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Probing Nuclear Structure Relevant for Neutrinoless Double-Beta Decay with Nuclear Resonance Fluorescence

Friman-Gayer, Udo (2020):
Probing Nuclear Structure Relevant for Neutrinoless Double-Beta Decay with Nuclear Resonance Fluorescence.
Darmstadt, Technische Universität, DOI: 10.25534/tuprints-00011385,
[Online-Edition: https://tuprints.ulb.tu-darmstadt.de/11385],
[Ph.D. Thesis]

Abstract

Neutrinoless double-beta (0νββ) decay is a hypothetical second-order process of the weak interaction, which, if observed, would reveal neutrinos as the first example of so-called Majorana particles which are their own antiparticles. Furthermore, since the decay rate for 0νββ decay is directly related to the effective mass of the electron neutrino, it would allow for a direct determination of the neutrino mass. However, an obstacle for the planning of future 0νββ-decay searches and for a quantitative extraction of the neutrino mass are currently the poorly constrained nuclear matrix elements which mediate the decay process. These matrix elements have the be supplied by nuclear theory, which is challenged with the phenomena of nuclear shape evolution and shape coexistence that prevail in regions of the nuclear chart where most 0νββ-decay candidates are located. A major problem is the lack of sensitive experimental data, which are required to fix the parameters of effective theories.

Based on a previous successful study, the nuclear structure of the candidate pairs 82Se/82Kr and 150Nd/150Sm was investigated in this work using the method of nuclear resonance fluorescence. The observables of interest were the decay channels of a low-lying collective nuclear excitation, the scissors mode, which are expected to be highly sensitive to the location of the candidate pairs in the phase diagram of nuclear shapes. The scissors mode can be studied selectively and with a high degree of model indepedence with the chosen method. The experiments were performed at the High-Intensity Gamma-Ray Source which currently provides the most intense, linearly polarized, quasi-monochromatic photon beam at the energies of interest. Using the high sensitivity of the polarized beam, magnetic dipole excitations, which are the manifestations of the scissors mode in even-even nuclei, were identified and their decay behavior was characterized. A known drawback of experiments with monoenergetic photon beams, namely the lack of a photon-flux calibration, was solved in the present work without any additional instrumentation by calibrating the flux on the nonresonant scattering of photons on the targets. For this purpose, a detailed Monte-Carlo particle simulation application was developed.

For all nuclei of interest, decay branches on the order of few percent could either be observed, or constrained to such small values. Two effective nuclear models, the shell model and the interacting boson model, which are also frequently used to predict 0νββ decay matrix elements, were used for a preliminary interpretation of the data. For the nucleus 82Se, the shell model gave a good description of the energies of excited 1+ states and the total observed strength. The good agreement allowed for an interpretation of the structure of the wave functions of the scissors mode candidates, which advised against a simple relation between the measured quantities and the shape coexistence in that nucleus. For the higher-mass isotopes, a careful parameter adjustment in the framework of the interacting boson model was able to reproduce the entire low-energy structure of 150Nd and, with minor exceptions also of 150Sm. The new parameter sets in this model were used together with our collaborator to update previous predictions of nuclear matrix elements for 0νββ decay.

Note that the analysis of the data on the A=150 nuclei was done by Jörn Kleemann. This work presents only his main results.

Item Type: Ph.D. Thesis
Erschienen: 2020
Creators: Friman-Gayer, Udo
Title: Probing Nuclear Structure Relevant for Neutrinoless Double-Beta Decay with Nuclear Resonance Fluorescence
Language: English
Abstract:

Neutrinoless double-beta (0νββ) decay is a hypothetical second-order process of the weak interaction, which, if observed, would reveal neutrinos as the first example of so-called Majorana particles which are their own antiparticles. Furthermore, since the decay rate for 0νββ decay is directly related to the effective mass of the electron neutrino, it would allow for a direct determination of the neutrino mass. However, an obstacle for the planning of future 0νββ-decay searches and for a quantitative extraction of the neutrino mass are currently the poorly constrained nuclear matrix elements which mediate the decay process. These matrix elements have the be supplied by nuclear theory, which is challenged with the phenomena of nuclear shape evolution and shape coexistence that prevail in regions of the nuclear chart where most 0νββ-decay candidates are located. A major problem is the lack of sensitive experimental data, which are required to fix the parameters of effective theories.

