  Quasielastic excitation function measurement has been carried out for the
$^{4}$He + $^{232}$Th system at $\theta_{lab}$=160$^\circ$ with respect to the
beam direction, to obtain a representation of the fusion barrier distribution.
Using the present data along with previously measured barrier distribution
results on $^{12}$C, $^{16}$O, and $^{19}$F + $^{232}$Th systems a systematic
analysis has been carried out to investigate the role of target and/or
projectile structures on fusion barrier distribution. It is observed that for
$^{4}$He, $^{12}$C, and $^{16}$O + $^{232}$Th, reactions the couplings due to
target states only are required in coupled channel fusion calculations to
explain the experimental data, whereas for the $^{19}$F+ $^{232}$Th system
along with the coupling of target states, inelastic states of $^{19}$F are also
required to explain the experimental results on fusion-barrier distribution.
The width of the barrier distribution shows interesting transition behavior
when plotted with respect to the target-projectile charge product for the above
systems.
