Z. Sujkowski

High-spin neutron excitations in neutron-deficient even-mass Pb isotopes

Abstract High-spin states in 194, 196 pb have been investigated using the 188 Os( 12 C, χn) 194, 196 Pb and the 198 Hg(α, 6n) 196 Pb reactions. The experiments included γ-ray excitation function, γ-γ coincidence, γ-∑γ coincidence, γ-ray angular distribution, lifetime and conversion electron measurements. The level schemes of 194 Pb and 196 Pb have been extended up to states with J π = 15 − ( E = 3955 keV) and J π = 20 + ( E = 5444 keV), respectively. All levels except two new isomers which have most likely spin and parity J π = 10 − ( 194 Pb ) and J π = 11 − ( 196 Pb ) can be interpreted as neutron excitations. The excitation energies of levels with spin J ⩽ 12 are well accounted for by the one-broken-pair model including 1p1h excitations. The B (E2, 12 + −10 + ) values reflect the systematic trend for a pure ( i 13 2 ) 2 configuration. Levels with spin 12 J i 13 2 neutron holes are coupled to core excitations in the A + 2 nucleus and also by calculations performed within the two-broken-pair model. Within this framework the J π = 20 + state observed in 196 Pb has a pure ( i 13 2 ) 4 configuration. The two new isomers have most likely a proton 2p2h structure. The excitation energy of the J π = 11 − isomer in 196 Pb agrees with that predicted for a deformed 1 2 + [404] − 29 2 − [514] 13 2 + [606] 11 − state.

K-SHELL IONIZATION OF DY ATOMS PRODUCED IN (ALPHA,CHI-N) AND (C-12,CHI-N) NUCLEAR-REACTIONS

Abstract The K-shell ionization cross sections, σ K Z , are determined for Dy atoms produced in bombardments of several Gd targets with α-particles in the energy range 47–130 MeV and of 148, 150 Nd targets with 12 C ions at 65 and 90 MeV. The determination is based on the measured intensity ratios of the characteristic K X-rays of the product atoms to those of the target elements. Analysis of the simultaneously measured discrete in-beam γ-ray spectra allows one to determine ionization cross sections for Dy product atoms, σ Z K ( der ), derived as a sum of the products of the γ-ray intensities times the corresponding K-shell internal conversion coefficients. The measured and the total derived values are found to be identical to within the experimental errors. The partial ionization yields per reaction are analyzed as functions of the average angular momentum left in each exit channel. Strong dependence of these yields on the nuclear structure is found. Moreover, the variation of the yields with the angular momentum is also strongly structure dependent. Properties of the quasicontinuum γ-radiation deexciting the reaction residues are considered. Upper intensity limits are determined for the low-energy magnetic dipole components in this radiation.

In-beam γ-ray studies of complex band structures and of isomeric states in 152, 153Dy nuclei

Abstract The high-spin level structures of 152 Dy and 153 Dy were studied experimentally with 154, 155 Gd(α xnγ ) in-beam reactions, and for 152 Dy also with 144, 146 Nd ( 12 C, x n γ) reactions. The experiments included measurements of singles γ-ray and conversion-electron spectra, γ-ray angular distributions and E γ - t and E γ - E γ - t coincidences. A multiplicity filter set-up was used to study the feeding and decay of isomeric states in 152 Dy. In 152 Dy about twenty so far unknown levels were found, including two high-spin isomeric states with T 1 2 ≈ 60 and ≈ 13 ns at excitation energies E x ≈ 5.04 and 6.08 MeV, respectively. These states are compared with recent calculations on yrast traps. The level scheme of 153 Dy contains 28 levels up to E x = 4.1 MeV and J π = ( 37 2 + ) . Band structures in both nuclei are discussed in comparison with other N = 86 and N = 87 isotones.

Experimental investigation of the quasi-continuum γ-ray cascades following 160Gd(α, xn)164 − xDy reactions

Abstract Multiplicity shape parameters have been determined for the 160 Gd( α , x n) 164 − x Dy reactions with x = 4–8 for E α = 40–110 MeV. Gamma-ray entry lines for the (α, 4n) and (α, 6n) reactions have been deduced from the side-feeding multiplicities as a function of γ-ray energy for various bombarding energies. The γ-ray multiplicities and the excitation energies of the entry lines increase steeply with the bombarding energy in the region of compound-nucleus formation and retain rather constant values when pre-equilibrium decay is important. These saturation effects locate the neutron “drip line” in the energy versus spin plane. The role of angular momentum and of excitation energy on the γ-ray decay process is investigated. The multiplicity data are compared with a combined statistical-master-equation-IBA calculation, which yields insight in the physical parameters that govern the decay process.

