Ultra-high precision high voltage system for PTOLEMY

TL;DR

The PTOLEMY project seeks to detect the cosmic neutrino background via electron capture on tritium bound to graphene, which demands energy resolution better than 0.5 eV at the 18.6 keV endpoint and thus ppm-level stability of the HV electrodes. The paper presents a precision HV system based on a chain of REF5010 voltage references, read by precision multimeters and monitored non-invasively by a field mill, combined with LOESS-based detrending to suppress long-term drifts. Results show per-board RMS noise around 0.2 mV, implying a theoretical full-chain precision near 0.05 ppm for 20 kV if uncorrelated, though present field-mill performance yields more conservative estimates of a few ppm; detrending appears effective and phase-coherence analyses indicate little cross-talk between boards. The work demonstrates the feasibility of achieving ppm-level HV control for PTOLEMY's demonstrator, quantifies current limitations, and outlines a fast-switching scheme to enable ms-scale HV modulation of the dynamic electromagnetic filter, informing a scalable path toward CνB detection.

Abstract

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 \SI{500}{meV} at the tritium $β$-spectrum endpoint of \SI{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 a \emph{field mill} device. The currently available precision on high voltage is, in the conservative case, as low as \SI{0.2}{ppm} per \SI{1}{kV} single board and $\lesssim$ \SI{50}{mV} over the \SI{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 \SI{0.05}{ppm} over \SI{20}{kV}.

Figures (13)

• Figure 1: Schematic of the PTOLEMY concept. From left to right: tritiated graphene target, RF detection antenna (CRES) and fast trigger, dynamic electromagnetic filter and high-precision calorimeter.
• Figure 2: REF5010 ultra-high precision voltage reference by Texas Instruments. Left: visual appearance of the electronic component. Right: corresponding circuit diagram.
• Figure 3: Schematic of the repeated chain module (top) developed at LNGS electronics laboratory. Drawing of the 1kV board (bottom).
• Figure 4: REF Chain. Left: schematic of the REF chain showing $V_{\rm in}$ with the four slow switch pins. Right: visual appearance of the twenty 1kV boards, arranged in two layers, as operated at LNGS during tests.
• Figure 5: Bench test of the HV system. According to the descriptions in the picture, the REF chain is monitored by an $81/2$-digit precision multimeter (for the 1kV board only) and by the field mill, read out by a $71/2$-digit precision multimeter, according to the slow switch configuration. Finally, the board temperature is read out by a 6.5-digit precision multimeter.