Blue glow

Observed effects

Image of a 60-inch cyclotron, circa 1939, showing an external beam of accelerated ions (perhaps protons or deuterons) ionizing the surrounding air and causing an ionized-air glow. Due to the similar mechanism of production, the blue glow is thought to resemble the "blue flash" seen by Harry Daghlian and other witnesses of criticality accidents.

Blue glow

See also: Ionized-air glow

Many criticality accidents have been observed to emit a blue flash of light. The blue glow of a criticality accident results from the fluorescence of the excited ions and atoms (or excited molecules) of air (mostly oxygen and nitrogen) falling back to unexcited states, which produces an abundance of blue light.^{[citation needed]} This is also the reason electrical sparks in air, including lightning, appear electric blue. The smell of ozone was said to be a sign of high ambient radioactivity by Chernobyl liquidators.
This blue flash or "blue glow" is often incorrectly attributed to Cherenkov radiation. It is a coincidence that the color of Cherenkov light and light emitted by ionized air are a very similar blue as their methods of production are different. Cherenkov radiation does occur in air for high energy particles (such as particle showers from cosmic rays)^[32] but not for the lower energy charged particles emitted from nuclear decay. In a nuclear setting, Cherenkov radiation is instead seen in dense media such as water or in a solution such as uranyl nitrate in a reprocessing plant. Cherenkov radiation could also be responsible for the "blue flash" experienced in an excursion due to the intersection of high-intensity gamma radiation with the vitreous body within the eyeballs of those in the presence of the criticality. This would also explain the absence of any record of blue light in video surveillance of the more recent incidents.

Heat effects

Some people reported feeling a "heat wave" during a criticality event.^[33][34] It is not known whether this may be a psychosomatic reaction to the terrifying realization of what has just occurred, or if it is a physical effect of heating (or nonthermal stimulation of heat sensing nerves in the skin) due to energy emitted by the criticality event.
A review of all of the criticality accidents with eyewitness accounts indicates that the heat waves were only observed when the fluorescent blue glow (the non-Cherenkov light, see above) was also observed. This would suggest a possible relationship between the two, and indeed, one can be readily identified. In dense air, over 30% of the emissions lines from nitrogen and oxygen are in the ultraviolet range, and about 45% are in the infrared range. Only about 25% are in the visible range. Since the skin feels infrared light directly as heat, and ultraviolet light is a cause of sunburn, it is likely that this phenomenon can explain the heat wave observations.^[35]