A direct measurement of the 17O(a,g)21Ne reaction in inverse kinematics and its impact on heavy element production
M.P. Taggart, C. Akers, A.M. Laird, U. Hager, C. Ruiz, D.A. Hutcheon, M.A. Bentley, J.R. Brown, L. Buchmann, A.A. Chen, J. Chen, K.A. Chipps, A. Choplin, J.M. D'Auria, B. Davids, C. Davis, C.Aa.Diget, L. Erikson, J. Fallis, S.P. Fox, U. Frischknecht, B.R. Fulton, N. Galinski, U. Greife, R. Hirschi, D. Howell, L. Martin, D. Mountford, A.St.J. Murphy, D. Ottewell, M. Pignatari, S. Reeve, G. Ruprecht, S. Sjue, L. Veloce, M. Williams
AA direct measurement of the O( α, γ ) Ne reaction in inverse kinematics and itsimpact 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 a Department of Physics, University of York, York, YO10 5DD, UK b TRIUMF, Vancouver, Canada V6T 2A3 c McMaster University, Hamilton, ON, Canada d Geneva Observatory, University of Geneva, Maillettes 51, CH-1290 Sauverny, Switzerland e Simon Fraser University, Burnaby, BC, Canada f Colorado School of Mines, Golden, CO, USA g Department of Physics, University of Basel, Klingelbergstrasse 82, 4056 Basel, Switzerland h Astrophysics Group, Lennard-Jones Labs 2.09, Keele University, ST5 5BG, Staffordshire, UK i Kavli Institute for the Physics and Mathematics of the Universe (WPI), University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa, 277-8583,Japan j SUPA, School of Physics and Astronomy, The University of Edinburgh, Edinburgh, EH9 3FD, UK k University of Victoria, Victoria, BC, Canada l UK Network for Bridging the Disciplines of Galactic Chemical Evolution (BRIDGCE) m Konkoly Observatory, Research Centre for Astronomy and Earth Sciences, Hungarian Academy of Sciences, Konkoly Thege Miklos ut15-17, H-1121 Budapest, Hungary n NuGrid Collaboration
Abstract
During the slow neutron capture process in massive stars, reactions on light elements can both produce and absorbneutrons thereby influencing the final heavy element abundances. At low metallicities, the high neutron capture rate of O can inhibit s-process nucleosynthesis unless the neutrons are recycled via the O( α ,n) Ne reaction. The efficiencyof this neutron recycling is determined by competition between the O( α ,n) Ne and O( α, γ ) Ne reactions. Whilesome experimental data are available on the former reaction, no data exist for the radiative capture channel at therelevant astrophysical energies.The O( α, γ ) Ne reaction has been studied directly using the DRAGON recoil separator at the TRIUMF Labo-ratory. The reaction cross section has been determined at energies between 0.6 and 1.6 MeV E cm , reaching into theGamow window for core helium burning for the first time. Resonance strengths for resonances at 0.63, 0.721, 0.81 and1.122 MeV E cm have been extracted. The experimentally based reaction rate calculated represents a lower limit, butsuggests that significant s-process nucleosynthesis occurs in low metallicity massive stars. PACS numbers ∗ Corresponding author
Email address: [email protected] (A.M. Laird) Present address: Department of Physics, University of Surrey,U.K. Present address: Rare Isotope Science Project, Institute for Ba-sic Science, Daejeon 34047, Republic of Korea Present address: Physics Division, Oak Ridge National Labora-tory, Oak Ridge TN 37831 USA Present address: Department of Physics, Faculty of Science andEngineering, Konan University, 8-9-1 Okamoto, Kobe, Hyogo 658-8501, Japan Present address: E. A. Milne Centre for Astrophysics, Depart-ment of Physics and Mathematics, University of Hull, HU6 7RX,United Kingdom
1. Introduction
Almost all the elements in the Universe heavier thaniron are produced by neutron-capture reactions, either viathe r-process (rapid neutron capture) or the s-process (slowneutron capture). While significant uncertainties remainin r-process nucleosynthesis, the s-process is consideredgenerally well understood. Here, the neutron flux is suchthat the timescales for neutron capture are longer than theassociated β -decays, and so the path of nucleosynthesis liesclose to the valley of stability. Most s-process elementsbetween iron and strontium are thought to have been pro-duced in massive stars, through the weak s-process, and Preprint submitted to Physics Letters B October 3, 2019 a r X i v : . [ nu c l - e x ] O c t hose between strontium and lead via the main s-processin Asymptotic Giant Branch (AGB) stars [1].However, abundance ratios (e.g. [Y/Ba]) observed inextremely metal poor stars and in one of the oldest globu-lar clusters in the galactic bulge, NGC 6522, cannot be ex-plained by the main s-process or the r-process. Chiappini et al. [2] show that massive rotating stars at low metallic-ity can provide an explanation for the unique abundancesobserved both in the galactic halo and NGC 6522 (see also[3] and [4]). For such