A direct measurement of the O( , ) Ne reaction in inverse kinematics and its impact on heavy element production

Article

Reference

A direct measurement of the O( , ) Ne reaction in inverse kinematics and its impact on heavy element production

CHOPLIN, Arthur (Collab.)

Abstract

During the slow neutron capture process in massive stars, reactions on light elements can both produce and absorb neutrons thereby influencing the final heavy element abundances.

At low metallicities, the high neutron capture rate of $^{16}$O can inhibit s-process nucleosynthesis unless the neutrons are recycled via the $^{17}$O(α,n)$^{20}$Ne reaction.

The efficiency of this neutron recycling is determined by competition between the

$^{17}$O(α,n)$^{20}$Ne and $^{17}$O(α,γ)$^{21}$Ne reactions. While some experimental data are available on the former reaction, no data exist for the radiative capture channel at the relevant astrophysical energies.The $^{17}$O(α,γ)$^{21}$Ne reaction has been studied directly using the DRAGON recoil separator at the TRIUMF Laboratory. The reaction cross section has been determined at energies between 0.6 and 1.6 MeV Ecm, reaching into the Gamow window for core helium burning for the first time. Resonance strengths for resonances at 0.63, 0.721, 0.81 and 1.122 MeV Ecm have been extracted. The experimentally based reaction rate calculated represents a lower limit, but suggests that [...]

CHOPLIN, Arthur (Collab.). A direct measurement of the O( , ) Ne reaction in inverse kinematics and its impact on heavy element production. Physics Letters. B , 2019, vol. 798, p. 134894

DOI : 10.1016/j.physletb.2019.134894

Available at:

http://archive-ouverte.unige.ch/unige:128791

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Physics Letters B

www.elsevier.com/locate/physletb

A direct measurement of the ¹⁷ O( α , γ ) ²¹ Ne reaction in inverse kinematics and its impact on heavy element production

M.P. Taggart

^a,1

, C. Akers

^a,2

, A.M. Laird

^a,l,n,∗

, U. Hager

^b

, C. Ruiz

^b

, D.A. Hutcheon

^b

, M.A. Bentley

^a

, J.R. Brown

^a

, L. Buchmann

^b

, A.A. Chen

^c

, J. Chen

^c

, K.A. Chipps

^a,3

,

A. Choplin

^d,4

, J.M. D’Auria

^e

, B. Davids

^b,e

, C. Davis

^b

, C.Aa. Diget

^a

, L. Erikson

^f

, J. Fallis

^b

, S.P. Fox

^a

, U. Frischknecht

^g

, B.R. Fulton

^a

, N. Galinski

^b

, U. Greife

^f

, R. Hirschi

^h,i,l,n

,

D. Howell

^b

, L. Martin

^b

, D. Mountford

^j

, A.St.J. Murphy

^j

, D. Ottewell

^b

, M. Pignatari

^k,m,l,n,5

, S. Reeve

^b

, G. Ruprecht

^b

, S. Sjue

^b

, L. Veloce

^b

, M. Williams

^a,b

aDepartmentofPhysics,UniversityofYork,York,YO105DD,UK bTRIUMF,Vancouver,V6T2A3,Canada

cMcMasterUniversity,Hamilton,ON,Canada

dGenevaObservatory,UniversityofGeneva,Maillettes51,CH-1290Sauverny,Switzerland eSimonFraserUniversity,Burnaby,BC,Canada

fColoradoSchoolofMines,Golden,CO,USA

gDepartmentofPhysics,UniversityofBasel,Klingelbergstrasse82,4056Basel,Switzerland hAstrophysicsGroup,Lennard-JonesLabs2.09,KeeleUniversity,ST55BG,Staffordshire,UK

iKavliInstituteforthePhysicsandMathematicsoftheUniverse(WPI),UniversityofTokyo,5-1-5Kashiwanoha,Kashiwa,277-8583,Japan jSUPA,SchoolofPhysicsandAstronomy,TheUniversityofEdinburgh,Edinburgh,EH93FD,UK

kUniversityofVictoria,Victoria,BC,Canada

lUKNetworkforBridgingtheDisciplinesofGalacticChemicalEvolution(BRIDGCE),UK

mKonkolyObservatory,ResearchCentreforAstronomyandEarthSciences,HungarianAcademyofSciences,KonkolyThegeMiklosut15-17,H-1121Budapest, Hungary

nNuGridCollaboration

a r t i c l e i n f o a b s t ra c t

Articlehistory:

