Neutron spectroscopy of 26Mg states: Constraining the stellar neutron source 22Ne(α,n) 25Mg

(1)

2017

Publication Year

2020-08-27T11:33:20Z

Acceptance in OA@INAF

Neutron spectroscopy of 26Mg states: Constraining the stellar neutron source

þÿ22Ne(±,n) 25Mg

Title

Massimi, C.; Altstadt, S.; Andrzejewski, J.; Audouin, L.; Barbagallo, M.; et al.

Authors

10.1016/j.physletb.2017.02.025

DOI

http://hdl.handle.net/20.500.12386/26872

Handle

PHYSICS LETTERS. SECTION B

Journal

768

(2)

Contents lists available atScienceDirect

Physics

Letters

B

www.elsevier.com/locate/physletb

Neutron

spectroscopy

of

26 Mg

states:

Constraining

the

stellar

neutron

source

22 Ne(

α

,

n)

25 Mg

C. Massimi

a

,

b

,

∗

,

S. Altstadt

c

,

J. Andrzejewski

d

,

L. Audouin

e

,

M. Barbagallo

f

,

V. Bécares

g

,

F. Beˇcváˇr

h

,

F. Belloni

i

,

E. Berthoumieux

i

,

J. Billowes

j

,

S. Bisterzo

k

,

l

,

D. Bosnar

m

,

M. Brugger

n

,

M. Calviani

n

,

F. Calviño

o

,

D. Cano-Ott

g

,

C. Carrapiço

p

,

D.M. Castelluccio

a

,

F. Cerutti

n

,

E. Chiaveri

i

,

L. Cosentino

r

,

M. Chin

n

,

G. Clai

a

,

q

,

N. Colonna

f

,

G. Cortés

o

,

M.A. Cortés-Giraldo

s

,

S. Cristallo

t

,

M. Diakaki

u

,

C. Domingo-Pardo

v

,

I. Duran

w

,

R. Dressler

x

,

C. Eleftheriadis

y

,

A. Ferrari

n

,

P. Finocchiaro

r

,

K. Fraval

i

,

S. Ganesan

z

,

A.R. García

g

,

G. Giubrone

v

,

I.F. Gonçalves

p

,

E. González-Romero

g

,

E. Griesmayer

aa

,

C. Guerrero

n

,

F. Gunsing

i

,

A. Hernández-Prieto

n

,

o

,

D.G. Jenkins

ab

,

E. Jericha

aa

,

Y. Kadi

n

,

F. Käppeler

ac

,

D. Karadimos

u

,

N. Kivel

x

,

P. Koehler

ad

,

M. Kokkoris

u

,

S. Kopecky

ae

,

M. Krtiˇcka

h

,

J. Kroll

h

,

C. Lampoudis

i

,

C. Langer

c

,

E. Leal-Cidoncha

w

,

C. Lederer

af

,

H. Leeb

aa

,

L.S. Leong

e

,

S. Lo Meo

a

,

q

,

R. Losito

n

,

A. Mallick

z

,

A. Manousos

y

,

J. Marganiec

d

,

T. Martínez

g

,

P.F. Mastinu

ag

,

M. Mastromarco

f

,

E. Mendoza

g

,

A. Mengoni

q

,

P.M. Milazzo

ah

,

F. Mingrone

a

,

b

,

M. Mirea

ai

,

W. Mondelaers

ae

,

A. Musumarra

r

,

C. Paradela

w

,

A. Pavlik

af

,

J. Perkowski

d

,

M. Pignatari

aj

,

L. Piersanti

t

,

A. Plompen

ae

,

J. Praena

s

,

J.M. Quesada

s

,

T. Rauscher

ak

,

al

,

R. Reifarth

c

,

A. Riego

o

,

M.S. Robles

w

,

C. Rubbia

n

,

M. Sabaté-Gilarte

s

,

R. Sarmento

p

,

A. Saxena

z

,

P. Schillebeeckx

ae

,

S. Schmidt

c

,

D. Schumann

x

,

G. Tagliente

f

,

J.L. Tain

v

,

D. Tarrío

w

,

L. Tassan-Got

e

,

A. Tsinganis

n

,

S. Valenta

h

,

G. Vannini

a

,

b

,

I. Van Rijs

ak

,

V. Variale

f

,

P. Vaz

p

,

A. Ventura

a

,

M.J. Vermeulen

ab

,

V. Vlachoudis

n

,

R. Vlastou

u

,

A. Wallner

am

,

T. Ware

j

,

M. Weigand

c

,

C. Weiß

aa

,

R. Wynants

ae

,

T. Wright

j

,

P. Žugec

m

a_Istituto_Nazionale_di_Fisica_Nucleare,_Bologna,_Italy

b_Dipartimento_di_Fisica_e_Astronomia,_Università_di_Bologna,_Italy c_{Johann-Wolfgang-Goethe}_{Universität,}_Frankfurt,_Germany d_Uniwersytet_Łódzki,_Lodz,_Poland

