An apparatus for determining density of fluid returning from a wellbore (“return fluid”) during drilling of the wellbore being drilled using a drill string having a drill bit at an end thereof is disclosed. The apparatus in one embodiment includes a first nucleonic densitometer placed on outside of a return line carrying the return fluid from the wellbore that includes drilling fluid supplied to the drill string and cuttings cut by the drill bit during drilling of the wellbore and a processor that determines the density the return fluid from measurements provided by first nucleonic densitometer.