Imanol Landa-Medra

Aitor Eguia-Barrio

Sus Sananes-Israel

Will Porcher

Khiem Tr

Arianna Moretti

Diogo Vieira Carvalho

Stefano Passerini

Iratxe De Meatza

Insights into the Electrochemical Performance of 1.8 Ah Pouch and 18650 Cylindrical NMC:LFP|Si:C Blend Li-ion Cells

Keywords: lithium-ion batteries, silicon graphite anodes, LFP, NMC, electrode manufacturing, cell formats

Silicon has become an integral negative electrode component for lithium-ion batteries in numerous applications including electric vehicles and renewable energy sources. However, its high capacity and low cycling stability represent a significant trade-off that limits its widespread implementation in high fractions in the negative electrode. Herein, we assembled high-capacity (1.8 Ah) cells using a nanoparticulate silicon-graphite (1:7.1) blend as the negative electrode material and a LiFePO 4 -LiNi 0.5 Mn 0.3 Co 0.2 O 2 (1:1) blend as the positive electrode. Two types of cells were constructed: cylindrical 18650 and pouch cells. These cells were subjected both to calendar and cycling aging, the latter exploring different working voltage windows (2.5-3.6 V, 3.6-4.5 V, and 2.5-4.5 V). In addition, one cell was opened and characterised at its end of life by means of X-ray diffraction, scanning electron microscopy, and further electrochemical tests of the aged electrodes. Si degradation was identified as the primary cause of capacity fade of the cells. This work highlights the need to develop novel strategies to mitigate the issues associated with the excessive volumetric changes of Si.

Introduction

Lithium-ion batteries (LIBs) have influenced the technological developments of the last 30 years, from portable electronics to electric vehicles (EVs). Regarding the latter, almost all car manufacturers offer an electric model based on different LIB chemistries [START_REF] Li | High-nickel layered oxide cathodes for lithium-based automotive batteries[END_REF][START_REF] Blomgren | The Development and Future of Lithium Ion Batteries[END_REF]. Nevertheless, it is well known that many car users foster concerns regarding 'driving range anxiety', which is caused by the limited mileage that EVs can achieve without recharging, the availability of recharging points, and the shorter driving range under certain climatic conditions (such as low temperature) [START_REF] Li | High-nickel layered oxide cathodes for lithium-based automotive batteries[END_REF]. As a result, it is necessary to develop novel materials capable of providing higher capacities at higher voltages, which translate into higher energy densities.

LiFePO 4 (LFP) is the safest, state-of-the-art cathode material for automotive applications. In fact, it has been selected by Tesla for the Model 3 [3]. LFP can provide 170 mAh•g -1 at an average voltage of 3.45 V vs. Li, roughly providing 586 Wh•kg LFP -1 [START_REF] Armand | Lithium-ion batteries-Current state of the art and anticipated developments[END_REF]. In order to increase the energy density of LIBs, many EV manufacturers have chosen layered metal oxides as cathode material instead of LFP. Among these layered oxides, LiNi x Mn y Co z O 2 (NMC) and LiNi x Co y Al z O 2 (NCA), both with x + y + z = 1, have been used in different EV models [START_REF] Li | High-nickel layered oxide cathodes for lithium-based automotive batteries[END_REF][START_REF] Li | 30 Years of Lithium-Ion Batteries[END_REF]. Many of these materials can provide higher capacities at higher voltages, leading to higher energy densities [START_REF] Busà | The effects of ambient storage conditions on the structural and electrochemical properties of NMC-811 cathodes for Li-ion batteries[END_REF][START_REF] Wood | Chemical stability and long-term cell performance of low-cobalt, Ni-Rich cathodes prepared by aqueous processing for high-energy Li-Ion batteries[END_REF]. In particular, research associated with NMCs has pursued a decrease in cobalt content in the material replacing cobalt with nickel, which is cheaper and can lead to higher capacities [START_REF] Ryu | Capacity Fading of Ni-rich Li[Ni x Co y Mn 1-x-y ]O 2 (0.6 ≤ x ≤ 0.95) Cathodes for High-Energy-Density Lithium-Ion Batteries: Bulk or Surface Degradation?[END_REF][START_REF] Sun | Impact of microcrack generation and surface degradation on a nickel-rich layered Li[END_REF]. In this context, NMC111 (Ni:Mn:Co = 0.33:0.33:0.33) has been subsequently replaced by NMC532, NMC622, and ultimately NMC811 [START_REF] Ma | A systematic study on the reactivity of different grades of charged Li[Ni x Mn y Co z ]O 2 with electrolyte at elevated temperatures using accelerating rate calorimetry[END_REF][START_REF] Manthiram | A reflection on lithium-ion battery cathode chemistry[END_REF]. Nevertheless, there is a trade-off between the high capacity resulting from high nickel content and the cycle life, as well as the thermal stability of these materials [START_REF] Ma | A systematic study on the reactivity of different grades of charged Li[Ni x Mn y Co z ]O 2 with electrolyte at elevated temperatures using accelerating rate calorimetry[END_REF][START_REF] Manthiram | A reflection on lithium-ion battery cathode chemistry[END_REF].

Regarding the negative electrode, graphite, which possesses a 372 mAh•g -1 capacity and a redox potential of 0.1 V vs. Li + /Li, has been the predominant material of the last 25 years [START_REF] Asenbauer | The success story of graphite as a lithium-ion anode material-fundamentals, remaining challenges, and recent developments including silicon (oxide) composites[END_REF]. Only Li 4 Ti 5 O 12 (LTO) has questioned the supremacy of graphite, mostly for power applications [START_REF] Ma | Comparison of LiVPO4F to Li4Ti5O12 as anode materials for lithium-ion batteries[END_REF]. However, its low discharge capacity (175 mAh•g -1 ) and high redox potential (1.55 V vs. Li + /Li) limit its implementation in high-energy applications [START_REF] Armand | Lithium-ion batteries-Current state of the art and anticipated developments[END_REF]. Nevertheless, graphite is insufficient to achieve the highest volumetric energy density goals [START_REF] Li | Degradation mechanisms of high capacity 18650 cells containing Si-graphite anode and nickel-rich NMC cathode[END_REF][START_REF] Salah | Pure silicon thin-film anodes for lithium-ion batteries: A review[END_REF]. Thus, it has been blended with silicon oxide (SiO x ) and silicon (Si) to enhance its capacity and energy density [START_REF] Asenbauer | The success story of graphite as a lithium-ion anode material-fundamentals, remaining challenges, and recent developments including silicon (oxide) composites[END_REF][START_REF] Chae | Confronting Issues of the Practical Implementation of Si Anode in High-Energy Lithium-Ion Batteries[END_REF]. Silicon offers an excellent capacity and works at ~0.4 V vs. Li + /Li, which makes it an ideal candidate as an anode material [START_REF] Difeng | Achieving a high loading Si anode via employing a triblock copolymer elastomer binder, metal nanowires and a laminated conductive structure[END_REF][START_REF] Chae | Integration of Graphite and Silicon Anodes for the Commercialization of High-Energy Lithium-Ion Batteries[END_REF]. Nonetheless, the drawback of this material is its low cycling stability; its immense capacity is associated with a significant volumetric expansion (+280%) that compromises the mechanical stability of Si anodes [START_REF] Wang | Highly Stretchable Conductive Glue for High-Performance Silicon Anodes in Advanced Lithium-Ion Batteries[END_REF][START_REF] Casimir | Silicon-based anodes for lithium-ion batteries: Effectiveness of materials synthesis and electrode preparation[END_REF]. The continuous expansion/contraction cycles during the lithiation/delithiation cycles lead to the thickening of the solid electrolyte interphase (SEI) and fracture of the Si particles, causing the electric disconnection of these particles [START_REF] Chae | Integration of Graphite and Silicon Anodes for the Commercialization of High-Energy Lithium-Ion Batteries[END_REF][START_REF] Zuo | Silicon based lithium-ion battery anodes: A chronicle perspective review[END_REF]. This loss of active material causes a gradual capacity decrease with the number of cycles; thus, Si displays a limited cycle life [START_REF] Chae | Confronting Issues of the Practical Implementation of Si Anode in High-Energy Lithium-Ion Batteries[END_REF][START_REF] Wetjen | Quantifying the distribution of electrolyte decomposition products in silicon-graphite electrodes by neutron depth profiling[END_REF]. Si is usually combined with graphite in small fractions to obtain a compromise between an increased energy density and an acceptable cycle life. Nevertheless, the exploitation of silicon as an active material cannot be based on the decrease in its concentration until it is functional; it is necessary to determine the reactions occurring in the material upon lithiation/delithiation to optimise its use.

Theoretically and at high temperatures, Si is sequentially lithiated from its original phase to crystalline phases Li 12 Si 7 , Li 7 Si 3 , Li 13 Si 4 , and Li 22 Si 5 , providing a total capacity of 4200 mAh•g -1 [START_REF] Mcdowell | 25th anniversary article: Understanding the lithiation of silicon and other alloying anodes for lithium-ion batteries[END_REF]. At room temperature and in real LIBs, however, Si undergoes a two-phase lithiation in which the intermediate phases are amorphous [START_REF] Ryu | Fundamental Understanding of Nanostructured Si Electrodes: Preparation and Characterization[END_REF]. By the end of the lithiation, no Li 22 Si 5 is formed; the metastable and crystalline Li 15 Si 4 is the silicon phase [START_REF] Mcdowell | 25th anniversary article: Understanding the lithiation of silicon and other alloying anodes for lithium-ion batteries[END_REF]. The capacity that can be obtained with the lithiation of Si to Li 22 Si 5 is 3579 mAh•g -1 [START_REF] Marinaro | Toward pre-lithiatied high areal capacity silicon anodes for Lithium-ion batteries[END_REF]. During subsequent delithiation, the crystalline Li 15 Si 4 is removed, and an amorphous phase is obtained [START_REF] Mcdowell | 25th anniversary article: Understanding the lithiation of silicon and other alloying anodes for lithium-ion batteries[END_REF]. Thus, analysis by means of X-ray diffraction (XRD) can provide valuable information on the degree of lithiation of Si [START_REF] Limthongkul | Electrochemically-driven solid-state amorphization in lithium-silicon alloys and implications for lithium storage[END_REF].