e_Institut_de_Physique_Nucléaire,_CNRS-IN2P3,_Univ._Paris-Sud_et_{Paris-Saclay,}₉₁₄₀₆_Orsay_Cedex,_France f_Istituto_Nazionale_di_Fisica_Nucleare,_Bari,_Italy

g_Centro_de_{Investigaciones}_Energeticas_{Medioambientales}_y_{Tecnológicas}_(CIEMAT),_Madrid,_Spain h_Charles_University,_Prague,_Czechia

i_Commissariat_à_l’Énergie_Atomique_(CEA)_Saclay_{- Irfu,}_{Gif-sur-Yvette,}_France j_University_of_Manchester,_Oxford_Road,_Manchester,_UK

k_INAF_{- Osservatorio}_astroﬁsico_di_Torino,_Torino,_Italy

l_JINA_(Joint_Institute_of_Nuclear_{Astrophysics),}_University_of_Notre_Dame,_IN_46556,_USA m_Department_of_Physics,_Faculty_of_Science,_University_of_Zagreb,_Croatia

n_European_Organization_for_Nuclear_Research_(CERN),_Geneva,_Switzerland o_Universitat_Politecnica_de_Catalunya,_Barcelona,_Spain

p_Instituto_Tecnológico_e_Nuclear,_Instituto_Superior_Técnico,_Universidade_Técnica_de_Lisboa,_Lisboa,_Portugal q_Agenzia_nazionale_per_le_nuove_tecnologie,_l’energia_e_lo_sviluppo_economico_sostenibile_(ENEA),_Bologna,_Italy r_Istituto_Nazionale_di_Fisica_Nucleare,_Laboratori_Nazionali_del_Sud,_Italy

s_Universidad_de_Sevilla,_Spain

t_INAF_{- Osservatorio}_Astronomico_di_Collurania,_Teramo,_Italy u_National_Technical_University_of_Athens_(NTUA),_Greece

*

Correspondingauthor.

E-mailaddress:cristian.massimi@bo.infn.it(C. Massimi). http://dx.doi.org/10.1016/j.physletb.2017.02.025

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2 C. Massimi et al. / Physics Letters B 768 (2017) 1–6

v_Instituto_de_Física_Corpuscular,_{CSIC-Universidad}_de_Valencia,_Spain w_Universidade_de_Santiago_de_Compostela,_Spain

x_Paul_Scherrer_Institut,_Villigen_PSI,_Switzerland y_Aristotle_University_of_{Thessaloniki,}_{Thessaloniki,}_Greece z_Bhabha_Atomic_Research_Centre_(BARC),_Mumbai,_India aa_{Atominstitut,}_Technische_Universität_Wien,_Austria ab_University_of_York,_Heslington,_York,_UK

ac_Karlsruhe_Institute_of_Technology,_Campus_Nord,_Institut_für_Kernphysik,_Karlsruhe,_Germany ad_Department_of_Physics,_University_of_Oslo,_N-0316_Oslo,_Norway

ae_European_Commission_JRC,_Institute_for_Reference_Materials_and_{Measurements,}_Retieseweg_111,_B-2440_Geel,_Belgium af_University_of_Vienna,_Faculty_of_Physics,_Austria

ag_Istituto_Nazionale_di_Fisica_Nucleare,_Laboratori_Nazionali_di_Legnaro,_Italy ah_Istituto_Nazionale_di_Fisica_Nucleare,_Trieste,_Italy

ai_Horia_Hulubei_National_Institute_of_Physics_and_Nuclear_Engineering_{- IFIN}_HH,_Bucharest_{- Magurele,}_Romania aj_E._A._Milne_Centre_for_{Astrophysics,}_Dept._of_Physics_&_Mathematics,_University_of_Hull,_United_Kingdom ak_Department_of_Physics_{- University}_of_Basel,_Basel,_Switzerland

al_Centre_for_Astrophysics_Research,_University_of_{Hertfordshire,}_Hatﬁeld_{AL10 9AB,}_United_Kingdom

am_Department_of_Nuclear_Physics,_Research_School_of_Physics_and_Engineering,_The_Australian_National_University,_Canberra,_Australia

a

r

t

i

c

l

e

i

n

f

o

a

b

s

t

r

a

c

t

Articlehistory:

Received7July2016

Receivedinrevisedform9February2017 Accepted12February2017

Availableonline16February2017 Editor:V.Metag

Keywords: s Process

α+22_Ne

Neutronspectroscopy

This workreports onaccurate, high-resolutionmeasurements ofthe 25_Mg(n_,

_γ

₎26_Mg _and 25_Mg(n_,_tot)

cross sections inthe neutronenergyrangefromthermal to about300 keV, leading toasigniﬁcantly improved25_Mg(n_,

