Mohammadreza Rezaei
email: mrezaei@ucm.es.

Pedro Martín-Holgado
email: pmartinholgado@us.es.

Yolanda Morilla
email: ymorilla@us.es.

Francisco J Franco
email: fjfranco@fis.ucm.es.

Juan Carlos Fabero
email: jcfabero@ucm.es.

Hortensia Mecha

Helmut Puchner

Guillaume Hubert
email: guillaume.hubert@onera.fr.

Juan Antonio Clemente
email: juanancl@ucm.es.

Evaluation of a COTS 65-nm SRAM Under 15 MeV Protons and 14 MeV Neutrons at Low VDD

Keywords: COTS, SRAM, proton tests, radiation hardness, low bias voltage, reliability, soft error, TID synergistic effect. I

des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

It is well-known that, when the bias voltage of a memory is lowered, a new problem comes up: the critical charge to trigger Single Event Upsets (SEUs) decreases [START_REF] Hiramoto | Ultra-low-voltage Operation: Device Perspective[END_REF]. In fact, recently, the authors made a characterization of the SEU sensitivity of various COTS bulk SRAMs (with 130-nm, 90-nm and 65-nm manufacturing processes) at ultra-low bias voltages under neutron irradiation that supports this point [START_REF] Clemente | SEU Characterization of Three Successive Generations of COTS SRAMs at Ultralow Bias Voltage to 14.2-MeV Neutrons[END_REF], [START_REF] Clemente | Sensitivity Characterization of a COTS 90-nm SRAM at Ultra Low Bias Voltage[END_REF]. SEUs fall into two classes: Single Bit Upsets (SBUs) and Multiple Bit/Cell Upsets (MBUs, MCUs). If a part of the device that is critical for its correct operation is affected, a more serious event (a Single Event Functional Interrupt or SEFI) arises. Many other experiments have been done in the literature investigating the effect of such bias voltage variation on the behavior of academic custom D Flip-Flops, SRAM cells and manufacturing materials [START_REF] Qazi | A Low-Voltage 1 Mb FRAM in 0.13µm CMOS Featuring Time-to-Digital Sensing for Expanded Operating Margin[END_REF]- [START_REF] Liu | Soft-Error Performance Evaluation on Emerging Low Power Devices[END_REF]. Proton irradiation can cause SEUs through Proton Direct Ionization (PDI) and/or by secondary particles [START_REF] Dodds | Hardness Assurance for Proton Direct Ionization-Induced SEEs Using a High-Energy Proton Beam[END_REF]. The dependence of the SRAM sensitivity on energy levels of protons has been investigated in various studies [START_REF] Dodds | The Contribution of Low-Energy Protons to the Total On-Orbit SEU Rate[END_REF]- [START_REF] Hubert | Radiation Effects Investigations Based on Atmospheric Radiation Model (ATMORAD) Considering GEANT4 Simulations of Extensive Air Showers and Solar Modulation Potential[END_REF]. The effect of lowenergy protons on SRAMs has been also examined on various SRAM technologies mostly at nominal bias voltage [START_REF] Heidel | Low Energy Proton Single-Event-Upset Test Results on 65 nm SOI SRAM[END_REF], [START_REF] Heidel | Single-Event Upsets and Multiple-Bit Upsets on a 45 nm SOI SRAM[END_REF]. These studies show that SRAMs are especially sensitive to low-energy protons in comparison with high-energy ones. In [START_REF] Rodbell | Low Energy Proton SEUs in 32-nm SOI SRAMs at Low Vdd[END_REF] the effect of low-energy protons on a 32-nm SOI SRAM at low voltage is investigated.

Standard error detection and correction techniques are usually implemented in modern COTS memories, but they are sensitive to the type of bitflips: indeed they are not as efficient in dealing with multiple events as with SBUs.

Other studies in the literature have demonstrated that there is another point that should be taken into consideration while working at low voltages. In [START_REF] Yao | The Impact of Total Ionizing Dose on Unhardened SRAM Cell Margins[END_REF] it is shown that the Total Ionizing Dose (TID) can affect power consumption and threshold working voltage of the SRAM cells. It has also been shown that some memories can suffer from TID-SEE synergistic effects, which increases the sensitivity to neutrons or protons [START_REF] Salvy | Total Ionizing Dose influence on the Single Event Effect sensitivity of active EEE components[END_REF].

This paper explores the behavior of the CY62167GE30-4 5ZXI COTS bulk 65-nm SRAM, manufactured by Cypress Semiconductor, when powered up slightly above the minimum threshold voltage that guarantees data retention. Static and dynamic tests were made under 15.6 MeV protons. This allowed the authors not only validate the effects of DVS on this COTS SRAM, but also to establish a comparison with results obtained with 14.2 MeV neutrons in a previous work [START_REF] Clemente | SEU Characterization of Three Successive Generations of COTS SRAMs at Ultralow Bias Voltage to 14.2-MeV Neutrons[END_REF]. The parts were not delidded, and it was estimated that the encapsulation layer reduced the energy of protons on the surface of the memory to something within the range 13.4-14.2 MeV.

The rest of the paper is structured as follows. First, in Section II, some information about the facility used for testing the memories and how tests have been run is included. Then the results of the experiments are reported in Section III. Section IV presents a discussion of the TID tests and the rationale behind the differences observed in the sensitivity between protons and neutrons. Finally, the conclusions are proposed in Section V.

II. EXPERIMENTAL SETUP

The Device Under Test (DUT) was controlled by an Arduino Due, which was connected to a computer system in order to provide power and store the results. The DUT supply voltage was controlled by means of a variable voltage generator. It can provide a tunable voltage ranging from 0.05 to 3.5 V.

In order to guarantee that the observed errors were due to irradiation and not consequences of a problem in the experimental setup, the system was tested before and after the irradiation and the memory retained information at bias voltages above 0.72 V. The microcontroller is connected to the memory through a dedicated connection, whereas the data transmission to the computer was made with a USB port that virtualized the serial communication through the Universal Asynchronous Receiver/Transmitter (UART) of the microcontroller. To prevent any influence during the tests, everything in the irradiation chamber except the DUT itself was shielded with thick aluminum plates, whereas the computer was in another room for controlling and monitoring the process.

The tests were run in static and dynamic mode. The static tests consisted of five successive steps:

1) In order to observe all the SEUs that actually occurred in the memory cells, by using proprietary information provided by the manufacturer, the Error Correction Code (ECC) provided by the SRAM hardware was turned off.

2) The data are written on the memory with a specific pattern. In this case, the checkerboard patterns (0x55 and 0xAA) were used for logistic reasons.

3) The bias voltage of the memory is reduced and the device stays on stand-by. 4) The memory is exposed to radiation. 5) After irradiation, the bias voltage is restored to nominal and the memory is checked for errors. When performing the tests on Step 3), in order not to activate over-voltage protection structures present in the memories, all the address and data buses, as well as the enable pins were set to 0. This was done only when it was necessary to change the supply voltage of the memory from nominal to low. In consequence, the data of the first address of the memory was always written to 0x00 (consequently lost for performing the experiments), whichever the pattern was. Considering the size of the memory and the fact that only 8 bits were lost, it is totally bearable to sacrifice these data to adjust the voltage.

Due to the time constraints, the dynamic mode was run only with 0x55 pattern. At first, the pattern was written on the memory in the absence of radiation. Read cycles were done for 32 minutes of irradiation and to avoid losing any event, a final read cycle was made without irradiation. Eq. (1) shows the scheme of the dynamic stress test algorithm which is similar to the March-C approach [START_REF] Tsiligiannis | Dynamic Test Methods for COTS SRAMs[END_REF]. The up arrow indicates that write and read cycles were performed from the lowest address to the highest one. A deeper analysis of the dynamic mode results is provided in Section III-B.

