Moessbauer Spectroscopy Using a Resonant Detector.

Mossbauer spectroscopy using a resonant detector (MSRD) has the advantages of enhanced signal to background and reduced linewidth. However, construction difficulties have previously prevented its implementation for ('57)Fe Mossbauer spectroscopy. We have designed and constructed two high effect resonant detectors (RD) suitable for ('57)Fe MSRD. The RDs discriminate the signal (resonant gamma) against the background (non-resonant gammas) by a factor of 10-17 to 1 by means of detecting the secondary internal conversion electrons emitted by the Mossbauer-excited nuclei in the backscattering geometry. The detector efficiency profile shows a pronounced peak centered at the stainless steel isomer shift. The width of this peak is comparable to the linewidth of iron peaks which is about 10('-8) eV. Therefore, the energy resolution of the RD is extremely good compared to the conventional detectors whose resolution is typically in the keV region. We built two versions of the RD and set up MSRD operated in two modes: (1) Move the absorber and keep the source and detector at rest. (2) Move the source and detector assembly and keep the absorber at rest which requires miniaturizing of the RD and a spring supporting system and therefore is more difficult to setup. We have taken natural iron foils with various thicknesses in order to test the enhancement and the linewidth reduction effects of the MSRD. The results are always a factor of about 2 enhancement of the signal to background ratio compared to spectra taken with conventional proportional counters. We also found a reduction of about 10% of the natural linewidth. For demonstration of the usefulness of the linewidth reduction, we also compared beryllium spectra taken by MSRD and conventional transmission Mossbauer spectroscopy (CTM). The two quadrupole splitting peaks of iron in beryllium are barely discernible in the spectrum taken by CTM but can be clearly identified in the spectrum taken by MSRD. Therefore, we conclude that the MSRD technique should be very useful in taking Mossbauer spectra involving small effect or overlapping peaks.