Received16May2019

Receivedinrevisedform23August2019 Accepted23August2019

Availableonline28August2019 Editor:W.Haxton

Duringtheslowneutroncaptureprocessinmassivestars,reactionsonlightelementscanbothproduce and absorbneutronstherebyinfluencingthe finalheavyelementabundances. Atlowmetallicities, the highneutroncapturerateof¹⁶Ocaninhibits-processnucleosynthesisunlesstheneutronsarerecycled via the¹⁷O(α,n)²⁰Ne reaction. Theefficiency of thisneutronrecycling isdetermined by competition betweenthe¹⁷O(α,n)²⁰Neand¹⁷O(α,γ)²¹Nereactions.Whilesomeexperimentaldataareavailableon theformerreaction,nodataexistfortheradiativecapturechannelattherelevantastrophysicalenergies.

The¹⁷O(α,γ)²¹NereactionhasbeenstudieddirectlyusingtheDRAGONrecoilseparatorattheTRIUMF Laboratory. Thereactioncrosssectionhasbeendeterminedatenergiesbetween0.6and 1.6MeVEcm, reachingintotheGamow windowforcorehelium burningforthe firsttime. Resonancestrengths for resonancesat0.63,0.721,0.81and1.122MeVEcmhavebeenextracted.Theexperimentallybasedreaction ratecalculatedrepresentsalowerlimit,butsuggeststhatsignificants-processnucleosynthesisoccursin lowmetallicitymassivestars.

©^{2019TheAuthors.PublishedbyElsevierB.V.ThisisanopenaccessarticleundertheCCBYlicense} (http://creativecommons.org/licenses/by/4.0/).FundedbySCOAP³.

*

Correspondingauthorat:DepartmentofPhysics,UniversityofYork,York,YO105DD,UK.

E-mailaddress:[email protected](A.M. Laird).

1 Presentaddress:DepartmentofPhysics,UniversityofSurrey,UK.

2 Presentaddress:RareIsotopeScienceProject,InstituteforBasicScience,Daejeon34047,RepublicofKorea.

3 Presentaddress:PhysicsDivision,OakRidgeNationalLaboratory,OakRidge,TN37831, USA.

4 Presentaddress:DepartmentofPhysics,FacultyofScienceandEngineering,KonanUniversity,8-9-1Okamoto,Kobe,Hyogo658-8501,Japan.

5 Presentaddress:E.A. MilneCentreforAstrophysics,DepartmentofPhysicsandMathematics,UniversityofHull,HU67RX,UnitedKingdom.

https://doi.org/10.1016/j.physletb.2019.134894

0370-2693/©^{2019TheAuthors.PublishedbyElsevierB.V.ThisisanopenaccessarticleundertheCCBYlicense}(http://creativecommons.org/licenses/by/4.0/).Fundedby SCOAP³.

2 M.P. Taggart et al. / Physics Letters B 798 (2019) 134894

1. Introduction

Almostall the elements inthe Universeheavier thaniron are produced by neutron-capture reactions, either via the r-process (rapid neutron capture) or the s-process(slow neutron capture).

While significant uncertainties remain inr-process nucleosynthe- sis, the s-process is considered generally well understood. Here, the neutron flux is such that the timescalesfor neutron capture are longer than the associated β-decays, andso the path of nu- cleosynthesis lies close to the valley of stability. Most s-process elements betweeniron and strontium are thought to have been producedinmassivestars,throughtheweaks-process, andthose betweenstrontiumandleadviathemains-processinAsymptotic GiantBranch(AGB)stars[1].

