Studies of an array of PbF2 Cherenkov crystals with large-area SiPM readout

Abstract

The electromagnetic calorimeter for the new muon (g − 2) experiment at Fermilab will consist of arrays of PbF 2Č erenkov crystals read out by large-area silicon photo-multiplier (SiPM) sensors. We report here on measurements and simulations using 2.0 -4.5 GeV electrons with a 28-element prototype array. All data were obtained using fast waveform digitizers to accurately capture signal pulse shapes versus energy, impact position, angle, and crystal wrapping. The SiPMs were gain matched using a laser-based calibration system, which also provided a stabilization procedure that allowed gain correction to a level of 10 −4 per hour. After accounting for longitudinal fluctuation losses, those crystals wrapped in a white, diffusive wrapping exhibited an energy resolution σ/E of (3.4 ± 0.1) %/ E/GeV, while those wrapped in a black, absorptive wrapping had (4.6 ± 0.3) %/ E/GeV. The white-wrapped crystals-having nearly twice the total light collection-display a generally wider and impact-position-dependent pulse shape owing to the dynamics of the light propagation, in comparison to the black-wrapped crystals, which have a narrower pulse shape that is insensitive to impact position.

Performance of the Muon g−2 calorimeter and readout systems measured with test beam data

“…A single crystal contains approximately 85% of the shower energy for a centered electron. corresponds to an energy calibration constant of ∼ 0.82 npe/MeV, consistent with the value reported in [5]. The accuracy of this technique is somewhat limited by the 0.5 × 1.0 cm 2 beam spot and the precision with which the calorimeter could be aligned to the center of the beam.…”

“…Based on this parameterization, the energy resolution at 2 GeV is 3.1%, surpassing the design target of 5%. While the constant term is consistent with that found in [5], the resolution is improved by about 11%.…”

“…Photon calibration is the step where the fitted pulse area can be interpreted as the number of photo-electrons (npe), or pixels, fired for a given pulse. The calibration constants are obtained by employing a procedure that uses the laser and a series of runs with varying neutral density filter settings, as described in [5,6,7]. An iterative sequence was used alternating between laser-calibration runs and adjustments to the PGA gain settings of the SiPMs and the SiPM group bias voltages.…”

“…where C E is a term accounting for the shower containment variance as studied in [5], and S E is a stochastic term describing the statistical fluctuation of the electromagnetic shower. The resultant fit is shown in Fig.…”

Next Generation Muong− 2 Experiments

“…To monitor this precisely, a laser calibration system is being developed [22] that will flash all crystals prior to each fill and, during special calibration runs, sparsely overlay calibration pulses on top of the real data in a pattern that leads to a series of 10 −4 gain measurements every 5 μs throughout the fill. The calorimeter system and gain stability were tested at SLAC and the resolution and overall performance was excellent, see [23].…”