_γ

₎26_Mg_{parametrization.}_The_relevant_resonances_for_n₊25_Mg_were_{characterized}_from

a combined R-matrix analysis ofthe experimental data. Thisresulted in anunambiguous spin/parity assignmentofthecorrespondingexcitedstatesin26Mg.Withthisinformationexperimentalupperlimits ofthereactionratesfor22_Ne(

_α

_,_n)25_Mg_and 22_Ne(

_α

_,

_γ

₎26_Mg_were_established,_potentially_leading_to_a

signiﬁcantlyhigher(

α

,n)/(

α

,

γ

)ratio thanpreviouslyevaluated.Theimpact oftheseresultshas been studiedforstellarmodelsinthemassrange2to25M.

1. Introduction

The stellar nucleosynthesis of elements heavier than iron oc-cursvia neutron capturereactionsandsubsequent

β

decays.The so-calledslowneutroncaptureprocessors process[1]takesplace inlowandintermediate massstarsduring theirAsymptoticGiant Branch (AGB) phase. An additional s-process contribution comes in thehelium-burning coreandsubsequent carbon-burning shell phasesofmassivestars.The22Ne(

α

,

n)25Mgreactionconstitutesa majorneutronsourceforthes process.InAGBstars,thissourceis activatedduring aseries ofconvectiveHe-shell burningepisodes, so-called thermal pulses, with temperatures as high as 400 MK (more than30 keVthermalenergy).Although itcontributesonly about 5% to the total neutron budget, this scenario determines theﬁnal abundance pattern. Hence, itis decisivefor deriving in-formation on neutron density, temperature, and pressure in the He-burninglayers of AGBstars [1].In massivestars, where ther-malenergiesof25and90keVarereachedduringtheHe-coreand C-shellburningphases,theneutronbudgetisbyfardominatedby the22_Ne(

_α

_,

_n)25_Mg_reaction.

The 22Ne(

α

,

n)25Mg cross section in the temperature regime ofthe s process isaffectedby unknown resonancecontributions. Alsothecompeting22Ne(

α

,

γ

)26Mgreaction,whosecrosssection isalso insuﬃcientlyknown,addsto thepersistentuncertainty of the neutron budget and the associated neutron densities during stellarnucleosynthesis.The (

α

,

γ

) reactioncontributesto the de-structionof22_Ne_during _the_entire_helium_burning_phase_because ofitspositive Q -value of10.615MeV, alreadybeforethe thresh-old for neutron production via 22_Ne(

_α

_,

_n)25_Mg _{( Q}

_{= −}

_{478 keV)} is reached. In this context, the 25_Mg(n

_,

_γ

₎26_Mg _reaction _plays _a non-negligibleroleaswell,becausethehighs abundanceof25Mg makesitasigniﬁcantneutronpoisonforthes process.Sofar,this reactionrateistoouncertainforaquantitative assessmentofthe poisoningeffect[2].

Experimentaldataforthe22_Ne(

_α

_,

_n)25_Mg_cross_section_are es-sentially limited to

α

energies above about 800 keV. At lower energiesthemostrecentdirectmeasurements[3–5]providedonly an upperlimit ofabout10−11 barn,corresponding tothe experi-mental sensitivity.The lowest resonanceobserved in direct mea-surementsisatELab

α

=

832

±

2 keV (e.g. Ref.[4]).

2. Indirectapproach

Since a direct approach is exceedingly diﬃcult, indirect and transfer reactionswere considered asan alternativewayfor con-straining the reaction rate at low energies. For instance, the

α

-transfer reaction 22_Ne(6_Li, _d)26_Mg _has _been _studied _[5–7] _to search for levels in 26_Mg _also _below _the _neutron _threshold _at 11.093 MeV.Thesestudiesare particularlyrelevantforestimating the 22Ne(

α

,

γ

)26Mgrate, wherelevels belowandabove the neu-tron thresholdareinvolved.Direct22Ne(

α

,

γ

)26Mgmeasurements have onlybeencarried outabove 0.8MeV. Inview ofthe scarce experimental information, evaluations had, therefore, to rely on theoretical estimates. Accordingly, the uncertainties of the stellar 22_Ne(

_α

_,

_n)25_Mg _and 22_Ne(

_α

_,

_γ

₎ _rates _are _still _dominated _by un-known propertiesofstatesinthecompoundnucleus26Mgabove the

α

thresholdat10.615MeV.

