Multinucleon-Transfer Reactions as a Gateway to Neutron-Rich Actinides and Nuclei near the N ...

159 Pages·2017·17.01 MB·English

Multinucleon-Transfer Reactions
as a Gateway to Neutron-Rich Actinides
and Nuclei near the
= =
N 82 and Z 50 Shell Closures
Inaugural-Dissertation
zur
Erlangung des Doktorgrades
der Mathematisch-Naturwissenschaftlichen Fakultät
der Universität zu Köln
vorgelegt von
Andreas Günter Heinz Vogt
aus Köln
Köln 2017
Berichterstatter: Prof. Dr. Peter Reiter
Prof. Dr. Andreas Zilges
Prof. Dr. Rolf-Dietmar Herzberg
Tag der letzten mündlichen Prüfung: 10. Juli 2017
| Abstract
Multinucleon-transferreactionsareacompetitiveandpromisingtooltoprovideaccesstohard-to-reach
andexoticnuclei. Inthepresentwork,reactionproductsinthe136Xe+238Umultinucleon-transfer
reaction at 1 GeV were investigated employing the high-resolution position-sensitive γ-ray track-
ing array AGATA coupled to the large-solid-angle mass spectrometer PRISMA at the Laboratori
NazionalidiLegnaro(INFN,Italy). Beam-likereactionproductswereidentifiedandselectedbythe
PRISMA spectrometer. Recoils and fission fragments were tagged by DANTE micro-channel plate
detectorsinstalledwithinthescatteringchamber. Fissionandtransfereventsarediscriminatedby
exploiting kinematic coincidences between the binary reaction products. Mass yields and relative
cross-section distributions are extracted and compared with calculations based on the GRAZING
modelformultinucleon-transferreactions. Furthermore,populationyieldsfornucleiintheactinide
regionareobtainedandcomparedtoX-rayyieldsmeasuredbyAGATA.Perspectivesandlimitations
for the production of the hard-to-reach neutron-rich isotopes are discussed. A Doppler correction
forthetarget-likenucleienablesγ-rayspectroscopyoftheheavyreactionpartner. Nuclearstructure
informationofneutron-richactinidenucleiareabenchmarkfortheoreticalmodelsprovidingpredic-
tions for the heaviestnuclei. An extension ofthe ground-state rotational band in240U is achieved
andevidenceforanextendedfirstnegative-paritybandin240Uisfound. Theresultsarecomparedto
recent mean-field and density-functional theory calculations. Furthermore, multinucleon-transfer
reactions are a gateway to nuclei in the vicinity of the Z = 50 and N = 82 shell closures. Nuclei
inthisregionserveasabenchmarkfornuclearshell-modelcalculationsbasedonmoderneffective
interactions,butaredifficulttopopulateduetoalackofsuitablebeam-targetcombinations. Excited
reactionproductsweremeasuredaftermultinucleontransferin136Xe+238Uat1GeVand136Xe+
208Pbat930MeVwiththeAGATAtrackingarraycoupledtoPRISMAatLNL(INFN,Italy)aswellas
inthe136Xe+198Ptmultinucleon-transferreactionemployingthehigh-efficiencyγ-rayspectrometer
GAMMASPHERE in combination with the gas-detector array CHICO at LBNL. Moreover, Xe and
Ba isotopes were populated in fusion-evaporation reactions using the HORUS γ-ray array at the
University of Cologne. The high-spin level schemes of 132Xe, 133Xe, 134Xe, 135Xe and 137Ba were
considerablyextendedtohigherenergies. The2058-keVstatein135Xeisidentifiedasa9.0(9)-ns
isomer,closingagapinthesystematicsalongthe N =81isotones. Latestshell-modelcalculations
reproducetheexperimentalfindings. Theexperimentally-deducedreducedtransitionstrengthsof
the isomer decays are compared to shell-model predictions. A detailed picture of the lower-mass
N 82, Z 50regionoftheSegrèchartisdrawn. Anotherpartofthisthesiscoversdouble-sided
sili≤constrip≥detectors(DSSSD)whichareemployedforthedetectionofchargedparticlesproviding
positionandenergyinformation. Intersectingareasofbothp-andn-sidestripsformindividualpixel
segments resulting in a high detector granularity. However, due to limitations in fabrication and
theresponseofthereadoutelectronics,theperformanceofdifferentchannelsmayvary. Typically,
chargedparticlesdonotilluminatehomogeneouslythedetectorsurfaceduringin-beamexperiments.
