Hydrophobic Silica Aerogel Thermal Protection for a Low-Power, Helium-3-Free Cosmic-Ray Neutron Soil Moisture Sensor in Hyper-Arid Rangeland.

Soil moisture sensors in hyper-arid rangeland face temperatures beyond the range over which their electronics are specified, yet insulation placed around a neutron detector is a scatterer and risks being read as a change in soil water. We evaluate this trade-off computationally for a helium-3-free, boron-10-lined cosmic-ray neutron sensor fitted with an external hydrophobic silica aerogel sleeve. A voxel model of the instrument in a domain of its own size gives response functions, normalized to a maximum of 1.0 and applied to the spectrum of a 1000 m field simulation with no detector. A 20 mm sleeve raises the weighted epithermal count by 3.0%, constant to 0.0007 across a factor of six in soil water. The wet-to-dry scaling factor N(0.05)/N(0.30) is 2.1533±0.0161 bare and 2.1530±0.0160 sleeved, a ratio of 0.9999. The sleeve changes the count rate and not the moisture response, and the offset is absorbed into the calibration constant when calibration follows assembly. A diurnal thermal calculation gives an interior peak of 59.9 °C bare against a specified limit of 45 °C, and 40.7 °C sleeved. Every result is from simulation; field deployment remains future work.