Springer Theses Recognizing Outstanding Ph.D. Research Lars Von der Wense On the Direct Detection of 229mTh Springer Theses Recognizing Outstanding Ph.D. Research Aims and Scope The series “Springer Theses” brings together a selection of the very best Ph.D. theses from around the world and across the physical sciences. Nominated and endorsed by two recognized specialists, each published volume has been selected foritsscientificexcellenceandthehighimpactofitscontentsforthepertinentfield of research. For greater accessibility to non-specialists, the published versions includeanextendedintroduction,aswellasaforewordbythestudent’ssupervisor explainingthespecialrelevanceoftheworkforthefield.Asawhole,theserieswill provide a valuable resource both for newcomers to the research fields described, and for other scientists seeking detailed background information on special questions. 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More information about this series at http://www.springer.com/series/8790 Lars Von der Wense On the Direct Detection 229m of Th Doctoral Thesis accepted by Ludwig-Maximilians-University, Garching, Germany 123 Author Supervisor Dr. LarsVon derWense Prof. PeterG.Thirolf Department ofMedical Physics Ludwig-Maximilians-University Ludwig-Maximilians-University Garching,Bavaria Garching,Bavaria Germany Germany ISSN 2190-5053 ISSN 2190-5061 (electronic) SpringerTheses ISBN978-3-319-70460-9 ISBN978-3-319-70461-6 (eBook) https://doi.org/10.1007/978-3-319-70461-6 LibraryofCongressControlNumber:2017957623 ©SpringerInternationalPublishingAG2018 Thisworkissubjecttocopyright.AllrightsarereservedbythePublisher,whetherthewholeorpart of the material is concerned, specifically the rights of translation, reprinting, reuse of illustrations, recitation, broadcasting, reproduction on microfilms or in any other physical way, and transmission orinformationstorageandretrieval,electronicadaptation,computersoftware,orbysimilarordissimilar methodologynowknownorhereafterdeveloped. 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Printedonacid-freepaper ThisSpringerimprintispublishedbySpringerNature TheregisteredcompanyisSpringerInternationalPublishingAG Theregisteredcompanyaddressis:Gewerbestrasse11,6330Cham,Switzerland Firstindicationofthedecayof229mTh(centralspot)asobservedintherawdataonOctober15, 2014.Thesignalwasobtainedwhencollecting229Th3+ions,producedintheadecayof233U,with lowkineticenergydirectlyonthesurfaceofamicrochannel-platedetectorconnectedtoaphos- phorscreen.ThepictureshowsaCCDcameraimageofthephosphorscreenwithafield-of-view of about 75 (cid:1) 75 mm2 and 400 s exposure time. Intense signals at the edge of the detector originatefromfieldemissions.Afteramultitudeofexclusionmeasurements,theobservedsignal couldbeunambiguouslyattributedtothethoriumisomericdecay. I dedicate this work to my grandparents Erna and Walter Liedtke as well as Eva-Maria and Herbert Wendorf ’ Supervisor s Foreword The ‘flow of time’ affects our life in all its aspects, from surrounding nature to structuringourdailyactivitiesortotheglobalmemoryofhumanhistory.Presently the precision of our ‘official’ timekeeping is provided by atomic clocks, where electronic transitions in the atomic shell serve as a frequency standard. Optical atomic clocks already achieve a superb precision of 1 second within 10 times the ageoftheuniverse,hardlyconceivabletobefurtherimproved. Nevertheless,there istheintriguing perspectiveofa nuclear clock outperformingeventhebest optical atomic clocks. Since atomic nuclei are about 5 orders of magnitude smaller than atoms with their spacious electron shells, they are much less sensitive to external perturbations like electric or magnetic fields. Therefore a considerably improved stability of a nuclear frequency standard can be expected, based on a suitable transitioninthenuclearlevelscheme.Andherecomestheproblem:amongstallthe presently ca. 3400 known atomic nuclei with their currently known ca. 178,000 excited energy levels, there is only one candidate that qualifies for a nuclear clock transition: the lowest excited state in the whole nuclear landscape, namely the isomeric first excited state of the actinide isotope 229Thorium with a presently assumed excitation energy of 7.8(5) eV (corresponding