In order to investigate the S-D truncation in the interacting boson model, s'- boson is considered to enlarge the collective subspace. The structures of the s, d and s'-bosons are determined in terms of one of the microscopic IBM approaches. The calculation and discussion of the s'-boson and the system including one s'-boson show that, although the s'-boson has little contribution to the excited states below 2 MeV which are already well described by the sd IBM, it is expected that the introducing of the s'-boson may extend the ...
