Investigation of Nuclear Structure with Relative Self-Absorption Measurements

229 Pages·2015·4.3 MB·English

by  Christopher Romig

Investigation of Nuclear
Structure with Relative
Self-Absorption Measurements
KernstrukturuntersuchungenmitrelativenSelbstabsorptionsmessungen
VomFachbereichPhysikderTechnischenUniversitätDarmstadtzurErlangung
desakademischenGradeseinesDoktorsderNaturwissenschaften(Dr.rer.nat.)
genehmigteDissertationvonChristopherRomig,M.Sc.ausDarmstadt
2015—Darmstadt—D17
FachbereichPhysik
InstitutfürKernphysik
InvestigationofNuclearStructurewithRelativeSelf-AbsorptionMeasurements
KernstrukturuntersuchungenmitrelativenSelbstabsorptionsmessungen
VomFachbereichPhysikderTechnischenUniversitätDarmstadtzurErlangung
desakademischenGradeseinesDoktorsderNaturwissenschaften(Dr.rer.nat.)
genehmigteDissertationvonChristopherRomig,M.Sc.ausDarmstadt
1.Gutachten:Prof.Dr.Dr.h.c.NorbertPietralla
2.Gutachten:Prof.Dr.JoachimEnders
TagderEinreichung:14.10.2014
TagderPrüfung:24.11.2014
Erscheinungsjahr:2015
Darmstadt—D17
Abstract
In this work, a special application of the nuclear resonance fluorescence tech-
nique has been used and further advanced to study nuclear structure effects:
so-called relative self-absorption experiments. They allow for the direct de-
termination of ground-state transition widths of excited states, an important
quantity for comparisons to model calculations, as well as the level width
and the branching ratio to the ground state of individual states in a model-
independent way. Two self-absorption measurements have been performed.
Ontheonehand,theabsoluteexcitationwidthsandthedecaypatternoflow-
lyingexciteddipolestatesintheenergyregimeofthepygmydipoleresonance
have been investigated in the nucleus 140Ce. The results of the nuclear res-
onance fluorescence measurement, which is part of a self-absorption experi-
ment, as well as of the actual self-absorption measurement are presented for
117and104excitedstatesof140Ce,respectively. Theyarecomparedtoprevi-
ous measurements and discussed with respect to the statistical model. On the
otherhand,ahigh-precisionmeasurementoftheselfabsorptionofthe T =1,
Jπ = 0+ level at an excitation energy of 3563keV in 6Li has been conducted.
1
A precisely measured level width of this state can serve as a benchmark for
modernab-initocalculations.
Zusammenfassung
Im Rahmen der vorliegenden Arbeit wurde die Methode der relativen Selbst-
absorption, die auf Kernresonanzfluoreszenzmessungen basiert, weiterentwi-
ckelt und zur Untersuchung von Kernstruktureffekten eingesetzt. Sie erlaubt
es, Grundzustandsübergangsbreiten, eine essentielle Größe für den Vergleich
mit Modellrechnungen, sowie natürliche Linienbreiten und Verzweigungsver-
hältnisse in den Grundzustand modellunabhängig für individuelle Zustände
zubestimmen.InsgesamtwurdenzweiSelbstabsorptionsexperimentedurchge-
führt.ZumEinenwurdenabsoluteAnregungsstärkenunddasZerfallsverhalten
i
tiefliegenderDipolzuständeimAnregungsbereichderPygmy-Dipolresonanzim
Kern 140Ce untersucht. Die Ergebnisse der Kernresonanzfluoreszenzmessung,
die ein Teil eines Selbstabsorptionsexperimentes ist, sowie jene der eigentli-
chen Selbstabsorptionsmessung werden für 117 bzw. 104 angeregte Zustände
von 140Ce vorgestellt. Sie werden mit früheren Messungen verglichen und im
Rahmen des statistischen Modells diskutiert. Zum Anderen wurde eine Hoch-
präzisionsmessung der Selbstabsorption des T =1, Jπ =0+ Zustands von 6Li
1
bei einer Anregungsenergie von 3563keV durchgeführt. Eine präzise Bestim-
mung der natürlichen Linienbreite dieses Zustands kann als Referenzwert für
aktuelleab-initioRechnungenherangezogenwerden.