As indicated in Fig. 1, these states can be studied via neu-tron resonance spectroscopy ofn

+

25_Mg. _In _particular, _a simulta-neousresonanceshape analysisoftotalandcapturecrosssection data can be used to determine the resonance energies, ER, spin

parities, J π ,as well as the capture andneutron widths

γ and

n [8].Informationfromprevious 25Mg(n

,

γ

)26Mgmeasurements

suffers from considerable uncertainties due to sample problems and countingstatistics. Moreover, theavailable transmission data havebeenobtainedwithnaturalMgsamples,aseriouslimitation in view of the dominant abundance of 24_Mg _in _natural

(4)

magne-Fig. 1. Levelscheme(nottoscale)of26_Mg_with _the_entrance _channels22_Ne₊_α

andn+25_Mg_and_the_exit_channel26_Mg₊_γ_._Excited_states_in26_Mg_with _natural

parity(0+,1−,2+,. . .)andenergiesbelowELab

α =850 keV (inred)canbe stud-iedvianeutronresonancespectroscopyofn+25_Mg_as_indicated_by_the_schematic 25_Mg(n_,_γ₎26_Mg_cross_section._(Color_online.)

sium.Thecorrespondingdifferencesbetweenpresentandprevious transmissionresultsarediscussedinRef.[9].

3. Measurementsofn

+

25_Mg

This work on n

+

25_Mg _combined _two _neutron _{time-of-ﬂight} measurements, using the CERN-n_TOF facility [10] for the (n

,

γ

) channelandtheGELINAacceleratoratIRMM/Geel[11]forthetotal crosssection. Toavoid thelimitations ofprevious attempts, both measurements were carried out with highly enriched, metallic samples(97.87%25Mg).Then_TOFfacility,whichprovidesawhite neutron spectrum from thermal energy to about 1-GeV neutron energy,was chosenbecauseitisparticularlywellsuitedfor accu-ratehigh-resolutionneutronmeasurementsduetoitsoutstanding instantaneousneutronflux (about106 neutronsper bunchatthe experimental area) andthe very long flight-path of 185 m. The experimentalsetupwas basedontwo deuteratedbenzene(C6D6) scintillation detectors (cylindrical cells withan active volume of about1 liter),placedoneither sideoftheneutron beam,9.0cm upstream of the sample. These detectors are characterized by a verysmall neutronsensitivity. The chosen geometrical configura-tionminimized thebackground dueto scatteredin-beam

γ

rays andreduced systematic effects due to the anisotropy in the an-gulardistribution fromprimary

γ

-rays following p-waveneutron resonances.Thetotalnumberofneutronsimpingingonthevarious samplesusedinthemeasurementwasdeterminedbymeansofa low-mass,6_Li-based, _neutron _detector_[12],_while _the_energy de-pendenceoftheneutronﬂuxderivedfromadedicatedstudy[13]. Thewell-establishedtotal energydetectionprinciple in combina-tionwith thepulseheight weighting technique [14] was applied tomakethe detectioneﬃciencyforacaptureeventdirectly pro-portionaltothetotal

γ

-rayenergyavailable inthecaptureevent. Moreoverthesaturatedresonancetechnique[14]wasusedto nor-malize the capture data. The measurement covered the energy rangefromthermalenergytoabout300keVwitharesolutionof 0.05%to0.3%intherelevantenergyrangebetween1and300keV. The total cross section of n

+

25Mg was measured witha 6 Li-glassdetectoratthe50mstationoftheGELINAfacilityfor ener-giesuptoabout300keV.AcombinationofLi-carbonateplusresin, PbandCu-collimatorswas usedtoreducetheneutronbeamtoa diameter of less than 35 mm at the sample position. The sam-ple was placed in an automatic sample changer ata distanceof 23.78m fromtheneutronsource. Closetothe sample positiona

10_B _anti-overlap_filter_was_placed_to_absorb_slow_neutrons _from_a previousburst.Inordertohaveawell-characterizedmeasurement condition,permanentNaandCoblackresonancefilters[14]were used to continuously monitor the background at 2.85 keV and 132 eV.Additional blackresonancefilterswereusedindedicated runs. Thanks to the small dimensions of the neutron-producing target and the excellent time characteristic (1-ns pulse width), GELINAisparticularlysuitedforhigh-resolutiontransmission mea-surementsinthekeVregion.

Essential improvements with respect to the previous capture measurement atn_TOF [2]arethe quality ofthehighly enriched 25_Mg _sample _and_a_{substantially} _reduced_background _of_in-beam

γ

rays[10].Thetotalcrosssectionmeasurementbeneﬁtsalso sig-niﬁcantlyfromthehighly enrichedsample.Inbothmeasurements theexperimentalbackgroundwasdeterminedindedicatedruns.

