Design and Linearity Assessment of the TRISTAN Detectors' Remote Analog to Digital Conversion DAQ System

TL;DR

This work addresses the challenge of searching for keV-scale sterile neutrinos via precision differential beta spectroscopy in the KATRIN framework by engineering a remote Analog to Digital Conversion (RADC) DAQ for the TRISTAN detector, comprising thousands of silicon drift detector pixels. The approach couples a magnet-and-high-voltage-tolerant front-end (Tile Main Board) to a high-performance back-end ( Serenity-S1) with four readout modes and DPS-based event processing, enabling high data rates and flexible analysis. It provides a detailed non-linearity characterization through differential non-linearity (DNL) measurements and validates an in-situ calibration method using detector leakage current, then demonstrates mitigation strategies (increased rate, higher PAE voltage, and Gatti slider) that reduce ADC-induced distortions on the energy spectrum to render the sterile neutrino sensitivity effectively unaffected at the projected statistical level, achieving exclusions down to $\sin^2(\theta) \lesssim 10^{-6}$ for keV-mass states. The study showcases a scalable, modular DAQ design suitable for large pixel arrays in high-rate, high-background environments and underscores the importance of robust linearity control for high-precision beta-spectroscopy searches. The results imply that ADC non-linearity is a manageable systematic in next-generation TRISTAN measurements and not a limiting factor for achieving the desired sensitivity.

Abstract

The TRISTAN detector is an upgrade to the KATRIN experiment to enable a differential measurement of the tritium $β$-decay spectrum to search for the experimental signature of keV scale sterile neutrinos. The TRISTAN detector upgrade consists of performing precision electron spectroscopy with over 1000 silicon drift detector pixels, each responsible for recording event rates of $10^5$ counts per second. A project specific data acquisition (DAQ) system is developed to meet the experimental challenges with a remote analog to digital conversion (RADC) design. In this work, the conceptual design of the RADC DAQ is presented along with the built system for operating the TRISTAN detector upgrade. The system includes flexible signal processing logic and data management that is optimized for the high-rate precision measurement. The non-linearity of the system's readout channels are measured and shown to be stable over time. The systematic effect of this non-linearity is propagated to the sensitivity to the sterile neutrino signature and is demonstrated to be reducible to a subdominant contribution.

Figures (6)

• Figure 1: Concept of an RADC DAQ system operating in the KATRIN detector section. Environmental constraints from the high voltage and magnetic field are outlined in gray and isolated to the front-end board which provides detector bias voltages and digitizes the detector signals with minimized analog paths. Connection via optical links to a back-end unit allows for flexible event filtering and readout mode implementation.
• Figure 2: Block diagram of the Tile Main Board (TMB). Green blocks indicate individual plug-on sub modules which populate the PCB. The TMB provides detector operating voltage and waveform readout through the four micro-D connectors and streams waveform data to the backend through fiberoptic transceivers.
• Figure 3: Tile Main Board (TMB) installed and operated at a replica of the detector chamber upgrade. 21 ADC cards are mounted perpendicular to the TMB to allow for dense PCB design. Fiber optic connections on the right side provide connections to back-end control and readout. Two mounting positions for additional TMBs are visible to the left; each a column of four micro-D vacuum feedthroughs.
• Figure 4: Example response of the trigger (top) and energy (bottom) filters to delta pulse and detector event waveforms. The filters are parameterized by widths $\tau_w$, $\tau_g$, and $\tau_p$. An event trigger is produced and the energy filter is evaluated at the first zero crossing of the trigger filter after a threshold crossing.
• Figure 5: Comparison of the measured differential non-linearity (DNL) for an 8-bit range of the ADC's digitization range. (a) Measured DNL using the UPV Audio Analyzer voltage source with errorbars calculated from ten independent measurements over a four month period. (b) Measurement of the same channel's DNL using the TRISTAN detector leakage current, demonstrating the consistency of the methods. (c) Comparison of the DNL of three ADC readout channels. Ch 1 and 7 are channels on the same ADC card and Ch 25 is from a separate ADC card.