Coulomb Excitation of Medium-Weight Nuclei

Abstract

We have studied gamma-ray transitions in nuclei from titanium to cesium which are Coulomb-excited by alpha particles of energy up to 7 Mev. We obtained energies and reduced transition probabilities for most $2^{+}$ first excited states of the even-even nuclei, using isotopically enriched targets. A number of these states had not been observed previously. Agreement is obtained with the lifetime values from resonance fluorescence in the two cases where such a comparison is possible at present (${\mathrm{Ge}}^{72}$ and ${\mathrm{Ge}}^{74}$). All even-even transitions are found to be systematically faster than the single-proton estimate by a factor ranging from 10 to 50, being always slowest near closed shells. Their interpretation as vibrational excitations of nuclei with spherical equilibrium shapes leads to values for the inertial parameters $B_2$ and surface tensions $C_2$ (in the vibrational Hamiltonian) showing strong shell structure effects. A number of thick-target excitation functions between 3 and 7 Mev are in excellent accord with semiclassical $E2\mathrm{}$ theory. No obvious systematic behavior is encountered in the odd-$A$ nuclei, although most transitions contain considerably enhanced $E2\mathrm{}$ components. Coincidence and angular distribution measurements were performed for ${\mathrm{Se}}^{77}$ and ${\mathrm{Ru}}^{101}$ to establish decay schemes and spins of excited states. The 17.5-second $E3\mathrm{}$ isomer in ${\mathrm{Se}}^{77}$ was excited indirectly via a higher state. We encountered no rotational spectra with the possible exception of ${\mathrm{Se}}^{77}$.