A. Djedid

Cold moving mice: A microfoil internal conversion electron detector for low and intermediate energy Mössbauer transitions

Abstract A microfoil internal conversion electron (MICE) detector is described which permits direct Doppler shifting of resonance radiation up to velocities of ± 20 cm/s, and gives large improvements in signal-to-background ratios for many Mossbauer isotopes, when compared with transmission geometry. The detector described has efficiency of nearly unity, and it allows for cooling the reciprocating microfoil module to 100 K, which improves signal-to-background substantially over room temperature operation. We give an analysis of the signal-to-background that can be expected for this MICE detector, and for a corresponding transmission experiment. In a table of neutron produced isotopes we find more than 10 cases which are favorable to the MICE approach as compared with the more conventional transmission geometry. The signal-to-background enhancement predicted for several Mossbauer isotopes is substantial for the MICE geometry compared to transmission geometry. Direct measurements of the Mossbauer conversion electron spectrum for the 46.5 and 99.1 keV transitions of 183 W and the 100.1 keV transition in 182 W are reported and compared with our analysis. In the case of 183 W (46.5 keV) we observe over 500% signal-to-background, and this experimental result agrees well with our analysis of the expected size of the effect. Satisfactory agreement is also found for the 99.1 keV 183 W and 100.1 keV 182 W spectra. Based on the analysis given it is possible to determine nuclear resonance cross sections and thereby infer internal conversion coefficients for the resonance transitions. Thus, we are able to determine the internal conversion coefficient for the 46.5 keV transition in 183 W to be α = 13 ± 3.

Representation of lineshape parameters and deconvolution of Mössbauer spectra

Abstract We report a rapidly convergent analytic representation for the Mossbauer effect (ME) lineshape and its Fourier transform, which gives an exact description of transmission and conversion electron cases. This representation permits the accurate determination of all Mossbauer effect (ME) parameters, including position, width, cross section, and interference and can be used to deconvolute hyperfine information contained in either source or absorber. The limitations of lorentzian and exponential-lorentzian fits to ME experiments are clarified.

Precise determination of Mössbauer lineshape parameters including interference

Using 100 Ci183Ta and 5 Ci182Ta sources, with LiF and NaCl crystal monochromating filters, we have measured the lineshape parameters for the 46.5 keV and 99.1 keV Mossbauer effect (ME) transitions of183W and the 100.1 keV transition of182W. Using an analytic representation of the convolution integral and utilizing asymptotic analyses of the lineshape, we find, for both transmission and microfoil internal conversion (MICE) experiments, accurate values of all ME parameters including width, position, cross section, and interference. This new approach allows deconvolution of source and absorber spectra and gives a simple analytic expression for both as well as their Fourier transforms. The line widths for the 46.5, 99.1, and 100.1 keV transitions are 3.10(10), 0.369(18), and 0.195(12) cm/s, respectively. The interference parameters are −0.00257(9), −0.0093(12), and −0.0107(12) in the same respective order. The agreement between transmission and MICE /1,2/ measurements for the above lineshape parameters is within the experimental errors. We believe these measurements are the first having sufficient precision to allow a quantitative comparison with dispersion theory /3,4/ and they indicate interference parameters 10 to 20% smaller than predicted. Our measured line widths are less than earlier reported values. This is because our analysis of the true lineshape and the study of line asymptotics permits a quantitative determination of the isomer lifetimes rather than the usual lower bound found in earlier ME experiments.

Asymmetry parameters in183W and182W using mice detection with a crystal monochromator

Using exceptionally high intensity Mossbauer sources (∼ 1–100 Ci) of182Ta and183Ta, we have measured the Mossbauer effect for the 46.5 and 99.1 keV transitions of183W and the 100.1 keV transition of182W. Using a microfoil internal conversion electron (MICE) /1/ detector capable of operation at low temperatures, and a LiF crystal monochromator, we obtain effects of nearly 600% for the 46.5 keV transition and 3 1/2% and 6% for the other two cases, while standard transmission measurements typically yield much smaller signal-to-background ratios. With this technique we have measured the asymmetry term in the conversion electron spectra. To our present level of accuracy the results are in agreement with theoretical calculations of interference parameters /2/. Our results do not agree with earlier measurements /3/ on this transition, which are grossly at variance with theoretical calculations of the interference parameter.

Elastic and inelastic scattering in silicon using Mössbauer diffraction

The Mossbauer diffraction instrument at the Missouri University Research Reactor has been used to measure the elastic and inelastic contributions to the 444 Bragg reflection in dynamic silicon at room temperature. These measurements used the 46.5-keV gamma rays from high intensity183Ta sources cooled to liquid nitrogen temperature. The main feature of this study compared to similar measurements on silicon is the significantly improved momentum space resolution. ΔQ values of 0.011 Å−1 and 0.11 Å−1 were measured for the transverse and longitudinal directions, respectively.