First measurement of the isoscalar excitation above the neutron emission threshold of the Pygmy Dipole Resonance in68Ni

Contents lists available atScienceDirect

Physics

Letters

B

www.elsevier.com/locate/physletb

First

measurement

of

the

isoscalar

excitation

above

the

neutron

emission

threshold

of

the

Pygmy

Dipole

Resonance

in

68

Ni

N.S. Martorana

a

,

b

,

∗

,

G. Cardella

c

,

E.G. Lanza

c

,

L. Acosta

d

,

c

,

M.V. Andrés

e

,

L. Auditore

f

,

c

,

F. Catara

c

,

E. De Filippo

c

,

S. De Luca

f

,

c

,

D. Dell’ Aquila

g

,

B. Gnoffo

b

,

c

,

G. Lanzalone

h

,

a

,

I. Lombardo

c

,

C. Maiolino

a

,

S. Norella

f

,

c

,

A. Pagano

c

,

E.V. Pagano

a

,

M. Papa

c

,

S. Pirrone

c

,

G. Politi

b

,

c

,

L. Quattrocchi

c

,

F. Rizzo

a

,

b

,

P. Russotto

a

,

D. Santonocito

a

,

A. Trifirò

f

,

c

,

M. Trimarchi

f

,

c

,

M. Vigilante

g

,

A. Vitturi

i a_{INFN-LNS,Catania,Italy}

b_{DipartimentodiFisicaeAstronomia,UniversitàdegliStudidiCatania,Catania,Italy} c_{INFN-SezionediCatania,Catania,Italy}

d_{InstitutodeFisica,UniversidadNacionalAutonomadeMexico,MexicoCity,Mexico} e_{DepartamentodeFAMN,UniversidaddeSevilla,Sevilla,Spain}

f_{DipartimentoMIFT,Messina,Italy}

g_{INFN-SezionediNapoliandDipartimentodiFisica,UniversitàdiNapoliFedericoII,Napoli,Italy} h_{FacoltàdiIngegneriaeArchitettura,UniversitàKore,Enna,Italy}

i_{DipartimentodiFisicaeAstronomia,UniversitàG.GalileiandINFN-SezionediPadova,Padova,Italy}

a

r

t

i

c

l

e

i

n

f

o

a

b

s

t

r

a

c

t

Articlehistory:

Received23January2018

Receivedinrevisedform7May2018 Accepted8May2018

Availableonline16May2018 Editor:V.Metag

Keywords:

PygmyDipoleResonance Unstablenuclei Isoscalarprobe

The excitation of the Pygmy Dipole Resonance (PDR) in the 68_{Ni nucleus, above the neutron emission} threshold, via an isoscalar probe has been observed for the first time. The excitation has been produced in reactions where a 68_{Ni beam, obtained by the fragmentation of a} 70_{Zn primary beam at INFN-LNS,} impinged on a 12_{C target. The}

_γ

_{-ray decay was detected using the CsI(Tl) detectors of the CHIMERA} multidetector sphere. The 68_{Ni isotope as well as other heavy ion fragments were detected using the} FARCOS array. The population of the PDR was evidenced by comparing the detected

γ

-ray energy spectra with statistical code calculations. The isotopic resolution of the detection system allows also to directly compare neutron decay channels with the 68_{Ni channel, better evidencing the PDR decay response} function. This comparison allows also the extraction of the PDR cross section and the relative

γ

-ray angular distribution. The measured

γ

-ray angular distribution confirms the E1 character of the transition. The

γ

decay cross section for the excitation of the PDR was measured to be 0.32 mb with a 18% of statistical error.

©2018 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/). Funded by SCOAP3_.

1. Introduction

Recently,muchrelevancehasbeengiventothecollectivestates in neutron-rich nuclei. The remarkable interest inthese statesis alsodrivenby thepresenceofan electricdipole responsearound thenucleonbindingenergy[1–3].Thismode,theso-calledPygmy DipoleResonance (PDR),although iscarryingfew percentofthe isovectordipoleEnergyWeightedSumRule(EWSR),hasbeen pre-dictedtobepresentinallthenucleiwithneutronexcess;in

par-*

Correspondingauthorat:UniversitàdegliStudidiCataniaandINFN-LNS,Via SantaSofia,62,Catania,Italy.

