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Laser Spectroscopy of the Boron Isotopic Chain Laser Spectroscopy of the Boron Isotopic Chain Laserspektroskopie in der Bor-Isotopenkette Zur Erlangung des Grades eines Doktors der Naturwissenschaften (Dr. rer. nat.) genehmigte Dissertation von Bernhard Maaß aus Hanau Tag der Einreichung: 05.11.2019, Tag der Prüfung: 20.01.2020 Darmstadt — D 17 1. Gutachten: Prof. Dr. Wilfried Nörtershäuser 2. Gutachten: Prof. Dr. Thomas Aumann Fachbereich Physik Institut für Kernphysik AG Nörtershäuser Laser Spectroscopy of the Boron Isotopic Chain Laserspektroskopie in der Bor-Isotopenkette Genehmigte Dissertation von Bernhard Maaß aus Hanau 1. Gutachten: Prof. Dr. Wilfried Nörtershäuser 2. Gutachten: Prof. Dr. Thomas Aumann Tag der Einreichung: 05.11.2019 Tag der Prüfung: 20.01.2020 Darmstadt — D 17 Erklärung gemäß §9 Promotionsordnung Hiermit versichere ich, dass ich die vorliegende Dissertation selbstständig angefertigt und keine anderen als die angegebenen Quellen und Hilfsmittel verwendet habe. Alle wörtlichen und paraphrasierten Zitate wurden angemessen kenntlich gemacht. Die Arbeit hat bisher noch nicht zu Prüfungszwecken gedient. Darmstadt, den 31. Oktober 2019 (B. Maaß) i Bitte zitieren Sie dieses Dokument als: URN: urn:nbn:de:tuda-tuprints-114846 URL: https://tuprints.ulb.tu-darmstadt.de/id/eprint/11484 Dieses Dokument wird bereitgestellt von tuprints, E-Publishing-Service der TU Darmstadt http://tuprints.ulb.tu-darmstadt.de [email protected] Die Veröffentlichung steht unter folgender Creative Commons Lizenz: Namensnennung – Weitergabe unter gleichen Bedingungen 4.0 International http://creativecommons.org/licenses/by-sa/4.0/deed.de Abstract The charge radius of a nucleus is a fundamental physical property that corresponds to the binding strength and the structure of the bound nuclear system. This observable is particularly important for the investigation of so-called halo nuclei, which consist of a compact nuclear core with typical nuclear density and a dilute cloud of halo nucleons. Neutron halo nuclei have been subject to numerous investigations, but little is known about proton-halo systems. The isotope 8B is believed to be a prototype of a proton-halo, which was concluded indirectly from measurements of its quadrupole moment and analysis of the momentum distribution of breakup fragments. High-precision laser spectroscopy can provide direct proof of the halo structure by measuring the nuclear charge radius. Such mea- surements were performed previously for the most prominent (neutron) halo isotopes up to Z = 4 (beryllium). = Extending these investigations to the boron isotopes (Z 5) is the subject of this thesis. It covers the design and 8 = installation of the experimental setup for the on-line measurement of the short-lived B t1=2 770 ms as well as 10 11 first off-line measurements of the charge radius difference between the two stable isotopes B and B. The halo-candidate 8B is produced in a 3He 6Li,8B n reaction in inverse kinematics and the energetic 8B ions are stopped in a gas cell and then transported to the collinear beamline by radiofrequency ion guides. The production of 8B was optimized by improving the cryogenic gas target to avoid saturation effects. It was observed that the 8B ions that leave the gas cell have water molecules attached. While this improves the transport process, it prohibits laser spectroscopy on the atomic system. Therefore, a molecular breakup station based on the transmission of the molecules through nanometer-thin car- bon foils was designed, built and tested. The apparatus to perform collinear laser spectroscopy was also installed at the experimental site during this work and the performance of its fluorescence detection region has been sig- nificantly improved. The progress achieved represents a significant step towards laser spectroscopy on the exotic proton-halo candidate 8B in the near future. The isotope shift between the stable isotopes of boron 10B and 11B was measured on a collimated atomic beam. Beams of two lasers crossed the atomic beam in perpendicular alignment to perform resonance ionization mass spectrometry. The first laser excited the 2p ! 