Radiation testing of a commercial 6-axis MEMS inertial navigation unit at ENEA Frascati proton linear accelerator
Introduction
Small satellite space missions are an important asset and their relevance is increasing in recent years, not only for educational purposes, but also as technological demonstrators, and for realizing constellations of a huge number of nanosatellites for communication and internet providers (Williams et al., 2018, Lal et al., 2017, Panga et al., 2016). This kind of missions would greatly benefit from the use of commercial components.
MEMS technology has made a wide variety of cheap, lightweight sensors for attitude determination and control available on the market, often combining multiple axes and multiple sensors on a single chip and providing digital IO interface through the embedded Application Specific Integrated Circuit (ASIC). These devices have been successfully used as sensors for attitude determination in University Cubesat missions. As an example, the Radio Aurora Explorer (RAX) is a CubeSat mission developed to study space weather in Earth’s ionosphere (Klesh et al., 2009, Springmann et al., 2011) which employed a COTS MEMS Inertial Measurement Unit (IMU) by Analog Devices.
While COTS parts are an attractive alternative to Rad-Hard space qualified components, due to their lower cost and shorter procurement time, their proper qualification following standard RHA protocols would be so expensive and time consuming as to eventually overcome the benefits. As highlighted in Gaillard et al. (2009), radiation resistance characterization of MEMS devices poses the additional challenge of probing both electrical and mechanical domain, further increasing the complexity of the procedure. As indicated in previous studies (Knudson, 1996), the specific architecture of the mechanical part of the sensor can lead to very different outcome in radiation resistance.
The burden of RHA on COTS components cannot be sustained in particular on the typical limited resources of Universities’ SmallSats projects, which is the framework of our research. Many authors in recent years have proposed novel RHA strategies, especially tailored for COTS parts and Low Earth Orbit (LEO), thin-shielded missions (García Alía et al., 2017, Rousselet et al., 2016, Sainclair and Dyer, 2013). These novel, simplified testing strategies also take into account the growing complexity of components (Di Mascio et al., 2018), which hinders the applications of standard testing guidelines, especially, as is often the case, when details of the device architecture are note made available by commercial components manufacturer.
Proton testing, probing simultaneously cumulative and stochastic effects, could provide a trade-off between rigorous, but expensive, standard qualification process and usage of untested parts with unpredictable fault modes. Additionally, protons as a radiation source are representative of the trapped particles environment that the device could actually be subjected to in LEO orbits, the most common for this kind of satellites.
Bryn Pitt et al. (2017) investigated the radiation response of a commercial 3-axes MEMS accelerometer to be used in a robotic system deployed in extreme radiation environments. The research aimed at highlighting the system performance degradation with increasing level of total ionizing dose due to gamma irradiation. Even without proper understating of the device-level degradation or failure mechanism, based on the irradiation data the authors could provide a mitigation strategy that increased the system useful operating time.
In this contribution we report a generalization of such a methodology with proton beams to evaluate the radiation response of a commercial 6-axis integrated inertial navigation system (accelerometer, gyroscope) LSM6DS33 of CMOS/MEMS technology manufactured by STMicroelectronics. The use of proton beams allows to investigate in the same session combined TID and DD degradation; additionally, qualitative and quantitative analysis of non-destructive SEE at a system level (namely Single Event Functional Interrupt) can be investigated and, possibly, destructive SEE screening. As far as the beam energy is concerned, higher energy protons of a few hundreds of MeV are more efficient in inducing SEEs through indirect ionization; conversely, protons of a few tens of MeV, thanks to their higher Linear Energy Transfer (LET) and hardness factor, are more effective in probing cumulative damage. In the present investigation, the choice of beam energy is limited by the current availability of the source. The procedure combines both offline measurements, to analyze the effect of cumulative degradation before and after the radiation exposure in terms of accelerometer calibration change, and online testing through I2C acquisition of the sensors parameters during the irradiation. Information thus collected allow component screening based on mission parameters: minimum required TID/DD resistance and acceptable frequency of SEFI (as an order of magnitude); additionally, acquisition of the online behavior of the component allows implementation and test of a fault identification and recovery strategy that could improve the component reliability.
The proton beam is provided by the TOP-IMPLART linear accelerator currently under development at ENEA Frascati Research Center: it is a pulsed fully linear machine aimed at active intensity modulated proton therapy with a final energy of 150 MeV. Presently the machine offers a beam extraction point on the horizontal line at 35 MeV, with a current up to 50 µA in 3 µs long pulses and maximum repetition frequency of 50 Hz. An in-air irradiation set-up 1.8 m downstream the accelerator exit window with an effective beam energy of about 30 MeV, ±5% transverse homogeneity on a 40 mm diameter beam spot and intensity repeatability within ±5% was employed in the experimental campaign.
