http://faculty.publicpolicy.umd.edu/sites/default/files/fetter/files/1990-SAGS-DNW.pdf
In the absence of shielding, "ordinary" nuclear weapons-those containing kilogram
quantities of ordinary weapon-grade (6 percent plutonium-240) plutonium or uranium-238-can be detected by neutron or gamma counters at a distance of tens of meters.
The two fissile materials used in US and Soviet warheads are weapon grade uranium (WgU) and weapon-grade plutonium (WgPu). The compositions of these materials assumed in this study are given in table 1.
Table 1: The compositions of weapon-grade uranium and weapon-grade plutonium
assumed In this study, In percentages of total weight
Weapon-grade uranium
Uranium-234 1.0% Uranium-235 93.3% Uranium-238 5.5% Other. 0.2%
Weapon-grade plutonium. Plutonlum-238 0.005% Plutonium-242 0.015% Plutonium-241 0.44% Plutonlum-239 93.3% Plutonium-240 6.0% Other. 0.2%
PASSIVE DETECTION
All isotopes of uranium and plutonium are radioactive. The detectability of this radioactivity varies widely from isotope to isotope, depending on the halflife and the types of radiation emitted during radioactive decay.
The two types of radiation that might be detectable a few meters or more from a
warhead are neutrons and gamma rays (high-energy photons).
Detection of Radiation
As one moves away from the weapon, the flux of neutrons and gamma rays (particles per second per unit area) decreases inversely with the square of the
distance. For example, the particle flux at 2 meters is four times smaller than at 1 meter; at 3 meters it is nine times smaller, and so on. At some distance, the emissions from the weapon will become undetectable because the flux will be small compared to the flux of natural background radiation.
The detectability of gamma-ray emissions, on the other hand, is subject to much greater uncertainties. First, the gamma-ray background is less predictable than the neutron background. Second, the rate of gamma-ray emission from a weapon is much more dependent on details of its design than is the rate of neutron emission. For example, some nongovernmental analysts assume that thermonuclear weapons have a casing made out of depleted uranium. If our weapon models each had a uranium case weighing 10 percent of the total mass, the case would be about 1 millimeter thick. Since the mean free path of 1-MeV gamma rays is much greater than that (14 millimeters), about half the gamma rays produced in the case would escape. The resulting gamma-ray flux would be about 10 times greater than that from a depleted uranium tamper, and would be detectable at a distance three times greater. As another example, consider a tamper made of beryllium instead of tungsten or depleted uranium. Because beryllium is much less absorptive than these heavy metals (especially at low energies), the gamma rays emitted by the uranium or plutonium in the center of the weapon would be far more detectable. For a WgU core, 186-keV gamma rays would be emitted from uranium-235 at a rate of about 70,000 per second, which would be detectableat a distance of 6 meters (for a counting time of 1 minute). In the case of a plutonium core, 414-keV gamma rays would be emitted from plutonium-239 at a rate of about 500,000 per second, which would be detectable at a distance of about 20 meters.
Moreover, it is possible that the WgU could be mixed or contaminated with uranium from reprocessed reactor fuel (see appendix A for details), If this is the case, the presence of uranium-232 in WgU could make such weapons far more detectable than is indicated by our analysis. Even if this isotope is present at concentrations of less than 1 part per billion, the highly penetrating 2.614-MeV gamma rays emitted during the decay of uranium-232 would be detectable at distances of tens of meters.