Design and Performance of Cross-Shaped Veto Scintillator Detector in the PRad-II Experiment

The PRad-II experiment at Jefferson Lab aims to provide a new, high-precision measurement of the proton electric form factor at very low momentum transfers, down to Q² = 4 × 10−5 GeV². The experiment is scheduled to start in January 2026 and is expected to improve the precision of the proton charge radius determination by about a factor of three compared to the original PRad experiment. The experimental setup includes a windowless hydrogen gas target, two planes of GEM tracking detectors for improved vertex and angle reconstruction, a high-efficiency and high-resolution calorimeter (HyCal), and a new cross-shaped veto scintillator detector. The veto scintillator, consisting of four thin panels with a central opening, is mainly used to separate Møller electrons from elastic ep events at very forward angles, up to 0.5°, where energy-based separation is difficult. To minimize extra material and reduce possible backgrounds, thinner scintillators were carefully chosen and optimized through simulations and tests. In this talk, I will focus on the design and expected performance of this veto scintillator, its integration with the rest of the experimental setup, and its importance for achieving a precise proton radius measurement.