Cross section of the reaction 18O(p, γ)19Fat astrophysical energies: The 90 keV resonance and the direct capture component

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Contents lists available atScienceDirect

Physics

Letters

B

www.elsevier.com/locate/physletb

Cross

section

of

the

reaction

18 O

(

p

,

γ

)

19 F at

astrophysical

energies:

The

90 keV

resonance

and

the

direct

capture

component

A. Best

d

,

e

,

∗

,

F.R. Pantaleo

f

,

g

,

A. Boeltzig

a

,

b

,

c

,

1

, G. Imbriani

d

,

e

,

M. Aliotta

h

,

J. Balibrea-Correa

d

,

e

,

D. Bemmerer

i

,

C. Broggini

k

, C.G. Bruno

h

,

R. Buompane

e

,

j

,

A. Caciolli

l

,

k

,

F. Cavanna

m

,

T. Chillery

h

,

G.F. Ciani

a

,

c

, P. Corvisiero

n

,

m

,

L. Csedreki

c

,

a

,

T. Davinson

h

,

R.J. deBoer

b

,

R. Depalo

l

,

k

,

A. Di Leva

d

,

e

, Z. Elekes

o

,

F. Ferraro

n

,

m

,

E.M. Fiore

f

,

g

_,

_{A. Formicola}

c

_,

_{Zs. Fülöp}

o

_,

_{G. Gervino}

p

,

q

_{, A. Guglielmetti}

r

,

s

_,

_{C. Gustavino}

t

_,

Gy. Gyürky

o

,

M. Junker

c

,

a

,

I. Kochanek

c

,

M. Lugaro

u

, P. Marigo

k

,

l

,

R. Menegazzo

k

,

V. Mossa

f

,

g

,

V. Paticchio

g

,

R. Perrino

g

,

1

,

D. Piatti

l

,

k

, P. Prati

n

,

m

,

L. Schiavulli

f

,

g

,

K. Stöckel

i

,

v

,

O. Straniero

w

,

c

,

F. Strieder

x

,

T. Szücs

i

, M.P. Takács

i

,

v

,

2

,

D. Trezzi

r

,

s

,

M. Wiescher

b

,

S. Zavatarelli

m

a_Gran_Sasso_Science_Institute,_Viale_F._Crispi_7,₆₇₁₀₀_L’Aquila,_Italy

b_The_Joint_Institute_for_Nuclear_{Astrophysics,}_Department_of_Physics,_University_of_Notre_Dame,_Notre_Dame,_{IN 46556,}_USA c_Istituto_Nazionale_di_Fisica_Nucleare,_Laboratori_Nazionali_del_Gran_Sasso_(LNGS),_Via_G._Acitelli_22,₆₇₁₀₀_Assergi,_Italy d_Università_degli_Studi_di_Napoli_“Federico_II”,_Dipartimento_di_Fisica_“E._Pancini”,_Via_Cintia_21,₈₀₁₂₆_Napoli,_Italy e_Istituto_Nazionale_di_Fisica_Nucleare,_Sezione_di_Napoli,_Via_Cintia_21,₈₀₁₂₆_Napoli,_Italy

f_Università_degli_Studi_di_Bari,_Dipartimento_Interateneo_di_Fisica,_Via_G._Amendola_173,₇₀₁₂₆_Bari,_Italy g_Istituto_Nazionale_di_Fisica_Nucleare,_Sezione_di_Bari,_Via_E._Orabona_4,₇₀₁₂₅_Bari,_Italy

h_School_of_Physics_and_Astronomy,_University_of_Edinburgh,_Peter_Guthrie_Tait_Road,_EH9_3FD_Edinburgh,_United_Kingdom i_{Helmholtz-Zentrum}_{Dresden-Rossendorf,}_Bautzner_Landstraße_400,₀₁₃₂₈_Dresden,_Germany

