Particle physics observatories in high-pressure salt caverns
Documents:- For physicists: Snowmass 2020 letter-of-interest, slides from Snowmass flash talk, Neutrino2020 poster, two-slide summary.
- For engineers: Solution Mining Research Institute paper from 2016
- For our very first and definitely outdated thoughts on this, Underground physics without underground labs: large detectors in solution-mined salt caverns
Neutrinos
Deep geological bodies of solid salt are common in North America and Europe. WIPP, Boulby, and the IMB site are in abundant "bedded" salt deposits. Tall, pure "domal" salt sites are common on the US Gulf Coast, northern Germany, and the Persian Gulf. Note that the edge of the "Pine Salt" formation is 1.5° off-axis from the DUNE neutrino beam, and salt beds underlay many nuclear reactor sites.The natural-gas industry routinely creates large, stable high-pressure caverns in salt via solution mining. Solution-mined caverns are made from the surface by pumping in fresh water and extracting brine; the cavern shape is managed by manipulating a cover gas. The process is far cheaper ($20/m3) than conventional lab excavation ($1000/m3) and volumes to 2x106 m3 are possible. Our hope is to piggyback on this cheap, existing technology by designing detectors that can deploy remotely via the otherwise-conventional 12"-24" well pipe.
Salt caverns' vast sizes and high pressures give us a unique path to ton- or kiloton- scale gas TPCs. High-pressure TPCs are a well-known detector medium, with longer tracks and better calorimetry than liquids. However, detectors like NEXT, NEWS-G, and HPgTPC are constrained to small target masses because they need heavy, expensive pressure vessels. Salt caverns have nearly-unlimited pressurized space. We can make use of it under a challenging constraint: can the detector be engineered to squeeze down a 24" well?
A kiloton-scale CNO neutrino experiment in neon
- 10 m diameter x 80 m tall inflated cathode balloon cylinder
- Cylindrical drift under 100-200 kV voltage
- 50 cm segmented anode cylinder. Conventional multiwire gain grid.
- Target: 500 t neon at 100 bar (77 kg/m3)
- Shielding/veto: >15 m of Ar-based gas mixtures
- Electron tracks are 10-20 cm long; cut on sun direction
- TPC resolves gamma ray interactions as multisite Compton scattering
- Dissolved (?) beta emitters (214Bi, etc.) often have a displaced gamma
- Cosmogenic 11C production is low on 20Ne target
- The major pp-neutrino background 14C is absent
Dark matter
At volumes large enough to self-shiel, salt-compatible inflatable high-pressure gas TPCs may be excellent dark matter detectors. We described some possible enabling tecnologies in: Sub-Penning gas mixtures: new possibilities for ton- to kiloton-scale time projection chambers(https://arxiv.org/abs/1512.04926) by Benjamin Monreal, Luiz de Viveiros, William Luszczak.