CNRS Université Bordeaux

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the surrogate method

This surrogate method technique consists the use of an alternative reaction (the surrogate reaction) to produce the same compound nucleus as in the direct neutron induced reaction and to measure the probability of different decay modes: fission or gamma cascade emission. This is particulary pertinent in the case of short lived target nuclides.

Here, the absorption of a neutron by the target nucleus of mass (A-1) leads to the formation of a compound nucleus of mass (A). Then the excited compound nucleus or CN decays via one of the possible modes, e.g. fission, gamma emission or neutron emission (these are the (n,f), n,γ and (n,n) channels). The CN of mass (A) can also be produced using a transfer reaction of a beam nuclide w on a target nuclide X associated to an ejectile y. The ejectile is detected, is identified via ΔE-E technique and its total kinetic energy is measured so that the associated CN is unambiguously identified and its excitation energy is determined from the kinematics of the reaction.

The decay probability of the CN is extracted from the ratio of coincidence (fission or gamma with an ejectile) to single (ejectile) events after taking in account the efficiency factors. This decay probability is determined as a function of its excitation energy. Then this decay probability of the CN is multiplyed by the cross section for the formation of this CN at this excitation energy via direct neutron absorption, which is computed from optical model calculations.

Note that the reliability of the surrogate method is based on the fact that the surrogate reaction proceeds via a CN formation. The decay probability at a given excitation energy is not to be sensitive to the its formation process if the distribution of the Jπ is the same in both cases or, if the decay process is not sensitive to the distribution of the Jπ of the CN. The relevance of these assumptions are tested for the different channels and reactions.