Abstract: A method for determining a concentration of an isotope in a fluid, the isotope absorbing neutrons, the method comprising placing a plurality of neutron detectors at various distances from the fluid; irradiating the fluid by a neutron-emitting source, the latter being placed so that emitted neutrons pass through the fluid before reaching the detectors; measuring, by each detector, a quantity representative of an amount of neutrons reaching the detector; and based on the measurements resulting from the measuring, estimating a concentration of the isotope in the fluid. Further, the estimating step includes taking into account a database containing an estimate of the quantity measured by each detector and based on the database, and on the measurements resulting from the measuring step, estimating the concentration of the isotope in the fluid.