ii Zusammenfassung
Contents
1. Introduction 1
2. Motivation 7
2.1. Self-AbsorptionMethod . . . . . . . . . . . . . . . . . . . . . . . . . 7
2.2. DecayPatternof140Ce . . . . . . . . . . . . . . . . . . . . . . . . . . 11
2.3. LifetimeMeasurementin6Li . . . . . . . . . . . . . . . . . . . . . . 14
3. Low-LyingElectricDipoleStrength 19
4. NuclearResonanceFluorescence 25
4.1. BasicPrinciple . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
4.2. MeasuredObservablesandExtractedQuantities . . . . . . . . . . 27
5. RelativeSelfAbsorption 41
5.1. BasicPrinciple . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41
5.2. AbsorptionandAttenuationEffects . . . . . . . . . . . . . . . . . . 43
5.3. Normalisation and Atomic Attenuation Correction - A new Ap-
proach . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48
5.4. DeterminationofSelfAbsorption . . . . . . . . . . . . . . . . . . . 50
5.4.1. ExperimentalSelfAbsorption . . . . . . . . . . . . . . . . . 50
5.4.2. CalculationofSelfAbsorption . . . . . . . . . . . . . . . . . 52
6. ExperimentalSetup 59
6.1. S-DALINAC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59
6.2. DHIPS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61
7. DirectDeterminationofTransitionWidthsof140Ce 65
7.1. TheMeasurements . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65
7.2. TheClassicalNRFMeasurement-Analysis . . . . . . . . . . . . . 68
7.3. TheClassicalNRFMeasurement-ResultsandDiscussion . . . . 75
iii
7.4. TheRSAMeasurement-Analysis . . . . . . . . . . . . . . . . . . . 88
7.4.1. DeterminationofR . . . . . . . . . . . . . . . . . . . . . . 88
exp
7.4.2. DeterminationofResultingQuantities . . . . . . . . . . . . 95
7.5. TheRSAMeasurement-Results . . . . . . . . . . . . . . . . . . . . 103
7.6. TheStatisticalModel . . . . . . . . . . . . . . . . . . . . . . . . . . . 126
7.7. Conclusions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 135
8. High-PrecisionSelf-AbsorptionMeasurementon6Li 137
8.1. TheMeasurement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 137
8.2. SelfAbsorption . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 139
8.3. DeterminationoftheLevelWidth . . . . . . . . . . . . . . . . . . . 147
8.4. Conclusions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 153
9. AspectsofRelativeSelf-AbsorptionMeasurements-AConclusion157
A. Spectra 163
B. ResultsoftheNRFMeasurementon140Ce 171
C. ResultsoftheRSAMeasurementon140Ce 187
Bibliography 199
ListofPublications 209
ListofFigures 211
ListofTables 215
Danksagung 217
Lebenslauf 221
iv Contents
1 Introduction
All along, mankind is driven by its curiosity and exploratory urge. From
early human civilisations to the modern world, from childhood to old age,
westriveafterexploringoursurroundings. Oneofthecentralquestionsplagu-
ing mankind aims at the fundamental nature of matter. What is the world,
whatarewecomposedof?
