Development of a second generation experiment to measure the 2(3)S(1) transition to 2(3)P(J) intervals in positronium.

Abstract

Positronium has proven to be a rich testing ground for Quantum Electrodynamics over the last half of the twentieth century. The 2<super>3</super>S₁ &rarr; 2<super>3</super>P_J intervals in positronium have previously been measured and disagreements currently exist amongst various experiments and between some experiments and theoretical predictions. Furthermore, recent advances in theory have produced predictions with much higher precision than the current experiments. Therefore, work has been done to develop a 2<super> nd</super> generation experiment to measure the 2<super>3</super>S₁ &rarr; 2<super>3</super>P_J intervals in positronium. Previous experiments used the enhancement of the Lyman-a radiation from the 2<super>3</super>P_J states in order to observe the 2<super>3</super>S₁ &rarr; 2<super>3</super>P_J resonance line. In the 2<super>nd</super> generation experiment the 2<super>3</super>S₁ positronium is observed by photo-ionizing the positronium and detecting the photo-positron. This signal is then depleted when microwave radiation is applied on resonance as compared to the signal off resonance. The ultimate goal of the 2<super>nd</super> generation experiment is to improve the precision of the measurement of the 2<super> 3</super>S₁ &rarr; 2<super>3</super>P₂ interval from 1.5 MHz to 100 kHz. This dissertation describes the current status of the development of this 2<super>nd</super> generation experiment. This includes the development of all of the major subsystems necessary for detecting the 2<super>3</super>S₁ &rarr; 2<super>3</super>P_J transition, including the accumulated positron beam, the pulsed laser beam, the microwave delivery apparatus, and the interaction region. Results are presented showing the first photo-positron signal using an accumulated positron beam. The rate of this signal is approximately 100 mHz as compared to 15 mHz in the 1<super>st</super> generation experiments. This rate is sufficient to produce a 2<super>3</super>S₁ &rarr; 2<super>3</super>P₂ interval measurement with precision of 3 MHz with 8 hours of data acquisition and 600 kHz after 30 days. Potential improvements to the technique are suggested to allow for a 100 kHz measurement. Additionally, results are presented for Doppler broadening measurements of thermalization of positronium in gases. The thermalization rate is measured in He, Ne, Ar, N₂, H₂, Isobutane, and Neopentane. These rates were found to be much slower than previously thought. Important implications on experimental measurements of both the 1<super>3</super>S₁ decay rate and the 1<super>3</super>S₁-1<super>1</super>S₀ interval are discussed.