Re: Are there elements/compounds that accept neutrons and then release them?

SK,

Thanks for the question.  I can say something about how elements
interact at the nuclear level with neutrons, and also say something
about the topic of neutron storage and transport in general.

As you may know, neutrons are very handy particles for starting a
nuclear reaction.  They are uncharged, which means that they can mostly
ignore atomic electrons, and they are not affected much by the charge on
protons in a nucleus either.  Neutrons can easily move up to a nucleus
and react with it.  If neutrons could be conveniently stored, then you
would have an efficient means of creating nuclear reactions on demand.

Unfortunately, the same properties that make neutrons good for starting
nuclear reactions also makes them rather hard to handle.  We humans know
how to control particles using electromagnetism.  Since a neutron does
not have an electric charge, it doesn't have a handle that can be easily
grabbed by our electromagnetic techniques.

Now on to your question.  If we had a substance like a sponge that could
soak up neutrons and then release them later that would be very
convenient.  There are some problems with this however.  If a substance
is going to combine with a neutron to hold it, there must be a nuclear
effect involved.  There are two ways that this could happen.  Either the
neutron combines with the nucleus and there is a release of energy, or
the neutron combines with the nucleus after some energy is added to the
neutron/nucleus system.  

For the case where energy is released the neutron is bound into the
nucleus with an energy equal to the energy released.  This means that to
get the neutron back out of the nucleus you will have to add energy back
into the system.  Often this energy is inconveniently large.  Also,
putting energy into the system may give you back something besides the
neutron, like a gamma ray.  
There are nuclei which do not bind neutrons very tightly.  These nuclei
can be made to eject a neutron, but they are often unstable.  An
isolated neutron outside of a nucleus will decay into a proton, an
electron, and a neutrino.  This takes about 10 minutes for a free neutron.
If a neutron is loosely bound to a nucleus, this beta decay is still 
possible, and so after a few seconds or minutes the easily recovered 
neutron changes into particles which are not so useful for starting nuclear 
reactions.

For the case where energy is required to get the neutron to stick to the
nucleus the neutron often will pop out of the system by itself after a
length of time.  This can be described as a metastable state.  These
states also lose neutrons to beta decay and are also difficult to make.

Here are some terms that you can look up on the web which will tell you
more about these types of nuclei.

neutron dripline
neutron halo
nucleosynthesis 
s-process (slow )
r-process (rapid )

In a sense fissionable materials can be though of as a reservoir of
neutrons that can be called up when needed.  A nuclear fission reactor
starts with a few neutrons and is able to create many neutrons through
the fission process.  Some of these created neutrons are prompt and come
out right away, while other neutrons are delayed and come out after a
few seconds or  minutes.

There are some ways to store or at least handle neutrons which don't
involve nuclear reactions, at least not the type that involve large
quantities of energy.  Low energy neutrons ( thermal or cold neutrons )
can be reflected at small angles from special surfaces.  Neutrons also
have weak magnetic properties which can be used to hold a few inside a
magnetic bottle.  These methods can hold only a small number of
neutrons, and even these neutrons beta decay in a few minutes. 

Another way of marginally containing neutrons is to use a moderator. A
moderator is a substance which bounces neutrons around, slows them up,
and hopefully doesn't react with the neutrons.  Neutrons diffuse like a
gas when they are inside a moderator of sufficient size.  If you have a
source of neutrons inside a large moderator, like a 100 ton block of
carbon, the neutrons will tend to pile up around the source as they
diffuse outward.  This would be a leaky container of neutrons, somewhat
like what happens when you try to inflate a sock by blowing into it.

I hope that I have adequately answered your question.

Regards,

Everett Rubel