Laser Assisted Nuclear Decay Spectroscopy: A New Method for Studying Neutron-Deficient Francium Isotopes

140 Pages·2015·8.781 MB·English

by  Kara Marie Lynch (auth.)

Springer Theses
Recognizing Outstanding Ph.D. Research
Kara Marie Lynch

Laser Assisted
Nuclear Decay
Spectroscopy
A New Method for Studying
Neutron-Deficient
Francium Isotopes
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
for its scientific excellence and the high impact of its contents for the pertinent
fieldofresearch.Forgreateraccessibilitytonon-specialists,thepublishedversions
includeanextendedintroduction,aswellasaforewordbythestudent’ssupervisor
explaining the special relevance of the work for the field. As a whole, the series
will provide a valuable resource both for newcomers to the research fields
described, and for other scientists seeking detailed background information on
specialquestions.Finally,itprovidesanaccrediteddocumentationofthevaluable
contributions made by today’s younger generation of scientists.
Theses are accepted into the series by invited nomination only
and must fulfill all of the following criteria
• They must be written in good English.
• ThetopicshouldfallwithintheconfinesofChemistry,Physics,EarthSciences,
Engineering andrelatedinterdisciplinaryfieldssuchasMaterials, Nanoscience,
Chemical Engineering, Complex Systems and Biophysics.
• The work reported in the thesis must represent a significant scientific advance.
• Ifthethesisincludespreviouslypublishedmaterial,permissiontoreproducethis
must be gained from the respective copyright holder.
• They must have been examined and passed during the 12 months prior to
nomination.
• Each thesis should include a foreword by the supervisor outlining the signifi-
cance of its content.
• The theses should have a clearly defined structure including an introduction
accessible to scientists not expert in that particular field.
More information about this series at http://www.springer.com/series/8790
Kara Marie Lynch
Laser Assisted Nuclear
Decay Spectroscopy
A New Method for Studying
Neutron-Deficient Francium Isotopes
Doctoral Thesis accepted by
the University of Manchester, UK
123
Author Supervisor
Dr. KaraMarie Lynch Dr. KieranFlanagan
School ofPhysicsand Astronomy School ofPhysicsand Astronomy
Universityof Manchester Universityof Manchester
Manchester Manchester
UK UK
ISSN 2190-5053 ISSN 2190-5061 (electronic)
ISBN 978-3-319-07111-4 ISBN 978-3-319-07112-1 (eBook)
DOI 10.1007/978-3-319-07112-1
LibraryofCongressControlNumber:2014943508
SpringerChamHeidelbergNewYorkDordrechtLondon
(cid:2)SpringerInternationalPublishingSwitzerland2015
Thisworkissubjecttocopyright.AllrightsarereservedbythePublisher,whetherthewholeorpartof
the material is concerned, specifically the rights of translation, reprinting, reuse of illustrations,
recitation,broadcasting,reproductiononmicrofilmsorinanyotherphysicalway,andtransmissionor
informationstorageandretrieval,electronicadaptation,computersoftware,orbysimilarordissimilar
methodology now known or hereafter developed. Exempted from this legal reservation are brief
excerpts in connection with reviews or scholarly analysis or material supplied specifically for the
purposeofbeingenteredandexecutedonacomputersystem,forexclusiveusebythepurchaserofthe
work. Duplication of this publication or parts thereof is permitted only under the provisions of
theCopyright Law of the Publisher’s location, in its current version, and permission for use must
always be obtained from Springer. Permissions for use may be obtained through RightsLink at the
CopyrightClearanceCenter.ViolationsareliabletoprosecutionundertherespectiveCopyrightLaw.
The use of general descriptive names, registered names, trademarks, service marks, etc. in this
publicationdoesnotimply,evenintheabsenceofaspecificstatement,thatsuchnamesareexempt
fromtherelevantprotectivelawsandregulationsandthereforefreeforgeneraluse.
While the advice and information in this book are believed to be true and accurate at the date of
publication,neithertheauthorsnortheeditorsnorthepublishercanacceptanylegalresponsibilityfor
anyerrorsoromissionsthatmaybemade.Thepublishermakesnowarranty,expressorimplied,with
respecttothematerialcontainedherein.
