Development of MMC-Based $^{63}$Ni $\beta$ Spectrometers for a keV Sterile Neutrino Search

We developed an MMC-based detector system to measure the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$^{63}$</tex-math></inline-formula>Ni beta spectrum, with the goal of searching for keV-scale sterile neutrinos. Two identical detectors with gold absorbers of the same size were constructed. One of the absorbers enclosed a small source foil containing radioactive <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$^{63}$</tex-math></inline-formula>Ni, prepared using an electrochemical deposition procedure developed in this work. An excellent linearity in the energy range of interest up to 70 keV was demonstrated after energy calibration with an Am-241 source. Furthermore, the detector setup with <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$^{63}$</tex-math></inline-formula>Ni deposition produced an energy spectrum consistent with the expected activity and energy response. The potential of this detector system for a future keV sterile neutrino search is discussed.