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Reactions with weakly bound Nuclei near the Coulomb barrier

Reactions with weakly bound Nuclei near the Coulomb barrier. A. Navin GANIL. POA. Open experimental issues in understanding fusion with weakly bound nuclei near the Coulomb barrier Need for identification of complete fusion products in these reactions

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Reactions with weakly bound Nuclei near the Coulomb barrier

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  1. Reactions with weakly bound Nuclei near the Coulomb barrier A. Navin GANIL

  2. POA • Open experimental issues in understanding fusion with weakly bound nuclei near the Coulomb barrier • Need for identification of complete fusion products in these reactions 4,6,8He 63,65Cu188,190,192Os from SPIRAL I (Illustration of particle-gamma correlations as a tool) • Origin of large cross section for non fusion Transfer/ breakup followed by fusion events 6,7Li+60Ni, 165Ho, • Implications on effect of weak binding on the fusion process • 1n/2n in borromean nuclei transfer to the continuum 4He 6He 8He Sn(MeV)20.58 1.863 2.583 S2n(MeV) 0.973 2.138 <r2>1/2 (fm) 1.67(01) 2.54(04) 2.49(04) 6Li 7Li Sa/d Sa/t 1.475 2.47 MeV

  3. EXPECTATION … Suppression in the fusion cross section - breakup Large Enhancement -large sizes (halo’s,skin)  Nuclear Potential Fusion with weakly bound nuclei near the Coulomb barrier : Stable and Unstable nuclei Tunneling through a multi-dimensional barrier V(r,x1,x2,x3 ....) (Tl (E) = [1 + exp{2p/hw (VBl – E)}] –1) Quantity to be compared with the model • Complete fusion (projectile+target) OR • Complete fusion+ (projectile-x+target) • Conditions under which this can be done • (projectile-x+target) breakup fusion or transfer • Reaction cross section

  4. SPIRAL DE/E ~ 10-3 no measurable impurity 75MeV/A 13C 1013 p/s SPIRAL CSS1 CSS2 SISSI SIRa Stable beam LISE Production Target SPIRAL Experimental Area GANIL

  5. Experimental Approach 16.5 MeV < Elab < 34 MeV a 19.5, 30 MeV 6He 27 MeV 8He g - ray detector array evaporate  4+ Ga 2+ compound nucleus a2n p2n 3n 0+ 68Zn 68Ga 65Cu 68Zn Projectile 4,6,8He Target 63,65Cu & 188,192Os (~ 1.1-3.2 mg/cm2) Intensity of low lying g transitions  sER S sE.R = sFUSION 6,7Li 60Ni & 165Ho Pelletron, Spiral Intensity(particles/s) 2x1010 a, 6,7Li 1x107 6He 4x104 8He Annular Si CD detector for charged particle detection (elastic,transfer & breakup) Particle gamma coincidence for transfer angular distributions

  6. Setup ………at SPIRAL Exogam Array ‘CLOVERS’ Target Annular Si “CD” detector Faraday Cup Plastic scintillator “First” Singles g-ray measurements with RIB to obtain fusion and transfer cross sections sfusion and stransfer from intensity of g-rays

  7. BUY ONE (FUSION) GET ONE FREE (TRANSFER) Comparison of evaporation residues with statistical model calculations (CASCADE) 4,6He + 65Cu Good agreement for a + 65Cu but Large discrepancy in cross section for an channel (66Cu) in 6He + 65Cu Evidence for non fusion process TRANSFER (Same residue as from fusion) s (6He+Cu) = (6He+Cu)+(a+Cu) + (n+Cu) Similar results for 4,6 He + 63Cu

  8. More evidence : Particle – g coincidence 6,8He Q-value spectra 6He+65Cu4He+(67Cu)* 8He+63Cu4He+(65Cu)* transfer followed by evaporation • Observed Q-value spectra consistent with transfer, peaking Qopt ~0 • Transfer (2n) to the continuum Measured ratio of residues consistent with expectation ~ 30% as compared to complete fusion Large transfer cross sections and greater than breakup cross sections Similar effects observed in 6He+188,190,192Os Do not mix with Fusion products (target like) Bottom line: Residues better be only from CN process PRC 70 044601 (2004)

  9. Origin of non compound events with weakly bound nuclei • Transfer - Medium mass targets • 6,7Li + 60Ni p,n,g correlations near and away from grazing • Angles below and above the Coulomb barrier • – Evidence for Nuclear Breakup • Large Transfer like with He isotopes • Contrary to the understanding for medium mass targets • Experimental signatures of • Break fusion - Heavy targets • 7Li + 165Ho 42 MeV – Coulomb Breakup? • Gamma rays in coincidence with alpha and tritons to probe the origin of the • residues • ( inclusive measurements PRL 88, 1702) • Relevant for a complete understanding of interaction with weakly bound nuclei.

  10. Non compound process breakup or transfer Alpha Spectra  gamma spectra 7Li + 60Ni Qopt 40o grazing 120o grazing Transfer ? Nuclear dominated breakup ?

