W. Andrejtscheff

In-band M1 and E2 transition rates and collective structures in 128Ba

Abstract Subpicosecond mean lifetimes of eight excited states in 128Ba populated via the 96Zr(36S,4n) reaction were measured by the Doppler-shift attenuation (DSA) technique using a line-shape analysis. The differential decay-curve method (DDCM) was applied for the lifetime determination. The B(E2) values in the yrast band indicate that the first band-crossing is with a proton S-band. The configuration πh 11 2 d 5 2 of the negative-parity semi-decoupled bands is confirmed by the measured B(E2, I → I − 2) and B(M1, I → I − 1) transition strengths. The higher-lying “dipole” band in 128Ba can be described as a high-K four-quasiparticle band built on the prolate configuration (πh 11 2 d 5 2 ) ⊗ (νh 11 2 g 7 2 ) .

Electromagnetic transitions in some doubly odd deformed nuclei

Abstract Nanosecond lifetimes of excited states in doubly odd deformed nuclei have been determined by in-beam measurements applying the method of delayed γ-γ coincidences as well as by experiments in the radioactive decay with the method of delayed γ-ce coincidences, respectively. Analysing the time distributions and delayed γ-ray spectra, the following half-lives of isomeric states could be obtained for the first time: T 1 2 (63.7 keV in 160 Tb ) = 60 ± 5 ns , T 1 2 (138.7 keV in 160 Tb ) = 5.7 ± 0.5 ns , T 1 2 (82.0 keV in 156 Ho ) = 1.25 ± 0.20 ns , T 1 2 (87.2 keV in 156 Ho ) = 58.5 ± 3.5 ns , T 1 2 (139.2 keV in 158 Ho ) = 1.85 ± 0.10 ns , T 1 2 (38.3 keV in 162 Ho ) = 1.2 ± 0.2 ns , T 1 2 (179.9 keV in 162 Ho ) = 8.7 ± 0.2 ns , T 1 2 (342.8 keV in 164 Ho ) = 2.6 ± 0.2 ns , T 1 2 (295.1 keV in 166 Ho ) = 1.10±0.15 ns , T 1 2 (44.6 keV in 162 Tm ) = 1.40±0.15 ns , T 1 2 ,(163.4 keV in 162 Tm ) = 1.1 ± 0.1 ns , T 1 2 (220.1 keV in 178 Ta ) = 8.5 ± 1.0 ns , T 1 2 (289.5 keV in 178 Ta ) = 2.0 ± 0.5 ns , T 1 2 (392.8 keV in 178 Ta) ≈ 1 ns, T 1 2 (316.5 keV in 186 Re ) = 0.20 ± 0.10 ns , T 1 2 (300.2 keV in 188 Re ) = 1.5 ± 0.2 ns and T 1 2 (482.2 keV in 188 Re ) = 0.26 ± 0.10 ns . Furthermore, upper limits for the half-lives of fifteen excited states in 160Tb, 164, 166Ho and 186, 188Re have been estimated. For eight isomeric levels in 186, 188Re, the lifetimes earlier determined have been remeasured. Unlike previous studies, the existence of isomeric states at 87.2 keV in 156Ho and at 179.9 keV in 162Ho is suggested. Absolute γ-ray transition probabilities are deduced and compared with single-particle estimates according to Weisskopf and Nilsson, the latter also including pairing correlations. The K-, Ω- and f-forbidden transitions can qualitatively be explained in terms of configuration mixings. Experimental El, ΔK= 1 transition matrix elements in odd-odd deformed nuclei are supposed to be appreciably influenced by higher-order vibrational admixtures coupled via RPC and p-n interaction mixings.

Electromagnetic transitions in some odd-proton deformed nuclei

In-beam measurements of nanosecond lifetimes applying the method of delayed γ-γ coincidences were performed in the (p, n) reaction. Analysing the time spectra with the centroid shift method, the following half-lives of excited nuclear states in the subnanosecond region could be found: T12(353.2 keV in 161Ho) = 0.52±0.15 ns, T12(252.7 keV in 161Ho) ≦ 0.2 ns, T12(579.4 keV in 161Ho) ≦ 0.2 ns, T12(431.2 keV in 163Ho) = 0.37±0.15 ns, T12(439.9 keV in 163Ho) = 0.35±0.15 ns, T12(471.3 keV in 163Ho) ⪅ 0.2 ns, T12(612.8 keV in 163Ho) ⪅ 0.3 ns, T12(295.6 keV in 171Lu) = 0.85±0.20 ns, T12(469.2 keV in 171Lu) ≦ 0.2 ns, T12(357.0 keV in 173Lu) = 0.40±0.08 ns and T12(449.0 keV in 173Lu) = 0.58±0.12 ns. Following half-lives in 173Lu have been remeasured: T12(425.3 keV) = 0.84±0.20 ns and T12(434.9 keV) = 0.38±0.10 ns. Absolute γ-ray transition probabilities are deduced and compared with Nilsson model predictions including pairing correlations. Coriolis mixing calculations are performed for K-allowed as well as for K-forbidden transitions.

