  The decay of the Isoscalar Giant Monopole Resonance (ISGMR) in nuclei is
studied by means of a nonlinear classical model consisting of several
noninteracting nucleons (particles) moving in a potential well with an
oscillating nuclear surface (wall). The motion of the nuclear surface is
described by means of a collective variable which appears explicitly in the
Hamiltonian as an additional degree of freedom. The total energy of the system
is therefore conserved. Although the particles do not directly interact with
each other, their motions are indirectly coupled by means of their interaction
with the moving nuclear surface. We consider as free parameters in this model
the degree of collectivity and the fraction of nucleons that participate to the
decay of the collective excitation. Specifically, we have calculated the decay
width of the ISGMR in the spherical nuclei $^{208}\rm{Pb}$, $^{144}\rm{Sm}$,
$^{116}\rm{Sn}$ and $^{90}\rm{Zr}$. Despite its simplicity and its purely
classical nature, the model reproduces the trend of the experimental data which
show that with increasing mass number the decay width decreases. Moreover the
experimental results (with the exception of $^{90}\rm{Zr}$) can be well fitted
using appropriate values for the free parameters mentioned above. It is also
found that these values allow for a good description of the experimentally
measured $^{112}\rm{Sn}$ and $^{124}\rm{Sn}$ decay widths. In addition, we give
a prediction for the decay width of the exotic isotope $^{132}Sn$ for which
there is experimental interest. The agreement of our results with the
corresponding experimental data for medium-heavy nuclei is dictated by the
underlying classical mechanics i.e. the behaviour of the maximum Lyapunov
exponent as a function of the system size.
