[en] The method applied represents poisoning with ''non-reciprocal v absorbers'' and is to define the thermalizing parameter M₂ of neutrons in a water lattice with hollow channels, 2 cm stepped and a porosity of 0,2442. The pulse experiments were carried out on a IRT - 2000 reactor in Sofia with the help of a very fast mechanical selector. The M₂ values recieved for water (5,097 cm^-1) were compared to the theoretical values, calculated by Honeck. The value for M₂, registered for the examined lattice (2,92 cm^-1), confirming to a rule, is lower than the experimental value for water, which is in connection with the relative reduction in the concentration of the dispersive water nucleuses and also, quite probably, with the changes in the neutron spectrum, caused by the diffusion cooling in the lattice