| dc.description.abstract | Over the decades of intense studies in the fission of actinide nuclei, it has been established
that the fragment mass distribution is asymmetric at lower excitation energies (E*). The macro-
microscopic model describes it as a manifestation of fragment shell effects [1]. With an increase in
nuclear excitation energy, the shape of the mass distribution changes from asymmetric to symmetric
Gaussian, pointing to the attenuation of shell effects with the increase of excitation energy.
Surprisingly, recent experimental results of mass distribution studies stand remarkably far from
these expectations [2]. In a seminal work, Hirose et al. studied the fission of neutron-rich actinides
formed through multinucleon transfer (MNT) [2]. They observed the presence of shell-mediated
asymmetric fission even above E* ≈ 60 MeV in a series of neutron-rich actinide nuclei. Subsequent
theoretical calculations incorporating multi-chance fission have yielded reasonably accurate mass
spectra.
The primary focus of the investigations presented in this thesis is to examine the influence of
multi-chance fission on the decay of compound nuclei (CN) formed through the conventional full-
momentum transfer (FMT) fusion process. To address the phenomena of multi-chance fission, we
have investigated the mass distribution and neutron multiplicity in an actinide nucleus at various
excitation energies. Detection of fast neutrons coinciding with fission events was accomplished
using the National Array of Neutron Detectors (NAND) facility [3]. NAND is a multi-neutron
detector facility comprising one hundred BC501A organic liquid scintillators mounted on a
geodesic dome structure and installed at the Inter-University Accelerator Centre (IUAC) in New
Delhi for the study of heavy ion-induced fission and associated phenomena. The thesis also
provides a comprehensive overview of the facility and includes Monte Carlo calculations performed
to assess its performance characteristics. Following optimal tuning and performance evaluation, the
neutron array has been utilized for heavy ion-induced fission research.
In the experimental approach, we conducted measurements of the mass distribution, average
neutron multiplicity, and correlations between fragment mass and pre-scission neutron multiplicity
in the 227Pa compound nucleus formed through the complete fusion reaction 19F+208Pb, in the
excitation energy range of ≈ 30 to 60 MeV. To analyze the data, we employed well-known methods
such as the velocity reconstruction method for deriving the mass distribution and the moving source
fit method for determining neutron multiplicity. The results were analyzed theoretically using GEF
model calculations [4], incorporating multi-chance fission. Analysis of the Mass−TKE correlation
spectra at higher energies suggests that multi-chance fission does not significantly influence the
fission modes. However, at energies ≈ 35 MeV, the measurements have shown evidence of
asymmetric fission. Theoretical analysis at these two energies reveals a significant occurrence of
multi-chance fission and resultant asymmetric fission driven by fragment shells. Thus, the observed
trend in Mass−TKE distribution is attributed to the presence of shell-influenced asymmetric fission
modes facilitated by higher chance fission. This conclusion is ascertained by Mass−νpre correlation
measurements performed at these energies.
In conclusion, our findings suggest that the correlation of higher pre-scission neutron multiplicity
with asymmetric fission is a signature of shell effects reinstated by multi-chance fission. | en_US |
