L.K. Kostov

Collectivity and the role of two-proton and two-neutron excitations in 82Kr

Abstract Excited states in 82 Kr have been studied in the reaction 80 Se(α, 2n) by using in-beam γ-ray spectroscopy. Measurements of γγ coincidences, excitation functions, angular distributions and the linear polarization of the γ-rays have been carried out. All together, 38 levels have been identified up to I = 12 h and excitation energies up to 6 MeV. For 23 of these levels the mean lifetime could be determined by Doppler shift and the pulsed-beam γ-timing methods. Most of the levels could be grouped into collective bands according to measured B (E2) values. Two sequences, I π = 8 + , 10 + , 12 + , are interpreted as rotation-aligned 2q.p. bands built up on g 9 2 protons and neutrons, respectively. Interaction matrix elements between these bands were deduced as V ≲ 10 keV. Additional states, I π = (6 + ), 7 + , 8 + , indicate coexistence of rotation-aligned and deformation-aligned 2q.p. excitations. An odd-parity ΔI = 1 band is ascribed to proton excitations of the type g 9 2 ⊗ P 3 2 f 5 2 . Several fast M1 transitions with B (M1) = 0.1–1 W.u. have been observe states of equal spin. They are interpreted as being due to configuration mixing.

Two-quasiparticle excitations and collectivity in the weakly-deformed transitional isotopes 104, 106, 108Cd

Abstract Using the generalized centroid-shift method on the Rutgers tandem, the following half-lives of 106 Cd excited states were measured in the reaction 93 Nb( 16 O, p2n): T 1 2 (3679.0 keV ) = 0.7 +0.1 −0.3 ns , T 1 2 (3507.8 keV ) = 1.2 ± 0.4 ns , T 1 2 (3044.2 keV ) = 0.4 ± 0.1 ns, and T 1 2 (2330.7 keV ) = 0.6 ± 0.2 ns . With the same method applied on the Rossendorf cyclotron, the following half-lives were measured in the reactions 102, 106 Pd (α, 2 n ): T 1 2 (2902.0 keV ) = 0.8 +0.2 −0.1 ns ( 104 Cd ) as well as T 1 2 (3737.3 keV ) = 0.2 ± 0.1 ns , T 1 2 (3223.7 keV ) = 0.2 ± 0.1 ns , T 1 2 (3057.4 keV ) = 0.10 ± 0.05 ns , T 1 2 (2975.3 keV ) = 0.15 ± 0.10 ns , T 1 2 (3110.5 keV ) = 0.3 ± 0.1 ns , and T 1 2 (2565.2 keV ) = 0.2 ± 0.1 ns ( 108 Cd ) . The results reveal the non-collective (two-quasiparticle) character of several states above 2.9 MeV in 104, 106, 108 Cd, in qualitative accordance with predictions of the slightly-deformed-rotor model. They concern completely aligned [h 11 2 g 7 2 ] (9 − −11 − -13 − , etc.) as well as semi-decoupled [h 11 2 d 5 2 ] (6 − -8 − -10 − , etc.) two-quasineutron band structures. Further, the possible character of 8 + (two-quasiproton) excitations, 5 + (two-quasineutron) states and of other intrinsic excitations is discussed. The experimental findings present a challenge to current theories of transitional nuclei for a quantitative treatment of absolute γ-ray transition strengths.

Electric quadrupole transition strengths of the type 6+1 → 4+1 in 106–112Sn

Abstract Nanosecond isomers in 106,110,112Sn and 107Cd are investigated by the bombardment of 106,110,112Cd with 29MeV 3He ions. Thereby, the following half-lives of states with Jπ = 6+1 are determined: T 1 2 (2323.6 keV in 106 Sn ) = 2.8 ±0.5 ns , T 1 2 (2477.0 keV in 110 Sn ) = 5.6 ± 0.4 ns and T 1 2 (2548.9 keV in 112 Sn ) = 13.8 ±0.4 ns . Further, T 1 2 (205.0 keV in 107 Cd ) = 0.7 ±0.1 ns was derived. Calculations of level energies and transition rates are performed within the quasiparticle-phonon model. The experimental B(E2, 6+1 → 4+1) values in 106–112Sn are reasonably reproduced within this model revealing thereby the importance of weak two-phonon components. Some level energies and transition rates in 106–112Sn (Z = 50) and their corresponding valence mirror partners with N = 82 are compared and discussed.