Positive- and negative-parity yrast bands in the transitional nucleide 150Sm and the interacting boson approximation

Abstract The 150Nd(α, 4n) reaction at Eα = 45 MeV has been used to study high spin states in the 15062Sm88 nucleide. The experiments comprised measurements of singles γ-ray and conversion electron spectra, γ-ray angular distributions, three-parameter (Eα−Eα−t) coincidences and a γ-ray anti-Compton spectrum. Predominantly populated were the ground-state band (g.s.b.) extending to Jπ = 12+ and tentatively to 16+ and a negative-parity odd spin band (n.p.b.) extending from Jπ = 5− through 15− and tentatively to 19−. The bands are interpreted in the framework of the interacting boson approximation model with a Hamiltonian appropriate for a transitional nucleus, far away from either the rotational or vibrational limit. It is found that the model gives a very satisfactory description of the energies of the two bands observed as well as of the branching ratios of the intra- and inter-band transitions. The gentle backbending of the 2ℷ h 2 versus (ħω)2 curve for g.s.b. above Jπ = 10+ is reproduced in the calculation without making use of the band-crossing concept. The absolute reaction cross sections have been determined at 45 MeV bombarding energy to be σtot = 1380 ± 170 mb and σ4n = 800 ± 100 mb.

Observation of strongly deformed shapes in 154-152Dy nuclei at medium temperatures☆

The gamma-decay of the giant dipole resonance (GDR) built on excited states of 154-152Dy nuclei is studied. The selection of GDR decay from high spin states leading to specific exit channels was made possible by triggering on high spin isomers. The deduced energy splitting of the GDR implies large deformations (\beta\ congruent-to 0.4-0.5). The resonance widths of the components are comparable to those of the GDR built on the ground state indicating small shape fluctuations.

Compound and precompound γ-ray entry lines from measured multiplicities and energies in α-induced reactions

Multiplicities of quasi-continuum γ-rays have been measured for the 160Gd(α, xnγ) reactions as a function of bombarding energy and for the 4n exit channel also as a function of γ-ray energy. Increase of beam energy causes initially an increase of the energy and angular momentum of the entry line in the residual nucleus until the neutron “drip line” is reached. This can be understood in terms of competing compound and precompound processes.

Negative parity bands in 100Ru and 150Sm and the interacting boson approximation

Ground-state band members up to Jπ = 10+ in 100Ru and up to 14+ in 150Sm and odd-spin negative parity states up to 15− in both nuclei have been identified from (α, 4n) in-beam γ-ray and conversion electron spectra. The data are interpreted in terms of interacting quadrupole and octupole bosons.

The angular-momentum dependence of the giant dipole resonance in 154Dy∗

Abstract The statistical γ-ray decay of the compound nucleus 154 Dy ∗ formed at an excitation energy of 69 MeV is studied in three angular-momentum windows 〈 J 〉 = 31,42 and 50ℏ. The GDR strength function extracted from the data indicates a constant centroid energy for the resonance E GDR = 15.2 ± 0.5 MeV, a small deformation of the nuclear shape for all spin regions and an increase in the resonance width with increasing angular momentum. The latter might indicate larger shape fluctuations at higher angular momenta. A systematical study of the dependence of the extracted strength function with different methods of analysis is presented, and their discrepancies discussed.

GDR DISSIPATION AND NUCLEAR SHAPE IN HOT FAST-ROTATING DY NUCLEI

Abstract The statistical γ-ray decay of the GDR built on excited states in Dy nuclei has been investigated for selected domains of angular momentum up to about 70ħ and temperatures in the range 1–2 MeV. The GDR strength distribution extracted from the data indicate large average nuclear deformations ( β ∼ 0.35) at high angular momentum and average temperatures T ⩾ 1.5 MeV. The experimental observation is supported by results from calculations in which thermal shape fluctuations are taken into account around an oblate equilibrium deformation β eq . Although this equilibrium deformation increases with angular momentum, the calculations show rather large and constant average deformations 〈 β ∼0.35.