The first surviving attempts to answer this question date back to Greek
philosophersinthefourthcenturyB.C.Atthistime,peoplehadnotmuchmore
thantheirownsensesattheirdisposaltostudythecompositionofmatter. The
philosopherswereaimingtoexplainthecomplexityofdifferentmaterialswith
fundamental building blocks. Empedocles, a pre-Socratic philosopher (∼ 494
to434B.C.)wastheveryfirstonepostulatingthateverythingcanbeascribed
tothecompositionandinteractionofonlyfour“roots”,namelyair,earth,fire,
and water: “But these (elements) are the same; and penetrating through each
othertheybecomeonethinginoneplaceandanotherinanother,whileeverthey
remainalike(i.e. thesame).”1Theseclassicalelements(Fig.1.1)areconserved
and changeless hinting already to modern concepts. At the same time, Leu-
cippusandparticularlyhisstudentDemocritus(both∼460to370B.C.), also
Greekphilosophers,followedadifferentapproach: “...byconventionsweetand
by convention bitter, by convention hot, by convention cold, by convention color;
but in reality atoms and void”.2 They postulated that no material can be end-
lesslysubdividedintosmallerparts. Atsomepointalimitisreached,asmallest
particlethatcannotfurtherbeassembled: theatom(fromἄτομος,atomos,an-
cient Greek for indivisible). In this context, different properties of materials,
such as taste or colour, were ascribed to different forms and shapes of the
correspondingatoms.
1 Empedocles,fromA.Fairbanks,ThefirstPhilosophersofGreece(London: K.Paul,Trench,
Trübner&CO.Ltd.,1898)
2 Democritus, from H. Diels and W. Kranz, Die Fragmente der Vorsokratiker, 6th edition,
Berlin: Weidmann, 1951, trans. C.C.W. Taylor, The Atomists: Leucippus and Democritus.
Fragments,ATextandTranslationwithCommentary,Toronto: UniversityofTorontoPress,
1999
1
air earth fire water
Figure1.1.:Alchemical symbols of the four classical elements air, earth, fire,
andwaterthatwereuseduntilthe18thcentury.
Ittookmanycenturiesbeforethistheory, calledatomism, wasfurtherstud-
ied and experimental scientists discovered chemical elements which, indeed,
havedifferentproperties. In1869,Mendelejewclassifiedthebythenknownel-
ementsaccordingtotheirpropertiesinaperiodicsystemofelements[Men69]
which basically endures till nowadays (Fig. 1.2). From then on, scientists, ex-
ploitingmoreandmoreadvancedtechnologies,triedtoinvestigateatomsand,
thus, the nature of matter, increasingly closer and the field of atomic physics
was born even before Einstein finally proved the existence of atoms in 1905
withtheexplanationofBrownianmotion[Ein05a]. However,athistime,new
observationsalreadyindicatedthatatomsarenotatallindivisible.
In 1896, Becquerel discovered radioactivity in Uranium [Bec96], a phe-
nomenon that Marie and Pierre Curie further studied, identifying more ra-
dioactivesubstancesin1898[Cur98]. Oneyearearlier,in1897,Thomsonhad
discovered the electron [Tho97] - the first sub-atomic particle. Based on his
discovery, he proposed that the neutral atom consists of a homogenous pos-
itively charged sphere in which negatively charged electrons are distributed
(see Fig. 1.3) [Tho04]. This model was disproved in 1911 when Rutherford
interpreted the famous α-scattering experiments on gold foils that Geiger and
Marsden [Gei09] conducted under his leadership at the Physical Laboratories
oftheUniversityofManchester. Fromtheirobservationsheconcludedthat“the
atom consists of a central charge supposed concentrated at a point,...” [Rut11].
ThisconclusionleadtoRutherfordsatomicmodel: thepositivechargeaswell
as the main part of the atomic mass is concentrated in a, compared to the
atom, very small nucleus (Fig. 1.3). Only a few years later, he discovered
one of the building blocks of nuclei, showing that also nuclei can be divided
into smaller parts. Irradiating nitrogen with α particles, he generated oxy-
gen and a new kind of particle radiation which he called proton [Rut19].