{↑ (W (0x55), R(0x55), R(0x55) , R(0x55), R(0x55), R(0x55))} (1)
The proton irradiation campaign took place in May 2019 at CNA (Centro Nacional de Aceleradores), Spain [START_REF]Centro Nacional de Aceleradores[END_REF]. The experiments were performed using the external beam line installed in the 18/9 IBA compact cyclotron laboratory. The DUTs were placed at 56 cm from the exit nozzle with 100 µm aluminum foil as window, so that the final energy at the surface was 15.6 MeV protons, whose estimated spread was on the order of 400 KeV. Irradiation was done at normal incidence to the front side of the memory. Given the thickness of the DUT packaging (∼362 µm), it was estimated that the energy received by the metal memory cells of the SRAM is in the range of 13.4-14.2 MeV. The final energy of the incident beam was obtained using the energy loss data calculated with the SRIM2013 code [START_REF] Ziegler | SRIM-The stopping and range of ions in matter[END_REF]. The uniformity of the flux was better than 90% in the exposed area of interest, maintaining the flux stability within 5% during each experimental run. The medium flux used for performing the tests never surpassed 2.5 × 10 8 (p/cm 2 /s).

The proton flux monitoring was performed measuring the beam current into an electrically isolated graphite collimator (1 cm of diameter) behind the exit window. A medium flux value was calculated in the base of the pulses registered by the counter. Finally, the fluence at the device under test was calculated depending on the exposure time for every run with an accuracy of 10%. Despite the low proton energy, the different device technologies were sensitive enough without delidding them. Previous tests also conducted by the CNA group [START_REF] Lindoso | Evaluation of the Suitability of NEON SIMD Microprocessor Extensions Under Proton Irradiation[END_REF] with similar devices showed that the covering (epoxy or similar) thickness on the order of 900 µm as maximum, allows working with incident proton energies on the order of 15 MeV to study single event effects in technologies below 130 nm.

The TID test was carried out in the CNA-RadLab facility, also at the CNA [START_REF] Morilla | New Gamma-radiation Facility for Device Testing in Spain[END_REF]. The irradiation system contains a Cobalt-60 gamma source with associated photon energies of 1.17 and 1.33 MeV (mean value: 1.25 MeV). One irradiation step was carried out during the campaign, achieving a total dose of 276.48 krad(Si). The dose rate was 194.2 rad(Si)/h, which is within the "low rate" window (36-360 rad(Si)/h) of the ESA standard, according to the TID Test Method [START_REF]Total Dose Steady-State Irradiation Test Method[END_REF]. The dose rate was obtained by measuring the charge with two Farmer ionization chambers (PTW-Freiburg, TM30013 model, Germany) and one multichannel electrometer, (PTW-Freiburg, T10004 model, Germany), and also considering the environmental correction factor.

The DUT was mounted on a printed circuit board which was placed into a 12 × 17 cm 2 filter box to be exposed to radiation, in compliance with the ESCC Basic Specification No. 22900 [START_REF]Total Dose Steady-State Irradiation Test Method[END_REF]. This container had 2 mm of aluminum, and 1.5 mm of lead in the outer layer and a 5 mm front cover of polymethyl methacrylate (PMMA) to achieve the chargedparticle equilibrium. The dose rate uniformity in the filter box was greater than 90%, so the uniformity throughout the memory (DUT), considering its size, was significantly larger.

III. EXPERIMENTAL RESULTS

A. Static tests

Table I shows the experimental rounds and the number of SBUs and N-bit MCUs observed in each round. To estimate the sensitivity of the device it was needed to extract the number of SBUs/MBUs from the whole set of errors. Proprietary unscrambling information from Cypress was used for this purpose, which allowed relating addresses involved in the same multiple events. Any isolated bitflip observed in the memory is presented as an SBU. Affected addresses located at a Manhattan distance equal or lower than 3 were grouped in the same MCU.

Concerning the false 2-bit events derived from the combination of two independent SBUs, in [START_REF] Franco | Influence of Randomness During the Interpretation of Results From Single-Event Experiments on SRAMs[END_REF], the authors presented a set of equations that estimate the number of such false 2-bit MCUs, as a simple function of the total number of bitflips per round, the memory size and the metric used to identify such MCUs (Manhattan distance, etc...). According to it, in the worst case, namely 0xAA at 0.75 V, the number of expected false 2-bit events was only 0.71, more than two orders of magnitude below the number of observed 2-bit events. Therefore, the occurrence of false events is negligible.

As mentioned before, two different patterns were tested and results do not show any significant dependence on them. However, due to the time constraints the two most extreme cases for patterns (namely 0x00 and 0xFF) were not tested and there might be a slight difference in the cross-sections in comparison with all these patterns [START_REF] Heidel | Single-Event Upsets and Multiple-Bit Upsets on a 45 nm SOI SRAM[END_REF]. At first, the 0x55 pattern was tested from 3 V down to 0.75 V. After that, the same sequence was tested with the 0xAA pattern. All types of events were more prone to occur as the bias voltage decreased.

The largest event observed in this memory was one 5-bit MCU for the 0x55 pattern and one 7-bit MCU for 0xAA. For both patterns, the multiplicity of the rest of MCUs was equal or less than 4. It has to be mentioned that no MBUs were found in the results. This is undoubtedly the result of interleaving the physical addresses by design. Also, based on [START_REF] Franco | Influence of Randomness During the Interpretation of Results From Single-Event Experiments on SRAMs[END_REF] the probability of observing any false MBUs with this number of bitflips is very low, which is in agreement with the results.

Fig. 1 shows the relationship between the bias voltage and SBUs/2-bit MCUs sensitivity for 15.6 MeV proton and 14.2 MeV neutron irradiation. The latter were obtained only with 0x55 pattern and previously presented and discussed in [START_REF] Clemente | SEU Characterization of Three Successive Generations of COTS SRAMs at Ultralow Bias Voltage to 14.2-MeV Neutrons[END_REF] by the authors. These were obtained in the GENEPI2 accelerator, available in Grenoble (France) [START_REF] Villa | Accelerator-Based Neutron Irradiation of Integrated Circuits at GENEPI2 (France)[END_REF]. The uncertainty margins of the cross-section for larger MCU sizes with proton irradiation were so wide that it was impractical to include them. This graph also includes error bars for the experimental cross-section calculations. Said error bars were calculated counting in the beam profile (±10%), fluence accuracy (±10%) and with 95% of confidence intervals as discussed in [START_REF] Autran | Real-time soft-error sate measurements: A review[END_REF].

One can clearly spot the dependency of the cross-section on the bias voltages. Both of the tested patterns with the proton irradiation showed almost identical behaviors. It is important to notice that lowering the bias voltage down to 1 V does not lead to a significant raise in memory sensitivity. However, it seems to increase drastically for the 0.75 V -1 V range for both irradiation sources. It is shown that the SRAM sensitivity triples by lowering the voltage from nominal to slightly above threshold voltage. This is a good hint to control the limits of the DVS technique.

It can be easily deduced that the device is more sensitive to neutrons, as SBU and MCU cross-sections are one order of magnitude larger than their respective proton results. 0 . 5 1 . 0 1 . 5 2 . 0 2 . 5 3 . 0 3.57 × 10 -15 cm 2 /bit is the highest SBU cross-section for proton irradiation which is observed at 0.75 V and with the 0x55 pattern. For neutrons the value is 4.63 × 10 -14 cm 2 /bit at 0.8 V. There are several reasons that can explain why proton and neutron cross-sections have almost one order of magnitude in difference, to be discussed in greater detail in Section IV.

