Experimental study of the microwave emission from electrons in air

We searched for the emission of microwave radiation in the Ku band generated by a 95 keV electron beam in air. We unequivocally detected the radiation, and measured its yield and angular dependence. Both the emitted power and its angular pattern are well described by a model, where microwave photons are generated via bremsstrahlung in the free-electron atomic-nucleus collisions, during the slowdown of the electrons. As a consequence, the radiation is not isotropic but peaked in the forward direction. The emission yield scales proportionally with the number of electrons. This contrasts a previous claim that the yield scales with the number squared, due to coherence. With a Monte Carlo simulation we extrapolate our results to the ultra high energy cosmic ray energy range.

Figure 1

A schematic view of the experimental setup (not to scale). The cylindrical plastic vessel is also shown, which is evacuated and used to measure possible spurious signals, and then removed during all other measurements.

Figure 2

Dependence of the microwave signal from the beam intensity, at $\theta =25°$, for the two orientations of the LNB dipole. The best least squares fits of all data are shown with a linear law (continuous line) and with a quadratic law $f(x)=A+B·x^2$ (dashed line).

Figure 3

Angular dependence of the MW emitted power for the two orientations of the LNB dipole, and result of the Monte Carlo simulation (full line). The error bars represent the measurement errors. Data from 70° to 85° could not be taken because the horn hit against the wall of the anechoic chamber.

Figure 4

Results of the Monte Carlo simulation. (a) Dependency of the energy ${\mathcal{E}}_{\mathrm{MW}}$ emitted in MW radiation on the electron energy $T$. The line is the best fit with a straight line ${\mathcal{E}}_{\mathrm{MW}}=a·T$, with $a=1.420\times {10}^{-23}\mathrm{J}/\mathrm{GeV}$. (b) Angular shape of the electromagnetic shower at different energies.