A neutron detector is what picked this up? Jeez. I know my internal alert chart went from orange to red in the instant I read your post.
If neutron radiation was detected, I share your concern. Ordinary radioactive materials (uranium, plutonium, etc.) in ordinary amounts are generally fairly neutron-cool; they emit alpha and gamma radiation to one degree or another, but their neutron emission rates are low -- that is, until you get enough of the stuff together in a small space. Then, neutron emissions from some of the atoms cause the nuclei of other nearby atoms to fission (split), releasing more neutrons, which causes still further fission -- the famous "chain reaction". Every time a nucleus splits, a small portion of its mass is converted to energy in the form of electromagnetic radiation, and thus (when the radiation interacts with matter) to heat, according to Einstein' famous formula E=MC^2 (energy equals mass times the speed of light squared). If this fission chain reaction occurs gradually, the energy will released at a controlled rate, and the reaction becomes "critical" (self-sustaining), producing heat; this is how nuclear power plants generate steam. However, if the fission chain reaction occurs quickly enough, and within nuclear materials that are smaller that a given volume (the "critical mass"), then an uncontrolled, runaway chain reaction occurs, releasing all the fission energy at once: E=MC^2. Boom. This is the principle behind an atomic bomb
Neutron radiation also has the effect of rendering surrounding materials radioactive over time. The stray neutrons from the radioactive material shoot out of their nuclei and slam into the nuclei of other atoms nearby, knocking out nuclear particles from them and changing their composition; certain of the resulting changed nuclei become unstable as a result, emitting electromagnetic radiation on their own. (These "changed" atoms are called radioisotopes.) By converting the atoms in nearby materials to radioisotopes over time, neutron-emitting material can cause non-radioactive materials to become radioactive. Thus, even if the ship in question has no nuclear or radioactive comb on board now, the presence of radiation might indicate that such a bomb was located on the ship at some time before. By analyzing the rate at which this induced radiation dissapates (its "rate of decay"), scientists might be able to tell how long ago it was irradiated and, thus, when the nuclear materials were onboard.
The presence of detectable neutron radiation onboard this ship would be a serious indication that a critical mass of radioactive material is or was on board. That would not bode well.