stars, rotation-induced mixing isconsidered to have a significant impact on nucleosynthe-sis of light elements, especially at low metallicities [5, 6].S-process abundances depend critically on the presenceof those light elements which can act as neutron sourcesand poisons (isotopes which capture neutrons, thus remov-ing them from contributing to s-process production). Atlow metallicities, the lack of secondary neutron poisons(e.g. N) and the large abundance of primary O re-sults in a high neutron capture rate to O. Thus Ocould act as a poison if these neutrons are not recycled viathe O( α ,n) Ne reaction. This recycling of neutrons isdetermined by competition between the O( α ,n) Ne and O( α, γ ) Ne reactions. However, these reaction rates arehighly uncertain at the relevant energies and the statusof O as a neutron poison, and the impact on s-processabundances, is therefore as yet undetermined.There are two theoretical calculations of the O( α, γ ) Neto O( α ,n) Ne reaction rate ratio. The first, from Caugh-lan and Fowler (CF88) [7], assumes the ratio to be around0.1 at low energies, dropping to 5 × − above about 1MeV. This assumption is based on experimental data onthe O( α, γ ) Ne reaction for the higher energies, and onHauser-Feshbach calculations at lower energies. The sec-ond prediction comes from Descouvemont [8], using theGenerator Coordinate Method, and suggests the ratio tobe of the order of 10 − at all energies. This huge dis-agreement at low energies results in significant differencesin the predicted s-process abundances. Models by Hirschi et al. [6] show the impact of the two different predictionson the abundances of the heavy elements. The variationis particularly marked (up to three orders of magnitude)between strontium and barium.For low metallicity massive stars, s-process nucleosyn-thesis is thought to occur during two stages of evolution,firstly core helium burning and then later shell carbonburning. The temperature for core helium burning is around0.2 - 0.3 GK, corresponding to an energy range of interest(Gamow window) between about 0.3 and 0.65 MeV in thecentre of mass (E cm ). For the onset of carbon shell burn-ing, temperatures are higher at around 0.8 to 1.3 GK,with a Gamow window between E cm = 0.7 to 1.8 MeV.The O( α, γ ) Ne reaction Q-value is 7.348 MeV [9] andthe relevant excited states, shown in Figure 1, lie between7.65 and 8.0 MeV excitation energy (E x ) in Ne for corehelium burning temperatures. However, the required par-tial width and spin-parity information for Ne levels inthe region of interest is poorly known, preventing reliable calculation of the contribution of individual resonances tothe reaction rate.
Figure 1:
Part of Ne level scheme. The Gamow window for the O( α, γ ) Ne reaction during core helium burning in massivestars is indicated by the bar on the left and the bars on theright show the energy regions covered by the present work.
Experimental data on the O( α ,n) Ne reaction areavailable covering the range E cm = 0.56 - 10.1 MeV [10,11, 12, 14], and there is only one published experimen-tal dataset on the O( α, γ ) Ne reaction [13]. Tradition-ally, experimental determinations of such ( α, γ ) reactioncross sections have relied on using an intense beam of α -particles and the detection of γ -rays from de-excitation ofthe products. For the O( α, γ ) Ne reaction, however, thehigh Q-value of the reaction results in the products hav-ing high excitation energies where many nuclear states arepopulated. Clean identification of these states is difficultto extract from the background, particularly at the as-trophysically interesting energies where the yield from thereactions of interest is extremely low, typically less than 1event for every 10 incident α -particles. Despite the ex-perimental challenges, measurements using this techniqueprovided the first direct data on the O( α, γ ) Ne reac-tion. Best et al. [13] measured the O( α, γ ) Ne reactionby in-beam spectroscopy, using a He beam on an im-planted target. The measurements spanned E cm between0.7 and 1.9 MeV but no yield was observed below E cm = 1.1 MeV ( ∼ cm = 0.811 MeV, believed to correspond to a state at8.159(2) MeV. Subsequently Best et al. [14] also stud-ied the competing O( α ,n) Ne reaction across the sameenergy range. Many resonances were observed and fit-ted using an R-matrix framework. Finally, using bothdatasets and estimates for the contribution from lower-2ying states, Best et al. [14] calculated new reaction ratesand concluded that the ( α, γ ) channel is strong enoughto compete with the ( α ,n) channel leading to less efficientneutron recycling. However, neither measurement had suf-ficient sensitivity to provide any experimental data in theenergy region relevant to the s-process during the core he-lium burning stage.
