Fig. 1: Energy-level scheme and experimental setup.
From: Measuring the α-particle charge radius with muonic helium-4 ions
Left: energy levels of interest in (μ4He)+. We drive the 2S → 2P transitions ν1 and ν2 (at wavelengths of 813 nm and 899 nm, respectively) and measure the 8.2-keV Lyman-α X-ray from the subsequent decay to the 1S1/2 ground state. Indicated are the Lamb shift (LS) and the shift due to the finite nuclear size (FNS), which is proportional to \({r}_{{\rm{\alpha }}}^{2}\). Right: sketch of the experimental setup (not to scale). On the way to the He target, the muon is detected, thereby triggering the laser system. After the muon is stopped in 2 mbar of He gas at room temperature, (μ4He)+ is formed. About 1 μs after the trigger, the laser pulse arrives at the target, is coupled into the multipass cavity and distributed over the entire muon stop volume (hatched area). The pulse is produced by a Ti:Sa oscillator seeded by a continuous-wave (CW) Ti:Sa laser and pumped by a frequency-doubled pulsed thin-disk laser. The continuous-wave Ti:Sa laser is stabilized to a Fabry–Pérot (FP) cavity and referenced to a wavemeter. The Lyman-α X-rays are measured via LAAPDs (not shown) mounted above and below the cavity. SHG, second harmonic generation.
