Cluster radioactivities of odd-mass nuclei

Available as short communication only. Experimental measurements evidenced 'fine structure' of cluster radioactivity. The transition toward the first excited state of the daughter nucleus 209 Pb was found to be favoured, relative to that reaching the ground state, which is hindered (similar to previous observations in spontaneous fission of odd-mass nuclei, or in fine structure of alpha-decay). Spontaneous fission rates of odd-A nuclei are slower than that of e-e nuclei, indicating a higher fission barrier due to shell effects. If the uncoupled nucleon is left in the same state both in parent and heavy fragment the transition is favoured. Otherwise the difference in structure leads to a large hindrance H=T(exp)/T(e-e) where T(exp) is the measured partial half-life for a given transition, and T(e-e) is the corresponding quantity for a hypothetical even-even equivalent. The favoured transitions are explained within the present version of the analytical super-asymmetric fission model (ASAFM) but the hindered ones need a larger action integral. Such an increase can be obtained with a larger potential barrier due to the so-called 'specialization energy' in a similar way to what has been done for spontaneous fission of odd-mass nuclei. A large hindrance factor is obtained for 24 Ne decay of 233 U and a good agreement of the half-life of the transition to the first excited state with the experimental one is obtained. Other hindered transitions could be: 23 F decay of 231 Pa to the ground state of 208 Pb (H=12), 14 C decay of 221 Fr to the ground state of Tl (H=8.5), 14 C decay of 221 Ra to the ground state of 207 Pb (H=9) and 24 Ne decay of 231 Pa to the ground state of 207 Tl. The 14 C transition from 225 Ac to the ground state of 211 Bi seems to be not hindered. (Author) 3 Refs