j_Università_degli_Studi_della_Campania_“L._{Vanvitelli”,}_Dipartimento_di_Matematica_e_Fisica,_Viale_Lincoln_5,₈₁₁₀₀_Caserta,_Italy k_Istituto_Nazionale_di_Fisica_Nucleare,_Sezione_di_Padova,_Via_F._Marzolo_8,₃₅₁₃₁_Padova,_Italy

l_Università_degli_Studi_di_Padova,_Via_F._Marzolo_8,₃₅₁₃₁_Padova,_Italy

m_Istituto_Nazionale_di_Fisica_Nucleare,_Sezione_di_Genova,_Via_Dodecaneso_33,₁₆₁₄₆_Genova,_Italy n_Università_degli_Studi_di_Genova,_Via_Dodecaneso_33,₁₆₁₄₆_Genova,_Italy

o_Institute_for_Nuclear_Research_of_the_Hungarian_Academy_of_Sciences_(MTA_Atomki),_PO_Box_51,₄₀₀₁_Debrecen,_Hungary p_Università_degli_Studi_di_Torino,_Via_P._Giuria_1,₁₀₁₂₅_Torino,_Italy

q_Istituto_Nazionale_di_Fisica_Nucleare,_Sezione_di_Torino,_Via_P._Giuria_1,₁₀₁₂₅_Torino,_Italy r_Università_degli_Studi_di_Milano,_Via_G._Celoria_16,₂₀₁₃₃_Milano,_Italy

s_Istituto_Nazionale_di_Fisica_Nucleare,_Sezione_di_Milano,_Via_G._Celoria_16,₂₀₁₃₃_Milano,_Italy t_Istituto_Nazionale_di_Fisica_Nucleare,_Sezione_di_Roma,_Piazzale_A._Moro_2,₀₀₁₈₅_Roma,_Italy

u_Konkoly_Observatory,_Research_Centre_for_Astronomy_and_Earth_Sciences,_Hungarian_Academy_of_Sciences,₁₁₂₁_Budapest,_Hungary v_Technische_Universität_Dresden,_Institut_für_{Kern- und}_{Teilchenphysik,}_Zellescher_Weg_19,₀₁₀₆₉_Dresden,_Germany

w_INAF_–_Osservatorio_Astronomico_d’Abruzzo,_Via_Mentore_Maggini,₆₄₁₀₀_Teramo,_Italy

x_Department_of_Physics,_South_Dakota_School_of_Mines_and_Technology,_501E_St_Joseph_Street,_Rapid_City,_{SD 57701,}_USA

a

r

t

i

c

l

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i

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Articlehistory: Received3July2019

Receivedinrevisedform26August2019 Accepted27August2019

Availableonline30August2019 Editor:W.Haxton

The observation ofoxygen isotopes ingiant stars sheds lightonmixing processesoperating intheir interiors.Dueto theverystrong correlationbetweennuclearburning andmixingprocessesit isvery important toreduce theuncertainty on thecross sections ofthe nuclearreactions that are involved. Inthispaper wefocus ourattentiononthereaction18_O₍_p_,

_γ

₎19_F._While_the 18_O₍_p_,

_α

₎15_{N channel} _is

thought to be dominant, the (p,

γ

) channel can still be an important component in stellar burning in giants, depending onthe low energycross section.So far onlyextrapolations from higher-energy

*

Correspondingauthorat:UniversitàdegliStudidiNapoli“FedericoII”,DipartimentodiFisica“E.Pancini”,ViaCintia21,80126Napoli,Italy. E-mailaddress:best@na.infn.it(A. Best).

