Instrumentation for laser-induced annihilation spectroscopy of metastable antiprotonic helium atoms

We have established a technique for laser resonance spectroscopy of metastable antiprotonic helium atoms (p̄He+) by successfully observing laser-induced annihilation. In this paper, we describe the full instrumentation of the experiment in detail, namely, the particle detectors, the laser system, the cryostat and the helium gas target, the data acquisition and analysis method. For an effective laser triggering selective to 3% metastable antiprotons, a highly efficient detection system for antiproton annihilation was required, with a suppressed inefficiency of less than a percent. Guided by simulations, we designed a system of seven lead-scintillator sandwich counters to detect the charged pions and π0-decay gamma rays from antiproton annihilation, and achieved 99.7±0.1% efficiency for the detection of annihilation. We employed two identical sets of excimer-pumped dye laser systems with 2–5 mJ output energy per pulse, which were triggered by every metastable antiproton formed in a helium gas target of 0.3–1.0 bar at 4.5–10 k. The resonant deexcitation of the metastable states was detected as a spike-like forced annihilation in the time spectrum, thus revealing the level structure of this exotic atom. This powerful technique enabled us to study also the lifetimes and populations of specific metastable states, by changing the timing and the wavelengths of the laser pulses.