How to block neutrons
Being bulk neutral neutrons participate only weakly in electromagnetic interactions which is the dominate interaction for charged particles.
Instead neutron scattering can be thought of as primarily a contact interaction with the nuclei of atoms in the way. Light atoms (and hydrogen in particular)
- have a larger cross-sectional area per nucleon than heavy ones
- take up more of the energy of the interaction in recoil than heavy ones
making them much more effective at reducing the kinetic energy of non-thermal neutrons per unit areal mass density.
Historically waxes, water and plastics have been the neutron shielding materials of choice, though concrete or rammed earth are cheap and not too bad.
Once down to thermal energies neutrons get as much kinetic energy as they lose on average and you just have to wait for them to decay or capture.
Doping your absorber with boron, chlorine or even gadolinium will help to capture the thermalized neutrons faster. PVC gets you the chlorine for free in your plastic, and boron can be added easily to concrete or to a number of plastics.
It should not be overlooked that it takes a lot of space to slow, thermalize and capture neutrons (that contact interaction thing means they go through more material before interacting than charged particles); especially if you need to get them all. They are notorious for penetrating large quantities of shielding, and distance is one of your best friends when it comes to neutron shielding.