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A. Brondi, G. La Rana, R. Moro, M. Trotta, E. Vardaci Università di Napoli Federico II, and Istituto Nazionale di Fisic

The Physics Opportunities with Eurisol Trento, January 16-20, 2006. Search for isospin effects on nuclear level density. A. Brondi, G. La Rana, R. Moro, M. Trotta, E. Vardaci Università di Napoli Federico II, and Istituto Nazionale di Fisica Nucleare, Napoli, Italy.

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A. Brondi, G. La Rana, R. Moro, M. Trotta, E. Vardaci Università di Napoli Federico II, and Istituto Nazionale di Fisic

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  1. The Physics Opportunities with Eurisol Trento, January 16-20, 2006 Search for isospin effects on nuclear level density A. Brondi, G. La Rana, R. Moro, M. Trotta, E. Vardaci Università di Napoli Federico II, and Istituto Nazionale di Fisica Nucleare, Napoli, Italy

  2. Why is it important to study the level density ? Level density is a basic ingredient for x-section calculations Astrophysical processes“Astrophysical Reaction Rates from Statistical Model Calculations”, ADNDT 75 (2000) 1-351 SHE’s production Capture of two nuclei in the attractive potential pocket. Probability of forming a compact compound nucleus (CN). Survival probability against fission. Fluctuation-dissipation dynamics: Fokker-Plank or Langevin equations Evaporative process: Statistical Model

  3. Study of isospin effects on level density through fusion-evaporation reactions P(Uo,Jo,e,l,U,J) r(U,J) . Tl(e) Temperature, Angular momentum, Pairing & Shell effects: a , s, d Isopin (?) Isospin comes in through: Isospin Distribution Symmetry Energy

  4. Isospin distribution Statistical mechanics A reduction of level density with increasing |T3| is predicted

  5. Level densities in n-rich and n-deficient nuclei Isospin distribution Form A: Form B: Form C: 20<A<70 ENSDF

  6. Level density in n-deficient Dy nuclei n-rich n-deficient 140Dy

  7. Study of the level density in n-deficient Dy nuclei • Which observables? • “… complete level schemes up to 2.5 MeV will be difficult to obtain for • higher A and for nuclei far off the valley of stability. Thus further tests of • this level density approach will likely be based on evaporation spectra…” • Al Quraishi et al., Phys. Rev. C 63, 065803 • Method: observation of evaporative xp channels • Observables: E.R. yields and energy spectra • To what extent such effects on level density can be observed? • Statistical model calculations (Lilita_N97) • 76Kr + 64Zn  140Dy Ex = 50 MeV - xp channels Standard a = A/8,

  8. Study of level density in n-deficient Dy nuclei Enhanced effects using Z-Zo prescription

  9. Study of level density in n-deficient Dy nuclei Best condition: Decay channels involving a small number of particles – Low Ex Owing to the higher average energy, 1particle decay channels are enhanced using Z-Zo prescription

  10. Why is it important to study the symmetry energy ? • Esym=bsym(T)(N-Z)2/A • As a part of the nuclear Equation Of State it may influence the mechanism of Supernova explosion • General theoretical agreement on its temperature dependence (LRT+QRPA vs. large scale SMMC) • Possible consequences of T dependence of Esym on core-collapse Supernova events still debated • Effects enhanced by the instrinsic isospin dependence of Esym Fusion-evaporation reactions: Esym affects the particle B.E.

  11. SYMMETRY ENERGY mw(T) 0 < T < 3 MeV - 98Mo, 64Zn, 64Ni -Hartree-Fock – coupling s.p.s. to c.v. -LRT – QRPA Decrease of the effective mass  Increase of Esym Esym(T)= bsym(T) x (N-Z)2/A bsym(T)=bsym(0)+(h2ko2m/6mk)[mw(T)-1 –mw(0)-1] mw(T)=m + [mw(0) – m]exp(-T/To)

  12. Study of the level density in n-rich Mo nuclei Method: observation of evaporative xn channels (1n, 2n) Observables: E.R. yields and energy spectra Statistical model calculations (Lilita_N97) 105Zr + 4He  109Mo Ex = 14-20 MeV - 1n, 2n channels 98Kr + 12C  110Mo Ex = 50-85 MeV - 5n, 6n channels Isospin distribution: Standard a = A/8, Symmetry energy: prescription of P. Donati et al.

