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Register a new userTheoretical study of the ${}^{4}rm{He}(gamma,p)^3rm{H}$ and ${}^{4}rm{He}(gamma,n)^3rm{He}$ reactions
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{it Ab initio} calculation of the total cross section for the reactions $^{4}rm{He}(gamma,p)^3rm{H}$ and $^{4}rm{He}(gamma,n)^3rm{He}$ is presented, using state-of-the-art nuclear forces. The Lorentz integral transform (LIT) method is applied, which allows exact treatment of the final state interaction (FSI). The dynamic equations are solved using the effective interaction hyperspherical harmonics (EIHH) method. In this calculation of the cross sections the three-nucleon force is fully taken into account, except in the source term of the LIT equation for the FSI transition matrix element.
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The cross-sections and analyzing powers for $(p,n)$ reactions on ${}^{3}{rm He}$ and ${}^{4}{rm He}$ have been measured at a bombarding energy of $T_p$ = 346 MeV and reaction angles of $theta_{rm lab}$ = $9.4^{circ}$--$27^{circ}$. The energy transfer spectra for ${}^{3}{rm He}(p,n)$ at large $theta_{rm lab}$ ($ge$ $16^{circ}$) are dominated by quasielastic contributions, and can be reasonably reproduced by plane-wave impulse approximation (PWIA) calculations for quasielastic scattering. By contrast, the known $L$ = 1 resonances in ${}^{4}{rm Li}$ are clearly observed near the threshold in the ${}^{4}{rm He}(p,n)$ spectra. Because these contributions are remarkable at small angles, the energy spectra are significantly different from those expected for quasielastic scattering. The data are compared with the PWIA calculations, and it is found that the quasielastic contributions are dominant at large $theta_{rm lab}$ ($ge$ $22^{circ}$). The nuclear correlation effects on the quasielastic peak for ${}^{4}{rm He}(p,n)$ are also discussed.
The astrophysical $^{3}{rm He}(alpha, gamma)^{7}{rm Be}$ and $^{3}{rm H}(alpha, gamma)^{7}{rm Li}$ direct capture processes are studied in the framework of the two-body model with the potentials of a simple Gaussian form, which describe correctly the phase-shifts in the s-, p-, d-, and f-waves, as well as the binding energy and the asymptotic normalization constant of the ground $p_{3/2}$ and the first excited $p_{1/2}$ bound states. It is shown that the E1-transition from the initial s-wave to the final p-waves is strongly dominant in both capture reactions. On this basis the s-wave potential parameters are adjusted to reproduce the new data of the LUNA collaboration around 100 keV and the newest data at the Gamov peak estimated with the help of the observed neutrino fluxes from the Sun, $S_{34}$(23$^{+6}_{-5}$ keV)=0.548$pm$0.054 keV b for the astrophysical S-factor of the capture process $^{3}{rm He}(alpha, gamma)^{7}{rm Be}$. The resulting model describes well the astrophysical S-factor in low-energy Big Bang nucleosynthesis region of 180-400 keV, however has a tendency to underestimate the data above 0.5 MeV. Two-body potentials, adjusted on the properties of the $^7$Be nucleus, $^3{rm He}+alpha$ elastic scattering data and the astrophysical S-factor of the $^{3}{rm He}(alpha, gamma)^{7}{rm Be}$ direct capture reaction, are able to reproduce the properties of the $^7$Li nucleus, the binding energies of the ground 3/2$^-$ and first excited 1/2$^-$ states, and phase shifts of the $^3 {rm H}+alpha$ elastic scattering in partial waves. Most importantly, these potential models can successfully describe both absolute value and energy dependence of the existing experimental data for the mirror astrophysical $^{3}{rm H}(alpha, gamma)^{7}{rm Li}$ capture reaction without any additional adjustment of the parameters.
Four light-mass nuclei are considered by an effective two-body clusterisation method; $^6$Li as $^2$H$+^4$He, $^7$Li as $^3$H$+^4$He, $^7$Be as $^3$He$+^4$He, and $^8$Be as $^4$He$+^4$He. The low-energy spectrum of each is determined from single-channel Lippmann-Schwinger equations, as are low-energy elastic scattering cross sections for the $^2$H$+^4$He system. These are presented at many angles and energies for which there are data. While some of these systems may be more fully described by many-body theories, this work establishes that a large amount of data may be explained by these two-body clusterisations.
Astrophysical $S$ factors and reaction rates of the direct radiative capture processes $^{3}{rm He}(alpha, gamma)^{7}{rm Be}$ and $^{3}{rm H}(alpha,gamma)^{7}{rm Li}$, as well as the primordial abundance of the $^{7}{rm Li}$ element, are estimated in the framework of a modified two-body potential model. It is shown that suitable modification of phase-equivalent $alpha-^{3}{rm He}$ potentials in the $d$ waves can improve the description of the astrophysical $S$ factor for the direct $^{3}{rm He}(alpha, gamma)^{7}{rm Be}$ radiative capture reaction at energies above 0.5 MeV. An estimated $^{7}{rm Li/H}$ abundance ratio of $(4.89pm 0.18 )times 10^{-10}$ is in very good agreement with the recent measurement of $(5.0pm 0.3) times 10^{-10}$ by the LUNA collaboration.
The ${^3{rm He}}(alpha,gamma){^7{rm Be}}$ and ${^3{rm H}}(alpha,gamma){^7{rm Li}}$ astrophysical $S$ factors are calculated within the no-core shell model with continuum using a renormalized chiral nucleon-nucleon interaction. The ${^3{rm He}}(alpha,gamma){^7{rm Be}}$ astrophysical $S$ factors agree reasonably well with the experimental data while the ${^3{rm H}}(alpha,gamma){^7{rm Li}}$ ones are overestimated. The seven-nucleon bound and resonance states and the $alpha+{^3{rm He}}/{^3{rm H}}$ elastic scattering are also studied and compared with experiment. The low-lying resonance properties are rather well reproduced by our approach. At low energies, the $s$-wave phase shift, which is non-resonant, is overestimated.
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