Development and application of a large area positronium annihilation lifetime spectrometer

Positronium (chemical symbol Ps) is a metastable atomic system composed of an electron bound to a positron. Since the positron is the antiparticle of the electron, this arrangement usually comes to a violent end; namely a flash of annihilation gamma radiation, as the electron and positron annihilate each other. This may make it seem as though Ps atoms cannot be studied in any detail, since they are prone to this self-annihilation process. In fact, this does not happen instantaneously, and depends on exactly how the electron and positron interact. The overlap of the positron and electron wavefunctions is actually quite small, and in the longest-lived state the pair can avoid touching (and thus annihilating) for a relatively long time, apprroximately 142 ns (or 0.142 microseconds). This is not what we humans think is a long time, but for an atom it is, and as a result it is possible to study the properties of Ps atoms before they disappear. This is fortunate, because Ps is a unique atomic system in that its properties embody the simplicity of single-electron (hydrogenic) systems, but also include complicated QED phenomena, such as real and virtual annihilation. A striking example of this is the ground state hyperfine splitting; in hydrogen this energy splitting (1.4 GHz, this is also what gives rise to the famous "21 cm line" used in astronomy) occurs because of the interaction of the electron and proton spins. The same physics occurs in positronium but produces a much larger effect, on the order of 100 GHz, because the positron magnetic moment is much larger than that of the proton. However, the actual hyperfine interval in Ps is closer to 200 GHz, with the additional splitting arising through virtual annihilation effects. Thus, QED processes can have a very profound impact on Ps properties, and therefore precision studies of Ps can be used to test quantum electrodynamics (QED) theory. Such tests can be conducted via optical or microwave spectroscopy, but here we wish to measure the annihilation lifetime with high precision, as this is also something that can be calculated very precisely. To do this we plan to build a large area positronium annihilation lifetime spectrometer that will use a beam of positrons to generate a beam of Ps atoms. By keeping these atoms away from any material objects we can measure how long it takes them to decay via self-annihilation using some large detector arrays. It is expected that we can improve on previous Ps decay measurements using new technological developments, such as trap based positron beams and laser-produced Ps atoms. Moreover, the large detector system will open the door to other measurements that make use of freely moving atoms. Often these atoms can move away from standard gamma-ray detectors and this prevents efficient detection of long-lived atoms (such as atoms in excited states). The new system will allow these kinds of atoms to be probed, not just for lifetime measurements, but also for other types of spectroscopy using lasers or microwave radiation.