Abstract: Two kinds of low-angle boundary (mosaic structure) in Ce:YAP were observed by etching figures and rocking-curve. From SEM figure, the boundary angle on (010) plane estimated is 51". The rocking-curve by four-crystal high-resolution XRD shows that the boundary angle on the plane paralleled with [010] is 58". The FWHM (Full Width-Half Maximum) of rocking-curve is 54", and the one of ideal YAP crystal calculated out is 17" according to Darwin s theory. The integrality of Ce:YAP sample was analyzed on the basis of these result.

Key words: Ce:YAP crystal, scintillating crystal, low-angle boundary