I still don't think this is right.
The "Spallation Neutron Source" produces neutrons from slamming mercury with protons.
Directly from Le Wiki-Pédia:
The spallation process at SNS begins with negatively charged hydrogen ions that are produced by an ion source. Each ion consists of a proton orbited by two electrons. The ions are injected into a linear particle accelerator, or linac, which accelerates them to very high energies (eventually to 90% the speed of light).[citation needed (no seriously, WTF!)] The ions pass through a foil, which strips off each ion's two electrons, converting it to a proton. The protons pass into a ring-shaped structure, a proton accumulator ring, where they spin around at very high speeds and accumulate in “bunches.” Each bunch of protons is released from the ring as a pulse, at a rate of 60 times per second (60 hertz). The high-energy proton pulses strike a target of liquid mercury, where spallation occurs. The spalled neutrons are then slowed down in a moderator and guided through beam lines to areas containing special instruments where they are used in a wide variety of experiments.
So that's the idea. Where do we get those neutrinos from?
In the extra-interesting presentation about "COHERENT Elastic Neutrino-Nucleus Scattering" by Kate Scholberg ("CEνNS is a possibility but those internal Greek letters are annoying.. CEvNS, pronounced “sevens”... spread the meme!")
we find the explanation on 21 ("Stopped-Pion (πDAR) Neutrinos"):
The proton hits the Hg nucleus, eliciting Pions from nature's infinite bag of probablistic stuff: a pion+ and a pion- (these are quark-antiquark pairs), unsure what happens to the Hg nucleus.
The pion- is captured by another Hg nucleaus with high probability.
The pion+ decays into a muon and a muon neutrino.
The muon then decays into a positron, an anti-muon-neutrino and an electron-neutrino. That last one is the one we want.
Photons are not involved anywhere!
The must be some additional trickery regarding focusing of the neutrinos, we want a beam after all.