Scintillation

After a helium atom is ionized, recombination of electron and ion occurs. Excited helium atoms are know to form He₂^* dimers in the liquid. Transitions from high lying excited states produce radiation in the infrared and visble in the following fashion:

• Dimers in spin singlet states can radiatively decay into the ground state emitting photons at 16eV. This is the source of the scintillation signal.
• Spin triplet dimers are have an extremely long radiative decay lifetime (~10s), therefore do not contribute to the scintillation signal.

and the principle source of electromagnetic radiation comes from radiative dissociation of the dimers undergoing a transition from the first excited state. This transition produces a band of radiation in the ultraviolet from 13 to 19 eV, with its peak at about 16 eV, thus these are often called “16 eV photons.” Since helium is transparent below 20 eV, this radiation is not absorbed.

These photons have equal probability of travelling in all directions, and undergo a minimal refraction passing through the helium-vacuum interface. Because ~35% of the energy of the initial recoil electron from a neutrino event will be converted to 16 eV photons, for every 100 keV initial energy, about 1000 photons will be emitted upwards toward the detector arrays. Considering the proposed HERON configuration, each neutrino event will produce hundreds to thousands of photon counts. Combined with coded aperture detector arrays, photon signals alone can determine the location of neutrino event to within a radius of 2 cm in a 5×5×5 meter detector for events with energy > 50 keV.

More information regarding scintillaions in helium can be found in the following articles:

• Scintillation and Quantum Evaporation Generated by Single Monoenergetic Electrons Stopped in Superfluid Helium
• Scintillation and Anisotropic Roton Generation by Charged Particles in Superfluid Helium