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In 1962, Gurgen Askaryan predicted that a high energy shower in a
dense material could emit
a very intense pulse of coherent cherenkov radiation. Several experiments
have been using this effect to instrument the largest volume cosmic
ray detectors. The effect relies on:
We set out to determine if the effect was real.
We used the Final Focus Testbeam at the Stanford Linear Accelerator to make a showers up to 1019 eV. We used a bremsstrahlung beam from 30 GeV electrons giving us ~3 GeV photons. So we could avoid transition radiation from the beam leaving the machine and could work with a shower that would develop its own charge asymmetry.
We dumped the electron beam into a "sandbox" containing 7000 pounds of silica sand. We used an S-band pyramidal "standard gain" horn and other antennas to measure the microwave energy from 200 MHz to 5 GHz
We saw enormous pulses (1 kilovolt/meter at one meter distance) consistent with theory. We also observed the quadratic dependence of emitted power on shower energy. The radiation was polarized and emitted at the cherenkov angle as expected.