Electromagnetic calorimeter BGOegg for quark nuclear physics

A 4π electromagnetic calorimeter, BGOegg, was constructed and installed at the SPring-8/LEPS2 beamline for Quark Nuclear Physics. One of the main subjects to be studied with the BGOegg calorimeter is to search for precursory phenomena of the chiral phase transition of the QCD vacuum. It would be a challenging study to measure γγ decay of mesons inside a nucleus, where strong decay channels are open. The spectral function of the η′(958) meson propagating in the nucleus provides us ample information on the effect of partial restoration of chiral symmetry to axial U(1) anomaly, being measurable through η′ → γγ decay process taking place in the nucleus. BGOegg consists of 1320 BGO single crystals, which subtend 86% of the 4π solid angle. Each crystal has a trapezoidal shape, the length of which is 220 mm corresponding to 20 radiation lengths. The measured energy resolution is 1.3% for 1 GeV positrons, which shows one of the world's best resolutions as an electromagnetic calorimeter to detect photons in the several hundred MeV region. The γγ invariant mass resolutions are σ(π⁰) approx. = 5.8MeV, σ(η) approx. = 13MeV, and σ(η′) approx. = 19MeV, which are obtained so far in the latest energy calibration task for the data obtained with a 20 mm thick Carbon target. The construction project of a 4π electromagnetic calorimeter started with the selection of scintillating crystals appropriate for the detector elements of the calorimeter. A Bismuth Silicate (BSO) crystal was chosen at first as the best candidate for a γ detector measuring several hundred MeV photons. Instead of BSO, however, BGO crystals were selected at the end of the crystal-selection procedure. It took a long time, about 10 years, to complete the construction project of the BGOegg calorimeter. (author)