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Topical applications of nucleon knockout reactions

Topical applications of nucleon knockout reactions. E. C. Simpson University of Surrey IOP NPPD Conference University of Glasgow 4 th April 2011. The Nuclear Chart. Stable β + decay β - decay. N=126. Z=82. N=82. Proton dripline. Astrophysical r -process. N=50. Z=50. N=28.

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Topical applications of nucleon knockout reactions

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  1. Topical applications of nucleon knockoutreactions E. C. Simpson University of Surrey IOP NPPD Conference University of Glasgow 4th April 2011

  2. The Nuclear Chart • Stable • β+ decay • β- decay N=126 Z=82 N=82 Proton dripline Astrophysical r-process N=50 Z=50 N=28 N=20 Neutron dripline Z=28 N=8 Z=20 Z Z=8 N Wapstra, Audi and Thibault, Nuc. Phys. A279, 129 (2003)

  3. Light Nuclei -1n, -2n Z=8 np correlations? -1p Halo nuclei -1p Reaction mechanism Bazinet al., PRC 57, 2156 (1998) Sauvanet al., PRC 69, 044603 (2004) Bazinet al., PRL 102, 232501 (2009) N=8

  4. Z=20 Proton Rich Nuclei -1n -2n -1n -2n -1n N=8 -1n -1p Reduction of spectroscopic strength -1n -2n -1n -1p -2n Isospin symmetry Yonedaet al., PRC 74, 021404(R) (2006) Gadeet al., PRC 76, 024317 (2007) Gadeet al., PRC 77, 044306 (2008)

  5. Gadeet al., PRL 99, 072502 (2007) Bastinet al., PRL 99, 022503 (2007) Kanungoet al., PRL 102, 152201 (2009) -2p Coexisting configurations -1p -2p The island of Inversion Quenching of N=28 closure -2p -2p -2p Residue momentum distributions -2p -2p -2p N=28 -1n -2p Z=8 Neutron Rich Nuclei Emergence of N=16 closure N=20

  6. Beyond… N=28 Isospin symmetry Z=28 -1n -2p -2p -2p New Island of Inversion (N=40) -1p -2p Z=20 Gadeet al., PRC 74, 021302(R) (2006) Adrichet al., PRC 77, 054306 (2008) Brown et al., PRC 80, 011306 (2009) Cross shell excitations

  7. Nucleon knockout reactions Absolute cross sections Beam directional Momentum distributions Removal of nucleons from a (secondary radioactive) beam at energies >80 MeV/nucleon on a light nuclear target (Be, C) Halos: 15C, 19C, 27P, 31Ne Magic numbers: 24O, 42Si Exotic Rs: 23Al, 23Si, 27P, 27S • Cross section proportional to spectroscopic strength • Suppression of spectroscopic strengths in asymmetric systems Orbital angular momentum – final state spins, evolution of shell ordering Width increases with nucleon binding energy Hansen et al., Annu. Rev. Nucl. Part. Sci. 53, 219 (2003)

  8. Knockout reactions [Fast spectator] κcprobed by KA+2 and KAin the lab frame

  9. A probe of surface structure • Projectile follows a straight line path passed the target • Projectile internal co-ordinates assumed fixed for the duration of the interaction • Core assumed to be undisturbed by the interaction • Reaction probes the projection of the (two-) nucleon wave function on the impact parameter plane • Final state of the valence nucleons and target unobserved Impact parameter plane Sc Core Target S2

  10. Two-nucleon overlap Shell-model LS-coupled two-nucleon overlap β1 β2 Two-nucleon wave function Single-nucleon wave function

  11. Direct 2N knockout: thresholds 28Mg(-2p) +5.583 υ -1p proton evaporation +13.300 π 30.03 26Ne neutron evaporation +6.750 -1p υ 1p knockout 16.73 27Na -2p Rs(2N)=0.5 +8.503 υ 28Mg Tostevinet al., PRC 70, 064602 (2004) Tostevinet al., PRC 74, 064604 (2006)