1 _Permanent_address:_Istituto_Nazionale_di_Fisica_Nucleare,_Sezione_di_Lecce,_Via_Arnesano,₇₃₁₀₀_Lecce,_Italy. 2 _Current_address:_{Physikalisch-Technische}_{Bundesanstalt,}_Bundesallee_100,₃₈₁₁₆_{Braunschweig,}_Germany.

https://doi.org/10.1016/j.physletb.2019.134900

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Keywords:

Experimentalnuclearastrophysics Undergroundnuclearphysics Hydrogenburning Stellarevolution

measurementsexistandrecentestimatesvarybyordersofmagnitude.Theselargeuncertaintiescallfor anexperimentalreinvestigationofthisreaction.Wepresent adirectmeasurementofthe18_O₍_p_,

_γ

₎19_F

cross section using a high-eﬃciency 4

π

BGO summing detector at the Laboratory for Underground Nuclear Astrophysics (LUNA). The reaction cross section has been directly determined for the ﬁrst time from140 keVdown to85 keV and the differentcross sectioncomponents have beenobtained individually.Thepreviouslyhighlyuncertainstrengthofthe90 keVresonancewasfoundtobe0.53± 0.07neV,threeordersofmagnitudelowerthananindirectestimatebasedonnuclearpropertiesofthe resonantstateandafactorof20lowerthanarecentlyestablishedupperlimit,excludingthepossibility thatthe90keVresonancecancontributesigniﬁcantlytothestellarreactionrate.

Theobservationofoxygen isotopesin evolvedgiant stars pro-videsuniqueinformationonvariousmixingprocessesoperatingin stellar interiors, among them those induced by convection, rota-tionandmagneticﬁelds[1–6].Oxygenisotopicratioscanbeused todeterminetheeﬃciencyofthesemixingprocesses.Forinstance, inthe innermost portionof theH-rich envelope ofa star climb-ing the red giant branch (RGB) or the asymptotic giant branch (AGB),wherethetemperatureexceeds

∼

20 MK,18Oisefficiently destroyed by protoncaptures(see figure 5 in[7]). Ifsome insta-bilityinducing a mixingprocess extends downto thisregion, an increaseofthe16O/18Oratioshouldbeobservedatthestellar sur-face(seefigures6and7in[7]).Theobservedratioisparticularly sensitiveto themaximumtemperatureattainedby theinstability responsible for the mixing. However, the effective application of thischemicalproberequiresapreciseevaluationofthelow-energy rates of the two 18_O _{proton-capture} _channels, _i.e., _the _reactions 18_O

₍

_p

_,

_α

₎

15_{N and}18_O

₍

_p

_,

_γ

₎

19_F.

Thesetworeactionsarealsoimportantinthelong-standing de-bate about the origin of galactic ﬂuorine (see, e.g. [8]). Fluorine

enhancements are commonlyobserved in thermally pulsingAGB

stars[9,10],butthecorrespondingnucleosynthesisscenarioisstill largely uncertain(see, e.g.[11,12]).The main productionchannel shouldbe 15N

(

α

,

γ

)

19F,operatingintheHe-burningregionofan AGBstar. Inthiscasethemain issueisthe amountof15_N avail-ablefortheﬂuorine production.Accordingtoan earlysuggestion by [13], 15N is produced by the 18O

(

p

,

α

)

15N reaction.3 An al-ternative path could be the direct production of ﬂuorine in the H-burningzonethroughthe 18_O

₍

_p

_,

_γ

₎

19_{F reaction.}_Current nucle-osynthesis models ﬁnd that this ﬂuorine source is prevented by the competing18O

(

p

,

α

)

15N reaction andthat ﬂuorine isinstead depletedinthe H-burningzone, mainlythrough the19_F

₍

_p

_,

_α

₎

16_O reaction.Sincethereaction18O

(

p

,

α

)

15N dominatesovertheentire energyrangeofastrophysical interest (the ₍(p_p_,,_γα)₎ rateratioranges from

∼

100to10000,dependingonthetemperature)onlyastrong increaseofthe(p,

γ

)channel crosssection couldchangethis oc-currence.

Motivatedbythisastrophysicalcontext,westartedadeep un-dergroundstudyofthetwo18_O_{proton-capture}_channels. _The re-sultsofourrecentmeasurementof18O

(

p

,

α

)

15N isdiscussedina separatepaper[14].Herewepresentourstudyofthe18O

(

p

,

γ

)

19F channel.