Based on a previous successful study, the nuclear structure of the candidate pairs 82Se/82Kr and 150Nd/150Sm was investigated in this work using the method of nuclear resonance fluorescence. The observables of interest were the decay channels of a low-lying collective nuclear excitation, the scissors mode, which are expected to be highly sensitive to the location of the candidate pairs in the phase diagram of nuclear shapes. The scissors mode can be studied selectively and with a high degree of model indepedence with the chosen method. The experiments were performed at the High-Intensity Gamma-Ray Source which currently provides the most intense, linearly polarized, quasi-monochromatic photon beam at the energies of interest. Using the high sensitivity of the polarized beam, magnetic dipole excitations, which are the manifestations of the scissors mode in even-even nuclei, were identified and their decay behavior was characterized. A known drawback of experiments with monoenergetic photon beams, namely the lack of a photon-flux calibration, was solved in the present work without any additional instrumentation by calibrating the flux on the nonresonant scattering of photons on the targets. For this purpose, a detailed Monte-Carlo particle simulation application was developed.

For all nuclei of interest, decay branches on the order of few percent could either be observed, or constrained to such small values. Two effective nuclear models, the shell model and the interacting boson model, which are also frequently used to predict 0νββ decay matrix elements, were used for a preliminary interpretation of the data. For the nucleus 82Se, the shell model gave a good description of the energies of excited 1+ states and the total observed strength. The good agreement allowed for an interpretation of the structure of the wave functions of the scissors mode candidates, which advised against a simple relation between the measured quantities and the shape coexistence in that nucleus. For the higher-mass isotopes, a careful parameter adjustment in the framework of the interacting boson model was able to reproduce the entire low-energy structure of 150Nd and, with minor exceptions also of 150Sm. The new parameter sets in this model were used together with our collaborator to update previous predictions of nuclear matrix elements for 0νββ decay.

Note that the analysis of the data on the A=150 nuclei was done by Jörn Kleemann. This work presents only his main results.

