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A colleague and I were discussing the fact that beta decay can emit neutrinos with arbitrarily low energies. We have neutrino detectors that can detect solar neutrinos -- and maybe extrasolar neutrinos or a cosmological component as well? -- but presumably any such detector has some energy threshold. E.g., if the working volume is made of chlorine, then is the detector only sensitive to neutrinos with enough energy to transmute 35Cl or 37Cl into 35- or 37- S or Ar? If so, then how far down in energy does our knowledge of the ambient neutrino environment go, and can we only guess about the spectrum below that based on extrapolation?

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    $\begingroup$ When you say "we have detectors [for] the cosmological neutrino background," are you thinking of the micro-eV relic neutrinos? I'm pretty sure there is no technology that has specifically detected those. A good starting place in the literature is this recent Borexino paper, which is (I think) the first experimental information on the energy spectrum from sub-MeV solar neutrinos originating in the p-p process. $\endgroup$ – rob Mar 5 '19 at 21:47
  • $\begingroup$ @rob: Thanks for the info. I'm ignorant about cosmological neutrinos, was probably making unjustified assumptions. I'll edit the question. $\endgroup$ – user4552 Mar 6 '19 at 6:10
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Rob's comment pointed me to some relevant information, so I thought I would write up a quick self-answer. Since this isn't my field, I may be getting stuff wrong here. A more authoritative answer from a specialist would be great, as would comments correcting any mistakes I may be making.

Borexino is the first neutrino detector to be able to measure neutrinos with energies below 3 MeV. Its energy range includes of the peak of the spectrum of solar neutrinos from the pp process. It consists of 278 tons of liquid organic scintillator, shielded by a large volume of water. Neutrinos scatter inelastically from electrons in the scintillator, so there is no minimum energy needed in order to interact. Below about 500 keV, it mostly sees background from 14C and 210Po.

For comparison, the detector Super-Kamiokande used both scattering from electrons and scattering from nuclei, which produced ultrarelativistic electrons and positrons. It was only sensitive to events that were energetic enough for the electron or proton to produce Cerenkov radiation.

Below is an energy spectrum:

borexino spectrum

Here are a couple of a recent papers on Borexino's results:

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  • $\begingroup$ Technically there will be a very small minimum energy that the scattered electron will need to have to excite the liquid scintillator. I don't know exactly what this energy is, but it must be greater than approximately a few eV. In practice, the real lower energy threshold is going to be a lot higher since a typical liquid scintillator produces ~500 detectable photons/MeV, so we're talking a few keV. $\endgroup$ – user545424 Mar 7 '19 at 18:41

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