Properties & Principles of Heron Detector

A generic HERON detector
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There are several physics and technological challenges to creating a detector for low energy solar neutrinos via elastic scattering. The maximum recoil energies of the electron for the pp and ⁷Be fluxes are 261 and 665 keV, respectively. Achieving as low a threshold as possible is also particularly valuable for rate and physics reasons. In comparison to detectors designed for the much more rare ⁸B neutrinos, those constructed for the pp flux can be much smaller: tens of tonnes relative to kilo-tonnes. Never the less, a signal of a few kilovolt energy deposition by a single recoil electron must be reliably detected, its energy measured and distinguished from potential backgrounds. Potential background sources include decay products of natural, primordial (U, Th, K) and cosmogenic isotopes in the target material itself, its container or external to the detector. The latter is minimized, as for all solar detectors, by external shielding and siting them at depth below ground. Extracting the signal and dealing with the other background sources must be accounted for in the design of the detector itself.

The HERON detector is centered around the use of superfluid helium as the ~20 tonne target material. Superfluid helium has the unique property of “self-cleaning”; that is, it expels every other atomic species from its bulk. Similarly, at the operating temperature (~ 30 mK) of the superfluid, thermal energy is sufficiently low relative to the gravitational potential energy that particulate matter falls out of the bulk. Consequently, there can be no background sources within the detector material itself. The principal background source is from the containment cryostat for the helium. Careful selection of construction materials and the development of a distinctive background signature different from that of the signal mitigate radiation from the containment vessel.

For the pp and ⁷Be neutrino scatters, the longest recoil electron path is 2 cm in the liquid helium. The electron loses its energy through ionization and atomic collisions not leading to ionization. Copious production of UV (~16 eV) scintillation photons and of phonons/rotons results from the ion recombinations and atomic collisions, respectively. The scintllation and phonon/roton channels can be used in a complementary manner to determine the position and energy of a signal event and aid in discrimination against background.

An array of low heat capacity sapphire (or silicon) wafers placed above the helium surface are used as the sensors for both the scintillation and phonon/roton signals. The phonon/roton signal is developed through quantum evaporation and adsorption on the wafer while the UV photons are directly absorbed in the thin wafers. Metallic magnetic calorimeters mounted on the wafers and read-out with SQUID electronics register the resulting heat pulses.

This group has studied extensively the details of these energy loss processes and pioneered the development of the magnetic micro-calorimeters. Further information can be found in the R & D portion of this website.

Proposed HERON construction
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For visualization purposes, the HERON detector can be thought of as a cylinder of six-meter diameter and six meter height. Consequently, on this scale the 2 cm recoil electron is a point source of radiation. In contrast, the energy deposition pattern in the helium by the background is quite different. It consists dominantly of a series of spatially distributed Compton scatters time-coincident with each other. To capitalize on these differences, the wafers are placed in two planes vertically separated. Together the two planes constitute a coded aperture array in which the wafers function as patterned pixels. Using a maximum likelihood analysis of the resulting energy depositions and timing a statistical separation of signal and background is made. Simulation studies on this method are being carried out and are also illustrated in the R & D section.

An additional possibility for use in developing a second contrasting signature for signal and background, and which the group is pursuing, involves the use of scintillation and drifted electron charge collection instead of scintillation and phonon/rotons. The method is based on another unique property of liquid helium; namely, that an isolated electron (for example, the stopped recoil Compton or neutrino event) forms a bubble. Under suitable circumstances, application of low electric fields can be used to control the natural rise of the bubble and to extract the charge that would also be detected by the wafers. Here also the combined hit patterns from both channels, electron multiplicity and timing would be used to discriminate signal and background. The principles of “e-bubble” detection is further discussed in the R & D section.