The most important low energy contributions are the direct capture (DC)cross section, the tail ofthe strong ER

=

143 keV4 andthe previouslyunmeasuredER

=

90 keVresonance.Thestate in 19_F _{corresponding} _to _the _latter _resonance _{( J π}

₌

3

2 +

, Ex

=

(

8084

±

3

)

keV) has been studied recently, yielding inconclusive

3 _According_to_this_scenario,_part_of_the15_N_is_produced_in_the_H-burning_shell

andtherestintheHeburningregion,whenthes-processnucleosynthesistakes placeandfreshprotonsarereleasedbythe14_N₍_n_,_p₎13_{C reaction.}

4 _All_energies_in_this_letter_are_given_in_the_{center-of-mass}_reference_frame,_unless

explicitlystatedotherwise.

results.Buckneretal.performedadirectmeasurementofthelow energy 18O

(

p

,

γ

)

19F cross section [15], resulting in both a much lowerDCcrosssectionthanmeasuredbyWiescheretal.[16] and in an upper limit of the resonance strength of

ωγ

<

7

.

8 neV, too low to contribute signiﬁcantly to the cross section. Shortly thereafter Fortune [17] calculated a resonant cross section based on known nuclear propertiesofthe state. Using literature values of the competing

(

p

,

α

)

channel strength and calculated proton widths, this publication arrived at a much higher recommended value for the resonance strength of

ωγ

= (

0

.

7

±

0

.

28

)

μeV. In this case, the90 keVresonancewould providea signiﬁcant con-tribution to the total reaction ratefor temperatures between 30 and80MK.Theresultingratecouldbeup toafactor100higher than that currently adopted in stellar andnucleosynthesis mod-els. Major effects are expected in the case ofmassive AGB stars

undergoing Hot Bottom Burning (HBB) at temperature of about

40-50MK.Inthisway,asolutionforsomepuzzling astrophysical cases maybe obtained. Forinstance, manySC stars5 are ﬂuorine rich, and some of them, such as WZ Cas, show the typical fea-turesofamoderateHBB,i.e.,substantialLienhancementtogether witharatherlowCisotopicratio(12_C/13_C

_∼

₄

₋

_5;_see_[₁₀_]). How-ever,theFenrichmentappearsincompatiblewithHBB,unlessthe branching ratio

<

18_O

₍

_p

_,

_γ

₎

19_F

_{> /}

_<

18_O

₍

_p

_,

_α

₎

15_N

_>

_is substan-tiallylarger thanusuallyadopted,asitwouldbe inthecaseofa strength of the 90keV resonance closeto the value reported by Fortune.

Inordertoresolvethisambiguitythe18_O

₍

_p

_,

_γ

₎

19_{F low}_energy cross section, covering the energy region of the 143 keV reso-nance and below, has been directly measured at the Laboratory for Underground Nuclear Astrophysics (LUNA) [19] at the Gran Sasso National Laboratory in Italy. The measurements discussed

herewere performedwitha largesegmentedbismuthgermanate

(BGO) detector surrounding the target in an almost 4

π

geome-try(asdetailedin[20]).Thedetectorwassurroundedonallsides by at least 10 cm of lead, reducing the low-energy background componentbyaboutanorderofmagnitude.ThesixBGOdetector segments were read out and time-tagged individually and coin-cident sum spectra were constructed oﬄine. This has the effect

of summing up gamma-rays that are emitted in a cascade

de-excitation of the resonant state to a single signal at the energy Q value (7993.6 keV [21]) plus the c.m. energy (here between 80 and 150keV) of the reacting particles. A major advantage of this technique isthe shiftingof thesignal out ofthe low-energy partofthegamma-rayspectrumthatisdominatedbynatural ra-dioactivity inthe laboratory environment intothe higher energy

region where the prevalent background stems from cosmic-ray

interactions with the detector. At LUNA this high-energy back-groundcomponentis reducedby overthreeorders ofmagnitude with respect to Earth’s surface. The BGO campaign was part of

(3)

awider-ranging investigation ofthe18_O

₍

_p

_,

_γ

₎

19_{F reaction,} _where thehigher-energyresonancesanddirectcapturecrosssection(up

to 400 keV) were also measured with a high purity germanium

(HPGe) setup that could be inserted in the same lead shield as

the one used for the BGO. The HPGe measurements, including

a much more detailed determination of the decay branchingsof several resonances, will be presented in a forthcoming publica-tion.

