We have developed a fast detector using stacked avalanche photodiodes (APDs) for x-ray diffraction experiments with synchrotron radiation. This detector has four independent channels of APDs, and the detector efficiency has reached 55&percent; in all for 16.53 keV x rays. Since the dead time of the counting system is shorter than 4 ns, output rates of up to4.5×108 counts/shave been obtained for 16.53 keV x rays. The dynamic range is more than1010in the multibunch mode of a storage ring. Pulse-height measurements at output rates of up to108 counts/swere successfully carried out by sequential single-channel discrimination. ©1998 American Institute of Physics.