Limits to the observation of the Unruh radiation via first-quantized hydrogen-like atoms

We consider ionized hydrogen-like atoms accelerated by an external electric field to detect Unruh radiation. By applying quantum field theory in the Rindler spacetime, we show that the first-quantized description for hydrogen-like atoms cannot always be adopted. This is due to the frame-dependent definition of particles as positive and negative frequency field modes. We show how to suppress such a frame-dependent effect by constraining the atomic ionization and the electric field. We identify the physical regimes with nonvanishing atomic excitation probability due to the Unruh electromagnetic background. We recognize the observational limits for the Unruh effect via first-quantized atomic detectors, which appear to be compatible with current technology. Notably, the non-relativistic energy spectrum of the atom cannot induce coupling with the thermal radiation, even when special relativistic and general relativistic corrections are considered. On the contrary, the coupling with the Unruh radiation arises because of relativistic hyperfine splitting.