E-mailaddress:martorana@lns.infn.it(N.S. Martorana).

ticular forthose farfrom thestability line. Ina pioneeringwork [4],ithasbeenshownthat,assoonasthenumberofneutrons in-creases,theisovectorandisoscalardipole responsesshowasmall bump at low energy in the strength distribution, which is well

separated from the well known peak of the Giant Dipole

Reso-nance(GDR).Thismode hasa strongrelationwiththe symmetry energyandithasbeenusedasafurthertooltoconstrainit[5,6]. Furthermore,thePDRmighthaveabiginfluenceonthe astrophys-icalr-processnucleo-synthesis,sincethepresenceofanevensmall dipolestrengtharoundtheneutronseparationenergystrongly en-hancestheneutroncapturecrosssections[7–9].

Assuming that a nucleus witha neutron excess is formed by a saturatedcore, withthe samenumberofneutronsandprotons,

https://doi.org/10.1016/j.physletb.2018.05.019

0370-2693/©2018TheAuthors.PublishedbyElsevierB.V.ThisisanopenaccessarticleundertheCCBYlicense(http://creativecommons.org/licenses/by/4.0/).Fundedby SCOAP3_.

andaneutronskin,thePDRcanbeconsideredasasurface mode associatedwiththe motionofthe neutronskin againstthe satu-ratedcore. Severalmicroscopic many body models [1,10–16] are abletodescribe the low-lyingdipole strength, andtheyall agree onthepropertiesofthetransitiondensities:theneutronsand pro-tonstransitiondensitiesareinphaseinsidethenucleusbutatthe surfacejusttheneutronpart,withoppositephase,survives.These propertiesgiverisetoatransitiondensitywheretheisovectorand isoscalarcharacteraremixed,andthereforethismodecanbe pop-ulatedbybothisoscalarandisovectorprobes[15].

Several experiments have been performed on stable nuclei

[1–3].Thecomparisonbetweentheexcitationoftheselow dipole statesinducedbyeitheralphaparticles,17_{Onucleiorrealphotons}

revealsaparticularpropertywhichisknownas“isospinsplitting” [2,3,17–25]:only the low energy region ofthe PDR ispopulated bybothisoscalarandisovectorprobes,whilethehigherpartis ex-citedmainlyviaelectromagneticinteraction.Thissharpseparation of pure isovector and mixed isoscalar-isovector states, in a very limitedenergy range, has to be better understood and it is cur-rentlya topicof interest [26,27]. Until now the isospin splitting hasbeenobserved foralmost all thestable nucleianalyzed.It is interesting toinvestigatewhetherthe sameeffect ispresentalso forthe PDRregionabove theneutronemission thresholdandfor unstablenuclei. The experimental studyofthe PDRatan energy above the neutron emission threshold, withunstable nuclei, has beendoneinpioneeringexperiments,carriedoutattheGSI,using relativisticCoulombexcitationson132_{Sn[28] and}68_Ni[29,30]

iso-topes.ThepresenceofthePDRinunstablenucleiwasinvestigated usingrelativisticCoulombexcitationsalsoinrefs. [31–33].

In this work, we report on the first observation of the PDR, abovethe neutronemission threshold,in the68Ninucleus, using anisoscalarprobe,obtainedinanexperimentcarriedoutat INFN-LNS of Catania.In order to excite the PDR we used an isoscalar targetof12C.Semiclassicalcalculations,basedonmicroscopicform factorbuiltwithmicroscopicRPAtransitiondensities[14–16] show that,attheenergyused(28 A MeV),mostofthetotalinelasticPDR crosssection(more than60%)isduetothepure nuclear interac-tion.TheCoulombcontributionamountsat9%,whileanother30% isgivenbytheinterferencebetweennuclearandCoulomb contri-butions.Such calculationshavebeendone accordingto the semi-classical model described in [13–15], where the real part of the optical potential hasbeen obtainedby thedouble folding proce-dure,whiletheimaginaryparthasbeenchosentohavethesame geometryofthe realpartwithhalfofits intensity.Thisisa kind ofstandardprocedurewhenonehasnoexperimentalinformation abouttheelasticscatteringcrosssection.