New positive-parity states and the shell-model description of 111Sn

Abstract The excited states in 111Sn have been studied by means of three different reactions using inbeam spectroscopic methods. Energies and intensities of γ-rays have been determined via the (α, n), (3He, 3n) and (6Li, 4n) reactions. Coincidences, angular and time distributions have been measured in the (α, n) and (3He, 3n) reactions. Conversion-electron spectra have been recorded in the (α, n) reaction. As a result, 9 levels of even parity have been identified with spin values between 9 2 and ( 17 2 ) and excitation energies up to 2.5 MeV, and three additional odd-parity states have been established up to Ex 11 2 − isomer, three further levels with ns lifetimes were found. The experimental data are compared with shell-model predictions. The hamiltonian parameters have been determined in a fit to 22 known levels of 111,112,113Sn. In the case of even-parity states a remarkable agreement between experiment and theory can be stated. For the description of odd-parity states a particle-core-coupling model is succesfully applied.

Absolute E1, ΔK = 1 transition probabilities between multi-quasiparticle high-spin states in 176Hf and 178Hf

Abstract Applying the generalized centroid shift method in (α, 2n) reactions, the half-lives of the 3080 keV 15 + state in 176 Hf and of the 1637 keV 5 − state in 178 Hf have been measured as T 1 2 = 0.20 +0.12 −0.08 ns and T 1 2 = 0.40 ± 0.10 ns , respectively. B (El) values of K -allowed E1 transitions n 9 2 + [624]→ 7 2 − [514] are derived, and together with other data on similar transitions in odd- A nuclei, compared with predictions of the Nilsson plus pairing model. In 176 Hf, the 15 + and 14 − states at 3080 and 2866 keV, respectively, appear as quite pure deformed 4QP configurations. In the 2QP state at 1637 keV in 178 Hf, possible strong mixing of vibrational components is discussed coupled via 2QP K -admixtures arising from the partial alignment of the i 13 2 neutron.

Electromagnetic properties of some odd-odd nuclei in the A≈ 100 region and IBFFM description of 106Ag

Abstract Electromagnetic properties of odd-odd transitional nuclei with A ≈ 100 are investigated experimentally and theoretically. Nanosecond isomers are studied in-beam by means of delayed γγ-coincidences and the generalized centroid-shift method. The reactions 98 Mo+30MeV 7 Li and 92,94 Mo+ 50 MeV 12 C are utilized. Following half-lives are determined: T 1 2 (547.2 keV in 102 Rh) = 0.25±0.07ns, T 1 2 (243.1 keV in 102 Rh) = 0.30±0.10ns, T 1 2 (181.1 keV in 99 Tc) = 3.8 ± 0.3ns, T 1 2 (2902.0keV in 104 Cd) = 0.6±0.1 ns, T 1 2 (118.7keV in 103 Pd) =0.8±0.2 ns, T 1 2 (131.1 keV in 105 Cd) = 1.5 ±0.3 ns, T 1 2 (211.8 keV in 104 Ag) = 1.4±0.1 ns, T 1 2 (181.0 keV in 102 Ag) = 3.5±0.2 ns. Additionally, several upper limits of level lifetimes are derived. The systematics of E1, M1 and E2 transition rates in 100,102,104 Rh and 102,104,106 Ag is presented and discussed. Fast M1 transitions within the 2qp π g 9 2 ν h 1 2 band appear to be associated with the strong coupling of the π g 9 2 proton and the rotational alignment of the h 11 2 neutron. Extensive IBFFM calculations of level energies and electromagnetic properties of 106 Ag as a typical case demonstrate that this model could account for the complex structural pattern of this type of nuclei. NUCLEAR REACTIONS 98 Mo( 7 Li, 3n), ( 7 Li2nα), E =30 MeV; 92,94 Mo( 12 C, pn), 94 Mo( 12 C, 2n), ( 12 C, 2pn), E = 50 MeV; measured E γ , I γ , γγ ( t ). 102 Rh, 99 Tc, 102,104 Ag, 104 Cd,

The elimination of prompt components in delayed coincidence time distributions

Abstract In the centroid-shift analysis of a delayed coincidence time distribution, an unknown prompt component may considerably influence the result. A new procedure is proposed to quantitatively identify and eliminate the prompt component in a complex time distribution provided the shape of the experimental apparatus response function is known. The procedure uses some features of the realistic complex distribution. Practical applications to experimental data obtained in different reactions with germanium detectors are demonstrated.

