Speaker
Description
We present the formation of positronium (Ps) through charge-exchange of magnetically confined positrons and background gas in the laboratory. A bunch of 10⁵ positrons is extracted from a buffer-gas trap and injected into the dipole field of a permanent magnet trap with E×B drifts [1]. Once injected, the positrons are confined through a combination of magnetic mirroring and electrostatic reflection off the biased magnet. A 21-BGO detector array situated in re-entrant ports 1cm from the confinement volume detects ~10,000 gammas per shot. FPGA processing timestamps detections to 8ns accuracy and records photon energy with 66keV resolution. The time evolution of the 511keV peak-to-valley ratio identifies Ps-formation through charge-exchange of the ~7eV positrons with impurities (H2O, O2, …) in the ~5·10-6 Pa vacuum as the dominant loss mechanism during the first ~500ms of confinement. An increase in the positron kinetic energy, achieved by reducing the electrostatic bias, leads to an increase in charge-exchange and associated triple coincidence detection. Inelastic collisions cool the positrons, enabling a ramp of the magnet bias to negative voltages to pull them into the loss cone. This cooling below the Ps-formation energy threshold cuts off the charge-exchange and allows transport to the wall through elastic collisions to become the dominant loss mechanism. A fraction of positrons created in solar flares will form positronium through charge-exchange. The resulting annihilation spectra depend on the traversed media’s constituents, their density and ionization fraction [2]. In the future, plasma relevant to solar environments could be generated in the dipole field prior to the injection of positrons so that the interactions with Ps and the effect on the flux ratio of the 511keV of 2-gamma line to the 3-gamma continuum could be studied.
[1] Stenson, E. V., Nißl, S., Hergenhahn, U., Horn-Stanja, J., Singer, M., Saitoh, H., Pedersen, T. S., Danielson, J. R., Stoneking, M. R., Dickmann, M., & Hugenschmidt, C. (2018). Lossless Positron Injection into a Magnetic Dipole Trap. Physical Review Letters, 121(23).
[2] Murphy, R. J., Share, G. H., Skibo, J. G., & Kozlovsky, B. (2005). The Physics of Positron Annihilation in the Solar Atmosphere. The Astrophysical Journal Supplement Series, 161(2), 495–519.
We gratefully acknowledge the support of the Advanced Beam Measurement Group at the National Institute of Advanced Industrial Science and Technology (AIST) in Japan.
The APEX collaboration receives/has received support from IPP/MPG; the European Research Council (ERC) under the European Union's Horizon 2020 research and innovation programme; the Helmholtz Association; the Deutsche Forschungsgemeinschaft (DFG); the Alexander von Humboldt Foundation, the UC San Diego Foundation; the United States Department of Energy, the Japan Society for the Promotion of Science (JSPS); and the National Institute for Fusion Science (NIFS).
