Nuclear Structure

In-beam gamma-ray and conversion electron spectroscopic studies have been performed on the 253 No nucleus. A strongly coupled rotational band has been identified and the improved statistics allows an assignment of the band structure as built on the 9/2 − [734]ν ground state. The results agree with previously known transition energies but disagree with the tentative structural assignments made in earlier work.

New high-statistics data have been obtained on the decay properties of 253 No and its daughters using the reaction 207 Pb(48 Ca, 2n) 253 No. This was made possible thanks to an improved transmission of fusion-evaporation residues through the VASSILISSA recoil separator and an increased efficiency of the GABRIELA detector setup. The decay schemes of 253 No and 249 Fm have been revisited. The known level scheme of 249 Fm has been confirmed, including a new level at 669 keV excitation energy. The observation of L X-rays in coincidence with the α decay of 249 Fm gives additional support to the ground-state configu

An in-beam study has been performed to further investigate the known isomeric decays and to identify T = 1 excited states in the medium-heavy N = Z = 33 nucleus 66 As. The fusion-evaporation reaction 40 Ca(28 Si,pn) 66 As was employed at beam energies of 75 and 83 MeV. The half-lives and ordering of two known isomeric states in 66 As have been determined with improved accuracy. In addition, several prompt γ-ray transitions from excited states, both bypassing and decaying to the isomeric states in 66 As, have been observed. Most importantly, candidates for the 4 + → 2 + and 6 + → 4 + transitions in the T = 1 band have been identified. The results are compared with shell-model calculations using the modern JUN45 interaction in the pf 5/2 g 9/2 model space.

Neutron-induced reactions on 196 Pt were studied over the neutron energy range from 1 to 250 MeV. A ''white'' neutron beam was provided by the spallation neutron source of the Weapons Neutron Research facility at the Los Alamos Neutron Science Center. The prompt reaction ␥ rays were measured with the large-scale Compton-suppressed Ge spectrometer GEANIE. The incident neutron energy was determined by the time-offlight technique. Excitation functions for ␥-ray transitions in 184,186,188,190Ϫ196 Pt and 189,191,193 Ir isotopes are compared with enhanced Hauser-Feshbach reaction modeling, as implemented in the reaction code GNASH. Overall, the model predictions agree well with the measured ␥-ray yields. Discrepancies appear, however, due to inadequate discrete-level information in the calculations describing the preequilibrium process.

Lifetime measurements have been made in the ground-state band of the transitional nucleus 138 Gd from coincidence recoil-distance Doppler-shift data. 138 Gd nuclei were produced using the 106 Cd (36 Ar, 2p2n) reaction with a beam energy of 190 MeV. Reduced transition probabilities have been extracted from the lifetime data collected with the Köln plunger placed at the target position of the JUROGAM-II array. The B(E2) values have been compared with predictions from X(5) critical-point calculations, which describe the phase transition between vibrational and axially symmetric nuclear shapes, as well as with IBM-1 calculations at the critical point. While the excitation energies in 138 Gd are consistent with X(5) predictions, the large uncertainties associated with the measured B(E2) values cannot preclude vibrational and rotational contributions to the low-lying structure of 138 Gd. Although experimental knowledge for the low-lying γ and β-vibrational bands in 138 Gd is limited, potential-energy surface calculations suggest an increase in γ softness in the ground-state band. In order to more fully account for the effects of γ softness, the X(5) and IBM-1 calculations need to be extended to include the γ degree of freedom for 138 Gd.

The quasicontinuous spectra associated with the decay of the superdeformed bands in 192,194 Pb have been extracted. The rapid rise in the ␥-ray intensity in these spectra for E ␥ Շ1.8 MeV is interpreted to arise from the dramatic increase in level density above the energy gap for even-even nuclei. This energy gap has been found to be Ϸ0.95(10) MeV at 6ប in 194 Pb and Ϸ0.4 Ϫ0.1 ϩ0.4 MeV at 10ប in 192 Pb.

An IN2P3-JINR collaboration has launched a project called GABRIELA at the Flerov Laboratory for Nuclear Reactions (FLNR) at the Joint Institute for Nuclear Research (JINR) in Dubna (Russia). The goal of the project is to perform gamma-ray and internal conversion electron spectroscopy of heavy nuclei produced in fusion-evaporation reactions and transported to the focal plane of the recoil separator VASSILISSA. During five experimental campaigns of GABRIELA, the detection system has gained in sensitivity and new spectroscopic information has been obtained for 249 Fm, 251 Fm, 253 No and 255 Lr.