to a transition wavelength of about 160 nm) and an expected half-life of a few thousand seconds. Thus an extremely narrow relative linewidth of the isomeric ground-state transition of DE=E(cid:3)10(cid:4)20 can be expected, which would be ideally suited to serve as an ultra-precise nuclear frequency standard. This so-called ‘thorium isomer’ was known from indirect measurements since about 40 years, however, and despite intenseworld-wideefforts,couldneverbedirectlydetected.Thusthenuclearclock stayed purely hypothetical. The long-searched direct detection of the isomeric deexcitationtothegroundstatewasnowforthefirsttimeachievedbyLarsvonder Wense in the framework of his Ph.D. thesis, paving the road towards a nuclear clock with far-reaching consequences in many fields, just mentioning here con- siderably improved satellite-based navigation (GPS), relativistic geodesy or the search for temporal variations of fundamental constants like the fine-structure constant. ix x Supervisor’sForeword Receiving his scientific education in physics and mathematics at the University of Hamburg, Lars earned his diploma in physics with a theoretical topic in string theory. After having moved to the University of Cambridge in the UK he passed with honors as a Master of Advanced Study in Applied Mathematics. Despite this strong theoretical background he bravely embarked on a new challenge when taking over the strongly technology-driven experimental task for his Ph.D. project in Munich to search for the direct decay of the thorium isomer. Using a techno- logically highly complex and multi-stage experimental setup he succeeded to generate a pure particle beam of the thorium isomer and could unambiguously detect a signal of the energetic de-excitation of this exotic excited nuclear state. It should be noted that final success was not achieved in the photonic decay branch, which is investigated in all competing projects worldwide (and also in the first phase of Lars’ thesis work), but he draw the right conclusions from the non-observation of decay photons and re-directed the experimental focus towards the internal conversion decay channel, searching for conversion electrons rather than ultraviolet photons, which finally led to a highly visible breakthrough. His thesis describes the impressive scope of experimental work (and theoretical foun- dations)thatwererequiredtoreachthisgoal.Thisachievementgainedwidespread attention worldwide and Lars von der Wense published this pioneering result in May 2016 in ‘Nature’, followed by various media coverages and conference invitations.Attheendof2016,theseresultswerechosenbytheeditorsof‘Physics World’ into the Top-10 list of most important breakthroughs in physics of 2016. IamverypleasedthatSpringerpublishesLarsvonderWense’sexcellentthesis andIamconvincedthatreaderswillalsoappreciatethisexperimental‘pieceofart’. Garching, Germany PD Dr. Peter G. Thirolf August 2017 Abstract Themeasurementoftimehasalwaysbeenanimportanttoolinscienceandsociety. Today’smostprecisetimeandfrequencymeasurementsareperformedwithoptical atomic clocks. However, these clocks could potentially be outperformed by a ‘nuclear clock,’ which employs a nuclear transition instead of an atomic shell transitionfortimemeasurement.Amongthe176,000knownnuclearexcitedstates, there is only one nuclear state that would allow for the development of a nuclear clockusingcurrentlyavailabletechnology.Thisistheisomericfirstexcitedstateof 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 detectionoftheground-statedecayof229mTh.Twodecaychannels(radiativedecay and internal conversion) are experimentally investigated. Only the investigation oftheinternalconversiondecaychannelhasledtothesuccessfulobservationofthe 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 circumventsthegeneralassumedrequirementofabetterknowledgeoftheisomeric energy value, thereby paving the way for nuclear laser spectroscopy of 229mTh. Manyofthepresentedresultshavesofarbeenunpublished.Thisincludesresults of the investigation of a potential radiative decay channel of 229mTh, a negative resultinthesearchforanisomericdecayduringextractionof229Th1+aswellasthe, investigation of the isomeric decay in thorium molecules and on an MgF -coated 2 surface. xi