Bentley, M.

Brown, J.

Buchmann, L.

Chen, A. A.

Chen, J.

Chipps, K.

D'Auria, J.

Davids, B.

Davis, C.

Diget, C. A.

Erikson, L.

Fallis, J.

Fox, S. P.

Fulton, B. R.

Galinski, N.

Greife, U.

Hager, U.

Herwig, F.

Hirschi, R.

Howell, D.

Hutcheon, D.

Laird, A.

Martin, L.

Mountford, D.

Murphy, A.

Ottewell, D. F.

Pignatari, M.

Reeve, S.

Ruiz, C.

Ruprecht, G.

Sjue, S.

Taggart, M.

Veloce, L.

An experimental campaign to perform the first accurate measurement of the 17O(a,g)21Ne reaction rate was carried out during May and November 2009 at the DRAGON recoil separator in the TRIUMF laboratory, Vancouver, Canada. The goals were to differentiate between two previous conflicting theoretical predictions, and to establish the (a,g)/(a,n) reaction rate ratio.Two distinct energy regions were scanned: a higher region, Ecm 1.5 MeV, where the recoil cone was sufficiently confined and the cross section sufficiently large so that a statistically significant yield could be achieved, and a lower region, Ecm 0.7 MeV, chosen to approach the astrophysicalenergies and reduce errors from extrapolation. The experiment, performed in inverse kinematics, used the highest intensity beam ever deliveredto DRAGON, 11012 pps of 17O incident on a 4He gas target. An array of 30 Bismuth Germanate (BGO) detectors [1] was used in conjunction with two microchannel plates (MCP) [2] for a local time-of-flight measurement, and ion chamber for coincident event detection

