On the Direct Detection of 229m Th

234 Pages·2018·6.328 MB·English

by  Lars Von der Wense (auth.)

Springer Theses
Recognizing Outstanding Ph.D. Research
Lars Von der Wense

On the Direct
Detection of
229mTh
Springer Theses
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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
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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