Another key aspect in the development of high-energy LIBs is the format of the cells. There are three main categories: prismatic, cylindrical, and pouch [START_REF] Armand | Lithium-ion batteries-Current state of the art and anticipated developments[END_REF]. Cylindrical cells are the most widely implemented format [START_REF] Quinn | Energy Density of Cylindrical Li-Ion Cells: A Comparison of Commercial 18650 to the 21700 Cells[END_REF]. Their dense container helps to prevent deformation due to swelling in the presence of side reactions [START_REF] Budde-Meiwes | A review of current automotive battery technology and future prospects[END_REF]. They are defined with a numeric code (XXYYY) in which the first two numbers (XX) represent the diameter in mm, and the remaining numbers (YYY) represent the height of the cell in tenths of mm [START_REF] Blomgren | The Development and Future of Lithium Ion Batteries[END_REF] Among them, the 18650 cells are the most popular; these cells were initially manufactured by Sony for their cameras, and the length of 65 mm is due to space limitations in such a device designed to be held in the palm of a single adult hand [29]. On the other hand, the diameter of 18 mm was selected due to safety reasons; it was determined as the maximum size to avoid thermal runaway for a cell of ~1 Ah capacity [29]. Recently, TESLA has announced the shift to 4860 cylindrical cells, despite the safety issues that can arise [START_REF] Tranter | Communication-Prediction of Thermal Issues for Larger Format 4680 Cylindrical Cells and Their Mitigation with Enhanced Current Collection[END_REF], which will probably have an impact on the cell size selected by other EV developers.

Even if cylindrical cells are the first option for industry, their low packing density and poor heat transport motivated battery developers to search for alternatives. Prismatic cells, with hard casings similar to those of cylindrical cells, provide safety towards swelling with increased packing density, but their energy density is ~20% lower than that of cylindrical cells [START_REF] Li | 30 Years of Lithium-Ion Batteries[END_REF]. In any case, most manufacturers select this format in their EVs [START_REF] Quinn | Energy Density of Cylindrical Li-Ion Cells: A Comparison of Commercial 18650 to the 21700 Cells[END_REF]. In addition, it is believed that pouch cells (prismatic cells with a soft packaging) will be able to outperform their competitors, becoming the primary option in the near future [START_REF] Budde-Meiwes | A review of current automotive battery technology and future prospects[END_REF].

In this work, high-energy cells with 1.8 Ah capacity were assembled in two different formats to assess the impact of the cell design and casing/packaging: 18650 cylindrical and pouch. The anode consisted of a graphite/Si mix material, while the cathode comprised of an LFP/NMC532 blend. A combination of materials was utilised to increase the energy density of the electrodes through the addition of NMC532 and Si to the stable-cycle-life LFP and graphite, respectively. NMC532 was selected due to its good compromise between high capacity and stability at high voltages [START_REF] Jung | Oxygen release and its effect on the cycling stability of LiNixMnyCozO 2 (NMC) cathode materials for Li-ion batteries[END_REF]. Both types of cells were assembled using the same batch of electrodes and subjected to the same cycling protocols. The calendar ageing of some cells was investigated, while the cycling age of the other cells was studied using three different working voltages. Lastly, one cell was opened and characterised at the end of its cycle life.

Materials and Methods (Experimental) 2.1. Anode Manufacturing

The negative electrode of this work was prepared at CIDETEC's electrode manufacturing line. The components of this electrode were nanoparticulate silicon (N-100, Tekna, Solli, Oslo) and graphite (MEG-2C, SGL Carbon, Meitingen, Germany) as anode active materials, Super C45 carbon (Imerys, Paris, France) as the conductive additive, carboxymethyl cellulose (CMC, Wallocel DOW, Midland, MI, USA) as the dispersant and binder, and styrene butadiene rubber (SBR, JSRmicro, Leuven, Belgium) as the co-binder. These components were mixed in a weight ratio of [Si/Gr/C45/CMC/SBR] = 10.4/74.6/5/5/5. The experiments performed to define the anode formulation are shown in Figure S1.

The components were water-processed in a planetary mixer. In addition, the procedure was adapted to eliminate agglomerates via pre-dispersion of Si in the CMC solution and addition of the solids (C45 and graphite) at different steps and the SBR latex at the end. Unexpected low slurry pH was measured (pH ~3), which could affect the polymer (CMC and especially the SBR) conformation. Thus, the slurry pH was adjusted to pH = 6-7 by addition of ammonia (NH 4 OH). Finally, a mirror-like wet coating with minimal fish-eye spots, straight edges, two-side alignment within <1 mm, and targeted loading (2.54 mAh/cm 2 ) of 3.7 mg/cm 2 within 0.3 mg/cm 2 deviation between faces was achieved.

Overall, 75 m was produced in two different coating widths (130 and 205 mm, onto 250 mm-width and 10 µm thick Cu foil, Schlenk, Roth, Germany) for each of the cell formats (cylindrical and pouch cells, respectively).

The anodes with 205 mm width for soft packaged pouch cells did not need slitting. Electrodes were die-cut directly (four anodes on 14 cm wide sheet) after calendering, for the stacked design of 100 × 61 mm coated area, by CIDETEC. The anode rolls manufactured in 130 mm width coating were slit by CEA for cylindrical cells. To limit waste, CEA used a lab slitting equipment to slit the anode coating.

The calendering step for the anodes was aimed at an expected optimum porosity of 32% (1.41 g/cm 3 ). Control of flexibility performed by bending test (no damage when the electrode was wound on mandrels with decreasing diameter) revealed no cracks on the 2 mm diameter mandrel. This coupled with the 90 • peel test strength (67 ± 2 N/m) provided satisfactory mechanical results with very high adhesion to the Cu current collector.

Cathode Manufacturing

The positive electrode in the current work was developed in CEA and then upscaled, adapting viscosity with coating equipment capability for 50 L of slurry and a coating machine with an oven of 5 m length (Megtec, De Pere, WI, USA). The positive electrode consisted of LiFePO 4 (LFP, beLife, Dnipropetrovsk, Ukraine) and LiNi 0.5 Mn 0.3 Co 0.2 O 2 (NMC532) as active materials, Super C65 carbon black (Imerys, Paris, France) as the conductive additive, and polyvinylidene fluoride (PVDF, Solvay Solef ® 5130, Brussels, Belgium) as the binder. The weight ratio of these components was [LFP/NMC/C45/PVDF] = 45.25/45.25/5/4.5. Finally, CEA coated 380 m of two-sided electrode from the slurry onto an aluminium current collector of 20 µm thickness and a width of 30 cm (Hydro, Oslo, Norway). The loading of this coating was 14.4 mg/cm 2 (2.3 mAh/cm 2 ). During the slitting step, the electrode width was adjusted by cutting the coils. Then, the cathode was calendered to 36% porosity (2.3 g/cm 3 ). After calendering, a control of flexibility (satisfactory at 4 mm diameter bending) and adhesion strength (260 ± 21 N/m) was applied.

Cell Manufacturing

In order to compare the two cell designs, both the cylindrical and the pouch cells consisting of the same components (except the separator which was specific to the assembly) were conditioned with the same protocol. The separator was a tri-layer Celgard 2325 grade (Charlotte, NC, USA) for the cylindrical hard-case cells, while the stacked soft packaging cells were assembled with a modified Celgard ECT-2015 grade (same thickness) suitable for the specific lamination/winding process on the cell-assembly line. The electrolyte was composed of ethylene carbonate and dimethyl carbonate (EC:DMC) in a volume proportion of 1:1 with 1 M lithium hexafluorophosphate (LiPF 6 ) and a blend of additives: 10% fluoroethylene carbonate (FEC), 2% lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), and 2% vinylene carbonate (VC). The electrolyte was purchased from UBE Industries.

Assembly of 18650 Cylindrical Cells

Cell assembly was performed on semiautomatic winding equipment inside a dry room with a dew point of -40 • C. Each cell consisted of a double-side coated 55 mm wide cathode and a 57 mm wide anode with two 60 mm wide separators. Electrodes and separators were wound around a mandrel, and the resultant jellyroll was dried in a vacuum oven overnight. After welding of the tabs on the bottom and the cap for the anode and cathode, respectively, and grooving, the cells were placed in an Ar-filled glove box for electrolyte filling and crimping. A picture of the components used for the assembly of 18650 cells is shown in Figure S2.

Assembly of Pouch Cells

Electrodes were cut to size in a semiautomatic die-cutting unit (MTI Corp., Richmond, CA, USA) to 14 cm sheets from the electrode rolls. The cathodes and anodes were cut to different sizes (10 cm × 6.1 cm and 9.8 × 5.9 cm for the anodes and cathodes, respectively). Pictures of the die-cutting unit, a schematic representation of the cells, and a picture of the final cell are shown in Figure S3.

The stacked soft-packaging cell was designed comprising eight cathodes and nine anodes per cell. The assembly was carried out in a dry room (dew point -50 • C) by manual stacking of the electrodes after vacuum-drying at 120-140 • C for 12 h. The process, using a guiding tool to guarantee stack alignment, is depicted by the photographic sequence in Figure S4.

Electrode flanges (tabs) were ultrasonically welded to terminal tabs (100 µm thick Al (+) and Ni-plated Cu (-)) and then placed between two half-shells of aluminium laminated foil (ALF) pouch material (without depth-forming) and heat sealed on three sides before the filling step.

The cells were filled with 11.5 g (9 mL) of electrolyte, and the remaining side was thermally sealed under -850 mbar using a vacuum chamber sealer. The cells were then ready to be formed (see Section 2.4) under external pressure applied by sandwiching the cell between two stainless-steel plates.

After this formation, the cells were degassed and finally sealed under full vacuum for grading characterisation.

Electrochemical Tests 2.4.1. Conditioning (Formation)

The conditioning experiments were performed inside a temperature chamber set at 45 • C. After a resting period of 2 h, a 1 C pre-charge pulse of 10 s was applied, which was followed by a 3 h rest period for the impregnation of the electrolyte. After that, a C/10 constant current cycle between 4.5 and 2.5 V was conducted, including a constant voltage step by the end of the charge at 4.5 V until the current decreased to C/20. Afterwards, the cells were removed from the chamber, waiting until their temperature dropped below 30 • C. After this formation, the pouch cells were degassed and finally sealed under full vacuum for grading characterisation.

Calendar Ageing

After conditioning, four cells per format were charged to 3.6 V (two cells per format) and 4.5 V (two cells per format) and stored at 25 • C for 58 days (for the pouch cells) and 96 days (cylindrical cells). The cells were kept at an open-circuit voltage state. The capacity evolution during calendar ageing was determined after two consecutive cycles with a 0.3 C charge and discharge current rate.