However, abundanceratios (e.g.[Y/Ba]) observed inextremely metal poor stars and in one of the oldest globular clusters in the galactic bulge, NGC 6522, cannot be explained by the main s-processorthe r-process.Chiappini et al.[2] show that massive rotatingstarsatlowmetallicitycanprovideanexplanationforthe unique abundances observed both in the galactic halo and NGC 6522(see also[3] and [4]).Forsuch stars,rotation-induced mix- ing isconsidered tohave asignificant impact onnucleosynthesis of light elements, especially at low metallicities [5,6]. S-process abundances depend critically on the presence of those light el- ements which can act as neutron sources and poisons (isotopes whichcaptureneutrons,thusremovingthemfromcontributingto s-processproduction). At low metallicities, the lack ofsecondary neutronpoisons(e.g.¹⁴N)andthelargeabundanceofprimary¹⁶O resultsinahighneutroncapturerateto¹⁷O.Thus¹⁶Ocouldactas apoison iftheseneutrons are notrecycledvia the¹⁷O(

α

,n)²⁰Ne reaction.Thisrecyclingofneutrons isdetermined bycompetition betweenthe¹⁷O(

α

,n)²⁰Neand¹⁷O(

α

,

γ

)²¹Nereactions.However, thesereaction ratesare highly uncertain atthe relevantenergies andthestatus of¹⁶Oasa neutronpoison,andthe impact ons- processabundances,isthereforeasyetundetermined.

Thereare two theoretical calculationsofthe¹⁷O(

α

,

γ

)²¹Neto 17O(

α

,n)²⁰Ne reaction rate ratio. The first, from Caughlan and Fowler (CF88)[7], assumesthe ratiotobe around 0.1atlow en- ergies,droppingto5×^{10−4 aboveabout1MeV.Thisassumption} isbased on experimental dataon the ¹⁸O(

α

,

γ

)²²Nereaction for thehigherenergies,andonHauser-Feshbachcalculationsatlower energies.ThesecondpredictioncomesfromDescouvemont[8],us- ingtheGeneratorCoordinateMethod,andsuggeststheratiotobe of the order of 10⁻⁴ at all energies. This huge disagreement at low energies results in significant differencesin the predicteds- processabundances.Models byHirschiet al.[6] showtheimpact ofthe two different predictions onthe abundances of the heavy elements.Thevariation isparticularlymarked(uptothreeorders ofmagnitude)betweenstrontiumandbarium.

Forlow metallicity massive stars,s-processnucleosynthesis is thought to occur during two stages ofevolution, firstly core he- liumburningandthenlatershellcarbonburning.Thetemperature forcoreheliumburningisaround0.2- 0.3GK,correspondingtoan energyrangeofinterest(Gamowwindow)betweenabout0.3and 0.65MeVinthecentreofmass(E_cm).Fortheonsetofcarbonshell burning, temperaturesare higherataround0.8to 1.3GK,witha GamowwindowbetweenEcm=⁰.7 to1.8MeV.The¹⁷O(

α

,

γ

)²¹Ne reactionQ-valueis7.348 MeV[9] andtherelevantexcited states, shownin Fig.1, lie between7.65 and8.0 MeV excitation energy (Ex) in²¹Neforcore heliumburningtemperatures. However,the requiredpartialwidthandspin-parityinformationfor²¹Nelevels intheregionofinterestispoorlyknown,preventingreliablecalcu- lationofthecontributionofindividual resonancestothereaction rate.

Fig. 1.Partof²¹Nelevelscheme.TheGamowwindowforthe¹⁷O(α,γ)²¹Nereac- tionduringcoreheliumburninginmassivestarsisindicatedbythebarontheleft andthebarsontherightshowtheenergyregionscoveredbythepresentwork.

Experimental data onthe ¹⁷O(

α

,n)²⁰Ne reaction are available covering the range Ecm=⁰.56 - 10.1 MeV [10–12,14], and there is onlyone publishedexperimental dataseton the¹⁷O(

α

,

γ

)²¹Ne reaction [13]. Traditionally, experimental determinations of such (

α

,

γ

)reactioncrosssectionshavereliedonusinganintensebeam of

α

-particles and the detection of

γ

-rays from de-excitation of theproducts.Forthe¹⁷O(

α

,

γ

)²¹Nereaction,however,thehighQ- valueofthereactionresultsintheproductshavinghighexcitation energies wheremanynuclear statesare populated.Cleanidentifi- cation ofthese statesis difficultto extract fromthe background, particularly at the astrophysically interesting energies where the yield from the reactions of interest is extremely low, typically lessthan 1eventforevery10¹² incident