2. Experiment

In Fig. 1 a sketch not to scale of the experimental apparatus ispresented.A 70Zn primary beamwas acceleratedtoan energy of 40 A MeV, using the Superconducting Cyclotron (CS), and it impinged on a 250 μm thick 9_{Be target to produce, with a}

pro-jectile fragmentation reaction, the 68Ni beam, delivered via the FRIBs@LNS fragment separator of the INFN-LNS [34,35]. In such a facility,a taggingsystem was used in order to eventby event characterizethecocktailbeam[36].AlargesurfaceMicro-Channel Plate(MCP)givesthestart oftheTimeofFlight(ToF)measurement ofthe beam. The stop of the ToFis delivered by a Double-Sided SiliconStripDetector (DSSSD),with32

×

32 strips2mm width,

and156 μm thick, placed at about 12.9m from the MCP, along

thetransport beamline, approximately 2m before the12_C

reac-tiontarget. The DSSSD provides alsothe energy lossinformation forthe



E–ToF identification method,moreover the DSSSD pro-ducesalsothefirst informationonthebeampositiontomeasure

Fig. 1. Sketch, not to scale, of the used experimental set up (color online).

Fig. 2. a)



E–EscatterplotobservedwiththeFARCOSarrayirradiatedbythe re-actionsproductsproducedwiththe wholecocktailfragmentationbeam.Isotopic identificationoffragmentswithZfrom27to30isobserved.Intheinsetthe



E– ToFidentificationscatterplotofthecocktailbeamisreported.b)Reactionproducts detectedwiththe FARCOSarrayincoincidencewith 68_{Niions,asshown inthe} blackcircleintheinsetofFig.2a(coloronline).

thetrajectory.Thesecond positioninformationwasprovidedbya ParallelPlateAvalanche Counter(PPAC)device, mountedatabout 80cmfromthetarget,andhaving1mmspatial resolution.A12C target, 75 μm thick, was used to induce the isoscalar excitation on the projectile nucleus. The scattered 68Ni ions were detected and identified in four prototype telescopes of the FARCOS array [37,38].During theexperimenttheFARCOS array was placedjust afterthesphereoftheCHIMERAmultidetector[39,40] andit cov-ered angles from 2◦ to 7◦, with approximately 70% coverage of azimuthal angles.TheFARCOSmodules aretripletelescopes,each

of them constituted by two stages of DSSSD 300 and 1500 μm

thick, followedby fourCsI(Tl)scintillatorsread bya photo-diode.

68_{Ni scatteredionswere stoppedinthe secondstage ofthe}

tele-scopes.

3. Analysisandresults

Asabove mentioned,we usedthetaggingsystemtogetevent byeventtheisotopiccompositionoftheexoticbeam.Intheinset ofFig.2aisreportedthe



E–ToF plotobtainedwiththistagging system.The 68_{Nibeam, evidencedbytheblackcircle,isthemost}

intensebeaminthecocktail(about20%ofthetotalintensity).The beamenergy,aftertheDSSSDdetector,isabout28 A MeV.Atthis beamenergytheionidentificationisdifficultbecauseofthe pres-enceofnotfullystrippedionsproduced afterthe9_{Be target.Just}

the 92% of the incomingZn beam is totally stripped,more than the7% oftheionshaveacharge statusof29+,andalsothe28+ charge statusis present.However, the configurationofthe trans-portbeamline,withthepresenceofmagnetsaftertheMCP,allows torejectthelargestamountofthisbackgroundbythestripping ef-fectofthemylarfoil.Withthismethodwewerethereforeableto correctlyidentify the68_Ni28+ _{beam. Amore quantitative analysis}

Fig. 3. PulseshapediscriminationplotinaCsI(Tl)detectoroftheCHIMERAsphere. Wenotethatonlylightparticlesandγ-raysareobservedduetotheforward focus-ingofthereactionproducts(coloronline).

ofthecontaminationoftaggedbeamswillbediscussedinthe fol-lowing.ThehighqualityoftheFARCOSsilicondetectorsresponse, togetherwiththecleaningeffectofthefragmentseparator,allows thefullisotopicidentificationof68Ni,asitisshowninFig.2a.In Fig.2b itis shownthe



E–E plot,obtainedwith theFARCOS ar-ray,incoincidencewiththe68_{Nibeam,shownasablackcirclein}

theinsetofFig.2a.FromtheFig.2bonecanbetterappreciatethe amountofthecontaminationofthetaggedbeamby66Ni27+(14%) produced at similar energy withrespect to the one of the main beam. A second contaminant of lower intensity (7%) is 66_Cu28+

producedatlowerbeamenergy.Thankstotheisotopicresolution ofthedetectionsystem, andto theenergydifferenceofthe vari-ousnuclei,reactionproductsgeneratedbythesecontaminantscan beeasilyremoved.