Electrochemical Ageing

The cycling ageing tests were applied on cylindrical and pouch cells. Cycle ageing was performed with a current of 0.3 C for both charging and discharging within three different voltage windows: (i) between the minimum and maximum voltage limits (2.5-4.5 V), which includes the transition between LFP and NMC and between two intermediate voltages, (ii) 3.6-4.5 V, and (iii) 2.5-4.5 V to investigate the ageing degradation on the different LFP and NMC voltage working range. Each test was carried out on two cells from the same batch to ensure the test result repeatability. Electrochemical tests were performed with a Basytec Cell Test System potentiostat at 25 • C ± 1 • C (at CIDETEC facilities, Donostia-San Sebastian, Spain), a Maccor cycler S4000 (at Helmholtz Institute Ulm, Ulm, Germany), and a PEC SBT0550 battery cycler (at CEA, Grenoble, France).

Post-Mortem Characterisation

One cylindrical cell was opened to conduct post-mortem characterisation of its electrodes. This cell was previously cycled at 25 • C and within the voltage range 2.5 V-4.5 V until it reached 70% state of health (SOH) after 44 cycles. The cell was then fully discharged (0% state of charge, SOC) and introduced in an argon-filled glove box (MBraun, München, Germany) with O 2 and H 2 O concentration below 1 ppm, respectively. The venting was pierced to evaluate the internal pressure, the free electrolyte was recovered by the venting, and the cell case was cut. Afterward, the electrode roll was extracted and unwound. The positive and negative electrodes were separated, and samples for post-mortem and extended electrochemical analyses were cut out from the middle part of the recovered sheets (avoiding the external parts of the electrodes). These samples were rinsed with DMC solvent; rinsing baths of solvent were used, in which each sample was soaked for approximately 30 s. Pristine samples were studied in parallel.

Scanning electron microscopy (SEM) imaging and energy-dispersive X-ray spectroscopy (EDX) was performed on a carbon-sputtered sample using a JSM 7600F (JEOL, Tokyo, Japan). The crystallographic analysis of the different samples was performed by means of powder XRD, using a Bruker D8 Discover diffractometer (Cu Kα radiation, λ = 0.154 nm, Billerica, MA, USA) equipped with a LynxEye PSD detector (Stockholm, Sweden). The diffractograms were recorded between 2θ = 10 • and 80 • at 0.003 • •s -1 . XRD and SEM analyses were performed using an inert transfer chamber to protect the sample from the external atmosphere.

Lastly, some samples were also used to assemble half coin cells (HCCs, CR2032 configuration) using lithium metal (Rockwood Lithium, 500 µm thick, Frankfurt, Germany) as the counter electrode. Electrodes of 1.13 cm 2 were punched and assembled in an argon-filled glove box (H 2 O < 0.1 ppm, O 2 < 0.1 ppm) versus lithium, using Whatman, GF/D separator (Maidstone, UK), and 120 µL of 1 M of LiPF 6 in EC:DMC (1:1) + 10% FEC, 2% LiTFSI, and 2% VC electrolyte. These cells were subjected to two cycles at C/20 followed by a rate capability test and 150 cycles at C/3 (for the half coin cells with recovered negative electrode) or 1 C (for the half coin cells with recovered positive electrode). The tests were conducted at 20 • C. The potential windows for positive and negative electrode HCCs were 4.3 V-2.6 V and 1.0 V-10 mV, respectively.

Three-Electrode Cells

Three-electrode cells were assembled to monitor the potential of each of the electrodes upon galvanostatic cycling. Three-electrode Swagelok cells were assembled in an MBraun argon-filled glove box with oxygen and water contents below 1 ppm. Lithium metal foil (Rockwood Lithium, Frankfurt, Germany) was used as reference electrode along with glass fibre separators (Whatman, Cytiva, Maidstone, UK), soaked with 1 M LiPF 6 in EC:DMC (1:1) + 10% FEC, 2% LiTFSI, and 2% VC electrolyte.

Results

Conditioning Results

Figure 1 shows a representative conditioning cycle of one of the cells. The conditioning cycle at 45 • C is divided into different steps in Figure 1a. Initially, a 1 C pulse of 10 s was introduced in between two resting periods of 2 and 3 h. The aim of these resting periods was to achieve an efficient impregnation of the electrodes and the separator with the electrolyte, while the pulse was applied to avoid copper oxidation at ~0 V. Afterwards, a C/10 C-rate was applied throughout the charge step. The potential initially increased rapidly until ~2.5 V, at which point the SEI was formed [START_REF] Moretti | A Comparison of Formation Methods for Graphite//LiFePO 4 Cells. Batter[END_REF]. The delithiation of the LFP, together with the lithiation of the anode, was the reaction corresponding to the plateau between 3.35 and 3.5 V. Even if it is easy to ascribe this plateau to LFP in the cathode, it is not trivial to identify the anode active material (Si or graphite) undergoing the reduction reaction. This analysis was performed using a three-electrode cell and is discussed in Section 3.3. The delithiation of NMC532 and the lithiation of the anode were the main reactions occurring above 3.5 V. Most of the charge capacity was obtained in this last region (~1.3 Ah), with the capacity in the LFP delithiation region being only ~0.45 Ah (Figure 1b).

The subsequent discharge was initiated with the delithiation of the anode and the lithiation of the NMC532 (~1.1 Ah), followed by a stable plateau between 3.1 and 2.5 V for LFP lithiation and the delithiation of the anode.

The discharge capacity and coulombic efficiency obtained for the pouch cells were 1.81 ± 0.05 Ah and 82.3% ± 0.5%, respectively, whereas 1.93 ± 0.02 Ah and 86.6% ± 0.3% were obtained for the 18650 cells (Table 1). The higher discharge capacity obtained in the 18650 cells probably originates from the different behaviour towards the residual water content during cell assembly. For the first point, all the cells were assembled in a dry room, paying attention to dry all the components before assembly. It is reasonable to consider that the cylindrical configuration is more resilient at maintaining pressure on the electrode and to avoid the particles from disconnecting even if the pouch cells are formed between two plates.

Calendar Ageing

Figure 2 shows the evolution of SOH at the end of the calendar ageing test of both the cylindrical and the pouch cells at 3.6 and 4.5 V. The SOH was calculated using Equation [START_REF] Li | High-nickel layered oxide cathodes for lithium-based automotive batteries[END_REF].

SOH (%) = 100 - (C 0 -C i ) C 0 × 100, (1)
where C i is the discharge capacity measured at the end of the ageing test, and C 0 is the discharge capacity measured before the ageing test (initial capacity), both obtained at a 0.3 C-rate. The capacity of the cells stored at 3.6 V seems to decrease more slowly than that of the cells stored at the higher voltage of 4.5 V. This is because the electrolyte degrades faster at higher voltage, i.e., when the cell is fully charged. On the other hand, the degradation is higher in pouch format when aged at 4.5 V. No notable differences were observed between the two cell formats at 3.6 V calendar aging. Similar to what was observed for the calendar ageing tests, the pouch cells seemed to degrade faster than the cylindrical cells.

Cycling Aging: Effect of the Voltage Cycling Window on the Capacity Fade Rate

For both cell formats, the cells cycled within the 3.6 to 4.5 V voltage window showed the slowest ageing rate, compared to the cells cycled within the voltage windows of 2.5 to 3.6 V and 2.5 to 4.5 V. For the latter two cases, there was a minimal difference between the capacity of the cylindrical cells, which showed slightly higher capacity retention. On the other hand, the pouch cells showed similar capacity degradation at those two voltage windows.

To provide further insight into the cause of the differences in the capacity retention of the cells depending on the voltage window, a three-electrode Swagelok cell with Li as the reference was assembled and cycled between 2.5 and 4.5 V. The electrochemical results obtained with this cell are shown in Figure 4.

The curve of the capacity evolution with the cycle count (Figure 4a) showed an almost linear and steep capacity decay after the two initial C/20 cycles. These two initial cycles are analysed in detail in Figure 4b, where the contribution of the anode and the cathode were obtained through the use of the reference electrode included in the cell. Both cycles displayed that the discharge capacity of the full cell was limited by the delithiation of the anode in the voltage window selected (2.5-4.5 V). The differential analysis of the lithiation and delithiation curves of the anode in these two cycles are shown in Figure 4c, where the peaks associated with the (de)lithiation of graphite and silicon are differentiated. It can be observed that the delithiation of silicon occurred at ~0.4 V, which is close to the lower cut-off voltage of the full cell (below 3 V in Figure 4b). This contribution disappeared in the DVA curve with the repetitive lithiation/delithiation steps at C/3, as highlighted in Figure 4d. Thus, the cells cycled within the voltage window of 3.6-4.5 V were cycled in a range that avoided deep Si lithiation/delithiation. These results explain the higher cycle life observed in cells cycled at the 3.6-4.5 V voltage window.

Post-Mortem Characterisation

One of the cylindrical cells cycled in the voltage window 2.5-4.5 V was dismantled (Figure S5) and subjected to material and electrochemical characterisation, particularly of the electrodes. On initial visual analysis, the negative electrode presented large de-bonding areas, as shown in Figure S6. The zones strongly adhered to the separator were white/grey, while no drastic colour change was observed for the separator which mainly remained white (Figure S7). The jelly roll was still well soaked by the electrolyte during dismantling. The positive electrode unexpectedly showed a very high degree of debonding, as shown in Figure S8. Usually, no (or very-low level) debonding occurs for the positive electrodes. Nevertheless, undamaged samples of this electrode showed a very high adhesion of around 100 N/m.

SEM-EDX

The SEM images of the positive electrode are given in Figure S9. The positive postmortem electrode was composed of a mixture of particles with small (Ø < 1 µm) and larger (1 µm < Ø < 5 µm) particle sizes. Some carbon fibres were also observed. The morphology of the positive electrode was homogeneous in large zones, as shown in the lowest-magnification SEM picture.

