sources, the character of the Z distributions, as illustrated by the r parameters, 
changes abruptly over the 60 to 80 MeV per nucleon range. The change in the T para.nieters 
is actually the result of the contribution of the heavier elements as illustraked in Fig. 2. 
Once again, while this change is quite a.pparent for the two sources mentioned above, the 
T parameters characterizing the Z distributions of IMFs from the non-equilibrium source 
do not exhibit this abrupt change in slope. 
O 0 
Figure 3. The results of a fit of the IhTF 
charge distributions using a, simple Z - T  2.5 
C- parameterization for (a) the ecluilibrium, A 
(b) non-equilibrium, and (c) the sum of 
the two, and sources. b. non-equilibrium 
1 - 5 1  c .  eq + neq 
Beam Energy ( M ~ V / A )  
The inclusive nature of this experiment precludes any clear statement concerning both 
the nature of and reason for this abrupt change in emission source topology. One possi- 
ble explanation would be that angular momentum is playing a more important role att 
higher energies, thus increasing the import a.nce of heavy element e~nission from the eqtiili- 
Grated compound nucleus. Another possibility would be that sequential or instantaneous 
multifragmentation, which should be characterized by a significantly different charge distri- 
bution, may be enhancing the yield of hea.vy elernents in the equilibrium emission source 
beyond E/A z 50 MeV. Clearly, further investigations of an exclusive nature utilizing 
detector systems with a large geometric efficiency must be performed to examine this 
y henomenon more closely. 
1. D.E. Fields, et al., MSU Cyclotron Laboratory Annual Report, 1987, p.27. 
2. D.E. Fields, et al., Phys. Lett. B220, 356 (1989). 
3. L.G. Moretto, et al., Nucl. Phys. A247, 211 (1975). 
4. A. Kiss, et al., Nucl. Phys. A499, 131 (1989). 
5. E. Renshaw, private communication. 
6. D.H.E. Gross, Nucl. Phys. A437, 643 (1985). 
OCTUPOLE CORRELATIONS IN 1457146Nd NUCLEI 
W. Urban, J.C. Bacelar, 0. Dermois and J. Jongman 
Kernfysisch Versneller Instituut, Groningen, The Netherlands 
P.P. Singh 
Indiana University, Bloomington, Indiana 4 7405 
For the 146Nd nucleus, calculations i11dica.te an enhancement of octupole correlations at 
medium spins.' It has been predicted that with increasing rotational frequency a reflection- 
asymmetric structure should become yrast. In this work we aimed to verify this prediction. 
To populate high spins in 146Nd, we utilized the '36Xe+'3C compound nucleus reaction 
using a solidified '36Xe target and 54 MeV I3C beam from the KVI cyclotron. The y - y 
coincidences were collected with 4 Ge detectors in anti-Compton shields. The level scheme 
resulting from these data is shown in Fig. la*. An interesting result of the present work is 
an extension of the ground state, alternatting-parity band up to spin I=14. It is possible, 
though at present still specrilative that the I=18 level a,t 5900.5 keV a.rld tlie I=19, level 
at 6195.0 keV are aaso members of this band. An analogous 18 and 19 levels have been 
found at similar excitation energies in other N=86 nuclei. 
The above observations support the prediction thaA the octupole correlations extend 
in '46Nd to high spins. As in 148Sm one observes here a competition between octupole 
and single-particle, reflection-symmet ric excitations. The presence of a short cascade 
built on top of the 8+, 2475.1 keV level and the ($/2)2+,4+,6+ excitations on top of the 


IMF Emission in the 14-N + nat-Ag, Au Reactions at E/A = 60-100 MeV

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IMF Emission in the 14-N + nat-Ag, Au Reactions at E/A = 60-100 MeV

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