Radiation, Electromagnetic Radiation Injury

An important facet of a forensic investigation is the determination of the cause of the injury or death. This may not always be self-evident, since some causes of trauma do not leave readily apparent external clues. This is especially so when the harmful agent originates at some distance from the scene. One example is electromagnetic radiation.

Any nuclear explosion 25 miles (40 km) or higher above the ground produces a high-altitude electro-magnetic pulse (HEMP), a short-lived, overlapping series of intense radio waves that blanket a large swath of ground. Electromagnetic bombs have also been developed and tested.

These radio waves can induce electrical currents in metallic objects and so cause damage to electrical and electronic equipment, including electrical power grids, telephone networks, radios, and computers. Since the basis of human physiology is the transmission of electrical impulses, disruption of the passage of currents in the body can have debilitating or even dire consequences to cardiac and neurological functions.

The electromagnetic pulse from a nuclear explosion consists of a series of overlapping radio pulses. When a nuclear weapon detonates, large numbers of gamma rays (high-energy photons with wavelengths less than .1 nm) radiate outward from the burst point. Many of these collide with atoms in the Earth's atmosphere, knocking electrons free. These free electrons are created almost simultaneously in a large volume of the atmosphere surrounding the explosion, and travel rapidly away from the burst point in all directions. Because any charged particle crossing magnetic field lines experiences a force at right angles to its direction of motion, the Earth's magnetic field forces these electrons to follow curved paths. Because charged particles following curved paths emit electromagnetic waves (synchrotron radiation), the explosion-liberated electrons spiraling through the Earth's magnetic field emit a strong radio pulse. Additional pulses, of longer duration but lower magnitude, are subsequently caused by scattered neutrons and gamma rays (radiation that has made one or more bounces, rather than following a straight radial path from the burst point) and by the expansion and ascent of the ionized nuclear fireball through the Earth's magnetic field. The electromagnetic pulse caused by the latter effect, termed the magnetohydrodynamic EMP or HD-EMP, is of low intensity but long duration, and is thought to be a particular threat to power transmission lines.

Two other forms of electromagnetic pulse may be caused by nuclear explosions. The first is generated inside electronic devices by the passage of ionizing radiation (e.g., neutrons and gamma rays) directly into metallic cases, circuit boards, semiconductor chips, and other components, where it can cause brief electrical currents to flow by knocking electrons loose from atoms. This effect is termed systems-generated electromagnetic pulse (SGEMP). The other form of EMP—source-region EMP or SREMP—occurs when a nuclear weapon explodes at low altitude. In this situation, a highly asymmetric electric field is produced in the vicinity of the burst (e.g., within a radius of 3–8 km) having intensities that are much greater than those produced by the high-altitude electromagnetic emission.

see also Electrical injury and death; Electromagnetic weapons, biochemical effects.