EDX analysis, corresponding to a rectangular zone (~40 × 55 µm), is presented in Figure S10. The positive electrode was mainly composed of C, O, Fe, and P elements. Mn and Ni were also detected, but in smaller amounts. The Co signal was not detected, but it could have been masked by the Fe main peak, as it appeared in the same energy domain. The EDX analysis was also performed in three different zones, as described in Figure S11. The EDX spectra are compared in Figure S12; Point 1 corresponds to an area with a large particle size (1 µm < Ø < 5 µm). EDX analysis revealed the main composition of Mn, Ni, and Co elements, thus corresponding to NMC532 particles. A small amount of P was also detected; however, because of the small amount of P along with peak overlap, it was difficult to deduce the chemical nature of the P-containing material. It could correspond to LiPF 6 or LiFePO 4 materials, as F or Fe peaks would have been masked by the Mn, Co, and Ni peaks. On the contrary, EDX analysis of Point 2 and Point 3 showed similar compositions. Here, Fe and P were the main elements, together with C and O, as shown in Figure S12. No other elements such as Ni, Mn, or Co were detected; thus, it corresponded to LiFePO 4 material.

The negative electrode (Figure S13) was composed of nanometric (Ø ~200 nm) and micrometric (Ø ~5-10 µm) particles. The micrometric particles were smooth. The electrode seemed to be homogeneous, as shown in the lowest-magnification (×200) SEM picture, corresponding to a rectangular area of 400 × 600 µm 2 . Furthermore, no deposited particle was observed on the surface of the electrode. EDX analysis of a rectangular area with dimensions of 150 × 200 µm 2 is shown in Figure S14. The main elements from the negative electrodes, namely, C, O, and Si, were detected, while F and P were also observed, likely derived from the SEI layer and/or LiPF 6 salt. Lastly, small amounts of Mn were observed, indicating dissolution of Mn from the positive electrode, followed by its reduction in the negative electrode. Similar EDX analysis and conclusions were also obtained with another area having smaller dimensions (40 µm × 55 µm). EDX analyses were also performed in three different zones of the electrode, as shown in Figure S15. Point 1 corresponds to the micrometric particles, whereas Point 2 and Point 3 correspond to the nanometric particle agglomerates. However, in the Point 2 area, the nanometric particles seemed to be 'naked', whereas they seemed to be covered by a 'layer' in Point 3. The respective EDX spectra are given in Figure S16. In all three areas, C, O, and Si were detected. Furthermore, Na, F, and P elements were also observed. The Na was attributed to the CMC binder, whereas F and P stemmed from LiPF 6 residues and/or the SEI layer. Mn from the dissolution of the positive electrode was only observed in Point 2. Lastly, a very small amount of S from LiTFSI additive in the electrolyte, close to the detection limit, was detected in Point 2 and Point 3. The intensity of the Si peak in Point 1 was the lowest. This is consistent with the morphology of the particle, which preferably corresponded to graphite (micrometric) rather than Si (nanometric) particles. However, Si particles could have been deposited on the surface or very close to the graphite particles, as Si was detected. In Point 3, the intensity of C and F peaks was higher than in Point 2. This likely corresponds to an SEI-like layer, which is in good agreement with the SEM observations, where a thin layer was observed. Its composition could have been LiF rather than Li x PO y F z , because the intensity of the P peak did not change between Point 2 and Point 3.

XRD

The samples of the positive and negative electrodes taken from the aged cells are compared with their corresponding pristine electrodes in Figure 5. The two active materials of the cathode blend, i.e., LiFePO 4 (JCPDS No. 83-2092) and NMC532 (JCPDS No. 00-85-1968), could be clearly indexed for the pristine sample (Figure 5a). On the other hand, the aged electrode showed no presence of the lithiated phosphate, and only the FePO 4 phase could be observed. This suggests significant lithium inventory loss in the cell; hence, the remaining cyclable lithium only allowed the lithiation of the NMC phase and was not available for the phosphate phase (reaction at lower voltage). This is also supported by the electrochemical characterisation of the harvested cathodes (Figure 6a). Additionally, there was no significant change in the NMC532 phase that would indicate structural ageing. Regarding the anode (Figure 5b), graphite (88 wt.%) and silicon (12 wt.%) could be clearly observed and indexed for the pristine sample. In the aged sample, however, while no major changes could be observed for the graphite (JCPDS No. 01-73-5918) phase (no shift of 00l reflection), the contribution of silicon (JCPDS No. 00-005-0565) nearly disappeared, as evidenced by the absence of the peaks detected in the pristine sample at 28.4 • and 47.3 • . Instead, several additional peaks with low intensity could be distinguished in the aged sample, particularly in the 30 • to 40 • range. The compounds leading to these reflections remain unidentified.

Electrochemical Characterisation of Aged Electrodes

Some electrodes disassembled from the aged cylindrical cell were punched and assembled in half coin cells. Figure 6 displays the results of the half-cells assembled using the aged electrodes. The voltage profile of the first and fourth cycles of the cathode half-cell subjected to galvanostatic cycling are shown in Figure 6a, while the rate capability test can be found in Figure 6b. The first delithiation of the aged cathode showed a high polarisation of up to ~3.8 V with a similar profile of NMC and absence of active LFP. Upon discharge, the first part of the voltage slope was attributed to the lithiation of NMC until a plateau at ~3.4 V, corresponding to the reactivation of LFP. The voltage profile of the fourth cycle clearly showed the voltage profiles of both NMC and LFP, confirming that the structure of both cathode materials was not damaged. Nevertheless, in Figure 6b, the long-term cycling of the cathode in the half-cell is also displayed, highlighting that, even when using excess fresh electrolyte and lithium metal as the anode, the cell suffered from a constant capacity fading. Overall, the NMC capacity was reduced upon cycling, while the plateau of LFP was less altered (inset of Figure 6b).

The residual capacity of the anode (Figure 6c) was below 2 mAh, and no charge plateau at 0.4 V characteristic of Si delithiation was visible. Its absence even during further cycling (and the low capacity but high stability of the anode for more than 100 cycles in Figure 6d) suggests the complete loss of Si activity.

Discussion

Overall, the performance of pouch cells was lower than that of cylindrical cells in cycling and calendar ageing tests. This can be attributed to two different factors. On the one hand, the hard casing of cylindrical cells can prevent excessive volumetric expansion. Berckmans et al. evidenced that the application of external pressure can provide higher capacity and lower cell resistance with extended cycle life [START_REF] Berckmans | Analysis of the effect of applying external mechanical pressure on next generation silicon alloy lithium-ion cells[END_REF]. They attributed this improvement to the limitation of the swelling of the Si particles. The coulombic efficiency was higher for cylindrical cells, evidencing the lower occurrence of irreversible reactions. In addition, calendar ageing experiments revealed that hard cylindrical casing was more efficient in avoiding capacity loss than soft pouch packaging. This could be associated with the easier deformability of the latter format, which could allow side reactions such as gassing. Although the pouch format could improve the packaging density of the cells, the energy and number of cycles that we could obtain from these cells remain unsatisfactory; hence, additional development is required, such as control of the optimum external pressure.

Furthermore, the electrochemical and post-mortem characterisation of the cells further support that the origin of the capacity fade of the cells could be attributed to the loss of cyclable Si. As discussed in Section 1 and evidenced by the three-electrode cell, the lithiation of bulk crystalline silicon proceeded via a two-phase reaction (ca. 0.17 V) and involved the formation of amorphous Li y Si. The complete lithiation resulted in crystalline Li 15 Si 4 . Reversing the process, amorphous Li z Si was formed via a two-phase reaction (ca. 0.45 V), followed by the complete delithation (via a solid solution mechanism) of amorphous silicon. This phase could be relithiated to amorphous Li x Si; while charging below 0.07 V vs Li + /Li, the crystalline Li 15 Si 4 was formed again. Rhodes et al. [START_REF] Rhodes | Understanding the Degradation of Silicon Electrodes for Lithium-Ion Batteries Using Acoustic Emission[END_REF] experimentally showed that the potential regions where extensive fracturing of Si takes place correspond to the twophase reactions of Li 15 Si 4 . Therefore, the faster decay of the cells cycled down to 2.5 V, corresponding to extensive formation and dealloying of Li 15 Si 4 , could be partially attributed to the silicon containing anode. The three-electrode experiment showed a progressive loss of Si activity upon cycling at C/3 between 2.5 and 4.5 V, most probably due to the typical fracturing upon large volume expansion/contraction. On the contrary, when the discharge cut-off was limited to 3.6 V, the capacity retention was higher than when using 2.5 V as the cut-off limit. This was associated with silicon utilisation, with a lower working voltage of the cell resulting in higher silicon utilisation. Furthermore, it was observed that cycling in voltage windows of 2.5-3.6 V and 2.5-4.5 V had similar ageing effects. As a result, it appears that the low cut-off voltage was associated with a full silicon reaction that accelerated the capacity fade in both cylindrical and pouch cell formats.

The hypothesis of silicon governing the capacity fade of the cells was further confirmed by the post-mortem characterisation. On the one hand, disappearance of the Si signal from the XRD pattern of the post-mortem negative electrode indicated the degradation of this material, likely pulverised and disconnected from the anode. In addition, assembly of HCCs with aged electrodes suggested that the remaining active materials continued being electrochemically active, while Si was the sole unrecoverable material.

Consequently, it is necessary to study the stabilisation of silicon in the repetitive volumetric expansion/contraction during lithiation/delithiation cycles. Many efforts have been directed toward developing silicon/carbon composites that can buffer these volumetric changes, enhancing the cycle performance of the cells [START_REF] Chae | Integration of Graphite and Silicon Anodes for the Commercialization of High-Energy Lithium-Ion Batteries[END_REF][START_REF] Luo | Surface and Interface Engineering of Silicon-Based Anode Materials for Lithium-Ion Batteries[END_REF]. The use of silicon oxide (SiO x ) instead of Si can also be a good alternative; even if its capacity is lower (1200-1500 mAh•g -1 ) [START_REF] Shen | Scalable synthesis of Si nanowires interconnected SiOx anode for high performance lithium-ion batteries[END_REF][START_REF] Suh | Electrochemical behavior of SiOx anodes with variation of oxygen ratio for Li-ion batteries[END_REF], it undergoes a lower volumetric expansion and can be added in higher fractions in the anode formulation [START_REF] Pan | Systematic electrochemical characterizations of Si and SiO anodes for high-capacity Li-Ion batteries[END_REF].