Consequently, radiation damages of the detector are distributed non-uniformly. Position-resolved
charge-collectionlossesforfront-andback-sidesegmentswereinvestigatedforanin-beamexper-
iment and by performing radioactive source measurements. A novel position-resolved calibration
methodforradiation-damagedDSSSDs,basedonmutualconsistencyofp-sideandn-sidecharges,
wasdeveloped. Ityieldsasignificantenhancementoftheenergyresolutionandtheperformanceof
radiation-damagedpartsofthedetector.
iii
| Zusammenfassung
MultinukleontransferreaktionensindeinvielversprechenderZugangzurProduktionschwerzugängli-
cherundexotischerAtomkerne.DerersteTeilderArbeitumfassteineReaktionsstudiezurSchwe-
rionenkollisionzwischen136Xeund238UbeieinerStrahlenergievon1GeV,durchgeführtmitdem
hochauflösendenundpositionssensitivenGamma-Tracking-ArrayAGATAamLaboratoriNazionalidi
Legnaro (INFN, Italien). Strahlähnliche Reaktionsprodukte wurden mit dem Massenspektrometer
PRISMA identifiziert und selektiert. Targetähnliche Teilchen und Spaltfragmente wurden mittels
DANTE-MikrokanalplattendetektoreninnerhalbderStrahlkammerdetektiert.KinematischeKoinzi-
denzenzwischendenverschiedenenReaktionsproduktenerlaubeneineDiskriminierungzwischen
SpaltungunderwünschtemMultinukleontransfer.MassenspektrenundrelativeWirkungsquerschnitte
wurdenausdenDatenextrahiertundmitModellrechnungendesGRAZING-Codesverglichen.Die
AusbeuteanüberlebendenAktinidenkernenwurdemitdervonAGATAgemessenenRöntgenstrah-
lung,einemcharakteristischenFingerabdruckdesjeweiligenElements,verglichen.DieDiskussion
derErgebnissezeigtPerspektivenundLimitierungenfürdieProduktionvonschwerzugänglichen
neutronenreichenIsotopeninderAktinidenregionauf.DieKernstrukturneutronenreicherAktinideist
einewichtigeRichtgrößezurBewertungtheoretischerModellezurBeschreibungschwersterTransak-
tinide.DieGrundzustandsbandevon240U konntezuhöherenEnergienerweitertwerden.Weiterhin
gelangderNachweiseinernegativenParitätsbande.DieErgebnissewerdenmitjüngstpublizierten
Mean-Field-undFunktionaldichtetheorie-Rechnungenverglichen.Multinukleontransferrreaktionen
eignen sich ebenfalls als Zugang zu Kernen in der Region der magischen Schalenabschlüsse bei
Z =50und N =82.Kernenordwestlichvon132SnsindwichtigePrüfmarkenfürmoderneeffektive
Schalenmodellinteraktionen.BedingtdurchdasFehlengeeigneterStrahl-Target-Kombinationenist
dieProduktiondieserKernejedochoftmalssehranspruchsvoll.Multinukleontransfer-Experimente
anAGATA+PRISMA(136Xe+238U sowie136Xe+208Pb),undanGAMMASPHERE+CHICOamLBNL
(136Xe+198Pt)erlaubeneinedetaillierteSpektroskopievondiversenschwerzugänglichenIsotopen
in dieser Region der Nuklidkarte. Ferner wurden Xenon- und Bariumisotope mittels Fusionsver-
dampfungsreaktionenamHORUS-AufbauanderUniversitätzuKölnspektroskopiert.DieHochspin-
Termschemata von 132Xe, 133Xe, 134Xe, 135Xe und 137Ba wurden signifikant zu höheren Energien
erweitert. Im Kern 135Xe wurde der Zustand bei 2058 keV als Isomer mit einer Halbwertszeit von
9.0(9)nsidentifiziert.DiesesErgebnisschließteineletzteverbliebeneLückeinderSystematikder
IsotonenkettemitNeutronenzahlN =81undergibteindetaillierteresBildderIsotopenregion.Dieex-
perimentellbestimmtenreduziertenÜbergangswahrscheinlichkeitenderisomerenZuständewerden
mitVorhersagenvonSchalenmodellrechnungenkonfrontiert.EinweitererTeilderArbeitbehandelt
doppelseitigsegmentierteSiliziumstreifen-Detektoren,welcheinderNiederenergie-Kernphysikhäufig
zurDetektiongeladenerTeilcheneingesetztwerden.DieseDetektorenstellennichtnureineEnergie-,
sondernaucheineOrtsinformationmithoherDetektorgranularitätzurVerfügung.Überkreuzende
Flächenvonp-undn-dotiertenStreifenformenhierbeieinzelnePixel.BedingtdurchFabrikations-
beschränkungen und Limitierungen der Ausleseelektronik kann die Leistungsfähigkeit einzelner
Kanäle drastisch variieren. Weiterhin illuminieren geladene Teilchen die Detektoroberfläche nicht
gleichmäßig.DieskannzuinhomogenenStrahlenschädenführen.PositionsabhängigeVerlustebeider
LadungssammlungwerdenineinemIn-Beam-ExperimentsowieineinemdediziertenTestexperiment
durchBestrahlungmiteinemradioaktivenPräparatuntersucht.IndiesemZusammenhangwurde
eine neuartige positionsabhängige Kalibrierungsmethode entwickelt, die auf der wechselseitigen
Abhängigkeit der Ladungssammlung in der jeweils p- und n-dotierten Detektorseite beruht. Diese
KalibriermethodeerzielteinesignifikanteVerbesserungderEnergieauflösungundMessbefähigung
vonstrahlengeschädigtenTeilendesDetektorsystems.