The data reduction forthe determination of thecross section startingfromdetectorscountingratewasperformedfollowingthe procedures described in[14].The captureyield (representing the fractionoftheneutronbeamundergoingcaptureevents)obtained atn_TOF and thetransmission (representing thefraction of neu-tron beam that traverses the sample without interaction) from GELINA arepresented inFig. 2 andcomparedwiththe resultsof a simultaneous R-matrix analysisof both data sets. More in de-tail,theexperimentalcaptureyieldY ,deducedfromtheresponse ofthecapturedetectionsystemandtheﬂux-monitorisrelatedto the total andcapturecross-sections,

σ

tot and

σ

γ respectively, by

thefollowingexpression[14]:

Y

(

En

)

= (

1

−

e−nCσtot(En)

)

σ

γ

(

En

)

σtot

(

En

)

+

YM S

(

En

),

(1)

while the transmission T , which was experimentally obtained fromthe ratioof Li-glass spectraresulting froma sample-in and a sample-out measurement, is related to the total cross-section by[14]:

T

(

En

)

=

e−nTσtot(En)

,

(2)

wherenC

= (

3

.

012

±

0

.

009

)

×

10−2 atoms/bdenotestheareal

den-sityofthecapturesampleandnT

= (

5

.

02

±

0

.

04

)

×

10−2 atoms/b

theoneofthetransmissionsample, En thelaboratoryneutron

en-ergy,andYM S thecontributionofmultipleinteractioninthe

sam-ple.Thislatterterm,togetherwithotherexperimentaleffectssuch asself-shielding,Dopplerbroadeningandresponseofthe time-of-ﬂightspectrometer,areproperlytakenintoaccountintheR-Matrix codes SAMMY[15] andREFIT[16] usedinthe presentcombined analysis.

4. Improved25_Mg(n

_,

_γ

₎26_Mg_cross_section

Resonance energies, ER, partial widths,

γ and

n, and the

spinsand parities, J π weredetermined by a combined R-matrix analysisin the energyregion En

<

300 keV, corresponding to an

incident

α

-particleenergyfromthresholdat Eα

=

570 keV to ap-proximately 850keV,wheredirect(

α

,

n)measurementsare diﬃ-cultandaccuratecrosssectiondataaremissing.

Theinformationon J π isparticularlyimportantforidentifying thestatesin26Mgthatcontributetothe22Ne(

α

,

n)25Mgcross sec-tion.Thisselection iscrucialbecause22_Ne_nuclei_and

_α

_particles have both J π

=

0+. Accordingly, (

α

,

n) reactions can only pop-ulate natural-parity states (0+

,

1−

,

2+

,

. . .

) in 26Mg, whereas the

n

+

25_Mg_reaction_proceeds_also_via _{J π}

₌

₁+

_,

₂−

_,

₃+

_,

_{. . .}

_states. The results of the combined R-matrix analysis of both cross sections are summarized in Table 1, including the deduced ﬁrm

J π assignments. Forthe natural-parity states, the

α

energies in thelaboratorysystem ELab

(5)

Fig. 2. Measuredneutroncaptureyieldandtransmissionof25_Mg._Experimental_data

arerepresentedbyredsymbolsandthesimultaneousR-matrixofbothdatasetsis givenbydashedlines.(Coloronline.)

Fig. 3. ResonanceshapeanalysisoftheneutronresonanceatEn=194 keV.Thespin parityassignment Jπ₌₄−_[17]_is_excluded_by_the_present_data,_which_clearly

in-dicate Jπ₌₃−_._Only_uncorrelated_{uncertainties,}_attributable_to_counting_statistics,

arereported.(Coloronline.)

withdirect

α

+

22_Ne _{measurements.} _The _{uncertainties} _on _E

x and

ELab

α are determinedby theuncertaintyinneutronenergy, which

never exceeds 0.1 keV. This is an important result in itself be-causeit allows one to resolve inconsistencies in charged-particle data.

Fig. 3demonstrates thesensitivityof thepresentdatato spin and parity, which clearly exclude the previous J π

=

4− assign-ment[17]fortheneutronresonanceat194keV.Inparticular,the lowest values ofthe chi-squared per numberof degreesof free-dom ofthe ﬁt, were 1.1, 5.6, 7.3 and17 assuming J π

=

3−, 4+, 4−and3+respectively.Inthismannerthe J π valueswere deter-minedforeachresonance.Intwocasesthespin-parityassignment waschangedwithrespecttothepreviousevaluation.Asshownin Ref.[9],thesetwo resonancescould notbe accurately studiedby usingthe transmission data onnatural magnesium (e.g. reported in[17])sincetheexperimentalsignaturewasdominatedbystrong resonancesinn

+

24_Mg.

Comparedto evaluated resonancedata [18], sixneutron reso-nanceshadtoberemovedintheenergyrangecoveredinTable 1. These weak resonances (

n

≈

0

.

