Objective: This study aims to calculate and estimate the stopping power, R CSDA , absorbed dose and cross sections for F18 simulated and propagated in water. Methods: In order to access the objective of this work, Monte Carlo Simulation -Geant4 code and Bethe-Bloch, Rudd, Seltzer, Berger, Wang and Rutherford equations are used. Finding: The simulation results are in a good agreement with many studies that deal with the same parameters and the same conditions. The change in the main dosimetric

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... rameters (mass collision stopping power and absorbed dose) is clearly indicated at the energy range below 50 keV. The collisional stopping power holds a special importance in the radiation dosimetry field of this energy range. It can represent the total F18 stopping power and it is showing a close relation with the absorbed dose. Application: This study is devoted to the development of numerical analysis of the stopping power, absorbed dose and cross sections for positron collisions with water. Such quantitative studies are very relevant for medical, in particular, PET applications to predict the patient's health status under exposure and for scientific knowledge on propagation of annihilation photons in media. This study analyzes a fundamental parameter of dosimetry in patients, the results will certainly be utilized to optimize patient exposure and to reduce effectively the radiation dose in the whole body without significantly sacrificing image quality.