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     Edinburgh Research Explorer                                      The first direct measurement of 17O(,)21Ne and its impact upons-process abundancesCitation for published version:Taggart, M, Hager, U, Laird, A, Ruiz, C, Hutcheon, D, Ottewell, DF, Fallis, J, Erikson, L, Bentley, M, Brown,J, Buchmann, L, Chen, AA, Chen, J, Chipps, K, D'Auria, J, Davids, B, Davis, C, Diget, CA, Fox, SP, Fulton,BR, Galinski, N, Greife, U, Herwig, F, Hirschi, R, Howell, D, Martin, L, Mountford, D, Murphy, A, Pignatari,M, Reeve, S, Ruprecht, G, Sjue, S & Veloce, L 2010, 'The first direct measurement of 17O(,)21Ne and itsimpact upon s-process abundances' Paper presented at 11th Symposium on Nuclei in the Cosmos, NIC XI,Heidelberg, Germany, 19/07/10 - 23/07/10, pp. 1-45.Link:Link to publication record in Edinburgh Research ExplorerDocument Version:Peer reviewed versionGeneral rightsCopyright for the publications made accessible via the Edinburgh Research Explorer is retained by the author(s)and / or other copyright owners and it is a condition of accessing these publications that users recognise andabide by the legal requirements associated with these rights.Take down policyThe University of Edinburgh has made every reasonable effort to ensure that Edinburgh Research Explorercontent complies with UK legislation. If you believe that the public display of this file breaches copyright pleasecontact openaccess@ed.ac.uk providing details, and we will remove access to the work immediately andinvestigate your claim.Download date: 28. Apr. 2017PoS(NIC XI)045The first direct measurement of 17O(α,γ)21Ne and itsimpact upon s-process abundancesM. Taggart∗a,1, U. Hager2, A.M. Laird1, C. Ruiz2, D. Hutcheon2, D. Ottewell2, J. Fallis2,L. Erikson3,4, M. Bentley1, J.R. Brown1, L. Buchmann2, A.A. Chen5, J. Chen5,K.A. Chipps1, J. D’Auria2, B. Davids2, C. Davis2, C.Aa. Diget1, S.P. Fox1, B. Fulton1,N. Galinski2, U. Greife3, F. Herwig6, R. Hirschi7, D. Howell2, L. Martin2,D. Mountford8, A. Murphy8, M. Pignatari2,6,9, S. Reeve2, G. Ruprecht2, S. Sjue2,L. Veloce2aE-mail: mpt504@york.ac.uk1Department of Physics, University of York, Heslington, York, UK2TRIUMF Laboratory, Vancouver, BC, Canada3Colorado School of Mines, Golden, CO, USA4Pacific Northwest National Lab, Richland, WA, USA5McMaster University, Hamilton, ON, Canada6University of Victoria, Victoria, BC, Canada7University of Keele, Keele, UK8School of Physics and Astronomy, University of Edinburgh, Edinburgh, UK9Joint Institute for Nuclear Astrophysics, University of Notre Dame, IN, USAAn experimental campaign to perform the first accurate measurement of the 17O(α ,γ)21Ne reac-tion rate was carried out during May and November 2009 at the DRAGON recoil separator in theTRIUMF laboratory, Vancouver, Canada. The goals were to differentiate between two previousconflicting theoretical predictions, and to establish the (α ,γ)/(α ,n) reaction rate ratio.Two distinct energy regions were scanned: a higher region, Ecm ∼ 1.5 MeV, where the recoil conewas sufficiently confined and the cross section sufficiently large so that a statistically significantyield could be achieved, and a lower region, Ecm ∼ 0.7 MeV, chosen to approach the astrophysicalenergies and reduce errors from extrapolation.The experiment, performed in inverse kinematics, used the highest intensity beam ever deliveredto DRAGON, 1×1012 pps of 17O incident on a 4He gas target. An array of 30 Bismuth Germanate(BGO) detectors [1] was used in conjunction with two microchannel plates (MCP) [2] for a localtime-of-flight measurement, and ion chamber for coincident event detection.11th Symposium on Nuclei in the Cosmos, NIC XIJuly 19-23, 2010Heidelberg, Germany∗Speaker.© Copyright owned by the author(s) under the terms of the Creative Commons Attribution-NonCommercial-ShareAlike Licence. http://pos.sissa.it/PoS(NIC XI)045The first direct measurement of 17O(α ,γ)21Ne M. Taggart1. IntroductionAbove the iron peak it becomes energetically unfavourable to produce more massive nucleithrough the process of nuclear fusion [3][4]. Most of the heavy elements beyond iron that weobserve in the solar system have been produced by the slow neutron capture process (or s process)and by the rapid neutron capture process (or r process) [5]. In massive stars, the so-called “weak”component of the s-process is responsible for the production of heavy isotopes up to the strontium-zirconium peak. In rotating models at low metallicity, the high neutron to seed ratio allows theproduction of isotopes beyond the Sr-Zr peak, up to barium depending on the neutron flux. Onecritical source of uncertainty for the abundance of these isotopes is the impact of parasitic lightneutron absorbers.In the specific case of massive rotating stars at low metallicity, 16O is potentially a significantneutron poison [6]. Its impact as a neutron poison depends upon the 17O(α ,γ)/17O(α ,n) reactionrate ratio: the (α ,n) reaction allows the neutrons to be recycled and be available to the s-processnucleosynthesis later on. The competition from the (α ,γ) channel reduces the neutron abundanceavailable for s-process nucleosynthesis.Prior to this work, experimental data were sparse. Pre-existing unpublished data on the17O(α ,n)20Ne reaction went as low as Ecm ∼ 0.5 MeV. However, the strength of the compet-ing 17O(α ,γ)21Ne channel is less well known, with no experimental data and conflicting pre-dictions from the two existing theoretical models. Work by Descouvemont [7] predicts that the17O(α ,γ)21Ne reaction should be 104 times weaker than the competing (α ,n) reaction. On the otherhand, an earlier prediction by Caughlan and Fowler [8] suggests only a factor of 10 should separatethe channels, resulting in much lower s-process abundances between strontium and barium. Theeffect on the final s-process yields using the two different rates mentioned above for the 17O(α ,γ)is shown in figure 1. The stellar model and the rest of the nuclear network are the same for the twocases [6].Figure 1: Comparison of the elemental abundances that result from Descouvemont and CF88 predictionstaken from [6]2PoS(NIC XI)045The first direct measurement of 17O(α ,γ)21Ne M. Taggart2. Experimental MethodAn experimental campaign to study the 17O(α ,γ)21Ne reaction in inverse kinematics was car-ried out in May and November 2009. The experiment used an 17O beam accelerated to selectedenergies over the range, Ecm ∼ 0.621-1.597 MeV. The lower of these energies approaches the re-gion of astrophysical interest of Ecm ∼ 0.3-0.5 MeV. The experiment was carried out at TRIUMFin Vancouver, Canada, using the DRAGON