In my last post, I described an array of tests that crystal oscillators must undergo, regardless of their application. In this post, I will discuss the radiation testing that must be undertaken in extraterrestrial applications.
Defining the Spacescape
To begin with, the kinds of radiation a part will be exposed to varies by type of space environment, particularly the orbital level. The Spacescape is divided into four regions:
- Many satellites have historically been located in Geosynchronous orbits(GEO), where a satellite can be continuously over one spot on earth due to its speed being equal to the earth’s rotational speed. These GEO orbits are about 36,000 km above sea level.
- There are also Middle earth orbits (MEO), of about 20,000 km, used for navigational satellites such as GPS.
- In recent times. Low earth orbit (LEO) orbits of about 100 km have become more common. This region is inhabited by thousands of small satellites, launched in clusters, that are doing a host of navigation and communications applications
- And then there are scientific deep space and planetary missions.
All of these space environments have unique radiation environments. There are no exact ways to duplicate the radiation exposure of each space environment, rather the best that can be done is to create analog radiation environments that can simulate the actual environments in space.
Defining Radiation Categories
While the number of ways these analog environments can be created is quite large, most of them fall into one of 4 different categories. Even within a given category, the levels of radiation produced can vary greatly, depending upon requirements and capabilities. The four categories of radiation testing are:
- Total Ionizing Dosage (TID) testing using gamma rays produced (usually) from radioactive cobalt.
- Dose Rate tests generated using Flash X-Rays or particles from a linear accelerator (LINAC).
- Heavy Ion testing using a linear accelerator.
- Neutron Bombardment from a fast neutron irradiation facility.
Radiation Testing Details
Total Ionizing Dose Testing
TID testing using gamma rays is the most common radiation test. Using cobalt to produce the gamma rays is less expensive than the other types of tests, and can adequately simulate the exposure over time in satellites. One caveat is that the TID type radiation exposure in space is at very low rate and that slow of an exposure rate cannot be entirely simulated on earth because it would take too long. For some types of electronic components, (BiCMOS in particular) it is therefore necessary to do what is called ELDRS testing, Extremely Low Dose Rate Radiation, but even then the ELDRS test is still faster than the actual exposure rate in space, which can complicate test results and analyses.
Dose Rate Testing
A second kind of radiation test uses flash X-Rays to do what’s called dose rate testing, which uses the very fast release of high energy X-Rays. The flash X Rays are used to expose the parts under test quicky to a very high dose rate of high energy of much higher energy levels than are used in TID tests, and which simulate the exposure in space to very high energy events. The actual energy and dose rate is always high but can vary a lot depending on the requirements. The specifications tested are called Single Event (SE) requirements and possible issues can come in many different varieties, from a very short “upset” that self-recovers, all the way up to ”Latchup” where the part under test dies permanently.
Heavy Ion Tests
Next, there are heavy Ion tests, where parts are bombarded with protons or heavier ions that impart extremely high energy doses to the parts under test. Heavy Ions from LINACs can impart very high energy doses in a very short time, to test high level single events, or even what is called a Prompt Dose, which usually simulates an exposure to a nuclear bomb.
Neutron Bombardment
Very high energy single event and prompt doses can also be simulated by neutron bombardment, with the difference (from Heavy Ion exposure) being that neutrons are electrically neutral so they can result in different effects than exposure to charged ions.
Testing Requires Expertise
All in all, radiation testing is a very complex and custom field, and must be planned, specified, and analyzed by a qualified radiation scientist. Also, radiation testing, to simulate the radiation in many different space environments, is a rapidly evolving field with changing methodologies and requirements being continuously developed.