iv
| Contents
1 Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
1.1 NuclearreactionscloseandabovetheCoulombbarrier . . . . . . . . . . . . . . . . . 7
1.1.1 Theoryofmultinucleon-transferreactions . . . . . . . . . . . . . . . . . . . . 10
1.2 Nuclearstructurenorthwestof132Sn . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
1.2.1 Reactionpathwaysforthestudyofhigh-spinstates . . . . . . . . . . . . . . 13
1.2.2 Isomericstatesnorthwestof132Sn . . . . . . . . . . . . . . . . . . . . . . . . . 16
1.2.3 Existingexperimentaldatainthe50 Z,N 82region . . . . . . . . . . . 19
1.2.4 Shell-modelinteractionsforthedescr≤iptiono≤f50 Z,N 82nuclei . . . 21
1.3 Outlineofthisthesis . . . . . . . . . . . . . . . . . . . . . . . .≤. . . .≤. . . . . . . . . . . 25
2 Light and heavy transfer products in the
136Xe + 238U multinucleon transfer reaction . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
Addendum: SimulationofthePRISMAspectrometer. . . . . . . . . . . . . . . . . . . . . . . 41
3 Spectroscopy of the neutron-rich actinide nucleus 240U . . . . . . . . . . . . . . . . . . . 47
4 High-spin structure of 134Xe . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59
5 Isomers and high-spin structures
in the N =81 isotones 135Xe and 137Ba . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73
6 Characterization and calibration of radiation-damaged
double-sided silicon strip detectors. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93
7 High-spin structures in 132Xe and 133Xe
and evidence for isomers along the N =79 isotones . . . . . . . . . . . . . . . . . . . . . 105
8 Summary and conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 121
Bibliography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 132
List of figures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 147
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 153
Curriculum vitae . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 157
Erklärung zur Dissertation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 158
v

| Introduction
1.1 Nuclear reactions close and above the Coulomb barrier
CollisionsbetweenheavyionscloseandabovetheCoulombbarrieryieldavastanddiversespectrum
of reaction modes. Nuclear reactions can be classified by timescales, involved masses and kinetic
energies,andbytheimpactparameterb. Simplifiedsketchesofthedifferentreactionmechanismswith
varyingpenetrationenergyandimpactparameteraregiveninFig.1andinFig.2. ElasticRutherford
scatteringandCoulombexcitationdominatefordistantcollisions,i.e.largeimpactparameters. Ifthe
energiesofthecollidingnucleidonotreachtheCoulombbarrier,thetrajectoriesaremostlygoverned
bytheelectromagneticinteractionandarecalculablewithahighprecision[1,2].
At low impact parameters, a di-nuclear system is formed as a basic transitional stage between the
entrance channel and the formation of reaction products [4]. At this initial phase of the reaction,
the two fragments are linked by a neck [5]. In a head-on collision scenario at energies above the
Coulomb barrier, fusion-evaporation reactions take place. The projectile is incorporated into the
targetnucleusandahotcompoundnucleus,inwhichalldegreesoffreedomarepopulated,isformed.
This highly-excited system reaches thermodynamic equilibrium in timescales of t > 10 20 s and,
−
subsequently,within10 15s,itevaporatesnucleonsandhigh-energyγraysuntiltheexcitationenergy
−
issmallerthantheparticle-separationenergyabovetheyrastline[6]. Dependingonthenumberof
emittedparticles,variousresidualnucleiarepopulatedwhichmayfurtherdecayviaγ-rayemission.
