The success of the MEG experiment relies on the possibilities to measure the photon energy with very high precision.
The current detector is the world’s largest LXe scintillation detector with 900l
LXe surrounded by 846 photomultiplier tubes (PMTs) submerged in the liquid to detect scintillation light in the VUV range (λ = 175 ± 5 nm).
The photon detector has a C–shape and it is placed outside the COBRA magnet with the innermost detector surface at 65cm from the target center.
The solid angle covered by the calorimeter is about 12%, for an extension of 120° in φ and for |cos θ| < 0.35. Nevertheless the presence of the magnet coil in front to the innermost calorimeter surface, the probability that photon reaches the calorimeter is 95%.
The LXe choice is due to either high light yield which entails high efficiency and good energy resolution that its fast signal response. In particular, a fast signal formation is essential in order to have a precise timing measurement and a minimal impact of the pile–up of accidental γ–rays under high rate background.
The energy release for photon of 52.8 MeV/c is measured from the sum of the signal from all PMTs with a resolution of 1.4% (FWHM).