Themeasurements presented inthisletterwere performed at thesolidtargetstationoftheLUNAfacility,usinganodizedTa2O5 targetsthat were produced withwaterenriched to 99% 18O. An-odizationoftantalumisastandardmethodthatisknownto pro-duce homogeneous targetsthat can withstand highprotonbeam currentsforextendedperiods[22,23].Targetswiththicknesses(in termsofprotonenergylossat100keV)ofapprox.6 keVand14 keV,respectively,wereemployedovertheexperimentalcampaign. Thethinnertargetswereusedforoff-resonancemeasurementsand thethickeronesaround theregionofthe90keVresonance. Pro-tonbeamsofupto 200μAwere deliveredonto thewater-cooled target and changes in target thickness were regularly monitored byrepeatedmeasurementsoftheyieldproﬁleofthe143keV res-onance. Targets were exchanged withfresh ones before reaching adecrease by 10% in maximumyield andthickness. A liquid ni-trogen cooled copper pipe extended to the face of the target to reduce carbondeposition on the target.A suppression voltageof -300 V was applied to the copper pipe in order to reﬂect back

sputtered electrons from the target during beam bombardment,

eliminatingapossiblesystematicoverestimationofthebeam cur-rent.

The eﬃciency of the BGO detector, when used in summing

mode, is not a simple function of the photon energy. Instead, all possible decay paths from the resonant state to the ground state have to be taken into account with the respective ener-gies and branching ratios of the gamma rays involved. For the setupusedherethishasbeendonewithaGeant4[24] simulation thatincludes allknowndeexcitationpaths oftheresonant decay. Thissimulationhasbeenemployedsuccessfullyforpast measure-ments at LUNA [25,20] and was revalidated during the current campaignusingcalibratedsources andwell-known resonancesin 27_Al

₍

_p

_,

_γ

₎

28_{Si and} 14_N

₍

_p

_,

_γ

₎

15_O. _In _general, _an _agreement _with the calibrationdata within a few % was found, butit should be stressed that the eﬃciency and its uncertainty depends on the knowledgeofthebranchingsofthereactionunderstudy.Thiswill bediscussedforthespeciﬁccaseofthepresentmeasurement be-low.

The excitation function ofthe 18_O

₍

_p

_,

_γ

₎

19_{F cross} _section _was measured in steps smaller than the target thicknesses between 85and150keV.Total depositedcharges rangedfroma few mil-licoulomb at the highest energies to

∼

40 C at 85 keV. Beam-induced backgroundwas checkedusing an inert target produced withnaturalH2O.The countrateintheregionofinterestaround 8 MeV was found to be consistent withthe naturalbackground (4

×

10−5 _s−1 _or ₄ _counts _per _day). _A _summed _spectrum _of _all runs taken inside the 90 keV resonance region (the sum of the runs done in the energy range between 90 and 96 keV) is dis-playedinFig.1,showingclearsignalsatthesumenergyforboth theresonantenergyandbelow.6

The datain the full energyrange studied hereare comprised of three components: the contributions of the 143 and 90 keV

6 _The_spectra_are_{normalized by}_measurement_time_to_allow_comparison_with

theroombackground.Forbothin-beammeasurements similarcurrentswereused, sotheratioofthetwospectraapproximatelyrepresentstheyieldratio.Thetotal numberofcountsintheregionofinterestfortheoff-resonancedatais85,ofwhich 21areexpectedbackgroundcounts.

