The inhalation of radioactive airborne particles is one of the most important routes of entry of radionuclides into the human body. As part of the internal dose assessment program in nuclear medicine laboratories, air monitoring is of utmost importance either to identify and monitor airborne radioactive material, either to evaluate the radiological conditions of all people involved in workplaces designated as controlled and supervised areas. For this purpose, systems composed by scintillating detectors inserted in Marinelli beakers are widely used in the clinical practice for the measurement of the activity of radioactive liquid solutions. In presence of gamma rays, originating directly from the decay of gamma emitters or by the positron annihilation for ß+ emitters, these systems measure the activity thourgh the number of photoelectric interactions occurring in the detector in a fixed amount of time, being them calibrated using a reference liquid solution of known activity and gamma emitter. In order to obtain the calibration constants to perform measurement of activity in gasses, the aim of this work is to use a Monte Carlo simulation to define correction factors to be applied in the measurement of radioactivity in air with Marinelli beakers systems calibrated with liquid solutions.
The system for the activity measurement available at IRST is composed by a polymethyl-methacrilate Marinelli beaker, a NaI(Tl) detector (NDI-65/50, Gamma Technical Corporation, comprised of readout and acquisition system) with cylindrical crystal of diameter and thickness equals to 50 mm, and a Pb shield with internal Cu coating. The Monte Carlo code Geant4 (version 10.0.p02) is used to simulate the experimental setup, the physics processes and the experimental signal, simulated starting from the energy deposited in the NaI block. Three reference experimental spectra of Tc99m, F18 and Lu177 water solutions are used to extract the full experimental energy resolution for photons with energy 140.5 keV, 511 keV and 208 keV. The same spectra are used for the validation of the simulated spectra. The air calibration scale factors are obtained by comparing the relative yield of the photoelectric peak in the simulation for gamma sources in air and water, for the same activity.
The correction to be applied for the measurement of the activity in air increases with the decreasing gamma energy. The photoelectric peak is negligibly affected by the material surrounding the detector and Marinelli beaker system, this is verified by comparing the results of the nominal simulation with one obtained by completely removing the lead shield surrounding the beaker.
The measurement of the activity in air by means of a Marinelli beaker system calibrated using radioactive solutions in water requires the application of correction factors that can be determinate by means a Monte Carlo simulation.

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