[Paper Review] Multi-Megaton Water Cherenkov Detector for a Proton Decay Search -- TITAND

Y. Suzuki|ArXiv.org|Oct 2, 2001

Radiation Detection and Scintillator Technologies20 citations

TL;DR

TITAND proposes a scalable, multi-megaton water Cherenkov detector for proton decay searches, designed to operate in shallow seawater using a semi-pressure vessel and 50 cm PMTs with 20% photo-coverage. It achieves a 3σ discovery sensitivity of ~7×10³⁴ years for a 1 Mton detector and ~4×10³⁵ years for a 10 Mton detector, significantly improving on current limits by leveraging reduced background rates through tight momentum cuts and buoyant, modular construction.

ABSTRACT

In this report, we will show a detector which can be used to search for proton decay in the lifetime region beyond 10$^{35}$ years. We will briefly review the current experimental status and discuss the sensitivity of the future proton decay detectors, and we specifically present a possibility of a scalable multi-megaton water Cherenkov detector immersed in the shallow water.

Motivation & Objective

• To develop a scalable, cost-effective multi-megaton water Cherenkov detector capable of probing proton decay lifetimes beyond 10³⁵ years.
• To overcome the limitations of current detectors by reducing background rates through tighter momentum cuts and improved event selection.
• To enable long-baseline neutrino oscillation experiments and supernova burst detection by making the detector movable and modular.
• To reduce construction costs to ~$250k per 2.0 Mton, making large-scale detectors feasible.
• To explore the feasibility of operating a large detector in shallow seawater using buoyancy and pressure-balanced vessel design.

Proposed method

• The detector uses a semi-pressure vessel design that maintains internal water pressure at 10 atm, eliminating the need for pressurized PMTs.
• It employs 50 cm diameter PMTs with 20% photo-coverage, arranged every meter, similar to Super-Kamiokande and Kamiokande.
• The detector is constructed from four 70×70×100 m modules built onshore, towed to site, and filled with ultra-purified water via desalination and purification systems.
• An outer 2 m thick layer acts as an active shield to detect cosmic ray muons and suppress backgrounds.
• A tight momentum cut of 100 MeV/c is applied to reduce backgrounds, improving signal-to-noise ratio despite lower detection efficiency.
• The detector is designed to float via ballast tanks and can be positioned with ~10 m accuracy using a self-positioning system.

Experimental results

Research questions

• RQ1 Can a multi-megaton water Cherenkov detector be built cost-effectively and operated in shallow seawater to probe proton decay lifetimes beyond 10³⁵ years?
• RQ2 How can background suppression be improved in proton decay searches without sacrificing detection efficiency?
• RQ3 What is the sensitivity of a 10 Mton detector with a tight momentum cut to discover proton decay in the e⁺π⁰ mode?
• RQ4 Can a large-scale detector be made movable and modular for use as a far detector in neutrino oscillation experiments?
• RQ5 What are the engineering and cost implications of constructing a 2.0 Mton detector with 20% PMT coverage and semi-pressure vessel design?

Key findings

• The 3σ discovery sensitivity for proton decay in the e⁺π⁰ mode reaches ~7×10³⁴ years for a 1 Mton detector over 10 years with a tight 100 MeV/c momentum cut.
• For a 10 Mton detector, the 3σ discovery sensitivity improves to ~4×10³⁵ years under the same conditions.
• Background rates are reduced to ~0.15 events per Mton·year with the tight momentum cut, down from ~2.2 events per Mton·year with standard cuts.
• The detector design allows for buoyant operation in seawater by balancing buoyancy forces, enabling deployment at any depth without pressurized PMTs.
• Construction cost is estimated at ~$250k per 2.0 Mton, significantly reducing the cost barrier for large-scale detectors.
• The detector is sensitive to supernova neutrino bursts, with ~200 events per Mton expected for a supernova in Andromeda (650 kpc away).