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Développement d’un système de mesure directe du débit d’émission de sources neutroniques

Florestan Ogheard 1
1 LNHB - Laboratoire National Henri Becquerel
DM2I - Département Métrologie Instrumentation & Information : DRT/LIST/DM2I
Abstract : The manganese bath technique is the reference method for neutron source emission rates calibration. It is used to calibrate neutron sources using radionuclides (AmBe, PuBe, 252Cf,…) in terms of neutron emission rate under 4π sr. As a complement to this technique, the anisotropy of the source is measured using a rotating source holder and a neutron long counter. The neutron source to be measured is immersed in a manganese sulphate solution whereby the emitted neutrons are captured within the bath contents. In a typical configuration (a 1m diameter sphere and a concentrated solution), approximately half of the neutrons lead to the creation of 56Mn via the 55Mn(n, γ) capture reaction. The 56Mn radionuclide has a half-life of approximately 2.6 hours and the bath reaches saturation when the number of nuclei decaying is equal to the number of nuclei created per unit time. The neutron emission rate from the source can then be deduced from the 56Mn activity at saturation, assuming proper modelling of the nuclear reactions occuring in the bath. The manganese bath facility at LNE-LNHB has been recently refurbished in order to comply with appropriate safety and radioprotection regulations. This has lead to the upgrading of both the measurement methodology and the modelling of the bath, and a study on the development of a new detector for the on-line measurement of the manganese activity was started. This new detector uses the β-γ coincidence measurement method. The bêta channel consists of two photomultipliers tubes which allow the detection of Cerenkov light, and the gamma channel uses a solid scintillation detector. The advantage of this measurement method is that it allows the determination of the bath activity without any prior calibration, unlike the former method which uses a gamma-ray detector calibrated using a high activity manganese source. The principle of the Cerenkov-gamma coincidence measurement has been validated by a prototype of the detector and via modelling of the system using the stochastic transport code GEANT4. The final detector has also been made and the results obtained have been compared to those from a primary measurement method already in use at LNE-LNHB. Furthermore, a comparison of the results from modelling the manganese bath with GEANT4, MCNPX and FLUKA have been undertaken to find the most reliable code. This comparison lead to the identification of various weaknesses, particularly in GEANT4, and several uncertainty factors, such as the modeling of the neutron emission and the choice of the cross-section library. Finally, neutron source calibration has been carried out with the Cerenkov-gamma method and the correction factors given by the new modeling of the bath using MCNPX. These results have been complemented with a comparison with the former method simultaneously undertaken, after calibration of the detector in the bath using a 56Mn source irradiated in a nuclear reactor. At the end of this study, several improvements have been proposed, from which a number are currently under development at LNE-LNHB.
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Florestan Ogheard. Développement d’un système de mesure directe du débit d’émission de sources neutroniques. Autre [cond-mat.other]. Université Paris Sud - Paris XI, 2012. Français. ⟨NNT : 2012PA112176⟩. ⟨tel-00740509⟩

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