Fig. 1. BGOsumspectraintheon-resonanceenergyregion(black),belowthe90 keVresonance(red)and aroombackgroundspectrum(green).Thetwovertical linesdelimittheregionofinteresttocalculatetheexperimentalyields.

resonances and the direct capture component. The contribution from tails of higher-energy resonances (the closest signiﬁcantly strong one beingat 316 keV) was calculated to be negligible. In order to disentangle the three components from each other the datawere ﬁt usingan incoherentsumoftwo Breit-Wigner reso-nantshapesandaconstant“Sfactor”modelingthedirectcapture (DC) cross section. The S factor is related to the cross section

σ

through

σ

(

E

)

=

1_ES

(

E

)

e−2π η , with the Sommerfeld parameter

η

=

μ

2EZ0Z1e 2_._The_Z

0,1e are thechargesofthereactionpartners and

μ

isthereducedmass.

ThestrongresonancewithEr=143keVcanbedescribedusing thestandardsingle-levelBreit-Wignerformula

σ

B W

(

E

)

=

λ

2 4

π

2 J

+

1

(

2 j0

+

1

)(

2 j1

+

1

)

p

(

E

)

γ

(

E

)

(

Er

−

E

)

2

+ (

E

)

2

/

4

.

(1)

J

,

j

0 and j1 are the spins of the resonant compound state andofthe two reactionpartners, respectively. The statistical fac-torinvolving thedifferentspinsiscommonlyabbreviatedas

ω

=

2 J+1

(2 j0+1)(2 j1+1).

λ

is the de Broglie wavelength and

p,γ are the

channel widths (in this case proton and

γ

) and

is the total width ofthe resonance.The 143 keVresonance is alsoobserved inthe

(

p

,

α

)

channelwithan

α

widthof123 eV[26] that com-pletely dominates the total widthof thestate. It is importantto note that the widths are energydependent andare functions of the Coulomb penetration factors forcharged particles andofthe multipolarityinthecaseofphotons.

Finally,dueto itsrelativeweakness,the90keVresonancehas

been modeled using a simpliﬁed Breit-Wigner description with

constant widths and introducing the resonance strength

ωγ

=

ω

p γ

.Inthiscasethecrosssectionbecomes

σ

B W

(

E

)

=

λ

2 4

π

ωγ

(

Er

−

E

)

2

+

2

/

4

.

(2)

TherelationshipbetweencrosssectionandmeasuredyieldYis

Y

(

E

)

=

E

E−E

η

σ

(

E

)

(

E

)

dE

,

(3)

(4)

where

is the effective stopping power of the target material7

andtheintegrationrangeisdeterminedbythetargetthicknessin termsoftheenergylossoftheprojectilewhiletraversingthe tar-get;

η

isthedetectioneﬃciencyofthesetup.Alsohereonlyc.m. valuesareusedinthecalculation.When extractingthecross sec-tionsoftheabovementionedthreecomponentsonehastoassume thateach ofthetworesonant statesandtheDCparthave differ-ent

γ

-ray branchings, andasthe detectioneﬃciencyofthe BGO isafunctionofthebranchingseachcomponentcontributestothe yieldweightedwithitseﬃciency

η

i.

Y

(

E

)

=

E

E−E

η

1

σ

B W1

(

E

)

+

η

2

σ

B W2

(

E

)

+

η

3

σ

DC

(

E

)

(

E

)

dE (4)

Onlythebranchingsofthe143keVresonancearewellknown andcouldbedirectlyusedfortheeﬃciencysimulation.Thereisno branching informationinthe literature on the90keV resonance, and only higher-energy direct capture branchings are available [16]. Forthelow energyresonance we adoptedthemethodology ofBuckneretal.:wecalculatedeﬃcienciesforthedetectionofthe cascadedecayofallstatesin19_F_with_known_branching_ratios, re-strictingtheselectiontostateswithEx

>

5500 keVandJ

<

9₂.The resultingeﬃciencydistributionwasthenusedfortheanalysis.The eﬃciencyatEx

=

8083 keV,correspondingtothe90keVresonance

was determined by a linear ﬁt through the data. The maximum

deviationbetweensimulation pointsandthefit(evaluatedatthe resonancestateenergy)wasadoptedastheuncertaintyofthis ef-ficiencydetermination,resultinginanefficiencyof

(

59

±

7

)

%.The eﬃciency forthe DC component was calculated to be

(

49

±

5

)

% usingtheWiescheretal.branchingsandassumingaconservative uncertaintyof10%.

