Amal Zeaiter
email: amal.zeaiter@etudiant.univ-lr.fr

Colette Besombes
email: colette.besombes@univ-lr.fr.

Larbi Rhazi

Amira Haddarah
email: amirahaddara@hotmail.com.

Tayssir Hamieh
email: tayssir.hamieh@ul.edu.lb.

Karim Allaf
email: kallaf@univ-lr.fr

How does instant autovaporization deepen the cold press-extraction process of sunflower vegetal oil?

Keywords: Sunflower Seeds, Instant Controlled Pressure-Drop (DIC), Cold-Press Extraction, oil quality, Central Composite Design (CCD), Response Surface Methodology (RSM)

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

Introduction

Sunflower is a famous oleaginous plant widespread on all continents due to its hardiness and low requirements for both inputs and water [START_REF] Castro | Main aspects of sunflower production in Brazil[END_REF]. Worldwide, its cultivation is one of the largest in the world in Americas (Canada, Brazil, Argentina, USA) [START_REF] Heiser | The sunflower (<Emphasis Type="Italic">Helianthus annuus</Emphasis>) in Mexico: further evidence for a North American domestication[END_REF], Asia (China, India, Indonesia) and Europe in particular in France [START_REF] Gunstone | The Chemistry of Oils and Fats: Sources, Composition, Properties and Uses[END_REF]. The sunflower seed consists of a shell and a kernel. The shell represents 70-90% of the dry matter formed from lignin, hemicellulose, cellulose fiber and a small amount of lipids and proteins [START_REF] Connor | Sunflower Physiology[END_REF], while the kernel, with about 20% of the dry matter of the seed, is the storage place of reserve for embryonic development [START_REF] Roche | Helianthus annuus L.) SEEDS WITHIN CULTURAL[END_REF]. Its lipid content is close to 44%, having a distribution of the main saturated fatty acids: palmitic acid (C16: 0) of 5%-7%, stearic acid (C18: 0) of <10%, and unsaturated: oleic acid (C18: 1) of 17%-20% and linoleic acid (C18: 2) of 50%-70% [START_REF] Ayerdi Gotor | Comparative analysis of fatty acids, tocopherols and phytosterols content in sunflower cultivars (Helianthus annuus) from a three-year multi-local study[END_REF].

Diversification of sunflower varieties results in differences in the levels of linoleic and oleic fatty acids. It generally meets the technological needs and requirements of the food and non-food industries. For the food sector, sunflower has beneficial effects on human health. It is a source of vitamin E, a dietary ingredient well-known for its anti-cholesterol properties [START_REF] Pelletier | A diet moderately enriched in phytosterols lowers plasma cholesterol concentrations in normocholesterolemic humans[END_REF]. It is also used in margarine [START_REF] Moreau | Phytosterols, phytostanols, and their conjugates in foods: structural diversity, quantitative analysis, and health-promoting uses[END_REF], canning, salad dressings, table oil, and infant formula. Moreover, in the non-food sector, oleic sunflower oil is valued because of its content of fatty acids as well as the presence of minor compounds favorable to the manufacturing of pharmaceutical or cosmetic products, lubricants (Al Mahmud et al., 2013), cleaning products, solvents in the paintings, etc. It can be used for the production of both edible products and biofuel for diesel engines as a result of esterification often with the methyl ester [START_REF] Rashid | Production of sunflower oil methyl esters by optimized alkali-catalyzed methanolysis[END_REF] [START_REF] Del Gatto | A comparative study of oilseed crops (Brassica napus L. subsp. oleifera and Brassica carinata A. Braun) in the biodiesel production chain and their adaptability to different Italian areas[END_REF], (Ayerdi-Gotor et al., 200811-12).

As an edible product, and because of the negative health effects of solvent traces, it is preferable that the industrial extraction of sunflower oil is obtained by pressing. However, press-cake would generally have a high residual oil content of 15 to 20%, which is too high for livestock feed or as a biomaterial [START_REF] Isobe | A new twin-screw press design for oil extraction of dehulled sunflower seeds[END_REF]. This point also has a significant economic impact due to the price difference between the oil and the residual solid cake. Since the consumer demands to avoid the use of solvent extraction, an increase in the extraction efficiency of the press becomes an essential objective in improving the performance of the industrial process.

From fundamental and phenomenological point of view [START_REF] Bouallegue | Phenomenological modeling and intensification of texturing/grinding-assisted solvent oil extraction: case of date seeds (Phoenix dactylifera L.)[END_REF], pressextraction is based on the gradient of total-pressure as driving force. This similar Darcysystem can be presented as following: = -.

Eq. 1

Where K is the oil-matrix permeability coefficient expressed in m 2 , ν the kinematic viscosity expressed in m 2 s -1 , A the normal oil-flux surface expressed in m 2 , and Pt the total pressure. A hydrostatic liquid behavior of the material, through for example high water content, induces a more homogeneous total pressure and, then, a too weak gradient of total pressure. Hence, the use of dried seeds is an inexorable condition in press-extraction operation [START_REF] Bouallegue | Texturing and Instant Cooling of Rapeseed as Pretreatment Prior to Pressing and Solvent Extraction of Oil[END_REF]. Therefore, to increase the pressed oil yield, one has to spread the pressure gradient, reduce both viscosity and product thickness. In addition, heating is well known to be capable of inducing increased yields through some thermal degradation of the cell walls. However, this operation inevitably induces a certain degradation of the main molecules and ingredients, many users recommend not to insert it. Hence, the objective of increasing the oil extraction yield must be coalesced with a perfect preservation of quality, and specialists advise against the use of heating as pretreatment and opt for a cold-press method to preserve the quality of both extracted oil and cake [START_REF] Boutin | Extraction from oleaginous seeds using supercritical CO2: Experimental design and products quality[END_REF].

