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Fusion of 32 S + 48 Ca near and below the Coulomb barrier

Fusion of 32 S + 48 Ca near and below the Coulomb barrier.

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Fusion of 32 S + 48 Ca near and below the Coulomb barrier

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  1. Fusion of 32S + 48Ca near and below the Coulomb barrier G. M.1, A.M. Stefanini2, C.L.Jiang3, L. Corradi2, S. Courtin4, H.Esbensen3, E. Fioretto2, A. Goasduff4, J.Grebosz5, F. Haas4, A.F.Kifle2, M.Mazzocco1, C.Michelagnoli1, T. Mijatović6, D.Montanari1, K.E.Rehm3, R.Silvestri2, PushpendraP. Singh2, F. Scarlassara1, S. Szilner6, X.D.Tang7, C.A.Ur1 1 Dipartimento di Fisica e Astronomia, Universitàdi Padova, and INFN, Sez. di Padova, Italy 2 INFN, LaboratoriNazionali di Legnaro, Legnaro (Padova), Italy 3 Physics Division, Argonne National Laboratory, Argonne, USA 4 IPHC, CNRS-IN2P3, Universitéde Strasbourg, Strasbourg, France 5 Institute of Nuclear Physics, Polish Academy of Sciences, Cracow, Poland 6 RuđerBoškovićInstitute, Zagreb, Croatia 7 University of Notre Dame, Notre Dame, USA

  2. Outline Fusion excitation function of 32S + 48Ca Experimental set-up and results CC calculations Couplings to transfer channels and barrier distribution Logarithmic slope and S factor Comparison with the similar system 36S + 48Ca Summary

  3. Electrostatic separator and E-ΔE-ToF telescope to detect evaporation residues at ≈ Oo scattering chamber MCP, IC & Si detectors beam E-ΔE-ToF telescope ER detected here HV _ _ + + target HV

  4. Detector set-up, experimental matrix ΔE-ToFand angular distributions degraded beam fusion on C, F Experimental Angular Distributions TOF (cha.) ER 40Ca + 40Ca lowest measurable cross section ≈ 0.5-1 μb ΔE (cha.)

  5. The fusion excitation function of 32S + 48Ca Nuclear Potential (Akyüz-Winther) Vo= 75.12MeV (61.81 MeV) a= 0.65 fm (0.65 fm) ro= 1.12 fm (1.17 fm) Barrier parameters Vb= 44.57MeV (43.12 MeV) rb= 9.61 fm (9.87 fm) ħω= 3.59 MeV (3.49 MeV) The lowest 2+ and 3- state in 32S and 48Ca b3=0.39 0.18 3- 3- 0.10 2+ b2=0.31 2+ 1phon = 32S: 2+, 3- 48Ca: 2+, 3- 2phon = 32S: (2+)2, 3- 48Ca: 2+, 3- gs 48Ca 32S

  6. 32S+48Ca g.s. transfer Q-values (MeV) Pair transfer Form Factor

  7. What happens above the barrier … At high energies the fusion cross sections for 32S+48Ca are explained nicely by the calculation in which the coupling to one pair transfer channel is schematically taken into account. Without transfer, the calculation clearly overestimates the experimental cross sections.

  8. Comparing with a similar system A.M. Stefaniniet al., PRC 78 (2008) 044607

  9. 32S+48Ca 36S+48Ca Qfus=+7.55MeV Qfus=+7.66MeV

  10. The barrier distributions also show interesting features Double peaked shape of BD for 32S+48Ca has been interpreted within a simplified CC calculation including an effective transfer channel coupling. The comparison with the near-by system 36S+48Ca (negative Q-value for transfer channels) that shows a BD with only one main peak supports CC calculation result.

  11. Fusion cross sections of three systems involving 48Ca C.L. Jiang et al., PRC 82 (2010) 041601(R)

  12. Summary Fusion cross sections of 32S + 48Ca have been measured in a wide energy range The fusion barrier distribution has an interesting double-peaked shape Below the barrier, the log slope of the exct. function is rather flat and no maximum of the S factor is observed The present data are nicely reproduced with CC calculations using a WS ion-ion potential The comparison with 36S + 48Ca shows differences that seem to correlate with coupling effects of Q>0 transfer channels These transfer couplings possibly push the threshold of hindrance below the lowest measured energy, but detailed calculations are needed before drawing firm conclusions

  13. The barrier distributions also show interesting features A.M.Stefanini et al., PLB 679 (2009) 95

  14. Our most results on 32S + 48Ca by coupling schematically a Q>0 transfer channel, the data are well fit the barrier distribution is very wide and and shows an unusual shape the logarithmic derivative is rather low and keeps increasing below the barrier

  15. ? S(E) Q>0 Q<0 E We recently measured the fusion excitation function of 36S + 48Ca (Q-value=+7.6MeV) 0 -Q For Q>0 S(E) may not show any maximum  no fusion hindrance!

  16. L(E) = d[ln(Eσ)]/dE dS/dE = S(E)[L(E) – πη/E] S has a maximum when dS/dE = 0, i.e. when L(E) = πη/E = LCS The energy E = ES where this happens has been usually taken as the threshold energy for hindrance. From the empirical systematics of Jiang et al. one obtains ES ≈ 0.356 [Z1Z2√μ]2/3 MeV C.L.Jiang et al., PRC 73, 014613 (2006)

  17. Scheme of CC calculations

  18. 36S+48Ca A-W V0= 61.34 MeV r0=1.17 fm a=0.65 fm Vb= 42.7 MeV V0=164.6 MeV r0= 0.90 fm a=0.95 fm Vb= 43.3 MeV

  19. Experimental details The sensitivity of the experimental set up allows us to measure down to about 600 - 650 nb ER were detected by two MCP detectors and finally stopped in a 600 mm2 silicon detector placed in the IC The total length of telescope was ≅ 100 cm with a geometrical solid angle DW ≅ 0.042 msr The electrostatic deflector transmission was measured for several systems in a wide range of masses, by switching off the filter and by detecting ER at q = 3o and 5o, then comparing the yields with the results of the corresponding measurement with the electrostatic deflector on. The interpolated value for the present case is: T=0.72±0.03

  20. 32S+48Ca g.s. transfer Q-values (MeV) 40Ca+48Ca transfer Q-values (MeV)

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