Formationanddecayofthecompoundnucleusareindependent;theinitialidentitiesofprojectile
and target are lost. The excitation energy depends on the initial kinetic energy and the Q value
Inelastic scattering
(Coulomb excitation) Deep-inelastic transfer
& incomplete Fusion
Complete fusion
b
Quasi-elastic scattering
Elastic (Rutherford) scattering
Figure 1: Reactionchannelsdependingontheimpactparameter. Inthetworegimesofmultinucleon
transfer,quasi-elasticanddeep-inelastictransfer,beam-andtarget-likefragmentsretaina
partialmemoryoftheinitialreactionchannel. Productioncrosssectionsaremaximized
aroundareaction-specificgrazingangle. AdaptedfromRef.[3].
7
n n
Entrance channel γ γ
γ γ
γ γ n
Spectator/participant
Quasi-elastic Quasi-fission
fragmentation
γ
Compound
n
thermalization
γ
Di-nuclear
γ γ
phase
n n
p
γ
γ
n
n γ n
γ γ n γ
Deep-inelastic
γ
n
transfer
γ
p n
Fusion
Fusion-fission Fragmentation Vaporization
evaporation
Excitation energy 0-3 MeV/nucleon 3-10 MeV/nucleon >10 MeV/nucleon
Figure 2: Regimesofnuclearreactionsbetweenheavyions. Quasi-elasticreactions,deep-inelastic
transferandquasi-fissionprocesseshappenwithinsmalltimescalesintheearlystagesof
thereaction,theso-calleddi-nuclearphase. Fusion-evaporationorfusion-fissionreactions
requiretheformationofacompoundsystem. Seetextfordetails.
forthecompoundformation. Ifthetransferredangularmomentumisaboveacertaincriticallimit
givenbythemassofthecompoundnucleus,thecompoundsystemmightalsoundergofission. Tobe
differentiatedfromthisstatisticalfissionistheso-calledquasi-fission(QF)process[7]whichoccurs
intheearlydi-nuclearstageofthecollisioninnon-equilibriumbeforeformingacompoundnucleus:
a nucleon transfer occurs from the heavy fragment toward the lighter one and the two fragments
re-separatewithagreatermasssymmetrythanintheinitialentrancechannel.
Between the two extremes of elastic scattering and fully-damped collisions leading to compound
nuclei,thereisawealthofpartially-dampedinelasticcollisionsatintermediateimpactparametersthat
areoftenreferredtoasmultinucleon-transferreactions[8]. Tworegimes,nonethelesstransitional
innature[9–12],canbedistinguished: (i)quasi-elasticprocessesincludinginelasticscatteringand
thetransferoffewnucleons,accompaniedbysmallenergylossesand(ii)deep-inelasticreactions
withsmallerimpactparametersenablinglarger-scalenucleonflowbetweenthefragments. Latteris
characterizedbyhighvaluesoftotalkineticenergyloss. Generally,essentialfeaturesarethetransfer
of nucleons (from simple one-step transfers to complicated multistep reactions) and the partial
conversionofkineticenergyoftheprojectileintointernalexcitationenergyofthereactionresidues.
8
The collisions keep the binary character of the system and the ejectiles retain some resemblance
with the initial nuclei. Therefore, the reaction products are often called beam- and target-like
fragments. Thereisafastredistributionandrearrangementofnucleonsamongthecollidingnuclei
(N/Z equilibration),governedbystrongdrivingforcesassociatedwithsurfacemodes,single-particle
degreesoffreedomandtunnelingprobabilitiesinthedi-nuclearcomplex. Thereactionstakeplace
withinapprox.10 22 s[6],similartotransferreactionsutilizingverylightions.
−
Sincemultinucleon-transferreactionsareperipheralcollisions,therelative-motionangularmomentum
is dissipated into intrinsic spin of the reaction partners, although not as efficiently as in fusion-
evaporation reactions. Experimentally, a precise knowledge of the grazing angle θ is crucial
grazing
sinceitisthescatteringangleatwhichthebinaryreactioncrosssectionismaximized,ratherthanthe
onesofinelasticCoulomborelasticinteractions. Thus,toproperlymeasurethehighestproduction
yieldsofreactionproducts,thesolid-angleofamassspectrometermeasuringeitherofthereaction
partnersshouldcoverarangearoundthatspecificangle. θ isapproximatedasthescattering
grazing
angle that corresponds to the impact parameter when the two nuclei are just touching each other.
