G. Schupp

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.

A gamma-ray diffraction instrument for high-intensity Mössbauer sources☆

A Mossbauer gamma-ray diffraction instrument has been developed which utilizes high intensity sources produced by neutron irradiation. Most of the early work has used 70 Ci, 5.1 d 183Ta sources produced by double neutron capture which yield 12 × 1010 photons/s for the 46.5 keV Mossbauer transition in 183W. A dewar is enclosed in the shielding cask which allows sources to be cooled to 77 K. Samples are located approximately midway between source and detector, which are separated by 155 cm. Collimators in front of the sample and detector commonly limit the beam to 3 mm wide by 25 mm high. A 27% minimum is seen in the velocity spectrum for the 46.5 keV photons scattered from the 200 Bragg reflection in LiF taken in transmission. Enriched, room temperature absorbers mounted on a rotor can be Doppler-shifted up to 17 meV. Q-space resolution of 0.01 A−1 is significantly better than that reported by other groups using Mossbauer scattering. When a microfoil internal conversion electron (MICE) detector is used, the instrument has the potential for directly measuring the energy transferred by the inelastically scattered photons, especially for low energy excitations in solids which require high energy resolution.

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.

Precision measurements of recoil-free fraction and interference with hundred Curie sources**

We discuss our experiences with exceptionally intense Mössbauer sources (of order 100 Ci) and their use in making precision determinations of line shape parameters. We review our measurements of interference and recoilless fraction using conventional transmission geometry. Fitting simultaneous sets of data with known constraints greatly improves the efficiency of line-shape determination. Measurements of interference have been made using a two-absorber technique first introduced by Mössbauer and coworkers. Our results indicate that the method can be useful for determining interference, but that incorrect line shape parameters are obtained if powder samples are used which do not comply well with the assumption of thickness uniformity in the absorber.

Using line shape to precisely determine recoil-free fraction: Application to tungsten

The analytic description of Mössbauer line shape based on the Fourier-transform method [1] makes it possible to precisely determine the recoil-free fraction,f, of a single absorber over a wide temperature range with a single measurement at each temperature. Most measurements of recoil-free fraction have been done with a Lorentzian representation of the Mössbauer line with semi empirical correction factors of uncertain precision. Our method, based on line shape rather than area, is exact for absorbers of any thickness, limited only by the number of terms taken in the expansion. In fact, our exact line-shape description makes it advantageous to use very thick absorbers. As an example, we report values for an absorber of effective thickness 12. The resonance thickness number of the absorber is an intrinsic parameter, so that a fit yields this quantity directly. The correlations that arise between certain variables may be reduced by fitting, simultaneously and with known constraints, spectra taken with different absorbers or with one absorber at various temperatures. Using this technique, we report precise values forf(T) in tungsten using the 46.5 keV transition in183W atT=297 K and 1067 K; these values are 0.300±0.009 and 0.0123±0.0002, respectively.

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.

Mössbauer spectra obtained by modulation of a scattering crystal

We have carried out numerous experiments with supersources having intensities in the 100 Curie range. These sources usually require massive shielding, and are not easily moved to carry out Mössbauer spectroscopy. Several of these sources can be used with microfoil conversion electron (MICE) detectors, but they cannot be moved easily either because of the delicate microfoils used, which ideally have thicknesses less than the range of the internally converted electrons. Here, we describe a technique for doing Mössbauer spectroscopy by oscillating a monochromating crystal parallel to the reciprocal lattice vector of the Bragg reflection, this being used to filter out extraneous photons from the beam. Specifically, an LiF crystal is used in diffraction experiments as a filter to scatter the 46.5-keV Mössbauer gamma rays from183Ta by setting it at the (200) Bragg reflection. In the present measurements, the LiF crystal was mounted in the transmission mode and oscillated with a crank along the scattering vectorQ to produce the velocity modulation, with the source, sample and absorber all at rest. The velocity components of the filtering crystal along the incident and scattered beams cause the measured linewidth to be equal to the usual Mössbauer width divided by 2 sinθ, whereθ is the Bragg angle. Measured widths for the (200), (400), (600) and (800) Bragg reflections agreed with the calculated values of 12.08, 6.04, 4.03 and 3.02 cm/s, within our experimental uncertainties. The technique could have applications not only to MICE detectors, but also to very narrow resonances such as67Zn, where the increased velocities required for small Bragg angles could be an advantage and lead to enhanced resolution.

Experiments and possibilities with super-intense Mössbauer sources

We have been able to prepare Mössbauer sources having intensities ranging from 10 to 100 Ci for several isotopes, using the Missouri University Research Reactor Facility, MURR. These sources offer rich opportunities for carrying out precision Mössbauer experiments as well as high collimation scattering experiments. Using such intense sources, we have been able to measure the interference parameter of the 46.5 keV Mössbauer transition to about 1% accuracy. We find it to be about 10% greater than predicted by theory, which may have significant implications for time reversal violation experiments. We have also been able to show that the Bragg scattered recoilless fraction scattered from (200) planes of sodium chloride crystals is 94% of that found for the (600) reflection, even though the integrated intensity of this reflection is more than a factor of 10 less than that of the (200) reflection. We highlight some of the experiments that we have carried out and discuss some of the enticing possibilities for the future with sources in the 10–1000 Ci range. One of these is the possibility of doing Mössbauer spectroscopy with a stationary source and absorber, moving only a monochromating crystal filter along the direction of the reciprocal lattice vector associated with the Bragg reflection being used.