Central part of the experimental setup

<p><strong>Figure 3.</strong> Central part of the experimental setup. Antiproton and laser beams coming from opposite sides are injected into the microwave cavity which also contains the helium gas. Microwaves are fed through a wave guide from the top, and the microwave power is measured by a small antenna. Outside the vacuum chamber, two Cherenkov counters are mounted to detect the pions resulting from the annihilations.</p> <p><strong>Abstract</strong></p> <p>In this work, we describe the latest results for the measurements of the hyperfine structure of antiprotonic <sup>3</sup>He. Two out of four measurable super–super-hyperfine (SSHF) transition lines of the (<em>n</em>, <em>L</em>) = (36, 34) state of antiprotonic <sup>3</sup>He were observed. The measured frequencies of the individual transitions are 11.125 48(08) GHz and 11.157 93(13) GHz, with the increased precisions of about 43% and 25%, respectively, compared to our first measurements with antiprotonic <sup>3</sup>He (Friedreich <em>et al</em> 2011 <em>Phys. Lett.</em> B <strong>700</strong> 1–6). They are less than 0.5 MHz higher with respect to the most recent theoretical values, still within their estimated errors. Although the experimental uncertainty for the difference of 0.032 45(15) GHz between these frequencies is large as compared to that of theory, its measured value also agrees with theoretical calculations. The rates for collisions between antiprotonic helium and helium atoms have been assessed through comparison with simulations, resulting in an elastic collision rate of γ<sub><em>e</em></sub> = 3.41 ± 0.62 MHz and an inelastic collision rate of γ<sub><em>i</em></sub> = 0.51 ± 0.07 MHz.</p>