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Caractérisation de HOPE un piège magnéto-gravitationnel à neutrons ultra-froids destiné à la mesure du temps de vie du neutron sur SUN2 une source superthermale de neutrons ultra-froids

Abstract : Neutron decay offers a plethora of observables which are useful for applications in nuclear and particle physics and tests of the standard model of the latter. One of them, the neutron lifetime (880.2(10) s in the PDG 2018), is a crucial input for estimation of the helium-4 content of the universe resulting from the primordial nucleosynthesis. In addition, it serves for a sensitive test of the consistency of the standard model through the CKM matrix unitarity. During the past decades, its measurement has revealed notorious difficulties and still unresolved inconsistencies between different methods. Projects for reliable measurements of the neutron lifetime are therefore of high interest.In this work we present the project HOPE, an ultra-cold neutron (UCN) magneto-gravitational trap coupled to a dedicated superthermal UCN source and aiming for a neutron lifetime measurement with accuracy below 1 s. The choice of magnetic trapping removes a major systematic uncertainty due to neutron interactions with material walls. However, magnetic trapping techniques have to ensure that marginally trapped neutrons that still are able to hit those walls are sufficiently well characterized not to induce significant systematic errors. Since the evolution of those neutrons in the trap is difficult to predict, we have developed simulations that analysed neutron trajectories existing in the trap. Analysis of many neutron histories provided information about the modification of the neutron spectrum during the filling of the trap, heating of neutrons due to vibrations, and the spectral composition of the emptying.These simulations accompany first experimental results obtained from the coupling of HOPE and the prototype helium-4 superthermal UCN source SUN2 set up at Institut Laue-Langevin. Our measurements have shown that, using the vertical implementation studied in this thesis, a measurement of a statistical precision of 0.75 s per reactor cycle is within reach, although sources of systematic uncertainty still need to be carefully analysed to provide an unbiased measurement. Still, we measured in two different procedures storage time constants of 889(19) s and 882(17) s in agreement with the current world-average value of the neutron lifetime, the error being due to counting statistics and the systematic uncertainty staying undetermined. Though not yet proven there are experimental indications, supported by simulations, that neutron heating from mechanical vibrations might limit the accuracy attainable with the current setup. A simplified new setup, with the trap implemented in horizontal orientation without cryostat (thus avoiding strong vibrations of the mono-stage GM cold head used in the current setup) is described at the end of this thesis. According to simulations such a configuration increases counting statistics by a factor 3, while strongly improving on a major systematic uncertainty by spectral heating due to vibrations.
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Loris Babin. Caractérisation de HOPE un piège magnéto-gravitationnel à neutrons ultra-froids destiné à la mesure du temps de vie du neutron sur SUN2 une source superthermale de neutrons ultra-froids. Physique Nucléaire Expérimentale [nucl-ex]. Université Grenoble Alpes, 2019. Français. ⟨NNT : 2019GREAY053⟩. ⟨tel-02527327⟩

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