1 0 -1 6 1 0 -1 5 1 0 -1 4 1 0 -1 3 P r o t o n S B U s 0 × 5 5 P r o t o n S B U s 0 × A A P r o t o n 2 -b i t M C U s 0 × 5 5 P r o t o n 2 -b i t M C U s 0 × A A N e u t r o n S B U s N e u t r o n 2 -b i t M C U s P r o t o n M U S C A S B U s P r o t o n M U S C A 2 -b i t M C U s N e u t r o n M U S C A S B U s N e u t r o n M U S C A 2 -b i t M C U s C r o s s -s e c t i o n ( c m 2 / b i t ) B i a s v o l t a g e ( V )
H 3 / V 3 K J 4 K J 3 K J 2 K J 1 D 1 3 5 D 4 5 V 2 V 1 H 2 H 1 a )
Fig. 1 also compares the results with theoretical crosssection predictions issued by the MUlti-SCAles Single Event Phenomena Predictive Platform (MUSCA-SEP3), a modeling tool developed at the French Aerospace Lab (ONERA) [START_REF] Hubert | Radiation Effects Investigations Based on Atmospheric Radiation Model (ATMORAD) Considering GEANT4 Simulations of Extensive Air Showers and Solar Modulation Potential[END_REF], [START_REF] Hubert | Operational SER Calculations on the SAC-C Orbit Using the Multi-Scales Single Event Phenomena Predictive Platform (MUSCA-SEP3)[END_REF]. MUSCA-SEP3 is used to predict cross-section at various VDDs showing similar behavior with the actual results, espe-cially for protons. The difference between cross-sections of proton and neutron irradiations is clear also in the predictions.

Investigating the origin of the 2-bit MCUs led the authors to analyze the shape of this kind of event. Fig. 2 (a) shows all the possibilities for 2-bit MCUs with a Manhattan distance equal to or less than 3. There can be vertical (V), horizontal (H), diagonal (D) and knight jump (KJ) shapes with variations. These shapes can be technology-dependent and vary from device to device.

Fig. 2 (b) shows the percentage of such 2-bit MCU shapes for the static and dynamic tests and the 0x55 pattern (the 0xAA one depicted a similar behavior). At the first look, it is obvious that most of the 2-bit MCUs have vertical shapes. MUSCA predictions match the experimental observations, with some small disagreements in the prediction of horizontal-shaped events. Lowering the bias voltage increased the chance of diagonal MCUs to occur. However, the vertical shapes are the dominant kind of 2-bit MCUs in all the tests. This point has been confirmed with the manufacturer, since the well stripes of the CY62167GE-4 SRAM are organized in columns, and it is very difficult to cross them since the well doping is very high. It is, in fact, considerably stronger in the 65-nm technology than in the preceding 90-nm one, thereby leading to an almost null probability of horizontal-shaped events to occur in this technology.

B. Dynamic tests

It is reported by the manufacturer of the DUT that this memory is functional over a wide range of 2.2-3.6 V. The authors' tests showed that data can be read and written successfully within this range. 5 rounds of dynamic tests making the device working at different voltages were done. For each round the DUT supply voltage was set to a new value within the mentioned working voltage range of the memory (Table II) following the scheme described in (1), and with event rewrite: after observing any event, it was noted and the original data were restored with the correct value. In this way, error pile up from one read cycle to the next one is prevented.

Table II shows the total number of SBUs and MCUs for each supply voltage in dynamic mode. For the dynamic tests, the DUT is read and irradiated at the same time, to do so, the I/O ports need to be fed by, at least, 2.2 V to work properly. This voltage level is much higher than the minimum one that guarantees data retention. It can be observed that a) the relation of the SBUs to total bitflips is almost 79% for every voltage and b) the 2-bit MCUs have a 19% rule in total bitflips collectivity. To compare this table with Table I, two points should be taken into account. First, the total irradiation time for each row of Table II was almost 32 times higher than each static test. Second, the results of 5 read cycles added up to generate the final result for each voltage. Since both static and dynamic tests were run under the same range of fluxes, the difference between the static and dynamic fluences (2 orders of magnitude higher for the dynamic tests) shows the contrast in the irradiation times. This can explain the higher abundance for the larger MCUs during the dynamic tests. Bitflips might be piled up within a given read cycle, which can be justified by comparing the read speed of the memory with the irradiation time. Therefore, it is postulated that many of the larger size MCUs are not originated from one single particle. Thus several SBUs or even smaller MCUs with different occurrence time would be seen as a larger MCU.

According to the estimations based on [START_REF] Franco | Influence of Randomness During the Interpretation of Results From Single-Event Experiments on SRAMs[END_REF], the number of false 2-bit MCUs for each row in Table II comparing to the total number is negligible. In the worst case, namely at 2.25 V, the number of expected false 2-bit events was 20, for the other voltages this number is always below 10. The estimation of false events of large multiplicities is much more complex and it still remains an open problem.

There is a difference for the 2 events labeled as ">9 bit" in Table II (at 2.50 V and 2.75 V), which are events with 18 and 20 bitflips, respectively. In order to investigate this in greater detail, a bitmap plot for the read cycle containing the largest MCU sizes was created. Fig. 3 shows this bitmap for the dynamic tests at 2.75 V and 2.5 V. It can be seen that the two largest MCUs observed at the dynamic tests are vertical lines of bitflips. These are similar to type-A MCUs that were introduced in [START_REF] Bosser | Investigation on MCU Clustering Methodologies for Cross-Section Estimation of RAMs[END_REF].

It is important to note that this type of error did not appear on the static tests. The authors also investigated the distribution of the events on the device. These were randomly scattered all over the SRAM at different bias voltages with different patterns. Therefore, plots of the events occurred at other voltages are similar to the plots in Fig. 3, with the exception of the large-scale ones.

Finally, Fig. 4 shows the SEU cross-section of the DUT for dynamic tests. In this case, it is evident that the cross-section for each SEU type is independent of the working voltage of the memory. Cross-sections for SBU and 2-bit MCU are comparable with the results of the static tests at nominal bias voltage (also shown in the figure).

Again, the shapes of 2-bit MCUs for dynamic mode results are mostly vertical with a few diagonal ones. These vertical shapes prevent the flipped bits from occurring in the same word, which would keep the memory safe from MBUs even without interleaving. Analyzing the MCU shapes with the 0xAA pattern on the memory showed almost the same behavior as 0x55.

IV. DISCUSSION

In this section, the authors try to investigate the reason behind the difference between cross-sections of the neutrons and protons with similar energies. Loss of proton energies from 15.6 MeV to almost 14.2 MeV reaching the silicon layer of the memory might be one possible explanation. However, previous studies show that SRAM sensitivity to proton irradiation does not differ by changing the proton energy in the range of 2 -100 MeV [START_REF] Dodds | Hardness Assurance for Proton Direct Ionization-Induced SEEs Using a High-Energy Proton Beam[END_REF], [START_REF] Dodds | The Contribution of Low-Energy Protons to the Total On-Orbit SEU Rate[END_REF], [START_REF] Heidel | Low Energy Proton Single-Event-Upset Test Results on 65 nm SOI SRAM[END_REF]. Hence, even if the energy that is reaching the memory cell is around 14.2 MeV, the cross-section for protons should be the same, therefore the authors believe that this is not the main reason for the gap between both sensitivities. Another reason that can 2 . 2 5 2 . 5 0 2 . 7 5 3 . 0 0 3 . 2 5

1 0 -1 8 1 0 -1 7 1 0 -1 6 1 0 -1 5 S B U s 2 -b i t M C U s 3 -b i t M C U s 4 -b i t M C U s S t a t i c S B U s S t a t i c 2 -b i t M C U s C r o s s -s e c t i o n ( c m 2 / b i t ) B i a s v o l t a g e ( V )
Fig. 4. Cross-section of the dynamic results at various voltages be postulated is the existence of background particles affecting the sensitivity of the memory. In order to verify this, a TID test was run to check for gamma particle contribution to these results.