Printedonacid-freepaper
SpringerispartofSpringerScience+BusinessMedia(www.springer.com)
Supervisor’s Foreword
Even though more than 100 years has now elapsed since Rutherford discovered
thenucleusthereremainmanyunansweredquestionsassociatedwiththeforcethat
binds protons and neutrons together. This directly impacts our understanding of
astrophysical processes and the properties of the early universe. This compelling
area of research requires both theoretical and experimental efforts. A key com-
ponentofthisworkisthemeasurementsofnuclearobservablesthatdonotrelyon
prior assumptions of a particular theoretical model. The interaction between
atomicelectronsandthenucleusissensitivetothesize,shape,andmagnetization
of the nucleus. Of the many methods that can be used to measure these nuclear
observables, laser spectroscopy is particularly well suited to studying unstable
nuclei.
It has now been nearly 40 years since the first laser spectroscopy experiments
were carried out on unstable isotopes. During the intervening years many tech-
niques have been developed to measure the nuclear moments, charge radii, and
spins of unstable nuclei. Many of these variations on the original concept have
beentailoredtosuittheconstraintsimposedbyaparticularelement.Twomethods
stand out in terms of their wide application, resolution, and sensitivity: collinear
and in-source laser spectroscopy. Collinear methods offer high resolution while
in-source methods offer the highest sensitivity. In 1982 a method for combining
theresolutionofthecollinearmethodwiththesensitivityofthein-sourcemethod
was proposed. At that time, the advantages of the collinear resonance ionization
spectroscopy (CRIS) technique were offset with significant technological chal-
lenges. A critical problem was the duty cycle loss associated with using pulsed
lasers to probe continuous beams. The introduction of ion-beam cooling and
bunching techniques in the last decade resolved this problem and resurrected the
collinear resonance ionization concept.
Kara Lynch joined this endeavour in 2010 and focussed her Ph.D. on a fasci-
natingregionofthenuclearchart.Theneutron-deficientisotopesoffranciumsitat
a pivotal point where a spherical shellmodel description evolves into a deformed
liquid drop. This region is characterized by a softening of the nuclear potential
v
vi Supervisor’sForeword
withdecreasingneutronnumberandyetmaintainingasphericalshapeandalmost
purewavefunctions.Astheproton-driplineisapproached,itisexpectedthatthere
will be an abrupt change in the ground-state structure from a spherical to highly
deformed shape as the occupation of intruder states becomes energetically
favourable. Kara simultaneously applied the techniques of laser spectroscopy and
decay spectroscopy to study the 202-206Fr. These measurements confirm for the
firsttimetheexistenceofthe10-isomericstatesin204,206Frandallowanalysisof
thecompositionofthenuclearwavefunction.Selectingtheisomersin204Frusing
the CRIS technique, Kara was able to perform decay spectroscopy on just the
excited state. This cleaning method provides the route to measuring unambigu-
ously the branching ratio and half-lives of states with overlapping alpha-decay
energies.Inordertostudytheseexoticspecies,Karaconstructedabespokedecay
spectroscopy station that is compatible with the UHV requirements of the
experiment. The experiments reported in Kara Lynch’s thesis represent the first
successful demonstration of the CRIS technique with a total experimental effi-
ciency of greater than 1 %. By measuring the hyperfine structure of 202Fr, with a
production rate of 100 atoms/s, Kara demonstrated that the method is more than
two orders of magnitude more sensitive than existing techniques. Kara’s research
has made a large impact within the community and she has been awarded the
prestigious IOP Nuclear Group Prize.
Manchester, June 2014 Dr. Kieran Flanagan
Abstract
Radioactive decay studies of rare isotopes produced at radioactive ion beam
facilities have often been hindered by the presence of isobaric and isomeric
contamination. The Collinear Resonance Ionization Spectroscopy (CRIS) exper-
iment at ISOLDE, CERN uses laser radiation to stepwise excite and ionize an
atomic beam in a particular isomeric state. Deflection of this selectively ionized
beam of exotic nuclei, from the remaining neutral contaminants, allows ultra-
sensitive detection of rare isotopes and nuclear structure measurements in
background-free conditions.
