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Laser spectroscopy of nobelium isotopes

Chhetri, Premaditya :
Laser spectroscopy of nobelium isotopes.
[Online-Edition: https://tuprints.ulb.tu-darmstadt.de/8204]
Technische Universität , Darmstadt
[Ph.D. Thesis], (2018)

Official URL: https://tuprints.ulb.tu-darmstadt.de/8204

Abstract

Laser spectroscopy is a versatile tool to unveil fundamental atomic properties of an element and the ground state information of the atomic nucleus. The heaviest elements are of particular interest as the ordering of their shell electrons is strongly influenced by electron-electron correlations, quantum electrodynamics and relativistic effects leading to distinct chemical behaviour. The elements beyond fermium (Z > 100) are accessible in fusion evaporation reactions at minute quantities and at high energies, hampering their optical spectroscopy. Recently, the RAdiation Detected Resonance Ionization Spectroscopy (RADRIS) technique was employed to explore the electronic structure of the element nobelium (No, Z = 102). The 1S0->1P1 ground state transition of this element was identified.

In this work, the pioneering experiment on laser spectroscopy of nobelium was extended to the isotopes of nobelium (252-254 No). These were produced in fusion-evaporation reactions by bombarding lead targets (206-208 Pb) with 48Ca projectiles. After separation from the primary beam by the velocity filter SHIP (Separator for Heavy Ion reaction Products), at GSI, the fusion products were stopped in 95 mbar high-purity argon gas and collected onto a thin tantalum filament. After a sufficient collection time, which depended on the half-life (T1/2) of the isotope under consideration, the primary beam was blocked in order to have a background free signal. During the beam-off period, the collected nobelium ions were re-evaporated as neutral atoms from the filament and were probed by two laser beams for ionization. The created photo-ions were detected by their characteristic alpha decay. With this technique the isotope shift of the transition was measured for the isotopes 252-254 No. A hyperfine splitting of the 1P1 level was resolved in 253No. These measurements in combination with state-ofthe-art atomic calculations provided a deep insight into the evolution of nuclear deformation of the investigated nobelium isotopes in the vicinity of the deformed shell closure at neutron number N = 152 along with an assesment of the magnetic moment, µ, and the spectroscopic quadrupole moment, Qs, for 253No.

Moreover, several high-lying Rydberg states were measured for the first time in 254No. These Rydberg states, populated in different ways enabled establishing different Rydberg series and the extraction of the first ionization potential of the element plus an additional low-lying atomic state in 254No that is optically inaccessible from the ground state.

Item Type: Ph.D. Thesis
Erschienen: 2018
Creators: Chhetri, Premaditya
Title: Laser spectroscopy of nobelium isotopes
Language: English
Abstract:

Laser spectroscopy is a versatile tool to unveil fundamental atomic properties of an element and the ground state information of the atomic nucleus. The heaviest elements are of particular interest as the ordering of their shell electrons is strongly influenced by electron-electron correlations, quantum electrodynamics and relativistic effects leading to distinct chemical behaviour. The elements beyond fermium (Z > 100) are accessible in fusion evaporation reactions at minute quantities and at high energies, hampering their optical spectroscopy. Recently, the RAdiation Detected Resonance Ionization Spectroscopy (RADRIS) technique was employed to explore the electronic structure of the element nobelium (No, Z = 102). The 1S0->1P1 ground state transition of this element was identified.

In this work, the pioneering experiment on laser spectroscopy of nobelium was extended to the isotopes of nobelium (252-254 No). These were produced in fusion-evaporation reactions by bombarding lead targets (206-208 Pb) with 48Ca projectiles. After separation from the primary beam by the velocity filter SHIP (Separator for Heavy Ion reaction Products), at GSI, the fusion products were stopped in 95 mbar high-purity argon gas and collected onto a thin tantalum filament. After a sufficient collection time, which depended on the half-life (T1/2) of the isotope under consideration, the primary beam was blocked in order to have a background free signal. During the beam-off period, the collected nobelium ions were re-evaporated as neutral atoms from the filament and were probed by two laser beams for ionization. The created photo-ions were detected by their characteristic alpha decay. With this technique the isotope shift of the transition was measured for the isotopes 252-254 No. A hyperfine splitting of the 1P1 level was resolved in 253No. These measurements in combination with state-ofthe-art atomic calculations provided a deep insight into the evolution of nuclear deformation of the investigated nobelium isotopes in the vicinity of the deformed shell closure at neutron number N = 152 along with an assesment of the magnetic moment, µ, and the spectroscopic quadrupole moment, Qs, for 253No.