A. TID test

As mentioned before, TID can affect SRAM sensitivity to SEUs or even directly cause errors on some devices [START_REF] Zheng | Total Ionizing Dose Influence on the Single-Event Upset Sensitivity of 130-nm PD SOI SRAMs[END_REF]- [START_REF] Koga | Synergistic Effects of Total Ionizing Dose on SEU Sensitive SRAMs[END_REF].

To study this phenomenon in more detail, the DUT was tested with gamma irradiation. The process took 60 days and the total accumulated dose reached 276.48 krad(Si) before dismounting the DUT. During this time, the SRAM was tested periodically and no bitflips were observed.

The same SRAM was tested before and after the dose accumulation and the results can be seen in Fig. 5. It is worth mentioning that the after-TID tests were done less than 2 hours after dismounting the memory from the gamma radiation source. It can be seen in this figure that, although the SBU cross-section for after-TID results are, in most of the cases, higher than the before-TID ones, they all fall within the 95-% confidence interval of each other. Also, it can be seen that for 2-bit MCUs and 3-bit MCUs there is almost no difference before and after the TID test. Thus, there may be hints of larger influence of TID on SBUs than on MBUs, specially at lower voltages, but further tests are necessary.

To have a deeper insight into the reason of why this is happening, the authors compared the number of each SEU type before and after the TID test, which is shown in the plots of Fig. 6. Comparing both plots shows a slightly higher contribution of SBUs in the total number of events for the after-TID test. On average, said contribution rises from 77.8% (before) to 81.5% (after), which is a 3.7% increase. For each plot, the maximum difference for SBU and 2-bit MCU contributions are about ±10. In general, it is hard to conclude from Figs. 5 and 6 whether or not TID had some effects in the SBU/MCU distribution of events. However, it can be concluded that the one order of magnitude difference in the cross-sections is not coming from the TID effects. The above subsection allowed concluding that gamma irradiation could not explain the significant sensitivity difference between protons and neutrons observed in Fig. 1. This subsection gives more insight into this phenomenon. In a recent study, Han et al. [START_REF] Han | Characteristics of energy deposition from 1-1000 MeV proton and neutron induced nuclear reactions in silicon[END_REF] compared the memory sensitivities against these particles in the range of 1-1000 MeV using GEANT4 simulations. This study can be used to extract some conclusions for the proton/neutron energies that are object of study in this paper in a qualitative way.

( c m 2 / b i t ) B i a s v o l t a g e ( V ) S B U s S B U s * 2 -b i t M C U s 2 -b i t M C U s * 3 -b i t M C U s 3 -b i t M C U s *
According to [START_REF] Han | Characteristics of energy deposition from 1-1000 MeV proton and neutron induced nuclear reactions in silicon[END_REF], the SEU cross-section of protons above and at 30 MeV can be considered equivalent to neutrons. However, below 30 MeV the cross-section of protons is lower than neutrons and the difference increases for lower energies. Several reasons are mentioned to explain this difference. First, a higher cross-section for neutron nonelastic reactions in silicon compared to the proton one. Second, neutron nonelastic events can deposit more energy than the proton ones on the whole. Finally, neutron SEUs have a smaller threshold energy than the proton ones do at this lower range of energies. Another interesting phenomena was observed in [START_REF] Han | Characteristics of energy deposition from 1-1000 MeV proton and neutron induced nuclear reactions in silicon[END_REF] that is more relevant for the results presented in this paper: there exists a bump in the neutron SEU cross-section at 14 MeV. Similar to previous observations, this can be related to the relatively large neutron nonelastic cross section around that energy. Thus, the equivalent LET dramatically increases for neutrons whose energy is close to 14 MeV.

In order to investigate the reason for the difference in the proton and neutron cross-sections in a quantitative way, the authors also performed simulations using GEANT4. The probability of interaction of 14 MeV protons and neutrons with Silicon nuclei is 1.01 × 10 -5 µm -1 and 8.69 × 10 -6 µm -1 , respectively. Therefore, the difference observed in the SEU sensitivities between both particles cannot implicate these interaction probabilities.

Conversely, the angular and energy properties of the secondary ions (SIs) generated by these nuclear reactions are very different depending on whether neutrons or protons are involved. Thus, characteristics of SIs from the 14 MeV neutrons and protons induced elastic and nonelastic reactions in silicon were investigated by using GEANT4 simulations. SIs created by such nuclear interactions can range from H to P, including several isotopes. 10 4 interactions were considered with normal p/n incidence and simulations provided the secondary ion count, atomic and mass numbers, their energies and directions. For the latter, a Cartesian coordinate system was considered, and the output direction of each particle interaction is a unit vector whose projections over the axes X, Y and Z are V x , V y and V z , respectively. It is assumed that the direction of the incident particle is (V x =0, V y =0, V z =1). Fig. 7 presents in (a) the cumulative relative frequency of V z and in (b) the relative frequency of SI energy restricted to atomic number higher than 11. V z values in the range (0-1] correspond to SIs emitted in the direction of the incidence angle, while angles less than 0 imply SIs emitted in an opposite direction. A 0-value would therefore indicate that the SI is emitted perpendicular to the incident particle. Results presented in Fig. 7(a) show that, for neutron interactions, SIs are emitted in all directions while proton ones induced exclusively SIs in the direction of the incidence angle. This shows a first difference between neutrons and protons, which one can imagine that the consequence on the SEU sensitivity is significant. However, this difference is not sufficient to explain the order of magnitude observed experimentally in Fig. 1 (and also confirmed by MUSCA-SEP3).

To complete the analysis, Fig. 7(b) shows that SIs induced by neutrons have a higher energy than protons. This difference is significant too, on the order of a factor of 2.5 for the highest energies. Thus, once a SEU sensitivity threshold is considered (i.e. for the MUSCA SEP3 simulations), it has an important impact in the neutron cross section. Considering the same particle fluence, while few protons will be likely to induce SEUs, many neutrons will be possible to do so.