2. Experimental details
Here we report on the first measurement of the O( α, γ ) Nereaction exploiting, instead, a beam of O ions incidenton a helium gas target. The Ne recoils from the reactionexited the target (unlike in the above case) with the unre-acted beam, allowing their detection in coincidence withprompt gamma rays from their de-excitation. The mea-surement was performed at the DRAGON recoil separatorin the ISAC facility, at the TRIUMF Laboratory, Canada,which is specifically designed to study such radiative cap-ture reactions relevant to nuclear astrophysics. It consistsof a windowless recirculating gas target, surrounded byan array of 30 bismuth germanate (BGO) gamma-ray de-tectors, and a two-stage electromagnetic recoil separator.Details of the DRAGON separator are given in Hutcheonet al. [15] and Engel et al. [16].The O beam with a typical current of 600 enA(corresponding to ∼ pps) impinged on thewindowless helium gas target. DRAGON was configuredto transmit 4 + 21 Ne recoils from the O( α, γ ) Ne re-action. These recoils were detected at the focal planeby an ionization chamber (IC). The IC anode consistedof four segments, providing energy loss and residual en-ergy (dE-E) information, and was filled with isobutaneat a typical pressure of 8 Torr. Two micro-channel plate(MCP) detectors upstream of the IC measured the localtime-of-flight (TOF) of the recoils over a distance of 60cm [17]. Recoils were then identified, and distinguishedfrom “leaky” beam transmitted through the separator, bytheir locus on an energy loss-vs-TOF graph, an exampleof which is shown in Figure 2. Further discrimination wasprovided by prompt γ -rays detected in the BGO array incoincidence with events in the IC. The time between theprompt γ -ray detection and subsequent MCP detection al-lowed for a separator TOF measurement, which was usedfor additional particle identification. When the detectionyields were too low to distinguish a clear Ne recoil locus,the profile likelihood technique [18] was used to calculatea confidence interval. In these instances, the MCP andseparator TOF regions of interest were extrapolated fromhigher yield data.For each beam energy delivered, an energy measure-ment was made both with and without target gas present.In combination with the measurements of the gas targetpressure, temperature and the known effective length [19],this allows the stopping power to be calculated. Beamenergy measurement is performed by centering the beam on a set of slits at the energy-dispersed ion-optical fo-cus after the first magnetic dipole field, using an NMRfield read back, where the energy-to-field relationship fora given mass-to-charge ratio has been calibrated by manywell-known, precise nuclear resonances [15, 19]. The beamintensity was measured every hour in three Faraday cups(FC), one located upstream of DRAGON, one after the gastarget and one after the first dipole magnet. Continuousmonitoring of the beam intensity throughout data takingwas achieved via recoiling α -particles, from elastic scatter-ing of the beam on the helium in the target, detected intwo surface-barrier detectors located within the gas targetassembly. These elastic scattering data were normalised tothe measured beam intensity at the start and end of everyrun [20]. Target pressures of between 4 and 8 Torr wereused. The energy loss of the beam, in the center of mass,across the gas target varied from 53 keV at 8 Torr for thelowest energy, to 30 keV at 4 Torr for the highest energy.For the five measurements around the E x = 8.155 MeVstate, target pressures between 4 and 6 Torr were used,with a corresponding center of mass energy loss of 28 to44 keV. dE (channel)
