Silica aerogel radiator for use in the A-RICH system utilized in the Belle II experiment

TL;DR

This paper reports on the development and mass production of large-area silica aerogels for the A-RICH detector in Belle II, demonstrating effective kaon-pion separation exceeding 4σ at 4 GeV/c.

Contribution

It introduces a new large-area aerogel fabrication process and its application in the A-RICH system for particle identification in Belle II.

Findings

Aerogel radiators achieved over 4σ K/π separation at 4 GeV/c.

Successful fabrication of large-area, hydrophobic silica aerogels.

Mass production process for aerogels was established and validated.

Abstract

This paper presents recent progress in the development and mass production of large-area hydrophobic silica aerogels for use as radiators in the aerogel-based ring-imaging Cherenkov (A-RICH) counter, which will be installed in the forward end cap of the Belle II detector. The proximity-focusing A-RICH system is especially designed to identify charged kaons and pions. The refractive index of the installed aerogel Cherenkov radiators is approximately 1.05, and we aim for a separation capability exceeding 4$\sigma $ at momenta up to 4 GeV/$c$. Large-area aerogel tiles (over 18 $\times $ 18 $\times $ 2 cm$^3$) were first fabricated in test productions by pin drying in addition to conventional methods. We proposed to fill the large end-cap region (area 3.5 m$^2$) with 124 water-jet-trimmed fan-shaped dual-layer-focusing aerogel combinations of different refractive indices (1.045 and 1.055). Guided by the test production results, we decided to manufacture aerogels by the conventional method and are currently proceeding with mass production. In an electron beam test undertaken at the DESY, we confirmed that the $K$/$\pi $ separation capability of a prototype A-RICH counter exceeded 4$\sigma $ at 4 GeV/$c$.