Spins and mixing ratios in 138Ba

The spin assignments to the 1899 (4⁺), 2308 (3⁺ or 4⁺) and 2446 (3⁺) keV levels in ¹³⁸Ba have been confirmed by γ-γ directional correlation measurements. In addition, the multipolarity and $\text{E2}\text{M1}$ mixing parameters for a number of transitions have been established as follows: 409 keV (M1+E2, ?0.75 < δ < ?0.45 or ?0.85 < δ < ?0.05 depending on the choice of J^π = 3⁺ or 4⁺ for the initial state), 463 keV (E2, 0 < δ < 0.15 for $\text{M3}\text{E2}$ admixture), 547 keV (M1+E2, ?0.06 < δ < ?0.015), 872 keV (M1+E2, δ undefined) and 1010 keV (M1+E2, ?0.015 < δ < +0.020).     

Giant M1 resonance in the direct-semidirect model for nucleon radiative capture

The direct-semidirect model for nucleon radiative capture proceeding via giant E1 and E2 resonance states is extended to account for capture through collective M1 excitation. The ²⁰⁸Pb(N, γ₀) and ¹⁴⁰Ce(N, γ₀) reactions are investigated in the 2–10 MeV energy interval. For the M1 and isoscalar E2 neutron capture processes, calculations provide cross section values of the same order of magnitude, as well as comparable effects on γ-ray angular distributions. The model proves to be an appropriate framework for discussing the E1-M1 and E1-E2 interference processes, giving useful suggestions as to effects arising from the presence of the M1 and E2 collectivities.     

Vibrational band structure in 184W populated in the decay of 184Re

The decay of ^184mRe has been investigated through γ-ray and conversion electron studies. The band head of the K^π = 2^? octupole band has been established at 1130.0 keV. The E2/M1 mixing ratios of three transitions from the γ-vibrational band to the ground state band have been determined by angular correlation measurements. A mixing of El, M2 and E3 multipolarity has been derived for the 921 keV transition combining angular correlation and conversion electron data. A value B(E3, 0⁺ → 3^? = (25 ± 5) × 10⁴e² · fm⁶ was obtained from the measured E2/M1 mixing of the 91 keV 3^? → → 2^? transition and γ-branchings. The data are discussed in terms of the collective model taking into account band mixing.     

Radiative Ml capture in light nuclei

The importance of semi-direct processes in E1 and E2 capture reactions has been well documented. Little is known, however, about M1 capture although a vast body of data indicates a non-statistical nature. We discuss here the importance of the semi-direct M1 process and concentrate in particular on recent experimental results claiming that the M 1 strength in²⁹Si isnearly one order of magnitude smaller than in ²⁸Si, the target nucleus. We use shell model arguments to demonstrate that this reduction is indeed consistent with the semi-direct reaction process and that other mechanisms like2p-2h fragmentation and phonon coupling are not likely to be instrumental in elucidating further the non-statistical nature of the²⁸Si+n reaction.     

The M2 radiative widths of nuclear levels at about 7 MeV excitation

The M2 widths and M2/E1 mixing ratios in transitions of the form 1⁻¹ → 2⁺ from nuclear levels at about 7 MeV excitation in ⁷⁴Ge, ¹⁰⁰Mo, ¹¹²Cd and ¹⁸⁶W have been measured. The M2 widths are found to be about the same average magnitude as the M2 widths in nuclei with A < 40.     

Nuclear orientation of 160Tb in Tb metal

The decay of ¹⁶⁰Tb oriented at low temperatures in Tb metal has been investigated, and angular distributions of 19 γ-rays have been determined. The reduction in anisotropy in polycrystalline metal samples is discussed. Multipole mixing ratios have been determined for many of the γ-rays. A critical discussion is given of the previously reported $\text{M2}\text{E1}$ mixing ratios, and previous nuclear orientation studies are re-analyzed in a consistent manner so that their results can be compared. It is shown that the nuclear orientation and angular correlation results for the 1178 keV transition are consistent only if E3/E1 mixing is present. The El branching ratios and $\text{M2}\text{E1}$ mixing ratios are analyzed on the basis of Coriolis-mixed K = 0, 1 and 2 character for the negative-parity levels of ¹⁶⁰Dy.     

Observation of the M1 giant resonance by resonance averaging in 106Pd

An investigation of capture of 2 keV and 24 keV neutrons in a ¹⁰⁵Pd target resulted in resonance-averaged intensities of primary gamma rays with energies between 5.2 and 9.5 MeV. From these intensities the gamma ray strength functions have been evaluated for E1, M1 and E2 radiation and compared with predictions of the giant resonance theory. The inclusion of an energy dependent spreading width for the E1 giant resonance is necessary. The energy distribution of M1 reduced strength is consistent with an interpretation of a broad resonance around 8.8 MeV. E2 data agrees satisfactorily with the giant resonance extrapolation.     

