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Transfer Reactions with Halo Nuclei

Transfer Reactions with Halo Nuclei. Barry Davids, TRIUMF ECT* 2 Nov 2006. S 17 (0): Remaining Issues. Cyburt, Davids, and Jennings examined theoretical and experimental situation in 2004 Extrapolation is model-dependent

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Transfer Reactions with Halo Nuclei

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  1. Transfer Reactions with Halo Nuclei Barry Davids, TRIUMF ECT* 2 Nov 2006

  2. S17(0): Remaining Issues • Cyburt, Davids, and Jennings examined theoretical and experimental situation in 2004 • Extrapolation is model-dependent • Even below 400 keV, GCM cluster model of Descouvemont and potential model based on 7Li + n scattering lengths differ by 7%

  3. Extrapolation

  4. The Data

  5. Concordance? • Using a “pole” model, fit radiative capture data below 425 keV • Allows data to determine shape, consistent with cluster and potential models • Junghans et alia result: 21.4 ± 0.7 eV b • All other radiative capture: 16.3 ± 2.4 eV b • Transfer reaction ANC determinations: 17.3 ± 1.8 eV b and 17.6 ± 1.7 eV b

  6. Mirror ANC’s • Timofeyuk, Johnson, and Mukhamedzhanov have shown that charge symmetry implies a relation between the ANC’s of 1-nucleon overlap integrals in light mirror nuclei • Charge symmetry implies relation between widths of narrow proton resonances and ANC’s of analog neutron bound states • Tested by Texas A & M group for 8B-8Li system • Ground state agreement excellent • 1+ 1st excited state shows 2.5s discrepancy between theory and experiments (Texas A & M and Seattle)

  7. The Experiment • Measure ANC’s of the valence neutron in 8Li via the elastic scattering/transfer reaction 7Li(8Li,7Li)8Li at 11 and 13 MeV • Interference between elastic scattering and neutron transfer produces characteristic oscillations in differential cross section • Amplitudes of maxima and minima yield ANC

  8. Calculations • DWBA calculations performed with FRESCO by Natasha Timofeyuk and Sam Wright • 8Li + 7Li Optical potentials from Becchetti (14 MeV 8Li on 9Be, modified to be appropriate for 7Li), two from Potthast (energy-dependent global fit to combined 6Li+6Li and 7Li+7Li data from 5-40 MeV) • 7Li + n binding potentials taken from Esbensen & Bertsch and from Davids and Typel

  9. Calculations by Sam Wright

  10. Advantages of the Method • Identical initial and final states => single vertex is involved • Statistical precision greater (compared with distinct initial and final states) • Single optical model potential needed • Elastic scattering measured simultaneously • More than one beam energy allows evaluation of remnant term in DWBA amplitude • Absolute normalization of cross section enters only as a higher-order effect in ANC determination

  11. Experimental Setup

  12. Target, Beam, & Detectors • Two annular, segmented Si detectors • 25 µg cm-2 7LiF target • LEDA detector covers lab angles from 35-61° • S2 detector covers 5-15° in the lab • 7Li cm angular coverage from 10-30° and 70-122° • 8Li beam intensities of 2-4  107 s-1

  13. Online Spectrum from S2 Detector

  14. Ground state structure of 9Li (N=6 new closed shell?) R. Kanungo et al. 9Li(d,t)8Li E ~ 1.7A MeV 8Liex2 8Ligs 8Liex1 PRELIMINARY ONLINE SPECTRUM Q-value for d(9Li,t)8Li [MeV]

  15. 11Li Transfer Studies • 11Li is the most celebrated halo nucleus but isn’t well understood because of its soft Borromean structure • In particular, the correlation between two halo neutrons is insufficiently studied experimentally • Two-neutron transfer reactions are known to be the best tool for studying pair correlations of nucleons in nuclei • TRIUMF, for the first time in the world, can provide a low energy beam of 11Li with sufficient intensity for such studies.