Conclusions

In this work, we assembled 1.8 Ah cells in 18650 cylindrical and pouch formats using NMC532/LFP and graphite/Si blends as the positive and negative electrodes, respectively. Cells of both formats were subjected to electrochemical testing, while a cylindrical cell was also opened at its end of life, and the electrodes were post-mortem characterised. The electrochemical performance of the cylindrical cells was superior to that of pouch cells in terms of both capacity and cycle life; a lower irreversible capacity loss was also reported. This was attributed to a better cell pressure management in the hard-cased cylindrical cells. In addition, it was observed that the main capacity decay mechanism of the cells was due to the degradation of Si. Furthermore, the voltage window selected for the galvanostatic cycling significantly affected the capacity retention. The cycle life was extended when the cells were cycled in the 3.6-4.5 V range due to the limited lithiation of silicon, as shown by the three-electrode cell tests. This was also confirmed by the post-mortem analysis of the cylindrical cell, which further evidenced the Si degradation. This supports the need to develop strategies that minimise the negative impact of silicon due to its volumetric expansion/contraction upon cycling.

Supplementary Materials:

The following supporting information can be downloaded at https: //www.mdpi.com/article/10.3390/batteries8080097/s1: Figure S1

Figure 1 .

 1 Figure 1. Characteristic conditioning cycle of the cells. (a) Voltage vs. time representation and (b) voltage vs. capacity representation. Electrochemical processes at each step of the profile are indicated in (a).

Figure 2 .

 2 Figure 2. SOH evolution upon calendar aging at 25 • C for cylindrical (circles) and pouch (squares) cells at 3.6 V (blue markers) and 4.5 V (red markers). Error bars indicate the standard deviation between the two cells per experiment.

Figure 3

 3 Figure 3 shows the evolution of the SOH versus the total capacity throughput (cumulative capacity during the cycle life) of the cylindrical and pouch cells at 25 • C. The circles, triangles, and squares indicate the cells cycled in the 2.5-3.6 V, 3.6-4.5 V, and 2.5-4.5 V voltage windows, respectively.

Figure 3 .

 3 Figure 3. SOH evolution with the total capacity throughput for cylindrical (circle markers) and pouch (square markers) cells cycled in the 3.6-4.5 V (blue markers), 2.5-3.6 V (orange markers), and 2.5-4.5 V (green markers) voltage windows.

Figure 4 .

 4 Figure 4. Experiments with a three-electrode cell consisting of an NMC-LFP cathode, a Gr/Si anode, and a Li reference electrode cycled at 25 • C between 2.5 and 4.5 V at a C/3 C-rate. (a) Discharge capacity with the cycle count. (b) Voltage profiles at C/20. Differential capacity plot at (c) C/20 and (d) C/3 of the anode voltage profiles. Delithiation features associated with graphite and silicon utilisation in (c) are highlighted in yellow and green, respectively. Evidence of a lack of silicon activity after 50 cycles in (d) is highlighted in purple.

Figure 5 .

 5 Figure 5. X-ray diffraction patterns of the pristine and aged electrodes. (a) Cathodes and (b) anodes. Blue, green, and pink dashed lines in (a) are ascribed to diffraction features of NMC532, LFP, and FePO 4 , respectively. Blue and green markers in (b) are ascribed to graphite and silicon, respectively.

Figure 6 .

 6 Figure 6. Comparison of the voltage profile and subsequent rate capability test from aged (a,b) cathodes and (c,d) anodes. Red and green galvanostatic curves correspond to cycles number 1 and 4, respectively. The inset in (b) shows the voltage vs. capacity curve of these cells for cycles 7 (red line), 25 (green line), and 140 (blue line) at 1 C discharge and 0.3 C charge C-rates.

  . (a) Electrochemical performance of anodes containing 12 and 15 wt.% Si in half coin cells (CR2032, separator Whatman GF/D, electrolyte 1 M LiPF 6 in EC:DMC (1:1 wt.%) + 2% FEC) and (b) results with reduced carbon content; Figure S2. Jelly roll and all components for assembling an 18650 cell; Figure S3. Die-cutting unit for the small pouch cell format (left), stacked electrode soft packaging cell design with dimensions (centre), and one of the final cells (right); Figure S4. Stacking process steps: lamination of cathode with separator, sequential stacking of electrodes, cell core wrapping, and tab ultrasonic welding; Figure S5. Picture of the aged cell opened in the glove box; Figure S6. Picture of the aged negative electrode. (a) Debonding and (b) white/grey areas, more strongly adhered to the separator; Figure S7. Picture of the aged separator facing the positive electrode; Figure S8. Picture of the aged positive electrode with large debonding area; Figure S9. SEM images of the positive electrode at different magnifications; Figure S10. EDX analysis of the positive electrode; Figure S11. Location of the three electrode areas at which the EDX analysis of the positive electrode was conducted; Figure S12. Results of the EDX analysis conducted at the three electrode areas of the positive electrode; Figure S13. SEM images of the negative electrode at different magnifications; Figure S14. EDX analysis of the negative electrode; Figure S15. Location of the three electrode areas at which the EDX analyses of the negative electrode were conducted; Figure S16. Results of the EDX analysis conducted at the three electrode areas of the negative electrode. Author Contributions: Conceptualisation, A.M., S.P. and I.d.M.; data curation, I.L.-M., A.E.-B., W.P., K.T., A.M., D.V.C. and I.d.M.; funding acquisition, S.P. and I.d.M.; investigation, A.E.-B., W.P., K.T., D.V.C. and I.d.M.; project administration, A.M. and I.d.M.; supervision, W.P., A.M., S.P. and I.d.M.;

Table 1 .

 1 Average first cycle discharge capacity (at C/10) and coulombic efficiency, second cycle discharge capacity (at 1 C), and AC resistance of the pouch and the 18650 cylindrical cells assembled.

		1st Cycle-Discharge C/10	1st Cycle-Coulombic Efficiency	2nd Cycle-Discharge 1 C	AC Resistance at 1 kHz
	18650	1931 ± 17 mAh	86.6 ± 0.3%	1807 ± 29 mAh	76 ± 7 mΩ (50% SOC)
	Pouch	1810 ± 50 mAh	82.3 ± 0.5%	1630 ± 30 mAh	26 ± 7 mΩ (30% SOC)

Acknowledgments:

The authors acknowledge SGL Carbon for the graphite supply. We would like to thank Carmen Palacios, Ibai Mujika, Andoni Contreras, and Iker Boyano for the anode electrode manufacturing at the CIDETEC pilot plant and Silvia Lijó for the pouch cell assembly.

Funding: This research was funded by the European Union's Horizon 2020 research and innovation programme, grant number 653373 (SPICY project) and grant number 814389 (SPIDER project). The Si nanoparticles were available from the SPICY project. The APC was funded by the SPIDER project.

Institutional Review Board Statement: Not applicable.

Informed Consent Statement: Not applicable.

Data Availability Statement:

The data presented in this study are available on request from the corresponding author. The raw data are not publicly available due to confidentiality provisions in project consortium agreements.

writing-original draft, I.L.-M.; writing-review and editing, A.E.-B., S.S.-I., K.T., S.P. and I.d.M. All authors have read and agreed to the published version of the manuscript.

Conflicts of Interest:

The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.


R De Stefano

A Elayeb

A Sari

H Makil

P Russo

F Carrel

Evaluation of linac-based delayed gamma neutron activation technique for copper characterization in scrap metal by means of Monte Carlo modeling

Keywords: Delayed gamma rays, Neutron activation analysis, linac, copper, MCNP6, detection limits

Delayed gamma rays following neutron activation, induced by a 6 MeV linear electron accelerator (linac) coupled to a deuterium oxide photo-neutron conversion target, are simulated with MCNP6.1. The 1039 keV copper activation gamma peak is detected with a 33 % relative efficiency hyper pure germanium detector. Two copper distributions were assessed for a 2-hour irradiation followed by a 2-hour counting configuration.

Homogeneously distributed copper in a planar scrap metal matrix shows 65 Cu detection limits of 10.01 g. A solid copper sample, modeled at five distinct positions in the planar scrap metal matrix, exhibits detection limits from 8.27 to 14.27 grams.

Introduction

Neutron Activation Analysis (NAA) is a legacy technique developed since the end of the '30s [START_REF] Meyer | L'analyse par activation de neutrons de réacteur[END_REF] [START_REF] Guinn | Instrumental Neutron Activation Analysis[END_REF]. Following its discovery, the method was used in several applied fields such as archelogy [START_REF] Glascok | Neutron activation analysis and provenance research in archeology[END_REF][4] and forensics [START_REF] Coleman | The application of neutron activation analysis to forensic science[END_REF]. More recently, the method is used for homeland security applications due to the growing need of threat detection in the past years [START_REF] Gierlik | SWAN -Detection of explosives by means of fast neutron activation analysis[END_REF] [START_REF] Carasco | Measuring hydrogen with fast neutrons : Application to organic materials identification in cargo containers[END_REF].

Several NAA techniques are also widely used in the framework of foundational research subjects [START_REF] Crocket | Neutron activation analysis for noble metals in geochemistry[END_REF][9] [START_REF] Cercasov | Investigation of atmospheric particulates deposited on leaves using instrumental neutron activation analysis[END_REF]. In regards to industrial applications NAA is still under development, for instance, for the non-destructive elemental characterization of specific manufactured pieces such as metals [START_REF] Popescu | Multielemental analysis of metallurgical samples by thermal neutron activations[END_REF][12] [START_REF] Hampel | Fast determination of impurities in metallurgical grade silicon for photovoltaics by intrumental neutron activation analysis[END_REF].

In the framework of its collaboration with ArcelorMittal R&D, LIST Institute from Commissariat a l'Energie Atomique et aux energies alternatives (CEA) is currently studying the applicability of a recent neutron activation technique that aims at estimating the amount of copper in scrap metal [START_REF] Sari | Detection and quantification of copper in scrap metal by linac-based neutron activation analysis[END_REF] [START_REF] Stefano | Simulation of photo-neutron induced activation delayed gamma rays for copper characterization in scrap metal[END_REF]. Using the 6 MeV mode, the Linatron ® M9 linac from Varex Imaging Corp. housed at the SAPHIR platform of CEA Paris-Saclay is coupled to a deuterium oxide photo-neutron target. The latter is placed inside a High Density Polyethylene (HDPE) neutron interrogation cell manufactured for neutron irradiation purposes [START_REF] Sari | Design of a Neutron Interrogation Cell Based on an Electron Accelerator and Performance Assessment on 220 Liter Nuclear Waste Mock-Up Drums[END_REF]. This technique presents several advantages in the scope of scrap metal characterization, due to important neutron emission intensities higher than 10 9 n.s -1 [START_REF] Sari | Characterization and optimization of the photoneutron flux emitted by a 6 or 9 MeV electron accelerator for neutron interrogation measurements[END_REF], and the possibility to use the same linac along with a X-ray radiography and photon activation analysis [START_REF] Carrel | Coupling high energy radiography and photon activation analysis (PAA) to optimize the characterization of nuclear waste packages[END_REF] [START_REF] Carrel | Characterization of old nuclear waste packages coupling photon activation analysis and complementary non-destructive techniques[END_REF].

