V. Stefánsson

Depth selective Mössbauer-effect measurements by means of scattered electrons

Abstract A method is described for performing ME-measurements in a scattering geometry where arbitrary depth layers in the scatterer can be examined. The method, which may have a depth resolution of better than 50 A, is based on a statistical treatment of the energy loss of the resonance scattered conversion electrons. Energy loss distributions of 7 keV electrons scattered in iron-films are determined and ME-measurements are performed on enriched iron and stainless steel. From these measurements spectra emanating from different depth layers in the scatterer are calculated.

Internal conversion of E2 transitions in 152Sm and 154Gd

Abstract In a recent investigation the K-conversion electron particle parameters for three E2 transitions in the deformed nuclei 152 Sm and 154 Gd were found to be anomalous; for the 245 keV transition in 152 Sm the reported deviation from the theoretical value was 39±9%. A new measurement has been carried out in this work. For the transitions of 122 keV in 152 Sm and of 123 keV in 154 Gd the values found, expressed in units of finite-size corrected theoretical values, are b 2 (122)/b 2 0 (122) = 0.99±0.04 , b 2 (123)/b 2 0 (123) = 0.99±0.03 . For the 245 keV transition, which is of low intensity, a fairly high accuracy in the value obtained for the particle parameter was made possible by use of the b 4 method. From the geometrical relation between b 2 and b 4 , we obtain b 2 (245)/ b 2 0 (245) = 1.02±0.04. It is shown that if only dynamic contributions to the tabulated values are assumed, this result corresponds to a value for the conversion coefficient of α K (245)/ α K 0 (245) = 1.01±0.015.

Gamma-gamma Directional Correlations in 103Rh

Gamma-gamma directional correlations in 103Rh were measured in the decay of 103Ru. No dependence on the chemical composition of the source was observed. The following directional correlation coefficients were determined: A2(557 - 53) = -0.064 ± 0.003, A2(444 - 53) = -0.035 ± 0.013. The lifetime of the 93 keV state was measured as T½ = 1.06 ± 0.05 ns. By use of this value for the lifetime and the rotation in an external magnetic field of the directional correlation pattern, the gyromagnetic factor of the 93 keV state is obtained as g = 1.07 ± 0.17. The properties of 103Rh were calculated on basis of the unified vibrational model. The adequacy of this model for describing the positive-parity states studied in this work is discussed.

Levels and transitions in136Ba

Excited states of136Ba are studied in the beta decay of136Cs. The gamma spectrum is measured with high-resolution and coincidence techniques. Several new transitions are suggested to be connected with two recently reported levels. Gamma-gamma directional correlations are performed in an attempt to determine the characteristics of these levels, and in order to determine mixing ratios. The lifetime of the 2207.5 keV state is remeasured, and the lifetime of the 2140.5 keV state is measured for the first time.

Levels and transitions in126Te

The decay of126Sb to126Te is studied by use of Ge(Li) detectors in singles and coincidence arrangements. The gamma-gamma directional correlations of six cascades are measured. A level scheme is constructed where all transitions observed are located. The characteristics of several of the levels are assigned, on basis of the logft values deduced and the directional correlation results. Mixing ratios are deduced for three transitions. From the comparison of the relative intensities ofE1 andE2 transitions depopulating the same levels, threeE1 transitions are concluded to be strongly retarded.

Transition probabilities in146Eu

Transition probabilities in the odd-odd nucleus146Eu are measured. A recent determination of the lifetime of the 115.6 keV level has indicated a strong retardation of the 115.6 keVM1 transition. In contrast, the present results areT1/2(230.3)≦165 ps,T1/2(115.6)≦160 ps. TheE2 cross-over transitions of 269.5 and 230.3 keV were not found in the Ge(Li) gamma spectrum. All results are in agreement with the single particle model.

Nuclear-Structure Effects on the Conversion Electron Particle Parameter of the 57.5 keV E1 Transition in 180Hf

Electron-gamma directional correlations involving electrons emitted in the 57.5 keV 8- → 8+ transition in 180Hf are measured for the first time. The result for the second-order correlation coefficient, expressed relative to the theoretical value for a pure E1 transition without nuclear-structure effects in the internal conversion process, is 0.77 ± 0.04. The result is analysed in terms of the nuclear current and nuclear charge penetration parameters λ1 and λ2 and the mixing ratio δ(M2/E1).

ELECTRON SCATTERING EFFECTS IN DIRECTIONAL CORRELATIONS.

Abstract Electron scattering effects in directional correlations are studied by use of a magnetic electron spectrometer. Foils of various metals are placed in front of the radioactive source. The results are compared with a theory by Frankel, which is generally used to correct for scattering effects in directional correlations. The theory is found to be approximately valid. However, it is shown that the scattering effect depends critically on the properties of the electron spectrometer used. Thus, the concept of scattering, as generally used, is ambiguous. Due to the difficulty of estimating the source thickness, it is suggested that the scattering correction is related to the readily determined energy loss of the electrons. Scattering versus energy-loss curves, valid for the lens type spectrometer used, are presented. Such curves should make feasible the use of thicker sources than hitherto have been found acceptable.

The E0 component of the 689 keV 2β → 2g transition in 152Sm

Abstract The electric monopole contribution to the 689 keV 2 β → 2 g transition in 152 Sm is studied by measuring the 689K-122γ directional correlation. The result for the directional correlation coefficient is a 2 (K γ ) = 0.10±0.05. This value is used together with earlier electron and γ-ray intensity data to evaluate the E0/E2 electron amplitude and the M1 penetration parameter. The corresponding E0/E2 mixing ratio is compared with recent theoretical predictions.

E0 — Contribution to the 689K-122γ Angular Correlation in 152 Sm

In some recent experiments (ref. 1 – 9) beta — vibrational states in 150Nd, 152Sm, and 154,156 Gd were investigated. The results were found to disagree with the collective — model description of these states. The relative magnitude of the E2 matrix elements for transitions connecting the beta and ground state bands cannot be described by introducing only one mixing parameter.