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Proton unbound states in 21 Mg and their astrophysical significance

Proton unbound states in 21 Mg and their astrophysical significance. Nova Herculis 1934: AAT. Nova Persei 1901: Herschel. Alex Murphy. X-ray burster in NGC 6624: HST. Motivation. Better knowledge of the level structure of 21 Mg is needed… Astrophysical

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Proton unbound states in 21 Mg and their astrophysical significance

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  1. Proton unbound states in 21Mg and their astrophysical significance Nova Herculis 1934: AAT Nova Persei 1901: Herschel Alex Murphy PROCON 2003 X-ray burster in NGC 6624: HST

  2. Motivation Better knowledge of the level structure of 21Mg is needed… • Astrophysical • Nucleosynthesis and energy generation • X-ray bursts • Novae • Reaction rates dominated by resonant contributions • Nuclear • Proton-rich nuclei far from stability • Large level shifts • Comparison of reaction mechanisms • Shell model studies The Experiment Resonant scattering reaction: 20Na(p,p)20Na (using TUDA at TRIUMF) PROCON 2003

  3. Astrophysical motivation Binary systems • Compact, evolved star (WD or neutron star) orbiting less evolved massive star (e.g. RG) • More massive star expands… • Material flows through L1 • Material is typically hydrogen • Accretion on compact object • Layer of H builds up on top of evolved material (e.g. C/O/…) Artist’s conception of accretion from a red giant on to a compact object PROCON 2003

  4. Astrophysical motivation… Subsequent evolution depends on compact object’s mass and accretion rate. • High rate  ‘stable’ H and He burning and mass increase beyond Chandrasekhar: SN 1a • Lower accretion rate  cataclysmic variable • Neutron star host  X-ray burst • White dwarf host  nova • Nuclear reactions: • explosive hydrogen burning • HCNO, rp-process • (p,g), (a,p) important • Material ejected to ISM HEAO light curve of X-ray burst MXB 1728-34 20Na(p,g)21Mg reaction significance X-ray bursters: a crucial link in the rp-process Novae: affects NeNa cycle. PROCON 2003

  5. 21Mg 22Mg 23Mg 20Na 21Na 22Na 18Ne 18Ne 19Ne 17F 17F 18F 18F 14O 14O 15O 15O 16O 16O 17O 17O Astrophysical significance: X-ray Bursters • T9 0.6 • (a,p) and (p,g) rates overtake b+ decays • Breakout via rp-process begins (required because of waiting points from). • Reaction flow dominated by 15O(a,g)19Ne(p,g)20Na(p,g)21Na… • Triggers subsequent explosion • T9~0.4 • Energy generation by HCNO cycles • Waiting points at 14O, 15O and 18Ne isotopes • Unable to generate required energy output 18Ne 17F 18F 14O 15O 16O 17O 13N 14N 15N 12C 13C PROCON 2003

  6. 21Mg 22Mg 23Mg 20Na 21Na 22Na 18Ne 19Ne 20Ne 17F 18F 14O 15O 16O 17O 13N 14N 15N 12C 13C Reaction path in X-ray burster • T9>0.8 • Alternative breakout path kicks in • Reaction flow dominated by 14O(a,p)17F(p,g)18Ne(a,p)21Na… PROCON 2003

  7. Astrophysical significance: ONeMg Novae • Temperatures achieved are too low for breakout • NeNa and MgAl cycles thought to provide necessary energy production. • NeNa cycle: • First stage is 20Ne(p,g)21Na. • Where does the 20Ne come from? • b-decay of 20Na feeds 20Ne. • Rate of 20Na(p,g) compared to the b+ decay of 20Na (448ms) determines abundance of 20Ne 21Mg 22Mg 23Mg 20Na 21Na 22Na 23Na 19Ne 20Ne NeNa cycle PROCON 2003