This paper aims to explore the achievable performances of such a variant of NAA in the field of copper characterization in scrap metal using the SAPHIR linac at 6 MeV. In this scope, Monte Carlo simulations have been carried out with MCNP6.1 and the Activation Control Card (ACT) [START_REF] Durkee | MCNP6 delayed-particle verification and validation[END_REF] to compute the 1039 keV delayed gamma ray from 65 Cu. This study focuses on a surface interrogation scenario by modeling a scrap metal planar sample with two very different copper distribution assumptions. Either copper is described with a homogeneous distribution, or with a solid disk placed at five positions in the metal matrix. This paper reports the neutron flux behavior in the measurement cell, the 1039 keV peak contribution detected with a hyper pure germanium computed with MODAR Software (MCNP Output Data Analysis with Root) [START_REF] Carasco | MCNP output data analysis with ROOT, (MODAR)[END_REF], and corresponding detection limits.

Numerical Approach

With the aim of evaluating the performance of linac-based neutron activation technique, a numerical model of the SAPHIR platform and the neutron irradiation cell formerly developed in [START_REF] Sari | Design of a Neutron Interrogation Cell Based on an Electron Accelerator and Performance Assessment on 220 Liter Nuclear Waste Mock-Up Drums[END_REF] have been adapted to MCNP6.1. This version of MCNP6 was used as it implements the ACT card which allows the simulation of neutron-induced activation gamma rays for which creation and temporal behavior were validated in [START_REF] Stefano | Simulation of delayed gamma rays from neutron induced fissions using MCNP 6.1[END_REF] [START_REF] Simon | Feasibility study of fissile mass quantification by photofission delayed gamma rays in radioactive waste packages using MCNPX[END_REF]. Fig. 1 illustrates the SAPHIR platform with a Linatron ® M9 linac and the HDPE cell developed for neutron interrogation applications. This figure also reports a perspective representation of the main building blocks of the simulation model for the case of the solid copper disk with each element numbered from (1.) to (8.) with :

(1.) a 126.6 g copper disk-shaped sample of 3 cm radius and 0.5 cm thickness for the case of the solid copper distribution model. This element is absent for the homogeneous distribution study.

(2.) A planar shaped iron matrix of 40 × 40 × 0.5 cm 3 dimensions for which 125.9 g of 65 Cu are added to this element for the homogeneous distribution study. (5.) The n-type coaxial Hyper Pure Germanium (HPGe) detector crystal, defined by an airfilled cylinder of 0.7335 cm radius and 4.285 cm length placed within a germanium cylindrical body of 2.548 cm radius and 5.505 cm length. (6.) The detector's aluminum external envelope of 3.66 cm inner radius, 6.94 inner length, and 1.5 mm thickness. (7.) The air filling the HDPE cell cavity.

(8. a.) The Linatron ® M9 tungsten collimator housing the (8. b.) primary (e -, γ) conversion target described as pure tungsten bodies of 19.3 g.cm -3 density. Dimensions (in cm) related to this numerical model are reported in Fig. 2; three section representations are shown including a view from the top, and two from the sides. Each simulated component, its respective mass, volume, density and associated chemical composition for the homogeneous and the solid copper descriptions are summarized in Table 1.

The incident neutron energy distribution calculated in [START_REF] Sari | Characterization and optimization of the photoneutron flux emitted by a 6 or 9 MeV electron accelerator for neutron interrogation measurements[END_REF], presented in Fig. 3, corresponds to the (e -, γ) conversion of the incident 6 MeV electron beam in tungsten followed by the (γ, n) reaction in the deuterium oxide canisters. The neutron source distribution is a 10 keVper-bin histogram from 0 to 3 MeV, emitting an average energy and neutron emission of 0.4 MeV and 2.03 × 10 9 n.s -1 in 4π sr. The related spatial definition corresponds to a homogenous distribution in the volume of the D2O secondary target [START_REF] Sari | Characterization and optimization of the photoneutron flux emitted by a 6 or 9 MeV electron accelerator for neutron interrogation measurements[END_REF]. Liquid deuterium oxide fills four separated equi-volume parallepiped-shaped adjacent canisters, justifying a single body source description. This case scenario models the copper sample with a pure 65 Cu abundance. This choice comes from the fact that 65 Cu is the second most abundant isotope in natural copper, up to an average 30.85 % [START_REF] Georgopoulos | Environmental copper : its dynamics and human exposure issues[END_REF] and the second most likely present in scrap metal. Also, this study focuses only on the 1039 keV activation delayed gamma ray associated to radiative capture 65 Cu(nth, γ) 66 Cu which is one of the most intense copper activation gamma ray [START_REF]NUCLEIDE-LARA on the web[END_REF]. In more details, for this reaction resulting radioactive 66 Cu undergoes a β -decay onto 66 Zn activation product with a 5.120(14) min half-life ; after which 66 Zn (produced in one of its excited states), emits the 1039 keV gamma ray of 9.23 % intensity to reach the Ground State (GS) [START_REF]NUCLEIDE-LARA on the web[END_REF].

With the aim of reaching 66 Zn nucleus secular equilibrium, the irradiation time period Tirrad has been fixed to two hours to reach a nucleus production rate higher than 99.99 %. In the same way, the 1039 keV gamma ray acquisition time Tacquisition was fixed to two hours to count the signal of interest with a rate higher than 99.99 %. Irradiation and counting times were rounded up to two hours rather than ten times the activation product half-life to simulate more realistic acquisition times with respect to industrial applications. The associated chronogram related to this study is presented in Fig. 4.

Creation and transport of activation gamma rays such as the 1039 keV signal from copper are simulated with the ACTivation (ACT) control card from MCNP6.1 in single-step computations. For this study, only Delayed Gamma rays (DG) from non-fissionable materials are of interest, thus, parameters related to the use of the ACT card are fixed to NONFISSION=P and DG=Lines. Although the ACT card allows single-step computations, which reduces biases related to classical two-steps calculations such as reaction rate simulation and its accurate description as a secondary source [START_REF] Nicol | Feasibility study of 235U and 239Pu characterization in radioactive waste drums using neutron-induced fission delayed gamma rays[END_REF] [27], using ACT engages an important computing power. In order to reduce computational time, activation gamma rays were evaluated with a variance reduction point photon flux F5:P tally placed in front of the HPGe detector model (see Fig. 2), and a cluster composed of around a thousand of processors, owned by CEA List Institute, has been used. Each computation was carried out using 10 10 source-neutrons to obtain a statistical uncertainty below 4% for the 1039 keV activation gamma ray of interest.

In order to simulate the signal detected by an HPGe detector, each point photon flux F5:P output was processed with the MODAR software [START_REF] Carasco | MCNP output data analysis with ROOT, (MODAR)[END_REF] developed by CEA IRESNE Institute. MODAR allows smearing the ideal F5:P output tally with the energy response function of a 33% relative efficiency HPGe detector following

𝑆 (𝐸) = ∫ 𝐹5: 𝑃 (𝐸 ′ ) × 𝑅𝐹 𝐻𝑃𝐺𝑒33% (𝐸 ′ , 𝐸)𝑑𝐸′ 𝐸′ (1)
Where S(E), given in counts-per-source-neutron, refers to the MODAR processed signal at energy E after smearing the MCNP F5:P output with the HPGe response function. F5:P (E') describes the point photon flux estimated at the entrance of the HPGe detector and finally the pre-calculated response function RFHPGe33%(E',E) defines the probability that an incident photon with energy E' leads to an energy deposit E in the 33% relative efficiency HPGe detector. RFHPGe33%(E',E), written in an independent ROOT ® [28] macro, was formerly estimated in [START_REF] Carasco | MCNP output data analysis with ROOT, (MODAR)[END_REF] by means of MCNP simulations where each E' energy deposit corresponds to a F8:P tally output computed in the germanium crystal volume for a given incident mono-energetic photon at energy E. Mono-energetic source photons emitted perpendicular to the front section of the crystal were defined by a disk-shaped surface of 2.725 cm radius parallel to the detectors front surface. Two thousand independent computations allowed covering the 0 to 100 MeV energy region, for incident photon energies defined each 50 keV.

Both raw F5:P output data and MODAR processed spectrum for the computation related to Fig. 2 are presented in Fig. 5. One can notice the presence of several activation gamma rays from the main chemical components in this simulation. Activation gamma rays with intensities above 1 % in the 100 to 1800 keV energy region are reported in Table 2. Fig. 6 shows the Region of Interest (RoI) of the delayed gamma ray at 1039 keV from 65 Cu(n, γ) 66 Cu reaction which is clearly visible above the active background continuum, allowing further investigations on the applicability of this method to copper characterization in scrap metal.

Performance assessment

This section focuses on two copper distribution assumptions with the aim of evaluating the performances for both described scenarios. The first scenario corresponds to a 125.9 g homogenous distribution of 65 Cu throughout a plate of scrap iron metal (report to Tab. 1

for the chemical and geometrical description). The second case investigates a solid 126.6 g pure 65 Cu disk at various positions within an iron metal plate. The solid copper disk is described for five positions in the matrix; a d0 = 0 cm centered position, and for respective distances at d1 = 4.23 cm, d2 = 7.75 cm, d3 = 14.04 cm and d4 = 24.04 cm from the center along the diagonal of the metal piece (report to Fig. 7).

All computed signals S(E) were scaled in order to obtain the number of counts for each energy bin C(E) written as

𝑪 (𝑬) = 𝑺(𝑬) × 𝑬 𝒏 × 𝑻 𝒊𝒓𝒓𝒂𝒅 (2)
Where En is the 2.03

For the homogenous distribution, the signal is lower than d2 solid distribution and close to d3. This signal behavior is explained as follows:

 (n, γ) activation rates in the plate (reported in Tab. 3) for the six studied computations are homogenous. Rates are 5340 ± 38.98 for the lowest, up to 5346 ± 5.881 act.s -1 for the highest.

