Heures thésards

Animé par Richard Dallier

vendredi 19 novembre 2021 à 11:00

Amphi Georges BESSE

Présentations des équipes Xénon et S.E.N.

11h00
Titre : Développement d’une détection par SPC pour la recherche de la désintégration double bêta sans émission de neutrinos
Présentation de Vincent Cecchini (équipe Xénon)

R2D2 est un programme de R&D dont l’objectif est d’évaluer la possibilité d’utiliser une chambre à projection temporelle (TPC) à haute pression de gaz dans le cadre des recherches de décroissance double bêta sans émission de neutrino. L’observation de ce processus extrêmement rare (si il existe) nécessite de réaliser des mesures avec une ex- cellente résolution en énergie, dans un environnement présentant un très faible bruit de fond radioactif. A ces conditions s’ajoute le besoin d’une grande quantité d’isotopes (1T) susceptibles de décroître via une radiation ββ, afin de compenser la rareté du phénomène.

Avant de travailler à échelle réelle, un premier prototype a été construit au CENBG pour étudier la résolution que ce type de détecteur peut atteindre. Celle-ci est notamment liée au comportement du gaz, à l’électronique et à la chaîne d’acquisition, ou encore aux conditions expérimentales. Pour évaluer ces différents paramètres, le prototype est utilisé avec un mélange d’ArgonP2 (98% Ar + 2% CH4) et calibré avec une source alpha de ²¹⁰Po. Dans le même temps, des travaux de simulation sont menés pour pouvoir travailler sur la forme des signaux et leur traitement, ainsi qu’orienter les développements futurs.
La prochaine étape sera de caractériser la résolution en utilisant du Xenon, gaz prévu pour la version finale puisqu’ayant un isotope émetteur de radiations ββ.

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11h30

Titre : First-forbidden β-decay study in the pnQRPA approach
Présentation de Arthur Beloeuvre (équipe SEN)

First-forbidden beta decays play an important role in several domains of physics. First, in astrophysics, where nuclear data such as the half-life govern stellar evolution and nucleosynthesis [1]. Second, they are of interest for nuclear reactors physics as first highlighted in 2014 [2]. In first-forbidden β-decays, the form factor of the leptonic spectra are not equal to one as for al- lowed decays. It has been shown that it could have a non negligeable impact on the shape of the antineutrino energy spectra. Among the models developed since then, which do not all tend to agree [3, 4, 5, 6], some even state that it could solve the reactor antineutrino shape anomaly. New theoretical calculations of the first-forbidden form factors associated to summation calcu- lations [7] and dedicated experimental measurements would be useful to corroborate or negate already existing predictions.
Charge-exchange excitations corresponding to beta-decay first forbidden transitions in nuclei have been studied in the self-consistent proton-neutron quasiparticle random-phase approximation (pnQRPA) using the finite-range Gogny interaction [8]. No parameters beyond those included in the effective nuclear force are included. Axial deformations are taken into account for both the ground state and charge-exchange excitations.
With this formalism, nuclear matrix elements have been computed for operators derived from the multipole expansion of the weak current [9]: spin-dipole, anti-analog dipole and pseudoscalar-axial vector and tensor-polar vector operators. Those operators come to complete the already existing Fermi and Gamow-Teller operators already considered in Ref. [8] in order to have a simultaneous description of the allowed and first-forbidden β-decays.

At this conference, first results of the charge-exchange operators will be presented for both spherical and axially deformed nuclei with a comparison to other theoretical models.

References :

[1] M. Arnould, S. Goriely, and K. Takahashi. The r-process of stellar nucleosynthesis: Astro- physics and nuclear physics achievements and mysteries. Phys. Rept., 450:97–213, 2007.
[2] A. C. Hayes, J. L. Friar, G. T. Garvey, Gerard Jungman, and Guy Jonkmans. Systematic Uncertainties in the Analysis of the Reactor Neutrino Anomaly. Phys. Rev. Lett., 112:202501, 2014.
[3] Dong-Liang Fang and B. Alex Brown. Effect of first forbidden decays on the shape of neutrino spectra. Phys. Rev. C, 91(2):025503, 2015. [Erratum: Phys.Rev.C 93, 049903 (2016)].
[4] X. B. Wang and A. C. Hayes. Weak magnetism correction to allowed β decay for reactor antineutrino spectra. Phys. Rev. C, 95(6):064313, 2017.
[5] X. B. Wang, J. L. Friar, and A. C. Hayes. Nuclear Zemach moments and finite-size corrections to allowed β decay. Phys. Rev. C, 94(3):034314, 2016.

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[6] J. Petkovic, T. Marketin, G. Martınez-Pinedo, and N. Paar. Self-consistent calculation of the reactor antineutrino spectra including forbidden transitions. J. Phys. G, 46(8):085103, 2019.
[7] M. Estienne et al. Updated Summation Model: An Improved Agreement with the Daya Bay Antineutrino Fluxes. Phys. Rev. Lett., 123(2):022502, 2019.
[8] M. Martini, S. Peru, and S. Goriely. Gamow-Teller strength in deformed nuclei within the self- consistent charge-exchange quasiparticle random-phase approximation with the Gogny force. Phys. Rev. C, 89(4):044306, 2014.
[9] Aage Bohr and Ben R Mottelson. Nuclear Structure. World Scientific Publishing Company, 1998.