This thesis outlines the work undertaken in the development of the novel
technique of laser-assisted nuclear decay spectroscopy. The isomeric ion beam is
selected usinganatomicresonanceofitshyperfinestructure,whereitisdeflected
to a decay spectroscopy station. This consists of a rotating wheel implantation
systemforalpha-decayspectroscopy,anduptothreegermaniumdetectorsaround
the implantation site for gamma-ray detection.
Laser spectroscopy provides a measurement of the spin, moments, and change
in mean-square charge radii of the ground and isomeric states in the parent
nucleus.Complementaryinformationonthelevelstructureofthedaughternucleus
comes from the decay spectroscopy, providing further information on the isotope
under investigation.
The new techniques of collinear resonance ionization spectroscopy and laser-
assisted nuclear decay spectroscopy have been developed and optimized in the
experimental campaign studying the neutron-deficient francium isotopes. In this
thesis, the hyperfine structure studies of 202-207,211Fr are presented, alongside the
radioactive decay studies of 202,204,218Fr and their isomers.
vii
Acknowledgments
To Kieran, for the enormous support and encouragement he has given me
throughout my Ph.D. I couldn’t have hoped for a better supervisor, nor a more
inspirational one.
To Thomas, for being a constant source of nuclear-physics knowledge, patience,
and gossip!
TotheCRIScollaboration,formakingitallpossible.Iconsidermyselfverylucky
to work with such a talented group of individuals.
ToJonBillowesandGerdaNeyens,forthedepthofknowledgeandunderstanding
they have brought to my work.
To Yorick Blumenfeld and Maria Garcia Borge, for the wonderful opportunity
their support and kindness has given me.
To the on-site team, for making ISOLDE such an inspiring place to work.
To Tom, for all the laughs we shared during our Ph.Ds.
To Andy Smith, for his invaluable help with the DSS design.
To the core, for allowing me to be a valence particle.
ToSarah,forthelife-longfriendshipthatstartedwhenweweresummerstudents.
To my parents, for their endless love and support. It means the world to me.
To Espen, for making me smile everyday.
To all my friends, family, and colleagues who have made these last 3 years so
special…
ix
Contents
1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
1.1 Physics Motivation for Studying Francium. . . . . . . . . . . . . . . . 2
1.2 Recent Studies in the Field. . . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.3 This Work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
2 Theoretical Considerations for Laser Spectroscopy. . . . . . . . . . . . 7
2.1 Hyperfine Structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
2.1.1 Nuclear Spin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
2.1.2 Magnetic Dipole Moment . . . . . . . . . . . . . . . . . . . . . . 9
2.1.3 Electric Quadrupole Moment . . . . . . . . . . . . . . . . . . . . 10
2.1.4 Hyperfine Anomaly. . . . . . . . . . . . . . . . . . . . . . . . . . . 10
2.2 Isotope Shift. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
2.2.1 Mass Shift . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
2.2.2 Field Shift . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
2.2.3 Total Isotope Shift . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
2.2.4 King Plot Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
3 Theoretical Considerations for Nuclear Decay Spectroscopy . . . . . 15
3.1 The Nuclear Landscape . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
3.2 Theoretical Nuclear Models . . . . . . . . . . . . . . . . . . . . . . . . . . 15
3.2.1 The Droplet Model . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
3.2.2 The Shell Model. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
3.2.3 The Deformed Models. . . . . . . . . . . . . . . . . . . . . . . . . 17
3.3 Studying Nuclear Structure. . . . . . . . . . . . . . . . . . . . . . . . . . . 19
3.3.1 Deformation of the Nucleus. . . . . . . . . . . . . . . . . . . . . 19
3.3.2 Shape Coexistence in the Region of 186Pb . . . . . . . . . . . 20
3.3.3 Proton Intruder States . . . . . . . . . . . . . . . . . . . . . . . . . 23
xi