Zahlavi

Astrofyzika

Basic research activities include experiments for nuclear astrophysics using (mainly) the indirect methods as well as the experiments concerning exotic nuclei, nuclei far from stability line. An important aspect of the basic experimental research is cooperation. It not only allows to concentrate efforts and competences of different group, but it also opens an access to various facilities and tools that otherwise would not be possible to operate by one sole group.

ANC for 14C isotope

The 14C(n, γ)15C reaction plays an important role in inhomogeneous big bang models. In Timofeyuk et al. [Phys. Rev. Lett. 96 162501 (2006)] it was shown that the 14C(n, γ)15C radiative capture at astrophysically relevant energies is a peripheral reaction, i.e., the overall normalization of its cross section is determined by the asymptotic normalization coefficient (ANC) for 15C→14C+n. Here we present new measurements of the 14C(d, p)15C differential cross sections at deuteron incident energy of 17.06 MeV and the analysis to determine the ANCs for neutron removal from the ground and first excited states of 15C. The results are compared with previous estimations.

ANC for 15N isotope

The 15N(p,γ)16O reaction provides a path from the CN cycle to the CNO bi-cycle and CNO tri-cycle. The measured astrophysical factor for this reaction is dominated by resonant capture through two strong Jπ=1- resonances at ER=312 and 962 keV and direct capture to the ground state. Asymptotic normalization coefficients (ANCs) for the ground and seven excited states in 16O were extracted from the comparison of experimental differential cross sections for the 15N(3He,d)16O reaction with distorted-wave Born approximation calculations. Using these ANCs and proton and α resonance widths determined from an R-matrix fit to the data from the 15N(p,α)12C reaction, we carried out an R-matrix calculation to obtain the astrophysical factor for the 15N(p,γ)16O reaction. The results indicate that the direct capture contribution was previously overestimated. We find the astrophysical factor to be S(0)=36.0±6.0 keV b, which is about a factor of 2 lower than the presently accepted value. We conclude that for every 2200±300 cycles of the main CN cycle one CN catalyst is lost due to this reaction.

  • A. M. Mukhamedzhanov et al., Phys.Rev. C 83, 044604 (2011), Phys.Rev. C 78, 015804 (2008)
  • Asymptotic normalization coefficients from the 14C(d, p)15C reaction

Astrophysical nuclear reactions - indirect methods

GAČR P203-10-310 /2010

Reactions 15N(p,g)16O, 18O(p,a)15N, 17O(p,a)14N reactions were measured. The reactions d(d,p)t a d(d,n)3He were studied in the energies down to 2 keV and it was the first experiment with charged spectator, the results show 15% change in reaction rates at BBN temperatures. 6Li(d,a)4He a 7Li(p,a)4He reactions were used to practically verify the independence of THM method on Trojan Horse particle. 11B(p,a)8Be reaction was analyzed usin Monte-Carlo techniques to show that it is possible to remove the influence of other background reactions from the data.ANCs were deduced for the lowest states of 18O(n,g)19O capture from 18O(d,p)19O reaction, the main contribution coming from capture to ground state and 1.471 MeV excited state.The reaction 17O(n,a)14C was studied, the THM method and reaction 17O(d,a)14C was used and the cross section was deduced downto subthreshold energies. It was shown for the first time, that THM can overcome the centrifugal barrier suppression effect and it opens a unique possibility to study n-induced reactions on short-lived nuclei.The reaction 14C(n,g)15C of inhomogeneous Big Bang nucleosynthesis was studied, earlier found contradictions for using charge symmetry 15F-15C for mirror reaction 14O(p,g)15F were overcome and with  14C(d,p)15C reaction and FR-ADWA approach there were obtained ANC coefficients that were fully compatible with measurements based on charge symmetry. It was shown that the approach also increased reliability and accuracy.The measurement of the 19F(p,a)16O process by THM  has revealed a resonant structure at low astrophysical energies that was not experimentally accessible before. This structure results in faster reaction rate up to 70% and depletion of surface 19F can reach 40% of previously accepted model values.In the frame of the project, the indirect ANC an THM methods were used to study stellar and Big Bang nucleosythesis, where an important role belongs to radiative capture processes (p,g) and (n,g) and (p,a) a (n,a).

Research

A cooperation with foreign partners is an important factor for the experimental basic research. It not only allows to concentrate efforts and competences of different groups from different countries but it also simplifies the access to various facilities and tools that would not be possible to operate by one sole group. Basic research activities include experiments for nuclear astrophysics using (mainly) the indirect methods as well as the experiments concerning exotic nuclei and nuclei far from stability line.


After a decade of experimental efforts, the eited state 02+ in 34Si isotope was found. This neutron rich nucleus is close to Island of Inversion, where classical magic numbers are eroded and a coexistence of spherical and deformed states appears at low excitation energies. The observed stated has an excitation energy 2719(3) keV and a half-life of 19.4(7) ns, the deformation beta was determined to 0.29(4). Moreover,the experimental method allowed to identify that this state is fed from newly observed 1+ state from 34Al with half-life of 26(1) ms.

  • collaboration - IFIN-HH Magurele,Bordeaux,GANIL
  • F. Rotaru et al., Phys.Rev.Lett.109, 092503 (2012)

The low lying states of 18Na were studied by means of resonant elastic scattering of H(17Ne,p)17Ne at energy of 4 A.MeV on radioactive ion accelerator SPIRAL. Supporting arguments for existence of two very narrow low lying states in 18Na were found. This would result in understanding of two-proton 19Mg decay proceeds simultaneously - as the so-called true 2p decay.

