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

Bracken, D.S.

Brzychczyk, J.

Kwiatkowski, K

Morley, K.B.

Renshaw, E.

Viola, V.E.

English

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ISOTOPIC YIELDS OF INTERMEDIATE-MASS FRAGMENTS EMITTED IN 
E/A = 50 MeV 4He + 116>124Sn REACTIONS 
J. Brzychczyk, K. Kwiatkowski, D.S. Bracken, 
K.B. Morley, E. Renshaw, and V.E. Viola 
Ind iana  Universi ty  Cyclotron Facility, Bloomington,  Indiana 4 74 08 
In the present work we investigate the influence of the target N/Z ratio on the prop- 
erties of intermediate-mass fragments (IMF's) formed in the reaction of E/A = 50 MeV 
4He ions with targets of l16Sn (N/Z = 1.32) and 124Sn (N/Z = 1.48). The experiment was 
performed with 4He beams of 180 and 200 MeV incident energy. Self-supporting l16Sn 
and 124Sn targets were used, each with areal densities of 1.4 mg/cm2 and isotopic purity 
of 95.6% and 96.7%, respectively. 
Reaction products (2  2 3) were detected with two detector telescopes positioned on 
rotatable arms inside a 162-cm diameter scattering chamber. The first telescope, employed 
primarily at forward angles, consisted of an axial-field gas-ionization chaxnber followed by 
90 pm and 1 mm silicon surface-barrier detectors (450 mm2), and a 2-cm thick CsI(T1) 
scintillator crystal operated with a photodiode. The first silicon element was placed 38 cm 
from the target and was also used for timing, with the beam rf signal as the time refer- 
' 
ence. Time-of-flight and AE-E measurements provided full isotope separation for Z < 6 
fragments with a low energy threshold. The second telescope consisted of a gas-ionization 
chamber followed by a 500 pm passivated silicon detector (25 cm2).l The gas-ionization 
chambers were operated at 20 Torr of CF4. Inclusive energy spectra of the intermediate- 
mass fragments were measured with the AE-E/TOF telescope at forward angles of 12", 35" 
and 65", and with the large solid-angle AE-E telescope at 160". Backward-angle isotopic 
yields were obtained by complementary TOFJAE-E measurements with the first telescope 
also. This measurement was performed at 154" and 180 MeV incident energy. For both 
systems the low energy thresholds were E/A E 0.4 MeV. 
The data demonstrate the influence of target composition on both the charge and 
isotopic distributions of the fragments. Elemental cross sections from the l16Sn + 4He 
system are distinctly enhanced relative to 124Sn + 4He (Fig. 1). This effect is strongest 
for the equilibrium-like fragments (160°), where the yields are 2-3 times larger for l16Sn 
compared to 124Sn. For nonequilibrium emission (12"), both targets produce similar yields 
for light IMF's; however, the ratio of the elemental cross sections from Sn to those from 
124Sn becomes larger as a function of increasing fragment charge. This result is understood 
in terms of the lower average neutron binding energies for the 4 ~ e  + 124Sn system, thereby 
favoring decay via neutron emission at the expense of IMF probability. 
For a given element the ratio of the cross section from the l16Sn system to that from 
124Sn for any series of isotopes is found to decrease with increasing neutron number (Fig. 2). 
This behavior reflects the larger N/Z ratio of the composite system formed from 124Sn and 
can be described in terms of a simple exponential function (slope=y) related to neutron 
binding energy differences. This isotopic dependence on target N/Z is strongest for the 
a(200 MeV)  + Sn + IMF 
Figure 1. Charge distributions at 12" (upper set) and 160" (lower set) for IMF's emitted 
from '16Sn (solid lines) and 124Sn (dotted lines). 
backward-angle component of the spectra, where the slope of the ratio o('16 S n ) f ~ ( ' ~ ~ S n )  
is identical for Li, Be, B and C ejectiles. Such behavior is consistent with emission from 
an equilibrated system. 
At forward angles the nuclidic yields are found to be sensitive to both the N/Z of the 
target and the Z of the ejectile. Neutron-excess IMF's are favored with the 124Sn target; 
however, the exponential slopes (7)  of the ratio c~("~Sn)/a('~*Sn) for the isotopes of a 
given element evolve from a rather flat behavior for Z = 3 isotopes to one for Z = 6 that 
is virtually identical to that for equilibrium-like emission. Thus, nonequilibrium emission 
of light IMF7s appears to be less sensitive to the target-projectile composition, indicating 
that only partial N/Z equilibration is achieved for these products. Another feature of the 
nonequilibrium spectra is that the isotope ratios depend on IMF kinetic energy. Near 
the Coulomb peak, the isotopic ratios at forward angles are similar to those at backward 
angles. However, for increasingly energetic IMF's, the yield of neutron deficient isotopes 
is enhanced with respect to the heavier isotopes. This effect is strongest for Li and Be 
isotopes. 
Fragment Mass Number 
Figure 2. Ratio of yield for a given isotope from '16Sn target to that from 124Sn target as 
a function of mass number for Li, Be, B and C, as indicated on the figure. The upper set 
shows nonequilibrium data at forward angles; the lower set is for data obtained at 154'. 
The dotted line is a fit to the data of an exponential function, exp(-~A); the value of y is 
given in each frame. 
When compared with the backward-angle data, a simple statistical model calculation 
which includes excited states of the fragments gives fair agreement. In general there 
is a tendency for the calculation to overpredict the yields of neutron-excess fragments. 
An accreting source calculation,' calibrated by the charge distributions, also provides a 
reasonable description of the nuclidic yield results for the entire data set. However, this 
calculation encounters difficulties in fitting spectral shapes at forward angles, as well as 
angular distributions. 
1. W.A. Friedman and W.G. Lynch, Phys. Rev. C 28, 16 (1983); 28, 950 (1983). 


Isotopic Yields of Intermediate-Mass Fragments Emitted in E/A = 50 MeV 4-He + 116,124-Sn Reactions

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Isotopic Yields of Intermediate-Mass Fragments Emitted in E/A = 50 MeV 4-He + 116,124-Sn Reactions

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