Marco La Cognata INFN-LNS Catania

FLUORINE DESTRUCTION IN STARS: FIRST EXPERIMENTAL STUDY OF THE 19F(p,α0)16O REACTION AT

ASTROPHYSICAL ENERGIES

Marco La CognataINFN-LNS Catania

Asymptotic Giant Branch (AGB) stars are the main s-process site. 13C(a,n)16O, active in the intershell, is the main neutron source in low-mass AGB stars

Origin of the heavy elementsRe

lativ

e ab

unda

nces

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.19F 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.19F 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 19F(p,a0)16O reaction

Below Ecm = 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Unpublished data (Lorentz-Wirzba 1978) suggests that no resonance occurs

a0 is the dominant channel in the energy region of astrophysical interest

s

cA

a

C

direct break-up

x

2-body process

Additional advantages:● reduced systematic errors due to straggling, background…● magnifying glass effectBut...● off-shell cross section deduced (x virtual particle)● no absolute units

FromA+a(xs) c+C+s @ 10-60 MeV

A + x c + C @ 5-20 keV

Though EA >> VCoul it is possible to measure at the Gamow peak since:

Ec.m.=EA-x-Qx-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 contributionTHM

reac

tion

Indirect Techniques: the THM

s

cA

a

C

direct break-up

x

2-body process

Additional advantages:● reduced systematic errors due to straggling, background…● magnifying glass effectBut...● off-shell cross section deduced (x virtual particle)● no absolute units

FromA+a(xs) c+C+s @ 10-60 MeV

A + x c + C @ 5-20 keV

Though EA >> VCoul it is possible to measure at the Gamow peak since:

Ec.m.=EA-x-Qx-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 contributionTHM

reac

tion

Indirect Techniques: the THM

See C. Spitaleri talk for more details

The THM for resonance reactionsIn the “Trojan Horse Method” (THM) the astrophysically relevant reaction, sayA(x,c)C, is studied through the 23 direct processA(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

xs

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 breakupMi(E) is the amplitude of the transfer

The experimentd: Trojan horse nucleus p+n B=2.2 MeV |ps|=0 MeV/cn: spectatorp: participant d(19F,a16O)n Q3b=5.889 MeV

19F(p,a) 16O Q2b=8.113 MeV

Butane gas 52mbar

(150 µg/cm2 )

The experiment

d

19F

pn

20Ne*16O

a

The experiment was performed at INFN-LNS Catania

Beam: 19F@50 MeV

Only in the coincidence 1-5 the a0 channel was populated significantly

a1 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. 2H(19F,a1

16O)n2. 19F(p,a)16O

3. 2H(19F,a016O)n

4. 19F(12C,15N)16O


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. 2H(19F,a1

16O)n2. 19F(p,a)16O

3. 2H(19F,a016O)n

4. 19F(12C,15N)16O

From energy conservation:Ebeam+Q=E1+E2+E3 Y=Ebeam-E2-E3

x=p32/2u

Events from d(19F,a16O)n must gather along a straigth line

a0 channel Statistically significant only coincidence PSD1-PSD5.

(1)

(2)

(3)(4)

(1)(2)

(3)

(4)


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

a0 channel Statistically significant only coincidence PSD1-PSD5.Selection of the reaction channel

Selection of the QF reaction mechanism

22222

2 11)()(1)(

sss pbpaba

baabp

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=0.2317 fm-1

b=1.202 fm-1

If a level is populated -> increase in reaction yield in correspondence of its excitation energy.

2H+19F -> 17O* + 4He -> 16O+n+4He

(a) Many horizontal loci appear -> presence of sequential mechanism corresponding to the population of excited states of 17O.

(b) No level from 5He sequential decay are present in the experimental plots.

Vertical loci -> excited states of 20Ne -> sequential or QF

Selection of the QF reaction mechanism

22222

2 11)()(1)(

sss pbpaba

baabp

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=0.2317 fm-1

b=1.202 fm-1

Only events with 0<pn<40 MeV/c will be selected.

Observed 20Ne 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: 12.957 and 13.048 MeV;

13.222, 13.224, and 13.226 MeV;

13.529, 13.586, and 13.642 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

E20Ne (MeV) Ecm (MeV) Jp

12.957 0.109 2+


Resonance with spin 4 and angular moment 4 suppressed in the direct measurement due to the centrifugal barrier


12.957 0.109 2+

13.048 0.2 4+


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.


12.957 0.109 2+

13.048 0.2 4+

13.22213.22413.226

0.3740.3760.378

0+1-3-


These resonances have already been measured and are therefore used for the normalization


12.957 0.109 2+

13.048 0.2 4+

13.22213.22413.226

0.3740.3760.378

0+1-3-

13.52913.58613.642

0.6810.7380.794

2+2+2+


Fitting the THM cross section

QF cross section of the 2H(19F,α016O)n

reaction in arbitrary units.

horizontal error bars p−19F-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 19F(p,a)16O cross sectionR-matrix parameterization of the 19F(p,α0)16O 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 12.957 MeV provide a significant contribution

Gamow window: 27-94 keV this level lies right at edge of the Gamow window for extramixing in AGB stars

19F(p,a)16O reaction reaction rate(a) Reaction rate for the 19F(p,α0)16O. Upper

and lower limits are also given, though they are barely visible because of the large rate range.

(b) Ratio of the reaction rate in panel (a) to the rate of the 19F(p,α0)16O 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, T9~0.04 in AGB stars)

Perspectives The contribution of the a0 channel only has been addressed since this is currently regarded as the dominant one at temperatures relevant for AGB stars

Spyrou et al. (2000)

a1 channel is even more uncertain!

An experimental campaign is scheduled to extract the cross section for the a1 channel and to improve the spectroscopy of the resonances discussed here.

Marco La Cognata INFN-LNS Catania

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