Nilsson Levels in Odd Mass Odd Z Nuclei in the Region Z = ( 99 ─ 105 )

Similarities in nuclear structure are observed for isotopes of odd-Z elements with even neutron numbers. These can be used to explain the α-spectra and to construct partial level schemes. Vice versa changes in the α-decay pattern may reveal a change of the ground-state (gs) configuration in the daughter nuclei. Results of previous studies were presented in the last year GSI Scientific Report [1]. In 2004 for two more Md-isotopes α-γ-coincidence measurements were performed: Md was produced via the reaction Bi(Ti,2n)Db ─ α → Lr ─ α → Md. In coincidence with α-decays of the latter isotope we measured a γline of 253 keV. A so far unknown α-γ-coincidence pair of (Eα = 7092 keV, Eγ = 352 keV) was observed in an irradiation of Pb with Ca at 4.60 AMeV. In this reaction predominantly the isotope No was produced. As a control we performed irradiations of Pb and Bi where mainly the isotopes No and Lr were produced. In these reactions the above mentioned activity was not observed. Thus it is seemingly connected to the production of No. However it cannot be attributed to No itself or any known daughter product of which α-decay has been reported so far. Since an EC branch of ≈30 % is reported for No [2], it is straightforward to assign the unknown activity (tentatively) to a so far not reported α-branch in the EC-daughter Md. The Eγ = 295 keV line in coincidence with α-decays of Md [1] has been confirmed by Theisen et al. [3] (although they give a somewhat different energy of 290 keV), who in addition observed one more line at 235 keV, which was confirmed by us in a recent experiment, however, our energy of E = 243 keV is slightly different. We measured an intensity ratio of I(295) : I(243) = 1 : 0.13. As discussed in [1] the strongest γ-lines may be attributed to the transition 7/2[514] → 7/2[633] in the Es daughter isotopes, where the final state may be regarded as the groundor a low lying state. The new results allow for a complete systematics of the 7/2[514] Nilsson level in odd mass einsteinium isotopes in the mass range A=243 to A=253. The experimental data are shown in fig. 1b. Evidently an increase of the level energies with increasing neutron numbers is observed until the maximum is reached at Es, whereas it drops from Es to Es by about 90 keV. The experimental results can be compared with theoretical calculations shown in fig. 1a. The calculations of Cwiok et al. [4] qualitatively represent the trend, although they predict a more smooth increase and locate the maximum at Es. Energies are, however, higher by several hundred keV. The energies according to the calculations of Parhomenko and Sobiczewki [5] are in general closer to the experimental values, but locate opposite the experimental trend the maximum already at Es. Both calculations predict the 7/2[633] level as ground states and locate the 3/2[521] – level above 100 keV, while from experimental side, as discussed in [1], the situation is not clear so far.