|
Recent development of high-energy heavy-ion accelerators enables us to obtain the secondary radioactive ion beam of the very neutron-rich nuclei, which provides us the unique oppotunity to investigate the structures of nuclei far from the stability line. For some light nuclei on the neutron drip-line, some exotic phenomena that have not been observed for the stable region have been found. ''Neutron halo'' is one of such phenomena. This nucleus has a novel twofold structure composed of a core with normal nuclear density and a halo with very weak binding of the valence neutron(s). Some halo nuclei have two-neutron halo system. This is a three-body system(``core+n+n'' where any two-body sub-systems(``n+n''and ``core+n'') are unbound. This type of nucleus is called ``Borromean'', whose binding mechanism is not well understood. In this study, we report on the invariant mass spectrum of unbound nucleus 13Be which is a component of Borromean halo nucleus 14Be. In the experiment, we measured the mass of 13Be produced in the one-neutron stripping reaction of 14Be at 70.18 MeV/nucleon on a carbon target. 13Be breaks up into 12Be + n system immediately, and we have measured the coincidence by using a magnetic spectrometer and neutron counters and deduced the invariant mass of 12Be + n system in order to search for the resonance of 13Be. In the relative energy spectrum we observed a asymmetric peak at about Erel=300 [keV]. The spectral shape indicates the non-resonant continuum of 12Be + n. This implies that 14Be has s-wave valence neutrons, and thus 12Be + n system is structureless due to no centrifugal barrier on this neutron. In this study, we have extracted the scattering length of 12Be + n to be a = -0.76 ± 0.07 [fm]. Such a small value indicates that the effective interaction between 12Be and n is small. This result may be important to understand the binding mechanism of 14Be. |