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Astrophysically Important Reaction Rates For Novae And Xray Bursts From Proton Breakup At Intermediate Energies A. Banu, L. Trache, F. Carstoiu, N. A. Orr, N. L. Achouri et al. Citation: AIP Conf. Proc. 1304, 339 (2010); doi: 10.1063/1.3527219 View online: http://dx.doi.org/10.1063/1.3527219 View Table of Contents: http://proceedings.aip.org/dbt/dbt.jsp?KEY=APCPCS&Volume=1304&Issue=1 Published by the American Institute of Physics.

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Astrophysically Important Reaction Rates For Novae And X-ray Bursts From Proton Breakup At Intermediate Energies A. Banua,b, L. Trachea,b, F. Carstoiub, N. A. Orrc, N. L. Achouric, A. Bonaccorsod, W. N. Catforde, M. Chartierf, B. Fernandez-Dominguezf, M. Freerg, L. Gaudefroyh, M. Horoii, M. Labichej,k, B. Laurentc, R. C. Lemmonk, F. Negoitab, S. Paschalisf, N. Pattersone, B. Pietrasf, B. Roederc,a, F. Rotarub, P. Roussel-Chomazh, E. Simmonsa, J. S. Thomase, R. E. Tribblea a Cyclotron Institute, Texas A&M University, College Station, Texas 77843-3366, USA National Institute of Physics and Nuclear Engineering, “Horia Hulubei” (IFIN-HH), R-077125 Magurele-Bucharest, Romania c Laboratoire de Physique Corpusculaire, IN2P3-CNRS, ISMRA et Université, F-14050 Caen, France d Instituto Nazionale di Fisica Nucleare, Sez. Di Pisa, I-56127 Pisa, Italy e Department of Physics, University of Surrey, Guildford GU2 5XH, UK f Oliver Lodge Laboratory, University of Liverpool, Liverpool L69 7ZE, UK g School of Physics and Astronomy, University of Birmingham, Birmingham B15 2TT, UK h Grand Accélérateur d’Ions Lourds, BP 55027, 14076 Caen Cedex 5, France i Department of Physics, Central Michigan University, Mount Pleasant, Michigan 48859, USA j Department ot Engineering and Science, University of the West of Scotland, Paisley PA1 2BE, UK k Nuclear Physics Group, STFC Daresbury Laboratory, Daresbury, Warrington, WA4 4AD, UK b

Abstract. We discuss the use of one-nucleon removal reactions of loosely bound nuclei at intermediate energies as an indirect method in nuclear astrophysics. The breakup reactions are proved to be good spectroscopic tools and can be used to study a large number of loosely bound proton- or neutron-rich nuclei over a wide range of beam energies. As peripheral processes, they can be used to extract asymptotic normalization coefficients (ANCs) from which non-resonant capture reaction rates of astrophysical interest can be calculated parameter free. In this talk, we present results of a proton-breakup experiment carried out at GANIL (France) with a cocktail beam centered around 23Al at 50 MeV/nucleon. Momentum distributions of the breakup fragments, inclusive and in coincidence with gamma rays detected by EXOGAM Germanium clover array, were measured in the focal plan of SPEG energy-loss spectrometer. We present in particular the investigations of reaction rates for 22Mg(p,γ)23Al and 23Al(p,γ)24Si important for novae and X-ray bursts, respectively. Keywords: one-proton removal, configuration mixing, spectroscopic factor, ANC, 23Al, 24Si PACS: 26.30.-k, 25.60.Tv, 21.10.Jx, 21.10.Hw

INTRODUCTION The proton-rich near drip line nucleus 23Al is an appealing nucleus to be studied CREDIT LINE (BELOW) TO BE structure INSERTED THE FIRST PAGE The OF EACH PAPER rising questions to both nuclear andON nuclear astrophysics. questions for CP1304, Carpathian Summer School of Physics 2010 edited by L. Trache, S. Stoica, and A. Smirnov © 2010 American Institute of Physics 978-0-7354-0859-3/10/$30.00

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nuclear structure are related to the ground state spin-parity of 23Al, information that until recently [1] was still missing generating controversies on a possible exotic proton-halo structure in 23Al [2]. The questions for nuclear astrophysics are related to the so-called “22Na puzzle”. Gamma-ray space-based telescope have shown the ability to detect γ-ray lines of cosmic origin (see the famous case of 26Al [3]) providing us with direct and strong evidence that nucleosynthesis is ongoing in our Galaxy. Among gamma-ray emitters of cosmic origin is also 22Na, sought to be synthesized in ONe novae [4]. Once ejected, 22Na could be observable by the characteristic 1.275 MeV γ ray following its β+ decay (2.6 yr half-life) [5]. In spite of considerable efforts undertaken in the search for a galactic 1.275 MeV γ line, so far only upper limits could be derived. To solve “this puzzle”, it has been proposed that 22Na itself and/or its precursor 22Mg could be depleted by 22Na(p,γ)23Mg and 22Mg(p,γ)23Al. The 22Mg(p,γ)23Al is dominated by non-resonance capture to the ground state of 23 Al and by resonant capture to its first excited state. We studied here the former component. With a fairly small proton-separation energy of Sp = 141.11(43) keV (most accurate value to date) [6], the yield of 23Al can be easily removed by photodisintegration at nova temperature of 0.2-0.4 billion degree Kelvin. However, it was pointed out that despite the strong photodisintegration of 23Al a considerable 2pcapture flow through 22Mg, leading to 24Si via 23Al(p,γ)24Si, could be established via proton capture on the small 23Al equilibrium abundance [7]. This mechanism could deplete 22Mg and thus could reduce the amount of 22Na produced in ONe novae, which would be in better agreement with observations. Evaluation of 22Mg(p,γ)23Al and 23 Al(p,γ)24Si reaction rates are necessary to clarify this questions. We present here the results of the rates of the corresponding non-resonant radiative components of the aforementioned capture reactions carried out via one-proton removal at intermediate energies. The parallel momentum distributions of the core breakup fragments measured in one-nucleon removal reactions are the spectroscopic tools available to determine the single-particle structure of loosely bound exotic nuclei, due to their sensitivity to the angular momentum of the orbital involved. From the shape of the momentum distributions, one can determine the quantum numbers of the single-particle orbital, and from the integrated total and partial cross sections one is able to extract the corresponding ANCs of the wave function’s components [8]. Hence, with the ANC from one-proton removal reactions, one can evaluate astrophysical S factor, information needed to determine astrophysical reaction rates for radiative proton captures (p,γ) that are outside the reach of other direct or indirect methods such as the ANC method involving transfer reactions [9]. Unlike the ANC method which requires radioactive beams of very good purity and intensity, this new method, one-nucleon removal at intermediate energies, has also the advantage that can be used for beams of low quality, such as cocktail beams, and intensities as low as a few pps, as was the case of our experiment at GANIL discussed next.