There,thedistance d isthesumoftheradiiofthenucleiparticipatinginthereaction:
d =(cid:18) ZtZpe2 (cid:19)(cid:18)1+cscθgrazing(cid:19) 1.2(cid:128)A1/3+A1/3(cid:138) [fm] (1.1)
4πε E 2 t p
0 kin ≈
Z e and Z e correspondtothenuclearchargesoftheprojectileandthetarget,A andA aretheir
p t p t
respective masses. E is the kinetic energy of the impinging beam. The angular distributions
kin
of the reaction products are bell-shaped around the grazing angle [14, 15]. The evaporation of
lightparticlesfromtheprimaryfragments,especiallyneutrons,stronglyinfluencesthefinalisotopic
yield distribution [8]. Figure 3 displays a typical production yield distribtion of the 40Ca+208Pb
multinucleon-transfer reaction at E = 235 MeV. Pure proton stripping and neutron pickup are
lab
)
s
el
n
n
a
h
c
(
Z
M (channels)
Figure 3: Distribution of mass and nuclear charge for the beam-like products of the 40Ca+208Pb
reactionatabeamenergyof235MeV.Thedashedlineslabelpureprotonstrippingand
neutronpickup. Thesolidlinedepictsthe N/Z chargeequilibration. Reprintedfigurewith
permissionfromRef.[13]. CopyrightbytheAmericanPhysicalSociety.
9
labeledbydashedlines;thesolidlinedepictstheconfinementimposedby N/Z chargeequilibration
as well as by the optimum Q-value of the reaction. In this case, massive transfer of protons is
accompaniedbyadrifttowardslowermasses,illustratingtheroleofparticle(inthiscaseneutron)
evaporation. Thus,aspecificreactionproductmaybeaccompaniedbyseveralbinarypartners. Inthe
caseofreactionsystemsinvolvingheaviernuclei,especiallyactinidetargets,thepresenceoffission
maycontaminatethegenuinetransferchannels[16].
For beam energies of 30 MeV/nucleon and above, so-called intermediate to relativistic energies,
nuclearfragmentationisfavored,populatingalargerangeofexoticnuclei. Theseviolentcollisions
can be described by the abrasion-ablation model in which the incoming projectile shears off the
overlappingsectorofthetargetnucleus(abrasion). Thenon-overlappingpartsremainundisturbedin
thecollisionandareoftencalledspectators. Thehighly-excited,hotfragmentoftheoverlapregion
(participant)decaysviatheemissionofnucleonsorfurtherfragments;thespectatorexcesssurface
energyafterthesuddenabrasionistransformedintoexcitationenergy[17]. Beam-likefragments
carry on with a velocity similar to that of the initial beam. Projectile fragmentation is of interest
in the production of radioactive ion beams employing large fragment separators [18] at in-flight
fragmentationfacilitieslikeRIKEN,Japan[19],atthefuturefacilityFAIRinDarmstadt,Germany[20]
oratitsU.S.counterpartFRIB.Highercenter-of-massenergiesleadtoevenmoreviolentreactions
suchasvaporization,leavingtheareaoflow-energynuclear-structurephysics.
1.1.1 Theory of multinucleon-transfer reactions
The transfer of several nucleons between beam and target isotopes is a non-equilibrium quantum
transport phenomenon which is not trivially computable. Several models aim for a description
of partially-damped reactions in both the quasi-elastic and deep-inelastic regimes. Models with a
reasonablepredictivepowershouldespeciallybeabletopredicthowthetotalreactioncrosssection
is shared among the different reaction channels. Multinucleon-transfer reactions have also been
extensively analyzed by direct-reaction theories such as GRAZING and complex Wentzel-Kramers-
Brillouin(CWKB)theory[21]. Inthesetheories,therelativemotionofthereactionpartnersistreated
inasemi-classicalapproximation. The GRAZING model,basedontheframeworkofdirectreaction
theory,isabletocomputetotalreactioncrosssections,angulardistributions,excitationfunctions,and
isotopicdistributionsforbothpre-andpost-neutronevaporation. Thefundamentalsofthismodel
arebrieflydiscussedinthefollowingsection.
Severalmodelshavebeenproposedforthedescriptionofdeep-inelasticheavy-ioncollisions,such
as Fokker-Planck [22] equations, the so-called master equations [23], or the improved quantum
moleculardynamics(ImQMD)model[24]. Asuccessfuldescriptionofmasstransferinmultinucleon
transferwasachievedwithinthetime-dependentHartree-Fock(TDHF)theory. Thismean-fieldtheory
offersamicroscopicframeworkyieldingafulldescriptionofnotonlytransfer,butallstagesinthe
evolutionofanuclearcollision. RecentdevelopmentswerepursuedbySekizawaandYabana[25,
26] employing the particle-number projection method [27, 28] for various reaction systems like
40Ca+208Pb, 58Ni+208Pb, 40,48Ca+124Sn, or 136Xe+198Pt. The colliding nuclei are treated as
compositesystemsofindividualneutronsandprotonsintheirrespectiveorbitals. Calculationsare
10