MOSSBAUER-SPECTRA OBTAINED BY MODULATION OF A SCATTERING CRYSTAL

We have carried out numerous experiments with supersources having inte nsities in the 100 Curie range. These sources usually require massive shielding, and are not easily moved to carry out Mossbauer spectroscop y. Several of these sources can be used with microfoil conversion elec tron (MICE) detectors, but they cannot be moved easily either because of the delicate microfoils used, which ideally have thicknesses less t han the range of the internally converted electrons. Here, we describe a technique for doing Mossbauer spectroscopy by oscillating a monochr omating 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 Mossbauer gamma rays from Ta-183 by setting it at the (200) Bragg reflection. In the present measuremen ts, the LiF crystal was mounted in the transmission mode and oscillate d with a crank along the scattering vector Q to produce the velocity m odulation, with the source, sample and absorber all at rest. The veloc ity components of the filtering crystal along the incident and scatter ed beams cause the measured linewidth to be equal to the usual Mossbau er width divided by 2 sin theta, where theta is the Bragg angle. Measu red widths for the (200), (400), (600) and (800) Bragg reflections agr eed with the calculated values of 12.08, 6.04, 4.03 and 3.02 cm/s, wit hin our experimental uncertainties. The technique could have applicati ons not only to MICE detectors, but also to very narrow resonances suc h as Zn-67, where the increased velocities required for small Bragg an gles could be an advantage and lead to enhanced resolution.