[en] The results of search done under the International Atomic Energy Agency Contract no 4121/R2/RB: ''Measurement of the thermal neutron macroscopic absorption cross-section on small rock samples'' at the Institute of Nuclear Physics during the period from July 1, 1987 to June 30, 1988 are presented. The report contains the determination of the correction curve based on the measurement of the solutions of H₃BO₃ (IRM) including the definite neutron parameter for plexiglass moderator; the theoretical approach to the application (as the external moderator) of teflon - the material having lower thermal neutron absorption cross section than the typical rock material. (author)