Speaker
Description
The next-generation KOTO II experiment at J-PARC will operate at significantly higher beam intensities than its predecessor, requiring detector upgrades to maintain the stringent background suppression necessary for the search for ultra-rare $K_L \to \pi^0 \nu \bar{\nu}$ decay. One of the most critical components is the beam-hole photon veto (BHPV), which must efficiently detect the two photons from the $\pi^0$ emitted at small angles while maintaining high discrimination power against accidental coincidences from beam-induced backgrounds. The lead–aerogel Cherenkov counter previously employed in KOTO has demonstrated excellent performance; however, its timing capabilities and radiation hardness represent limiting factors for the demanding KOTO II conditions [1]
To address these challenges, we developed a novel, compact small-angle calorimeter (SAC) featuring fine granularity, excellent timing resolution, and robust radiation tolerance. The calorimeter is based on ultrafast lead tungstate (PWO-UF) crystals, complemented by radiation-hard photomultipliers optimized for operation in a high-rate and high-radiation environment. The use of PWO-UF crystals provides several key advantages: fast scintillation response, high density for compact shower containment, and intrinsic radiation resistance [2]. Moreover, the high segmentation of the SAC design enhances spatial resolution and allows effective neutron–photon discrimination, reducing false vetoes and improving overall detection efficiency.
Extensive R&D has been carried out on crystal properties and readout technologies, and dedicated test campaigns with both PWO-UF and PbF₂ samples have confirmed high photoelectron yields and sub-nanosecond timing performance, making them suitable candidates for the fast response required in the KOTO II beam environment [3].
In August 2025, we carried out a dedicated beam test of the SAC prototype at CERN T9, which included arrays of full-size PWO-UF crystals read out with different types of fast photomultipliers. In particular, we tested Hamamatsu R9880 and R14755 miniature metal-channel PMTs, which demonstrated excellent timing capabilities. Complementary to the beam campaign, the crystals were characterized at the University of Ferrara using X-ray diffraction (XRD). This allowed us to determine their crystallographic orientation with respect to the beam axis and to exploit coherent effects of charged particles traversing aligned crystals [4]. The resulting measurements provided valuable insights into both the energy response and the timing characteristics of the SAC under conditions where coherent effects may influence light production, improving the timing performance.
This contribution presents an overview of the prototype R&D and the first beam-test results obtained with oriented PWO-UF crystals and advanced ultra-fast PMTs.
[1] J.Fry, et. al., Proposal of the KOTO II experiment, arXiv:2501.14827
[2] M. Korzhik et al., NIMA, 1034 (2022) 166781
[3] D.Paesani, et. al., Front. Phys. 11:1223183.
[4] L.Bandiera, et. al., NIMA 936 (2019) 124–126
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