On the decay of compound nuclei following alpha-particle and12C induced reactions

Multiple coincidence rates have been measured using a detector system consisting of a Ge(Li) spectrometer and eight NaI(Tl) or eight liquid scintillators. Reactions induced byα-particles with energies of 51–55 MeV and 118 MeV¹²C ions are studied. The data are analysed to give the first and second central moments of the distribution of the number ofγ-rays feeding individual levels in the final nuclei. When these numbers are compared to spin distributions calculated with the statistical model code GROGI the relative importance of dipole and quadrupole deexcitation modes can be ascertained. In particular, in the¹²²Te(α, 4n)¹²²Xe reaction theγ-decay prior to the entry into the ground band is well described as a statistical process proceeding to 50% by dipole and 50% by quadrupole radiation. In the¹⁶⁶Er(α,4n)¹⁶⁶Yb and¹⁹²Os(α,4n)¹⁹²Pt reactions the relative amount of quadrupole radiation is larger and it seems that the dipole and quadrupole decay takes place via separate cascades. In the¹⁶⁴Dy(¹²C, 7-8n) reactions the average multiplicity is independent of spin, suggesting that the nucleus forgets the spin of the entry state before the process enters into the ground band. In the¹⁷⁶Yb(¹²C, 8n)¹⁸⁰Os reaction, finally, the nucleus definitely retains memory of the entry state during the decay. In this last case the multiplicity measurement is combined with aγ-ray singles measurement to give an average excitation energy prior to theα-decay and the average moment of inertia characterising the decay of the high-spin states.