ThePDRcan decaybothbyneutronorby

γ

-rayemission(the lastonewithlowerprobability).Inordertodetect

γ

-raysweused thesecond stage ofthetelescopes oftheCHIMERA multidetector sphere,composedofCsI(Tl)scintillatorsreadbyaphoto-diode. De-tails on the

γ

-rays detection and identification can be found in refs. [41,42]. The energy calibrationof CsI(Tl) detectors was per-formed using 24 MeV proton elastic and inelastic scattering on

12_C, 197_{Au and CH}

2 targets. More in detail scatteringof protons

andpulser signalswere used to calibrate all detectors inproton equivalentenergy,verifyingthelinearityoftheresponsefunction. Theprotonenergycalibrationwasconvertedin

γ

-rayenergy cal-ibration using,asa referencepoint, the

γ

-rayfrom thedecay of the4.44MeVlevelofthe12C.These

γ

-raysprovidedalsoacheck for the angular distribution and efficiency calibration, as shown in [42]. The p

+

12_{C collision was also usefulin order to verify}

that theprobability to produce

γ

-rays fromthedecay ofexcited

12_{C isnegligible intheregion around 10MeV, wherethePDRis}

expected.Allexcitedlevelsinthisenergyrangehaveinfacta neg-ligible

γ

-decaybranchingratio[43].

Inordertoidentifythe

γ

-rays,weusedthesocalledfast-slow methoddescribedin[41].InFig.3anexampleofpulseshape dis-criminationplot,obtainedinthisexperiment,isshown.Thetiming oftheMCPwasusedasareferencetimeforthepulseshape anal-ysis.

Duetothelargeenergyspreadofthefragmentationbeam(2%), itisdifficult fromionenergymeasurements to evaluatethetotal kineticenergylossandfromthistoextracttheaverageexcitation energyofthe system.However, asit was seeninFig. 2b,68_Ni

₊

12_{C reactions mostly produce, atforward angles,two kind of}

re-actions: reactions in which the mass of 68Ni projectile does not change,andreactions in whichone ortwo neutrons are emitted and66, 67Ni ionsaredetected. Theelasticchannel isnotobserved dueto theFARCOS angular coverage;thegrazing angleis infact around 0

.

7◦.In Fig.4awe report, asbluedots, the

γ

-ray energy spectrum,Dopplershiftcorrectedbyassuminganemissionbythe projectile,andmeasuredincoincidencewiththeNifragments de-tected by the FARCOS array (Fig. 2b). We note the presence of

Fig. 4. a)Thebluedotsrepresentthe γ-rayenergyspectrum,Dopplershift cor-rected,incoincidencewithNifragmentsdetectedbytheFARCOSarray(Fig.2b). Theredsquaresrepresentthebackground.b)γ-rayspectrum(bluedots)obtained bysubtractingthetwospectrashowninFig.4a.Thebluedashedlineistheγ-ray energyspectrumproducedbythe68_{Nistatisticaldecay,foldedwiththedetector} response function,obtainedusingthe standardCASCADE code.Thered continu-ouslineistheCASCADEcalculation,foldedwiththedetectorresponsefunction,by includingthepresenceofthePDR,GDRandtheGQR.IntheinsettheCASCADE cal-culationbyincludingthepresenceofthePDR,theGDRandtheGQR,withoutthe folding,isreported(coloronline).

γ

-rays yield extending up to 20 MeV. However, the most ener-getic

γ

-raysdetectedareessentiallyduetospuriouscoincidences. Thisbackgroundhasbeenevaluatedmovingthefast-slowcutson both sides around the

γ

line and it is plotted as red squares. The difference between the coincidence spectrum and the back-groundisreportedasbluedotsinFig.4b.Inordertoevidencethe populationofthePDR,inthisfigure,arealsoreportedtwo calcula-tions performedusingthestatisticalcodeDCASCADE[44,45]. The bluedashedlineisobtainedwithastandardstatisticalcalculation, whiletheredfulllineincludesthestrengthduetothepopulation ofthePDR,thestandardGDRandtheGiantQuadrupoleResonance (GQR).The populationoftheIsoscalarGiantMonopoleResonance (ISGMR) andIsoscalar GiantQuadrupole Resonance (ISGQR) have beenstudiedinthe68Niinrefs. [46,47].Attheenergyof28 A MeV, thecrosssectionforthepopulationoftheISGMRisverylowand moreover this mode can not decay, atleast at the groundstate, by

γ

-raysemission;forthesereasonsweneglectedthistransition intheCASCADEcalculation.Whereas,weincludedinthestatistical calculationalsothepresenceoftheGQR,byconsideringthevalues reportedinrefs. [46,47].Wealsounderlinethat whileresonances must be explicitly inserted, standard acceptedconstant strengths ofothertransitionsareincludedinCASCADE.

