Radiation Flux

Crew members are not as durable as spacecraft, since they are vulnerable to neutron radiation. A one megaton Enhanced-Radiation warhead (AKA "neutron bomb") will deliver a threshold fatal neutron dose to an unshielded human at 300 kilometers. There are also reports that ER warheads can transmute the structure of the spacecraft into deadly radioactive isotopes by the toxic magic of neutron activation. Details are hard to come by, but it was mentioned that a main battle tank irradiated by an ER weapon would be transmuted into isotopes that would inflict lethal radiation doses for up to 48 hours after the irradiation. So if you want to re-crew a spacecraft depopulated by a neutron bomb, better let it cool off for a week or so.

For a conventional nuclear weapon (i.e., NOT a neutron bomb), the x-ray and neutron flux is approximately:

Fx = 2.6 x 1027 * (Y/R2)

Fn = 1.8 x 1023 * (Y/R2)

where: There are notes on the effects of radiation on crew and electronics here.
 * Fx = X-ray fluence (x-rays/m2)
 * Fn = Neutron fluence (neutrons/m2)
 * Y = weapon yield (kilotons TNT)
 * R = range from ground zero (meters)