Albedo neutrons were also considered as they also significantly increase the neutron cross section, as shown in recent works [START_REF] Hubert | Experimental Evidence of Ground Albedo Neutron Impact on Soft Error Rate for Nanoscale Devices[END_REF], [START_REF] Hubert | Study of Secondary Scattering/Albedo Neutron Fields and Their Impacts on SER as Function of Scene Topologies[END_REF]. Indeed, neutrons are attenuated as they interact with nuclei in the matter, and they can induce the re-emission of particles able to reinforce the local spectra. Cascade neutrons (i.e., with energy higher than 20 MeV) colliding with nuclei produce fast neutrons (on the order of 1 MeV) which are moderated to epithermal and thermal ones as they travel and interact with the elements of the room. One of them is concrete, which is used to build the walls of facility test rooms. Another one is polyethylene, typically C u m u l a t i v e r e l a t i v e F r e q u e n c y V z ( a n g u l a r d i r e c t i o n ) used to protect some parts of the SEE test boards. The latter was used for the neutron experiments of [START_REF] Clemente | SEU Characterization of Three Successive Generations of COTS SRAMs at Ultralow Bias Voltage to 14.2-MeV Neutrons[END_REF], fairly close to the DUT (∼40-50 cm.), which was not the case for the proton tests. A first study, made in [START_REF] Hubert | Study of Secondary Scattering/Albedo Neutron Fields and Their Impacts on SER as Function of Scene Topologies[END_REF], revealed that the neutron spectra ranging from 0.1 MeV to a few MeV are reinforced because of the albedo neutrons issued from collisions with concrete and polyethylene. These low-energy neutrons have a high impact in the device SEE sensitivity, as recent studies have pointed out [START_REF] Lambert | Single Event Upsets Induced by a few MeV Neutrons in SRAMs and FPGAs[END_REF]. Another preliminary analysis of the SEU contribution of 14 MeV neutrons made with MUSCA-SEP3 on this COTS SRAM showed a 1.9X increase if albedo is taken into account. This study is out of the scope of this work, but it is clear that it should be investigated in depth. Therefore, the difference in cross-sections between protons and neutrons observed experimentally can be attributed firstly to nuclear reaction properties and secondly by the scattering/albedo neutrons induced by polyethylene shielding and concrete chamber walls. Thermal neutrons were not considered in these investigations. Nevertheless, if the device contains a borophosphosilicate glass (BPSG) passivation layer, it is likely that thermal neutrons induced by material environments (facility, protection of SEU tester board, etc.) further increases the SEU sensitivity under the neutron beam.

The existence of a bump for 14 MeV neutrons was predicted in [START_REF] Han | Characteristics of energy deposition from 1-1000 MeV proton and neutron induced nuclear reactions in silicon[END_REF]. Even if the neutron tests were not performed over a wide range of energies, a comparison between cross-sections of 14 MeV neutrons and 14 MeV protons will surely reveal a very similar phenomena. According to [START_REF] Han | Characteristics of energy deposition from 1-1000 MeV proton and neutron induced nuclear reactions in silicon[END_REF], this can lead to important mispredictions in the assumption that the SEU cross-sections vary with energy following the Weibull fit (or the Bendel model).

V. CONCLUSIONS Experimental proof of sensitivity of the CY62167GE30-4 5ZXI bulk 65-nm COTS SRAM, when powered up at low bias voltages at static and dynamic modes against 15.6 MeV protons is presented in this paper. Under natural conditions, the memory is fully functional with bias voltages above 0.72 V and up to the nominal voltage established by the manufacturer. SBUs and MCUs were detected at all the bias voltages. Vertical shapes are shown to be the dominant 2-bit MCUs and no MBUs were observed with this memory.

An interesting result was the 15.6 MeV proton/14.2 MeV neutron SEU sensitivities being 1 order of magnitude apart, for particles with almost the same energies. Comparisons with MUSCA-SEP3 show a very good agreement between experimental results and predictions. The difference observed between both particles was explained by higher energies of secondary ions produced by neutrons and their direction (studied with GEANT4), as well as a bump found and analyzed at about 14 MeV neutrons in a previous work [START_REF] Han | Characteristics of energy deposition from 1-1000 MeV proton and neutron induced nuclear reactions in silicon[END_REF].

Fig. 1 .

 1 Fig.1. SBU/2-bit MCU sensitivity against 15.6 MeV protons and 14.2 MeV neutrons[START_REF] Clemente | SEU Characterization of Three Successive Generations of COTS SRAMs at Ultralow Bias Voltage to 14.2-MeV Neutrons[END_REF] at various bias voltages.

  M C U s o b s e r v e d B i a s v o l t a g e ( V )

Fig. 2 .

 2 Fig. 2. (a) Possible shapes for a 2-bit MCU (Vertical, Horizontal, Diagonal, Knight Jump) and (b) 2-bit MCU shapes for 0x55 at various bias voltage

Fig. 3 .

 3 Fig. 3. (a) Observed type-A events [31] at 2.75 V and (b) at 2.5 V for the dynamic test (>9-bit in TableII). Each point is the physical position of a bitflip in the DUT, indicating the axes as X and Y in the ranges of 0-5600 and 0-4100 respectively. Expanded regions include the type-A events with dimensions of 30 × 25 (a) and 30 × 35 (b).

  s s -s e c t i o n x 1 0 -1 5

Fig. 5 .Fig. 6 .

 56 Fig. 5. Cross-section of the memory before and after TID on various voltages. * The tests after the TID test

  E n e r g y p r o p e r t i e s R e l a t i v e F r e q u e n c y E n e r g y o f s e c o n d a r y i o n ( Z = 1 4 , 1 3 , 1 2 ) , i n M e V

Fig. 7 .

 7 Fig. 7. (a) Cumulative relative frequency of Vz and (b) relative frequency of secondary ion energies, restricted to atomic numbers higher than 11

TABLE I RUN

 I SEQUENCE OF THE IRRADIATION CAMPAIGN INCLUDING THE NUMBER OF SBUS AND N-BIT MCUS

	Voltage (V)	Pattern	Fluence (p/cm 2 ) ×10 9	SBUs	2-bit MCUs	3-bit MCUs	>3-bit MCUs	Affected addresses
	3.00		7.4	155	35	4	0	237
	2.00		9.3	229	49	7	1	353
	1.50		7.4	170	32	0	0	234
	1.00		9.3	296	76	2	1	457
	0.95	0x55	9.3	264	58	4	0	392
	0.90		9.3	345	56	2	1	467
	0.85		9.3	418	71	7	1	585
	0.80		9.3	508	91	7	0	711
	0.75		9.3	560	60	7	3	713
	3.00		10.6	212	45	4	0	314
	2.00		12.2	260	72	12	0	440
	1.50		12.0	290	60	10	0	440
	1.00		11.2	345	70	6	1	506
	0.95	0xAA	12.2	468	107	7	1	707
	0.90		12.8	523	118	11	3	798
	0.85		12.6	601	116	18	0	895
	0.80		12.8	626	112	9	1	880
	0.75		13.0	719	121	14	0	1003

TABLE II THE

 II NUMBER OF N-BIT MCUS OBSERVED IN DYNAMIC TESTS AT VARIOUS VOLTAGES

	Voltage (V)	Total bitflips	SBUs	2-bit	MCUs 3-bit 4-bit 5-bit 6-bit 7-bit 8-bit 9-bit >9-bit	Fluence (p/cm 2 )×10 11
	2.25	10940	6884	1614	182	24	8	0	0	1	0	0	3.8
	2.50	4352	2760	670	61	9	1	2	0	0	0	1	2.0
	2.75	6031	3829	897	103	12	0	2	2	0	1	1	2.4
	3.00	7477	4688	1155	116	24	2	3	0	0	1	0	2.6
	3.20	7718	4965	1149	100	10	1	2	1	1	0	0	2.7

This work was supported in part by the Spanish MINECO project TIN2017-87237. The work at the CNA has been supported in part by the Spanish Ministry of Economy and Competitiveness under project ESP2015-68245-C4-4-P.


Cesar Segovia

Mahdi Hedri

Hermine Huot

Isabelle Zigler-Devin

Chang Liu

Yetao Tang

Rongliang Qiu

Jean Louis Morel

Nicolas Brosse
email: nicolas.brosse@univ-lorraine.fr

Industrial Ramie growing on reclaimed ion-adsorption Rare Earth Elements mine tailings in Southern China: Defibration and fibers quality

Keywords: Natural fiber, ramie, degumming, steam explosion, rare earth elements, phytoremediation

Ramie plants (Boehmeria nivea) have been cultivated on ion-adsorption rare earth elements (REEs) mine tailings and showed a low ability to accumulate REEs, especially in the stems (REE= 24±17 mg kg -1 ) with higher concentration in leaves and roots (REE= 57 to 66 mg kg -1 ). Ramie individual fibers were isolated using three degumming processes: a traditional alkali treatment (AT), a steam explosion (SE) treatment after a neutral (SEN) or after a basic (SEB) impregnation. Cellulose-rich (> 80%) fine fibers with very low amounts of non-cellulosic sugars (<5%), low REE concentrations (REE ≈ 4-6 mg kg -1 ) and a high mechanical strength (tensile strength ≈ 260-540 cN/fiber, breaking point ≈ 20-50 cN/fiber) have been produced. However, SE degumming resulted in more degraded fibers with cracks slightly affecting their tensile properties.