The statisticalcalculationsreportedinFig.4b wereperformed

assuming a range of excitation energies with a maximum at

26.5 MeV. Therange ofexcitation energies hasbeen fixed in or-der to reproduce the charge distribution observed (Fig. 2b) and the slope of the spectrum from 2 to 6 MeV. Excitation energies larger than28MeV are excluded becausewe donot detectmass smallerthan66.Thebluedashedlinestatisticalcalculation repro-ducesjustthefirst regionofthe spectrum(from2 to6MeV),in fact,duetothecompetitionofparticleemission,ifnoresonanceis excited,thehighenergy

γ

-decayprobabilityisverysmall.The ex-perimentalspectrumisreproducedonlyincludingresonancesand in particularwe inserted inthe CASCADEcalculation a PDR con-tributionwithacentroidaround10MeV,awidthof2MeVanda strengthof9%

±

2%EWSR, aGDRcontributionwithacentroidof

17MeV,awidthof6MeVandastrengthof91%

±

2%EWSRand

aGQRwithacentroidaround16MeVandawidthof1.5MeV. Theparameters ofthePDRandGDR resonancesarein reason-ableagreement,insideourerrorsbars,withthevaluesreportedin ref. [30]. These statisticalcalculationsare folded with theCsI(Tl) responsefunction,evaluatedbyperformingGeant4[48,49]

simula-Fig. 5. a)γ-rayenergyspectraDopplershiftcorrected.Thebluedotsrepresentthe coincidencewith68_{Ni. Theredsquaresrepresentthe γ-rayenergyspectrumin} coincidencewith66, 67_{Ni.Thesespectrahavebeennormalizedatlowenergy.The} bluefulllineistheγ-raysfirststepspectrumobtainedwithCASCADE(seetext). b)CrosssectionofthePDRobtainedwiththesubtractionofthespectrashownin Fig.5a(coloronline).

tion.Inordertobetterunderstandtheeffectofthefolding proce-dure,intheinsetofFig.4bisreportedthestatisticalcalculationby includingthePDR,theGDRandtheGQR,withoutthefoldingwith theCsI(Tl)responsefunction.There,onecanclearly seethebump correspondingtotheexcitationofthelow-lyingdipolestate.

Due to the energy resolution of CsI(Tl) detectors the popula-tionofthePDRinFig.4bisevidenced onlybyasmallchangeof slopearound10MeVandbythecomparisonwithstatistical calcu-lations.AbetterexperimentalevidenceofthePDRpopulationcan beobtainedbycomparingthe

γ

energyspectrameasuredin coin-cidencewiththe68_Nichanneland66, 67_{Nichannels.Infact,atthis}

lowexcitationenergythehighenergy

γ

-decayofthePDRhinders furtherparticledecay.Onthecontrary,ifoneormoreneutronsare emittedbythesystem,thehighenergy

γ

-raysdecayisinhibited.

For these reasons we compare in Fig. 5a the

γ

-rays energy spectrum(blue dots)in coincidence withthe68_{Ni (Fig. 2b), with}

the one measured in coincidence with neutron decay channels

66, 67_{Ni. The two spectra are normalized in the low energy}

re-gion.AlsothesespectraareDopplershiftcorrectedbyassumingan emission fromthe projectile. The background was evaluated and subtracted withthefast-slow cuts, asshowninFig. 4a. We note inFig. 5a the enhancement around 10MeV, asit was expected, due to the PDR decay in the 68_{Ni channel. We observe a}

rela-tively smallyield inthe GDRhigh energyregion aspredictedby thesemiclassicalcalculations.Indeed,thesecalculationsshowthat, attheserelativelylowincidentenergiesandwiththelowCoulomb fieldofthe target,the excitationprobability forthe PDRmodeis higherthantheonecorresponding totheGDR energyregion.The smallyield ishoweveralso dueto thelower detection efficiency inthehigherenergyregion.Weunderlinethatitisnotpossibleto comparetheexclusiveenergyspectraofFig.5withinclusive CAS-CADEcalculations.However,onecancomparethe

