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Development of calibration sources for proton spectroscopy

Abstract : Proton spectroscopy in neutron beta decay gives a complementary access to $lambda$ (ratio of the weak coupling constants gA/gV and enables new searches for physics beyond the Standard Model. In experiment, low-energy protons (E< 751.4 eV) are usually guided and selected using electromagnetic fields. Precise knowledge of the electrostatic potentials is mandatory as it can drastically bias proton selection. For instance, electrostatic potentials have to be known with an accuracy of a few mV in the proton spectroscopy instruments aSPECT and PERC in order to reach their aspired precision. As experimental conditions can directly impact the field (temperature effects on electrode position and work function, surface conditions, charging traps, etc.), it is mandatory that electrostatic measurements are performed in-situ. Further systematic effects are related to proton detection. In addition to prior off-line detector characterization, it is important to verify detector resolution, homogeneity, backscattering probability etc. in-situ. The protons from neutron decay itself are not suitable for this purpose because of their broad energy distribution and insufficient localization.The goals of this project were to create calibration sources for in-situ electrostatic measurements and proton detector characterizations and to build a dedicated test setup to characterize and optimize the sources.For electrostatic measurements, the process of positron moderation was identified as most promising. Positron moderation creates a beam of positively charged particles with a very narrow energy spread (FWHM of a few tens of meV) and with a well-defined emission angular distribution. Such a beam would allow to directly compare, inside the spectrometer, potential differences between different electrodes.For detector characterization, available commercial proton sources are difficult to couple to high magnetic fields and often induce a deterioration of the vacuum quality in experiments, making it complicated to use detection systems at high voltage (-15 to -30 kV). Electron Stimulated Desorption (ESD) of hydrogen adsorbed on a crystal surface was found to provide the desired properties: a sharp and well-defined energy distribution of the created proton beam and compatibility with ultrahigh vacuum.The aSPECTino spectrometer was built as test setup. It is a MAC-E filter which uses electro-magnetic fields to guide and select low-energy charged particles before detecting them in a solid-state detector. The detector is set at high voltage to post-accelerate the selected particles. The resistive coils of the spectrometer produce a magnetic field between 3.5 and 16 mT which is sufficient to confine low-energy positrons. Low-energy protons with a small radial momentum component can also be effectively guided onto the detector.CALIPSO, which stands for CALIbration Positron/proton SOurce, is a two-in-one calibration source: one apparatus is designed to provide, not at the same time, both positrons and protons. Its core is a tungsten (110) crystal. In the case of positrons the tungsten crystal is coupled to a 22Na positron source. The crystal serves as positron moderator and re-emits a fraction of the primary positrons from the Na source, with low energy and a small angular and energy spread. In the proton configuration the same crystal is used as substrate for adsorbed hydrogen. ESD is induced by electrons emitted from a hot cathode and hitting the tungsten crystal.This thesis introduces the physical processes used to create the low-energy positron and proton beams of CALIPSO as well as the design and the development of both the aSPECTino spectrometer and the CALIPSO source. It presents first experimental results of the preliminary characterizations of aSPECTino and CALIPSO. The expected performances of CALIPSO in both configurations and the sensitivity for comparisons of electrode potentials are derived and demonstrated by simulations.
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Romain Virot. Development of calibration sources for proton spectroscopy. Instrumentation and Detectors [physics.ins-det]. Université Grenoble Alpes, 2017. English. ⟨NNT : 2017GREAY048⟩. ⟨tel-01731369⟩

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