5 eV) were assigned in previous

n

+

25_Mg_experiments,_but_were_not_identiﬁed_in_this_work.

Table 1

n+25_Mg_resonance_parameters_and_{corresponding}_excitation_energies_of_the26_Mg

compoundnucleus.ThequoteduncertaintieswereobtainedbytheR-Matrixﬁt.

En Ex ELabα Jπ γ n

(keV) (keV) (keV) (h)¯ (eV) (eV)

19.92(1) 11112 589 2+ 1.37(6) 2095(5) 62.73(1) 11154 1+ 4.4(5) 7(2) 72.82(1) 11163 649 2+ 2.8(2) 5310(50) 79.23(1) 11169 656 3−a ₃_.₃₍₂₎ ₁₉₄₀₍₂₀₎ 81.11(1) 11171 5(1) 1–30 100.33(2) 11190 3+ 1.3(2) 5230(30) 155.83(2) 11243 2− 4.7(5) 5950(50) 187.95(2) 11274 779 2+ 2.2(2) 410(10) 194.01(2) 11280 786 3−a ₀_.₃₍₁₎ ₁₈₁₀₍₂₀₎ 199.84(2) 11285 2− 4.8(4) 1030(30) 203.88(4) 11289 0.9(3) 3–20 210.23(3) 11295 2− 6.6(6) 7370(60) 243.98(2) 11328 (843) 2.2(3) 171(6) 260.84(8) 11344 1.0(2) 300–3900 261.20(2) 11344 >3 3.0(3) 6000–9000

a _Parity_change_with_respect_to_previous_evaluations.

Below the lowest directly observed resonance in the

22_Ne(

_α

_,

_n)25_Mg _reaction _{cross-section} _at _ELab

α

≈

830 keV (En

≈

235 keV), ﬁve natural-parity states have been identiﬁed corre-sponding to energies ELab_α

=

589, 649, 656, 779 and 786 keV. The doublets at 649/656 and 779/786 keV were resolved owing to the good energy resolution in the neutron channel, which is signiﬁcantlybetterthanincharged-particleexperiments.The low-est observed22_Ne(

_α

_,

_n)25_Mg_resonance_at _ELab

α

=

832

±

2 keV[4]

was not observed in the presentneutron data at En

=

234 keV,

while a resonancewith a width compatible to the one reported in [4] (

=

250

±

170 eV) is located at En

=

243

.

98 keV, i.e.

ELab

α

=

843 keV. One possible explanation is that the two

res-onances correspond to the same state, in which case the en-ergies are inconsistent with each other. Another possibility is that they correspond to two different levels: in this case the width of the

(

α

,

n

)

resonance at 832 keV is lower than quoted in [4], being below the sensitivity of the presentmeasurements (

≈

20 eV),whiletheobservedneutronlevelisnotanatural par-itystate.

The 25_Mg(n

_,

_γ

₎26_Mg _cross _section _was _also _{signiﬁcantly} im-proved by an R-matrix analysis of the combined data sets for 25_Mg(n

_,

_γ

₎_and_(n

_,

_tot),_resulting_in_rather_accurate_Maxwellian av-eraged cross section (MACS) for thermal energies between kT

=

5 and 100 keV.Thoughhigherresonancesupto900keVwere con-sideredaswell, theMACS valuesare dominatedby contributions from theresonances in Table 1. Thepresent resultsandthe cur-rentlyrecommendedvaluesintheKADoNiS[22] compilationand in Ref.[2] arecompared inTable 2.While thenewMACS values are compatiblewiththeprevious measurement[2]exceptforthe lowest and highest temperature, the uncertainties were reduced byalmostafactorofthreeonaverage.Theseuncertaintiesarethe sumofuncorrelatedorstatisticaluncertaintiesandsystematic un-certainties.

5. Impactonreactionratecalculations

As mentioned above, the reaction channels for

α

+

22_Ne _and

n

+

25_Mg_open_at_excitation_energies _E

x

=

10

.

615 and11.093MeV

inthecompoundnucleus26Mg.Apartfromthelevelsbetween

α

-andn-channel, thepresent studyprovides accessto the relevant states in the energy region between the 22_Ne(

_α

_,

_n)25_Mg thresh-old and the lowest resonance reached in direct

α

+

22_Ne experi-ments. Therefore the new resonance information has been used forcalculating theupperlimit ofthereactionrates,based purely on experimentally-available information for 22_Ne(

_α

_,

_n)25_Mg _and

(6)

Table 2

25_Mg(n,_γ₎_{Maxwellian-averaged}_cross_sections_(in_mb),_compared_with_a_previous_work_and_recommended_values._Experimental_values_include_the_{contributions}_from_direct

radiativecapture[2].

