S. J. Robinson

Level structure of 191Os: Coexisting oblate and prolate configurations

Abstract An extensive level scheme for the transitional nucleus 191 Os has been constructed on the basis of (n, γ), (n, e − ) and (d, t) reaction data. γ-rays following thermal capture and average resonance capture (ARC) were detected with pair spectrometers. Secondary γ-ray spectra were obtained with the GAMS bent crystal spectrometers. The high precision of the resultant energies was the critical factor in developing a level scheme on the basis of Ritz combinations. γγ coincidences were obtained from both primary and secondary transitions. Conversion electrons, detected with the BILL spectrometer, gave multipolarity information. The (d, t) reaction measured with a Q3D magnetic spectrometer gave additional information on the hole states in 191 Os. A discussion of the resulting level scheme briefly reviews the fragmentation of negative-parity strength but focuses mostly on a rare example of an anti-aligned decoupled set of coexisting positive-parity states. For these, triaxial particle-plus-rotor calculations are carried out. Reasonable agreement is obtained and the calculations also explain the unusually strong population in (n, γ) of this set of mostly medium spin states.

E1 and M1 transition strengths from two-quasiparticle structures in 168Er

Abstract Lifetime measurements in 168 Er were performed in the (n, γ) reaction using the generalized centroid-shift method with the following results: T 1 2 (JK π = 54 − at 1193.0 keV ) = 0.70±0.07 ns and T 1 2 (JK π = 33 3 − at 1999.2 keV ) = 0.44 −0.08 +0.12 ns . The size of some matrix elements of the ty 〈0 + | M 10 E |0 − 〉(Δ K = 0) and 〈0 + | M 1−1 E |1 − 〉(ΔK = 1) in 168 Eris estimat The evolution of the strenghts of E1 transitions linking some negative-parity bands with the ground band in N = 100 isotones with varying spin and neutron number is considered. For K -forbidden transitions, ΔK = 0 admixtures in the E1 matrix elements are found to be dominant in 168 Er and 170 Yb, while in 174 W ΔK = 1 admixtures become at least similarly important. In 168 Er, K π = 2 γ + admixtures in the ground band appear responsible for up to 45% of the size of the E1 transition matrix elements. A surprisingly small mixing (∼0.01%) of the structure { 7 2 − [523]− 1 2 + [411]} p (K π = 3 3 − ) into the s 7 2 + [633]− 1 2 − [5]} n (K π = 3 1 − ) and vice versa is found to account for the measured B (M1 strenght of the band-head 33 3 − → 33 1 − transition.

High-resolution spectroscopy of 32P: (II). Level density and primary transition strengths observed after thermal neutron capture in 31P☆

Abstract The -γ-ray spectrum emitted after thermal neutron capture in 31P was studied at the ILL high flux reactor with a pair spectrometer and an intrinsic Ge detector. A total of 212 transitions were assigned to the decay of 32P and 155 of these, representing 96.7% of the observed flux, were placed in a level scheme of 38 states. The neutron binding energy was determined as 7935.74 (16) keV. The densities of states observed in this reaction and in a recent (d, p) study are analyzed in the constant temperature Fermi-gas model. The primary E1 and M1 transition strengths in 32P are discussed and are compared to other sd-shell nuclei.

Nuclear structure of 156Gd studied with (n, γ), (n, e−), (d, p), (d, t) reactions and lifetime measurements

Abstract The nucleus 156 Gd was studied with (n, γ) and (n, e − ) reactions at the Institut Laue-Langevin (ILL) in Grenoble. The (d, t) and (d, p) reactions were investigated at the Munich Tandem Accelerator. Primary gamma rays following average resonance capture were observed at the Institut for Nuclear Research in Kiev. Summed coincidences measurements were carried out at the Reactor IBR-30 in Dubna. An extensive level scheme was constructed up to 2.35 MeV including 413 transitions and 18 band assignments. Lifetimes of 16 selected levels were measured with the Gamma-Ray Induced Doppler broadening method at the ILL. Low-lying bands of positive and negative parity were interpreted in the framework of the spdf-IBA. The agreement was found to be quite good.

Level scheme and electromagnetic transition strengths in 177Lu

Abstract In the 176 Lu(n th ,γ) 177 Lu reaction, γ-ray and conversion electron spectra were measured using the bent crystal spectrometers GAMS and the spectrometer BILL, respectively. An extension of the low-lying level scheme of 177 Lu is achieved. Further, γγ delayed-coincidence lifetime measurements were performed and the following half-lives of excited states in 177 Lu were determined for the first time: T 1 2 (811.543 keV ) = 1.0(1) ns , T 1 2 (1356.930 keV ) = 11(2) ns and T 1 2 (1544.277 keV ) = 0.8 −0.1 +0.2 ns . The strengths of the observed M1, E2 and E1 transitions are compared to rotor-plus-quasiparticle calculations. The enhancement of the newly established K -forbidden E1 transitions linking the 5 2 + [402] and 9 2 − [514] bands may be explained by the influence of the octupole modes. The quadrupole deformation associated with the 1 2 − [541] band is deduced from the measured B( E2 , 9 2 − 1 2 → 5 2 − 1 2 transition strength and found to be larger than that of the ground state. The intrinsic configurations of isomeric three-quasiparticle states are discussed.

GRID lifetime measurements in59, 61, 63Ni following thermal neutron capture

The GRID-method has been used to measure the lifetimes of ten excited states in59, 61, 63Ni following thermal neutron capture in Ni targets of natural isotopic composition. Four of the lifetimes have been determined for the first time, the other six lifetimes can be compared with the results of conventional DSA-measurements following charged particle induced reactions. Cascade feeding effects have been included in the analysis. Level energies and electromagnetic properties of negative parity states in59Ni have been compared with the results of shell model calculations in 3p0h and 4p 1h model spaces. Statistical model estimates of the lifetimes as function of excitation energy and spin are also given.

The experimental structure of199Au and the interacting boson-fermion model

Gamma rays of199Au obtained after double neutron capture in197Au were measured at the ILL high flux reactor. A level scheme up to 1770 keV excitation energy is established. The result is compared with IBFM and Boson-Fermion-Symmetry calculations.

Electromagnetic transition strengths in the transitional doubly odd nucleus 198Au

Abstract The half-lives of low-lying levels in 198Au are determined by means of delayed coincidences and generalized controid-shift analysis in the (n, γ) reaction as follows: T 1 2 (3 + , 381.2 keV ) = 2.3(2), ns , T 1 2 (1 − , 192.9 keV ) = 0.7(2) ns , T 1 2 (1 − , 247.6 keV ) = 0.4(1) ns , T 1 2 (4 − , 215.0 keV ) = 0.4(2) ns . In some cases, decompositions of complex distributions were carried out in order to determine the corresponding time centroids. Upper limits are derived for the lifetimes of several other low-lying levels. The deduced electromagnetic transition probabilities B(σL) are compared to results of a new IBFFM calculation.

Nuclear Structure Investigations and Lifetime Measurement in 156Gd

The level structure of 156Gd has been studied using the neutron capture reaction at the ILL in Grenoble with the Gams 2/3 spectrometer and the pair spectrometer PN4. The conversion electron spectrum has been measured with BILL. Lifetimes have been measured using the GRID‐Method. ARC measurements have been carried out at the Institut for Nuclear Research in Kiev. The direct reactions 155Gd(d,p)156Gd and 155Gd(d,t)156Gd have been investigated at the Munich Tandem Accelerator.