Nuclear Data for Astrophysics
Experimental and Theoretical Investigations of Proton-and Alpha-Particle-Induced Nuclear Reactions on Light Nuclei for Astrophysics
Tech Area / Field
- PHY-ANU/Atomic and Nuclear Physics/Physics
8 Project completed
Senior Project Manager
Lapidus O V
National Nuclear Center of the Republic of Kazakstan / Institute of Nuclear Physics, Kazakstan, Almaty
- University of Colorado at Boulder, USA, CO, Boulder\nOhio State University / Department of Physics and Astronomy, USA, OH, Athens
Project summaryA very actual problem of nuclear astrophysics is explanation of the light element (Li, Be, B) abundance in the Universe. This interest is stipulated by the understanding that these elements can not be produced in stars in the reactions of nuclear synthesis since they are intensively destroyed in reactions (p,g) and (p,a) at even comparatively low temperatures 2-5ґ106 K. At such conditions, as the calculations show , content of the Li, Be, B isotopes has to be Ј 10-13 compared to that of hydrogen. The observed abundances are in fact for 2-3 orders of magnitude higher. To resolve the contradiction helps Big Bang model that now provides the most correct predictions for the occurrences of light elements.
One of the sources for 6Li production in Big Bang model is the reaction D(a,g)6Li. But the absence of reliable direct measurements of its cross sections and corresponding astrophysical S-factors in the region of rather low energies (< 700 keV) makes the model predictions quite ambiguous . Direct measurements of the cross sections were performed at E і 710 keV [2, 3]. Upper limit of the cross section measured in  at 53 keV is much higher than the same in theoretical estimations [3, 5]. It is obvious that obtaining more accurate data was restricted by used thick target and comparatively high g-background. Data on cross sections at 100-600 keV were obtained from the process reverse to radiative capture with measurements of triple differential cross sections of 6Li dissociation to deuteron and a-particle in Coulomb field of 208Pb nucleus . But precision of the data is limited due to the large errors at measurement of the triple differential cross sections (і 30%), and with the necessity to take correctly into account the different multipole contributions and the nuclear part of the break up amplitude. The cross sections for the reaction D(a,g)6Li. calculated in [3, 7] at extremely low energies lie within a wide range. Thus there is need to carry out new direct measurements of the D(a,g)6Li. reaction cross sections and obtain more precise reaction rates on the basis of reliable theoretical approaches.
Radiative capture reaction 6Li(p,g)7Be is of crucial importance for 7Li production, the isotope that is formed by electron capture by a terminal nuclei 7Be. Values of the astrophysical S-factor for this reaction were obtained at Ep > 150 keV with 30% error in low-energy region . Calculated rates of this reaction at T9 Ј 1.5 were higher than those recommended in . This discrepancy can arise due to large experimental errors and uncertainties in the extrapolation procedure. The problem is that the binding energy of 7Be in the channel p + 6Li is quite high (e = 5.609 MeV) and, correspondingly, the reaction 6Li(p,g)7Be is not completely peripheral. That is why a method is required to calculate S-factors correctly taking into account the contributions of both low and high relative distances of colliding particles to total cross sections of 7Be formation. To solve this problem one should obtain the values of nuclear vertex constant for virtual decay 7Be ® p + 6Li. This provides us to take into account correctly the periphery part in the amplitude of direct radiative capture.
The present Project is devoted to experimental and theoretical investigation of the reactions of direct radiative capture D(a,g)6Li and 6Li(p,g)7Be at energies below 600 keV. As well, there will be measured differential cross sections for particle transfer reactions 6,7Li(a,6,7Li)a, 6,7Li(d,6,7Li)2H and 6Li(3He,d)7Be at energies 10-15 MeV/nucleon. These data are a valuable addition to radiative capture; with their help there will be obtained the values for nuclear vertex constants of virtual decays 6Li ® a + d, 6Li ® a + n + p, 7Li ® a + t and 7Be ® p + 6Li in framework of the developed method [10, 11] that includes correctly three-particle Coulomb dynamics. The obtained values of nuclear vertex constants will be used in calculations of astrophysical S-factors of the reactions D(a,g)6Li and 6Li(p,g)7Be in the region of extremely low energies.
Experimental and theoretical teams of the Nuclear Physics Department of INP NNC RK will take part in the Project. The experimental group consists of famous specialists experienced in investigations of particle transfer reactions and radiative capture at low energies [12-14]. The theoretical group unites the specialists famous for their calculations performed within the cluster model and experienced in calculations of complicated nuclear processes, including calculation of radiative capture reaction rates at astrophysical energies [15, 16].
At present time the majority of the Project participants works to finish the ISTC Project K-497, in which on the beam of linear accelerator Rechargable Accelerator Complex 2-1 (RAC-2-1) using an improved experimental method measured differential cross sections of radiative capture reaction protons on some 1p-shell nuclei with precision to 10%. There was supplemented a library of experimental and evaluated data on interaction of charged particles with light nuclei ranging from hydrogen to oxygen.
Thus, the present Project is a logical continuation of the project K-497 which is aimed to expand the database on nuclear constants with the data required for astrophysics and nuclear fusion.
Availability in the State Enterprise INP NNC RK of the unique accelerating facility RAC-2-1 that produces two crossed beams with energy spread 150 eV in the diapason 0.2 – 1.5 MeV (protons) and the isochronous cyclotron facilities U-150M with energy up to 60 MeV makes it possible to carry out the precision experiments in a wide range of energies for nuclear constants obtaining.
- There will be advanced experimental methods for measurements of differential cross sections in the aforesaid reactions of radiative capture and transfer reactions what will make it possible to lower the measurement errors below 15% and 7%, respectively.
- There will be developed theoretical methods of experimental data analysis on the basis of multicluster model and asymptotic method of nuclear processes analysis to obtain needed information about reliable values of nuclear vertex constants.
The obtained within the Project new data will enable to clarify specific parameters of the evolutionary model of the Universe and make on this basis more reliable predictions concerning the abundances of light elements in stars and cosmic radiation. The Project will serve for integration of weapon scientists from NIS into international scientific community.
Scope of activity
- Measurement of differential cross sections for the reactions D(a,g)6Li, 6Li(p,g)7Be in the energy range 170 – 600 keV and for the transition reaction 6,7Li(d,6,7Li)2H, 6,7Li(a,6,7Li)a and 6Li(3He,d)7Be at energies 10 – 15 MeV/nucleon.
- Calculation of the astrophysical S-factors and reaction rates for D(a,g)6Li, 6Li(p,g)7Be at extremely low energies using of obtained nuclear vertex constant for virtual 2- and 3-body decay 6Li ® a+d, 6Li ® a+n+p and 7Be ® p+6Li, with taking into account three-particle Coulomb dynamics.
- Completion of database with new nuclear data for astrophysics and nuclear fusion.
Role of foreign collaborators
- Exchange of information within the Project;
- Comments to technical reports on the Project;
- Joint workshops and working seminars.
Technical approaches and methodology
Measurements of differential cross sections for transfer reactions will be based on a DЕ-Е method using silicon semiconductor detectors. Registration of g-quanta from the radiative capture reactions will be performed by germanium and scintillation detectors.
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/14/. N. Burtebaev, A. Duisebaev, S.B. Sakuta. Refractive effects in the reaction 13C(3He,t)13N at an energy 60 MeV. // Phys. At. Nucl. V.60, N 1, 2003 P. 47-55
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