Introduction

In recent years, natural bast fibers (such as hemp, flax, jute, ramie) have received an increasing attention due to their economic importance and their renewable and sustainable character [START_REF] Pickering | A review of recent developments in natural fibre composites and their mechanical performance[END_REF]. The main advantages of using natural fibers include local availability, low density, biodegradability and carbon sequestration. Bast fibers are composed of elementary fibers which are glued together by a cementing material composed of lignin and polysaccharides [START_REF] Bismarck | Plant Fibers as Reinforcement for Green Composites[END_REF]. Regardless of the final application concerned (textile or composite), the removal of these non-cellulosic components (lignin, hemicelluloses, pectins) is required prior to downstream processes through a degumming process. In fact, the gummy components negatively affect the spinability of the fibers and, in composite applications, result in poor strength properties of resulting materials. Typically, chemical methods are used for the degumming process using large quantities of sodium hydroxide and other hazardous chemicals [START_REF] Adamsen | Chemical retting of flax straw under alkaline conditions[END_REF].

Steam explosion (SE) is an efficient environment-friendly process for bast fibers degumming.

In a previous paper, we described the optimization of fine hemp fibers extraction using SE [START_REF] Sauvageon | Toward the cottonisation of hemp fibers by steam explosion -Part 1 -Defibration and morphological characterization[END_REF]. This method allowed an efficient degumming of the technical hemp fibers and the production of a cotton-like material. Recently, Jiang et al. [START_REF] Jiang | A green degumming process of ramie[END_REF] reported a green degumming process of ramie using steam explosion in comparison with traditional chemical methods. It has been concluded from this study that SE has a great potential for industrial developments for economic and environmental reasons.

Compared to other natural fibers like flax or hemp fibers, ramie (Boehmeria nivea) fibers display outstanding properties such as silk-like appearance, high mechanical strength and high moisture absorption capacity and are among the most popular raw materials for textile application [START_REF] Jiang | A green degumming process of ramie[END_REF][START_REF] Meng | Rapid and energy-saving preparation of ramie fiber in TEMPO-mediated selective oxidation system[END_REF][START_REF] Li | High-efficiency ramie fiber degumming and self-powered degumming waste water treatment using triboelectric nanogenerator[END_REF]. Ramie is grown almost exclusively in China and one of the major limitation of the large scale development of ramie industry is the utilization of arable land for the production of non-food biomass. The utilization of plants for the management of polluted land has been described in order to limit the dissemination of pollutants (phytostabilisation). In addition, the cultivation of economic plants, such as fiber or bioenergy crops, can be considered to contribute to the socio-economic development of contaminated areas [START_REF] Ding | Ecosystem services provided by heavy metalcontaminated soils in China[END_REF]9].

Ramie, characterized by a vigorous root system, a high biomass and a good tolerance to heavy metals, has been shown to be a good candidate for the phytostabilisation of contaminated soils [START_REF] Yang | Constitutional tolerance to heavy metals of a fiber crop, ramie (Boehmeria nivea) and its potential usage[END_REF].

The demand for rare earth elements (REEs) is increasing worldwide due to their importance in many high-tech products. In Southern China, the ion-adsorption type REE deposits are a strategic resource. For decades, these deposits have been exploited by heap leaching, which consists of the removal of the vegetation, the excavation of the soil layer enriched in REEs and subsequent leaching by ammonium sulfate to recover REEs by ionic exchange. This mining activity has generated important environmental damages, such as large areas of mine tailings subjected to erosion, water contamination by ammonium, sulfate and metals, and landslides. For instance, in Ganzhou city, Jiangxi province, mining activity to recover REEs since 30 years has left more than 300 abandoned mines, 190 million tons of tailings and destroyed more than 150 km² of forest [START_REF] Yang | China's ion-adsorption rare earth resources, mining consequences and preservation[END_REF]. Investigation of watersheds impacted by this mining activity in the Dingnan county of Ganzhou city showed the contamination of river waters and sediments and agricultural soils irrigated with contaminated water [START_REF] Liu | Water, sediment and agricultural soil contamination from an ion-adsorption rare earth mining area[END_REF].

Main challenges for the revegetation of these tailings are the strong erosion and unfavorable physical properties (sandy material, poor structure), the low levels of organic matter and nutrients and the relatively high residual REE concentrations [START_REF] Liu | Limiting Factors for Restoration of Dumping Sites of Ionic Rare Earth Mine Tailings[END_REF]. Phytoremediation using amendments and pioneer or tolerant plants have been showed to improve the physico-chemical properties of the tailings and to increase biological activity and diversity, suggesting the potential of phytoremediation for the restoration of such mining areas [START_REF] Zhou | Restoration of rare earth mine areas: organic amendments and phytoremediation[END_REF][START_REF] Wei | The effects of phytoremediation on soil bacterial communities in an abandoned mine site of rare earth elements[END_REF].

In this context, the utilization of mining areas for growing industrial ramie could be a promising approach not only for the revegetation and phytostabilization of ion-adsorption REE mine tailings but also for biomass production. As a result, the mobilization of non-food phytotechnology ramie fibers for industrial developments could be of particular interest in order to reduce the impacts on the food markets.

In a previous work, we evaluated SE to achieve simultaneously the pretreatment and the decontamination of Zn and Mn contaminated willow harvested from an experimental phytoremediation plot for bioethanol production [START_REF] Ziegler-Devin | Steam explosion pretreatment of willow grown on phytomanaged soils for bioethanol production[END_REF]. In the present study, we described the defibration by SE and characterization of ramie fibers harvested on an ion-adsorption REE mining area in Dingnan county (Jiangxi province) in China. REE concentrations in the different compartment of the plant have been examined. The impact of the degumming process used (SE or traditional chemical method) on the ramie fiber quality in terms of chemical and mechanical properties has been examined.

Materials and Methods

Raw material

The site of demonstration for the reclamation of ion-adsorption REE mine tailings is located in the Dingnan county of Ganzhou city, Jiangxi province. Details regarding the location, reclamation program and growing conditions are given in the Supplementary Material section S1. Small plants (1 year old, about 35 cm in height) of the ramie cultivar Zhongzhu No.1 were planted in rows in the amended tailings with a density of 3 to 5 plants per m² in 2015. In 2018, ramie stems (1-2 meters high, ~1 cm in diameter at the base) were sampled for the analysis of fibers, cut and air-dried for several days.

Carding and degumming

Technical fibers were extracted from the ramie stems using a carding machine Bonino, type 250 HL520, with a lining adapted to bast fibers.

Chemical degumming: 10 g of technical fibers in 200 mL of NaOH 1% solution in water were heated at 90°C with string during 1 h. The treated fibers were filtrated, washed with a dilute acetic acid solution and then water. SE degumming: Technical fibers were impregnated with a solution of water or of sodium hydroxide (2 wt %). The fibers were macerated during 15 hours at room temperature using a weight ratio water/biomass of 15/1 after what the biomass was filter-pressed (6.8 atm = 100 psi) to remove the excess water. The fibers were transferred into a 2 L steam gun where about 100 g (dry basis) of biomass was cooked at temperatures ranging from 170°C for 3 minutes by addition of steam in a pressure-resistant reactor. After the cooking period, a pneumatic valve was opened and the vapor phase exited the reactor through a nozzle entraining as well the fibers.

