Experimental Investigation of the Stellar Nuclear Reaction ^{12}C + ^{12}C at Low Energies

Abstract

The reactions 12 C( 12 C,a) 20 Ne, 12 C( 12 C,^) 23 Na, and 12 C( 12 C,w) 23 Mg have been studied to provide reliable predictions of their cross-sections in the energy region of interest in stellar carbon burning. Many excited-state transitions have been observed for the a-particles and protons. Measurements for these particles have been made in the center-of-mass energy range 3.23-8.75 MeV. The relatively small yield of the 12 C( 12 C,w) 23 Mg reaction has been investigated from 4.25 to 6.25 MeV. The new results for these reactions lead to lower cross-sections than previously estimated for the region of astrophysical interest.The 12 C + 12 C reaction in stars has aroused new interest recently, owing to strong evidence that in many cases a substantial fraction of 12 C produced in the helium-burning process is not converted to 16 0 (Stephenson 1966;Loebenstein et al. 1967). The carbon burning is expected to occur at temperatures near 10 9 ° K, which corresponds to an effective thermal energy Eq of approximately 2 MeV. At such a temperature, energy losses due to the proposed neutrino process e + + p + P may dominate the radiation loss and considerably reduce the lifetime of the carbon-burning stage. This problem has been discussed in detail by Hayashi, Höshi, and Sugimoto (1962).The experimental study of the reaction 12 C + 12 C is quite complicated because of the large number of final states, as shown in the energy-level diagram of Figure 1. The possible end products are: 23 Na + p, 23 Mg + n, 20 Ne + a, 16 0 + 2a, and 24 Mg + 7. The Coulomb barrier for 12 C + 12 C is about 7 MeV in the center-of-mass system. For convenience, all energies in the 12 C + 12 C system will be given in the center-of-mass system. Previous experimental investigations by Almqvist, Bromley, and Kuehner (1960) and by Almqvist et al. (1964) have shown that the a-particle and proton channels produce the major yield while the neutron channel makes only a small contribution. In their experiment, the a-particle-plus-proton cross-section was measured between 5.0 and 12.5 MeV. All charged particles were counted in a solid-state detector at several angles, and the angular distributions were integrated to obtain the total cross-section. The number of excited states detected, however, was limited by the fairly high detector-cutoff energies of 7.5 and 6 MeV (laboratory) for a-particles and protons, respectively. Reeves (1966) has used these data for an extrapolation of the cross-section down to astrophysical energies. His results will be discussed later. Arnett and Truran (1969) have reexamined the process of carbon burning with regard to nucleosynthesis and energy generation. They have set up a nuclear-reaction network among the elements between carbon and sulfur and have solved the coupled non-linear equations numerically under a variety of conditions. Models for carbon-burning stars including evolutionary effects have also been studied recently by Rakavy, Shaviv, and Zinamon (1967), Murai et al. (1968), Vila (1966), and Beaudet and Salpeter ...

Type Ia supernovae and the12C+12C reaction rate

“…The evolution of the progenitor WD during the simmering phase (which is difficult to address because it is on the borderline between hydrostatic and hydrodynamic phenomena) is controlled by the competition between cooling processes (neutrino emission and convection) and heating by a dominant nuclear reaction: the fusion of two 12 C nuclei at temperatures in the range 2 × 10 8 −10 9 K. A&A 535, A114 (2011) The experimental measurement of the cross-section of the 12 C + 12 C reaction (carbon fusion reaction) at low energies has advanced slowly in the past few decades. At the time of formulation of the carbon detonation model for SNIa (Arnett 1969), the latest known experimental results extended down to a center of mass energy E cm = 3.23 MeV (Patterson et al 1969). Seven years later, when the deflagration model was proposed, the reference 12 C + 12 C reaction rate evaluated by Fowler et al (1975, hereafter FCZ75) was based on experimental measurements at E cm > 2.45 MeV (Mazarakis & Stephens 1973).…”

Constraining the12C+12C fusion cross section for astrophysics

“…The primary impact of the 12 C+ 12 C→ 23 Mg+n reaction (hereafter referred to as CCN) surrounds the component of s-process nucleosynthesis which occurs during convective shell-carbon burning of massive stars. The so-called weak s-process is responsible for the elements between iron and strontium and occurs also during convective core helium burning in massive stars with the neutrons coming mainly from the reaction 22 Ne(α,n) 25 Mg (see Ref.…”

“…The fusion reaction proceeds through the formation of the 24 Mg compound nucleus at high excitation energy (∼15 MeV) followed by alpha, proton, or neutron decay to states in the corresponding residual nuclei ( 20 Ne, 23 Na, and 23 Mg respectively). The proton and alpha decay channels are the most probable due to their positive Q-values (Q p =+2.2 MeV and Q α =+4.6 MeV), while the neutron channel comprises less than 1% of the total yield at astrophysical energies [4] due to a negative Q-value (Q n =-2.6 MeV).…”

The12C(12C,α)20Ne and12C(12C,p)23Na reactions at the Gamow peak via the Trojan Horse Method

“…12 C + 12 C is the sys-tem with more evidence for quasimolecular structure, derived mainly from excitation function measurements for many of the possible reaction channels [1][2][3][4][5][6][7]. The vast amount of data accumulated in this respect still awaits a sound and comprehensive theoretical model capable of explaining all the observed details.…”

“…Below a center of mass energy E cm of 2.5 MeV there is not enough energy to feed 23 Mg even in its ground state and α and p channel are the only relevant ones at low energies. Considerable efforts have been devoted to measure the 12 C + 12 C cross section at astrophysical energies, involving both, charged particle [11][12][13] and gamma ray spectroscopy [14][15][16][17][18][19]. Nevertheless, it has only been previously measured down to E cm = 2.5 MeV, still at the beginning of the region of astrophysical interest.…”