On the direct detection of 229mTh Lars von der Wense M¨unchen 2016 On the direct detection of 229mTh Lars von der Wense Dissertation von Lars von der Wense geboren in Elmshorn Fakult¨at f¨ur Physik Ludwig-Maximilians-Universitat¨ Munchen¨ M¨unchen 2016 Erstgutachter: PD Dr. Peter G. Thirolf Zweitgutachter: Prof. Dr. Thomas Udem Tag der Abgabe: 12.12.2016 Tag der m¨undlichen Pr¨ufung: 03.02.2017 Zur direkten Detektion von 229mTh Zusammenfassung Zeitmessung ist seit jeher ein wichtiges Werkzeug in Wissenschaft und Gesellschaft. Die derzeit pr¨azisesten Zeit- und Frequenzmessungen werden mit Hilfe optischer Atomuhren durchgef¨uhrt. Jedoch k¨onnte die Pr¨azision dieser Uhren von einer Kernuhrubertroffen ¨ werden, welche einen Kern¨ubergang anstelle eines atomaren H¨ullen¨uberganges zur Zeitmes- sung verwendet. Von den 176 000 bekannten Kernanregungen erlaubt nur ein einziger die Entwicklung einer Kernuhr unter Verwendung bereits existierender Technologie. Hier- bei handelt es sich um den isomeren ersten Anregungszustand von 229Th, bezeichnet als 229mTh. Trotz 40 Jahren Suche konnte der direkte Zerfall dieser Kernanregung bisher nicht beobachtet werden. Diese Doktorarbeit befasst sich mit Messungen, welche zur erstmaligen direkten Detek- tion des Grundzustandzerfalls von 229mTh gef¨uhrt haben. Zwei Zerfallskan¨ale (strahlender Zerfall und Innere Konversion) wurden experimentell untersucht. Ausschließlich der Zer- fallskanaluber ¨ Innere Konversion hat zur erfolgreichen Detektion des ersten angeregten isomeren Kernzustandes von 229Th gef¨uhrt. Aufbauend auf dieser direkten Detektion wird ein neues Konzept zur laserbasierten Kernanregung von 229mTh vorgeschlagen. Dieses Konzept umgeht die allgemein angenommene Anforderung eines genauer bekannten iso- meren Energiewertes und schafft damit die Voraussetzung f¨ur laserbasierte Kernspek- troskopie von 229mTh. Viele der pr¨asentierten Resultate sind bisher unpubliziert. Dieses beinhaltet die Ergeb- nisse der Untersuchung des strahlenden Zerfallskanals von 229mTh, ein negatives Resultat auf der Suche nach einem isomeren Zerfall w¨ahrend der Extraktion von 229Th1+, die Un- tersuchung des isomeren Zerfalls in Thorium-Molek¨ulen und auf einer MgF2-beschichteten Oberfl¨ache, sowie eine erstmalige Bestimmung der isomeren Halbwertszeit f¨ur neutrales 229Th. I II On the direct detection of 229mTh Summary The measurement of time has always been an important tool in science and society. To- day’s most precise time and frequency measurements are performed with optical atomic clocks. However, these clocks could potentially be outperformed by a “nuclear clock”, which employs a nuclear transition instead of an atomic shell transition for time mea- surement. Among the 176 000 known nuclear excited states, there is only one nuclear state that would allow for the development of a nuclear clock using currently available technology. This is the isomeric first excited state of 229Th, denoted as 229mTh. Despite 40 years of past research, no direct decay detection of this nuclear state has so far been achieved. In this thesis, measurements are described that have led to the first direct detection of the ground-state decay of 229mTh. Two decay channels (radiative decay and internal con- version) are experimentally investigated. Only the investigation of the internal conversion decay channel has led to the successful observation of the first excited isomeric nuclear state of 229Th. Based on this direct detection, a new nuclear laser excitation scheme for 229mTh is proposed. This excitation scheme circumvents the general assumed requirement of a better knowledge of the isomeric energy value, thereby paving the way for nuclear laser spectroscopy of 229mTh. Many of the presented results have so far been unpublished. This includes results of the investigation of a potential radiative decay channel of 229mTh, a negative result in the search for an isomeric decay during extraction of 229Th1+, investigation of the isomeric decay in thorium molecules and on an MgF2-coated surface, as well as a first report of the isomeric half-life for neutral 229Th. III IV spacehold First indication for the decay of 229mTh (central spot) as observed in the raw data on the 15th of October 2014. The signal was obtained when collecting 229Th3+ ions, produced in the α decay of 233U, with low kinetic energy directly on the surface of a microchannel-plate detector connected to a phosphor screen. The picture shows a CCD camera image of the phosphor screen with a field-of-view of about 75 × 75 mm2 and 400 s exposure time. Intense signals at the edge of the detector originate from field emissions. After a multitude of exclusion measurements, the observed signal could be unambiguously attributed to the thorium isomeric decay [1]. V VI spacehold I dedicate this work to my grandparents Erna & Walter Liedtke as well as Eva-Maria & Herbert Wendorf VII VIII Contents Introduction 1 1 Theoretical background 5 1.1Nuclearshellmodels.............................. 