MRI: Development of a High-Resolution Gamma Ray Spectrometer for Time Resolved Temperature Measurements of Confined Positronium.

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Broad significance. Positronium is an exotic matter-antimatter atom composed of an electron bound to its antiparticle, the positron. Scientists at the University of California Riverside will develop a new type of instrument that will measure the particles of light (photons) that are emitted by the positronium. The instrument (spectrometer) will have capabilities beyond those commercially available and will allow scientists to measure the temperature of large bursts of positronium atoms as a function of time. The new spectrometer will be the enabling technology leading to the production and observation of a dense gas of cold positronium in the "Bose-Einstein condensed state". This unusual state of matter will be a concentrated energy source which may lead to a variety of applications, including a new type of laser. The spectrometer will be used in a variety of applications in collaboration with scientists from around the world who need to measure radiation damage on short times scales, obtain information about the structure of materials, and measure the densities and temperatures of plasmas. Technical description. The proposed instrument is a high angular resolution, multi-detector two-gamma annihilation angular correlation spectrometer that will measure the velocity distribution of positronium atoms annihilating in the singlet state. The individual detectors will have time and double-pulse resolutions much less than the ~100 ns annihilation lifetime of positronium, enabling the efficient measurement of the temperature and thermalization rates of large bursts of positronium atoms as a function of time after their formation in micro-cavities and porous materials. The primary function of the proposed instrument will be to measure the amplitude of the low momentum component of a dense positronium gas as it undergoes a phase transition into the Bose-Einstein condensed (BEC) state. The use of the penetrating annihilation gamma radiation as a diagnostic allows external access to the momentum profiles of annihilating electron-positron pairs deep within the apparatus required for these experiments. The measurements will be unaffected by local conditions in the positronium-forming target, such as wall or inter-particle interactions, which would not be the case for optical spectroscopy. The combined capabilities of the proposed spectrometer will benefit other positronium applications using dense cold positronium not related to the BEC. In particular, measuring the thermalization rates of positronium would aid in the positronium characterization of porous materials such as zeolites and low-k dielectrics, and the ability to measure low momentum annihilations at short time scales may be useful for evaluating damage to materials used in fusion reactors.