γ

-raysspectrum incoincidencewith68Ni with

γ

-rays first stepspectragenerated withCASCADE,namelywiththe

γ

-raysemittedasfirstparticlein thedecayprocess.Infact,sucheventsatthelowexcitationenergy of the system produce only 68_{Ni nuclei. This calculation, folded}

withthe CsI(Tl) response function, is plotted asblue full line in Fig.5a. The comparison is quite good andthe calculation repro-ducesthebump in theenergyregion of thePDR.Clearly, inthis calculation,thelowerenergyspectrumisnotreproducedbecause inthisenergyregionthecontributionofthedecayofdiscrete lev-elsinthefinaldecaystepsismissing.

InFig. 5bwe report the PDRenergy

γ

-spectrumobtained by subtractingthetwonormalizedspectraofFig.5a.Thismethod al-lowsto give a lower limit to thePDR yield, becausesome small PDRcontributioncould bepresentalsointhe66, 67Ni coincidence spectrum.However thisis thesimplest waytocompare thePDR

Fig. 6. a)Themeasuredγ-rayangulardistribution.ThelineistheexpectedE1 an-gulardistribution.b)The68_{Niangulardistributionmeasuredincoincidencewith} γ-raysintheregionofthepygmyresonance(coloronline).

yieldwithpreviousexperimentalresults[29] andlooktothe pres-ence of isospin splitting above particle emission threshold. The cross section is obtained taking into account the detection effi-ciency, as better specified in the following. It is very important toverifytheE1characteroftheobservedbumpataround10 MeV incoincidencewiththe68_{Nichannel,inordertoprovethedipole}

characterofthetransition. Toprovethiswe extractedtheangular distribution ofthe emitted

γ

-rays inthe region of the enhance-ment ataround10 MeVshown inFig.5b.The granularityof the spherical region of the CHIMERA multidetector, covering angles from 30◦ to 176◦, in step of 8◦ up to 146◦, allows to extract the angular distribution. Because of the relatively low statistics, we were forced to sum

γ

-rays detected in two rings ofthe ap-paratus; therefore theeffectivelaboratory angular resolutionwas

±

8◦,beingnegligibletheerrorintheevaluationofthe68_Ni

scat-teringangleassumedasreferenceaxis.Thisangulardistributionis showninFig. 6a.Notwithstandingthescarce statistics,the angu-lardistributionshowsthetypicaldistributionexpectedforadipole transitionwithamaximumaround90◦(fullblueline).Theangular distribution was corrected for the effective

γ

-ray detection effi-ciency evaluated usingGeant4simulation at10 MeV,taking into account the thickness of CsI(Tl) scintillators of the sphere (from 8 to 4cm ) mounted atdifferent angles andmalfunctioning de-tectors.Onaveragethetotal

γ

-raydetectionefficiencywasofthe orderof25%.Inordertoevaluatetheoveralldetectionefficiencyit isalsoimportanttodeterminetheangulardistributionofthe emit-ted68_{NiincoincidencewiththePDRenhancement.Unfortunately,}

duetolossesinefficiencyofthePPACdetectorwedidnothavethe informationonthebeamtrajectoryforalltheevents.However,the beamdivergencewasratherlow,oftheorderof0

.

5◦;thereforeby assumingthiserrorintheangulardistribution(andafurther0

.

2◦ duetotheangularstraggling inthethick12_Ctarget),and

correct-ing for theaverage beam directionthat can be evaluated by the impingingpositionintheDSSSDdetector,wewereabletoevaluate theangulardistribution whichisplottedinFig.6b.We notethat thelargestpartoftheeventsiscollected from12◦ to 20◦,witha maximumaround15◦intheCMreferenceframe.Theoverallbeam efficiencywascalculatedwithaMonte-Carlo simulation,by using themeasuredbeamdistribution.Takingintoaccounttheobserved angulardistribution,onecanevaluateanaverageefficiencyforthe

68_{Nidetectionoftheorderof52%.}

We cannot extract muchmore physicalinformationfromthe

68_{Ni angular distribution, shown in Fig. 6b. In fact, to perform}

meaningfulDWBAcalculationsweshouldalsohavethemeasured elastic scattering angular distribution in order to fix the optical potential. However, the68Ni angular distributionis importantfor evaluatingthe68_{Nidetectionefficiencyanditmaybeareference}

pointforfuturemeasurements.