200 400 600 800 1000 1200 1400 M C P T A C ( c hanne l ) Ne RecoilROI E =822 keV cm Figure 2: (Color online) MCP local time of flight TAC (timeto amplitude converter output) versus dE for data at E cm =822 keV. Singles events are indicated in blue and coincidentdata, with a detected gamma-ray energy above 2 MeV, in red.The events in red in the top right of the figure are randomcoicidences between a gamma-ray and a scattered beam ion. At each energy, the raw yields were corrected for theseparator efficiency, the charge state fraction for 4 + re-coils exiting the gas target, the effective efficiencies of theIC and MCP detectors, and the data acquisition deadtime.As γ -ray coincidences were required for particle identifica-tion, the BGO array efficiency was also taken into account.The separator efficiency was determined from Monte Carlosimulations of DRAGON using GEANT3 [21]. For centreof mass energies below ≈ Ne recoil exceeds the DRAGON acceptance of 21mrad. If a resonance is located upstream of the target cen-tre, this limit is reached at higher energies. Similarly, theefficiency of the BGO array [22] depends on the location ofthe reaction in the target. For most energies studied in thiswork neither the width of the resonance, nor the angular3istribution or decay scheme of the subsequent decay of Ne are known, and the measured statistics were too lowto determine these values from the observed γ -ray energiesand distributions. Simulations were, therefore, conductedassuming three decay schemes (direct to ground state,via the 3.74 MeV state, and via the 1.75 and 0.35 MeVstates) and three angular distributions (isotropic, dipole,and quadrupole). For each simulated scenario (reactionlocation in gas target, assumed decay scheme, etc.) thecorresponding separator transmission and BGO detectionefficiencies were extracted, and the differences between thevarious scenarios used to determine the systematic errorson both values.Charge state distributions of Ne were measured atbeam energies of 160, 202, 290, and 360 keV/u and the4 + charge state fraction was estimated using an empiricalformula from [23]. This formula was used to interpolatethe 4 + charge state fraction for each of the recoil ener-gies. The efficiency of the end detectors was taken from acomparison of MCP and IC event rate data using atten-uated beam, together with the geometric transmission ofthe MCP detector grid.The effective cross sections ( σ ) and effective astrophys-ical S-factors ( S ) were then calculated from: σ = N r N b AN t (1) S ( E ) = Ee − πη σ (2)where N r N b is the corrected yield, AN t is the reciprocal targetnuclei per unit area, e − πη is the Gamow factor and E isthe center of target center of mass energy. The resonancestrength of the excitation level of interest was calculatedvia the equation [24] ωγ = 2 π(cid:15) ( E r ) Yλ ( E r ) × (cid:20) arctan (cid:18) E − E r Γ / (cid:19) − arctan (cid:18) E − E r − ∆ E Γ / (cid:19)(cid:21) − (3)where λ is the system’s de Broglie wavelength, (cid:15) is thetarget stopping power, E r is the resonance energy, E isthe initial center of mass energy and ∆ E is the beam en-ergy loss across the entire length of the target. The targetstopping power was calculated from (cid:15) ( E ) = − VN t d E d x (4)where VN t is the reciprocal target density and d E d x is the rateof ion energy loss in the target. For the runs where theresonance was fully contained within the target, the thicktarget yield was used to calculate the resonance strength: ωγ = 2 (cid:15) ( E r ) λ ( E r ) Y max . (5) The stated errors include both systematic and statis-tical uncertainties. The main sources of systematic un-certainty were the BGO detection efficiency (10%), sepa-rator transmission (between 20-30% for the lower energyruns, 2-10% for the 811 keV runs), detector efficiency andtransmission (between 4-5%) and integrated beam inten-sity (between 0.6-6%). Uncertainties in stopping power(3.7%) and recoil charge state fraction (1.6-4.1%) were alsoaccounted for. The range in uncertainies reflects the rangeof beam energies, populated states, recoil angular distri-bution and duration of the runs.
