Search

High-spin states of 194 Pb have been populated by the reaction 184 W( 16 O, 6n) at 113 MeV beam energy and studied using the EUROGAM-1 array. The level scheme has been extended up to spin of 33 ℏ and excitation energy of 11.1 MeV. The high-spin part of the level scheme is dominated by bands of dipole transitions. New dipole bands have been observed and the configurations of their band heads are discussed. Besides the well-known coupling of the K π =11 − two-proton state to several νi 13/2 rotation-aligned neutron states, the K π =8 + two-proton state is also involved. Such a coupling is identified for the first time in a lead isotope. Moreover, the K π =11 − two-proton state is coupled to the rotation-aligned proton pair ( πh 11/2 ) 2 . Thus many multiparticle configurations involving all the high- j subshells located around the proton and neutron Fermi levels, πh 11/2 , πh 9/2 , πi 13/2 , and νi 13/2 have been identified in this nucleus.

The nucleus195Pb has been populated with the184W+16O reaction at 113 MeV. Two dipole γ-ray cascades have been observed using the EUROGAM spectrometer. A comprehensive level scheme related to these structures has been established. Multiparticle configurations are attributed to these high-spin structures from detailed comparison with the neighbouring nuclei,193Hg,194Pb and195Tl. Band crossings due to i13/2 neutron pair breaking are observed in195Pb at about the same frequencies as in the isotone193Hg.

The six superdeformed bands in {sup 193}Pb have been studied with the EUROGAM 2 {gamma}-ray spectrometer using the {sup 168}Er({sup 30}Si,5{ital n}){sup 193}Pb reaction. The results are discussed in terms of cranked-Hartree-Fock-Bogolioubov-Lipkin-Nogami calculations. From the @FF {sup (2)} moment of inertia behavior as function of the rotational frequency and the {ital M}1 and {ital E}2 decay competition of the superdeformed states, the bands are interpreted as three pairs of signature partners based on quasineutron excitations. Dipole transitions linking two signature partner superdeformed bands have been observed and, for the first time in lead isotopes, the branching ratio {ital B}({ital M}1)/{ital B}({ital E}2)=0.15{plus_minus}0.04 {mu}{sup 2}{sub {ital N}}/{ital e}{sup 2}b{sup 2} has been extracted. {copyright} {ital 1996 The American Physical Society.}

Dipole transitions linking the superdeformed Yrast signature-partner bands in 195Tl have been observed using the EUROGAM 1 spectrometer. From the measurements of the photon decay branching ratios, taken together with the average quadrupole moment measured in neighbouring superdeformed nuclei, we find that the two signature partner superdeformed bands involve the excitation of the 81 st proton into the i 13 2 Ω= 5 2 intruder orbital

The192Hg nucleus was populated in the160Gd(36S, 4n) reaction at a beam energy of E=159 MeV. Emittedγ-rays were detected with the EUROGAM array comprising 43 Compton-suppressed large volume Ge detectors. The level scheme of192Hg has been extended up to an excitation energy of E=10.4 MeV and spin I=34 ħ. Two new structures, made of competing ΔI=1 and ΔI=2 transitions have been observed and their connexions with the low-lying levels established. Their lowest levels are located at 6.304 MeV and 6.879 MeV excitation energy. The experimental results are compared with mean-field HF+BCS calculations. It is proposed that the new structures originate from deformation-aligned quasi-proton excitations π(i13/2 * h9/2)K=11 andπ (h9/2) K=8 2 , coupled to rotation-aligned quasi-neutron ν(i13/2)n and quasi-proton π(h11/2)2 excitations.

Two regular dipole gamma -ray cascades have been observed in 194Pb. A comprehensive level scheme related to these structures has been built and first members of rotational bands built on K=8 and K=11 two-proton oblate states have been identified. From the excitation energies, spin values, behaviours of the moments of inertia and alignments, and from the comparison with the isotone 192Hg, multiparticle configurations can be attributed to three high-spin structures of 194Pb built respectively on the 4966 keV, 4334 keV and 6198 keV levels: ( pi 9/2(505) pi 13/2(606))(vi13/2)2, ( pi 9/2(505) pi 13/2(606))(vf5/2vi13/2), ( pi 9/2(505) pi 13/2(606)) (vf5/2(vi13/2)3). Within this interpretation, the regular dipole bands correspond to the presence of two or three i13/2 decoupled neutrons.