  • collaboration - GANIL/SPIRAL2
  • M. Assie et al., Phys.Lett. B 712, 198 (2012)

Astrophysical S(E) factor for 11B(p,alpha)8Be reaction was studied in an energy range from 600 keV down to zero by indirect THM method, which repsents the first measurement of astrophysical S factor at the Gamow peak. This energy region covers region of stellar burning of light elements LiBeB and future aneutronic fusion power plants using 11B+p cycle. The measured S factor 2.07 (41) MeVb support the pviously extrapolated one and the experimentally determined screening potential is compatible with the pvious value, confirming that theoretical adiabatic limit gives lower value.

  • collaboration - INFN LNS Catania
  • L. Lamia et al., J. Phys.(London) G39, 015106 (2012)

The cross section of the radiative proton capture reaction on the drip line nucleus 12N was investigated using the asymptotic normalization coefficient (ANC) method. The 14N(12N, 13O)13C proton transfer reaction at 12 MeV/nucleon was used to extract the ANC for 13O → 12N + p and calculate from it the direct component of the astrophysical S factor of the 12N(p, γ)13O reaction. The optical potentials used and the distorted-wave Born approximation analysis of the proton transfer reaction were discussed. For the entrance channel, the optical potential was inferred from an elastic scattering measurement carried out at the same time as the transfer measurement. From the transfer, the square of the ANC, C2 (p1/2,13Og.s.) = 2.53 ± 0.30 fm-1 was determined, and hence a value of 0.33(4) keVb was obtained for the direct astrophysical S factor at zero energy. Constructive interference at low energies between the direct and resonant captures leads to an enhancement of Stotal(0) = 0.42(6) keVb. The 12N(p, γ)13O reaction was investigated in relation to the evolution of hydrogen-rich massive Population III stars, for the role that it may play in the hot pp-chain nuclear burning processes, possibly occurring in such objects.

  • Phys. Rev. C79, (2009), 025805


Shape coexistence in heavy Si isotopes

After a decade of experimental efforts, the excited state 02+ in 34Si isotope was found. This neutron rich nucleus is close to Island of Inversion, where classical magic numbers are eroded and a coexistence of spherical and deformed states appears at low excitation energies. The observed stated has an excitation energy 2719(3) keV and a half-life of 19.4(7) ns, the deformation beta was determined to 0.29(4). Moreover, the experimental method allowed to identify that this state is fed from newly observed 1+ state from 34Al with half-life of 26(1) ms.

  • collaboration - IFIN-HH Magurele,Bordeaux,GANIL
  • F. Rotaru et al., Phys.Rev.Lett.109, 092503 (2012)

Search for 18Ne excited states for two-proton decay of 19Mg

The low lying states of 18Na were studied by means of resonant elastic scattering of H(17Ne,p)17Ne at energy of 4 A.MeV on radioactive ion accelerator SPIRAL. Supporting arguments for existence of two very narrow low lying states in 18Na were found. This would result in understanding of two-proton 19Mg decay proceeds simultaneously - as the so-called true 2p decay.

  • collaboration - GANIL/SPIRAL2
  • M. Assie et al., Phys.Lett. B 712, 198 (2012)

S factor in 11B p,alpha reaction by THM method

Astrophysical S(E) factor for 11B(p,alpha)8Be reaction was studied in an energy range from 600 keV down to zero by indirect THM method, which repsents the first measurement of astrophysical S factor at the Gamow peak. This energy region covers region of stellar burning of light elements LiBeB and future aneutronic fusion power plants using 11B+p cycle. The measured S factor 2.07 (41) MeVb support the previously extrapolated one and the experimentally determined screening potential is compatible with the previous value, confirming that theoretical adiabatic limit gives lower value.

  • collaboration - INFN LNS Catania
  • L. Lamia et al., J. Phys. (London) G39, 015106 (2012)

Direct radiative capture 12N(p,gamma)

The cross section of the radiative proton capture reaction on the drip line nucleus 12N was investigated using the asymptotic normalization coefficient (ANC) method. The 14N(12N, 13O)13C proton transfer reaction at 12 MeV/nucleon was used to extract the ANC for 13O → 12N + p and calculate from it the direct component of the astrophysical S factor of the 12N(p, γ)13O reaction. The optical potentials used and the distorted-wave Born approximation analysis of the proton transfer reaction were discussed.

For the entrance channel, the optical potential was inferred from an elastic scattering measurement carried out at the same time as the transfer measurement. From the transfer, the square of the ANC, C2 (p1/2,13Og.s.) = 2.53 ± 0.30 fm-1 was determined, and hence a value of 0.33(4) keVb was obtained for the direct astrophysical S factor at zero energy. Constructive interference at low energies between the direct and resonant captures leads to an enhancement of Stotal(0) = 0.42(6) keVb. The 12N(p, γ)13O reaction was investigated in relation to the evolution of hydrogen-rich massive Population III stars, for the role that it may play in the hot pp-chain nuclear burning processes, possibly occurring in such objects.

  • collaboration - Texas A&M University
  • A. Banu, et al. Phys. Rev. C79, (2009), 025805