3. Results
Figure 3 shows the measured S-factors at each cen-tre of mass energy for the present work in comparisonwith the calculation from Descouvemont [8]. It should benoted that the direct capture contribution is expected tobe lower than the cross section from Descouvemont, andis thus considered negligible. Data were initially taken atseveral energies above 1 MeV, where the yield is muchhigher, to allow a comparison with the Descouvemont cal-culation. Measurements were then pushed lower towardsthe astrophysically interesting energy range. Table 1 givesthe resonance strengths from the present work, comparedto literature values where available.The data point around 1.1 MeV covers the state at8.470 MeV. A resonance strength of 1.9 ± ± cm = 0.81MeV corresponds to a known J π =(9/2) + state in Ne atan excitation energy of 8.155(1) MeV [25]. This resonanceappears to be of comparable strength in both gamma andneutron channels [10, 14]. The weighted average of thefive highest yield data points, where the resonance is fullywithin the gas target, gives a measured resonance strengthof 5.4 ± ± π = 3/2 + state of total width 8keV [25] at 8.069 MeV (E cm = 0.721 MeV). This statecontributes to both the 0.695 and 0.748 MeV data points,with each measurement covering approximately half of therelevant yield. This resonance has not previously beenobserved in ( α, γ ) and a strength of 8.7 +7 . − . µ eV was found.The quoted uncertainty does not include the uncertaintyon the energy or the width of the state.Between the measurement at 0.695 MeV and the lowestdata point, there is a gap in the measured energy range,from 0.648 to 0.667 MeV, and so no constraint can beplaced on the contribution of the 1/2 − resonance at 0.66MeV (E x = 8.009(10) MeV). However, as this state cor-responds to an f-wave resonance and was observed as aneutron resonance, it is unlikely that this state will playany significant role in the ( α, γ ) rate.The lowest data point measured lies inside the Gamowwindow for core helium burning. Three known states are4 ff e c t i v e Figure 3: (Color online) Effective astrophysical S-factor fromthe present work, compared to the calculation for the O( α, γ ) Ne reaction from [8]. Each data point representsthe energy at the centre of the gas target and the horizontalerror bar corresponds to the energy loss in the target. Targetpressures of between 4 and 8 Torr were used. E CM ωγ (meV) Literature value [13](keV) (meV)633 (4.0 +3 . − . ) × − -721 (8.7 +7 . − . ) × − -810 5.4 ± ± ± ± Table 1: O( α, γ ) Ne resonance strengths from the present workcompared to literature values. covered by the energy thickness of the gas target at thisbeam energy (see Figure 1). Given the low yield, it is notpossible to determine which state dominates and so a com-bined resonance strength of 4.0 +3 . − . µ eV is reported here.This value is a factor of around 10 lower than the 0.03-0.05 meV upper limit given in [13]. The calculations by[14] suggest that the 7.982 MeV level makes the dominantcontribution here and so it is assumed that the observedstrength comes from this state and a resonance energy of0.633 MeV is used in the reaction rate calculation. How-ever, if the full observed strength lies instead in the 0.612MeV resonance, then the calculated reaction rate for theresonance would be 2.25 times higher.
4. Astrophysical impact
Using the narrow resonance formalism, the contribu-tions to the reaction rate from the resonances at 0.633and 0.81 MeV were calculated (the resonance at 0.721MeV contributes less than 10% to the total rate). Thesum of these two contributions (green) is shown in Fig-ure 4, in comparison with the recommended (black) ratefrom Best et al. [14], as a ratio to that of CF88. Thecross section from the present work excludes the predic-tion of Descouvemont [8]. However, the present rate is -1 Temperature (GK) -3 -2 -1 › σ v fi / › σ v fi r e f He-c C-sh O( α,γ ) Ne CF88 (ref) [7] (ref)Best et al. [14]present work
Figure 4: (Color online) Ratio of O( α, γ ) Ne reaction rates toCF88[7]. The lowest curve (green) is from the present work andis a lower limit on the rate (see text). The upper curve (black) isthe recommended rate of Best et al. [14]. The two shaded areaindicate the approximate temperature in the helium burningcore and carbon burning shell of massive stars. still around 100-1000 times lower than that of CF88 [7].It should be noted that within the Gamow window forhelium core burning, there are 6 known states, giving atypical level density of around 1.5 per 100 keV. This iswell below that assumed for a statistical model approachand thus the Hauser-Feshbach treatment of this reactionat low energies used by CF88 [7] may be expected to sig-nificantly overestimate the reaction rate.It must be emphasised that the present rate should beconsidered as a lower limit. There are two known states inthe energy region of interest whose spin and parity are notknown, and none of the states below E x = 7.96 MeV haveexperimentally constrained resonance strengths or partialwidths. Due to a lack of direct experimental data, the con-tribution of these states has not been included here. Therecommended rate from Best et al. [14] includes the