Those Neutrons are Stable, they are inside of atoms.
There isn’t any protection against Neutrons, they are seriously bad suckers that lead, radiation suits, nothing is going to protect you.
Look up the energy dependence of the cross section for neutron capture by U-235 in nuclear fission to illustrate the point...nuclear reactions go at different rates depending on various things, and the energy of the particles emitted can vary (at the point of origination so to speak).
My point was, if the neutrons are being released as the result of a nuclear reaction, *and* there are distinct sets of products with different energies, or different internal states of the products, then the emerging neutrons might have a range, or multiple discrete ranges, of kinetic energy.
Incidentally, the authors mention using boron-infused water -- what is the neutron capture cross section of their apparatus (depends on pressure, concentration of boron, temperature, volume, etc. etc.)
Cheers!