The Two-Photon Decay of the 11-/2 Isomer of 137Ba and Mixed-Symmetry States of 92,94Zr and 94Mo

169 Pages·2016·7.894 MB·English

by  Christopher Walz (auth.)

Springer Theses
Recognizing Outstanding Ph.D. Research
Christopher Walz

The Two-Photon Decay
of the 11–/2 Isomer
of 137Ba and Mixed-
Symmetry States of
92,94Zr and 94Mo
Springer Theses
Recognizing Outstanding Ph.D. Research
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Christopher Walz
The Two-Photon Decay
(cid:1)
11 137
of the Isomer of Ba
2
and Mixed-Symmetry States
92,94 94
of Zr and Mo
Doctoral Thesis accepted by
TU Darmstadt, Germany
123
Author Supervisor
Dr. Christopher Walz Prof. NorbertPietralla
Institut für Kernphysik Institut für Kernphysik
TU Darmstadt TU Darmstadt
Darmstadt Darmstadt
Germany Germany
ISSN 2190-5053 ISSN 2190-5061 (electronic)
SpringerTheses
ISBN978-3-319-27181-1 ISBN978-3-319-27183-5 (eBook)
DOI 10.1007/978-3-319-27183-5
LibraryofCongressControlNumber:2015958081
©SpringerInternationalPublishingSwitzerland2016
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’
Supervisor s Foreword
The discoveries of various types of radioactivity are most closely related with the
names of giants of physics, such as Henri Becquerel, Ernest Rutherford, Marie
Curie, Paul Villard, Enrico Fermi, Otto Hahn and others. We read about their
scientific contributions as historical achievements in a field that evolved in a
breath-taking way a century ago and which today, in the more general scope of
sub-atomic physics, represents the research topic of several national and interna-
tional large-scale research laboratories with several hundred millions of Euros as
annualoperatingbudgets.Fromthisperspectiveitisastounding—andformemost
fascinating—tohavewitnessedhowsignificantlyadedicatedandcourageousjunior
scientist in a university laboratory can contribute to this field, namely by discov-
eringanewvariantofradioactivity—thecompetitivedouble-gammadecay—some
85 years after its theoretical formulation, as Dr. Christopher Walz has done.
It is with a sense of pride, satisfaction and admiration that I write this foreword
tothepublicationofDr.Walz’sthesis.Ishouldadmitfranklythatthe‘discoveryof
anewmodeofradioactivity’wasnottheinitialproblemthatIencouragedDr.Walz
to work on. Instead, like many other achievements in science, it evolved from a
subsidiary task as a result of the inspiring atmosphere at the university and due to
advanced experimental skills and extraordinary dedication. This happened also to
Dr.Walz.Finally,hehassubmittedathesiswithan“…and…”-titlecombiningtwo
distinct research parts. Both parts would each have been enough for a very good
doctoral graduation.
The first topic Dr. Walz worked on addressed the proton–neutron balance of
collective excitations of the nuclear valence shell. From earlier work, it was clear
that there exist low-energy nuclear quantum states, so-called ‘mixed-symmetry
states’, in which protons and neutrons beyond the nuclear shell closures partly
moveoutofphase.Thisoriginatesfromarelativephasebetweenthecontributions
of valence protons and valence neutrons to the wave functions of these nuclear
quantum states. The scientific significance of mixed-symmetry states lies in their
particularsensitivitytoaspectsoftheeffectiveproton–neutroninteractioninnuclear
matter. That peculiar structure manifests itself in the occurrence of enhanced
v
vi Supervisor’sForeword
magnetic dipole radiation in their radioactive decay transitions, the observation of
which was previously considered as the best, although to some extent an indirect,
signature of mixed-symmetry character.
Dr. Walz’s thesis demonstrates a more direct experimental confirmation of the
out-of-phase motion of valence protons and valence neutrons in nuclear
mixed-symmetrystates.Hecomparedinelasticscatteringreactionsofboth,protons
and electrons, on nuclear mixed-symmetry states of selected nuclides in the mass
rangeofmassnumber90.Hemadeuseofthefactthattheseprobesreactdifferently
with the nuclear constituents. Electrons interact electromagnetically with the
nuclearcharges,i.e.withprotons,only.Incontrast,protonsinteractasprobeswith
the entire nuclear matter and even more sensitively with the neutrons due to the
dominant effective proton–neutron interaction. From a comparison, Dr. Walz
managedtodisentangletheprotoncontributionandtheneutroncontributiontothe
wave functions of nuclear mixed-symmetry states. He established a characteristic
differenceintheexcitationtransition radiiofmixed-symmetry states ofthestudied
nuclei in electron-scattering or proton-scattering reactions, respectively, and man-
aged to trace it back to the relative phase of protons and neutrons in the nuclear
wave function. This is a major experimental achievement and puts the research on
nuclear mixed-symmetry states on firm ground.
The second—and even more significant—achievement of Dr. Walz originated
from a topic initially tangential to his work. He was supposed to set up a newly
acquired array of radiation spectrometers that are distinct from conventional
detectors in having a significantly better time resolution along with a good energy