Electromagnetic transitions in some odd-neutron deformed nuclei☆

Abstract Using the reactions 155, 157 Gd(α,2n), 178 Hf(n,γ) and 177Hf(α, 2n , the following half-lives of excited nuclear states have been measured: T 1 2 (188.1 keV in 157 Dy ) = 1.00 ± 0.15 ns , T 1 2 (161.9 keV in 157 Dy ) = 1.3 ± 0.2 μ S , T 1 2 (177.6 keV in 159 Dy ) = 9.0 ± 0.5 ns , T 1 2 (614.3 keV in 179 Hf ) = 0.50 ± 0.15 ns , T 1 2 (720.7 keV in 179 Hf ) ≦ 0.3 ns , T 1 2 (516.4 keV in 179 Hf ) ns and T 1 2 (309.0 keV in 179 W ) = 1.53 ± 0.10 ns . A Ge(Li) timing system was employed. Electromagnetic transition probabilities are calculated in the Nilsson model including pairing and band mixing effects. Comparisons between theoretical and experimental results are performed.

Electromagnetic transition probabilities in odd-mass erbium isotopes

Abstract Nanosecond lifetimes of 163, 165 Er excited states have been measured by means of delayed γ-γ coincidences in the (α, 2n) reaction on enriched Dy targets and delayed γ-ce coincidences in the 163,165 Tm decay. The following half-lives unknown so far were obtained: T 1 2 (84.3 keV ) = 0.92 ± 0.08 ns , T 1 2 (104.3 keV ) = 0.52 ± 0.05 ns and T 1 2 (443.8 keV ) = 0.58 ± 0.10 μ s for 163 Er levels as well as T 1 2 (296.1 keV ) ≦ 0.24 ns , T 1 2 (297.3 keV ) = 0.70 ± 0.08 ns , T 1 2 (356.5 keV ) = 0.35 ± 0.06 ns , T 1 2 (507.3 keV ) = 0.70 ± 0.12 ns , T 1 2 (551.0 keV ) = 0.25 ± 0.03 μ s , T 1 2 . (589.7 keV ) ≦ 0.6 ns and T 1 2 (745.7 keV ) = 1.00 ± 0.15 ns for 165 Er levels. From the investigation of γ-ray spectra populating the 11 2 − [505] isomer, the rotational levels at 1024.0 keV (J π = 17 2 − ) and 1259.8 keV (J π = 19 2 − ) in 163 Er could be confirmed as well as the level at 1078.1 keV with J π = 17 2 − in 165 Er could be established. In order to obtain the wave functions of K-mixed nuclear levels, Coriolis coupling calculations taking into account the δN = 2 mixing in 165 Er are performed for 161, 163, 165 Er including in the fit all the experimentally known band-head and rotational energies of positive-parity and negative-parity states, respectively. Partial γ-ray half-lives of E1, M1 and E2 transitions in 161,163,165,167,169 Er are deduced from the present results and earlier data. Electromagnetic transition probabilities are calculated within a non-adiabatic approach and compared with the experimental values. The influence of pairing correlations and configuration admixtures on the transition probabilities is demonstrated using hindrance factors and numerical values of transition matrix elements.

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.

Picosecond lifetime measurements and collective transition strengths in 128Ba

Abstract Mean lifetimes of 14 excited states in 128 Ba populated via the 118 Sn( 13 C, 3n) and 114 Cd( 18 O, 4n) reactions were measured using the recoil-distance Doppler-shift (RDDS) technique and the pulsed- beam method. The differential decay-curve method (DDCM) was applied to extract the lifetimes from the experimental RDDS data. The nuclear deorientation effect on the transitions depopulating the 2 1 + and 4 1 + levels was measured and taken into account in the lifetime determination. The resulting B (E2) values in the ground-, gamma- and two-quasiproton bands are compared to theoretical predictions of collective models.

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.

Nanosecond isomeric transitions in the 89-neutron nucleus 153Gd

Abstract In 153Tb decay, the half-lives of the excited states at 41.6 keV and 129.2 keV in 153Gd have been determined as T 1 2 (41.6 keV ) = 4.1 ±0.1 ns and T 1 2 (129.2 keV ) = 2.50±0.15 ns . Nilsson characteristics for some levels are suggested. Comparisons to Nilsson and Weisskopf estimations for γ-ray transition probabilities are performed. Conclusions about the 153Gd level structure are drawn.

Evidence for strong ground-state shape asymmetry in even germanium and selenium isotopes

Abstract The asymmetry of even-even nuclei with 46≤ A ≤82 (22≤ Z ≤34) is studied by the sum-rule method applying recently introduced approximations. The uniquely high eccentricities of 72−76Ge and 74–78Se point at the most pronounced (effective) triaxiality of ground states emerging so far from the experimental data for 46≤ A ≤192.