[en] Study of x-ray conversion efficiency in laser produced plasmas and its optimization with respect to laser and target parameters has drawn considerable attention of research workers in the last few years. An important reason for this motivation is the use of sub-keV x-ray emission as a driver for the inertial confinement fusion. Here, experimental measurements of the spectral distribution of keV x-ray energy conversion of the M-shell emission from gold plasma produced by a 4 GW, 5 nS Nd:glass laser are presented. At a laser intensity of 2 x 10¹³ W/cm², the overall x-ray yield for 2.0 ≤ hν ≤ 3.6 keV is determined to be 0.1% of the laser energy. The effect of the keV x-ray spectral distribution on radiation preheat is discussed