resolutionforthedetectionofgammaradiation.Thedetectorarraywassupposedto
becalibratedwithgammaradiationfromastandardradiationsourceofthekindone
encounters in every gamma radiation laboratory. This is a routine task which does
not promise outstanding science at first glance. At the same time, Dr. Walz’s
academic co-supervisor, Dr. Heiko Scheit, raised Dr. Walz’s interest in the phe-
nomenon of double-gamma decay, a radioactive decay mode of second order in
electromagneticinteraction,whichhasneverbeforebeenobservedtocompetewith
aquantum-mechanicallyallowedsingle-gammatransition.Consequently,Dr.Walz
opted to search for the competitive double-gamma decay mode and to detect it for
the first time with the new spectrometer array at hand. I frankly admit my initial
scepticism about this heroic challenge, but Dr. Walz convinced meby quantitative
arguments that he could be successful. We agreed upon giving it a try for a while.
After several unsuccessful attempts to reduce the prohibitively large measurement
background he was close to giving up the project. But the quantitative arguments
were still standing and we decided to continue and to improve the set-up further.
Finally, clear evidence for the competitive double-gamma decay was observed in
the data and the superior time resolution of the new detectors made the interpre-
tation of the data unambiguous.
Supervisor’sForeword vii
Besidestheadvancedinstrumentation,Dr.Walz’soptimism,dedicationandskill
were key factors in our discovery. It has been a great pleasure to have been part
of the research team and to work with him as his supervisor—and as a colleague.
Darmstadt Prof. Norbert Pietralla
October 2015
Abstract
Thefirstpartofthisthesiscoversthetwo-photondecayofthe11(cid:1) isomerin137Ba.
2
Clearevidenceofthisexoticdecaymodeisfound,whichopensupthepossibilityto
usethetwo-photondecayprobabilityasageneraltooltoinvestigatethestructureof
atomic nuclei. Details on the experiment are given, and the major challenges to
successfully perform this difficult experiment are outlined. The two-photon decay
experimentwasconductedwiththenewLaBr -detectorballwhichwassetupfrom
3
scratch in the framework of this thesis. The properties and performance of this
powerful detector system—consisting of 18 large LaBr -detectors—are discussed.
3
Furthermore, proton scattering at 200 MeV has been performed on the nuclei
92,94Zr,94,96Moand70Zn.Themeasuredcrosssectionsarecomparedtocalculations
in the framework of the Quasi-Particle Phonon Model (QPM). Conclusions about
the validity of the QPM results are drawn. A comparison of the excitation cross
þ
sectionsofthe2 statesandtheone-phononquadrupolestateswithmixedneutron
1
and proton symmetry (2þ) allows to prove the special character of the 2þ states
ms ms
−
independentlyofelectromagnetictransition strengths.The possible existence of3
and 4+ states with mixed proton and neutron symmetry is discussed. Furthermore
theoriginofcollectivityoflow-lyingstatesinsphericalnucleiisinvestigatedinthe
QPM.
ix
Acknowledgments
(cid:129) I am particularly indebted to my supervisor Prof. Dr. Norbert Pietralla for
guiding me through the years of my bachelor, master and doctoral thesis. His
motivational capabilities and readiness for discussions were very important for
the successful completion of this thesis.
(cid:129) Theworkaboutthetwo-photondecayandtheset-upoftheLabr -detectorarray
3
would not have been possible without the excellent support, good ideas and
knowledgeofDr.HeikoScheit.Furthermorehekindlyagreedtobethesecond
reviewer of this work.
(cid:129) I am indebted to Dr. V. Yu. Ponomarev for the QPM calculations and
numerous fruitful discussions on the interpretation of the results of the
proton-scattering experiments.
(cid:129) I want to thank Dr. Retief Neveling, Dr. Ricky Smith and all other members
of the nuclear physics group at iThemba LABS for their great support with the
successful proton-scattering experiment in South Africa. Furthermore Retief
Neveling helped with the analysis of the proton-scattering data.
(cid:129) Many thanks go to Prof. Dr. Franco Camera, Dr. Oliver Wieland and
Stefano Riboldi for many useful suggestions on the LaBr -detector array and
3
for sharing their constructional drawings of the active voltage divider.
(cid:129) I want to thank Dr. H.T. Johansson for his ucesb and ggland software, H.
Schaffner for his mbs-readout files and Dr. N. Kurz for his support to get the
mbs-DAQ running.
(cid:129) IwishtothankDr.JohnMillenerforgenerouslysharinghisexperimentaland
theoreticalresultsonthetwo-photondecayin137Ba.Hisadviceandsuggestions
helped me significantly to conduct my two-photon decay experiment
successfully.
(cid:129) I am grateful to all current and former members of the Tagger-group. In par-
ticular Dr. Linda Schnorrenberger, Dr. Dmytro Symochko, Ronan Lefol,
Bastian Löher and Diego Semmler.
(cid:129) I would like to thank my officemates Christian Stahl, Michael Reese and
ChristophKrämerforapleasantworkingatmosphereandtheirsupportduring
the proton-scattering experiment in South Africa.
xi