  16. 11Li Halo Wave Function • Admixture of 2s1/2 and 1p1/2 waves dominate the halo wave function • Change of shell structure in nuclei far from the stability line? How about other waves such as 2d5/2 and other higher orbitals? --> pairing near the continuum • The spectroscopic factor of (2s1/2)2 would reflect the strength of other components • Unfortunately, but interestingly, 10Li is not bound • The single particle structure of halo neutrons is difficult to study. s1/2 does not make clear resonance state. • Measurements of neither the fragment momentum distribution nor the single particle transfer reactions (p,d) and (d,p) have provided conclusive results

  17. Cross Section Calculations by Ian Thompson direct two-neutron transfer only including two-step transfer 1. Bertsch-Esbensen, 2. Thompson-Zhukov, 3. Yabana (No three body correlation) 1.6 A MeV 11Li(p, t)

  18. Correlation of Neutrons in Halos • Interesting suggestion from three body calculation • Mixing of di-neutron and cigar -type configurations in 6He

  19. Recent Density Correlation Studies rn-n r2n rc rc-2n=rc+r2n <r1•r2>

  20. Three Methods • HBT interferometry measurement • Fragmentation, fusion of core • Electromagnetic dissociation • Matter and charge radii

  21. 11Li result

  22. ISAC@TRIUMF ISAC II ISAC I

  23. Too Low Beam Energy? • 1.6A MeV is appropriate for the study. • The effect of Coulomb barrier is extremely small for halo neutrons.1.6A MeV is much higher than the separation energy (~180 keV) . • Energy-momentum matching is not bad because of the narrow internal momentum distribution of the halo neutrons. • 6A MeV is conventional transfer reaction energy and thus analysis tools were well developed.

  24. MAYA K active zone zone of amplification and detection wall of Si + CsI gassiplex

  25. 11Li(p,t)9Li at TRIUMF • The first run is planned at the end of November 2006 • We expect 5000 (p,t) reactions to ground state of 9Li • Reactions populating the excited state of 9Li are also expected • Will measure other channels such as (p,d) • Be ready for data (Ian)

  26. Acknowledgements • 7Li(8Li,7Li)8Li: Derek Howell (M.Sc. Student, Simon Fraser University) • d(9Li,t)8Li: Rituparna Kanungo (TRIUMF) • p(11Li,t)9Li: Isao Tanihata (TRIUMF) and Hervé Savajols (GANIL)

  27. 6A MeV Kinematics of p(11Li, 9Li)t 1.6A MeV

  28. Typical events 20 CsI Array C4H10 gas 20 Si Array

  29. Differences between 6He and 11Li • 11Li is much less bound than 6He • 6He: mostly 1p3/2 wave • 11Li has mixed waves of 1p1/2, 2s1/2, … • The core of 11Li (9Li) may be much softer than that of 6He (4He)

  30. 6He results

  31. HBT measurement Rn-n= 5.9 ± 1.2 fm Rn-n= 6.6 ± 1.5 fm Rn-n= 5.4 ± 1.0 fm

  32. EMD 11LI (70A MeV)+Pb ->9Li+n+n 1MeV E(9Li-n) Nakamura et al. 2006 1MeV E(9Li-n)

  33. He and Li Radii Li He

  34. RMS radii and the configuration

  35. MAYA MAYA is essentially an ionization chamber, where the gas plays also the role of reaction target. It could be used with H2, d2, C4H10, between 0-2 atm. the projectile makes reaction with a nucleus of the gas. there are two little drift chambers before MAYA, to monitor the beam. the light scattered particles do not stop inside, and go forward to a wall made of 20 CsI detectors, where they are stopped, and identified. cathode wall of CsI detectors anode: amplification area. segmented cathode the recoil product leaves enough energy to induce an image of its trajectory in the plane of the segmented cathode. we measure the drift time up to each amplification wire. The angle of the reaction plane is calculated with these times. Φ tn t1

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