Mimicking Lightning in a Dielectric
It has long been known that thunderstorms can generate bursts of gamma rays from activated regions about a hundred meters across. Scientists explain this phenomenon as a positive-feedback cascade in which relativistic electrons emit high-energy photons that liberate more electrons (see Viewpoint: Chance of Thunder—and Gamma-Ray Flashes). Now Victor Pasko of Pennsylvania State University and colleagues have predicted that a similar mechanism can cause high-energy photon emission from centimeter-scale regions in dielectric solids [1]. They suggest that the phenomenon could be used to make compact x-ray sources.
In a thunderstorm, the cascade begins when cosmic rays liberate electrons from atmospheric molecules. These electrons are boosted to relativistic speeds by the storm’s immense electric field. They then slow down over tens or hundreds of meters because of friction-like interactions with the atmosphere, releasing their energy as gamma rays. Some of this radiation propagates back along the electrons’ trajectories, ionizing the atmosphere and injecting more electrons into the emission region.
Pasko and colleagues simulated this process in acrylic, quartz, and bismuth germanate. A 5-MeV electron beam fulfilled the role of the cosmic-ray trigger, while an electric field across the solid stood in for the thunderstorm. The frictional force in these solids is much greater than in the atmosphere, which shrinks the spatial scale of the process. The researchers found that, to sustain the positive-feedback mechanism, the electric field needed to be strong enough to accelerate the electrons to relativistic energies. It also needed to be applied over spatial scales large enough to generate sufficient numbers of feedback photons. They say that, to exploit this process in future x-ray sources, researchers will need to study the nanosecond-time dynamics of the emission and the effect of different material compositions.
–Sophia Chen
Sophia Chen is a freelance science writer based in Columbus, Ohio.
References
- V. P. Pasko et al., “Relativistic feedback discharges in dielectric solids,” Phys. Rev. Lett. 136, 095301 (2026).