Place of Publication: Darmstadt
Divisions: DFG-Collaborative Research Centres (incl. Transregio)
DFG-Collaborative Research Centres (incl. Transregio) > Collaborative Research Centres
DFG-Collaborative Research Centres (incl. Transregio) > Collaborative Research Centres > CRC 1245: Nuclei: From Fundamental Interactions to Structure and Stars
Profile Areas
Profile Areas > Matter and Radiation Science (MaRS)
05 Department of Physics
05 Department of Physics > Institute of Nuclear Physics
05 Department of Physics > Institute of Nuclear Physics > Experimentelle Kernphysik
05 Department of Physics > Institute of Nuclear Physics > Experimentelle Kernphysik > Nuclear Science
Date Deposited: 26 Jan 2020 20:57
DOI: 10.25534/tuprints-00011385
Official URL: https://tuprints.ulb.tu-darmstadt.de/11385
URN: urn:nbn:de:tuda-tuprints-113857
Referees: Pietralla, Prof. Dr. Norbert and Nörtershäuser, Prof. Dr. Wilfried
Refereed / Verteidigung / mdl. Prüfung: 16 December 2019
Alternative Abstract:
Alternative abstract Language
Der neutrinolose doppelte Betazerfall (0νββ-Zerfall) ist ein hypothetischer Prozess zweiter Ordnung in der schwachen Wechselwirkung, der, im Falle einer experimentellen Beobachtung, Neutrinos als das erste Beispiel von sogenannten Majorana-Teilchen identifizieren würde, welche ihre eigenen Antiteilchen sind. Außerdem würde es eine direkte Bestimmung der Neutrinomasse ermöglichen, da die Zerfallsrate für den 0νββ-Zerfall im direkten Zusammenhang mit der effektiven Masse des Elektron-Neutrinos steht. Ein Hindernis für die Planung von zukünftigen Experimenten, die nach dem 0νββ-Zerfall suchen, und für eine quantitative Bestimmung der Neutrinomasse sind momentan die nicht ausreichend eingeschränkten Kern-Matrixelemente die dem Zerfallsprozess innewohnen. Diese Matrixelemente müssen von der Kernstrukturtheorie bereitgestellt werden, die vor dem Problem der Beschreibung von Phänomenen wie der Entwicklung von Kerngestalten und der Koexistenz von Kerngestalten steht, welche in Regionen der Nuklidkarte vorherrschen, in denen auch die Kandidatenpaare für den 0νββ-Zerfall zu finden sind. Ein Hauptproblem ist der Mangel an aussagekräftigen experimentellen Daten, die benötigt werden um die freien Parameter von effektiven Theorien einzuschränken. Basierend auf einer vorherigen Studie wurde die Kernstruktur der Kandidatenpaare 82Se/82Kr und 150Nd/150Sm in dieser Arbeit mit der Methode der Kernresonanzfluoreszenz untersucht. Zerfallskanäle einer niedrigliegenden, kollektiven Kernanregung, der Scherenmode, waren die wichtigen Observablen, denn sie sind erwartungsgemäß höchst sensitiv auf die Lage der Kandidatenpaare im Phasendiagramm der Kerngestalten. Mit der gewählten Methode kann die Scherenmode selektiv und mit einem hohen Grad an Modellunabhängigkeit untersucht werden. Die Experimente wurde an der High-Intensity Gamma-Ray Source durchgeführt, die momentan im interessanten Energiebereich die intensivsten linear polarisierten quasi-monochromatischen Photonenstrahlen zur Verfügung stellt. Durch die hohe Sensitivität des polarisierten Strahls konnten magnetische Dipolübergänge, als welche sich die Scherenmode in gerade-gerade - Kernen manifestiert, identifiziert und ihr Zerfallsverhalten charakterisiert werden. Ein bekannter Nachteil von Messungen mit monoenergetischen Photonenstrahlen, der Mangel an Möglichkeiten zur Kalibrierung des Photonenflusses, wurde in der vorliegenden Arbeit dadurch umgangen, dass, ohne Zuhilfenahme weiterer Messaufbauten, der Photonenfluss anhand der nichtresonanten Streuung von Gammastrahlung an der Probe kalibriert wurde. Zu diesem Zweck wurde eine detaillierte Monte-Carlo Simulationsanwendung entwickelt. Für alle betrachteten Kerne konnten Zerfallskanäle in der Größenordnung von wenigen Prozenten entweder beobachtet, oder auf solch kleine Werte eingeschränkt werden. Zwei effektive Kernmodelle, das Schalenmodell und das Modell wechselwirkender Valenzbosonen, die auch häufig benutzt werden um 0νββ - Zerfallsmatrixelemente vorherzusagen, wurden für eine vorläufige Interpretation der Daten benutzt. Das Schalenmodell lieferte eine gute Beschreibung der Anregungsenergien von 1+-Zuständen und der gesamten beobachteten Stärke für den Kern 82Se. Die gute Übereinstimmung erlaubte eine Interpretation der Struktur der Wellenfunktionen der mutmaßlichen Scherenmodenfragmente, die jedoch gegen einen einfachen Zusammenhang zwischen den Messgrößen und der Koexistenz von Kerngestalten in diesem Kern spricht. Bei den Isotopen mit höherer Masse konnte eine sorgfältige Anpassung der Parameter des Modells wechselwirkender Valenzbosonen die gesamte Niederenergie-Kernstruktur von 150Nd und, mit Abstrichen, auch die von 150Sm reproduzieren. Die neuen Parametersätze für dieses Modell wurden in Zusammenarbeit mit unserer Kollaborateurin benutzt, um bisherige Vorhersagen von Kernmatrixelementen für den 0νββ-Zerfall auf den neusten Stand zu bringen. Es wird angemerkt, dass die Analyse der Daten für die Kerne mit der Massenzahl 150 von Jörn Kleemann durchgeführt wurde. In dieser Arbeit werden lediglich seine Hauptergebnisse aufgeführt.German
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