recoil separator [9] in the ISAC I experimental hall.An 17O3+ beam was delivered from the Offline Ion Source (OLIS) and accelerated to thedesired energy. The resulting maximum beam intensity of 1×1012 pps was the most intense beamever delivered to DRAGON. The beam intensity was recorded through hourly Faraday cup readingsat several points along the separator, and was continually monitored by elastic scattering within theDRAGON gas target by two surface barrier detectors, positioned at 30° and 57°.The beam was delivered to the DRAGON windowless gas target filled with 4He with differentpressures used in order to vary the target thickness, although the standard setting was 4 Torr. Thewindowless gas target uses a differential pumping system to maintain a sharp density profile despitethe lack of entrance or exit windows, thus minimising the effect of beam straggling.Several methods of particle identification were utilised for the experiment. The end recoildetector, an ion chamber (IC) [10] comprising 4 anodes, provided ∆E-E information. Correlatingthe arrival time of an ion in the IC with a corresponding γ ray detected with the BGO array resultsin additional beam suppresion. In addition, two MCP detectors, located 60 cm apart, provided alocal time-of-flight (ToF) measurement which could be combined with the IC information. Theresulting IC energy Vs MCP ToF spectra became those primarily used in the following analysis.The different length anodes of the IC allowed for a ∆E-E method to be used when the separation inthe ICsum (total energy deposited in the IC) Versus MCP ToF spectrum was not sufficient. Usingdifferent combinations of anodes, various ∆E thicknesses could be used to obtain the optimumseparation. The relative timing signals of γ and ion signals for coincident events were also used forparticle identification.An example of the IC energy versus MCP time of flight particle ID analysis is shown in figure2. The 21Ne nuclei can clearly be separated from those beam particles that have not been removedby the DRAGON separator.3. Results and DiscussionWhile the results presented in this work are preliminary, the experimental analysis is near-ing completion. Calculation of the raw yields is complete, from these the absolute cross sectionsand astrophysical S-factors can then be derived. The S-factor is useful in that it removes the de-pendency on the non-nuclear component, allowing for a more accurate extrapolation down to thelower energies, whereas the cross section tails off exponentially with decreasing energy, makingextrapolation unreliable.We have observed a new, strong resonant structure in the 17O + α system at Ecm ∼ 0.8 MeV,this can be seen in figure 3. This structure is under analysis; it should be noted that in the (α ,n)data [11], there are two states in this region, which do not correspond to known states in 21Ne. Thedata suggests that the relative strengths of the (α ,n) and (α ,γ) channels are comparable. This is3PoS(NIC XI)045The first direct measurement of 17O(α ,γ)21Ne M. TaggartFigure 2: Particle ID example for runs 20146-48, Ecm ∼ 0.823 MeV. Blue events show gamma-ray co-incidences, red depicts single IC hits and the green polygon is the cut applied should further analysis berequired.Figure 3: Plot of raw yields from the experiments performed in May and November 2009. It should benoted that the line is a guide to the eye and not in fact a fit of the data points.4PoS(NIC XI)045The first direct measurement of 17O(α ,γ)21Ne M. Taggartsurprising given that the ratio of the (α ,γ) to (α ,n) channels at the higher energies suggested thatthe (α ,γ) yield should be orders of magnitude lower than observed. This structure was observed ontwo separate occasions, in both the May and November experimental runs.Establishing the S-factor at the relevant energies is not the end of the work. These S-factorscan then be used with astrophysical reaction rate codes to see the effect that this new (α ,γ)/(α ,n)ratio has on the later s-process abundances, specifically in the relevant region from Sr to Ba.In order to do this the cross-sections will be required across the full temperature range of thestar, which for the He-burning stage of stellar evolution means 0.1 GK and above. This correspondsto an energy of Ecm ∼ 0.247 MeV, below present realistic detection limits. For comparison, theenergy range measured during the experimental campaign, Ecm ∼ 0.621 - 1.597 MeV, correspondsto stellar temperatures of 0.40 - 1.65 GK respectively. As a result some extrapolation will berequired in order to establish cross sections at these very low, astrophysically relevant energies.4. AcknowledgementsThe authors would like to thank all those who took part in the experiment, particularly theTRIUMF staff, specifically the OLIS experts and ISAC operators who made the experiment workdespite the short notice. Thanks should also be given to the UK STFC funding council, the Cana-dian NSERC DRAGON research grant and M.P. and F.H. acknowledge support from NSF grantsPHY 02-16783 (JINA) and EU MIRG-CT-2006-046520.References[1] D. Hutcheon et al. The DRAGON facility for nuclear astrophysics at TRIUMF-ISAC: design,construction and operation. Nucl. Instr. and Meth. A, 498:190–210, 2003.[2] M.J. Lamey. A microchannel detection system for DRAGON. Master’s thesis, Simon FraserUniversity, 2004.[3] Claus E. Rolfs and William S. Rodney. Cauldrons in the Cosmos. The University of Chicago Press,1988.[4] C. Iliadis. Nuclear Physics of Stars. Wiley, 2007. Chapter 6.[5] E. Margaret Burbidge, G. R. Burbidge, William A. Fowler, and F. Hoyle. Synthesis of the elements instars. Reviews of Modern Physics, 29:547–650, 1957.[6] R. Hirschi et al. Nugrid: s process in massive stars. Proceedings of the 10th Symposium on Nuclei inthe Cosmos, July 27 - August 1 2008, Mackinac Island, Michigan, USA, PoS 083, (2008)[7] P. Descouvemont. Microscopic three-cluster study of 21-nucleon systems. Physical Review C,48:2746, 1993.[8] G. R. Caughlan and W. A. Fowler Thermonuclear Reaction Rates V. Atomic Data and Nuclear DataTables, 40:283-334, 1988.[9] J.M. D’Auria et al. Astrophysics with a DRAGON at ISAC. Nuclear Physics A, 701:625–631, 2002.[10] C. Vockenhuber et al. Improvements of the DRAGON recoil separator at ISAC Nucl. Instr. and Meth.B, 266:4167-4170, 2008.[11] European compilation of reaction rates for astrophysics.http://pntpm.ulb.ac.be/Nacre/nacre.htm. August 18, 20105

The first direct measurement of 17O(α,γ)21Ne and its impact upon s-process abundances

The first direct measurement of 17O(α,γ)21Ne and its impact upon s-process abundances

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