A subset of the Eurogam array has been used to investigate the decay out of the superdeformed (SD) band in 194Pb. Two new band members and a new decay pattern into the low lying states at normal deformation (ND) have been established for this band. In the level scheme at normal deformation, the linking transitions between the previously known floating ΔI = 1 sequence and the lower states have been established for the first time. In this talk we will also report on the preliminary results of the search for the linking transitions between the SD and ND states in 192Hg with the fully operational EUROGAM array.

Small area chemically-milled parallel-grid avalanche counters (PGAC) with isobutane counter gas and thin entrance and exit windows, have been developed as transmission timing detectors for use in heavy-ion experiments. The performance of the counters has been investigated with α particles and elastically scattered 35Cl ions. To optimize the PGAC capabilities it is best to operate at pressures greater than 14 Torr where good timing and efficiency are available well below the breakdown threshold. A system timing resolution, using a silicon surface barrier stop detector, of 360 ps was achieved.

The nucleus 193Pb was populated via the 168Er(30Si,5n) reaction at a beam energy of 159 MeV and studied with the EUROGAM II spectrometer. Five new dipole Δ I = 1 cascades have been found. These structures have been connected to the level scheme which has been considerably extended up to a spin of ħ and an excitation energy of about 8 MeV. Angular distribution coefficients, a2, have been measured and confirm the dipole character of the in-band transitions. B(M1)/B(E2) ratios have been extracted for the two most intense cascades. The 193Pb dipole bands are discussed in comparison with those known in odd lead isotopic series and the structure of the band heads is analyzed in terms of microscopic HF+BCS calculations. The proposed configurations are based on a high-K two quasiproton excitation, π ([505]9/2t? ⊗ [606]13/2+)K=11t-, coupled to one or three rotation aligned quasineutrons involving the i13/2, p3/2, f5/2 and/or h{9/2} subshells. The main difference, compared with the heavier lead isotopes, is the presence of large Ω orbitals from the ν (i13/2) shell near the Fermi surface which are responsible for the increasing band-head spin as A decreases.

An excited superdeformed (SD) band has been observed in 192Hg with the UROGAM spectrometer. It is interpreted as a two quasi-neutron excitation in the 192Hg SD nucleus. Nuclear level lifetimeshave been measured using the Dopple-shift attenuation method. An average transition quadrupole moment of 19.5 ± 1.5 b has been obtained which is similar to the value measured in the yeast SD band. These results indicate that the deformation in the 192Hg SD nucleus is stable despite changes in the occupancy of high-N-orbitals.

A controversy has arisen over the existence, or not, of a superdeformed rotational band in192Pb. We have carried out an experiment using the168Er(3OSi, 6n)192Pb reaction at 159 MeV and have observed the coincidentγ rays in the EUROGAM 2 spectrometer. We confirm the original result of Henry et al. that a superdeformed band exists in192Pb.

Lifetimes of states in the superdeformed band in 192Hg have been measured using the Doppler-shift attenuation and the recoil-distance methods for the high-spin and the low-spin regions, respectively. Line shapes and decay curves were obtained from high-fold γ-ray coincidence spectra measured with the EUROGAM array. The transition quadrupole moments, derived from the lifetimes, are constant with an average value of Qt = 18.6 b down to the bottom of the band where it decays to the normal states. This suggests that the superdeformed potential-energy minimum is still present at that point and that the decay proceeds by tunneling.

The nucleus 193Pb was populated via the 168Er(30Si, 5n) reaction at a beam energy of 159 MeV and studied with the EUROGAM II spectrometer. Five new dipole ΔI = 1 cascades have been found. The 193Pb dipole bands are discussed in terms of microscopic HF+BCS calculations. The proposed configurations are based on a high-K two-quasiproton excitation coupled to rotation aligned quasineutrons. Parallel to the dipole bands the six superdeformed bands have been discussed in the framework of microscopic mean-field calculations. The bands are interpreted as three pairs of signature partners based on quasineutron excitations. Cross-talk transitions linking two signature partner superdeformed bands have been observed and, for the first time in lead isotopes, a mean B(M1)/B(E2) ratio value of 0.15±0.04 μ N 2 /e2b2 has been extracted.