  11. Particle-g coincidence for 7Li + 165Ho Gamma rays in coincidence with alpha particles t fusion DE • Beam : 7Li, 42 MeV; • Target : 165Ho , 2.68 mg/cm2 • Telescopes @ 40o, 50o near grazing angle • 4 Clovers E (channel no) 184.6 a + 165 Ho 228 270.8 all lines in 167Tm 297 354 Gamma rays in coincidence with tritons a fusion Similar Spectra with a beam 384 511 454 552

  12. Coincidence g-spectra for Ea gates Different Et fusing ! 184.6 166Er 2 4 1 280.4 166Er 184.6 166Er 280.4 166Er 365.6 166Er 205 165Er 306,315 165Er 438.2 166Er Agreement with stat. model Breakup followed by fusion ? Coulomb dominated breakup

  13. Different Routes  166Er from t fusion/transfer and p transfer 7Li+165Ho4He+(168Er)* t fusion/transfer 6He+166Er proton transfer 184.6 166Er p transfer t-transfer/fusion 280.4 166Er 184.6 166Er 365.5 166Er 280.4 166Er Side feeding pattern Angular momentum E (keV)

  14. Implications - non compound events • 6,7Li,9Be+208Pb,209Bi ANU/Brazil 18O+198Pt…7Li+209Bi • sCF+sNC = sF bound nuclei (same CN) • Complete fusion+ break up fusion + transfer • Fusion cross section or reaction cross section?

  15. N-wall & EXOGAM+Particles Coming Soon in a beamhall near you 6He+65Cu4He+(67Cu)* 66Cu(g)+4He+neutron To decompose 1n /2n neutron transfer Transfer to the continuum (Followed by evaporation) 8He+63Cu fusion + transfer

  16. Perspectives Need for a) for exclusive experiments to disentangle reaction mechanism before comparing with models especially for medium mass nuclei b) Identify source of the non compound process transfer- breakup fusion c) Improved calculations for the understanding of reactions near Vc Higher intensity low energy 8He beams + more from SPIRAL and of course SPIRAL2

  17. SPIRAL 2 Workshops • Neutrons for science (on the web) • Nuclear Astrophysics • Low energy physics • Direct reactions • Gamma spectroscopy

  18. The Collaboration …. D.Bazin,Y. Blumenfeld,, J.M. Casandjian,A.Chatterjee,M Dasgupta, G. de France S. Kailas,R. Lemmon, K. Mahata, T. Nakamura, V. Nanal,R.G Pillay,E.C.Pollacco, R. Raabe , K. Ramachandran, M. Rejmund,A. Shrivastava, J.L Sida, V.Tripathi,E.Tryggestad GANIL BHABHA ATOMIC RESEARCH CENTER IPN, IN2P3-CNRS Tata Institute of Fundamental Research TIT, Japan CEA/ SACLAY NSCL, MSU Australian National Univ. Daresbury Lab

  19. Investigations in 7Li+60Ni system n p t d a + gamma coinc Near and away from grazing angle Near and away from the barrier 40o,50o 30 MeV 120o,140o 15 MeV Presence of two components in alpha Spectra snc~430mb and scn=350mb a angular distbn Ea (Vb ~14 MeV)

  20. triton gate alpha Fusion/ transfer 62Cu, 62Ni,61Cu,61Ni,58Co Alpha gate Triton Fusion/ transfer 63Zn, 60Ni

  21. Transfer followed by breakup 7Li +65Cu @25 MeV particle coincidences 1n stripping: 6Li (gnd)+ 66Cu* 6Li(3+) +66Cu* Break up cross-sections a+t and a+d And more A. Shrivastava et al (work in progress)

  22. 1n transfer or 2n transfer 6He + 65Cu Elab + 19.5 MeV

  23. Analogous for Ea gates n fusing ! 186.01 66Cu Similar E* mostly 66Cu 315.71 66Cu 65Cu 30 MeV 186.01 19.5 MeV 1115.5 65Cu 315.71 CN 186.01 higher E* more 65Cu 1115.5 65Cu

  24. Transfer Q-values and Sn’s • a+63Cu3He+64Cu -12.66 a+63Cu2He+65Cu - 6He+63Cu 5He+64Cu + 6.052 7.91, 18.768 6He+63Cu 4He+65Cu + 16.852 9.91, 17.826,7.45(Sp) 6He+65Cu 5He+66Cu + 5.20 7.07, 16.98 6He+65Cu 4He+67Cu + 15.21 8He+63Cu 7He+64Cu + 5.332 8He+63Cu 6He+65Cu + 15.687 8He+63Cu 4He+67Cu + 30.897 9.12,16.18

  25. elastic angular distbn s/sR

  26. Direct Reactions Elastic scattering angular distributions Reaction cross section sR relative contribution of transfer and breakup Angular distribution and Q value spectra for alpha particles gated with 66Cu consistent with transfer Direct evidence for transfer, Raabe et al Nature 2004 sR - (sfus+strans) = sbreakup strans ~ 1.5 sbreakup Large transfer cross sections and greater than breakup cross sections Also for 8He+63Cu at 27 MeV a elastic scattering NPA 147 107 (1970); NPA 388 573 (1982)

  27. Fusion with weakly bound nuclei  E.R. In beam -rays Fission Offline x rays n: What do I do now Alpha decay & fission 6He+209Bi, 9Be+208Pb, 11Be+208Pb Fission + coincidence6He + 238U, n+U Characteristic X-ray, Gamma activity measurements6He+64Zn Online gamma-ray measurements6,7Li+59Co,165Ho Combined with particle coincidence6,8He+Cu,Os Experimentallimitations? Broken up a populates the same channels (as p+t) difficult to distinguish experimentally Differentiate transfer and fusion products (Especially for Acn ~ <100)

  28. 6He+ Heavier targets…. Os isotopes: 6He+190Os 4He+(192Os)* Features Gate on 2+ 0+ of 190Os Shows inelastic and transfer contributions Target like products so not mixed up with residues unlike for lower ACN! Comparison with a induced reaction sdir_6He> sa sf ~ sa like Cu Bottom line: Residues better be only from CN process PRC 70 044601 (2004)

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