One of the things we have to worry about when designing electronic systems is the effect of radiation. Sad to relate, a lot of new designers really don’t understand as much about radiation as one might hope, so I thought it might be useful to provide a brief overview.
The first point to note is that radiation comes in two forms: electromagnetic waves and subatomic particles. We will focus on the former in this column.
In physics, electromagnetic radiation (EM radiation or EMR) refers to waves of an electromagnetic field propagating (radiating) through space carrying electromagnetic radiant energy. The force carrier for the electromagnetic force is the photon, which carries energy proportional to the radiation frequency. Although the photon is a type of elementary particle, it has zero rest mass.
Classically, electromagnetic radiation consists of electromagnetic waves, which are synchronized oscillations of electric and magnetic fields. In homogeneous, isotropic media, the oscillations of the two fields are perpendicular to each other and perpendicular to the direction of energy and wave propagation, thereby forming a transverse wave.
Electromagnetic waves are emitted by electrically charged particles undergoing acceleration, and these waves can subsequently interact with other charged particles, exerting force on them. EM waves carry energy, momentum, and angular momentum away from their source particle and can impart those quantities to matter with which they interact.
The term “electromagnetic spectrum” refers to the range of frequencies (the spectrum) of electromagnetic radiation and their respective wavelengths and photon energies. This spectrum covers electromagnetic waves with frequencies ranging from below one hertz to above 10²⁵ hertz, corresponding to wavelengths from thousands of kilometers down to a fraction of the size of an atomic nucleus. The limit for long wavelengths is the size of the universe itself, while it is thought that the short wavelength limit is in the vicinity of the Planck length.
Broadly speaking, we tend to divide the electromagnetic spectrum into seven major bands: radio waves, microwaves, infrared (IR), visible light, ultraviolet (UV), X-rays, and gamma rays. Each of these bands can be further subdivided. In the case of infrared, for example, we might talk about near infrared (NIR), mid infrared (MIR), and far infrared (FIR), where near infrared is closest to red in the visible spectrum. Similarly, ultraviolet can be classified as near ultraviolet (NUV) and extreme ultraviolet (EUV), where near ultraviolet is closest to violet in the visible spectrum. (Scientists subdivide the ultraviolet spectrum in a different way — speaking of UV-A, UV-B, and UV-C — when discussing the impact of UV radiation on the environment and human health.)
At some point that we typically class as the boundary between NUV and EUV, we move into the realm of ionizing radiation, in which photons carries sufficient energy to strip electrons away from any atoms or molecules with which they interact, thereby “ionizing” them; that is, leaving them as positive ions.
Now, you may think that it’s only ionizing radiation that causes problems in electronic systems, but you’d be wrong. The term electromagnetic compatibility (EMC) refers to the ability of electrical equipment and systems to function acceptably in their electromagnetic environment by limiting the unintentional generation, propagation, and reception of electromagnetic energy, which may cause unwanted effects such as electromagnetic interference (EMI) or even physical damage in operational equipment.
A stray electromagnetic wave may be picked up and amplified, subsequently causing all sorts of mischief. Radio waves, for example, can cause a wide variety of problems, which is why it’s so difficult to augment legacy safety-critical installations like nuclear power stations with modern wireless solutions (see Chirp Unlocks the Power of Sound).
As one example, I remember hearing the story about a big oil refinery in which an electronically controlled valve mysteriously activated at around the same time every night. When investigators arrived at the scene, nothing appeared to be untoward. The system managers were close to pulling their hair out before the cause of the problem was discovered — a security guard pausing at the same spot every night to smoke a cigarette and use his walky-talky to call into the security office to say, “All’s well” (it was ironic that this simple greeting caused things to become unwell).
In my next column, we will consider the effects of radiation in the form of subatomic particles. Until then, I welcome your comments and questions.