Positronium first experiment

Table 7.1. Summary of the available theoretical and experimental data for the 23Si-23Pj transitions of positronium. Key experiment, (a) Hagena et al. (1993), (b) Hatamian, Conti and Rich (1987) (the first-quoted errors are statistical whilst the second-quoted are systematic) theory, (c) Pachuki and Karshen-boim (1998)...

We report here the details of the first experiment to excite optically Ps from the n=l triplet ground state to the 2 S state[l]. The accuracy of our measurement of the 2 S - l Sj interval is approximately 0.8 ppm. Since the experiment was done with thermal positronium in vacuum, straightforward extension of the now proven technology should allow us to improve the accuracy of this measurement by several orders of magnitude. 

Laser spectroscopy of the 1S-2S transition has been performed by Mills and coworkers at Bell Laboratories (Chu, Mills and Hall, 1984 Fee et al, 1993a, b) following the first excitation of this transition by Chu and Mills (1982). Apart from various technicalities, the main difference between the 1984 and 1993 measurements was that in the latter a pulse created from a tuned 486 nm continuous-wave laser with a Fabry-Perot power build-up cavity, was used to excite the transition by two-photon Doppler-free absorption, followed by photoionization from the 2S level using an intense pulsed YAG laser doubled to 532 nm. Chu, Mills and Hall (1984), however, employed an intense pulsed 486 nm laser to photoionize the positronium directly by three-photon absorption from the ground state in tuning through the resonance. For reasons outlined by Fee et al. (1993b), it was hoped that the use of a continuous-wave laser to excite the transition would lead to a more accurate determination of the frequency interval than the value 1233 607 218.9 10.7 MHz obtained in the pulsed 486 nm laser experiment (after correction by Danzmann, Fee and Chu, 1989, and adjustment consequent on a recalibration of the Te2 reference line by McIntyre and Hansch, 1986). 

We now briefly review experimental evidence for the existence of some simple positronium compounds more detailed accounts have been given for early lifetime experiments by Goldanskii (1968) and for the liquid phase by Mogensen (1995). In the case of PsCl we shall see how traditional positron experiments using lifetime and ACAR techniques have provided strong evidence for the stability of this compound, in accord with theory. The first direct experimental evidence of the existence of PsH came from a positron-beam experiment (Schrader et al, 1992). 

One of the first studies of PsCl was that of Tao (1965), who used a lifetime experiment (subsection 1.3.1) in this experiment positronium was formed in argon or N2 gases to which small quantities of CI2 vapour had been added. The intensity of the long-lived ortho-positronium component was found to decrease as the CI2 concentration was increased,... 

The rate of positronium formation has been increased by 2-3 orders of magnitude using recently developed accelerator based slow positron sources. This opens the possibility of improvements of precision experiments on the Ps atom as well as new experiments on excited states First evidence for enhanced metastable Ps formation is presented and future possibilities are discussed. 

In the following I shall discuss a number of recent laser experiments on two-body systems namely, hydrogen, positron urn and muonic atoms (u He). In describing these experiments I will be introducing several applications of Doppler-free laser spectroscopy and of frequency-doubled tunable radiation. I will spend first a little time on the theory of the hydrogen atom, contrasting it with that of positronium and muonium. I will then make one or two remarks about frequency calibration of Doppler-free spectra and then consider in some detail laser experiments performed in Oxford and Stanford on hydrogen. 

In order to determine the effect of the microstructure of the solution on the beta counting efficiency in toluene -Triton mixtures in a first series of experiments the positron annihilation parameters were determined in toluene - Triton mixtures containing various amounts of water. As can be seen from Eigs. 11 and 12, where the Parameter l2 which is correlated to the formation probability of thermal positronium, is plotted as a function of Triton (in the presence of 0 and 2 % water) concentration, increasing amounts of Triton reduce I2 to a semi-plateau value, while X2, the annihilation rate of the thermal positronium, changes only slightly. A more detailed plot of I2 at lower Triton concentration reveals that I2 remains constant up to 20 mM or 10 mM Triton in solutions containing 0 or 2 % water, respectively. 

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