Probably not too many people are familiar with the name Mössbauer, so there’s no shame in having to look it up to see what it relates to. The Mössbauer effect refers to a gamma ray absorption phenomenon by certain nuclei, bound in a solid, that is used in Mössbauer spectroscopy. This technique studies the properties of atomic nuclei and their interactions with the surrounding environment through the absorption and re-emission of gamma rays. Measuring the absorption rate and energy shifts can help scientists gain insights into the chemical bonding and electronic environment around the nucleus, magnetic material properties, the oxidation states of elements, and crystalline structures and defects, to name a few applications.
In Mössbauer spectroscopy, the channel-to-velocity relation in the acquisition of the spectrum is fundamental to its quality. The velocity of the radioactive source is usually controlled by a servo-amplifier, from which two signals are distinguished: the Monitor signal, which is proportional to the required velocity in a secondary coil, and the Error signal, which is proportional to the difference between actual secondary coil voltage and required voltage. The determination of these signals allows control of the correct functioning of the spectrometer and can be used to improve the determination of the channel-velocity relationship.
In their article Embedded Device for Monitor and Error Velocity Signals in Mössbauer Spectroscopy, Matías J. Oliva and colleagues from the Universidad Nacional de La Plata (Argentina) describe how, building further on previous work where data acquisition was done via a standard digital oscilloscope, they built a new experimental setup, taking full advantage of many of the features a STEMlab 125-14 can offer. In their setup, its FPGA is responsible for data acquisition, while the CPU takes care of system control and user interfacing, as shown in the simplified block diagram below. Therefore, no other hardware was required for this project.
The FPGA process can be seen in the following diagram, and includes key operations such as oversampling and linear averaging, coherent averaging, and trigger control. FPGA-sampled and -processed data is stored in the STEMlab memory and accessed by the CPU for further analysis or transmission, thereby minimizing noise, increasing resolution, and reducing manual calibration time, leading to improved efficiency of the Mössbauer spectroscopy experiment. System control was done via a C-program, running on the STEMlab’s CPU, managing the acquisition, averaging and visualization of the results.
Systems must obviously be validated, so an acquisition run was compared with another one obtained using a Tektronix oscilloscope. The Red Pitaya ADCs produced some offset and gain errors that were estimated during the comparison, obtaining a gain error of approximately 1.131 and an offset error of around 0.015V. The digitalization of Monitor and Error signals, shown in the figure below, leads to significantly faster data acquisition with a comparable resolution. The remaining small alinearities will probably be the subject of a future upgrade.
System validation: a) raw data; b) offset and gain error correction; c) and d) partial zooms of the Error signal with evidence of alinearities
The description of how the Red Pitaya board was integrated into the project and the conclusion of the original paper demonstrate the perfect fit of the STEMlab for ambitious scientific projects, improving system performance along the way. Not only that, but it seems the authors are also considering taking advantage of some of the features they have so far neglected in future applications, such as the Ethernet connection. All very promising indeed, and further proof that you never know how far a Red Pitaya board will take you.