To make decision if some electronic device may be sent to space, how to protect it against space radiation and how it may affect execution of our software, we need to have a set of quantities which describe the device susceptibility to radiation.
Description of radiation environment
Space radiation is a sum of different particles which moves very fast, comes from different sources and carry different energies. To describe structure of radiation we use models which presents spectrum of fluxes of particles and their energy.
Flux and fluence of particles
Flux is a measure which tells us how densely the space is filled with moving particles. We can imagine that we put a square frame in space and starting to count particles which fly through the frame. The intense of appearing particles inside the frame is the flux – number of particles flying through the frame per second. It is usually given in units .
Fluence is a total number of particles which have flayed through the frame in given time, its unit is .
Resistance for Total Ionization Dose (TID)
During exposition on radiation semiconductors absorbs energy. There is a level of the absorbed energy (TID) which cause permanent failure of the device. TID is defined as an energy absorbed by unit of the material’s mass during its exposition for radiation. The most common unit is rad .
Linear Energy Transfer (LET)
When a heavy ion hits an electronic device, then it loses some energy to ionization while it traverse the device’s matter. Lose of the energy depends on heavy ion and on the type of hit material. The ion’s lost energy is responsible for parasitic events in the lattice structure. To describe how the radiation particle lose energy in the matter we use quantity named Linear Energy Transfer (in short LET). LET is simply defined as a portion of lost energy per distance on which it happen:. Newton is not convenient unit to describe LET, first of all it is better to describe lost energy in eV, to be more specific in MeV, what will better corresponds with description of ionization radiation and its particles energy. Secondary, when we want to compare different LET levels on the same material it is more practical to use surface field under radiation rays and the mass of the material in which the particle lost its energy, thus helps to better describe radiation treatment during experiments on devices. We can express length of the partition path in the material using density as follows:
Because density is constant for the material we can eliminate it:
Higher LET means that the material good stops the radiation particles, but then larger amount of absorbed energy may generate parasitic effects. LET for different materials and heavy ions can be computed by calculator like this one.
Cross section is a general quantity which describe device susceptibility for radiation, it is defined as a ratio of probability of event occurrence to fluence of particles:
SEU cross section
To describe how the digital device is sensitive for Single Events Upsets, the SEU cross section is used. If during experiments we treat a device with some fluence of particles, and we detect some number of bits affected with SEU, then ratio of number of affected bits to the fluence is named SEU cross section . We can present the SEU cross section as a value per device. If we know how many memories are included in the device, then we can divide cross section per device by number of bits inside the device to compute SEU cross section per bit .
It was assumed than cross section characteristic is taken for fluence equals to , what implicates sampling rates during experiments.
For different type of particles the device my response differently, thus experiments with different flux of different types of particles are required to find cross sections for each of them. Especially separated tests for neutrons and protons are required, since the response of chips for those particles is different for energy less than 50MeV.
During experiments on electronic chips the researchers treat the device with different flux of particles and measure cross sections. For experiments the flux of particles is prepared to cause expected LET in the device material. As a result of experiments we got characteristic of LET levels vs cross sections.
As we can see on the example above, for small LET levels the SEU cross section tends to be 0, it means those levels do not induce effects on the device. The smallest LET level which cause an effect is a crucial element of the device characteristic and is named “LET threshold()”. In practice it is hard to make experiments which threat the device with continuous full range of LET level from 0 to hundreds of , so the threshold is computed with extrapolation of characteristic taken during experiment. For example above we can imagine that the curve will reach cross section 0 for LET about 1
SEFI, SEL and other kind of
Apart of study SEU, we can also investigate other kinds of SEE. Similar characteristics may be prepared for SEL or SEFI, and different may be extrapolated for different kind of events.
Decision about suitability a device to a space mission
We can use to make assumption about susceptibility of the device for radiation which is present in the area of the space, in which the spacecraft will work. NASA divides levels of for three ranges:
|–||Device Threshold||Environment to be Assessed|
|1||Cosmic Ray, Trapped Protons, Solar Proton Events|
|2||Galactic Cosmic Ray Heavy Ions, Solar Heavy Ions|
|3||No analysis required|
The first group is a set of devices which are very sensitive for radiation, they cannot be considered to use outside the protection of Earth’s magnetosphere, moreover it must be checked how much they will suffer for SEE even on relatively benign environment of LEO orbits. The second group is immune for small energy particles, but if the device will be use far away from Earth we need to study its susceptibility for effects caused by heavy ions. The third group contains devices which are immune for the radiation.
Soft error rate (SER)
When exposition on radiation cause change of logical state of logic elements like latches, SRAM cells, or gates, then the logical error occurs. Because this kind of error is not destructive for the circuit it is named soft error.
Soft error rate (also known as SEU rate) is a quantity which describe chip response to a particular type of radiation environment, so for the same device it is different for different location (eg. terrestrial vs LEO orbit).
The are two common units of soft error rate:
- FIT – Failures In Time, which is a number of events per one billion of hours
- MTBF – Mean Time Between Failures in hours
Soft error rate must be predicted for the device and environment in which it will work. There are models which helps to predict SER based on space radiation structure and characteristic of the device. At the moment the best model is CREME96 – The Cosmic Ray Effects on Micro-Electronics (1996 Revision). CREME96 is a set of computer programs to creating numerical models of the ionizing radiation environment in near-Earth orbits, evaluating the resulting radiation effects on electronic systems in spacecraft and in high-altitude aircraft and estimating the high LET radiation environment within manned spacecraft. It requires to known cross section vs LET characteristics of the devices. CREME96 is available by www here.
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