Radiative capture reactions via indirect method
  In this paper we address the indirect method, which can provide a powerful
technique to obtain information about radiative capture reactions at
astrophysically relevant energies. The idea of the indirect method is to use
the indirect reaction $A(a, s\,\gamma)F$ to obtain information about the
radiative capture reaction $A(x,\,\gamma)F$, where $a=(s\,x)$ and $F=(x\,A)$.
The main advantage of using the indirect reactions is the absence of the
penetrability factor in the channel $x+A$, which suppresses the low-energy
cross sections of the $A(x,\,\gamma)F$ reactions and does not allow to measure
these reactions at astrophysical energies. A general formalism to treat
indirect resonant radiative capture reactions is developed when only a few
intermediate states do contribute and statistical approach cannot be applied.
Angular dependence of the triple differential cross section at fixed scattering
angle of the spectator $s$ is the angular $\gamma-s$ correlation function.
Using indirect resonant radiative capture reactions one can obtain the
information about important astrophysical resonant radiative capture reactions,
like $(p,\,\gamma), \,\,(\alpha,\,\gamma)$ and $(n,\,\gamma)$ on stable and
unstable isotopes. The indirect technique makes accessible low-lying
resonances, which are close to the threshold, and even subthreshold bound
states located at negative energies. In this paper, after developing the
general formalism, we demonstrated the application of the indirect reaction
${}^{12}{\rm C}({}^{6}{\rm Li},d\,\gamma){}^{16}{\rm O}$ proceeding through
$1^{-}$ and $2^{+}$ subthreshold bound states and resonances to obtain the
information about the ${}^{12}{\rm C}(\alpha,\,\gamma){}^{16}{\rm O}$ radiative
capture at astrophysically most effective energy $0.3$ MeV what is impossible
using standard direct measurements.