α

-particles.Despitethe experimental challenges, measurements usingthistechnique pro- vided the first direct data on the ¹⁷O(

α

,

γ

)²¹Ne reaction. Best et al.[13] measuredthe¹⁷O(

α

,

γ

)²¹Nereactionbyin-beam spec- troscopy, usinga⁴Hebeamonan implantedtarget.Themeasure- ments spanned Ecm between 0.7 and1.9 MeV but no yield was observed belowE_cm=¹.1 MeV(∼^{1 GK) exceptforastrong res-} onance at Ecm=⁰.811 MeV,believedto correspond toa state at 8.159(2) MeV.SubsequentlyBestet al. [14] alsostudiedthecom- peting¹⁷O(

α

,n)²¹Nereactionacrossthesameenergyrange.Many resonanceswereobservedandfittedusinganR-matrixframework.

Finally,usingbothdatasetsandestimatesforthecontributionfrom lower-lyingstates,Bestetal.[14] calculatednewreactionratesand concluded that the (

α

,

γ

) channel is strong enough to compete withthe(

α

,n)channel leadingtolessefficientneutronrecycling.

However, neither measurement had sufficient sensitivity to pro- vide any experimental data in the energy region relevant to the s-processduringthecoreheliumburningstage.

2. Experimentaldetails

Herewereportonthefirstmeasurementofthe¹⁷O(

α

,

γ

)²¹Ne reactionexploiting, instead,abeamof¹⁷Oionsincidentonahe-

Fig. 2.MCP localtimeofflightTAC(timetoamplitudeconverteroutput)versus dEfordataatEcm=^{822 keV.Singleseventsareindicatedinblueandcoincident} data,withadetectedgamma-rayenergyabove2MeV,inred.Theeventsinredin thetoprightofthefigurearerandomcoincidences betweenagamma-rayanda scatteredbeamion.

lium gas target. The ²¹Ne recoils from the reaction exited the target (unlike in the above case) with the unreacted beam, al- lowing their detection in coincidence with prompt gamma rays fromtheirde-excitation. The measurementwas performedatthe DRAGONrecoilseparatorintheISACfacility,attheTRIUMFLabora- tory,Canada,whichisspecificallydesignedtostudysuchradiative capturereactionsrelevant tonuclearastrophysics.It consistsofa windowlessrecirculatinggastarget,surroundedby anarrayof30 bismuthgermanate (BGO) gamma-raydetectors, anda two-stage electromagneticrecoilseparator.DetailsoftheDRAGONseparator aregiveninHutcheonetal.[15] andEngeletal.[16].

The¹⁷O³⁺beamwithatypicalcurrentof600enA(correspond- ingto∼¹.25×^{1012pps)impingedonthewindowlessheliumgas} target.DRAGON was configuredto transmit4^{+ 21}Nerecoils from the¹⁷O(

α

,

γ

)²¹Nereaction.Theserecoilsweredetectedatthefo- cal plane by an ionization chamber (IC).The IC anodeconsisted offoursegments,providingenergylossandresidualenergy(dE-E) information, andwas filled with isobutane at a typical pressure of8 Torr. Two micro-channel plate (MCP) detectors upstream of theIC measuredthe localtime-of-flight (TOF) oftherecoils over adistanceof60cm[17].Recoilswerethenidentified,anddistin- guishedfrom“leaky” beamtransmittedthroughthe separator,by theirlocuson an energyloss-vs-TOFgraph,an exampleofwhich isshowninFig.2.Furtherdiscriminationwasprovidedbyprompt

γ

-raysdetectedintheBGOarrayincoincidencewitheventsinthe IC.Thetimebetweentheprompt

γ

-raydetectionandsubsequent MCP detection allowed for a separator TOF measurement, which wasusedforadditionalparticleidentification.Whenthedetection yields were toolow to distinguish a clear ²¹Ne recoil locus, the profilelikelihoodtechnique[18] wasusedtocalculateaconfidence interval.Intheseinstances,theMCPandseparatorTOFregionsof interestwereextrapolatedfromhigheryielddata.