Moreover, several high-lying Rydberg states were measured for the first time in 254No. These Rydberg states, populated in different ways enabled establishing different Rydberg series and the extraction of the first ionization potential of the element plus an additional low-lying atomic state in 254No that is optically inaccessible from the ground state.

Place of Publication: Darmstadt
Divisions: 05 Department of Physics
05 Department of Physics > Institute of Applied Physics
05 Department of Physics > Institute of Applied Physics > Laser und Quantenoptik
Date Deposited: 18 Nov 2018 20:55
Official URL: https://tuprints.ulb.tu-darmstadt.de/8204
URN: urn:nbn:de:tuda-tuprints-82047
Referees: Thomas, Prof. Dr. Walther and Michael, Prof. Dr. Block
Refereed / Verteidigung / mdl. Prüfung: 16 July 2018
Alternative Abstract:
Alternative abstract Language
Laserspektrosopie ist ein universelles Werzeug für die Bestimmung fundamentaler Eigenschaften der Elektronenstruktur in der Atomhülle aber auch von Eigenschaften des Atomkerns speziell die Laserspektroskopie an den schwersten Elementen ist durch ihre hohe Kernladungszahl von großem Interesse und zusätzlich durch die geringe Verfügbarkeit extrem herausfordernd. Da die Elektronenhülle stark durch Elektron-Elektron-Korrelationen sowie QED- und relativistische Effekte beeinflusst wird, beeinflusst die hohe Ladung und die Vielzahl der Elektronen in den schweren Elementen die Elektronenkonfigurationen und somit auch ihre chemischen Eigenschaften. Die Erzeugung von Elementen schwerer als Fermium (Z > 100) ist lediglich in geringsten Mengen durch Schwerionen-Fusions-Reaktionen möglich und stellt daher eine große Herausforderung für laserspektroskopische Verfahren dar. Erst kürzlich wurde die RADRIS (von engl. RAdiation Detected Resonance Ionization Spectroscopy) Methode erfolgreich verwendet, um die Elektronenstruktur des Elements Nobelium (No, Z = 102) zu untersuchen, bei der unter anderem der 1S0->1P1 Grundzustandsübergang identifiziert werden konnte. Im Rahmen dieser Arbeit wurde diese Technik auf weitere Nobelium-Isotope ausgeweitet, die über die Fusions Reaktion eines 48Ca-Strahls mit einer Bleifolie (206-208Pb) an der GSI Darmstadt erzeugt wurden. Nach ihrer Separation vom Primärstrahl durch das Geschwindigkeitsfilter SHIP (Separator for Heavy Ion reaction Products) werden die Ionen in 95 mbar hochreinem Argongas gestoppt und auf einem Tantal-Filament gesammelt und dadurch neutralisiert. Nach einer geeigneten Akkumulationszeit, die der Halbwertszeit des jeweiligen Isotops angepasst ist, wird der Primärstrahl geblockt, um hintergrundfreie Messungen zu ermöglichen. Während dieser Zeit werden die neutralisierten Nobelium-Atome über ein Aufheizen des Filaments abgedampft, anschließend durch zwei Laserpulse ionisiert und über ihren charakteristischen Alpha-Zerfall nachgewiesen. Mit dieser Methode wurd ein Grundzustandsübergang in den Isotopen 252-254No detailiert vermessen und die Hyperfeinstruktur von 253No bestimmt. Die experimentellen Befunde in Kombination mit modernsten atomaren Berechnungen ermöglichen den Zugang zu Kerneigenschaften und geben somit Aufschluss über die Entwicklung der Deformation der Nobelium-Isotope in der Region des deformierten Schalenabschlusses bei N = 152, dem magnetischen Moment µ und dem spektroskopischen Quadrupolmoment Qs von 253No. Zusätzlich konnten verschiedene hochliegende Rydberg-Zustände in 254No vermessen werden, die von zwei verschiedenen angeregten Zuständen angeregt werden. Über die Konvergenz dieser Zustände konnte das Ionisationspotantial von 254No bestimmt werden.German
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