Indirect techniques for astrophysical reaction rates determinations. Faïrouz Hammache (IPN-Orsay)

Transcription

1 Indirect techniques for astrophysical reaction rates determinations Faïrouz Hammache (IPN-Orsay) Nucleus Nucleus 015 Catania, 1-6 June 015

2 Why indirect techniques? Astrophysics reactions Weak energies : few kev to few MeV E C (E) very weak (fb-pb) (E) (exponential) when E 1 E S E exp( ) E Direct measurements of (E) at high energies then extrapolation at stellar energies Problems with extrapolation: sub-threshold resonances, resonances at very low energy Radioactive nuclei (novae, r or s-process ): low intensities (~10 5 p/s) or targets with few atoms/cm direct measurements difficult or impossible? Subthreshold resonance S(E) E r Extrapolation? Gamow peak low energy direct measurements Low energy tail of large resonances Non resonant capture high energy E cm

3 Ingredients needed to evaluate & N A v Resonant reactions: A+xC*B+y or A+xC*C+ E cm l S A + x J E R p n Resonant reaction J C Resonant capture only possible for energies: E cm =E R =E x -Q E cm B+y v kt R 3 J c R exp kt J A 1 J x 1 tot Resonant reaction rates can be calculated if the resonant parameters (E R, J, i, i / tot,...) are known Indirect techniques & reactions can be used to extract these spectroscopic information: Transfer, THM, Coulomb dissociation, ANC, inelastic scattering, charge exchange reactions 1 E R x y

4 Massive stars & 1 C(,) 16 O reaction Crucial for energetics, nucleosynthesis, final fate of the stars, 1 C(,) 16 O 40% incertainty T=0. GK Gamow peak ~300 kev, (E 0 ) ~ 10-8 nb Direct 300 kev impossible Direct E cm 900 kev Need of precise data at high energies & extrapolation at 300 kev (R-matrix formalism) BUT Overlap of various contributions:? E1 & E transitions to the gs: Effect of the high energy tail of the 1 - & + sub-threshold resonances (S, γ α?) S (1 - ) !? S ( + ) !? & Interference effect & cascades? Study of 6.9 & 7.1 MeV states 16 O via the transfert reaction 1 C( 7 Li,t) 16 O

5 Experimental Set-up Split-Pole spectrometer (Tandem/ALTO) W ~ 1.7 msr E/E ~ 5 x H 3 H target 7 34,8MeV Faraday cup Position gas chamber, 4 + =11, -, 1 -, + Plastic scintillator (E) - d/dw measurements up to 45 E proportional gas counter - Strong population of the cluster + & 4 + state direct transfer mechanism - Population (weak) of the -, 8.87 MeV state compound nucleus? FR-DWBA & HF calculations

6 Results: Comparison exp & calculations r=6.5fm FR- DWBA HF FR-DWBA+HF Good description of the data by DWBA (6.05, 6.13, 6.9, 7.1, 9.58 et MeV) Direct transfer mechanism Disagreement at 10 for the 7.1 MeV Multi-step transfer? d/dw No (CDCC calculations of Keeley) S ( + )= kev S (1 - )= kev ~ C ~ C ( (1 ) ) (.07 ( ) ) fm fm 1 1 In agreement with ANC experiments Brune et al. 001 Avila et al. 014

7 Fit R-matrix calculations E & E1 components phase shifts data 1 C(,) 1 C measurements 1 C(,) 16 O astrophysical S-factors (direct data) γ α of the + & 1 - sub-threshold states are fixed parameters Tischhauser et al. =1.0 Tischhauser et al. =3.5

8 Fit R-matrix calculations E & E1 components phase shifts data 1 C(,) 1 C measurements 1 C(,) 16 O astrophysical S-factors (direct data) γ α of the + & 1 - sub-threshold states are fixed parameters Tischhauser et al. =1.0 Tischhauser et al. =3.5 =3.6 =.3 S E (300 kev)=5019 kev-barn S E1 (300 kev)=1008 kev-barn In excellent agreement with ANC results of Brune et al. 001 (γ α ( + ) & γ α (1 - ) fixed) In agreement with other works (γ α ( + ) & γ α (1 - ) free parameters) within error bars but central values 0% (E1) & 30% (E) N. Oulebsir, F. Hammache et al. PRC 85, (01)

9 WMAP Big-Bang Nucleosynthesis & 7 Li problem Primordial nucleosynthesis (BBN) of light elements is one of the three observational pillars of the Big Bang model together with the expansion of the Universe & the Cosmic Microwave Background radiation Observations He shell When T 10 9 K BBN begins : D, 4 He, 3 He, 7 Li synthesized via nuclear reactions Abundances depend on W B h W B h = (WMAP) (Komatsu et al. 011) (BBN+CMB) predictions (D, 3 He, 4 He) observations But: ( 7 Li/H) BBN / ( 7 Li/H) obs 3!!! (Coc et al. 013) 3 7 Li problem. Metal poor Halo dwarf stars

10 Nuclear solution to 7 Li problem? 7 Li production via EC of 7 Be: 7 Be production: 3 He( 4 He,) 7 Be known better than 8% (Adelberger et al. 010, Di Leva et al. 013) 7 Be destruction: 7 Be(n,p) 7 Li(p,) very well known (Descouvemont et al. 004) Missing 7 Be destruction channels? Chakraborty et al. 011, Civitarese et al (+) 7 Be+d 9 B* MeV (5/ + ) state Angulo et al. 005, O'Malley et al. 011, Scholl et al Be+ 3 He 10 C* hypothetical state at ~ 15 MeV (1 -, - ) 7 Be+ 4 He 11 C* hypothetical state at ~ 7.8 MeV Be+ 3 He Existence? ( -, 1 - ) ( -, 1 - ) ( + ) C states? p? B+p Be+

