[en] An experimental method was used to evaluate the primary isotope yields of semi-central collisions in the reaction system "6"4Zn+"1"1"2Sn at 40A MeV, The characteristic nature of the hot nuclear matter at the time of the isotope formation was studied. The multiplicities of light particles (LPs) associated with intermediate mass fragments (IMFs) were determined experimentally by using a kinematical focusing technique. The primary isotope distributions, reconstructed by a Monte Carlo method, were compared with those of the AMD-Gemini simulations. a_c/T=0.11 and a_s_y_m/T=3.34 were extracted from the reconstructed primary fragments yield. These are consistent with those of the primary fragments of the AMD simulation. (authors)