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Extensive high-spin band structures in 82 Sr have been established using proton- γ - γ coincidence techniques. On the basis of the Woods-Saxon cranking calculations with pairing, four of these bands are interpreted to have prolate, oblate, or triaxial shapes. Pairing correlations are predicted to be very weak at high spins in this nucleus, and calculations with no static pairing successfully reproduce the experimentally observed crossing frequencies and alignments despite the fact that none of the bands displays a rigid-rotor behavior. It is concluded that observation of rigid-rotor behavior is neither necessary nor sufficient for the disappearance of static pairing in nuclei.

Emission barriers and subbarrler anisotropies from a decay of Sn* and Y'b* compound nuclei are examined in the light of calculations incorporating deformation effects in the decay process. For the Yb* systems deformation which increases with spin is necessary to explain the data. For the Sn* systems the spectral shapes and auisotropies can be explained without deformation. For systems lighter than Sn this probe is not sensitive to the deformation. Energy spectra and angular correla52 34 82 tions of evaporated protons from the Cr( S, 2n2p) Sr reaction were measured in coincidence with discrete transitions. Large shifts in proton spectra were observed when high spin states in different rotational bands are populated. They are interpreted as due to near-yrast stretched proton emission preferentially populating the yrast hand by subbarrier protons. Simulations show that channel selected proton spectra cannot be used as probes of deformation.

Cratering and plastic deformation induced by individual MeV ions on the surface of polystyrene thin films of different molecular weights (Mw) (from 3250 to 2 X 10⁷u) are investigated using scanning force microscopy. 20 MeV, 85 MeV and 197 MeV gold ions are used to bombard the targets at grazing incidence (79° to the surface normal). Induced surface tracks consist of an elliptical crater followed by a hillock elongated in the direction of the ion incidence. For a given ion energy, the crater size is largest on the lowest Mw film. Crater dimensions are systematically reduced on films of heavier macro-molecules, up to a molecular weight of about 1.6 X 10⁵u. For Mw>1.6 X 10⁵u, the crater size remains approximately constant. The difference observed for the lateral dimensions of the craters are about 50% when comparing the lowest and the highest Mw films at a fixed energy. The observed saturation of the crater size for high Mw values coincides with the onset of entanglement effects in the polymer, which influences the viscosity and the compliance of the material. Moreover, the curve of the crater size versus Mw follows the same trend as the reciprocal viscosity (ηˉ¹) versus Mw , indicating that the viscosity is governing the final lateral dimensions of the craters. The hillock dimensions present a weak dependence on Mw , above a threshold at 3250u. The different behavior observed for craters and hillocks is discussed based on the viscoelastic properties of the polymer at different Mw and on the transient heating occurring close to the ion impact site.

Energy spectra and angular distributions of evaporated protons from the reaction $^{52}\mathrm{Cr}$${(}^{34}$S, 2p2n${)}^{82}$Sr at 130 MeV were measured in coincidence with discrete \ensuremath{\gamma} transitions. Large shifts and changes in the shape of the proton spectra were observed when high-spin states in different rotational bands are populated. They are interpreted as due to near-yrast stretched proton emission, which preferentially populates the yrast band by subbarrier protons.

Low-emission barriers and large subbarrier anisotropies in the α-particle decay, with respect to the spin direction, of Sn and rare-earth compound nuclei are examined in the light of recent calculations incorporating deformation. For the rare-earth systems deformation which increases with spin is necessary to explain the data. Energy spectra and angular correlations of evaporated protons from the 52Cr(34S, 2p2n)82Sr reaction were measured in coincidence with discrete transitions. Large changes in the shape of the proton spectra were observed when high-spin states in different rotational bands are populated. These effects cannot be explained by phase space arguments that do not include explicitly structure in the deexcitation process. They are interpreted as resulting from near-yrast to near-yrast stretched proton emission, which preferentially populates the yrast band by subbarrier protons.

Isomeric transitions, of E3, M2, and E1/M2 character, depopulating pi-tildeh/sub 9/2/xnu-tildei/sub 13/2/ bands in /sup 186,188,190/Tl, respectively, have been found with use of ..gamma..-ray spectroscopic techniques. The changes in multipolarity reflect successive changes in the bandhead spin values corresponding to level crossings within the pi-tildeh/sub 9/2/xnu-tildei/sub 13/2/ system. These crossings were predicted in the framework of the two-quasiparticle plus rotor model and interpreted as a direct consequence of the depletion of the i/sub 13/2/ shell.

