In this first “lecture” on this important topic, the Professor will define what is meant by radiation, what types of radiation are potential causes of damage and how radiation levels are characterized and quantified.
First, what do we mean by radiation? For our purposes, radiation is the transmission of energy through space in the form of either subatomic particles or electromagnetic waves. Subatomic particles include electrons, neutrons, protons and ions. Just for confusion purposes, electrons are sometimes referred to as beta particles and helium ions as alpha particles. Electromagnetic waves, in ascending order of frequency and descending order of wavelength include: radio waves, microwaves, infrared, visible light, ultraviolet, x-rays and gamma radiation.
The kinds of radiation we are concerned with are called Ionizing Radiation because they carry sufficient energy to damage electronic components by ionizing atoms or molecules by detaching electrons from them (thereby making the atom or molecule in question electrically charged). Considering electromagnetic waves, the energy carried by EM waves increases as the frequency of the wave increases (or as the wavelength decreases). Lower frequency waves such as visible light, microwaves and radio waves do not normally carry enough energy to damage electronic circuits (or living beings) but as the frequency goes up, X-Rays and especially gamma rays, can and do cause damage. Keep in mind that the electromagnetic waves can also be described as made up of particles called photons. A photon of visible light carries a certain amount of energy that is not problematic, but a gamma ray photon carries a much higher energy level that is problematic. Any radiated particles with actual mass (photons are massless), such as electrons, protons, neutrons and ions all carry enough energy to be a problem.
Key Radiation Measurements
Given the importance of the amount of energy involved in exposure to radiation, we need to know how it is characterized and quantified. There are two scales and units of measurement commonly used. First, we have what is called energy fluence, which is used to characterize the amount of exposure over a period of time, which is in units of MeV per centimeter squared, or just MeV (million electron volts). Next is a measurement of the accumulated dosage of radiation (TID) absorbed by a material, which is in units of kRads (kilorads). Don’t worry too much about the details at this point, but it is just useful to know what terms we will be using.
Quartz crystal resonators and crystal oscillators for use in space must take into account the effects of radiation. There are many types of radiation and the analysis of whether any specific electronic component is acceptable for use in any specific space environment and application is detailed and complex.
This series of lectures only outlines the big picture, bird’s eye view of what’s involved and what to watch out for, concerning only one specific component type, crystal oscillators. In my next lecture, I’ll discuss what it takes to produce crystals and oscillators that are suitable for space applications.