Jeudi 1er octobre 2009, à 14h00
Astrophysical reactions proceeding through compound states represent one of the crucial part of nuclear astrophysics. However, due to the presence of the Coulomb barrier, it is often very difficult or even impossible to obtain the astrophysical S(E) factor from measurements in the laboratory at astrophysically relevant energies.
The Trojan Horse method (THM) provides a unique tool to obtain the information about resonant astrophysical reactions at astrophysically relevant energies. The information about the resonant reaction x + A —> b + B is obtained via the THM reaction a + A —> y + b + B, where the Trojan Horse (TH) nucleus a=(yx). In the THM the compound system F*=x + A= b + B is formed in the subsystem of the reaction, while the outgoing particle y is treated as a spectator. The quasi-free kinematics provides the best conditions for it.
In this talk I will address the following issues : introduction to THM, kinematical conditions, why it works, new R matrix approach to analyze the TH reactions. The TH reaction can be considered as the direct transfer reaction a + A—> y + F* populating the compound state F* with its subsequent decay F* -> b + B. Since the transferred particle x is virtual, there is no Coulomb barrier in the initial channel of the binary sub-reaction x + A —> b + B. It allows one to measure the resonance states at very low (even at negative) energies. However, the R matrix analysis becomes more complicated and a new generalized multi-level, multi-channel R matrix approach, which allows to take into account the off-shellity of particle x, will be presented. In this approach the entry reduced width is replaced by the direct transfer reaction amplitude populating the compound state F*. The real application will be demonstrated for key astrophysical resonant reactions and . It will be shown that the THM allows one to obtain the astrophysical factors at low energies for isolated and overlapping resonances. Moreover, it allows us to determine the energy, width and spin-parity of the unknown compound states.