The spin of the 1082.8 keV level in 152 Sm was measured as 0 by directional correlation experiments. Earlier a level at 1081 keV was assigned a spin and parity of 1 ± or 2 + from the decay of 152 Pm. Our results indicate that either these two levels are different or the previously assigned 1081 keV ground state transition is misplaced in the decay studies of 152 Pm.

Coulomb excitation by $^{4}\mathrm{He}$ ions was studied to deduce the electric quadrupole and hexadecapole reduced transition matrix elements of deformed gadolinium and hafnium nuclei. Our values of $〈{0}^{+}\ensuremath{\parallel}M(E2)\ensuremath{\parallel}{2}^{+}〉$ for $^{154,156,158,160}\mathrm{Gd}$ are 1.962(20), 2.136(7), 2.230(12), and 2.269(14) eb. For $^{176,178,180}\mathrm{Hf}$ they are 2.281, 2.204, and 2.176 eb. The values of $〈{0}^{+}\ensuremath{\parallel}M(E4)\ensuremath{\parallel}{4}^{+}〉$ for $^{156,158,160}\mathrm{Gd}$ are 0.42 \ifmmode\pm\else\textpm\fi{} 0.08, $0.34_{}^{+0.20}{}_{\ensuremath{-}0.22}{}^{}$, and $0.35_{}^{+0.09}{}_{\ensuremath{-}0.07}{}^{}$ e${\mathrm{b}}^{2}$. For $^{176,178,180}\mathrm{Hf}$ they are 0.26 \ifmmode\pm\else\textpm\fi{} 0.12, $0.23_{}^{+0.11}{}_{\ensuremath{-}0.20}{}^{}$, and $0.21_{}^{+0.11}{}_{\ensuremath{-}0.16}{}^{}$ e${\mathrm{b}}^{2}$, if positive values are taken, and -0.76 \ifmmode\pm\else\textpm\fi{} 0.15, -0.67 \ifmmode\pm\else\textpm\fi{} 0.20, and -0.65 \ifmmode\pm\else\textpm\fi{} 0.16 e${\mathrm{b}}^{2}$ if they are negative. We compare our measurements with other experimental values and with theoretical predictions of quadrupole and hexadecapole moments.

The vibrational states of /sup 154/ /sup 156/ /sup 158/ /sup 160/Gd and /sup 176/ /sup 178/ /sup 180/Hf have been studied via Coulomb excitation. Thin high-purity targets were prepared in an isotope separator. Coulomb excitation of these nuclei was studied by the scattering of 11--17 MeV ..cap alpha.. particles. The particles were detected in an Enge split-pole spectrograph. The reduced transition probabilities B (Elambda;0/sup +//sub g./ ..-->.. I/sup ..pi../K), were obtained for I/sup ..pi../K = 2/sup +/2 states in each nucleus, and for I/sup ..pi../K = 2/sup +/0 and I/sup ..pi../ = 3/sup -/ states in several nuclei. The K/sup ..pi../ = 0/sup +/ levels are reasonably constant in energy and B (E2) strength while the K/sup ..pi../ = 0/sup +/ states change rapidly in energy and excitation strength with neutron number. In both the softly deformed Gd nuclei and the more strongly deformed Hf nuclei, the lightest nucleus has the lowest energy and largest B (E2) for an I/sup ..pi../K = 2/sup +/0 state, despite the difference in the sizes of the deformations at these lightest masses.

The 0$sup +$' state at 937 keV in $sup 72$Se is unusual in that it is only 75 keV above the first excited 2$sup +$ state. The half-life T$sub 1$/$sub 2$ of the 0$sup +$' state was determined to be 15.8 +- 0.9 ns by delayed coincidence techniques. The ratio of the E0/E2 decays was measured to be 0.37 +- 0.23 and the reduced monopole matrix element rho was calculated to be 0.176/sub - 0.070//sup +0.048/. The B(E2) value of the 75-keV transition was deduced to be 36 +- 7 single-particle units. This large B(E2) value indicates that the 0$sup +$' state is collective. These data are interpreted in terms of a coexistence of spherical and deformed shapes.

