Positron Cyclotron Excitation

The first evidence of positron trapping was obtained through microwave excitation of the positron cyclotron resonance near 166 GHz. Waveguides carry the microwave radiation into the magnet bore close to the trap center. The microwaves heated the positrons by increasing their cyclotron energy. Through the Coulomb interaction the positrons then increased the 9Be+ ion energy which changed the level of the 9Be+ ion resonance fluorescence.

Figure 3 (a) shows a resonance curve of the detected fluorescence at 313 nm while the microwave frequency was stepped near the positron cyclotron frequency. We believe the ~200 kHz resonance width was probably caused by power broadening, because a significant positron excitation appeared to be required to observe the resonance. First, there was a sharp threshold in the microwave power required to observe the resonance. Second, an increase of a few dB in the applied microwave power above this threshold was sufficient to rapidly annihilate the positrons, presumably through excitation of the positrons to a least a few volts energy where positronium formation with background gas atoms could take place. The significant excitation of the positron cyclotron motion was probably necessary because of the weak coupling between the positron cyclotron and 9Be+ ion motions, and the low rate of energy transfer between the positron cyclotron and axial energies in the high magnetic field of our trap [29]. Other potential sources of broadening include the relativistic mass shift (~10 kHz for each 300 K in energy), first-order Doppler broadening from positron motion within the trap, and magnetic field instability and inhomogeneity. Sections 3.2 and 3.3 show that, when cold, the positrons were typically confined in the Lamb-Dicke limit where first-order Doppler broadening occurs as side-bands. We did not observe any axial or rotational side-bands. Finally the measured magnetic field instability and inhomogeneity were too small to produce the observed broadening.

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