However, the nucleus seemed to consist out of further constituents. Atomic
2 1. Introduction
1 2
1 H He
alkalinemetals half-metals
alkalineearthmetals metalloids
3 4 5 6 7 8 9 10
2 Li Be lanthanides non-metals B C N O F Ne
actinides halogens
11 12 transitionmetals nobelgases 13 14 15 16 17 18
33 Na Mg Al Si P S Cl Ar
19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36
44 K Ca Sc Ti V Cr Mn Fe Co Ni Cu Zn Ga Ge As Se Br Kr
37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54
55 Rb Sr Y Zr Nb Mo Tc Ru Rh Pd Ag Cd In Sn Sb Te I Xe
55 56 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86
66 Cs Ba Hf Ta W Re Os Ir Pt Au Hg Tl Pb Bi Po At Rn
87 88 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118
77 Fr Ra Rf Db Sg Bh Hs Mt Ds Rg Cn Uut Fl Uup Lv UusUuo
57 58 59 60 61 62 63 64 65 66 67 68 69 70 71
La Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu
89 90 91 92 93 94 95 96 97 98 99 100 101 102 103
Ac Th Pa U Np Pu AmCm Bk Cf Es Fm Md No Lr
Figure1.2.:Theperiodicsystemofelements. Theelementsareorderedaccord-
ingtotheiratomicnumberanddividedinto18groupscorrespond-
ingtothecolumnsandsevenperiodscorrespondingtotherows.
Figure1.3.:In the Thomson model the atom is given by a homogenous posi-
tively charged sphere in which the electrons are distributed (left-
handside). Rutherforddisprovedthismodelandshowedthatthe
mainpartoftheatomicmassaswellasthepositivechargeiscon-
centratedintheatomicnucleus(right-handside). Beingfiveorders
ofmagnitudesmallerastheactualatom,thenucleusiscomposed
outofneutronsandprotonsandorbitedbytheelectrons.
3
nucleiwerefoundtohaveonlyhalfthechargenumberaswasexpectedifthey
were completely composed out of positively charged protons [Bar11]. Thus,
in 1920, Rutherford postulated the existence of a neutrally charged build-
ing block of nuclei which may explain this discrepancy [Rut20]. Indeed, in
1930,BotheandBeckerfoundanewstronglypenetratingradiationwhenthey
irradiated light elements, such as lithium or boron, with α particles [Bot30].
However,theymisinterpretedtheradiationasakindofγradiation. Eventually,
in 1932, Chadwick, one of Rutherfords students, disproved this interpretation
andshowedthattheobservedradiationconsistsofunchargedparticleswitha
mass comparable to that of the proton [Cha32]. He named the new particle
neutron.
Nowadays, we know that the two constituents of nuclei, protons and neu-
trons, are no fundamental particles either. They consist out of even smaller
particles, the quarks, that do not only form nucleons but also many other
particles subsumed as mesons and baryons. Nevertheless, with their discov-
eriesandconclusions,thephysicistsintheearly20thcenturyandinparticular
Ernest Rutherford shaped our nowadays view of atomic nuclei and laid the
foundationfornuclearphysicswhichistilltodayanimportantfieldofphysics.
Nuclear structure physics treats the weak, strong, and electromagnetic in-
teraction of nuclear systems. It aims at a consistent description of all nuclei
throughout the nuclear chart - 254 stable and about 6000 unstable ones (see
Fig.1.4)-basedonthejustmentionedthreeoffourfundamentalforces. How-
ever, the interactions within nuclei are difficult to describe. On the one hand,
nuclei are often too complex to model the complex interactions exactly. On
the other hand, the number of nucleons that nuclei consist of (e.g., 208 for
theheavieststablenucleus208Pb)ismuchtoosmallforastatisticalapproach.
Thus,nowadaysnuclearstructurephysicsreliesonmanydifferentmodelsthat
often describe only selected characteristics of nuclei. Furthermore, they are
oftenvalidonlyinlimitedmassregions. However,withintheirrangeofappli-
cability many models are already in very good agreement with experimental
observations. The area of experimental nuclear structure physics provides ob-
servationsandresultsthatserveasfoundationandbenchmarkofsuchmodels.
Systematicstudiesofthepropertiesofnucleiareusedtotestandimprovemod-
els which on the other hand provide important input for the analysis of many
experimentssuchthatexperimentandtheoryaredrivenbyeachother.
Beyond the fundamental understanding of nuclei and their characteristics,
nuclear structure physics also have a strong impact on further research fields
suchasparticlephysics,radiationtherapyorastrophysics. Forinstance,nuclear
4 1. Introduction