Multiple-particle $\ensuremath{\gamma}$-ray coincidence techniques have been used to study the high spin structure of $^{75}\mathrm{Kr}$ excited through the $^{40}\mathrm{Ca}$($^{39}\mathrm{K}$,3pn) reaction at $E=120$ MeV. From $\ensuremath{\gamma}$-ray angular distribution and $\ensuremath{\gamma}\ensuremath{-}\ensuremath{\gamma}$ coincidence experiments two $\ensuremath{\Delta}I=1$ bands were found. These bands are interpreted as being built on the ${\frac{3}{2}}^{\ensuremath{-}}[301]$ and ${\frac{5}{2}}^{+}[422]$ Nilsson orbitals, which have been identified in adjacent odd-$A$ nuclei in this region.

K-X-ray-γ directional correlations following the decay of54Mn,65Zn,75Se,85Sr,133Ba and207Bi have been performed. The experimental set-up consisted of a set of four scintillation counters, one for detecting X-rays and the three others for detecting γ-rays, placed at 90°, 180° and 270° with respect to the source to X-ray detector direction. Multiple coincidence units and a logic system were used in order to display three X-ray spectra in three 128-channel submemories of a 512-channel analyser, corresponding to coincidences between X-rays and γ-rays detected in each counter, respectively. For the case of the133Ba X-γ angular correlation, the scintillation counters were replaced by one Ge(Li) detector in order to select with high resolution the γ-ray of 356 keV. Corrections for chance coincidences were made and aχ2-test of the data was performed. From theK-X-γ directional correlations following the above-mentioned decays a value of the anisotropy was derived. In all cases this value is consistent with zero, to within a one percent error. These results are in agreement with the theoretical predictions of Dolginov. A discrepancy with the anisotropy value reported by Perepelkin appeared. The values corresponding to the decays of65Zn,85Sr and133Ba are consistent with the ones reported by Ramaswamy and McDonnell and Ramaswany.

The first-forbidden beta transitions 0− → 0+ and 0− → 2+ from the decay of 144Pr were studied. The corresponding spectrum shape factors and end-point energies were determined by means of a simultaneous fit of the data on both beta spectra with a theoretical function. The contribution of the induced pseudotensor term has been investigated analyzing the available information on spectrum shape factors, angular correlation, and longitudinal polarization.The data corresponding to both transitions were handled simultaneously. The introduction of the induced pseudotensor term is not indispensable for the interpretation of the experimental data. Besides, the nuclear matrix element [Formula: see text] for the 0− → 0+ transition was determined.

A 4+, 1.3 s isomer has been found at 102.6 keV in 76Br using the 75As(α, 3n) reaction. A previously reported band structure is interpreted to decay into this isomer of probable π g 9 2 ⊗ ν g 9 2 parentage.

As the properties of the nuclear weak interaction are well understood, the study of the nonunique first forbidden beta transitions is mostly oriented at the present time to the investigation of the nuclear structure. Within this line of thought, the 7/2 − →7/2 + beta transition from the decay of 141Ce is investigated in this paper., Facultad de Ciencias Exactas

International audience; Theoretical prediction of nuclear masses is analyzed as a pattern recognition problem on the N-Z plane. A global pattern is observed by plotting the differences between measured masses and Liquid Drop Model (LDM) predictions. After unfolding the data by removing the smooth LDM mass contributions, the remaining microscopic effects have proved difficult to model, although they display a striking pattern. These deviations carry information related to shell closures, nuclear deformation and the residual nuclear interactions. In the present work the more than 2000 known nuclear masses are studied as an array in the N-Z plane viewed through a mask, behind which the approximately 7000 unknown unstable nuclei that can exist between the proton and neutron drip lines are hidden. Employing a Fourier transform deconvolution method these masses can be predicted. Measured masses are reconstructed with and r.m.s. error of less than 200 keV. The existence of an island of stability around (Z\approx$ 116, N\approx 194) is strongly suggested. Other potential applications of the present approach are outlined.

International audience; Capture cross-sections on isotopes relevant to accelerator driven systems for energy production and nuclear waste transmutation, and to stellar nucleosynthesis have been studied at the innovative neutron time of flight facility n_TOF at CERN. The extremely high instantaneous neutron flux and the low background conditions in the experimental area make this facility unique for accurate measurements on low-mass or radioactive samples. The n_TOF facility is described, together with the features of the experimental apparata used. First results of the experimental campaign 2002-2004 are presented.

International audience; Recent experimental data showing the interplay between fusion hindrance effects and the onset of the quasi-fission mechanism are presented. The aim of such investigations is understanding the role of entrance channel properties on the competing fusion-fission and quasi-fission processes, also in connection with the possible implications for superheavy element production. In this context, the role of mass asymmetry, nuclear deformation and shell effects is discussed.