The BigBite Calorimeter for the Super Bigbite Spectrometer Program at Jefferson Lab

TL;DR

The BBCal is a two-part lead-glass electromagnetic calorimeter (PS+SH) built for the SBS program at Jefferson Lab to act as the primary electron trigger and measure electron energy with high resolution. It employs a detailed trigger-sum architecture, beam- and cosmic-based calibrations, and real-time monitoring, all supported by a robust Geant4-based simulation (g4sbs) for validation and optimization. The calorimeter achieves an energy resolution of about $\sigma_{E'}/E'_e \approx 6.2\%$, a position resolution of $\sim$1.2 cm, and a timing resolution of $\sim$0.4–0.5 ns, enabling precise electron identification and event selection in high-luminosity, high-$Q^2$ neutron form-factor measurements. The combination of careful HM shielding, HV monitoring, and online diagnostics ensured stable performance across multiple SBS experiments, making BBCal a critical component of the program’s success.

Abstract

We report features of the design, construction, installation, and performance of the BigBite Calorimeter (BBCal), a lead-glass electromagnetic calorimeter constructed as part of the BigBite Spectrometer (BBS), which served as the electron arm for the Super Bigbite Spectrometer (SBS) program of high-precision neutron electromagnetic form factor measurements in Hall A at Jefferson Lab. As a total-absorption calorimeter, BBCal provided the primary electron trigger for BBS, detecting (quasi-) elastically scattered electrons in the 1-4 GeV energy range with an energy resolution of approximately 6.2%, position resolution of 1.2 cm, and timing resolution of 0.5 ns.

Figures (24)

• Figure 1: Side view CAD drawing of the BBS detector stack installed in experimental Hall A. Scattered electrons travel through the detector stack from left to right in the drawing. Here, BB Magnet is the BigBite magnet, GEMs are the Gas Electron Multiplier detectors, GRINCH is the Gas Ring Imaging CHerenkov detector, PS is preshower, TH is Timing Hodoscope and SH represents the shower detector.
• Figure 2: PS detector map (back view). A schematic of an individual PS module is included above the detector map. The Epoxy used to connect the PMTs to the LG blocks was UV-cured.
• Figure 3: PS cluster energy distribution showing a prominent low-energy peak at approximately $89$ MeV, characteristic of MIPs. This feature enables effective rejection of pions from electrons using a simple threshold cut, indicated by the red vertical line. The data shown were obtained at $Q^2=4.5$ GeV$^2$ with $E_{\text{beam}}=4.0$ GeV during the E12-09-019 experiment and are typical of PS energy spectra obtained throughout the SBS program. For this setting, the scattered electron energies of interest ranged from 1.4-1.9 GeV. Figure adapted from Datta:2024vwq.
• Figure 4: SH detector map (back view). A schematic of an individual SH module is included above the detector map. The Epoxy used to connect the PMTs to the LG blocks was UV-cured.
• Figure 5: Sheets of mu-metal installed between each row of SH PMTs. This image is from the back of the SH layer during construction.