So far, many experiments have been carried out with gamma rays to obtain some information of the matter such as density. The work of Ryabov and others in 1976 is one of these experiments, which was using gamma densitometry to study flooding intervals in a borehole in order to enhance the efficiency of well logging. In this work, simulations have been carried out using MCNP code on soil and sedimentary rocks, and information is obtained about the substance by using flux of back-scattered gamma rays from the material. Since the irradiation of existing sources in nature causes disturbance in measurements, it should be a way to eliminate this disorder, therefore, in this work, the increase in the activity of the source used in simulations in relation to the radioisotopes existing in nature will be proposed, to reduce or eliminate this disruption, which increases the specific activity of the Cs¹³⁷ source relative to the specific activity of the sources in nature and examines the fluctuation fluxes. Using the results obtained in these simulations, it can be deduced that by increasing the special activity of the Cs¹³⁷ source in relation to the radioisotopes existing in nature, the amount of disruptions caused by radioisotopes existing in nature was reduced, until in special activity about 3.09946 MBq/gr this disorder will be close to zero, and it can be said that in a special activity equal to 3.09946 MBq/gr or greater than that, the disorder of radioisotopes existing in nature, can be eliminated to find materials in this way.