While not designed for those faced with low-yield neutron bombs or for those who are, in general, so close to the nuclear fireball that prompt/initial radiation would be life-threatening in the short-medium term, ducking and covering would nevertheless slightly reduce exposure to the "initial" gamma rays, specifically the portion emitted after the first flash of visible light. The initial gamma rays are defined as those emitted from the fireball & following mushroom cloud which can reach personnel on the ground for a total of approximately 1 minute, at which point the intensity of the radiation has diminished and the atmosphere itself is thick enough to act as full shielding.