Theabsolutecrosssectionsofthepygmy

γ

-raydecayreported inFig.5b,Fig.6aandFig.6bareobtainedthankstotheeventby

eventcounting of the beamintensity and the knowledge of tar-get thickness, once determined the detection efficiency. In total, we countedabout1

.

4

×

109 68Ni beamparticles.Taking into ac-counttheaveragebeamdetectionefficiency(52%)andtheaverage

γ

-raydetectionefficiency(25%),thetotalcrosssectionofthePDR

γ

-decayamountsat0.32mbwitha statisticalerroroftheorder of18%.

Beingsureof theE1 character ofthe resonancewe can com-pare themeasured

γ

-rayspectrum tothe one obtainedusingan almostpure isovectorprobe,likeintherelativisticCoulomb exci-tationinref. [29].Comparingourresultswiththeresultsobtained inref. [29] there is almost an orderof magnitudedifference be-tween the two cross sections, which is probably caused by the different reactions regimes. Furthermore, in the GSI experiment [29] themaximumofthestrengthdistributionislocatedataround

11 MeV, while in thiscase it seems peaked at around 10 MeV,

more in agreement with the resultobtained in ref. [30]. Due to the response function of CsI(Tl) detectors, the enhancement ob-served at10MeVis generatedby

γ

-rayswithenergyof approx-imately 10.5 MeV. The differentslope ofstatistical

γ

-ray spectra underlyingtheresonanceandpropagationerrors,inthecalibration procedure,canaccountfortheresidualdifferenceontheobserved energystrength ofthe resonance.Withinthe smallstatistics and therelativelyscarceenergyresolutionofthepresentmeasurement andprevious experiments[29,30],thiscomparisonseems to indi-cate that the outcome of the excitation process due to the two isoscalarandisovectorprobesissimilaralongthePDRenergy re-gion.Therefore the isospinsplitting seems notto be observed at theenergy abovethe neutronemission threshold;howevermore precisemeasurements arenecessaryto betterprovethis observa-tion.

4. Conclusions

Insummarywehaveobserved,forthefirsttime,the

γ

-ray de-cay of the pygmy resonance populated by an isoscalar probe in the 68_{Ni. The measured}

_γ

_{-ray angular distribution showsthe E1}

characteroftheresonance.Themeasuredcrosssectionofthe pro-cess amounts at 0.32 mb with 18% of statistical error. We have shownthat12Cisagoodprobefortheisoscalarexcitation. More-over,thetarget contributiontothe

γ

-rayenergyspectruminthe regionaround10MeVisnegligible.Wehavealsoshownthatthe E1states at the low lying energy region around 10 MeV can be excited by bothisoscalarandisovector probeswhich isin agree-mentwiththetheoreticalfindings[16] aboutthemixedcharacter ofthesedipolestates.Fromthecomparisonwithprevious experi-mentsperformedwithisovectorprobesitseemsthattheso-called isospinsplittingisnotpresentforthelowlyingdipolestatesfound atexcitationenergiesabovetheneutronemissionthreshold. How-ever,dueto thelimitsofthe relativesmall statisticsandrelative scarceenergyresolutionofthepresentmeasurementandprevious experiments[29,30],itisnotpossibletodrawdefiniteconclusions. Moreover,theroleofmultistepcontributionstothepopulationof thePDR,whichisnotpresentinthe(

α

,

α



,

γ

)experimentsin sta-blenuclei, maymodify the shapeof theenergy distribution.The multistep contribution has been taken into account in ref. [50], forCoulomb relativisticexcitation of Tin isotopes, where this ef-fect hasbeen estimatedfor the132_{Snto be of theorder of10%.}

In order to further improve this observation, we are planning a new experimenttoexcite the pygmydipole resonance withboth isoscalarandisovectorprobes.

Acknowledgements

Special thanks are due to the technical staff of the LNS cy-clotronandespeciallytheheadofacceleratordivisionG.Cosentino for theproductionof thefragmentationbeam. A particular men-tion forthe helpintheunderstanding ofthetransmissionof dif-ferent charge status of the beam is due to C. Nociforo of GSI, Darmstadt. We also thank M.N. Harakeh for the fruitful sugges-tions.

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