kT (keV) 5 10 15 20 25 30 40 50 60 80 100

KADoNiS 4.8 5.0 5.5 6.0 6.2 6.4(4) 6.2 5.7 5.3 4.4 3.6

Ref.[2] 3.5(4) 5.1(6) 4.9(6) 4.6(4) 4.4(6) 4.1(6) 3.5(6) 2.9(5) 2.5(4) 1.9(3) 1.4(2)

this work 2.8(2) 4.4(2) 4.3(2) 4.2(2) 4.0(2) 3.9(2) 3.6(2) 3.4(2) 3.0(2) 2.5(3) 2.2(3)

22_Ne(

_α

_,

_γ

₎26_Mg. _In _particular, _they _were _deﬁned _using: _(i) _data froman

α

-transferreaction fortheresonancebelow theneutron threshold[7],(ii) data from thiswork, and (iii)data fromdirect measurementsabove ELab_α

=

800 keV[4].Thecalculation,basedon thesimplenarrow-resonanceformalism[23],requiresasinputthe resonanceenergies and the so-calledkernels (also referred to as resonancestrength),knorkγ , kn

=

g

α

n

,

kγ

=

g

α

γ

=

kn

γ

n

;

(3)

where

isthetotalwidthandg

= (

2 J

+

1

)/

[(

2I

+

1

)(

2i

+

1

)

]

the statisticalfactorand J , I and i the spin of theresonance, target and projectile, respectively. Because

α

γ

n

, the kernels

canbereducedtokn

=

g

α .

Apartfrom

α ,allquantitiesweredeterminedinthiswork.For

thepresentanalysisweestimatedtheupperlimitsof

α basedon

the experimental constrainton the 22Ne(

α

,

n)25Mg cross section (

≤

10−11barn,correspondingto

α

≈

10−8 eV).

ThecontributionsoftheﬁveresonancesbelowELab

α

≈

800 keV

tothe22Ne(

α

,

n)25Mgratedependonthetemperatureofthe spe-ciﬁc s-process site. Particularly important is the 30% decrease of the upper limit of the rate compared to Ref. [7] around kT

=

25 keV (0.25to0.3GKinFig. 4),whichaffectstheneutron econ-omyduringtheHeshellburninginlow-massAGBstarsandduring coreHeburninginmassivestars.

The neutron resonance data obtained in this work were also used to determine the upper limit of the 22Ne(

α

,

γ

)26Mg rate. As shown in Fig. 4, these results differ signiﬁcantly compared to recent evaluations [7,19,20], also in the important tempera-turerangeabove0.3GK.Part ofthesediscrepancies werecaused by amisinterpretation ofprevious neutrondata,in particularfor the doublet at En

=

79 keV. While the impact of the 79-keV

doublet on the 22_Ne(

_α

_,

_n)25_Mg _rate _is _relatively _small _(at _most 3%), it plays a major role for the (

α

,

γ

) rate. As discussed in Ref. [7],

α

-transfer measurements provide experimental evidence of a natural parity state in 26_Mg _{corresponding} _to _an _energy _of about En

=

80 keV. Prior to this measurement the resonance at

En

=

79 keV was tentatively assigned to be of unnatural

par-ity whereas the closest resonance at En

=

81 keV was

consid-ered as the one with natural parity. This misinterpretation re-sulted in a biased 22_Ne(

_α

_,

_γ

₎26_Mg _reaction _rate, _since _the reso-nancestrength of the (

α

,

γ

) channel scales with the

n

/

γ

ra-tio (see Eq. (3)). With the present spin-parity assignment, the contribution to the reaction rate of this level became negligi-ble since the

n

/

γ ratio of the resonance at En

=

79 keV is

abouta factor of 2200 larger than the ratio of the resonanceat

En

=

81 keV.

In summary, the present resonance analysis of the neutron-inducedcross sectionsof25Mg hasledto decisiveimprovements forthepoorly knownstatesin26Mgwithexcitationenergies be-tween11112and11344 keV.Thisconcernsaccuratelevelenergies, ﬁrmspin/parityassignments,andthedeterminationofreliable

γ

and

n values. The conclusive identiﬁcation of their properties

wereusedtodeterminethecontributions oflevels belowthe

en-Fig. 4. Reactionrateratiooftheupperlimitsforthe22_Ne(_α

,n)25_Mg_(top_panel)

fromthisworkcomparedtotheupperlimitsinRefs.[7,20,19].Thesameratiosare presentedforthe22_Ne(_α_,_γ₎26_Mg_(bottom_panel)._Below_0.3_GK_the_present_data

areindicatinganenhanced(α,n)rate,whereasthe(α,γ)rateisstronglyreduced abovethattemperature.(Coloronline.)

Table 3

Upper limitsof thereaction rates,given incm3_/(_{mole s}₎_, _determined _from_the

presentmeasurements.

