This book describes Monte Carlo simulations of the processes in proportional counters used for absolute measurements of activity concentration of Krypton-85. It also offers a method for estimation of counting losses based on the results from Monte Carlo simulations. Each part of the book focuses on different aspects of the operation and/or modeling of proportional counters. Chapter I describes the experimental method for absolute measurement of activity concentration of Krypton-85 developed at LNHB/CEA-Saclay with emphasis on the origin of counting losses and their estimation. Chapter II...