Electronic Designs for Space: Radiation

Nuvation recently completed a complex data acquisition design for a space application.  The project consisted of an FPGA design with a soft-core ARM processor, and large banks of non-volatile data storage memory. This would be a fairly involved project for a terrestrial application, but the fact that it was for space imposed some significant constraints on the electronic design.

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One of the most important concerns is the radiation tolerance of the electronics.  Radiation can have a severe impact on electronic devices, with effects like digital circuits with flipping states, and spikes and transients in analog circuit outputs.  The problem is complicated by the fact that these effects can occur randomly at any time and may even destroy poorly protected devices.  Careful consideration had to be given to component selection, FPGA design, circuit design and firmware design, as well as chassis design and shielding, in order to assure that the system would perform reliably and meet its performance characteristics when subjected to the radiation environment of outer space.

Of course, radiation-tolerant design isn’t only necessary for space applications; the same principles and design challenges apply to any electronic system that is intended to operate in a radiation-prone environment, whether in space or on earth.  For example, electronics for high-altitude flight, nuclear reactors, particle accelerators, and military equipment that must withstand radiation from a nuclear explosion all require designs considerations similar to those for outer space.

In this series we’ll examine the types of radiation present in space, their effect on electronic components, and design techniques we use to mitigate the radiation effects.

Types of Radiation

The term ‘radiation’ refers to energy or matter moving through space, and can either occur as electromagnetic radiation or particle radiation.  Radiation is further classified into two categories: ionizing radiation and non-ionizing radiation.  Ionizing radiation is radiation with sufficient energy to knock electrons out of atomic orbits, which can change the properties of materials and the behavior of semiconductor devices.  Examples of such radiation include x-rays, gamma-rays, and high energy particle radiation.  Ionizing and non-ionizing radiation have different effects on electronics and require distinct design considerations.

Electromagnetic Radiation

Electromagnetic radiation is the transfer of energy via massless (when at rest) particles called photons, traveling at the speed of light.  Electromagnetic radiation is classified in terms of wavelength as shown in the diagram below.  A lot of the radiation present in the electromagnetic spectrum is harmless to both humans and electronics (for example, visible light).

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The Electromagnetic Spectrum

 

The energy of a photon is given by the Planck-Einstein equation:

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Where h is Planck’s constant, c is the speed of light, is the frequency and  is the wavelength.  Therefore, the energy of the electromagnetic wave is proportional to its frequency and inversely proportional to its wavelength; the higher the frequency, the higher the energy.  The high energy electromagnetic radiation in the ultraviolet, x-ray and gamma-ray spectrums pose the most danger to humans and electronics.

Electromagnetic radiation is emitted by various sources.  For example, in space, x-rays are emitted by astrophysical objects such as galaxy clusters, black holes, and stars. Gamma rays, on the other hand, are predominantly emitted as a result of radioactive decay processes such as those found in nuclear explosions, nuclear reactors, and various astronomical processes.

Particle Radiation

As opposed to electromagnetic radiation, which is composed of particles that have no intrinsic mass (photons), particle radiation is composed of fast moving sub-atomic particles that do have intrinsic mass, such as protons, electrons, and nuclei of atoms.  These particles can travel at speeds of hundreds of thousands of kilometers per hour and some close to the speed of light.  Common examples of particle radiation are alpha particles (helium nuclei) and beta particles (fast-moving electrons).  Particle radiation can result from radioactive decay or some other form of a nuclear reaction.  Cosmic Rays are a form of particle radiation that pose some of the greatest risk to electronics operating in space.  Cosmic rays are predominantly composed of very high-energy protons that mainly originate outside the Solar System.  Although the particles that make up this type of radiation are extremely light, their speed results in large kinetic energies that can damage materials and human tissue on impact.  In addition, some of these particles have great penetrating power, which makes protection via shielding more challenging.

In the next post we’ll look at the radiation environment on earth and in space!