We have carried out an experiment using the reaction168Er (30Si, 5n)193Pb at a beam energy of 159MeV with EUROGAM 2 to study the superdeformed (SD) bands in193Pb. We have observed some discreteγ rays which are possible candidates for links between the yrast SD band and the low-lying levels in this odd lead isotope. These data suggest for the lowest state of the SD band 1 an excitation energy of 4217±2 keV and a spin of (23/2−).

New results on the192Hg superdeformed band have been obtained with EUROGAM. The experiment has been performed with the160Gd(36S,4n) reaction at 159 MeV. Above 800 keV the γ-ray energies differ from the previously published ones. Thus the rise of the dynamical moment of inertia\(\mathfrak{F}^{(2)} \) above ħω=0.4 MeV is no longer observed. This is in better agreement with recent cranked Hartree-Fock-Bogoliubov calculations.

Recent data from the EUROGAM array have revealed the population of the yrast superdeformed (SD) band of192Hg in the α4n exit channel of the16O+184W reaction at 113 MeV beam energy. The nucleus assignment was made on the basis of the SD band transition energies, and the observation of characteristic X-rays and lowlying yrast γ-transition of192Hg in coincidence with the SD band γ-rays. Both the feeding and decay-out patterns of the observed SD band have been found similar to the ones previously measured in the (36S,4n) reaction.

The low-energy fusion cross section for 20 Ne+ 20 Ne has been measured. The data extend the systematics on fusion oscillations for such light symmetric systems. A simple extension of the usual barrier penetration model displays the decisive role of the symmetrisation of the system. It is also capable of qualitatively explaining the trends in this effect, in particular its decreasing importance as the mass of the system increases. The structure is essentially due to the sharpness of the cut-off of transmission coefficients as a function of angular momentum. The curvature of the Coulomb barrier is important in determining these coefficients and it is shown that the required values of this quantity are consistent with reasonable values of the surface diffuseness of the nuclear potential.

Angular distributions for the elastic scattering of /sup 40/Ca by /sup 40/Ca at E/sub c. m./ = 64.8, 71.8, 88, 93, 112.5, and 120 MeV and excitation functions at 60/sup 0/, 70/sup 0/, and 90/sup 0/ (c.m.) over the c.m. energy range 55--120 MeV have been measured. Strong-absorption-model and optical-model analyses are presented. The predictions of several theoretical interaction potentials are compared with the data. The absence of structure in the excitation functions is studied and explained by the large mass and charge of the colliding nuclei.

High spin states in 80 Sr have been studied using the techniques of in-beam γ-ray spectroscpy. The ground state band has been investigated up to (26 + ) and three new side-bands have been discovered. The deduced level scheme is compared with cranking model calculations; the predicted transformation to mostly non-collective excitations of an oblate shape is not observed. The behaviour of the J (1) and J (2) moments of inertia is discussed and presented as evidence for γ-softness in the light Sr isotopes.

Out-of-plane correlations between α-particles and projectile-like heavy ions have been measured for the system32S+197Au at ELAB=373 MeV. The angular widths of the correlations range from 24° to 40° (FWHM) depending on theZ of the heavy ion. In the framework of a model calculation these widths are shown to be very sensitive to the outof-plane momentum of the α-particle at emission. It is concluded that only little heating of the reaction partners occurs prior to α-particle emission.

In-plane angular correlations between α-particles and projectile residue nuclei ( Z =5−22) emerging from 32 S+ 197 Au collisions have been measured at 373 MeV incident energy. In the combined Coulomb field of the projectile residue and the target nucleus coincident α-particles are strongly focused into a narrow cone. Model calculations indicate that α-emission originates from the region of the projectile which is facing the target nucleus during the collision.