To calculate the cross sections from the experimental data a least-squares ﬁt was performed in which parameters inﬂuencing thedifferentcrosssectioncomponentswereallowedtovaryfreely andthenumericallyintegratedyieldwascomparedtothe experi-mentalone.Thevariedparameterswerethe

ωγ

ofthelow-energy resonance,a scaling factor inthe caseof the 143keV resonance andaconstant Sfactortodescribe theDCcomponent. Thelatter choicecanbejustiﬁedbytheratherlowrangeinenergybetween the lowest andhighest datapoint andthe fact that the S factor determined overa much larger energyrangein thework of Wi-escheretal.[16] isnotstronglyenergydependent.Theresonance energyof the low-energystate was set to 90.6keV andits total widthto121

±

12 eV[14]. Resultsofthe ﬁtareshowninFig. 2. Thereducedchi-squareis1.22.

The uncertainties inthe minimization results were calculated asfollows.The low energyresonancewidthwas ﬁxed torandom valuessampledfromaGaussiandistributionwiththemeanvalue andFWHMgivenaboveandforeachvalue alargenumberofﬁts wereperformed, wheretheexperimental yielddatawereallowed tovaryaccordingtotheir individualstatisticaluncertainties,again sampledfromaGaussiandistribution.Thissamplingwasrepeated manytimes.Thebestvalueforeachcrosssectioncomponentwas determinedasthe meanvalue oftheresulting distributionwhile thecorresponding uncertaintywasadoptedfromtheaverage dis-tancebetweenmeanvalueandthe16thand84thquantilesofthis distribution.

The results and a comparison with the literature are shown inTable 1.The followingfactors were includedinthe systematic

7 _SRIM-2013_[₂₇_{] was}_used_to_calculate_the_stopping_power_of_the_Ta

2O5targets.

At90.4keV

(Ta18

2O5)=2.81·10−14eV cm2.Thestoppingpowerhasbeenscaled

bythefactormtarget/(mtarget+mbeam).

(a)

(b)

Fig. 2. (a)Experimentalyieldandﬁttedcrosssectionof18_O₍_p_,_γ₎19_F._The_total_cross

sectionandthedirectandresonantcomponentsareshownaslines.Theleftaxis referstotheyieldpoints,therightonetothecrosssections(lines).Displayedin theinsetisazoomintothelowenergyresonanceregion.(b)Experimentaldatavs. ﬁtresultsinthelow-energyregion.

Table 1

Quantitiesdeterminingthethreecomponentsofthelow-energy18_O₍_p_,_γ₎19_{F cross}

section.

Fit parameter Value Literature

ωγ(143 keV) [meV] 0.88±0.07 (sys.) 0.92±0.06 [28] 1.0±0.1 [16]

ωγ(90 keV) [neV] 0.53±0.07 stat.

0.07 sys.

<8 [15] 0.7₋+00..5638·103[17]

S0(DC) [keV b] 23.0±22..7 stat7 sys..

7.06 [15] 15.7 [16]

uncertainty:chargeintegration3%and5%fromtheliterature stop-ping power. 3% were assignedtotake into account stoichiometry changes duetotarget degradation,and5% tothethickness deter-mination oftheused targets.Thelattertwowere includedinthe individual datapoints andunderwentthe MonteCarloprocedure described above. The efficiencyfor thedetection ofthe DC com-ponent was assignedasystematicuncertaintyof10%; dueto the much better knownbranchingsofthe 143keVresonancewe as-signed5% ofuncertaintytoitsdetectionefficiency.Thestrengthof the 143 keV resonance is in very good agreement with the lit-erature; the best-fit value of the resonance energy is 143.3 (

±

(5)

Fig. 3. Top:ReactionraterelativetoIliadisetal.[30].Bottom:fractional contribu-tionsoftheindividualcrosssectioncomponentstothetotalrate.

