Colin Rochat
The goal of this work was the improvement of the capabilities of a miniature neutron detector, for easing its use, and extending its range of applicability. Firstly, the Red Pitaya STEMlab 125-14 was the object of an exploratory study in order to propose an alternative method to the two current acquisition electronics based on photon counting, using analog and digital means. The study allowed to establish a new methodology based on the direct use of the SiPM pre-amplifier output, requiring the hypothesis that neutron events can be discriminated directly by pulse amplitude. It was first tested in the low neutron flux of the CARROUSEL facility, and the results were compared to the analog electronics. The agreement in count rate, spectrum, and linearity demonstrates the validity of this new methodology. Moreover, its application in CROCUS allowed demonstrating that it is working in reactor condition up to a flux of 1.13∗108 s−1cm−2. Secondly, an array of six miniature neutron detectors with different sensitivities was designed and tested. The qualification study in CARROUSEL allowed to show that all detectors behave normally, and that the sensitivity of the detectors increases as a function of the scintillator’s volume at low neutron flux, as expected. The conclusion is limited by the knowledge in effective dimensions of each scintillator. In a first experiment in CROCUS using the same setup, including digital threshold (50), a loss of linearity was observed simultaneously for all detectors at a power ofabout 4W(i.e. 7∗107s−1cm−2). As the cause was expected to be a higher gamma background, the experiment was repeated with a higher threshold (120): consistently with the CARROUSEL test, count rate and loss of linearity depends on the scintillation’s volume. Further studies are needed to conclude on the phenomenon at hand, but since the main parameter for gamma discrimination proved efficient, the gamma contribution increased at higher powers seems a good candidate. In conclusion, the results are globally satisfying for the new methodology with Red Pitaya STEMlab 125-14. However, the experimental campaign of this work was only a feasibility study. The results have to be confirmed with further studies. Moreover, this system could be improved. In particular, it can be adapted firstly for both RF inputs of Red Pitaya and secondly for dynamical measurements. The results of the multi-sized array of miniature neutron detector were also encouraging. However, the behaviour of certain detectors prevent to deduce with certainty some expected trends. Further studies have to be carried out by multiplying the number of detectors of each dimension.
2021
