Photon-Veto Counters at the Outer Edge of the Endcap Calorimeter for the KOTO Experiment

TL;DR

The paper details the design, performance, and calibration of the Outer-Edge Veto counters in the KOTO experiment, crucial for background photon detection in rare decay measurements at J-PARC.

Contribution

It introduces the design and performance evaluation of the OEV counters, including calibration methods and stability monitoring under high load conditions.

Findings

Average light yield of 20.9 photo-electrons for 1 MeV deposit

Time resolution of 1.5 ns for the counters

Effective gain stability monitoring with cosmic rays

Abstract

The Outer-Edge Veto (OEV) counter subsystem for extra-photon detection from the backgrounds for the? $K^0_L\rightarrow\pi^0\nu\bar{\nu}$ decay is located at the outer edge of the endcap CsI calorimeter of the KOTO experiment at J-PARC. The subsystem is composed of 44 counters with different cross-sectional shapes. All counters are made of lead and scintillator plates and read out through wavelength-shifting fibers. In this paper, we discuss the design and performances of the OEV counters under heavy load ($\sim8$ tons/m$^2$) in vacuum. For 1-MeV energy deposit, the average light yield and time resolution are 20.9 photo-electrons and 1.5 ns, respectively. Although no pronounced peak by minimum-ionizing particles is observed in the energy distributions, an energy calibration method with cosmic rays works well in monitoring the gain stability with an accuracy of a few percent.