Foreach beamenergydelivered, an energymeasurement was madeboth withand without target gas present. In combination with the measurements of the gas target pressure, temperature and the known effective length [19], this allows the stopping powerto be calculated. Beamenergy measurement is performed bycentring thebeamonasetofslitsattheenergy-dispersedion- optical focus after the first magnetic dipole field, using an NMR fieldread back,wheretheenergy-to-fieldrelationshipfora given mass-to-charge ratio has been calibrated by many well-known, precisenuclear resonances[15,19]. Thebeam intensitywas mea- suredeveryhourinthreeFaradaycups(FC),onelocatedupstream

ofDRAGON,oneafterthegastargetandoneafterthefirstdipole magnet. Continuousmonitoringofthebeamintensitythroughout data taking was achieved via recoiling

α

-particles, from elastic scattering of the beam on the helium in the target, detected in twosurface-barrierdetectorslocatedwithinthegastargetassem- bly.Theseelasticscatteringdatawerenormalisedtothemeasured beamintensityatthestartandendofeveryrun[20].Targetpres- suresofbetween4and8Torrwere used.Theenergylossofthe beam, inthecentre ofmass,acrossthegastarget variedfrom53 keV at8 Torrforthe lowest energy, to 30keV at4 Torrfor the highestenergy.Forthefivemeasurements aroundthe Ex=⁸.155 MeVstate,targetpressuresbetween4and6Torrwereused,with acorrespondingcentre ofmassenergylossof28to44keV.

At each energy,the rawyields were corrected forthe separa- tor efficiency, the charge state fraction for4⁺ recoils exiting the gas target, the effectiveefficiencies of the IC andMCP detectors, andthedataacquisitiondeadtime.As

γ

-raycoincidenceswerere- quiredforparticleidentification,theBGOarrayefficiencywasalso takenintoaccount.Theseparatorefficiencywas determinedfrom MonteCarlosimulationsofDRAGONusingGEANT3[21].Forcentre ofmassenergiesbelow≈^{1 MeV,themaximumconeangleofthe}

21NerecoilexceedstheDRAGONacceptanceof21mrad.Ifareso- nanceislocatedupstreamofthetargetcentre,thislimitisreached athigher energies.Similarly, theefficiency oftheBGO array [22]

depends on the location of the reaction in the target. For most energies studiedinthiswork neitherthewidthoftheresonance, nor the angular distribution ordecay scheme of the subsequent decay of ²¹Ne are known, and the measured statistics were too low to determinethese valuesfromthe observed

γ

-ray energies and distributions. Simulations were, therefore, conductedassum- ingthreedecayschemes(directtogroundstate,viathe3.74MeV state,andviathe1.75and0.35MeVstates)andthreeangulardis- tributions (isotropic, dipole, andquadrupole). For each simulated scenario (reaction location in gas target, assumeddecay scheme, etc.)thecorresponding separatortransmissionandBGO detection efficiencies were extracted, andthe differences betweenthe var- ious scenarios used to determine the systematic errors on both values.

Chargestate distributionsof²¹Newere measuredatbeamen- ergies of 160,202, 290,and360 keV/u and the 4⁺ charge state fractionwasestimatedusingan empiricalformulafrom[23].This formula was used to interpolatethe 4⁺ charge state fraction for eachoftherecoilenergies.Theefficiencyoftheenddetectorswas takenfromacomparisonofMCPandICeventratedatausingat- tenuated beam, together with the geometric transmission of the MCPdetectorgrid.

