A radio-isotope decays by emitting alpha and beta particles, and gamma rays, of various energies, into other isotopes, some of which may also be radioactive.

These "daughter" nuclei also emit specific particles at specific energies, and they have specific half-lives, so any radioactive material will eventually reach equilibium and form a family of different nucleides emitting radiation of various types and energies, at various concentrations.

The result will be a spectrum of radiation which will immediately identify the material, it is as specific as a fingerprint, and it will remain constant over time, not in intensity, but in shape. Even two or more decay chains going on simultaneously will create a composite spectrum which can be used to ID the species present.

I am puzzled by your "neutron source". Radiation is either alpha(an electron), beta (a helium nucleus) or gamma (like a highly energetic X-ray). I thought neutrons were only produced by fission, by a Uranium, Plutonium or Thorium nuceus spontaneously exploding. If you have enough fissionable material piled up in certain shape, (a "critical mass") the chances of enough neutrons not escaping the pile and striking another nucleus, (causing it to fission) increases, and you run the risk of a chain reaction! You won't necessarily get a nuclear explosion, but you may get a small "pop" and a flash of neutrons. There were stories running around Oak Ridge of something like this actually happening once, when a pile of uranium slugs on a table collapsed and accidentally achieved criticality. Several men died a horrible death just a few weeks later.

Fissionable material is usually shipped in sub-critical containers, stacked inside metal frames call "birdcages" which ensures the spacing of the containers is also sub-critical.

Do you recall what your Neutron Source was, and do you know how penetrating the neutrons were, and how much shielding was considered adequate?

PS. There is nothing special about lead shielding. Sheer matter is what stops penetrating radiation. Lead is only useful where you need a lot of shielding in a small volume, otherwise, metal, concrete or (providing you have enough of it) water (in a watermelon, too) can do the job just as well.