  13. Effect of symmetry energy Effect EX

  14. Symmetry energy and isospin distribution effects Esym effect dominates at low Ex

  15. Moving to higher excitation energies Higher cross sections Higher angular momenta s(mb) J(h) More channels involved, including charged particle emission

  16. Single effects on different evaporative channels No effects are observed for Esym

  17. Isospin effects on energy spectra shapes 5n channel 1n channel No effects are observed for Esym

  18. In program….. - Refinements of the model: microscopic level density based on single-particle level schemes obtained from Hartree-Fock calculations on the basis of the Gogny effective interaction, taking into account for parity, angular momentum, pairing corrections as well as collective enhancements. S. Hilaire et al., Eur. Phys. J. A, 169 (2001) - Estention of calculations to other exotic nuclei - Measurements with existent RIB and SB facilities

  19. Summary and perspectives • The availability of n-rich and n-deficient RNB’s will allow to study isospin effects on fusion-evaporation reactions. These may have strong implications in nuclear astrophysics and affect the estimation of SHE’s production cross sections. • Such studies require: - High intensity n-rich and p-rich beams • - High selectivity, high granularity, high efficiency detectors • Such tasks may be accomplished using: • - SPES, SPIRAL II, EURISOL beams • - Neutron + Charged particle detectors; High efficiency, large solid angle ER separators (PRISMA in GFM, VAMOS); Gamma tracking arrays (AGATA).

  20. Testing realistic effective interactions on exotic nuclei around closed shells • Covello, L. Coraggio, A. Gargano, and N. Itaco • Università di Napoli Federico II, • and Istituto Nazionale di Fisica Nucleare, Napoli, Italy

  21. 134Sn Coulex (Oak Ridge) B(E2;0+2+) = 0.029(4) e2b2 Theory B(E2;0+2+) = 0.033 e2b2 Lowest first-excited 2+ level in semi-magic even-even nuclei 726 keV Theory Expt.

  22. 136Te Coulex (Oak Ridge) B(E2;0+2+) = 0.103(15) e2b2 Theory B(E2;0+2+) = 0.18 e2b2 New measurement larger value: ~ 0.13(15) e2b2

  23. N/Z = 1.65: most exotic nucleus beyond 132Sn for which there is information on excited states B(M1;5/2+ 7/2+) = 0.29 x 10-3 (n.m.)2 (OSIRIS, Studsvik) Theory with free g-factors: B(M1) = 25 x 10-3 (n.m.)2 Theory with effective M1 operator: 4 x 10-3 (n.m)2 From 133Sb: ε5/2 = 962 keV 282 keV

  24. Mechanism of Supernova explosion • When the n-rich core of a massive star reaches a mass limit, it begins to collapse. • The increased density induces electron captures on both free protons and protons bound in nuclei, driving the matter in the core towards successively more n-rich nuclei. • As long as the density remains lower than the “trapping density”, the neutrinos produced escape freely from the core, releasing energy. • Influence of symmetry energy: • The larger the symmetry energy, the more difficult is to change protons into neutrons. • Via the EOS, the symmetry energy influences the amount of free protons in the core, that in the late stage of the collapse are believed to be the main source for electron capture. Larger symmetry energy  smaller electron capture rate  less energy lost by neutrino escape  stronger shock wave  Supernova explosion

  25. Isospin effects on …… 105Zr + 4He  109Mo 98Kr + 12C  110Mo

  26. Isospin effects on 1n and 2n channel yields 105Zr + 4He  109Mo

  27. Single effects on different evaporative channels No effects are observed for Esym

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