Design and Mechanical Integration of Scintillation Modules for SUB-Millicharge ExperimenT (SUBMET)

TL;DR

SUBMET addresses the search for millicharged particles in a largely unexplored region of parameter space by implementing a robust, modular scintillator-based detector optimized for low-charge interactions at J-PARC. The design combines long EJ-200 bars read out by PMTs in two aligned layers to maximize interaction probability while enabling coincidence timing and background suppression, protected by mu-metal shielding and a rigid aluminum frame to withstand static and seismic loads. Detailed treatment of scintillator properties, module assembly, and the mechanical support architecture (Supermodule, Cage, Table) is complemented by weight and seismic analyses that show a safety factor around 15 and displacements below 2 mm under anticipated loads. The fully assembled system was validated with cosmic muons, transported to the site, and installed by May 2024, entering data-taking in June 2024 and enabling sensitivity to millicharged scenarios with $m_\chi<1.6$ GeV/$c^2$ and $Q<10^{-3}e$.

Abstract

We present a detailed description of the detector design for the SUB-Millicharge ExperimenT (SUBMET), developed to search for millicharged particles. The experiment probes a largely unexplored region of the charge-mass parameter space, focusing on particles with mass $m_χ< 1.6~\textrm{GeV}/c^2$ and electric charge $Q < 10^{-3}e$. The detector has been optimized to achieve high sensitivity to interactions of such particles while maintaining effective discrimination against background events. We provide a comprehensive overview of the key detector components, including scintillation modules, photomultiplier tubes, and the mechanical support structure.

Figures (21)

• Figure 1: A 3D model of the submet detector. Each module consists of a $50 \times 50 \times 1500~\textrm{mm}^3$ plastic scintillator bar coupled to a pmt. Four modules are grouped in a $2 \times 2$ configuration to form a supermodule. Each layer comprises $5 \times 4$ supermodules. The detector consists of two such layers, aligned to ensure that a millicharged particle, shown as a red line, passes through both layers from left to right within a narrow time window.
• Figure 2: The mean number of simulated scintillation photons reaching one end of the scintillator bar ($\bar{N}_\gamma$) as a function of bar length. The gray band represents the rms for each length.
• Figure 3: (a) 3D model of the scintillator wrapping layers. (b) Photograph of a partially wrapped scintillator bar. The wrapping is intentionally left exposed to show the individual layers.
• Figure 4: 3D models of (a) the "pmt Support" and (b) the "led Support" components.
• Figure 5: 3D models illustrating (a) the assembly of a pmt, pmt support, and scintillator, and (b) the attachment of the led support to the opposite end of the scintillator. The pmt is shown unwrapped for clarity.