0.3) keV.Our directmeasurement of the strength of the 90keV resonance is in agreement with the Buckner et al. upper limit, which,beingmuch lowerthanthe Fortunecalculation[17],leads toanegligible astrophysicalimportance. TheDCS factorwas de-termined directly forthe ﬁrst time at very low energy. We ﬁnd its value to be 23.0 keV b (corresponding to a DC cross section at99.4keVof5.2μb),somewhathigherthan theWiescheretal. extrapolationandmuch higher thanthe three sigmaupper limit givenbyBuckneretal.Itisnotentirelycleariforhowinthe lat-terworkthetailofthe143keVresonancewas consideredinthe determinationof theupper limit, butit doesnot appeara likely candidate to resolve the difference. The experimental cross sec-tionspresentedherewerecorrectedforelectronscreeningeffects [25,29].8

The top part of Fig. 3 shows the low-temperature reaction

rateusing the experimental results of the present work relative to the literature rate from[30].9 _Our _rate, _while _overall _being _a

bit lower, still agrees very well with the literature. The rate in the higher-temperature range is dominated by the 140 keV res-onance. It is lower than the literature due to both the reduced strengthandtheslightlyhigherenergythantheoneusedin[30]. The contribution of the individual components to the total rate is shown in the bottom part of Fig. 3. The main result is that the90 keV resonanceonly contributesat mosta few percent to the reaction rate and does not play an important astrophysical role.

In summary we were able to perform a direct measurement

ofthe verylowenergycrosssection ofthereaction18O

(

p

,

γ

)

19F.

We can exclude a very strong 90 keV resonance, in agreement

with [15], ruling out the possibility that it plays a role in the 19_F _production _in _AGB _{stars, and} _through _the _{determination} _of theS factorto below 90keV, whereit is the dominant contrib-utortothetotalcrosssection,we areabletoconﬁrmthepresent pictureoftheastrophysicalroleof18_O

₍

_p

_,

_γ

₎

19_F,_improving_the un-certainty at the lowest energies. The low energy resonance and thestrengthoftheDCcomponentarenowknownwithprecisions ofapprox.20% and15%,respectively.Wehaveconﬁrmedthatthe stellar reaction ratefor the conditions present inlow mass AGB

stars is dominated by the DC component and the 143 keV

res-8 _A_screening_potential_of_0.66_keV_was_used_for_the_calculation._Resulting

correc-tionsrangebetween∼10%and4%,dependingontheenergy.

9 _This_reference_was_chosen_because_it_appears_that_the₂₂_keV_resonance_was

notincludedtoproducetheratetablein[15],leadingtoamuchlowerrateatthe lowesttemperatures.

onance, as the low-energyresonance is too weak to play a role. Thanks to our new results it will be possible to perform more ﬁrm nucleosynthesis calculations regarding the origin of oxygen polluters in the universe and mixingprocesses in RGB andAGB stars.

Acknowledgements

This work was supported by the INFN. The authors

acknowl-edgethesupportattheGranSassoNationalLaboratory,andwould liketothankD.CiccottiandL.Roscilliaswell astothe mechani-calworkshopsoftheINFNsectionsofLNGSandNaples.Following authorsacknowledgefundingfromseveralinstitutionsandgrants: Z.E., Z.F.,G.Gy.:NKFIHK120666; C.Bru., M.A.,T.D.: STFCUK;D.B., M.T.,T.S.,K.S.:DFG(BE4100/4-1),HelmholtzAssociation(NAVIand ERC-RA-0016), COST (ChETEC, CA16117); A.B.,J.B., A.D.L., G.I: the UniversityofNaples/CompagniadiSanPaolograntSTAR.

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