FLUORINE DESTRUCTION IN STARS: FIRST EXPERIMENTAL STUDY OF THE 19 F(p,α 0 ) 16 O REACTION AT ASTROPHYSICAL ENERGIES Marco La Cognata INFN-LNS Catania

Asymptotic Giant Branch (AGB) stars are the main s-process site. 13 C( ,n) 16 O, active in the intershell, is the main neutron source in low-mass AGB stars Origin of the heavy elements Relative abundances Elements heavier than iron are produced by neutron capture. Slow neutron capture (s- process) is responsible of the nucleosynthesis along the stability valley

Fluorine in astrophysics In AGB stars, the largest observed fluorine overabundances cannot be explained with standard AGB models. A possible lack of proper accounting for the contribution from C-bearing molecules might provide an interpretation in Population I stars (Abia et al. 2010). S-nuclei are produced and brought to the surface thanks to mixing phenomena, together with fluorine that is produced in the same region from the same n-source. 19 F is a key isotope in astrophysics as it can be used to probe AGB star mixing phenomena and nucleosynthesis. But its production is still uncertain!

Fluorine in astrophysics In the case of metal poor AGB stars our understanding is far from satisfactory (Lucatello et al. 2011, Abia et al 2011). We note that a significant fraction of the upper limits are located under the predicted lines (Lucatello et al. 2011) S-nuclei are produced and brought to the surface thanks to mixing phenomena, together with fluorine that is produced in the same region from the same n-source. 19 F is a key isotope in astrophysics as it can be used to probe AGB star mixing phenomena and nucleosynthesis. But its production is still uncertain!

The 19 F(p,   ) 16 O reaction Below E cm = 460 keV, where data do not exist, a non resonant contribution is calculated for s-capture. The S- factor was adjusted as to the lower experimental points between 460 and 600 keV The S(E) factor shows several resonances around 1 MeV. Breuer (1959) claimed the occurrence of two resonances at around 400 keV Unpublished data (Lorentz- Wirzba 1978) suggests that no resonance occurs  0 is the dominant channel in the energy region of astrophysical interest

s c A a C direct break-up x 2-body process Additional advantages: ●reduced systematic errors due to straggling, background… ●magnifying glass effect But... ●off-shell cross section deduced (x  virtual particle) ●no absolute units From A+a(x  s)  MeV A + x  c keV Though E A >> V Coul it is possible to measure at the Gamow peak since: E c.m. =E A-x -Q x-s Coulomb barrier  exponential damping of the cross section at astrophysical energies + electron screening  low-energy, bare-nucleus cross section is experimentally available only through extrapolation OR indirect measurements By selecting the QF contribution THM reaction Indirect Techniques: the THM

s c A a C direct break-up x 2-body process Additional advantages: ●reduced systematic errors due to straggling, background… ●magnifying glass effect But... ●off-shell cross section deduced (x  virtual particle) ●no absolute units From A+a(x  s)  MeV A + x  c keV Though E A >> V Coul it is possible to measure at the Gamow peak since: E c.m. =E A-x -Q x-s Coulomb barrier  exponential damping of the cross section at astrophysical energies + electron screening  low-energy, bare-nucleus cross section is experimentally available only through extrapolation OR indirect measurements By selecting the QF contribution THM reaction Indirect Techniques: the THM See C. Spitaleri talk for more details

The THM for resonance reactions In the “Trojan Horse Method” (THM) the astrophysically relevant reaction, say A(x,c)C, is studied through the 2  3 direct process A(a,cC)s: Standard R-Matrix approach cannot be applied to extract the resonance parameters of the A(x,c)C reaction because x is virtual  Modified R-Matrix is introduced instead (A. Mukhamedzhanov 2010) a A x s F* C c In the case of a resonant THM reaction the cross section takes the form The process is a transfer to the continuum where x is participant, e.g. a proton or an alpha particle and s is the spectator Upper vertex: direct a breakup M i (E) is the amplitude of the transfer

The experiment d: Trojan horse nucleus p+n B=2.2 MeV |p s |=0 MeV/c n: spectator p: participant d( 19 F,  16 O)n Q 3b =5.889 MeV 19 F(p,  ) 16 O Q 2b =8.113 MeV d( 19 F,  16 O)n Q 3b =5.889 MeV 19 F(p,  ) 16 O Q 2b =8.113 MeV Butane gas  52mbar (150 µg/cm 2 )

The experiment d 19 F p n 20 Ne* 16 O  The experiment was performed at INFN-LNS Catania Beam: 19 MeV Only in the coincidence 1-5 the  0 channel was populated significantly   1 channel in the 1-4

Selection of the reaction channel Identification of the particles with Z=8 through the  E-E tecnique. Three-body reaction in the exit channel -> coincidence events give rise to a kinematic locus. Kinematic locus of the reactions: 1. 2 H( 19 F,  1 16 O)n F(p,  ) 16 O 3. 2 H( 19 F,  0 16 O)n F( 12 C, 15 N) 16 O

