Design and Operation of the Shower-max Detector for the MOLLER Experiment.

The MOLLER experiment at Jefferson Lab, scheduled to start commissioning in late 2026, will measure the weak charge of the electron through parity-violating electron scattering and provide a powerful search for physics beyond the standard model. The apparatus includes a main detector system and several auxiliary detector systems, including the Shower-max. Shower-max is a Cherenkov based sampling calorimeter designed and built to make an independent measurement of the parity violating asymmetry in Moller scattering and provide a powerful cross check of the main detector measurement.

Constructed at Idaho State University and currently undergoing final testing at Jefferson Lab, the system consists of 28 detector modules around the beamline. The active area of each module consists of four layers of high purity tungsten interleaved with four layers of high purity fused silica (quartz). The first layer of tungsten initiates the electromagnetic shower, which grows as it passes through successive tungsten layers creating relativistic, charged particles that produce Cherenkov light in the quartz layers. The light is totally internally reflected inside the quartz until it exits a bevel cut and is directed to a photo-multiplier tube using an air-core aluminum light guide.

Using Geant4-based simulations, we studied and optimized the detector design: geometry, material configuration, and light collection in order to meet the physics requirements for the system, such as kinematic acceptance, energy resolution, event rates, yields, etc. Cosmic-ray tests combined with several electron beam tests have been used to calibrate optical simulation results and help validate the performance of the detector modules. This presentation covers the development, simulation, and testing of the Shower-max detector system.