Analog Cherenkov detectors used in laser spectroscopy experiments on antiprotonic helium

We describe some acrylic Cherenkov detectors read out by gateable fine-mesh photomultipliers, used in laser spectroscopy experiments of metastable antiprotonic helium (p̄He+) atoms carried out at the LEAR and AD facilities at CERN. The atoms were produced by stopping pulsed antiproton beams in a helium target. Charged particles emerging from the antiproton annihilations produced Cherenkov light in the detector, the time envelope of which consisted of a strong flash from the promptly-annihilating antiprotons, followed by a much longer but less intense tail from the delayed annihilations of the metastable atoms. The photomultiplier was turned off during the initial light flash by reversing the electric potential on its dynodes, thus allowing only the delayed annihilations to be recorded as an analog pulse. The atoms were irradiated with a laser pulse tuned to the characteristic wavelength which stimulated antiproton transitions from a metastable state to a state with a short lifetime against annihilation. The resonance condition between the laser beam and the atom was thus revealed as a sharp spike superimposed on the Cherenkov light pulse. We tested Cherenkov radiators of various sizes and different types of photomultipliers in order to (i) suppress the spurious afterpulsing in the photomultiplier signal caused by the strong flash of Cherenkov light during the prompt annihilation, (ii) evaluate the background caused by π+→μ+→e+ decay, and (iii) optimize the linearity, dynamic range, and response time of the detector.