In the simulation of intermediate energy heavy ion collisions by using theisospin dependent quantum molecular dynamics, the isospin effect on the processof multifragmentation and dissipation has been studied. It is found that themultiplicity of intermediate mass fragments $N_{imf}$ for the neutron-poorcolliding system is always larger than that for the neutron-rich system, whilethe quadrupole of single particle momentum distribution $Q_{zz}$ for theneutron-poor colliding system is smaller than that of the neutron-rich systemfor all projectile-target combinations studied at the beam energies from about50MeV/nucleon to 150MeV/nucleon. Since $Q_{zz}$ depends strongly on isospindependence of in-medium nucleon-nucleon cross section and weakly on symmetrypotential at the above beam energies, it may serve as a good probe to extractthe information on the in-medium nucleon-nucleon cross section. The correlationbetween the multiplicity $N_{imf}$ of intermediate mass fragments and the totalnumer of charged particles $N_c$ has the behavior similar to $Q_{zz}$, whichcan be used as a complementary probe to the in-medium nucleon-nucleon crosssection.
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