Large volume CsI(Tl) scintillators for exotic particle search in /sup 90/Zr

The E0 transition of first excited /sup 90/Zr nucleus has been studied with an octagonal configuration of 30 cm length CsI(Tl) scintillators and a strong /sup 90/Sr /spl beta/ source. To study a contribution of yet unknown neutral particles or phenomena in this transition, the precision measurement of even conventional transition processes with electron- and photon pairs is necessary. The rates for these transition modes are substantially small, and the background reduction from the data is of primary importance. Based on the present observation, we introduce somewhat nobel attempt to the efficient rare decay experiment as well as a method for position measurement of low energy /spl gamma/ rays.