Patent Number: 
Section: claims

1. Count rate measurement device of at least one fission chamber that contains fissile material, said count rate measurement device comprising:a measurement cell that contains the fission chamber,a neutron generator operable to emit neutrons, at a predetermined periodic pulse frequency, towards the fission chamber;a measurement cable, which collects, via a connector, a signal delivered by the fission chamber following the interaction of the neutrons with the fissile material;a neutron counter that delivers a neutron count signal corresponding to the neutrons emitted by the neutron generator at the predetermined periodic pulse frequency;a processing system that processes both the signal delivered by the fission chamber and a corresponding neutron count signal delivered by the neutron counter, and provides, during a predetermined time interval contained within a period defined by two successive pulses of neutrons emitted by the neutron generator, respectively, a signal representative of the signal delivered by the fission chamber and a signal representative of the neutron count signal, anda circuit for computing the count rate of the fission chamber normalized with reference to the neutron count signal, from the signal representative of the signal delivered by the fission chamber and the signal representative of the neutron count signal. 2. Count rate measurement device according to claim 1, wherein the measurement cell further comprises:a structure in which is formed a cylindrical cavity that opens via an opening into a wall of the structure,a first cylindrical jacket disposed on a wall of the cylindrical cavity,a second cylindrical jacket disposed inside the cylindrical cavity, at a distance from the first cylindrical jacket, the second cylindrical jacket containing the fission chamber, the connector and a first part of the measurement cable, the first and the second cylindrical jackets each having a first end fixed, in the structure, by a first ring situated on a side where the cavity opens into the wall of the structure and a second end fixed, in the structure, by a second ring,a third cylindrical jacket situated at an end of the structure and aligned with the second cylindrical jacket, the third cylindrical jacket containing a second part of the measurement cable that extends the first part and a centering ring, with a central opening in which a part of the second part of the measurement cable is placed, thereby maintaining the second part of the measurement cable in the third cylindrical jacket. 3. Count rate measurement device according to claim 2 further comprising:a sheet of material that covers the second cylindrical jacket, anda block of material that is placed in a space separating the sheet of material and the first cylindrical jacket. 4. Device according to claim 3, wherein the sheet of material is a sheet of cadmium. 5. Device according to claim 3, wherein the material of the block of material is made of boron and polyethylene. 6. Device according to claim 2, wherein the first cylindrical jacket and the second cylindrical jacket are separated by an air filled space. 7. Device according to claim 2, wherein the structure in which the cavity is formed is made of graphite. 8. Device according to claim 2, wherein the neutron generator is integrated in the structure of the measurement cell. 9. Device according to claim 2, wherein the neutron counter is integrated in the structure of the measurement cell. 10. Calibration device operable to measure an effective mass of fissile material contained in at least one fission chamber, characterised in that it comprises a count rate measurement device according to claim 1 and a circuit for computing the effective mass of the fissile material from the count rate delivered by the circuit for computing the count rate.