Investigation of positron and positronium annihilation for positrons colliding with nitrogen and carbon monoxide using the PsARS method .

摘要

The PsARS method is used to probe nitrogen and carbon monoxide and reveals evidence of resonance annihilation near their initial positronium formation thresholds. The experimental approach uses a positron beam, derived from a radioactive 22Na source, which undergoes scattering from room temperature atoms and molecules in a two chamber high vacuum system. The resulting annihilation signature from a positron scattering event is then detected using PsARS. A computer is used to control the experiment and perform calculations pertinent to the experiment. PsARS has an advantage over other spectroscopy techniques in that it looks at the ratio of two different signals coming from positron annihilation. Coincident pairs of gamma rays within the energy range of 300 to 460 keV consist of the first signal, which primarily come from the annihilation of triplet positronium, and coincident pairs of gamma rays within the energy range of 460 to 560 keV consist of the second signal, which primarily come from the annihilation of singlet positronium as well as other annihilation processes. By looking at trends in the ratio of these two signals verses positron incident energy for these gases near their corresponding initial positronium formation thresholds, we detect an unexpected enhancement of 2gamma annihilation. The onset of the detected enhancement of 2gamma annihilation appears at the targets' initial thresholds for electronic excitation by positrons. It appears that the incident positron is electronically exciting the target molecule, then temporarily binding to the excited target in a resonance-like process. This binding increases the amount of time the positron spends in the vicinity of the target, which leads to an increased chance that the positron can annihilate with one of the target's opposite-spin electrons, resulting in two 511 keV gamma-rays. Resonances were found for both nitrogen and for carbon monoxide at their respective initial thresholds for electronic excitation by positrons.