Shashlyk electromagnetic calorimeter modules with an energy resolution of about 3%/sqrt{E (GeV)} for 50-1000 MeV photons has been developed, and a prototype tested. Details of these improved modules, including mechanical construction, selection of wave shifting fibers and photo-detectors, and development of a new scintillator with improved optical and mechanical properties are described. How the modules will perform in a large calorimeter was determined from prototype measurements. The experimentally determined characteristics of the calorimeter prototype show energy resolution of sigma_E/E=(1.96+-0.1)% \oplus (2.74+-0.05)%/sqrt{E}, time resolution of sigma_T = (72+-4)/sqrt{E} \oplus (14+-2)/E (ps), where photon energy E is given in GeV units and \oplus means a quadratic summation. A punch-through inefficiency of photon detection was measured to be \epsilon = 5*10^{-5} (\Theta >5 mrad).
@article{arxiv.0709.4514,
title = {An Improved Shashlyk Calorimeter},
author = {G. S. Atoian and G. I. Britvich and S. K. Chernichenko and S. Dhawan and V. V. Issakov and O. V. Karavichev and T. L. Karavicheva and V. N. Marin and A. A. Poblaguev and I. V. Shein and A. P. Soldatov and M. E. Zeller},
journal= {arXiv preprint arXiv:0709.4514},
year = {2008}
}