Design of an active magnetic field compensation system for MiniCLEAN

MiniCLEANis a single-phase noble liquid scintillator experiment designed to detect nuclear recoils due to weakly interacting massive particles hypothesized to constitute the dark matter. The principle of the detector is to monitor scintillation light resulting from ionizing radiation using 92 photomultiplier tubes surrounding a spherical target. Photomultiplier tube response is known to be affected by sub-Gauss magnetic fields, so that the Earth's magnetic field has a non-negligible effect on the photomultiplier tube efficiency. In this experiment, the crucial nuclear recoil energy threshold depends on the ability to detect very small amounts of scintillation light; high photomultiplier tube efficiency is critical. Therefore, the MiniCLEAN collaboration has designed active compensation coils to mitigate the Earth's local magnetic field. Two features of the experimental environment make this situation unique: first, the underground laboratory (SNOLAB) is located in a nickel mine, so that direct measurement of the potentially distorted geomagnetic field is mandatory. Second, the close proximity of another experiment based on photomultiplier tubes (DEAP-3600) makes the compensating field outside our detector a concern. An additional complication is that MiniCLEANis surrounded by a steel water tank needed for shielding and a muon veto composed of four strings of 12 photomultipliers suspended in the water. We describe our design based on these considerations, survey data, field calculations and simulations of the photomultiplier tube response.