Performance Studies of the Mu2e Cosmic Ray Veto Detector

TL;DR

The paper evaluates the Mu2e Cosmic Ray Veto (CRV) performance using cosmic-ray test stands, focusing on single-layer efficiency, 3/4-layer muon veto efficiency, and long-term aging of CRV modules. It demonstrates that, at suitably chosen PE thresholds, the CRV can achieve and maintain an overall efficiency above $99.99\%$, with aging rates around $2.5$–$3\%/\mathrm{yr}$ for both 1.4 mm and 1.8 mm WLS fibers, ensuring background suppression to below one event over Mu2e run time. Monte Carlo studies with CRY/Geant4 reproduce the efficiency behavior and plateau observed at low thresholds, validating the modeling approach. The work also measures a longitudinal position resolution of about $30.4$ cm along a counter, and concludes that CRV performance meets the requirements for Mu2e operation and background control, with further updates planned for the overall background assessment.

Abstract

The cosmic ray veto (CRV) detector of the Mu2e experiment consists of four layers of plastic scintillation counters that surround the detector solenoid. These counters are embedded with wavelength-shifting fibers and are read out by silicon photomultipliers (SiPMs). The performance of a subset of the CRV counters was studied in a cosmic-ray test stand. Using data taken over a two-year period, we report the single-layer muon detection efficiency and the rate at which the light yield degrades due to the aging of the plastic scintillation counters.

Figures (12)

• Figure 1: The cosmic ray veto (gray) consists of 83 modules that are mounted on the top and sides of concrete shielding blocks (red) that surround the Mu2e apparatus, which is enclosed within a solenoidal magnet downstream of another S-shaped solenoidal magnet that is seen at the right.
• Figure 2: Exploded view of the manifold at the end of a di-counter showing the fiber guide bar, SiPM mounting block, SiPM carrier boards, SiPMs, and counter motherboard. The flasher LEDs, thermometer, and pogo pins are not shown.
• Figure 3: Top: the cosmic-ray test stand at Fermilab Wideband. Bottom: Cartoon of the cosmic-ray test stand end-view at Fermilab Wideband.
• Figure 4: Example waveform from two SiPM channels of a CRV counter, 127 ADC samples each. The pre-signal region shows dark-count pulses of 1 PE in both channels at 400 ns and 600 ns. The signal region shows signal pulses with 24 PEs and 30 PEs in the respective channels. All pulses get fitted with a Gumbel function.
• Figure 5: The single-layer inefficiency ($1-\varepsilon_{SL}$) of a CRV-T module as a function of the PE threshold. Note how each layer has a very similar efficiency.