Report on the Depth Requirements for a Massive Detector at Homestake

TL;DR

This report evaluates the depth requirements for a large underground neutrino detector at Homestake, concluding that depths up to 4300 mwe are sufficient for physics goals, with a recommendation to start geotechnical studies at 4850 ft.

Contribution

It provides a detailed analysis of depth requirements for a mega-ton scale neutrino detector at Homestake, integrating physics needs with practical site considerations.

Findings

Depths up to 4300 mwe are adequate for key physics processes.

A minimum depth is necessary to mitigate cosmic ray contamination.

Recommendation to initiate geotechnical studies at 4850 ft level.

Abstract

This report provides the technical justification for locating a large detector underground in a US based Deep Underground Science and Engineering Laboratory. A large detector with a fiducial mass in the mega-ton scale will most likely be a multipurpose facility. The main physics justification for such a device is detection of accelerator generated neutrinos, nucleon decay, and natural sources of neutrinos such as solar, atmospheric and supernova neutrinos. In addition to the physics justification there are practical issues regarding the existing infrastructure at Homestake, and the stress characteristics of the Homestake rock formations. The depth requirements associated with the various physics processes are reported for water Cherenkov and liquid argon detector technologies. While some of these physics processes can be adequately studied at shallower depths, none of them require a depth greater than 4300 mwe which corresponds to the 4850 ft level at Homestake. It is very important to note that the scale of the planned detector is such that even for accelerator neutrino detection (which allows one to use the accelerator duty factor to eliminate cosmics) a minimum depth is needed to reduce risk of contamination from cosmic rays. After consideration of the science and the practical issues regarding the Homestake site, we strongly recommend that the geotechnical studies be commenced at the 4850ft level in a timely manner.