Relativistic many-body calculations of E1, E2, M1, and M2 transitions rates for the 1 s 2 l′ 2 l′ ′ – 1 s 2 2 l lines in Li-like ions

Reduced matrix elements, oscillator strengths, and transition rates are calculated for all allowed and forbidden 1s2l′ 2l′ ′ – 1s ²2l electric dipole (E1), magnetic dipole (M1), electric quadrupole (E2), and magnetic quadrupole (M2) transitions in lithiumlike ions with nuclear charges ranging from Z?=?6 to 100. Relativistic many-body perturbation theory (RMBPT), including the Breit interaction, is used to evaluate retarded E1, M1, E2, and M2 matrix elements. The calculations start with a Dirac potential and include all possible 1s2l′ 2l′ ′ configurations, leading to seven odd-parity and nine even-parity states. First-order perturbation theory is used to obtain intermediate coupling coefficients. Second-order RMBPT is used to determine the matrix elements, which are evaluated for the 22, 20, 16, and 18 possible E1, M1, E2, and M2 transitions, respectively. A detailed discussion of the various contributions to the energy levels and E1, M1, E2, and M2 matrix elements is given for lithiumlike iron, Z?=?26. The transition energies used in the calculation of oscillator strengths and transition rates are evaluated using second-order RMBPT. Trends of E1, M1, E2, and M2 transition rates as functions of nuclear charge Z are shown graphically for 1s2l′ 2l′ ′ – 1s ²2l transitions.     

Coriolis coupling and electromagnetic properties of rotational bands in odd-a Yb nuclei

Properties of the rotational bands in¹⁶⁷Yb are studied using the quasiparticle-phonon model with the Coriolis interaction included. The level energies,B(E2) andB(M1) values, E2/M1 mixing ratios, and magnetic moments are calculated. In particular, theB(E2) values and the E2/M1 mixing ratios measured by using delay coincidence and nuclear orientation methods, respectively, are well reproduced.Dedicated to Professor Ivan Úlehla on the occasion of his sixtieth birthday.     

Nuclear orientation below 1 mK: Development and applications

The Oxford top-loading PrNi₅ nuclear demagnetisation stage /1/ has been further developed to reach temperatures below 1 mK. The hyperfine field for⁷¹As in nickel has been measured by low temperature nuclear orientation as 14.61(36)T. Measurements on transitions in⁷¹Ge yield spin values 3/2 (1299 kev and 1139 kev) and 5/2 (1027 kev) and 5 E2/M1 mixing ratios are given.     

EPM calculations of E2/M1 mixing ratios for even samarium isotopes 146–152Sm

The M1 operator and the E2/M1 mixing ratios, Δ(E2/M1), are discussed in the framework of the extended phonon projection model (EPM). The model is applied to ^{146, 148, 150, 152}Sm to describe their M1 properties. Comparison is made between the EPM, the consistent-Q formalism (CQF) of the basic interacting-boson approximation (IBA-1) and the experimental Δ(E2/M1) data. Both theoretical approaches give reasonably good results and parameter systematics.     

Magnetic properties of 166Er and the collective model

The E2/M1 admixtures for nine γ-ground band transitions in ¹⁶⁶Er were measured. Mixing ratios, δ(E2/M1), were calculated based on a general treatment of magnetic properties in the framework of the collective model. The agreement with the experimental values is generally good.     

Configuration assignments and decay of 126La high-spin bands

High spin states of ¹²⁶La have been populated using the reaction ¹¹⁶Sn+¹⁴N at 68 MeV. γ-rays and conversion electrons were detected with the GAREL array. Multipolarities of the lowest-lying in-band dipole transitions have been determined from the deduced internal conversion coefficients. Experimental B(M1)/B(E2) ratios have been derived for the bands and compared with calculated values using the D?nau-Frauendorf geometrical model. Configurations are proposed for the bands comparing them with cranked shell model calculations and on the basis of the measured B(M1)/B(E2) ratios. The β-decay of ¹²⁶La has also been revisited. The population of the ¹²⁶Ba levels gives a probable spin value of five for the decaying high-spin ¹²⁶La state with T _1/2≈ 64 s which may indicate a signature inversion in the πh_11/2νh_11/2 band. Received: 12 October 1999     

Signature-dependent ml and e2 transition probabilities in 155ho and 157ho

¹⁵⁵Ho and ¹⁵⁷Ho have been populated in the reactions ¹⁴¹Pr(¹⁸O,4n) and ¹⁴⁶Nd(¹⁵N, 4n) at 85 and 74 MeV, respectively. In both nuclei bands built on the $\text{7}\text{2}$?[523] configuration were established to spin values considerably above the first backbend. A signature dependence in the excitation energies as well as in the ratio of M1 to E2 transition rates is observed below, but not above, the backbend in both nuclei. In ¹⁵⁷Ho lifetimes were measured with the recoil-distance method. The ΔI = 2; E2 transition probabilities obtained show very little variation with either signature or spin and no irregularity at the backbend. The signature dependence and strong rise in the ratio B(M1)/B(E2) observed at the backbend in ¹⁵⁷Ho therefore must be caused by the B(M1) values. A signature dependence in the B(E2, I → I ?1)/B(E2, I → I ?2) ratios also found in ¹⁵⁷Ho below the backbend is mainly the result of signature dependence in the ΔI = 1 ; E2 transition rates. Qualitatively, most of the features observed can be explained by nonaxial deformations, which change from large negative to slightly positive values of γ at the backbend.     