The treated fibers were opened in the carding machine previously described (see below).

Opening the tufts into individual fibers: Card opened the tufts into the stage of individual fibers. This is essential to enable the elimination of impurities and performance of the other operations to be achieved as elimination of dust and removal of short fibers. Mainly for this study to see the effects of the treatment on the opening of the fibers. The schematic of the whole process is given in Fig. 1.

Chemical analysis

The experimental details regarding Klason lignin and simple sugars % determination have been given in previous papers. In short, extractive free fibers were ground and hydrolyzed in 2-steps procedure with sulfuric acid (72% and 4%). The insoluble residue (Klason lignin) is Analyse des Roches et des Minéraux (CNRS, Vandoeuvre-lès-Nancy, France). Trace element concentrations were analysed using ICP-MS after lithium metaborate attack and acid digestion [START_REF] Carignan | Routine Analyses of Trace Elements in Geological Samples using Flow Injection and Low Pressure On-Line Liquid Chromatography Coupled to ICP-MS: A Study of Geochemical Reference Materials BR, DR-N, UB-N, AN[END_REF]. Details regarding the soil analysis are given in Supplementary Material section S2.

SEM analysis

Scanning electron microscopy (SEM) analysis was carried out with Hitachi 3000 microscope.

All images were taken at an accelerating voltage of 15 kV.

Mechanical properties

For measuring single fiber properties, Favimat (Textechno Herbert Stein GmbH and Co. KG, Möchengladbach, Germany) was used. The main principle is that both single fiber ends are clamped between two sets of jaws. The displacement is insured by a constant speed motor with interchangeable equipment to vary the rate of elongation. For the tensile tests, the data of the load and elongation were transferred to a computer to be plotted and analyzed. The percentage of elongation and breaking point were determined according to the procedure described in NF EN ISO 5079 1996 standard method (Textiles fibres -Determination of breaking force and elongation at break of individual fibres). The tensile strength was determined according NF T25-501-2 2015 (Reinforcement fibres -Flax fibres for plastics composites-Part2: Determination of tensile properties of elementary fibres) standard method.

SEM analysis

Scanning electron microscopy (SEM) analysis was carried out with Hitachi 3000 microscope.

All images were taken at an accelerating voltage of 15 kV. Prior to the analysis, the specimens were coated with a thin layer gold/palladium alloy using a sputter coater Quorum SC7620

Results and discussion

Technical fibers were extracted in 32% yield from the ramie stems and separated from the wood using a mechanical decortication process (see section 2). Three degumming methods have been then compared: (1) a traditional alkali treatment, a SE treatment after (2) a neutral or (3) after a basic impregnation (Fig. 1). The experimental conditions used for the degumming steps were based on our previous studies using hemp technical fibers [START_REF] Sauvageon | Toward the cottonisation of hemp fibers by steam explosion -Part 1 -Defibration and morphological characterization[END_REF].

The compositions of ramie stem, wood, technical fibers and degummed fibers are given in Table 1. Ramie stems displayed relatively high glucans (primarily from cellulose), lignin and xylans contents. After decortication, the woody part retained most of the lignin and xylans, the fibers recovered being cellulose-rich and relatively poor in lignin (< 10%). Glucomannan has been identified as one of the hemicelluloses of ramie [START_REF] Hongshu | The glucomannan from ramie[END_REF]. From the mannose content in the different compartment of the plant listed in Table 1, we can assume that glucomannan is closely associated to the cellulosic fibers whereas xylan is one of the main components of wood. GluA, a marker of pectic components, was recovered in the fiber fraction as well. From Table 1 and in accordance with previous works dealing with ramie fibers degumming, it can be seen that the steam explosion with a basic impregnation is an efficient degumming process, producing cellulose-rich fibers (≈ 85%) with very low amounts of non-cellulosic sugars (<2%). In our study, the relatively high residual lignin contents obtained are justified by the degumming conditions applied, without bleaching agent.

The composition of the stems and extracted wood, technical fibers and fine fibers in terms of Trace Elements (TE) including Rare Earth Elements (REEs) is given in Table 1. The distribution of trace elements in the soil and organs of ramie grown on amended tailings is reported in Supplementary material section S2. In the plants, the average REE concentrations were higher in leaves and roots than in the stems. The major REEs detected in the plant were La, Ce and Nd as well as Y; the other REEs were present in low concentrations (<1 mg.kg -1 ).

The REE bioconcentration factor (leaf concentration/soil concentration) was 0.12±0.07 and the REE translocation factor (leaf concentration/root concentration) was 1.3±0.7, attesting that ramie has relatively poor ability to accumulate REEs. Main other trace elements in the plant were Ba, Sr, Rb and Zn (see Supplementary material section S2).

Table 1

After degumming, it can be observed a strong decrease of REE concentrations in the fine fibers (Table 1), which could be explained by the removal of a part of the pectic polysaccharides which are known to exhibit properties of ions exchange with high affinity for metal ions.

As previously proposed, in the SE process the combination of the steaming step (which provoked the hydrolysis of non-cellulosic components) and the shearing forces (applied during the explosive decompression) increased the extraction ability of TE in the aqueous stream [START_REF] Ziegler-Devin | Steam explosion pretreatment of willow grown on phytomanaged soils for bioethanol production[END_REF]. As a result, the degumming step appears to be an efficient process for both [START_REF] Pickering | A review of recent developments in natural fibre composites and their mechanical performance[END_REF] cleaning TE-enriched phytoremediation-borne ramie fibers and (2) producing superior quality fine fibers with low variability.

Degumming processes produced isolated ramie single fibers having an average diameter of 30 (±5) µm as can be seen in the SEM images given in Fig. 2. In contrast with soda treatment which produced smooth and intact fibers (Fig. 2A), SE degumming resulted in more degraded fibers with cracks and a rough surface (Fig. 2B andC). This can be explained by a degradation under the harsh SE conditions used as previously observed for hemp fibers [START_REF] Sauvageon | Toward the cottonisation of hemp fibers by steam explosion -Part 1 -Defibration and morphological characterization[END_REF].

The tensile properties of the fine ramie fibers produced were also analyzed in terms of elongation, breaking point and tensile strength. All the results obtained are gathered in Table 2. The data for cotton fiber was also used as a reference in order to evaluate the ramie fibers produced in this work in textile area. The results obtained compared well with literature: fibers extensional strain at break is lower for ramie (≈ 4 %) than for cotton fibers (≈ 9 %) but regarding Breaking Point and Tensile Strength (in cN), ramie fibers displayed higher properties [START_REF] Bismarck | Plant Fibers as Reinforcement for Green Composites[END_REF][START_REF] Jiang | A green degumming process of ramie[END_REF][START_REF] Li | High-efficiency ramie fiber degumming and self-powered degumming waste water treatment using triboelectric nanogenerator[END_REF]. The Elongation and Breaking point of ramie fibers produced by a chemical degumming process (≈ 4.5 % and ≈ 47 cN respectively) displayed significantly higher values than exploded fibers (≈ 3.4-3.9 % and ≈ 22-30 cN respectively). These results can be related with the aspect of the fibers given in the Fig. 2, the presence of damaged fibers after SE degumming having probably a negative impact on the tensile properties.

