Airbus Avionics & the Institut Laue-Langevin: working together for safety in the air
Single-event effects (SEEs), which are mainly induced by neutron particles, can be responsible for soft errors. SEEs are taken into account during design phase of Avionics Equipment to avoid impact on reliability and safety. As the functionality of avionic systems increases, aircrafts are embarking more and more advanced microprocessor and semiconductor memory devices. Complex SEE failures have come and gone over the last decades, depending on the materials used in these devices (see text box).
In 2019 Airbus Avionics launched a new project to characterize the risks associated with thermal neutrons particles for avionics equipment.
The SEE qualification for components to be used in aircraft equipment relies on the characterisation of cross-sections (i.e. the probability of interaction, for high-energy neutrons (already well known) and thermal neutrons).
Simulations have found that this thermal neutron environment could be significantly enhanced due to the thermalisation of high-energy neutrons at the time of their collision with the hydrogen atoms in fuel, baggage, or passengers, but the phenomenon is very difficult to predict using models.
Semiconductors and SEEs: developing together
Neutron fluxes at cruising altitudes are about 300 times greater than at sea level, and the interactions between thermal neutrons and the boron-10 (10B) isotope found in semiconductor materials produce an alpha particle and a lithium ion capable of triggering soft errors in avionic systems.
These “single-event effects” (SEEs) have been a concern for avionics since the late 1980s when they first began to appear in aircraft electronic systems; they had only previously been observed in orbiting satellites.
During the late 1990s thermal neutrons became a source of soft error, as borophosphosilicate glass (BPSG) was used in the production of dielectrics found in SRAM and DRAM memories: the soft error rate was suddenly multiplied by eight.
Integrated circuit manufacturers have more recently removed the BPSG, thus reducing thermal-neutron-induced soft error. The issue of thermal neutrons nevertheless reappeared in sub-65 nm memory and logic devices, correlated to 10B contamination in the tungsten plugs or p-type region in semi-conducting elements.
The only way to estimate the real thermal neutron fluxes inside an aircraft is to perform direct measurements during flights at cruising altitude. In 2021 the Institut Laue-Langevin became part of the Airbus Avionics project: it provided thermal neutron detectors for on-board use, while sharing its technical expertise with thermal neutrons detectors. The design, development and implementation of highly advanced neutron detectors is at the heart of the ILL’s activity, as all forty of its cutting-edge scientific instruments require detectors with unique technical specifications.The ILL neutron detector installed in an Airbus aircraft consists of 3He cylindrical proportional counters. There are 24 counters in total, mounted on 2 blocks of 12 counters each. The blocks are connected together and a charge amplifier and multi-channel analyser are used for the read-out. Pulse-height spectra are acquired for ten minutes and recorded on a computer.
More than 30 hours of in-flight tests will be performed at an altitude of 12 km - where the neutron flux is about 300 times greater than at sea level. The first flights took off in November 2021 and the campaign continued through the first half of 2022.
The data recorded provide a precise definition of the thermal neutron environment, thus helping Airbus Avionics assess the impact of thermal neutrons on its new-generation electronics and address the risks for all future Airbus Avionics projects.
A drive up the mountains as proof of concept
Since proportional counters contain thin anode wires at high voltage, they can be sensitive to vibration. It was important to prove that the system aboard an Airbus aircraft would be just as effective at detecting thermal neutron fluxes when being moved or otherwise disturbed.
The ILL is located at the foot of the French Alps, and a simple drive to the nearest ski resort was sufficient to test the robustness of the mobile installation and check that it continued to record the rising intensity of the thermal neutron fluxes, as expected during the 1500m rise in altitude.
This collaboration is the result of over a decade of experiments carried out by the experts at the Institut Laue Langevin, in Grenoble, France for Airbus Avionics R&D. The ILL’s instruments were used to predict thermal neutron risk on state-of-the art semiconductor technologies. These initial measurements allowed the Airbus Avionics Group to get in touch with ILL’s experts in neutron science and technology, thus gaining a better understanding of their skills and services in the field of thermal neutrons.
Both parties are now celebrating the success of the partnership, which has again demonstrated how collaboration between academic and industrial research can bring benefits to both: throughout the project, the industry develops with the help of academic expertise, while creating a new field of application for academic research and know-how.