 Detection efficiencies reported in Tab. 3 decrease with respect to the disk distance from (2.622 ± 0.01783) ×10 -4 for d0 position to (1.613 ± 0.01274) ×10 -4 for d4. The homogenous description result of (2.144 ± 0.01458) ×10 -4 is close to that for d3 of (2.294 ± 0.01514) ×10 -4 .

All six rates were computed with a F4:N neutron flux tally evaluated in the entire volume metal plate (i.e scrap metal and solid disk volumes for the positions study). This last was convoluted with the neutron induced (n, γ) activation reaction cross section using a FM4

tally multiplier coupled to the MT=102 parameter and SD option in order to scale this factor with the plate's and disk's volume. The results of these computations were finally scaled with the neutron emission and irradiation time. Homogenous rates for each studied configuration are explained on the one hand by the fact that iron and copper neutron activation cross-sections follow similar trends. For instance, accordingly to ENDF/B-VIII recent data libraries, 65 Cu and 56 Fe (n, γ) cross sections are 2.149 and 2.605 barns at 25.3 meV [START_REF] Brown | ENDF/B-VIII.0: The 8 th major release of the nuclear reaction data library with CIELO-projections cross sections, new standards and thermal scattering data[END_REF]. On the other hand, incident thermal neutron flux is homogenous across the XZ section at the planar metallic sample within the HDPE cavity for the homogenous and for d0 to d4 solid sample position in the matrix as presented in Fig. 9. To illustrate this result, thermal neutron flux cartographies where computed with a RMESH:N tally with low and high-energy thresholds of 10 -12 and 0.025 eV fixed with the ERGSH option.

Detection efficiencies calculations were done with the 1039 keV gamma ray energy deposit for the full-energy peak, computed with a F8:P tally in the HPGe crystal volume. For the solid disk description, sources were defined as a mono-energetic 1039 keV gamma rays isotropically emitted by cylinder shaped volumes of 3 cm radius and 0.5 cm thickness placed at respective d0, d1, d2, d3, and d4 distances from the middle of the matrix along the diagonal. For the homogeneous spread copper description, the photon source was defined as mono-energetic 1039 keV gamma rays isotropically emitted in the volume of plate shaped matrix.

This section aims at investigating the performance of linac-based neutron activation technique in terms of copper detection limits. In this scope, total counts from the 1039 keV peak have been integrated over six bins from 1036 to 1041 keV. In order to calculate the useful net counts of the 1039 keV signal over the active background, the former were evaluated with linear extrapolations of the Compton continuum with the (a ; b) couple of parameters in the 1036 to 1041 keV region (report to Tab. 3). The standard deviation 𝜎 on the 1039 keV gamma peak over the active background continuum is calculated following a Poisson law

𝜎 = √ 𝑁 + 2 × 𝐵 𝐶𝑜𝑚𝑝𝑡𝑜𝑛 (4)
Where N defines the net peak area and BCompton the Compton continuum active background.

The associated uncertainty on the total signal (described by T) corresponds to the MCNP6 statistical uncertainty (see Tab. 3) for the 1039 keV F5:P output.

Detection Limits in counts (DLcounts) were calculated to cover a detection for  and  factors [START_REF]Détermination du seuil et de la limite de détection en spectrométrie gamma[END_REF] (respectively describing a false-alarm and no-alarm probability for the signal of interest) of 2.5%

𝐷𝐿 𝑐𝑜𝑢𝑛𝑡𝑠 ≈ 3.92 × √ 2 × 𝐵 𝐶𝑜𝑚𝑝𝑡𝑜𝑛 (5)
Uncertainty over detection limits u(DLcounts) divided by the detection limits in counts is calculated as the ratio of the uncertainty on the active background Compton continuum 𝑢(𝐵 𝐶𝑜𝑚𝑝𝑡𝑜𝑛 ), estimated as √ 𝐵 𝐶𝑜𝑚𝑝𝑡𝑜𝑛 , over two times the Compton continuum

𝑢(𝐷𝐿 𝑐𝑜𝑢𝑛𝑡𝑠 ) 𝐷𝐿 𝑐𝑜𝑢𝑛𝑡𝑠 = 𝑢(𝐵 𝐶𝑜𝑚𝑝𝑡𝑜𝑛 ) 2 × 𝐵 𝐶𝑜𝑚𝑝𝑡𝑜𝑛 (6)
Detection limits in counts 𝐷𝐿 𝑐𝑜𝑢𝑛𝑡𝑠 were finally divided by the sensitivity coefficients SCu, resulting into the detection limits in mass 𝐷𝐿 𝑚𝑎𝑠𝑠 . SCu is calculated as the ratio of the signal net counts divided by the 125.9 g and 126.6 g homogeneous and disk-shaped copper distributions (see Tab. In this study, the 65 Cu detection limits reported in Tab. 3 increase from 8.272 ± 0.2085 g to 14.27 ± 0.5293 g for solid planar-disks positioned from d0 to d4 distances, in accordance with the calculated detection efficiencies trend, which decrease from d0 to d4. This is consistent with the notable homogeneity of activation rates for each position study. For a solid mass positioned in a planar matrix, the technique's performance is thus mainly dependent on the copper mass distance towards the detector. For the homogeneous spread copper in the metal plate, the detection limit is 10.01 ± 0.2895 g. This exhibits that the technique alone cannot discriminate homogenous copper spread on a scrap metal piece from a piece with heterogeneities on it.

Discussion

The detection limits for 65 Cu of several grams are encouraging results. It proves the detectability of the signal of interest above the Compton continuum with no interference with other gamma rays. It also shows the applicability of the method for a primary rough copper mass estimation.

Additional experimental measurements of the platform's active background should be studied to justify that the 1039 keV gamma ray can be detected above the Compton continuum. Additional contributions from activation gamma rays produced in the metal matrix, modeled as a pure iron plate, but also the walls of the facility or other bodies surrounding the neutron cell, could decrease the reported performances. Also, a copper chemical description integrating 63 Cu present with a 69.15 % abundance in natural copper could influence these results. The high energy activation gamma ray at 1345 keV is emitted following 63 Cu(n,γ) 64 Cu activation with a 12.7 hours half-life. Even though the branching ratio of this ray stands at 0.48 % [START_REF]NUCLEIDE-LARA on the web[END_REF], its contribution to the active background continuum could decrease the net signal of the 1039 keV peak. Furthermore, self-absorption and attenuation effects could alter this signal in presence of 63 Cu. In addition, second order effects associated to the numerical model geometrical approximations might influence the present results.

The following axis of investigation can enhance the reported performances. Detection limits can be optimized by operating the linac at 9 MeV with the same D2O secondary target for which neutron emission was evaluated at 7.99 × 10 9 n.s -1 [START_REF] Sari | Characterization and optimization of the photoneutron flux emitted by a 6 or 9 MeV electron accelerator for neutron interrogation measurements[END_REF]. Almost four times higher than the 2.03 × 10 9 n.s -1 emission for the 6 MeV mode. A heavier mass of deuterium oxide could be used to achieve higher neutron emissions. The neutron measurement cell can be optimized by widening the irradiation cavity and using complementary graphite blocks to enhance neutron flux thermalisation similar to [START_REF] Jordan | Differential die-away analysis system response modeling and detector design[END_REF][32] [START_REF] Antoni | Reduction of the uncertainty due to fissile clusters in radioactive waste 23 characterization with the Differential Die-away Technique[END_REF]. Additional lead bricks surrounding the outer walls of the neutron cell could limit the contribution of the active background produced outside the neutron cell, such as tungsten gamma rays reported in Tab. 2. A detector of higher relative efficiency such as the 104 % n-type coaxial HPGe used in [26][34] for similar Delayed Gamma Neutron Activation Analysis (DGNAA) applications could be deployed. Also several HPGe detectors instead of a single detector would enhance the signal. In addition, such as 764 keV and 1114 keV inelastic scattering copper (n, n'γ) gamma rays [START_REF] Holmqvist | Inelastic Neutron Scattering Cross Sections of 63 Cu and 65 Cu in the Energy Region 0.7 to 1.4 MeV[END_REF] emitted during neutron irradiation can be detected. Also prompt activation gamma rays such as high-energy gamma rays at 7640 and 7910 keV [START_REF] Charbucinski | Prompt gamma neutron activation analysis method and instrumentation for copper grade estimation in large diameter blast holes[END_REF], just after the end of each linac pulse could be detected complementarily to the 1039 keV gamma ray. In addition, the 1039 keV delayed gamma ray can be detected in inter-pulse mode rather than post-irradiation only, such as delayed gamma rays measured in [START_REF] Stefano | Pulsed neutron interrogation with PVT plastic scintillators to detect nuclear materials[END_REF] in the field of SNM (Special Nuclear Material) detection. For instance, the Linatron ® M9 linac can be used in inter-pulse mode with 2.5 µs long pulses and a frequency ranging from 40 to 400 Hz at 6 MeV and 40 to 385 Hz at 9 MeV.

In terms of experimental applications, linac based NAA present an advantage with respect to classical techniques. For instance, the use of high-energy 14 MeV neutron sources such as a D-T generator can deteriorate the resolution of the HPGe detector [START_REF] Gilmore | Practical gamma ray spectrometry[END_REF]. This study shows that for linac based NAA, 93.5 % of the interrogating source are thermal neutrons and 6.14 % are epi-thermal. The remaining 0.36 % proportion of fast neutrons entering the HPGe range at 8.04 × 10 6 n.cm -2 number-of-neutrons -per-surface-unit for the 2-h irradiation time, thus below the 10 9 n.cm -2 threshold recommended by ORTEC constructor [START_REF] Ametek-Ortec | Detecting High Energy Gamma Rays from Neutron Interactions: Neutron Damage and HPGe Detectors[END_REF]. With the present irradiation parameters, the latter would nevertheless be reached after 124 characterizations. This estimation was computed with a F4:N tally at the entrance of the HPGe crystal volume, segmented for the respective 0 to 25.3 meV; 25.3 meV to 0.9 MeV; and 0.9 to 10 MeV regions for thermal, epithermal and fast neutrons. In the same way, the experimental application of the present method should pay a particular attention on electronics dead-time, similar to [START_REF] Stefano | Pulsed neutron interrogation with PVT plastic scintillators to detect nuclear materials[END_REF]. Indeed, this study shows acquisitions during and between each irradiation pulse suffer an important data loss up to 57.9 %. A particular attention should focus on the copper (n, n'γ) and prompt activation gamma rays signal reconstruction. Also, experiments with more realistic industrial scrap metal matrixes shape's and distributions should be carried as it will influence the reported performances.

