Neutron activation time for Boron, Silver, and possibly other materials How do i justify/explain that the time needed to activate Boron is 2 hours and the time needed to activate Silver in water is 45 minutes?
Response to comment:
In a neutron cross section experiment, I have to determine the cross section for neutron capture in Boron. Before the start of the experiment I had to activate indium plates enclosed with boron in AmBe source, while silver was activated in water.
 A: "Activation" in the neutron business refers to transmutation of stable isotopes to unstable isotopes by neutron capture, such as $\rm^{27}Al + n \to {}^{28}Al + \text{photons}$,
or  to other nucleon-exchange processes such as $\rm^{14}N + n \to {}^{14}C + p$.  Sometime later the unstable isotope emits detectable beta (and possibly gamma) radioactivity.  The induced activity depends in a complicated way on the neutron exposure.  
However if you have a neutron source with a given neutron flux, spectrum, and geometry, the activity of your target material will eventually come into secular equilibrium.  The amount of time required is "a few half-lives" for the unstable isotope.
In your boron-indium experiment it sounds like you're using boron-loaded cladding (perhaps boron-doped plastic? pure boron isn't easily machinable) as shielding to keep your neutrons out of the room while you activate the indium.  Boron is an shielding for neutrons because it's a strong absorber that doesn't activate; B-11 basically ignores neutrons, and the fragmentation $\rm^{10}B + n \to {}^4He + {}^7Li$ sometimes emits a soft gamma ray but doesn't have any delayed activity from unstable daughter products.  However neutrons on indium will mostly capture via $$\rm^{115}In + n \to {}^{116}In + \text{photons}.$$
Indium-116 has an isomer with spin $5^+$ and a 54 minute half-life; after two hours of exposure this isomer will be 80% of the way towards its secular equilibrium.
For silver in water I don't immediately see an answer.  Water doesn't activate due to neutron exposure (all the neutrons go into making deuterium, which is stable).  Silver-107 and -109 share the neutron capture about equally, but the relevant lifetimes in the mass-108 and -110 systems are either shorter than five minutes or longer than 200 days.  However pure metallic silver doesn't dissolve in water. I predict you're dissolving silver iodide, since iodine-128 has half-life 25 minutes and that's again roughly half your "activation time."  Could also be silver bromide (Br-80 18 minutes, Br-82 and Br-82m less convenient decay times but a very small neutron capture cross section for Br-81.)
If you are putting thermal neutrons on silver iodide (or, for that matter, silver bromide) about 90% of the neutron capture will be on the silver.  After you remove the material from the neutron source your decay curve will have a 25s component from silver-110, a smaller 2.5m component from silver-108, and a 25m component from the iodine.  
My source for half-lives and cross-sections is the National Nuclear Data Center.
