High-energy neutron multiplier

Question

Are there any materials that can function as efficient high-energy neutron multipliers? I have read about the utilization of Beryllium and Lead as neutron multipliers, but they seem to only be discussed in the context of neutrons with energies in the tens of MeV. Do these materials have similar properties for neutrons with energies near 1 GeV, or are there other materials that perform neutron multiplication better for such energies?

I am asking this because I read that neutrons can be used in accelerators to produce pions which can then decay into muons which can then be used to catalyze fusion reactions. (in the reference above, deuterium and tritium are the beam particles due to the neutrons in their nuclei) However, one problem with using muon-catalyzed fusion is the energy cost of producing muons. If there is a material that can multiply the number of neutrons coming from an accelerator, could that lead to greater pion production and thus greater muon production?.

Answer 1

Most nuclides have a "nucleon separation energy" of around 10 MeV. Bound or quasi-bound nuclear states whose excitation energy is more than the separation energy can decay by emitting nucleons. You're not going to get much more energy than tens of mega-eV from any collective nuclear excitation where the heavy nucleus is at rest.

For example, at ORNL's Spallation Neutron Source, a beam of giga-eV protons stops in a mercury target. The excited mercury nuclei boil off neutrons. Each proton in the primary beam produces something like twenty or thirty neutrons. (The neutrons don't all come from the same mercury nucleus.) Conservation of energy tells you that, if a giga-eV proton makes thirty spallation neutrons, the average neutron energy must be somewhere under 30 MeV.

If you want a beam of giga-eV neutrons, you're going to need single-particle interactions with a beam of giga-eV-scale charged particles. For example, you might accelerate a beam of deuterium (or even tritium) to a couple of GeV and then pass the beam through a relatively thin solid target with a heavy nucleus. Deuterium doesn't have any stable excited states, an if the deuterium were to dissociate, the proton and neutron would each have roughly half of the beam energy. You'd then steer the charged particles to some beam dump, and put your neutron experiment in front of the neutrons.

The phrase "neutron multiplier" makes sense in a fission context, but not so much in an accelerator context. If you want GeV neutrons, you're working with an accelerator.