Selection of the reaction channel Identification of the particles with Z=8 through the  E-E tecnique. Full Q-value spectrum, imposing that Z=8 nuclei are detected in the telescope. Further cuts are needed to remove background reactions Kinematic locus of the reactions: 1. 2 H( 19 F,  1 16 O)n F(p,  ) 16 O 3. 2 H( 19 F,  0 16 O)n F( 12 C, 15 N) 16 O

From energy conservation: E beam +Q=E 1 +E 2 +E 3  Y=E beam -E 2 -E 3 x=p 3 2 /2u Events from d( 19 F,  16 O)n must gather along a straigth line  0 channel  Statistically significant only coincidence PSD1-PSD5. (1) (2) (3) (4) (1) (2) (3) (4) Selection of the reaction channel Q-value of the three-body reaction is linked to the mass of the particles and not to any kinematical variable.

Good agreement  confirms the accuracy of the performed calibration Comparision of the experimental Q-value with the theoretical one Kinematic locus obtained by introducing all the mentioned cuts  0 channel  Statistically significant only coincidence PSD1-PSD5. Selection of the reaction channel

Selection of the QF reaction mechanism Experimental study of the neutron momentum distribution inside deuteron. If the process is direct, neutron should keep the same momentum as inside deuteron  the momentum distribution bears information about the kind of process Theoretical distribution: Hulthen function in momentum space (red line): a= fm -1 b=1.202 fm -1

If a level is populated -> increase in reaction yield in correspondence of its excitation energy. 2 H+ 19 F -> 17 O* + 4 He -> 16 O+n+ 4 He (a) Many horizontal loci appear -> presence of sequential mechanism corresponding to the population of excited states of 17 O. (b) No level from 5 He sequential decay are present in the experimental plots. Vertical loci -> excited states of 20 Ne -> sequential or QF

Selection of the QF reaction mechanism Experimental study of the neutron momentum distribution inside deuteron. If the process is direct, neutron should keep the same momentum as inside deuteron  the momentum distribution bears information about the kind of process Theoretical distribution: Hulthen function in momentum space (red line): a= fm -1 b=1.202 fm -1 Only events with 0<p n <40 MeV/c will be selected.

Observed 20 Ne states Normalized coincidence yield obtained by dividing the selected coincidence yield by the product of the phase-space factor and of the p−n momentum distribution Three resonance groups corresponding to 20Ne states at: and MeV; , , and MeV; , , and MeV. The normalized yield was fitted simultaneously with four Gaussian curves to separate the resonance contributions

Resonance at lower energy, next to the region of astrophysical interest, with important consequences on the fluorine destruction E 20Ne (MeV)E cm (MeV)JpJp Observed 20 Ne states

Resonance with spin 4 and angular moment 4  suppressed in the direct measurement due to the centrifugal barrier E 20Ne (MeV)E cm (MeV)JpJp Observed 20 Ne states

The dominant resonance is the one with spin 3 therefore, as in the previous case, we do not expect a contribution in the direct measurement due to the centrifugal barrier. E 20Ne (MeV)E cm (MeV)JpJp Observed 20 Ne states

These resonances have already been measured and are therefore used for the normalization E 20Ne (MeV)E cm (MeV)JpJp Observed 20 Ne states

These resonances have already been measured and are therefore used for the normalization E 20Ne (MeV)E cm (MeV)JpJp Observed 20 Ne states

Fitting the THM cross section QF cross section of the 2 H( 19 F,α 0 16 O)n reaction in arbitrary units. horizontal error bars  p− 19 F-relative- energy binning. The vertical error bars  statistical and angular-distribution integration uncertainties. Red band  the cross section calculated in the modified R-matrix approach, normalized to the peak at about 750 keV and convoluted with the experimental resolution.

The 19 F(p,a) 16 O cross section R-matrix parameterization of the 19 F(p,α 0 ) 16 O astrophysical factor. Above 0.6 MeV, the reduced partial widths were obtained through an R- matrix fit of direct data Below 0.6 MeV, the resonance parameters were obtained from the modified R-matrix fit The non-resonant contribution is taken from NACRE (1999). Because of spin-parity, only the resonance at MeV provide a significant contribution Gamow window: keV  this level lies right at edge of the Gamow window for extramixing in AGB stars

19 F(p,a) 16 O reaction reaction rate (a)Reaction rate for the 19 F(p,α 0 ) 16 O. Upper and lower limits are also given, though they are barely visible because of the large rate range. (a)Ratio of the reaction rate in panel (a) to the rate of the 19 F(p,α 0 ) 16 O reaction evaluated following the prescriptions in NACRE (1999). The red band arises from statistical and normalization errors.  A reaction rate enhancement up to a factor of 1.8 is obtained close to temperatures of interest for astrophysics (for instance, T 9 ~0.04 in AGB stars)

Perspectives The contribution of the  0 channel only has been addressed since this is currently regarded as the dominant one at temperatures relevant for AGB stars  Spyrou et al. (2000)  1 channel is even more uncertain! An experimental campaign is scheduled to extract the cross section for the  1 channel and to improve the spectroscopy of the resonances discussed here.