Speaker
Description
Nuclear fusion is a complex process involving a complete re-arrangement of quantum systems with many degrees of freedom. Heavy ion fusion exhibits a strong entrance channel dependence at near fusion barrier energies and could be reasonably explained by coupled channels formalism [1,2] that explicitly includes the effects of internal degrees of freedom. Though tremendously successful in describing the data at near barrier energies, coupled channels theories fail to describe fusion at deep sub barrier and well above the fusion barrier energies [3]. Alternate theories [4,5] proposed recently also could not give a reasonable explanation to the experimental observations till date. Scarcity of experimental data spanning deep sub barrier to well above the barrier also adversely affect the theoretical developments to a great extent. It is pointed that instead of a coherent coupled channels approach, a model with a gradual onset of decoherence [6] between the superposed states may be required to describe fusion.
Evaporation residue (ER) excitation function measurements have been performed using the heavy ion reaction analyzer (HIRA) [7] at the Inter University Accelerator Center, New Delhi. The measurements were performed for the $^{16}$O+$^{142,150}$Nd reactions in the beam energy range 12% below the barrier to 50% above the barrier. Pulsed beams of $^{16}$O with a pulse separation of 4 μs, was used to bombard the isotopically enriched $^{142}$Nd and $^{150}$Nd targets. The measured ER cross section represents the total fusion cross section at near barrier energies, as fission cross section is negligibly small.
The fusion cross section at different energies is estimated by using the standard expression [8]. Coupled channels code CCFULL has been used to analyse the measured fusion cross sections for the two reactions. Nuclear potential parameters V$_0$, $r_0$ and $a$ were first fixed using the Akyüz-Winther parameterisation [9]. The measured excitation function is found to be significantly enhanced relative to the one-dimensional barrier penetration model (1-DBPM). Vibrational coupling of $2^+$ and $3^-$ states of $^{142}$Nd is found to explain the sub barrier fusion enhancement in $^{16}$O+$^{142}$Nd reaction. Vibrational effects of $^{16}$O seem to play no role in sub-barrier fusion in this reaction. The degree of fusion enhancement is larger for the $^{16}$O+$^{150}$Nd reaction, compared to $^{16}$O+$^{142}$Nd. Rotational couplings of ($2^+$ and $4^+$states) of the deformed target reproduced the fusion cross sections reasonably well in $^{16}$O+$^{150}$Nd reaction at sub- and near barrier energies.
Though CC calculations assuming AW potential parameters reasonably reproduce the fusion cross sections at below barrier energies, they overpredict fusion at energies well above the barrier. This difference is observed to increase with increasing beam energy. Careful analysis indicates that the observed difference is not due to fission or α particle emission. Diffuseness parameter in the range 1.0 - 1.1 fm is required to fit the cross sections. These values are significantly larger than the values obtained from elastic scattering measurements. The inadequacy of AW potential parameters hints the role of dynamical effects in fusion at higher energies.
.png)
.png)