Pre-equilibrium effects in fusion ofC12andGd158

Cross sections and the first three moments of the

γ

-ray multiplicity distribution were measured for many transitions in a series of

{}^{170 - x}{Yb}

and

{}^{166 - x}{Er}

products from bombardments of

{}^{158}{Gd}

with

{}^{12}C

at five energies corresponding to excitation in

{}^{170}{Yb}

between 94 and 143 MeV. The average multiplicity for each

xn

channel increases with energy at first and then levels off at ∼ 21. For the

α xn

channels saturation values between 16 and 20 are found. The excitation functions for

{}^{166 - x}{Er}

indicate that the isotopes with small

x

are formed primarily by

2 p (x + 2) n

evaporation and the ones with large

x

α xn

emission. A corresponding discontinuity in the average multiplicity vs

x

observed. These features are absent in

{}^{20}{Ne}

{}^{150}{Nd}

reactions at the same

{}^{170}{Yb}

excitation, but are present in the

{}^{166}{Er} ({}^{4}{He}, x n)

reactions at excitations from 63 to 92 MeV. The present results are clear indications of pre-equilibrium emission of neutrons and possibly of

α

particles, setting in at bombarding energies ∼ 10 MeV per nucleon.

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