First evidence of the magnetic character of dipole transitions in the “oblate bands” of199Pb

Internal conversion coefficients have been determined from online measurement of electron- and y- ray emission related to the dipole transitions in the so-called oblate collective bands in ¹⁹⁹Pb.The results strongly support the M1 (or M1+E2) character of these transitions.     

High-resolution (e,e') study of isovector M1 and M2 transitions in the oxygen isotopes: (I). 16O

The M1 and M2 transition strength distribution for ¹⁶O in the excitation energy range from 16 to 20 MeV has been measured in a high-resolution electron scattering experiment. The M1 strength is concentrated in three sharp states at E_x = 16.22, 17.14 and 18.79 MeV (± 0.01 MeV) with B(M1, k)↑ = 0.20 ± 0.02, 0.32 ± 0.03 and 0.13 ± 0.03 μ_N², respectively. An additional strength of 0.35 ± 0.09 μ_N², distributed over eight weakly excited states with excitation energies E_x = 17.4 to 18.0 MeV, brings the total measured M1 strength to B(M1, k)↑ = 1.0 ± 0.1 μ_N². The experimental M2 strength is distributed over states at E_x = 16.82, 17.78, 18.50 and 19.0 MeV (± 0.01 MeV) with B(M2, k)↑ = 19 ± 2, 13 ± 2, 59 ± 7 and 341 ± 51 μ_N² · fm², respectively. Electric transitions were also measured to states at E_x = 16.45 MeV (2⁺, E2), 17.30 MeV (1⁺, E1) and 18.20 MeV (2⁺, E2). Calculations were performed using the modified surface delta interaction in a 2p-2h shell model for the M1 transitions and the random phase approximation for the M2 transitions. The results show the sensitivity of the M1 strength as a measure of ground-state correlations and compare well with results from the ¹⁵N($\text{p}$, γ) reaction.     

Gamma rays from resonance neutron capture in 115In

Gamma rays in the range 5.4–6.7 MeV have been studied for 31 s-wave neutron resonances of ¹¹⁵In, selected by time of flight in the range 3–430 eV. In a subsidiary experiment, spin J = 5 has been assigned to 17 and J = 4 to 14 resonances by measuring intensity variations of some strong low-energy transitions. The reduced widths averaged over all initial states of the same spin have been estimated for 41 primary transitions: these values have provided information on the spin and parity of the corresponding ¹¹⁶In final states. Overall mean values of E1 and M1 radiative strength have been calculated. The width distribution has been fitted with a χ² function with ν = 1.10^+0.27_?0.09 degrees of freedom for M1 and ν = 1.42 ^{+ 0.014}_?0.08 for E1 radiation. An estimate of the spin cut-off parameter σ = 3.6_?0.4^{+ 0.8} has been derived. A non-statistical effect already evidenced in previous measurements has been confirmed, consisting of a strong modulation of the radiative strength against resonance energy, correlated also with the local neutron strength function. In addition, it has now been shown that this structure is due to E1 radiation only.     

Nuclear orientation study of the decay of 171Lu

The levels and γ-ray transitions in ¹⁷¹Yb have been studied by radioactive decay of oriented ¹⁷¹ Lu. The orientation was produced using the hyperfine field in ferromagnetic gadolinium to polarize an ensemble of ¹⁷¹Lu nuclei at a temperature of 14mK. The directional distribution of the γ-rays was measured using high-resolution Ge(Li) detectors. Anisotropies of 30 γ-rays in ¹⁷¹Yb were measured. Multipole mixing ratios were deduced for most of the mixed γ-ray transitions and unambiguous spin assignments were made for several levels.The energy and structure of the rotational band levels, reduced E2 and M1 transition probabilities and E2/M1 mixing ratios were calculated using the semi-microscopic quasiparticle-phonon model with a Coriolis interaction. In general, good agreement is obtained between the theoretical predictions and experimental results.     

Conversion-electron-gamma and gamma-gamma directional angular correlations in 160Dy

Gamma-gamma and conversion-electron-gamma angular correlations in ¹⁶⁰Dy have been measured for the 298 keV–966 keV and 298 keV–879 keV cascades. Particle parameters of the 966 keV E2 transition were determined to be b₂(E2; e_K) = +1.23±0.08 and b₂(E2; e_{ΣL + ΣM) = +1.27±0.23}. The multipole mixing ratio for the 298 keV radiation was determined to be δ(M2/E1) = +0.04±0.01 and we have confirmed the value δ(E2/M1) = ?13±2 for the 879 keV. The E0–E2 mixture parameter for the 879 keV transition was determined as q_K = ?0.03±0.09 assuming penetration effects to be negligible.