During the seventies and eighties a long experimental research program on the quasi-free reactions at low energy was carried out by a small group of nuclear physicists, where Claudio Spitaleri was one of the main protagonists. Nowadays, a posteriori, the results of these studies can be considered an essential step preparatory to the application of the Trojan Horse Method (THM) in Nuclear Astrophysics. Everything began in the seventies, with the publication on Nuclear Physics A of a paper

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... ng the results of an experiment carried out by a group of Japanese researchers on the three alpha-particles produced in the 3 He + 9 Be reaction at incident energy of 4 MeV [1]. The coincidence detection of two of the outgoing alpha particles showed that, in addition to the usual sequential decay through states of 8 Be, another mechanism contributed to the spectra, giving place to a bump centered at the minimum momentum of the undetected alpha. Angular correlations and energy sharing spectra showed that the shape of this bump was consistent with a contribution of the 5 He( 3 He, α) 4 He virtual reaction occurring in the frame of a quasi-free process. In this process the undetected alpha cluster αs acts as spectator of the process and keeps the momentum it owns at the time of the interaction, as schematized in fig. 1 .