Digital pulse processing in Mössbauer spectroscopy

A. VEIGA; C. GRUNFELD

Opatija

Conferencia; International Conference on the Applications of the Mössbauer Effect; 2013

Ruđer Bo?ković Institute of Croatia

High quality Mössbauer spectroscopy necessarily involves precise selection of resonant photons based on its energy. This selection traditionally requires sophisticated analog instrumentation whose standards have been well defined over time. Gaseous proportional counters provide efficient detection of low energy gamma radiation under certain conditions. They require a low noise preamplifier (where the small current pulse from the detector is integrated to an energy-proportional voltage step) followed by an elaborate analog linear processing stage in a shaping amplifier (where the voltage step is converted to a low noise nearly gaussian pulse whose amplitude is proportional to the step magnitude). This traditional configuration enables a relatively simple selection process based on pulse amplitude, usually provided by a single-channel analyzer. Low performance preamplifier and amplifier systems introduce noise and distortions throughout the analog processing, reducing the quality of the subsequent discrimination. This fact implies a diminished resonant photon proportion, reducing the detected Mössbauer effect and consequently extending the required recording time for a given signal to noise ratio in the spectrum. We propose that the performance of the preamplifier and amplifier system can be relaxed still preserving the spectrum quality, provided that a more versatile selection technique is implemented. The strategy presented here involves a combination of pulse height selection with time over threshold, rise time and pulse area techniques, resulting in a more versatile multi-parameter pulse-shape discrimination. In this way relevant events can be better identified, enhancing the detection efficiency. A real-time FPGA-based algorithm that implements these functionalities has been developed and tested. We report details about the design and organization of the FPGA module, including considerations about multi-parameter operation. Fig. 1 layout was used to test the performance of the FPGA-based algorithm under different conditions. Co57 source spectrum and Mössbauer calibrations are presented, taken with Cremat CR110 and CR200 modules and compared with those obtained with Ortec 142A preamplifier, 572A shaping amplifier and 551 single-channel analyzer, under equivalent laboratory conditions.