Design and characterization of the POKERINO prototype for the POKER/NA64 experiment at CERN

TL;DR

This work addresses the challenge of achieving a precise, high-resolution, compact PbWO$_4$ calorimeter for missing-energy searches of light dark matter in the NA64-$e$ and NA64-$+$ programs at CERN. The authors design and test the POKERINO prototype, a 3×3 PbWO$_4$ crystal matrix with a novel hybrid SiPM readout, to study SiPM saturation, gain stability under high-rate conditions, and energy-linearization strategies. Through cosmic-ray commissioning, a 120 GeV/$c$ muon calibration, and a high-rate H6 test-beam campaign, they demonstrate that the energy resolution can meet the target after saturation corrections, and that the negative-feedback mechanism from bias resistors remains sub-dominant under nominal beam conditions. The results validate the PKR-CAL design choices and establish a practical path toward deploying the full PKR-CAL calorimeter for precise missing-energy measurements in the CERN NA64 program, enabling sensitive exploration of light dark matter scenarios. The study also provides a quantitative framework for saturation correction and rate-dependent gain stabilization in SiPM-based homogeneous calorimeters.

Abstract

The NA64 experiment at CERN H4 beamline recently started a high-energy positron-beam program to search for light dark matter particles through a thick-target, missing-energy measurement. To fulfil the energy resolution requirement of the physics measurement $σ_E/E\simeq2.5\%/\sqrt{E\mathrm{[GeV}]} \oplus 0.5\%$ and cope with the constraints and performance requests of the NA64 setup, a new high-resolution homogeneous electromagnetic calorimeter PKR-CAL has been designed. The detector is based on PbWO$_4$ crystals, each read by multiple SiPM sensors to maximize the light collection. The PKR-CAL design has been optimized to mitigate and control unavoidable SiPM saturation effects at high light levels, as well as to minimize the gain fluctuations induced by instantaneous variations of the H4 beam intensity. The $R\&D$ program culminated in the construction of a small-scale prototype, POKERINO. In this work, we present the results from the experimental characterization campaign of the POKERINO aiming at demonstrating that the obtained performances are compatible with the application requirements.

Figures (12)

• Figure 1: (a) CAD drawing of the the POKERINO mechanical assembly. The PbWO$_4$ crystals (in cyan) are inserted in the copper-based mechanical assembly, formed by four plates (brownish) kept together by copper bars (yellow). The prototype is kept in position by two lightweight aluminium supports (green). The PKR-CAL-SiPM sensors are glued on the opposite crystals faces. b) A picture of the POKERINO detector, seen from the readout side. The copper braids are used to thermally couple the PKR-CAL-SiPM devices to the external copper structure.
• Figure 2: The typical light emission spectrum of a PbWO$_4$ crystal (black), compared with the quantum efficiency of the Hamamatsu S14160-6010 SiPM (blue) for a bias voltage of $V_{BD}+5$ V.
• Figure 3: Schematic of the PKR-CAL-SiPM assembly, with four SiPM detectors connected together in the hybrid configuration.
• Figure 4: Left: picture of a PKR-CAL-SiPM printed circuit board, not yet populated with the photo-detectors. The passive components and the output connector are hosted on the opposite PCB face. The large rhomboidal pads are connected to the SiPMs thermal junctions, to extract heat from the devices during operation. Right: a fully-assembled PKR-CAL-SiPM board. SiPMs are hosted on the opposite face. The central hole is used to host a SMA connector to attach to the sensor an optical fibre from the laser monitoring system.
• Figure 5: Picture of the setup employed for laboratory characterization of the PKR-CAL-SiPM device. A PbWO$_4$ crystal with a PKR-CAL-SiPM sensor was inserted in the aluminium frame, and the light pulses from a pulsed laser were transported to it via an optical fibre. The two plastic scintillator counters shown in the picture were employed for a different set of measurements exploiting cosmic-rays, not discussed in this work.