Deep inelastic fragment production in the symmetrical reaction 40 Ca on 40 Ca has been studied at E lab = 278 MeV, high above the Coulomb barrier. Kinetic energy distributions and angular distributions have been obtained for products in the Z = 8−22 region. The two-body nature of the mechanism is discussed. Sequential decay of primary fragments is shown to be important.

Inelastic scattering of 84 Kr on 232 Th has been measured at E =371 and 450 MeV and at different angles using the particle-gamma coincidence technique. It is found that for distances of closest approach smaller than about 20 fm the scattering of Kr on Th is mostly inelastic. At forward angles, the data are in agreement with predicted values from a Coulomb excitation program, but some differences are observed at backward angles.

The light product nuclei from the reaction 40 Ar + 232 Th at 295 MeV and at laboratory angles of 18° and 40°, were identified using combined ΔE  E and magnetic analysis techniques. The yield distributions and the kinetic energies of the reaction products have very distinct properties at these two angles. The data are consistent with the assumption of a predominant “quasi-elastic” reaction mechanism at 40° and a “deep inelastic” one at 18°. The isotopic dependence of the cross sections is not correlated with the ground state Q -values of the reaction. The experimental kinetic energies at 18° favour the interpretation that the observed nuclei are close to primary reaction products.

The elastic scattering of 84Kr from 208Pb and 232Th has been studied; strong-absorption radii R are deduced. A decrease of the radius parameter r, defined by R = r(A 1 1 3 + A 2 1 3 , is found with regard to reactions with lighter ions, which can be quite well fitted by an optical spherical model. Differences observed in scattering from 208Pb and 232Th may be due to deformation effects. Interaction barriers are estimated.

The low-lying excited states of 194, 196, 198, 200 Pb have been studied through the de-excitation γ-rays following the reactions Os( 12 C, 4n), W( 16 O, 4n) and Re( 14 N, 5n). Prompt and delayed spectra, γ-γ coincidences, γ-ray angular distributions and γ-ray time distributions have been obtained. Half-lives were measured for the 12 + level in the four isotopes, for the 9 − level in 194, 196, 200 Pb, for the 7− level in 200 Pb and for the 5− level in 196, 198 Pb. The results of microscopic calculations, performed with either a Gaussian interaction or a surface delta interaction, are compared to the experimental level schemes and transition probabilities, and discussed.

Coulomb excitation experiments have been performed on the two stable odd-mass tellurium isotopes using a N 3+ beam from the Orsay cyclotron. The reduced excitation probabilities of the following levels: 439.2 keV (3/2 + ) and 505.2 keV (5/2 or 3/2 + ) in 123 Te, 443.5 keV(3/2 + ), 463.4 keV (5/2 + ) and 671.3 keV (5/2 + ) in 125 Te were measured by this method. Some B (E2) and B (M1) transition probabilities are deduced from experiments. The results are discussed in terms of de Shalit's model and Kisslinger and Sorensen's calculations.

Evaporation-residue cross sections for the 40 Ca + 40 Ca system have been measured with a position-sensitive E − δE telescope. From the deduced values of the barrier and critical radii, it does not appear that the shell closure in 40 Ca has any influence on the fusion phenomenon.

The time dependence of 40Ca+40Ca deep inelastic collisions is studied by means of the fragment change and mass distributions. Two time components are unfolded. The “fast” one exhibits all features of “quasi-fission”. The characteristics of the “slow” component cannot be distinguished from compound nucleus fission.

The yrast superdeformed (SD) structure of 194Pb has been studied using the EUROGAM γ-ray spectrometer. New results have been obtained at low as well as at high spin values. At low frequency, the SD band has been extended to the lowest-spin SD state ever observed in the A ∼ 190 region. The energies of 194Pb and 192Hg SD bands are identical for ω < 0.25 MeV. However, they diverge for higher frequency, which rules out the idea that these two SD bands are identical. Moreover, the SD band of 194Pb is populated at lower spin values than that of 192Hg and de-excites towards normally-deformed (ND) states at a spin value lower than that of 192Hg. This suggests that the barrier separating the SD and ND minima in the potential energy surface is higher in 194Pb than in 192Hg.