The results of the first experimental tests of the modernized VASSILISSA separator with the use of accelerated 22 Ne ions are presented. Data has been obtained on the transmission coefficients of recoil nuclei synthesized in asymmetric combinations of the incident ion and target nuclei. Estimates from ion opti cal calculations performed in the design phase of the project of modernizing the separator are completely confirmed.

The Kr isotopes are considered to be among the best cases for shape coexistence studies in the mass A ∼ 70 region. Our campaign to investigate in detail the development of the shape coexistence in the neutron deficient Kr isotopes was started with the stable nucleus 78 Kr. To obtain the information about the intrinsic shape, Coulomb excitation experiments were performed. A total of 26 matrix elements were determined for 78 Kr. Simple geometrical and algebraic models do not reproduce all details of the electromagnetic structure. A good interpretation of the complex structure of the nucleus with competing oblate and prolate shapes comes from a study within the complex EXCITED VAMPIR approximation using Hartree-Fock-Bogoliubov transformations with symmetry projection before variation. The measured matrix elements are compared with the predictions of this model.

An experiment aiming to study the shape coexistence in 193 Bi has been performed at the Accelerator laboratory of the University of Jyväskylä, Finland (JYFL). Many new states have been found, hugely extending the previously known level scheme of 193 Bi. The I π = 29

Excited states in 197 Pb and 198 Pb, produced in the 186 W (18 O,xn) reaction, were studied using the EUROGAM phase 2 γ ray spectrometer array. Two new superdeformed bands have been observed in 197 Pb, and the existence of a superdeformed band in 198 Pb has been confirmed. The increased statistics provided by the latest generation of arrays has resulted in the reordering of some of the previous known cascades of magnetic dipole transitions, with the first observation of the extremely weak E2 crossover transitions.

An experiment aiming to study shape coexistence in 193 Bi has been performed. Due to its transitional character, it has an exceptionally large number of structures identified close to the yrast line. Many new states have been found, significantly extending the previously known level scheme of 193 Bi, including several new rotational bands. The πi 13/2 band was extended to I π = 45/2 +. The I π = 31/2 + member of the πi 13/2 band was found to de-excite also to a long-lived isomeric state. This isomeric state is located at 2350 keV and has a spin and parity of 29/2 +. The half-life of the isomeric state was measured to be 85(3) μs and it decays via the emission of an 84 keV E2 transition. A level structure feeding this isomeric state was constructed. A low-energy, 49 keV transition has been identified to depopulate the (29/2 −) isomeric state, which places it at an energy 2405 keV in the level scheme. This is the first time such a decay has been observed in the neutron-deficient Bi isotopes. A superdeformed band almost identical to that present in the neighboring isotope 191 Bi, based on the 1/2[651] Nilsson orbital, has also been identified.

An in-beam study of excited states in the transfermium nucleus 252 No has been performed using the recoil separator RITU together with the JUROSPHERE II array at the University of Jyväskylä. This is the second transfermium nucleus studied in an in-beam experiment. Levels up to spin 20 were populated and compared to levels in 254 No. An upbend is seen at a frequency of 200 keV/ប corresponding to spin 16. We also use an improved systematics to connect the energy of the lowest 2 ϩ state with its half-life and find that the deformation of both 252,254 No is slightly larger than previously assumed.

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Isomeric states in 253 No have been investigated by conversion-electron and γ-ray spectroscopy with the GABRIELA detection system. The 31 µs isomer reported more than 30 years ago is found to decay to the ground state of 253 No by the emission of a 167 keV M2 transition. The spin and parity of this low-lying isomeric state are established to be 5/2 +. The presence of another longer-lived isomeric state is also discussed.

Using state-of-the-art γ-ray spectroscopic techniques, the first rotational band of a superheavy element, extending up to a spin of 20h, was discovered in the nucleus 256 Rf. To perform such an experiment at the limits of the present instrumentation, several developments were needed. The most important of these developments was of an intense isotopically enriched 50 Ti beam using the MIVOC method. The experimental setup and subsequent analysis allowed the 256 Rf ground-state band to be revealed. The rotational properties of the band are discussed and compared with neighboring transfermium nuclei through the study of their moments of inertia. These data suggest that there is no evidence of a significant deformed shell gap at Z = 104.

␥ rays following the 160 Dy(36 Ar,4n͒ 192 Po reaction have been identified by employing a high-transmission gas-filled separator in recoil decay tagging measurements. The deduced level scheme reveals a flattening of the energy systematics, when going towards the neutron midshell indicating that the deformed intruder structures have become yrast. ͓S0556-2813͑96͒50212-8͔