The effective cross sections (

σ

) and effective astrophysical S- factors(S)werethencalculatedfrom:

σ =

^Nr N_b

A

N_t (1)

S

(

E

) =

^E

e^{−2π η}

σ

(2)

where _N^Nr

b isthecorrectedyield, _N^A

t isthereciprocaltargetnuclei per unit area, e⁻²π η isthe Gamow factorand E is the centre of targetcentre ofmassenergy.Theresonancestrengthoftheexcita- tionlevelofinterestwascalculatedviatheequation[24]

ωγ =

²

π (

E_r

)

Y

λ

²

(

E_r

) ×

arctan

E0

−

Er

/

2

−

^arctan

E0

−

Er

−

E

/

2

₋₁ (3)

4 M.P. Taggart et al. / Physics Letters B 798 (2019) 134894

Fig. 3.Effective astrophysicalS-factorfromthepresentwork,comparedtothecal- culationfor the¹⁷O(α,γ)²¹Nereactionfrom[8].Eachdatapointrepresentsthe energyatthecentreofthegastargetandthehorizontalerrorbarcorrespondsto theenergylossinthetarget.Targetpressuresofbetween4and8Torrwereused.

whereλisthesystem’sdeBrogliewavelength,

isthetargetstop- ping power, E_r is the resonance energy, E₀ is the initial centre of mass energy andE is the beam energyloss across the en- tirelengthofthetarget.Thetargetstoppingpowerwascalculated from

(

E

) = −

^V N_t

dE

dx (4)

where _N^V

t isthereciprocaltargetdensityand ^dE_dx istherateofion energyloss in thetarget. For the runswhere theresonance was fullycontainedwithin the target,the thick target yieldwas used tocalculatetheresonancestrength:

ωγ =

²

(

Er

)

λ

²

(

Er

)

^Ymax

.

(5)

Thestatederrorsincludebothsystematicandstatisticaluncer- tainties.ThemainsourcesofsystematicuncertaintyweretheBGO detectionefficiency(10%),separatortransmission(between20-30%

forthe lowerenergy runs, 2-10%forthe 811keVruns), detector efficiency andtransmission (between 4-5%)and integrated beam intensity(between0.6-6%).Uncertaintiesinstoppingpower(3.7%) andrecoilchargestatefraction(1.6-4.1%)werealsoaccountedfor.

The range in uncertainties reflects the range of beam energies, populated states, recoil angular distribution and duration of the runs.

3. Results

Fig. 3 shows the measured S-factors at each centre of mass energy for the present work in comparison with the calculation from Descouvemont [8]. It should be noted that the direct cap- ture contributionis expected to be lower than the cross section fromDescouvemont,andisthus considerednegligible.Data were initially taken at severalenergies above 1 MeV, where the yield is much higher, to allow a comparison with the Descouvemont calculation. Measurements were then pushed lower towards the astrophysically interesting energy range. Table 1 gives the reso- nance strengths from the present work, compared to literature valueswhereavailable.

Thedatapoint around1.1MeVcovers thestate at8.470 MeV.

Aresonancestrength of1.9±⁰.4 meVwas determined,whichis slightlyhigherthanthatreportedby[13] whofoundthestrength tobe1.2±⁰.2 meV.

Table 1

17O(α,γ)²¹Neresonancestrengthsfrom thepresentwork com- paredtoliteraturevalues.

EC M(keV) ωγ(meV) Literature value [13] (meV) 633 (4.0⁺₋³₂^._.¹₀)×10⁻³ –

721 (8.7⁺₋⁷₃^._.⁰₇)×^{10−3 –}

810 5.4±^{0.8 7.6}±^0.9

1122 1.9±^{0.4 1.2}±^0.2

The most prominent feature at around E_cm=⁰.81 MeV cor- responds to a known Jπ=(9/2)⁺ state in ²¹Ne at an excitation energyof8.155(1)MeV[25].Thisresonanceappearstobeofcom- parablestrengthinbothgammaandneutronchannels[10,14].The weighted averageofthe fivehighestyielddatapoints, wherethe resonance is fully within the gas target, gives a measured reso- nancestrengthof5.4±⁰.8 meV.Thisvalueisslightlyweakerthan the7.6±⁰.9 meVreportedby[13].

There isaknownJπ=³/2⁺ stateoftotalwidth8keV[25] at 8.069 MeV (Ecm=⁰.721 MeV).This statecontributes toboth the 0.695 and0.748 MeVdatapoints, witheachmeasurement cover- ingapproximatelyhalfoftherelevantyield.Thisresonancehasnot previously beenobservedin(

α

,

γ

)andastrengthof8.7⁺₋⁷₃^._.⁰₇ μeV was found. The quoted uncertainty does not include the uncer- taintyontheenergyorthewidthofthestate.

