The Feynman-α experiments were carried out using light-water-moderated and -reflected cores loaded with highly enriched uranium fuel at the Kyoto University Critical Assembly. An experimental technique using a multichannel scaler was developed to improve the accuracy of measurement and to shorten measuring time. Then, the βeff/l values of single and coupled cores with different neutron spectra were measured to demonstrate the capability of the present technique for measuring the prompt neutron decay constant α. Moreover, the Feynman-α method was applied to measuring large subcriticalities. Through these experiments, it is found that the present technique greatly improves the accuracy of a measurement, and the one-point reactor approximation is applicable to a tightly coupled core. It is also found that the subcriticality down to approximately -35 $ can be measured by this method if the position of the neutron detector is chosen carefully, and the present Feynman-α method can be applied to a subcriticality monitoring system.