Time-of-flight apparatus for the measurement of slow positronium emitted by nanochannel converters at cryogenic temperatures

Silicon nanochannel plates (NCPs) are efficient positron to positronium converters and are employed as sources of abundant cold positronium. Characterization of the emitted positronium energy spectrum is crucial to assess the NCP performance and support their further optimization. We present here a Time-of-Flight (ToF) system optimized to measure the energy spectrum of positronium produced by NCP converters held at cryogenic temperatures. The ToF-apparatus was tested by implanting 7 keV positrons into NCPs at room temperature before performing ToF experiments using positron implantation energies of 7 and 11 keV into NCPs held at 20K. At these conditions we succeeded in determining the energy spectrum of the positronium escaping from the nanochannels into the vacuum down to an energy of 10 meV.