In terms of complementarity with other nondestructive methods, reported signals for the homogeneous copper distribution and the five solid positions in the iron plate show the need to deploy this method with additional discrimination techniques. For instance, a primary collimated photon activation analysis using the linac at 9 MeV and the same detector could allow the detection of a copper hot spot. Indeed, (γ, α) reaction thresholds range respectively at 6.79 and 7.61 MeV for 65 Cu and 56 Fe [START_REF] Iaea | Handbook on photonuclear data for applications -Cross-sections and spectra[END_REF]. The presence of photoninduced copper activation gamma rays following 65 Cu(γ, α) 61 Co reaction in the spectrum for a particular scrap metal scanned position could provide additional information of a solid copper presence in the matrix. Moreover, a primary X-ray radiography performed using the same photon source, as already studied at the SAPHIR platform in the field of nuclear waste packages characterization [18][19], could be used to complete this study.

Nevertheless, in the scope of scrap metal characterization this method will be limited due to the close densities of copper (dCu=8.96 g.cm -3 ) and scrap metal, for which the matrix is mainly composed of iron (dFe=7.87 g.cm -3 ).

Conclusion

This study aimed to explore the potential of copper characterization in a scrap metal piece

( 3 .

 3 ) The HDPE cell with 5 cm thick walls and a 70 × 90 × 92 cm 3 internal volume. (4.) The (γ, n) deuterium oxide (D2O) conversion target of around 16 kg described by the parallelepiped of 54.4 × 13.6 × 19.6 cm 3 dimensions.

Fig. 1 :

 1 Fig. 1: a) View of the SAPHIR platform showing the Linatron ® M9 linac; b) perspective representation of the MCNP6 model of the measurement cell dedicated to neutron irradiation applications; c) internal computed bodies as noted in the text.

Fig. 2 :

 2 Fig. 2: Representation of the MCNP6 model (not to scale) from respectively: a) YZ view, b) XY view and c) XZ view of the neutron measurement cell. Colors are associated to the density of each model component.

Fig. 6 :

 6 Fig. 6: Zoomed-in section in the 0.8 to 1.2 MeV region of interest for the study of the 1039 keV copper activation gamma ray.

  × 10 9 n.s -1 neutron emission of the deuterium oxide secondary conversion target, and Tirrad the two hour-irradiation time. The spectra, in counts-per-bin, considering the homogeneous distribution and each position of the solid Cu disk zoomed in the 1039 keV RoI are reported Fig. 8. This figure shows a decreasing trend towards the distance d from the middle of the planar matrix (in fine, from the HPGe crystal) explained by a classical decrease in detection efficiencies. Indeed, the 1039 keV number of counts C(E) follow a quadratic law 𝑪(𝑬) = -𝟐. 𝟗𝟐𝟐𝟑𝒅 𝟐 -𝟕𝟑. 𝟎𝟖𝒅 + 𝟏𝟑𝟏𝟓𝟒. 𝟒

  3). Relative error u(DLmass) for detection limits in grams were u(SCu) corresponds to the standard deviation 𝜎 divided the copper homogenous and disks-shaped masses. The total (T) and net counts (N) for the 1039 keV peak of interest for this study, as well as the active background Compton continuum (BCompton) in the 1036 to 1041 keV RoI and associated detection limits, are reported in Tab. 3.

Fig. 7 :

 7 Fig. 7: XZ view from the numerical model of the metal matrix with a solid copper disk placed at a) d0; b) d1; c) d2; d) d3 and e) d4 positions as noted in the text.

Fig. 9 :

 9 Fig. 9: Thermal neutron flux XZ representation for: a) the homogeneous copper distribution; respective solid copper mass placed at b) d0 in the middle of the scrap metal sample; and c) d1; d) d2 ; e) d3 ; f) d4 distances from the middle of the metal piece.

  by means of linac-based delayed gamma neutron activation technique. This study focused on the use of a 6 MeV linac coupled to a 16 kg deuterium oxide secondary target emitting 2.03 × 10 9 n.s -1 in 4π sr within a HDPE neutron measurement cell. The analysis of the 1039 keV delayed gamma ray from65 Cu, and its detection with a 33 % relative efficiency HPGe was evaluated in the post-irradiation mode. Two assumptions regarding the distribution of copper in the scrap metal piece were compared: 125.9 g homogeneously spread and 126.6 g located in a disk-shaped solid mass. The solid mass was studied for five distinct positions in the matrix at increasing distances from the middle of the planar sample along the diagonal. Pure 65 Cu was modeled with MCNP6.1 and the signal of interest was simulated with the ACT card coupled to the MODAR software. All signals show the detectability of the 1039 keV delayed gamma ray above the active Compton Continuum background with no other activation gamma ray interferences. Mass detection limits of copper were evaluated from 8.272 ± 0.2085 g to 14.27 ± 0.5293 g for the solid disk positioned in the middle of the matrix, and positioned 33.94 cm far from the middle, following the diagonal of the plate. For the homogeneous spread copper in the metal matrix, detection limit are 10.01 ± 0.2895 g, which are encouraging performances in view of further investigations conducted with an optimized measurement protocol. Furthermore, performances obtained with the two case scenarios and for each studied solid copper position show the interest and the technical potential of the linac-based neutron activation technique with respect to classical NAA.

Table 1 :

 1 Description of the modeled components and associated chemical atomic and weight fractions.

						Chemical composition
		Modeled parts
					Solid distribution	Homogeneous distribution
	Body	Volume (cm 3 )	Mass (g)	Density (g.cm -3 )	Element	W. fraction	Element	W. fraction
	(1.)	14.14	126.6	8.96	65 Cu	1.	-
	(2.)	785.9	6.180×10 3	7.87	54 Fe 56 Fe 57 Fe	0.05845 0.91720 0.02435	54 Fe 56 Fe 57 Fe 65 Cu	0.05845 0.91720 0.004250 0.02010
						Element	W. fraction
	(3.)	234.9×10 3	223.2×10 3	0.95		1 H	0.143682
						2 H	0.000033
						12 C	0.856285
	(4.)	14.45×10 3	15.99×10 3	1.107		2 H 16 O	0.20118 0.79882
						70 Ge	0.2120
						72 Ge	0.2760
	(5.)	105.0	558.8	5.32		73 Ge	0.0776
						74 Ge	0.3594
						76 Ge	0.0750
	(6.)	27.16	73.35	2.7		27 Al	1.
						1 H	0.0011
	(7.)	1.500×10 3	1.830	1.22×10 -3		14 N	0.7925
						16 O	0.2064
						180 W	0.0012
	(8.a.)	8.026×10 3	154.9×10 3			182 W	0.2630
				19.3		183 W	0.1428
	(8.b.)	0.02260	0.4370			184 W	0.3070
						186 W	0.2860

Table 2 :

 2 Main present activation gamma rays[START_REF]NUCLEIDE-LARA on the web[END_REF].

	Activated Element	Associated reaction	Half-life		Activation delayed gamma rays (keV)		Associated intensities (%)
	copper	65 Cu(n, γ) 66 Cu	2.241 min		1039			9.23
	aluminum	27 Al(n, γ) 28 Al	5.120 min		1779			100
				1368	784.8	558.0	264.4	3.30	1.32	16.1	53.9
				1193	781.3	461.4	215.5	2.57	1.01	1.27	28.6
	germanium	76 Ge(n, γ) 77 Ge	11.30 hours	1085	714.4	419.8	211.0	6.05	7.17	1.23	30.8
				928.9	634.4	416.3	194.8	1.05	2.08	28.8	1.77
				810.4	631.8	367.4		2.27	6.95	14.0
					160.5			5.12
		182 W(n, γ) 183m W	5.20 sec		107.9			18.9
					102.5			2.42
	tungsten	184 W(n,γ) 185m W	1.670 min		173.7 131.6			3.26 4.33
				772.9	551.5	4.12		5.08
		186 W(n, γ) 187 W	23.72 hours	685.7	479.5	27.3		21.8
				618.3	134.2	6.28		8.80

Table 3 :

 3 Detection limits considering the homogeneous distribution assumption and the 282 solid copper mass at positions d0 to d4. 283

	Dist.	Homogeneous spread	Solid for d0	Solid for d1	Solid for d2	Solid for d3	Solid for d4
	Act. Rate (s -1 )	5346 ± 0.5883	5341 ± 3.247	5341 ± 3.248	5341 ± 3.247	5341 ± 3.248	5341 ± 3.247
	Det. Eff.	2.144	2.622	2.576	2.325	2.294	1.613
	(×10 -4 )	± 0.01458	± 0.01783	± 0.01597	± 0.01534	± 0.01514	± 0.01274
	F5:P Stat. Unc. (%)	1.540	2.170	2.230	2.360	2.680	3.680
	T (counts)	5452 ± 83.96	6314 ± 137.0	6221 ± 138.7	5981 ± 141.2	5433 ± 145.6	4491 ± 165.3
	(a;b)
	param.	(-2.427; 2885) (-2.889; 3378)	(-2.849; 3339)	(-2.759; 3242)	(-2.728; 3201)	(-2.569; 3033)
	BCompton (counts)	2189 ± 47.79	2271 ± 47.66	2284 ± 47.79	2262 ± 47.56	2206 ± 46.97	2190 ± 46.80
	N (counts)	3263 ± 87.41	4043 ± 92.66	3937 ± 92.22	3719 ± 90.79	3227 ± 87.40	2301 ± 81.74
	D.L. (counts)	259.4 ± 2.832	264.2 ± 2.772 264.9 ± 2.771	263.7 ± 2.772	260.4 ± 2.772	259.4 ± 2.772
	SCu (g -1 )	25.92 ± 0.6943	31.94 ± 0.7319	31.10 ± 0.7284	29.37 ± 0.7171	25.49 ± 0.6904	18.18 ± 0.6457
	D. L. (g)	10.01 ± 0.2895	8.272 ± 0.2085	8.518 ± 0.2185	8.979 ± 0.2387	10.22 ± 0.2974	14.27 ± 0.5293

Acknowledgment

Particular acknowledgement to Dr. Cédric Carasco from CEA IRESNE for his feedback and advice regarding MODAR Software source codes.