3s ground state transition and the second laser was used for non- resonant ionization of the excited atoms. Exact control of the overlap angle of laser and the atomic beam is essential to eliminate the first-order Doppler effect, which otherwise introduces significant uncertainties for the spectroscopy of these light ions. Therefore, an elaborated double-pass scheme has been used where the uncertainty in the overlap angle was minimized together with other sources of systematic uncertainties that are typically prevalent in laser spectroscopy. The isotope shift between 10B and 11B was measured for the first time with sufficient accuracy to extract the difference in the mean-square nuclear charge radius. Results are compared with predictions from state- of-the-art ab-initio nuclear structure theories and show reasonable agreement with the no-core shell model as well as Green’s function Monte Carlo calculations. iii Zusammenfassung Der Kernladungsradius ist eine fundamentale physikalische Observable, welche Aufschluss über die Größe und die Bindungsstärke eines Atomkerns gibt. Besondere Aussagekraft besitzt sie bei sogenannten Halokernen, welche aus einem kompakten Kern typischer Kerndichte bestehen, der von einer „diffusen Wolke“ verdünnter Kernmaterie, bestehend aus einem oder mehreren Halo-Nukleonen, umgeben ist. Neutronen-Halokerne wurden in einer Vielzahl an Experimenten untersucht, Protonen-Halos aber sind noch wenig erforscht. Das Isotop 8B wird als Prototyp eines Protonen-Halokerns gehandelt, dies aber nur auf Basis indirekter Messungen: sein großes Quadrupolmoment und die schmale Impulsverteilung der Komponenten in Aufbruchreaktionen sind Indizien für diese Zuordnung. Einen direkten Nachweis der Halo-Struktur kann eine laserspektroskopische Untersuchung liefern, wie sie bereits an Neutronen-Halokernen von Elementen mit Kernladungszahl Z = 2 (Helium) bis Z = 4 (Beryllium) durchgeführt wurde. Das Thema dieser Arbeit ist die Ausweitung dieser Messungen auf die Isotope von Bor (Z = 5). Dazu wurde 8 = der für die Messung des kurzlebigen Isotops B (Halbwertszeit: t1=2 770 ms) notwendige Aufbau entworfen und installiert. Weiterhin gelang eine erste präzise Messung der Differenz der Ladungsradien der beiden stabilen Isotope 10 11 B und B. Das Isotop 8B wird in der Reaktion 3He 6Li,8B n in inverser Kinematik produziert, anschließend in einer Gaszelle gestoppt und mit Radiofrequenzquadrupolen zum Experimentieraufbau transportiert. Die Produktion von 8B wurde optimiert, indem ein neues kryogenes Target entworfen wurde, welches Sättigungseffekte unterdrückt. Allerdings wurde festgestellt, dass 8B die Gaszelle als Molekül im Verbund mit Wassermolekülen verlässt. Tatsächlich er- höht dies die Transporteffizienz aufgrund der höheren Masse, Laserspektroskopie aber kann nur am reinen Atom durchgeführt werden. Deshalb wurde eine Molekülaufbruchstation entwickelt, aufgebaut und getestet, in welcher die Molekülionen durch eine nur wenige Nanometer dünne Folie transmittiert werden. Zugleich wurde der Exper- imentaufbau für die kollineare Laserspektroskopie vor Ort in Betrieb genommen, und die Effizienz der Fluoreszen- znachweisregion signifikant erhöht. Der erzielte Fortschritt ist wegweisend für die Messung des Protonen-Halo Kandidaten 8B in der nahen Zukunft. Die Isotopieverschiebung zwischen den beiden stabilen Bor-Isotopen 10B und 11B wurde an einem kollimierten Atomstrahl gemessen. Zwei Laserstrahlen wurden dafür mit einem Atomstrahl in lotrechter Geometrie überlagert um Resonanz-Ionisationsspektrometrie durchzuführen. Einer der beiden Laser regt den 2p ! 3s Grundzustands- übergang an, während der zweite Laser die resonant angeregten Atome ionisiert. Die Kontrolle des Winkels zwis- chen Atom- und Laserstrahl ist von Bedeutung, da ein Winkel, der von 90° abweicht, eine Dopplerverschiebung verursacht, welche gerade für leichte Ionen große Unsicherheiten verursacht. Um dies zu umgehen, wurde ein Rücküberlagerungsschema verwendet, in welchem die Unsicherheit der Winkelbestimmung sowie weitere typische systematische Unsicherheiten umgangen werden. Dadurch konnte die Isotopieverschiebung zwischen 10B und 11B zum ersten Mal mit einer Genauigkeit gemessen werden, welche die Extraktion der Differenz zwischen den beiden Kernladungsradien erlaubt. Dieses Ergebnis wird mit modernen theoretischen Kernstruktur-Modellen verglichen und zeigt eine gute Übereinstimmung mit Modellrechnungen des sogenannten „Green’s function Monte Carlo“ und des „no-core shell model“. v Contents List of Figures xi List of Tables xiii List of Acronyms xv 1. Motivation 1 1.1. Historical context . 1 1.2. Light and exotic: halo nuclei . 2 1.3. The interplay between atomic and nuclear physics . 3 1.4. Laser spectroscopy of the boron isotopic chain . 3 2. Nuclear Theory 5 2.1. The size and properties of boron . 5 2.1.1. Point-proton and nuclear charge radius . 5 2.1.2. The proton halo of 8B............................................. 6 2.1.3. The halo distance and charge radii . 7 2.1.4. Value and uncertainty estimates . 8 2.2. Advanced many-body models . 9 2.2.1. Nucleon-nucleon potentials . 9 2.2.2. Green’s function Monte Carlo . 11 2.2.3. The no-core shell model . 12 3. Atomic Theory 15 3.1. Atomic spectra . 15 3.1.1. Isotope shift . 16 3.1.2. Hyperfine structure . 17 3.1.3. Nuclear observables in atomic spectra . 18 3.2. Boron atoms . 20 3.2.1. Exact calculations in the boron system . 20 4. Experimental Methods 23
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