Conclusion

In terms of phytomanagement, ramie can be cultivated on ion-adsorption REE mine tailings and showed a low ability to accumulate REEs, especially in the stems and in the fibers. From this study we can conclude that SE appeared to be an efficient and green process for both degumming and cleaning phytotechnology ramie fibers. However, the relatively harsh conditions used during the SE degumming step resulted in more degraded fibers slightly affecting their tensile properties. This first field trial on REE ion-adsorption mine tailings showed the feasibility of producing industrial ramie fibers from plants growing on such substrate but further work is needed to optimize the crop management, the degumming step and the utilization of the woody part for a sustainable whole valorization of the ramie plant.

Acknowledgement

LERMAB is supported by the French National Research Agency through the Laboratory of Excellence ARBRE (ANR-12-LABXARBRE-01). This work was supported by the international joint laboratory ECOLAND and by ECOS Sud-CONICYT (France-Chili).

Figure 1 .

 1 Figure 1.

  recovered and weighted. The monosaccharide contents were determined from the liquid fraction by HAPE-PAD (ICS-3000 Dionex) equipped with a Dionex CarboPacTM PA-20 (3 x 150 mm) analytical column. For experimental details see Simangunsong et al., 2018. The stems, wood and fibers after treatment were analysed for trace elements at the Service d'

Fig. 2 .

 2 Fig. 2.

Figure 1 .Fig. 2 .

 12 Figure 1. Schematic of the process.

Table 1 . Chemical composition of ramie stems, and extracted wood and fibers and distribution of trace elements including Rare earth elements (REEs) in the organs of ramie plants.

 1

					Chemical composition				Trace elements
		K Lignin %	4	Glc 5		Xyl 6	GalU	7		Man	8	Rb	Zn	REE	9
	whole stems		19.7±1.4		48.9±1.2	9.3±0.3	2.7±0.1	1.5±0.1	53±1.9	70±2.1	17.4±0.4
	wood		21.4±1.2		51.8±0.9 13.3±0.7 3.2±0.2	0.3±0.0	49±1.1	60±1.8	14.9±0.3
	Raw fibers		9.7±0.7			77.4±1.3	0.4±0.0	3.1±0.2	2.0±0.1	71±2.2	100±3.5	6.4±0.1
	Fine fibers
	AT 1		7.6±1.1			82.5±0.6	0.4±0.0	2.1±0.1	2.0±0.2	1±0.0	76±3.2	5.7±0.1
	SEN 2		7.2±1.6			81.6±1.1	0.5±0.0	2.3±0.2	2.7±0.2	3±0.0	61±3.0	4.5±0.1
	SEB 3		9.9±0.2			84.1±0.9	0.3±0.0	0.6±0.0	0.7±0.1	<1	53±2.7	4.4±0.1
	1 alkali treated fibers,	2 steam exploded fibers after water impregnation,	3 steam exploded fibers after basic
	impregnation,	4 Klason lignin,	5 Glucose,	6 Xylose,	7 galacturonic acid,	8 mannose, other sugars (Ara, Rha, Gal and
	GlcU were also detected at low levels (< 1%)

9

sum of the concentrations of the 14 lanthanides and Y.

Table 2 . Physical properties of ramie fibers extracted with tree different degumming processes, compared with cotton fiber.

 2

	347
	348
	349
	350
	351
	352

SUPPLEMENTARY MATERIAL

Industrial Ramie growing on reclaimed ion-adsorption Rare Earth Elements mine tailings in Southern China: Defibration and fibers quality Cesar Segovia 1 , Mahdi Hedri 1 , Hermine Huot 2,3,4 , Isabelle Zigler-Devin 1 , Chang Liu 2, 3, 4 , Yetao Tang 2, 3 , Rongliang Qiu 2, 3 , Jean Louis Morel 4 , Nicolas Brosse

S1-Reclamation Program and growing conditions

The site is located in the Dingnan county of Ganzhou city, Jiangxi province (latitude: Ramie was planted on several terraces (1250 m² in total) in July 2015. Fresh organic amendment was added with a rate of 50-60 t ha -1 and mixed within the first 12 cm of the tailings by tillage. One-year plants (about 35 cm in height) of the ramie cultivar Zhongzhu No.1 were brought from the Institute of Bast Fiber Crops (Chinese Academy of Agricultural Sciences) and planted in rows in the amended tailings with a density of 3-5 plants per m². In June 2016, the aboveground part of the plants was cut and a granulated NPK fertilizer was added with a rate of 450 kg ha -1 . In March 2018, invading plants and aboveground part of ramie were cut and amendment was added to improve the growth of ramie plants. Plants and soil associated to the root system were sampled in May (5 samples), July (3 samples) and November 2016 (3 samples) at different locations of the planted terraces. Soil associated to roots was collected by shaking, air-dried and sieved at 2 mm. All the plants were divided into roots, stems and leaves and washed with tap water using ultrasound bath then rinsed with deionized water.

Then they were dried at 60°C and ground. In November 2018, the stems of several plants were sampled for the analysis of fibers. Some stems reached 2 m in height. The stems were cut and air-dried for several days.

S2-Distribution of trace elements in the soil and plant samples collected in 2016 (mean±standard deviation, n=11)


Reiko Oda

Jeanne Crassous

Nina Berova

Gennaro Pescitelli

Oliver Trapp

ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Special issue: Proceedings of 31st International

Symposium on Chirality, Bordeaux, 2019 R. Oda, J. Crassous, N. Berova, G. Pescitelli, O. Trapp

The 31st International Symposium on Chirality-Chirality 2019, held in Bordeaux, France, from 14 to 17 July 2019, was a great success with the participation of 340 scientists from 28 countries. The symposium started with two short courses given by four renown professors on "Chiroptical Spectroscopy Methods for Structural Analysis of Chiral Molecules" (G. Longhi and P. Polavarapu) and "Materials" (A.

Palmans and G. Markovich). During the following 3 days, we had wonderful invited Plenary and Keynote lectures for which 40% were women lecturers, as well as 58 oral lectures and two poster sessions (162 posters). Very lively discussions and exchanges between the researchers were also observed. We were very happy to have Prof. Larry Nafie who received the Chirality Medal for his seminal contributions to the development and application of vibrational optical activity (VOA). This year, we also had special presentations on "Enantioseparations" honoring some of the founders of the Chirality conferences. We are extremely grateful to the generous academic and industrial sponsors and to the local organizing commit-tee without whom this event would not have been possible.

During the symposium, with the great participations from BioTools, Organic Biomolecular Chemistry Journal, INITIO European project, and Association Française de Cristallographie, six poster prizes have been awarded to the following PhD students and post-doctorants: Hinako Okutsu (Nagoya University, Japan), Kakali Santra (Weismann Institute, Israel), Benjamin Martial (Laval University, Canada), Caoimhe Bond (Queen's University Belfast, UK), Ghislaine Vantomme (Eindhoven University, The Netherlands), and Flavia Pop (University of Angers, France). We do hope that all the participants appreciated the friendly atmosphere, the social program (French cuisine, Bordeaux wine Chateau, Beautiful city center, and so forth) and that we could soon welcome all of them back in Bordeaux! We are very proud of this special virtual issue with eight very high-quality articles by experts in chirality related domains embracing (i) a minireview from our Chirality Medal winner (L. Nafie), (ii) a tribute to Bill Pirkle (C. Welch), and articles on (iii) chiral encaged Verkade's Superbases (A. Martinez), (iv) chiral supra-molecular assemblies in polymer matrices (H. Jintoku), (v) computational aspects of ROA (P. Bour), (vi) hierarchical expression of chirality in aggregates (R. Oda), (vii) autocatalytic chiral amplification effects (M. Amedjkouh), and (viii) solid-state synthesis of chiral cyclodipeptides (A. Zehnacker).