INSTITUTO DE FISICA LA PLATA
Unidad Ejecutora - UE
congresos y reuniones científicas
Self-Tuning Digital Mössbauer Detection System
A. VEIGA; C. GRUNFELD; G. PASQUEVICH; P. MENDOZA ZÉLIS; N. MARTÍNEZ; F. H. SÁNCHEZ
Conferencia; XIII Latin American Conference on the Applications of the Mössbauer Effect; 2012
Universidad de Antoquia
Mössbauer detection system is a classical pulse height selector composed of four main elements: proportional counter, charge preamplifier, shaping amplifier and single channel analyzer, as shown in figure 1A. Long term stability is an important factor in high resolution experiments, being critical for the success of constant-velocity applications , where transmission is recorded at a single energy for a long period of time. In these conditions, an output rate variation can not be distinguished to be the effect of the resonant process under study or a consequence of a shift in spectrometer operating point . The primary source of long term instability is temperature dependence of hv supply and gaseous detector gain coefficient . A temperature change displaces the emission line out of the window center, modifying the background operating rate. As a consequence, in precision constant-velocity measurements, either spectrometer temperature control must be implemented or background rate must be periodically recorded, with the consequent loss in spectrometer efficiency. Digitization of the detection system, as shown in figure 1B, provides a powerful environment where a compensation algorithm for temperature drifts can be devised. In a classical digital arrangement, output of the shaping amplifier is sampled by a proper analog to digital converter followed by a digital processing module. In a more sophisticated array the shaping amplifier can be omitted by a careful sampling of the preamplifier output. In both cases every detected event is digitized, translated into energy by a proper algorithm and followed by digital output generation if selection criteria is met (i.e. amplitude window). We propose that with a moderate amount of digital resources the processing module can be designed to automatically monitor the status of the emission spectrum, detect potential temperature drifts and adjust the selection criteria in order to keep a constant background rate. An implementation is presented, where the source energy spectrum is continuously recorded by the digital processing module, an algorithm is applied to extract information of optimal selection criteria and window levels are dynamically adjusted. An automatic optimal window criteria and a window tracking strategy are also presented and the main aspects of field-programmable gate array (FPGA) implementation of the proposed techniques are included. Experimental results illustrate the performance of the processing module when detector thermal drifts are induced.  A. Veiga et al., Hyperfine Interactions, vol 167 (2006) 905.  G.A. Pasquevich, et al., Physica B: Condensed Matter 354 (2004) 369.  V. Vanha-Honko, Nuclear Instruments and Methods 176 (1980) 213.