Between themeasurement at 0.695 MeV andthe lowest data point, there is a gap in the measured energy range,from 0.648 to 0.667 MeV, andsono constraintcan be placedon thecontri- bution ofthe 1/2⁻ resonanceat0.66MeV (Ex=⁸.009(10)MeV).

However,asthisstatecorrespondstoanf-waveresonanceandwas observedasaneutronresonance,itisunlikely thatthisstatewill playanysignificantroleinthe(

α

,

γ

)rate.

ThelowestdatapointmeasuredliesinsidetheGamowwindow for corehelium burning. Three known statesare covered by the energythicknessofthegastargetatthisbeamenergy(seeFig.1).

Given the low yield, it is not possible to determine which state dominatesandso acombinedresonancestrengthof4.0⁺₋³₂^._.¹₀ μeV isreportedhere.Thisvalueisafactorofaround10lowerthanthe 0.03-0.05 meVupperlimit givenin[13].The calculationsby[14]

suggestthatthe7.982MeVlevelmakesthedominantcontribution hereandsoitisassumedthat theobservedstrengthcomesfrom this state and a resonance energy of 0.633 MeV is used in the reactionratecalculation.However,ifthefullobservedstrengthlies instead inthe0.612 MeV resonance,then thecalculated reaction ratefortheresonancewouldbe2.25timeshigher.

4. Astrophysicalimpact

Usingthenarrowresonanceformalism,thecontributionstothe reactionratefromtheresonancesat0.633and0.81MeVwerecal- culated (the resonance at 0.721 MeV contributes less than 10%

to the total rate). The sum ofthese two contributions (green) is showninFig.4,incomparisonwiththerecommended(black)rate fromBest et al.[14], asa ratiotothat ofCF88.Thecrosssection from the presentwork excludes the predictionof Descouvemont [8].However,thepresentrateisstillaround100-1000timeslower than thatofCF88 [7].It shouldbe notedthatwithin theGamow window for helium core burning, there are 6 known states, giv- ing atypicalleveldensityofaround1.5per100keV.Thisiswell belowthatassumedforastatisticalmodelapproachandthusthe Hauser-Feshbach treatment of thisreaction at low energies used by CF88[7] maybeexpectedtosignificantlyoverestimate there- actionrate.

Itmustbeemphasisedthatthepresentrateshouldbeconsid- ered asa lower limit.There are two known statesin theenergy regionofinterestwhosespinandparityarenotknown,andnone

Fig. 4.Ratio of¹⁷O(α,γ)²¹NereactionratestoCF88[7].Thelowestcurve(green)is fromthepresentworkandisalowerlimitontherate(seetext).Theuppercurve (black)istherecommendedrateofBestet al.[14].Thetwoshadedareasindicate theapproximatetemperatureintheheliumburningcoreandcarbonburningshell ofmassivestars.

ofthestatesbelowEx=⁷.96 MeVhaveexperimentallyconstrained resonancestrengths orpartialwidths.Due toa lackofdirectex- perimentaldata,thecontributionofthesestateshasnotbeenin- cludedhere.TherecommendedratefromBestet al.[14] includes thecontributionsfrom12resonanceswhichwerenotobservedin that work, but whose resonance strengths have been calculated based on estimates of the

α

-particle widths, branching between the

γ

- andneutron channels, andan assumedspectroscopicfac- torof0.01.Itisthereforeexpectedthattheratefromthepresent work, based only on observed resonances, is significantly lower.

However, withinthe Gamow window thedifference betweenthe presentrate and the recommended rate from Best et al. [14] is dominated by the estimated contribution from the resonance at 0.305MeV.Ifthisresonanceisnotasstrongassuggestedthenthe measuredresonanceat0.633 MeVmaymakeasignificant contri- butionandthe¹⁷O(

α

,

γ

)²¹Nereactionratewouldbeclosertothe lowerlimitpresentedhere.