While the proton beam main characteristics (energy, intensity, average fluence) are the same of conventional cyclotron-based cancer treatment facilities, already used for space components qualification, its time structure is significantly different, with high instantaneous dose rate and flux.
Proton beams produced by particle accelerators are, together with heavy ions, the main radiation source in standard radiation hardness assurance protocols for single event effects in electronic components (ESCC, 2014, JEDEC, 2013). While specialized facilities exist for proton and heavy ions radiation hardness assurance qualification, more often SEE testers are not the primary users of such accelerators.
Beam requirements for proton therapy largely superimpose with standard SEE high energy proton testing recommendations in terms of available particle energies, intensities and transverse homogeneity at the target position. From a beam delivery point of view, dosimetry prescriptions for patient therapy are even more stringent than for electronic equipment, thus resulting in well characterized and accurate irradiation.
While beam time availability for non-medical use is limited, typically on weekends only, the use of such facilities for electronic components testing is growing as more and more proton therapy centers enter operation stage (as tracked by the Particle Therapy Co-Operative Group https://www.ptcog.ch/index.php/facilities-in-operation). This is especially true in the USA where SEE testing relies more and more on proton therapy facilities availability, as highlighted in National Academies of Science, Engineering, and Medicine, (2018).
From the facility point of view, a synergy with the radiation hardness community might be beneficial since maximization of beam time usage is crucial in covering the hefty installation and running costs of the accelerator. A better understanding of the specific needs, methods and strategies of RHA with proton beams in commissioning or even design phase of accelerators for medical application could become increasingly relevant.
The remainder of the paper is organized as follows: in Section 2 the TOP-IMPLART accelerator characteristics and layout are described in details; in Section 3 beam diagnostic equipment is presented; in Section 4 irradiation set-up is described; Section 5 and Section 6 report the experimental data and their analysis. Conclusions and outlook are reported in Section 7.
Section snippets
The TOP-IMPLART accelerator
Proton therapy is an advanced radiotherapy technique. Compared to traditional photon based radiotherapy it allows higher conformation of the tumor volume and healthy tissues dose sparing. Unfortunately, the higher installation and running costs of the facilities have so far limited the availability of proton therapy.
High frequency pulsed LINear ACcelerators (LINAC) have been proposed, in the first nineties of the previous century (Hamm et al., 1991, Nightingale et al., 1992), as possible
Beam monitoring for the TOP-IMPLART LINAC commissioning and operation
Different monitoring devices, both interceptive and non-interceptive, provide online measurements of the beam position and intensity.
For the purposes of this work we will only focus on a set of detectors used for beam characterization at the exit of the last SCDTL module, set-up and operation for irradiation procedures.
An AC Current Transformer (ACCT) is installed after SCDTL4 and operated in air. It is followed by a small, thin Ionization Chamber (IC), a Faraday Cup (FC) and a fluorescent
Irradiation experiments with the 35 MeV proton beam
The intrinsic modularity of a LINAC accelerator allows experimental irradiation activities to complement the machine commissioning at different energy levels as new accelerating modules enter operation stage. Experimental activity is mainly focused on the development of a dedicated dose monitoring system, tailored to the high instantaneous dose rate of the TOP-IMPLART accelerator, and characterization of the machine properties in terms of reproducibility and stability even at pre-clinical
MEMS inertial navigation system irradiation
In summer 2019 two samples of Mini IMU-9 v5 were irradiated at ENEA Frascati Research Center. The IMU board mounts a LSM6DS33 3-axes gyroscope and 3-axes accelerometer of CMOS/MEMS technology and a LIS3MDL 3-axes magnetometer, both manufactured by STMicroelectronics.
Tests focused on accelerometer and gyroscope behavior of the LSM6DS33 chip. LSM6DS33 sensors can operate in different ranges and operative modes, the one used for the irradiation test are summarized in Table 3 (LSM6DS33 datasheet, //www.st.com/resource/en/datasheet/lsm6ds33.pdf
Sensors data discussion
Experimental data of gyroscope and accelerometer can be interpreted as a combination of deterministic and stochastic effects. The first category includes progressive sensor degradation due to combined TID/DD cumulative effects and eventual device failure, which was not observed. In the latter category are recoverable SEEs, as no disruptive events were recorded.
Conclusions
In this contribution we have discussed the possible application of the TOP-IMPLART accelerator, a prototype of a novel generation of proton-therapy machines, as proton source in standard SEE test procedure specifications. We have shown that full-LINAC accelerators, capable of delivering currents up to 50 µA, can fulfill most, if not all, requirements on a wide range of proton energies and with high energy uniformity. Dedicated experimental campaigns during the accelerator commissioning have
Declaration of Competing Interest
The author declare that there is no conflict of interest.
Acknowledgement
Research carried out within the TOP-IMPLART Project, funded by Regione Lazio, Italy.
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