11 Search for missing 10 C & 11 C states & Experimental Results 10 B( 3 He,t) 10 C & 11 B( 3 He,t) 11 C charge-exchange 35 MeV Tandem-ALTO SP =7,10, B( 3 He,t) 11 C spectrum 10 B( 3 He,t) 10 C spectrum He shell No additional state in 11 C at ~ 7.8 MeV Only statistical fluctuations All known 11 B mirror nucleus states below E x = 9 MeV have their counterpart in 11 C No obvious additional state in 10 C at ~ 15 MeV Contamination peaks due to 16 O Same results at other angles (7º and 15º)

12 10 C state 95% confidence level 7 Be+ 3 He reaction rates & impact on 7 Li Simulation of a state in the region of interest with various tot & d/dw 3 He,t & same target thickness, number of incident 3 He ions & energy resolution as experiment Er= 10-, 100-, 500- kev tot = 590 kev tot = 00 kev He shell exclusion zone Typical ( 3 He,t) cross-sections to 1 -, - 10 d/dw > 5 b/sr If present total 590 kev (95% CL) F. Hammache, A. Coc et al. PRC 88, 0680 (R) 013 3He from Wigner limit ( W = 1.84 MeV) in all cases ( 3 He) << tot tot = p or BBN calculations: no impact on 7 Li 7 Li problem not due to resonant states in 10 C & 11 C

13 10 C state 95% confidence level 7 Be+ 3 He reaction rates & impact on 7 Li Simulation of a state in the region of interest with various tot & d/dw 3 He,t & same target thickness, number of incident 3 He ions & energy resolution as experiment Er= 10-, 100-, 500- kev tot = 590 kev tot = 00 kev He shell exclusion zone Typical ( 3 He,t) cross-sections to 1 -, - 10 d/dw > 5 b/sr If present total 590 kev (95% CL) F. Hammache, A. Coc et al. PRC 88, 0680 (R) 013 3He from Wigner limit ( W = 1.84 MeV) in all cases ( 3 He) << tot tot = p or BBN calculations: no impact on 7 Li 7 Li problem not due to resonant states in 10 C & 11 C

14 6 Al observations & Nucleosynthesis Massive stars: (> 8 M ) Wolf-Rayet phase and ccsne + Observations of 6 Al (T 1/ = y) 6 Al 6 Mg* 6 Mg MeV Credit: MPE Garching / R. Diehl Allende meteorite Source of 6 Al? Astrophysical context of Solar System formation Cameron et al., 1977 Tatischeff et al., Al nucleosynthesis in massive stars: Core H burning Ne/C convective shell burning explosive Ne burning Limongi et al., 006 & Woosley et al., 007, Iliadis et al Al yield depends crucially on 6 Al(n,p) and 6 Al(n,) reactions Rates x 6 Al yield/

15 7 Al levels above neutron threshold 7 Al(p,p') 7 Al* inelastic 18 MeV TANDEM-ALTO SP = 10, 5, 40, 45 Deconvolution above neutron threshold (S n = MeV): energy resolution 14 kev Q lab = 40 Comparison with known levels: Very good agreement with 3 Na(,) 7 Al and 6 Al(n,) 3 Na (de Voigt et al. 1971, de Smet et al. 007, Whitmire et al. 1974) about 30 new resonances above S n! Resonance's spins? (,) and (n,) preferentially populate high-spins (J>7/) new resonances probably have low-spins S. Benamara, N. de Séréville et al. PRC89, (014)

16 S n = MeV 7 Al(p,p ) Q3D (MLL) S n kev E x S n Q3D E/E = x 10-4 W = 14 msr Split-Pole E=18 MeV I = 300 na t= 6h Q3D E=0 MeV I = 1 A t= h Confirmation of SP data with better energy resolution (~7 kev) & precision (~ kev) New states observed

17 Coincidence Orsay Experimental conditions: 3 W-type DSSSDs strips (E ~ 0 kev FWHM) close geometry around the target d ~ 11 cm, e ~ 4% Low background environment Beam intensity ~ 100 na! Coincidence spectrum: 6 Al(n,p) 6 Mg dominated by (n,p 1 ) channel

18 Coincidence Orsay Experimental conditions: 3 W-type DSSSDs strips (E ~ 0 kev FWHM) close geometry around the target d ~ 11 cm, e ~ 4% Low background environment Beam intensity ~ 100 na! Coincidence spectrum: 6 Al(n,p) 6 Mg dominated by (n,p 1 ) channel

19 Collaborators IPN-Orsay: F. Hammache, N. de Séréville, I. Stefan, P. Roussel, S. Ancelin, M. Assié, L. Audouin, D. Beaumel, S. Franchoo, B. Le Crom, L. Lefebvre, I. Matea, A. Matta, G. Mavilla, P. Morfouace, L. Perrot, D. Suzuki, M. Vandebrouck University of Béjaïa: N. Oulebsir University of Tizi-Ouzou: S. Benamara CSNSM-Orsay: A. Coc, C. Hamadache, J. Kiener, A. Lefebvre-Schuhl, V. Tatischeff University of York: P. Adsley, A. Laird, C. Barton, C. Diget, S. Fox, C. Nsangu University Libre de Bruxelles: P. Descouvemont University of Huelva: G. Marquinez Duran, A. M. Sanchez-Benitez GANIL-Caen: F. de Oliveira, P. Ujic UPC Barcelona: A. Parikh ASCR-Rez, Czeh Republic: J. Mzarek