The reaction rate from the present work was tested in a 25 solarmassstellarmodel,atametallicityofZ=^{0.001inmassfrac-} tion,andwith an initialequatorial velocity of70% of thecritical velocity(thevelocityatwhichthegravitationalforcebalancesthe centrifugalforce).ThemodelwascomputedwiththeGenevastel- lar evolution code up to the core oxygen burning stage, with a networkof737isotopes,fullycoupledtotheevolution(detailscan be found in [4] and [26]).Fig. 5 shows the yields of thismodel (greenline)plustwoadditionalmodelswiththesameingredients exceptthatoneiscomputedwiththerecommendedratefromBest et al.[14] (blackline) andtheother withtherecommended rate dividedby10(redline).Thelatterratewaschosentoillustratethe impactofthe0.305MeVresonancebeingweakerthanestimated.

Significant differences are observed between yields from the present rate and the recommended rate above strontium. These differences increase at higher atomic masses, with more than a factorof10aroundbarium.Thenewrateleadstoresultscloserto thoseusingtherecommendedrateofBestet al.dividedbyafac- torof10though thepresentrateleads tostill higherproduction ofelementsaroundbarium.Itisclearthatthecurrentuncertainty inthe¹⁷O(

α

,

γ

)²¹Nereactionratehasastrongimpactonthestel- larmodelpredictions.Itisthereforecrucialthat,intheabsenceof direct measurements, the missing spectroscopic information (i.e.

Fig. 5. S-process yields ofa fastrotating25 M at Z=^{0.001 whenusingthe} presentrate,therecommendedratefrom[14] andrecommendedrate/10forthe 17O(α,γ)²¹Nereaction(seetextforfurtherdetails).

spin/parity,reducedenergyuncertainty,partialwidths) oftherel- evantstatesin²¹Neisdeterminedtoallowthereactionratetobe betterconstrained.

5. Conclusions

In conclusion,a directmeasurement, ininversekinematics, of the¹⁷O(

α

,

γ

)²¹NereactionhasbeenperformedattheDRAGONfa- cility,attheTRIUMFlaboratory,Canada.Measurementsweremade ofthereactionyieldintheenergyrangeE_cm=⁰.6 - 1.6MeV,pro- vidingtheonlyexperimentaldataintheGamowwindowforcore helium burning. This work is over an order of magnitude more sensitive thanprevious work dueto theenhanced discrimination provided by thecoincident detection of both recoils and

γ

-rays.

Moreover, the eventidentification does not require prior knowl- edgeofthe associated

γ

-rayenergies. Theabundances calculated with stellar models using the lower limit on the ¹⁷O(

α

,

γ

)²¹Ne reactionratefromthepresentworkshowthemaximumcontribu- tiontos-processproductioninlowmetallicitymassivestars.

Acknowledgements

We would like to thank the beam delivery and ISAC opera- tions groupsat TRIUMF. In particular we gratefully acknowledge the invaluableassistance inbeamproductionfromK. Jayamanna, for delivering the high intensity beam. UK personnel were sup- ported by the Science and Technology Facilities Council (STFC).

CanadianauthorsweresupportedbytheNaturalSciencesandEn- gineering Research Council of Canada (NSERC). TRIUMF receives federal funding via a contribution agreement through the Na- tional Research Council of Canada. Authors acknowledge support from the “ChETEC” COST Action (CA16117), supported by COST (EuropeanCooperationinScienceandTechnology).A.Choplinac- knowledges funding fromthe Swiss National Science Foundation under grant P2GEP2-184492.RH acknowledges support fromthe WorldPremierInternationalResearchCenterInitiative(WPIInitia- tive), MEXT, Japan. The Colorado School of Mines group is sup- ported via U.S.Department ofEnergy grant DE-FG02-93ER40789.

MPacknowledgessupporttoNuGridfromNSFgrantPHY-1430152 (JINACenterfortheEvolutionoftheElements)andSTFC(through theUniversityofHull’sConsolidatedGrantST/R000840/1),andac- cesstoviper,theUniversity ofHullHighPerformanceComputing Facility. MP acknowledges the support from the “Lendulet-2014”

ProgrammeoftheHungarianAcademy ofSciences(Hungary). MP acknowledgessupportfromtheERCConsolidatorGrant(Hungary) fundingscheme(projectRADIOSTAR,G.A.n.724560).

6 M.P. Taggart et al. / Physics Letters B 798 (2019) 134894

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