Two-quasiproton and two-quasineutron excitations in the transitional nuclei 102,104,106Pd

Abstract Using the generalized centroid-shift method on the Rossendorf cyclotron, the half-lives of following levels were measured in the (α, 2n) reaction for the first time: T 1 2 = 1.1 ± 0.1 ns (E lev = 2294.4 keV ) in 102 Pd ; 0.5 ± 0.2 ns (2298.0keV), 0.40 ± 0.05 ns (3151.8keV) and 0.25 ± 0.10 ns (3368.1 keV) in 104 Pd ; 2.0 ± 0.5 ns (2306.1 keV ), 0.5 ± 0.1 ns (2699.3 keV ), ⩽0.2 ns (2998.7 keV ), 0.2 ± 0.1 ns (3289.6 keV ) and 0.25 ± 0.10 ns (3461.7 keV ) in 106 Pd . Further, 0.8 ± 0.2 ns (118.6 keV) in 103 Pd was determined confirming an earlier result. In all three Pd isotopes, J π = 4 − states at ≈2.3 MeV are considered as two-quasiproton excitations. Two-quasineutron structures of the type ( h 11 2 d 5 2 ) and ( h 11 2 g 7 2 ) are discussed for the above-mentioned Pd isotopes and their corresponding Cd isotones revealing common features in these Z = 46 and 48 nuclei.

Some M1 transition strengths in odd-A nuclei away from closed shells

Abstract By the in-beam application of the generalized centroid shift method, nanosecond half-lives have been determined for the first time: in 101 Pd T 1 2 (1337.4 keV ) = 1.2 +0.6 −0.3 ns and T 1 2 (261.0 keV ) = 0.7 ± 0.2 ns using the reaction ( 12 C ,x n ) , in 71 As T 1 2 (147.5 keV ) = 0.85 ± 0.25 ns using the reaction (16 O, αp), in 91 Nb T 1 2 (5455.3 keV ) = 1.2 ± 0.3 ns using the reaction (16O,2np), in 103 Pd T 1 2 (244.0 keV ) ns and in 91 Nb T 1 2 (3110.2 keV ) ns using the reaction P(α, xn). Some known nanosecond isomers in different nuclei produced as by-products have also been detected. In the nuclei investigated far away from closed shells with complex wave functions, M1 transitions are considered which would be l-forbidden in the pure shell model. A retarded Ml (+ E 2) 25 2 + → 23 2 + transition in 91 Nb is considered as proceeding between possible multiparticle-hole configurations.

Electromagnetic transition probabilities in the doubly odd N = 91 nucleus 154Eu

Abstract Nanosecond lifetimes of isomeric states in 154 Eu excited in the (n, γ) reaction have been determined by the generalized centroid-shift method using a plastic scintillator and a planar thin-window germanium detector in delayed γγ-coincidence experiments. The following subnanosecond half-lives of excited states have been obtained for the first time: T 1 2 (203.8 keV ) = 0.80 ± 0.10 ns , T 1 2 (239.3 keV ) = 0.96 ± 0.15 ns and T 1 2 (281.7 keV ) = 0.25 ± 0.10 ns . For eleven excited states, upper limits of the half-lives have been estimated, among them T 1 2 (99.9 keV ) ≦ 2 ns in contradiction to a previous result. From these and earlier half-life data, absolute transition probabilities are deduced and compared with Weisskopf and Nilsson-plus-pairing-model estimates. The Nilsson model without extended configuration mixings turns out to be incapable of reproducing satisfactorily the experimental B (E1) strengths of allowed E1 transitions. By comparison of the experimental dipole transition rates between states of similar structure in 152 Eu and 154 Eu, stronger influence of configuration mixings in 154 Eu is revealed. Several experimental transition moments of allowed El transitions deviate from the relevant systematics in strongly deformed rare-earth nuclei. It is concluded that even at low excitation energies and low angular momenta the N = 91 nucleus 154 Eu behaves as a transitional nucleus similar to the N = 89 neighbour 152 Eu.