con-tributions from 12 resonances which were not observed inthat work, but whose resonance strengths have been calcu-lated based on estimates of the α -particle widths, branch-ing between the γ - and neutron channels, and an assumedspectroscopic factor of 0.01. It is therefore expected thatthe rate from the present work, based only on observedresonances, is significantly lower. However, within theGamow window the difference between the present rateand the recommended rate from Best et al. [14] is dom-inated by the estimated contribution from the resonanceat 0.305 MeV. If this resonance is not as strong as sug-gested then the measured resonance at 0.633 MeV maymake a significant contribution and the O( α, γ ) Ne re-action rate would be closer to the lower limit presentedhere.The reaction rate from the present work was tested ina 25 solar mass stellar model, at a metallicity of Z = 0.001in mass fraction, and with an initial equatorial velocity5
Atomic number (Z) [ X / F e ] CoNiCuZnGaGeAsSeBrKrRbSrY ZrNbMo RuRhPdAgCdInSnSbTeI XeCsBaLaCePrNd SmEuGdTbDyHoErTmYbLuHfTaWReOsIrPtAuHgTlPb
Best et al. [14]Best et al. [14]/10present work
Figure 5: (Color online) S-process yields of a fast rotating 25M (cid:12) at Z=0.001 when using the present rate, the recommendedrate from [14] and recommended rate/10 for the O( α, γ ) Nereaction (see text for further details). of 70% of the critical velocity (the velocity at which thegravitational force balances the centrifugal force). Themodel was computed with the Geneva stellar evolutioncode up to the core oxygen burning stage, with a networkof 737 isotopes, fully coupled to the evolution (details canbe found in [4] and [26]). Figure 5 shows the yields of thismodel (green line) plus two additional models with thesame ingredients except that one is computed with therecommended rate from Best et al. [14] (black line) andthe other with the recommended rate divided by 10 (redline). The latter rate was chosen to illustrate the impactof the 0.305 MeV resonance being weaker than estimated.Significant differences are observed between yields fromthe present rate and the recommended rate above stron-tium. These differences increase at higher atomic masses,with more than a factor of 10 around barium. The newrate leads to results closer to those using the recommendedrate of Best et al. divided by a factor of 10 though thepresent rate leads to still higher production of elementsaround barium. It is clear that the current uncertainty inthe O( α, γ ) Ne reaction rate has a strong impact on thestellar model predictions. It is therefore crucial that, in theabsence of direct measurements, the missing spectroscopicinformation (i.e. spin/parity, reduced energy uncertainty,partial widths) of the relevant states in Ne is determinedto allow the reaction rate to be better constrained.
5. Conclusions
In conclusion, a direct measurement, in inverse kine-matics, of the O( α, γ ) Ne reaction has been performedat the DRAGON facility, at the TRIUMF laboratory, Canada.Measurements were made of the reaction yield in the en-ergy range E cm = 0.6 - 1.6 MeV, providing the only ex-perimental data in the Gamow window for core heliumburning. This work is over an order of magnitude moresensitive than previous work due to the enhanced discrim-ination provided by the coincident detection of both re-coils and γ -rays. Moreover, the event identification does not require prior knowledge of the associated γ -ray ener-gies. The abundances calculated with stellar models usingthe lower limit on the O( α, γ ) Ne reaction rate from thepresent work show the maximum contribution to s-processproduction in low metallicity massive stars.
6. Acknowledgments
We would like to thank the beam delivery and ISACoperations groups at TRIUMF. In particular we gratefullyacknowledge the invaluable assistance in beam produc-tion from K. Jayamanna, for delivering the high intensitybeam. UK personnel were supported by the Science andTechnology Facilities Council (STFC). Canadian authorswere supported by the Natural Sciences and EngineeringResearch Council of Canada (NSERC). TRIUMF receivesfederal funding via a contribution agreement through theNational Research Council of Canada. Authors acknowl-edge support from the ChETEC COST Action (CA16117),supported by COST (European Cooperation in Scienceand Technology). A. Choplin acknowledges funding fromthe Swiss National Science Foundation under grant P2GEP2-184492. RH acknowledges support from the World Pre-mier International Research Center Initiative (WPI Ini-tiative), MEXT, Japan. The Colorado School of Minesgroup is supported via U.S. Department of Energy grantDE-FG02-93ER40789. MP acknowledges support to Nu-Grid from NSF grant PHY-1430152 (JINA Center for theEvolution of the Elements) and STFC (through the Uni-versity of Hulls Consolidated Grant ST/R000840/1), andaccess to viper, the University of Hull High PerformanceComputing Facility. MP acknowledges the support fromthe Lendulet-2014 Programme of the Hungarian Academyof Sciences (Hungary). MP acknowledges support fromthe ERC Consolidator Grant (Hungary) funding scheme(project RADIOSTAR, G.A. n. 724560)
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