Effects of radiation on the Laser Interferometer Lunar Antenna (LILA)

Abstract

The Moon provides a promising environment for gravitational-wave (GW) observations in the midband. The Laser Interferometer Lunar Antenna (LILA) combines a lunar deformation channel (LDC) and a geodesic deformation channel (GDC) to cover the midband. The LDC uses the Moon's resonant response to GW for the lower portion of the midband while the GDC measures displacement of suspended test masses to measure the latter. Radiation on the lunar surface can produce spurious signals through test-mass charging, momentum transfer, and thermoelastic deformation. We estimate these contributions for a notional 100 kg fused-silica optic using Geant4 transport, a finite-element electrostatic force map, and a spatial thermoelastic response kernel. Solar-minimum galactic cosmic rays and lunar albedo neutrons define a background case; a worst-day solar energetic particle spectrum (1989 solar flare conditions) defines an extreme, but rare, case. Aluminum-equivalent shielding from 50 to 1000 mils (1.27 to 25.4 mm) and two electrostatic actuator voltage configurations are compared. In the uniform-charge approximation, balanced drive with electrode configuration (B) substantially reduces linear charge-to-force coupling. Ideal continuous compensation of the mean charging current suppresses coherent charge accumulation while introducing negligible discharge shot noise. With compensation, charge-related noise dominates the low-frequency response, whereas thermoelastic deformation dominates at higher frequencies. Accumulated-dose and photodetector transient estimates identify additional component-qualification considerations. Estimates indicate that even under solar flare conditions, radiation noise is not a significant contributor to LILA's overall noise budget as long as charge-aware designs are followed.