  8. Nuclear significance Specific case • Unusual level shifts observed in previous measurements • Thomas – Ehrman shift. Breaking of isospin symmetry due to Coulomb induced differences in proton and neutron distributions. • Selectivity of 24Mg(3He,6He)21Mg measurement. • could mean there are other states so far unobserved Wider case • Few studies on proton rich nuclei in this mass region • Level shift in p-rich nuclei • Comparison of reaction models • our (p,p) vs (3He,6He) • Shell model… • poor performance near drip lines (M. Horoi, private comm.). PROCON 2003

  9. Previous knowledge of 21Mg • Above proton threshold, only data are Kubono et al. (NPA 537 (1992) 53) • 24Mg(3He,6He)21Mg PROCON 2003

  10. Previous knowledge of 21Mg • Ex uncertain to ~15 keV • L from fits to DWBA. J then from comparison with 21F • Parity by (–1)L • Widths calculated (from Kubono et al., Alburgeret al. PRC 23 (1981) 2217 and Wiescher et al. A&A 160 (1986) 56) PROCON 2003

  11. 20Na(p,p)20Na Experimental details Thick target method: Scan through region of excitation in 21Mg to look for resonances. • Primary beam: 20 mA, 500 MeV, protons • Secondary beam: 20Na from ISAC • intense sodium beam surface ion source,  SiC primary target • Currents up to 5x107 pps achieved • 3.50 < Ex (21Mg) < 4.64 MeV • 2 energy settings • Target: 795 mg/cm2 CH2 foil • Detectors: 1.5 x LEDA (130° < qcm< 170°) TUDA PROCON 2003

  12. Experimental set-up 9.5 or 13 mm Mylar High sensitivity Faraday cup 6 or 9.5 mm Mylar 800mg/cm2 CH2 LEDA Recoil proton LEDA ~1 epA 20Na • 1.25 MeV/u • 1.60 MeV/u 60.5 cm 4.7o < qlab <33.7o 19.5 cm PROCON 2003

  13. The run: Successful experiment ran in November 2002 • 5 days of stable 20Ne calibration beams • 7 days of radioactive 20Na beams: up to 5x107 pps. • Expect Rutherford + resonances. • Resonance depends on Ex, Gp and Ltr • Interference • Two–body kinematics, low energy loss of protons in target • For a selected angle  energy of detected protons reflect the energy the reaction occurred at. • Hence, proton spectrum is just an excitation function. PROCON 2003

  14. Data… • Three resonances observed • Ex(21Mg) = 4.01MeV Primary aim of the experiment • Ex(21Mg) = 4.26 MeV Previously only Ex known (no width, spin information) • Ex(21Mg) = 4.44 MeV (approx) Previously unknown  R-matrix Analysis PROCON 2003

  15. Other recent data taken with TUDA 21Na(p,p)21Na 22Na observation in novae • 21Na(p,g)22Mg(b+)22Na*(1.275 MeV g-ray) • observability in, e.g., INTEGRAL (launched Oct ’02) Synthesis of 22Na in ONe novae • 20Ne(p,g)21Na(p,g)22Mg(b+)22Na or 20Ne(p,g)21Na(b+)21Ne(p,g)22Na Ph.D. Thesis (Edinburgh, 2003) PRC 65 Chris Ruiz et al. (2001) 042801(R) PROCON 2003

  16. Future directions: • Resonant scattering: a powerful technique • Requires beam development • Limited to states of width > few keV • Other channels • (p,a), (a,p), (d,p)… • Many of these are directly of astrophysical significance • Maybe used as indirect probes of low cross-section reactions • 2 proton unbound states? • Facility development • Driven by the physics programme • Source development • Crucial to future programme • ECR / Laser ion sources • Accelerator development • Charge state booster: A < 30  A < 60 • ISAC II: up to 6.5 MeV/u, A < 150 PROCON 2003

  17. Many thanks to… M.Aliotta, T.Brown, L.Buchmann, T.Davinson, J.D’Auria, M.Eaton, S.Engel, B.Fulton, D.Groombridge, D.Hunter, D.Hutcheon, A.Laird, J.Pearson, R.Pitkin, I.Robinson, J.Rogers, F.Sarazin, P.Walden, P.Woods Edinburgh TRIUMF York SFU Langara PROCON 2003

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