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EXPERIMENT We present the E491 experiment carried out at GANIL that investigated reactions produced by a cocktail beam around 23Al at 50 MeV/nucleon. A primary beam of 32S at 95 MeV/nucleon impinged on a thick carbon target. The SISSI device coupled to the beam analysis spectrometer tuned for a magnetic rigidity of 1.95 Tm was used to separate the 14 secondary ion species produced in this experiment. They impinged on a secondary carbon target (chosen to minimize the Coulomb dissociation contributions) positioned at the entrance of the energy-loss spectrometer SPEG. In the focal plane of SPEG the momentum distributions of the breakup fragments of interest were measured on a event-by-event basis using two large-area tracking drift chambers. An ionization chamber and a thick plastic scintillator detector were used to identify the breakup fragments. EXOGAM detectors were position around the reaction target to detect the gamma lines de-populating the core fragments left in excited states after one-proton removal reaction occurred. Momentum distributions of the core breakup fragments, inclusive and in coincidence with γ rays, were measured. The experimental setup is illustrated in Figure 1 (a), while (b) shows the composition of the secondary cocktail beam along with the production rates for some of the 14 ion species, among them the two nuclei under investigation here: 24Si produced with an intensity of about 30 pps and 23Al produced with an order of magnitude more intense beam.

a)

b)

FIGURE 1. (a) Experimental setup for the E491 run at GANIL (see text for details); (b) Composition of the secondary cocktail beam: 14 species were produced with rates from 30 pps (24Si) to 300 pps (23Al) or as high as 7000 pps (21Na).

RESULTS Here we present results for the structures of 24Si and 23Al. In Figure 2 (a), we present 23Al one-proton removal inclusive differential cross-section in the center-ofmass reference compared with theoretical calculations performed with an extended version of the Glauber model with second-order noneikonal corrections [9]. Double folding semi-microscopic potential proton-target and core-target were calculated using

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the JLM effective nucleon-nucleon interaction. These potentials were used to calculate the scattering matrix elements needed to describe the nuclear breakup process. We obtained that the measured momentum distributions for the 22Mg core fragments agree in shape with calculations that involve a 1d5/2 orbital (red curve in Figure 2 (a)) at variance with calculations done with 2s1/2 orbital (blue curve in Figure 2 (a)). This confirms the ground state spin-parity for 23Al to be J π = 5 / 2 + , and not 1/2+ , a relevant distinction for the rate of the astrophysical radiative capture 22 Mg(p,γ)23Al, and in agreement with the results from beta decay and magnetic moment measurements for 23Al [1].

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Al breakup (inclusive)

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(a) FIGURE 2. 23Al one-proton removal: (a) Experimental inclusive momentum distribution for 22Mg determined in the center-of-mass reference (see text for details); (b) Configuration mixing measured for 23 Al ground state structure; Doppler-corrected gamma-ray spectrum in coincidence with identified 22Mg residues in SPEG (see text for details).

By measuring the γ-ray decays of excited states in 22Mg residues after the oneproton removal occurred, we were able to disentangle for the first time the configuration mixing structure of 23Al ground state. Figure 2 (b) shows the gamma lines detected by EXOGAM in coincidence with 22Mg fragments. The spectrum is corrected for the Doppler shift and an add-back procedure was applied event-by-event. We observed clearly three gamma lines: 1247 keV ( 21+ → 0 +gs ), 2061 keV ( 41+ → 21+ ), and the unexpected 1985 keV ( 4 2+ → 41+ ). Although the 23Al configuration mixing consists of four configuration, the only one relevant for the 22Mg(p,γ)23Al reaction in stars corresponds to the 22Mg ground state. The momentum distribution corresponding to 22Mg ground state was determined by subtracting the measured exclusive momentum distributions from the measured inclusive distribution.

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24Si breakup 0.3 data

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FIGURE 3.

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Si one-proton removal cross section determined in the center-of-mass frame compared with theory.

We did not observe γ ray lines from 24Si one-proton removal reaction, implying that the ground state of 24Si is built upon a configuration in which 23Al core is left in its ground state taking into account the fact that there are not bound excited states in 23 Al. Figure 3 shows the corresponding momentum distribution for 24Si breakup. By comparison data-theoretical calculations for breakup cross sections, one can extract the corresponding ANCs, and from them the S factors and the astrophysical reaction rates are evaluated. Details about these evaluations can be found here [10].

ACKNOWLEDGMENTS This work was supported in part by the US DOE under Grant DE-FG0293ER40773 and the Robert A. Welch Foundation under Grant A-1082.

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