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					                  Proton unbound states in 21Mg and
                    their astrophysical significance
                                                                      Nova Herculis 1934: AAT




    Nova Persei 1901: Herschel

                                                                  Alex Murphy




                                 X-ray burster in NGC 6624: HST
Alex Murphy                           PROCON 2003                                        1
              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)

Alex Murphy                                      PROCON 2003              2
                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                Artist’s
                                                             conception
       material (e.g. C/O/…)                                 of accretion
                                                             from a red
                                                             giant on to a
                                                             compact object



Alex Murphy                                    PROCON 2003              3
                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                                HEAO light curve of X-ray burst MXB 1728-34
             (p,g), (a,p) important
      Material ejected to ISM
                                  20Na(p,g)21Mg  reaction significance
                            X-ray bursters: a crucial link in the rp-process
                                    Novae: affects NeNa cycle.

Alex Murphy                                     PROCON 2003                                         4
               Astrophysical significance: X-ray Bursters

   T9~0.4                                    T9  0.6
         Energy generation by HCNO                (a,p) and (p,g) rates overtake b+ decays
          cycles                                   Breakout via rp-process begins (required because
         Waiting points at 14O, 15O and            of waiting points from).
          18Ne isotopes
                                                   Reaction flow dominated by
         Unable to generate required               15O(a,g)19Ne(p,g)20Na(p,g)21Na…
          energy output                            Triggers subsequent explosion
                          18Ne
                                                                         21Mg   22Mg   23Mg



                          17F    18F
                                                                         20Na   21Na   22Na



              14O   15O    16O   17O
                                                                  18Ne   19Ne



              13N   14N    15N
                                                                  17F    18F



              12C   13C
                                                      14O   15O    16O    17O




Alex Murphy                                    PROCON 2003                                        5
              Reaction path in X-ray burster


       T9>0.8
                                                               21Mg   22Mg   23Mg
          Alternative breakout path kicks in
          Reaction flow dominated by                          20Na   21Na   22Na
            14O(a,p)17F(p,g)18Ne(a,p)21Na…

                                                        18Ne   19Ne   20Ne




                                                        17F    18F




                                            14O   15O    16O    17O



                                           13N    14N    15N




                                           12C    13C




Alex Murphy                                PROCON 2003                              6
                  Astrophysical significance: ONeMg Novae

         Temperatures achieved are too low
          for breakout
                                                             21Mg   22Mg   23Mg
                 NeNa and MgAl cycles thought to
                  provide necessary energy production.
                                                             20Na   21Na   22Na   23Na
         NeNa cycle:
                 First stage is 20Ne(p,g)21Na.              19Ne   20Ne
                                                                              NeNa cycle
                     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




Alex Murphy                                    PROCON 2003                                 7
               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.).



Alex Murphy                                      PROCON 2003                              8
              Previous knowledge of 21Mg

        Above proton threshold, only data are Kubono et al. (NPA 537 (1992) 53)
          
              24Mg(3He,6He)21Mg




Alex Murphy                             PROCON 2003                                9
                 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., Alburger et al. PRC
         23 (1981) 2217 and Wiescher et al. A&A 160 (1986) 56)

     Ex (MeV)       Er(MeV)            Jp           Gp (eV)
         3.086        -0.130       (3/2+, 5/2+)
         3.244        0.028        (3/2+, 5/2+)     3.04x10-19
         3.347        0.131          (7/2+)         1.23x10-6
         3.643        0.427        (7/2+,9/2+)      1.58x10-4
         3.752        0.536        (1/2-,3/2-)        99.4
         3.901        0.685          (7/2+)           0.872
         4.010        0.794          (1/2+)           4720
         4.261        1.045             ?               ?
         4.824        1.608             ?               ?




Alex Murphy                                       PROCON 2003         10
              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                                                     TUDA

     Target: 795 mg/cm2 CH2 foil
     Detectors: 1.5 x LEDA (130° < qcm<
      170°)



Alex Murphy                                PROCON 2003                                     11
              Experimental set-up
                                         9.5 or 13 mm Mylar




                     6 or 9.5 mm Mylar                                 High sensitivity
                                                                         Faraday cup



                 800mg/cm2 CH2
                                               Recoil proton   LEDA




                                          LEDA
   ~1 epA 20Na
      • 1.25 MeV/u
      • 1.60 MeV/u
                                                                4.7o < qlab < 33.7o



Alex Murphy                                PROCON 2003                                    12
              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.




Alex Murphy                                 PROCON 2003                     13
                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
Alex Murphy                                   PROCON 2003                                      14
                 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)


Alex Murphy                                        PROCON 2003   15
                  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




Alex Murphy                                       PROCON 2003                          16
              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




Alex Murphy                            PROCON 2003                             17

				
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posted:9/19/2012
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