Temperature Upper limits

109_K 22_Ne(_α_,_n) 22_Ne(_α_,_γ₎ 0.06 2.97×10−44 ₄_.₁₃_×₁₀−21 0.08 5.56×10−34 ₈_.₄₅_×₁₀−28 0.10 7.56×10−28 ₃_.₂₃_×₁₀−24 0.12 8.96×10−24 ₁ .55×10−21 0.13 3.29×10−22 ₁ .67×10−20 0.14 7.16×10−21 ₁_.₂₇_×₁₀−19 0.15 1.04×10−19 ₇_.₃₅_×₁₀−19 0.16 1.07×10−18 ₃_.₃₉_×₁₀−18 0.18 5.32×10−17 ₄_.₂₇_×₁₀−17 0.19 2.78×10−16 ₁_.₂₃_×₁₀−16 0.20 1.25×10−15 ₃ .21×10−16 0.25 4.99×10−13 ₁ .43×10−14 0.30 4.52×10−11 ₅_.₈₂_×₁₀−13 0.35 1.44×10−09 ₁_.₈₂_×₁₀−11 0.40 2.09×10−08 ₂_.₆₆_×₁₀−10 0.45 1.74×10−07 ₂_.₁₂_×₁₀−09 0.50 1.03×10−06 ₁_.₁₅_×₁₀−08 0.60 2.24×10−05 ₁ .50×10−07 0.70 3.36×10−04 ₁ .18×10−06 0.80 3.19×10−03 ₇_.₃₈_×₁₀−06 0.90 1.95×10−02 ₃_.₆₇_×₁₀−05 1.00 8.45×10−02 ₁_.₄₃_×₁₀−04

ergy rangecoveredby direct(

α

,

n)measurements. Whilewaiting forthe crucialestimate of

α , which willrequire alarge

experi-mentaleffort,thepresentresultscanleadtoimprovementsinthe calculationofthestellar22Ne

+

α

ratesintheastrophysically rele-vantenergyrangesubstantially.Inparticular,itwasfoundthatthe (

α

,

γ

)channelhadbeenstronglyoverestimatedsofar.Thepresent resultsaresummarizedinTable 3.

(7)

Fig. 5. Ratiosofthes abundanceofY,La,andPbfromstellars-processmodelsusing thepresentupperlimitsofthe22_Ne(_α

,n)25_Mg_rate_and_from_the_FRUITY_reference

set[24].

6. Astrophysicalimplicationsandconclusions

Theastrophysicalconsequencesofthepresentresultshavebeen studiedusingtheupperlimitsestablishedinthiswork.Their im-pact on the weak-s process in massive stars was explored for a 25 M[21]star.Thecalculatedabundancesareconsistentwiththe latest evaluations [7,19]. The present uncertainty of the

α

+

22_Ne ratecauseslargedifferencesintheweaks-processabundances,up toafactor50intheSrregion.

Noticeable changes are also found in intermediate-mass AGB models(IMS-AGBs,3

<

M

/

M

<

7). Asinthesestarshigher tem-peraturesarereachedduringthermalpulsesthaninlow-massAGB stars,thestrongpreponderanceofthe13C(

α

,

n)16Oneutronsource isprogressivelysubstitutedbythe22_Ne(

_α

_,

_n)25_Mg_reaction._This_is particularlyevidentatlowmetallicities.

Using the present upper limits for the 22_Ne(

_α

_,

_n) _rates, _the

s-process yields of AGB stars between 2 and 5 M and an ini-tial iron fraction corresponding to 0.7% of the solar value were calculated for comparison with reference AGB models from the FRUITY data base [24]. The resulting s abundances of Y and La wereselectedforcomparisonwithPb,becauseYandLaare char-acterizing the ﬁrst and second s-process abundance peak. As il-lustratedinFig. 5 theproductionofY and– especially–ofLa is increasingwithAGB massasaconsequenceofthemoreeﬃcient 22_Ne(

_α

_,

_n)25_Mg _source, _whereas _the _Pb _abundance _remains basi-callyfrozen.ForaﬁxedamountofPbwe obtain,therefore,larger abundances in the ﬁrst and, especially, in the second s-process

peak. This pattern in higher-mass stars may help to explain the

surface distributions in s-process enriched globularcluster stars, which exhibit a comparably low Pb abundance [25]. Whether a mixofenhanceds contributionsfromIMS-AGBsattheexpenseof thePbproducingLMS-AGBscouldprovideaplausiblesolutionwill beanalyzedinadedicatedpaper.

Acknowledgements

The isotopeusedinthisresearchwere suppliedby theUnited StatesDepartmentofEnergyOﬃceofScienceby theIsotope Pro-gramintheOﬃceofNuclearPhysics.

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