  12. Momentum distributions Width --> Final state spin 28Mg(-2p) 0+ Inc. Sp = 16.8 MeV Sn= 8.5 MeV Beam energy E = 82.3 A MeV Broadening in thick reaction target 9Be 375 mg/cm2 ΔKA = 0.29 GeV/c 4+ Counts All KA KA Bazinet al., PRL 91 012501 (2003); Simpson et al., PRL 102, 132502 (2009)

  13. 26Si(-2n) 2+ states 26Si(-2n) +3.300  1n +15.002 109 MeV/nucleon 376 mg/cm29Be target  34.042 24Si +3.408  First 2+ (1.86 MeV) and 2+/4+ doublet (3.41 MeV) observed Second state assumed 2+ on basis of cross sections – momentum distribution would clarify 1n 19.040 2n 25Si +5.517  26Si Shell model (sd-shell, USD) two-nucleon amplitudes Yonedaet al., PRC 74, 021303(R) (2006)

  14. Structure Sensitivity: 26Si(-2n) 26Si(-2n) Second2+ E=3.41 MeV FWHM of second state is ~25% larger, attributed to the different underlying structure – can this be verified by experiment? Does the SM accurately described the underlying structure? First 2+ E=1.86 MeV Simpson et al., PRC 82, 044616 (2010)

  15. Odd-even nuclei Population 25Si via one- or two-neutron knockout Reynolds et al., PRC 81, 067303 (2010) 26Si(-1n) -> 25Si 27Si(-2n) -> 25Si

  16. The island of inversion The island of inversion defines a region of nuclei near 31Na, where the ground states are dominated by intruder configuration. • 38Si(-2p) -> 36Mg Ratio of experimental to pure 0 ħω theoretical cross sections gives indications of fraction of 2 ħω components, agreeing with MCSM calculations 0+ 2+ Counts 13.6 13.8 14.0 14.2 p// (MeV/c) Gade et al., PRC 77, 044306 (2008); see also 32Mg(-2p), Fallon et al., PRC 81, 041302 (2010)

  17. Island of inversion near N=40 Two-proton knockout from 68Ni, 66Fe and 70Ni A new island of inversion is predicted, associated with the quenching of the N=40 shell gap, centered on 64Cr. Removing two-protons causes the two-proton removal cross sections to fall by an order of magnitude. The small cross section is symptomatic of suppressed structural overlap, hence rapidly changing structure Adrich et al., PRC 77, 054306 (2008) Inelastic scattering using 9Be targets: cross section increases for 60Cr – 64Cr, symptomatic of changing shapes? Gade et al., PRC 81, 051304(R) (2010)

  18. np-knockout up the N=Z line? 36Ar(-2N) thresholds +4.66  1n 2n KO +12.74  30.00 +11.04 +5.14  2 34Ar -np +5.90  34Cl np 1n KO 15.26 2n 35Ar sd-shell cross sections: 36Ar -n Direct 14 mb Plus many more states 0+,T=1, 0.7 mb 1+,T=0, 1.2 mb -np Indirect 19 mb 1.8 mb

  19. Knockout from 12C Particle Separation Thresholds Direct: Good Indirect: Bad

  20. 12C(-2N) ● WBP △ PJT Fragmentation of 16O and 12C projectiles at 2.1 GeV/nucleon, measured at LBL (1975) np-knockout underestimated Lindstrom et al., LBL Report 3650 (1975); Greiner et al., PRL 35, 152 (1975) Subedi et al., Science 320, 1476 (2009); Simpson and Tostevin, PRC 83, 014605 (2011)

  21. Outlook • Knockout reactions allow the study of nuclear structure for a wide range of isotopes, one- two-nucleon knockout offer complementary probes • New facilities offer new opportunities and challenges: description of knockout with deformed nuclei remains major task • Two-nucleon knockout still requires verification on stable nuclei; studies on 12C and 16O (in progress) indicate the need for new experiments • Different structure models (ab-initio, NCSM)

  22. Acknowledgements J. A. Tostevin, P. H. Regan Zs. Podolyak, S. J. Steer D. Bazin, B. A. Brown, A. Gade J. Lee PetrNavratil UK STFC Grants EP/D003628 and ST/F012012 UK EPSRC Grant EP/P503892/1

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