K-shell X-ray emission from a Cu nanowire target irradiated by an ultraintense femtosecond laser pulse is studied using an elliptically bent quartz crystal and imaging plate. The designed bent crystal spectrometer has better spectral resolution, which is higher than 1000. The absolute Ka radiation photon yields are obtained from the experimental results and the Monte-Carlo model. The conversion efficiency of the Cu Ka line is estimated to be 0.019% from the interaction of 4 J, 50-fs laser pulse irradiated on a Cu nanowire target. The high vield of K shell X-ray has important applications in X-ray emission source.
