The performance study of an electromagnetic calorimeter BGOegg

It is considered that the mass of the $\eta^\prime$ meson decreases in nuclear matter due to the partial restoration of chiral symmetry. As a result of the effect, the $\eta^\prime$ mesic nucleus, the bound state of an $\eta^\prime$ meson and a nucleus, is expected to be formed. We mainly search for an $\eta^\prime$ mesic nucleus in the missing mass spectrum of forward going protons using a photon beam with energies from 1.4 to 3.0 GeV and a nuclear target. The experimental background can be reduced by tagging the $\eta$ meson generated in the $\eta^\prime$N$\to\eta$N conversion process, where the $\eta$ meson is identified by detecting 2$\gamma$ with an electromagnetic calorimeter BGOegg. The BGOegg consists of 1320 BGO crystals and covers a polar angle from 24 to 144 degrees around the target. The energy and position resolutions of the BGOegg are previously measured to be 1.3\% and 3.1mm at 1GeV, respectively. We study the performance of the BGOegg with data collected up to now from December 2013. In this talk, we will present the result of the performance study of the BGOegg calorimeter.