5 1.1.1 Nuclearsingleparticleshellmodel................... 5 1.1.2 TheNilssonmodel........................... 7 1.1.3 Rotationalbands............................ 10 1.2 Nuclear γ decayandinternalconversion................... 11 1.2.1 Nuclear γ decay............................. 11 1.2.2 Internalconversion........................... 17 1.3Nuclearlaserexcitation............................. 22 1.3.1 TheopticalBlochequations...................... 22 1.3.2 Laser excitation of 229mTh....................... 29 1.3.3 A comparison to 235mU......................... 31 2 The history of 229mTh 33 2.1Firstpredictionofexistence.......................... 33 2.2 Further evidence for 229mTh.......................... 35 2.3 Constraining the transition energy ....................... 37 2.3.1 Firstenergyconstraints........................ 37 2.3.2 Improved energy determination .................... 38 2.3.3 Acorrectedenergyvalue........................ 39 2.4 Potential applications . ............................ 40 2.4.1 A nuclear clock based on 229mTh.................... 40 2.4.2 Search for temporal variations of fundamental constants ...... 43 2.4.3 A 229mTh-basednuclearlaser...................... 45 2.4.4 Further potential applications ..................... 46 2.5 229mThexcitationanddecay.......................... 47 2.5.1 Basictheoreticalinvestigations.................... 47 2.5.2 Excitation and decay under special conditions ............ 50 2.5.3 Other processes of isomer excitation and decay ............ 51 2.5.4 Coherentcontrolofnuclei....................... 51 2.6Searchforadirectdecay............................ 51 2.6.1 Firstclaimofadirectdetection.................... 52 2.6.2 Search for 229mTh via α decay..................... 52 IX X CONTENTS 2.6.3 Search for 229mThdecayinVUVtransparentmaterial........ 53 2.6.4 Search for 229mThdecayinaPaultrap................ 54 2.6.5 Search for 229mThdecayviainternalconversion........... 55 2.7Furtherexperimentalinvestigations...................... 57 2.7.1 An improved energy determination .................. 57 2.7.2 Probing the 229mThhyperfinestructure................ 57 2.7.3 The search for 229mThatstoragerings................ 58 3 Experimental setup 59 3.1Experimentalconcept.............................. 60 3.1.1 Conceptoftheion-beamformationsystem.............. 60 3.1.2 Searching for a photonic decay ..................... 62 3.1.3 Searching for an internal-conversion decay .............. 64 3.2Theion-beamformationsystem........................ 66 3.2.1 The 233Usource............................. 66 3.2.2 The buffer-gas stopping cell ...................... 69 3.2.3 TheDCcage.............................. 72 3.2.4 The RF+DC funnel ........................... 73 3.2.5 TheLavalnozzle............................ 74 3.2.6 The extraction RFQ . ........................ 75 3.2.7 TheQuadrupoleMass-Spectrometer................. 76 3.2.8 The triode extraction system ...................... 78 3.3Theopticalsystem............................... 80 3.3.1 Thecollectionsurface.......................... 82 3.3.2 Thefirstparabolicmirror....................... 84 3.3.3 The second parabolic mirror ...................... 86 3.3.4 Opticalfilters.............................. 88 3.3.5 Theopticalchamber.......................... 89 3.4Thedetectionsystem.............................. 90 3.4.1 TheMCPdetector........................... 91 3.4.2 TheCCDcamera............................ 94 3.5Efficiencyestimates............................... 95 3.5.1 Efficiency estimate for the ion extraction ............... 95 3.5.2 Efficiencyestimateforaphotonicdecay............... 96 3.5.3 Efficiencyestimateforaninternal-conversiondecay......... 97 4 Measurements 99 4.1Preparatorymeasurements........................... 99 4.1.1 Extraction rate for 229Thions..................... 99 4.1.2 Verificationoftheioncollection....................103 4.1.3 Verificationoftheopticalperformance................106 4.2 Searching for the 229Thisomericdecay....................109 4.2.1 Investigationofthephotonicdecaychannel..............109 4.2.2 Investigationoftheinternal-conversiondecaychannel........113 4.2.3 Investigationoftheisomericproperties................117 XI 4.3Confirmationmeasurements..........................119 4.3.1 Backgroundsignalsoriginatingfromsetupcomponents.......119 4.3.2 Background effects caused by ionic impact . .............120 4.3.3 Nuclear decay signals other than 229mTh...............122 4.3.4 Signalscausedbyexcitedshell-states.................126 4.3.5 Estimation of the 229,228Thintrinsicactivity.............127