Ultra-high precision high voltage system for PTOLEMY

The PTOLEMY project is prototyping a novel electromagnetic filter for high-precision β spectroscopy, with the ultimate and ambitious long-term goal of detecting the cosmic neutrino background through electron capture on tritium bound to graphene. Intermediate small-scale prototypes can achieve competitive sensitivity to the effective neutrino mass, even with reduced energy resolution. To reach an energy resolution better than 500 meV at the tritium β-spectrum endpoint of 18.6 keV, and accounting for all uncertainties in the filtering chain, the electrode voltage must be controlled at the level of a few parts per million and monitored in real time. In this work, we present the first results obtained in this effort, using a chain of commercial ultra-high-precision voltage references, read out by precision multimeters and afield mill device. The currently available precision on high voltage is, in the conservative case, as low as 0.2 ppm per 1 kV single board and ≲ 50 mV over the 10 kV series, presently limited by field mill read-out noise. However, assuming uncorrelated Gaussian noise extrapolation, the real precision could in principle be as low as 0.05 ppm over 20 kV.