A specific intensification of the process can be obtained by texturing by Instant Controlled Pressure-Drop (DIC), which normally increases the value of the initial permeability coefficient through higher porosity and better tortuosity of the matrix. Moreover, DIC texturing may also result in breaking the cell-walls, thus allowing the oil availability to be higher. However, once the consolidation stage of pressing is reached, porosity disappears and only some tortuosity effect and, much more, the oil availability issued from broken cellwalls persist. However, the DIC technique proposed for texturing sunflower seeds may possibly affect the functional properties of sunflower seeds, while probably preserving the quality of the oil extracted and the residual solid cake [START_REF] Van | Maîtrise de l'aptitude technologique des oléagineux par modification structurelle : applications aux opérations d'extraction et de transestérification in-situ[END_REF].

Indeed, the challenge of using instant pressure-controlled DIC coupled with cold press to well extract the sunflower seed oil must is worth to be studied to obtain the benefits of increasing oil yields and preserving the high quality of the oil [START_REF] Ixtaina | Characterization of chia seed oils obtained by pressing and solvent extraction[END_REF], [START_REF] Cường | IMPACT OF TEXTURING BY INSTANT CONTROLLED PRESSURE DROP (DIC) ON SOLVENT EXTRACTION OF JATROPHA CURCAS OIL[END_REF]. Thus, the oil from DIC-assisted cold pressing extraction must be analyzed to prove that it has the same quality as conventional cold-pressed oil, in terms of biochemical composition, sensory characteristics, and color. The oil must meet to all international oil standards.

Materials and methods

Raw materials and chemicals

The sunflower seeds were supplied by the company "Presse de Gascogne" in Cologne (France). This company specializes in the field of organic French-origin sunflower food oil.

Two varieties of sunflower: linoleic and oleic types were used. They were transported to the laboratory and well packaged, and stored at room temperature. The initial moisture content dry basis (db) (Wi) was about 0.07 ± 0.005 g H2O/g db for the linoleic sample and 0.09 ± 0.005 g H2O/g db for the oleic sample. Our study on cold-pressing extraction has concerned the whole sunflower seeds, thus maintaining their hulls.

The chemicals such as 99.99% purity n-hexane solvent used for oil extraction were provided by the University of La Rochelle-France. While for fatty acid analysis, 2,3,5trimethylhydroquinone (TMH) and iso-octane used to prepare the stock solution for the standards: α, β, and γ-Tocopherols for the calibration range of tocopherol analysis with acetone as solvent, were provided by the Institut Polytechnique UniLasalle Beauvais-(France), and kept at -40°C for chromatography analyses.

Treatment process and Experimental Protocol

The treatment protocol of oil press-extraction process was achieved at laboratory-scale (Figure 1). In the first part of our experimental study, sunflower seeds were subjected to an instant controlled pressure-drop (DIC) treatment aiming at greatly intensifying oil cold pressextraction. Some part of the experiments and assessments required a laboratory-scale n-hexane solvent extraction. Following the DIC texturing treatment and prior to the properly-said pressing, a drying step was performed. The sunflower seed samples underwent an airflow drying at 40 °C and 1 m/s, until reaching a final moisture content of 0.060 ± 0.005 g H2O/g db. This level is well-known as the most appropriate moisture content for the cold press-extraction. For an assessment requiring a solvent extraction process, the seeds were ground using a laboratory knife mill (RETSCH, Grindomix model GM 200-F; Kurt Retsch GmbH & Co. KG, Haan, Germany) at 8000 rpm for 40 seconds.

DIC treatment

The DIC unit was presented in various articles (Figure 2) (Allaf and Allaf, 2014;[START_REF] Mounir | Characterization of pure cheese snacks and expanded granule powders textured by the instant controlled pressure drop (DIC) process[END_REF]and Albitar et al., 2011). The research works carried out on instant controlled pressure-drop technology (DIC) at both fundamental levels and industrial applications have the specificity of a theoretical foundation based on the thermodynamics of instantaneity (Allaf et al., 2014): far from the equilibrium, processes occur between the two-asymptotic behaviors of quasi-static (classical thermodynamics) and instantaneous limits. Processes and transformations occurring in a theoretically zero time (transport of molecules in dozens of milliseconds), generally imply a decrease in the fluctuation degree of the random translational movement of the particles, at the same time an equally instantaneous decrease of the temperature. The relaxation time (Δt of dozens of ms) is an essential parameter in the considered processes; it implies a very high pressure-drop rate ΔP/Δt often greater than 5 10 5 Pa s -1 . The cooling rate can reach exceptional levels of 1500 to 2000 kW/m 2 of the product (Allaf et al., 2018b). DIC has been inserted as relevant process or effective intensification mean for many unit operations of decontamination, drying, extraction, chemical or enzymatic reactive transformations, etc. (Allaf et al., 2014[START_REF] Mounir | Characterization of pure cheese snacks and expanded granule powders textured by the instant controlled pressure drop (DIC) process[END_REF] [START_REF] Mounir | Swell Drying: Coupling Instant Controlled Pressure Drop DIC to Standard Convection Drying Processes to Intensify Transfer Phenomena and Improve Quality-An Overview[END_REF]) (Berka-Zougali et al., 2010) (Albitar et al., 2011). Practical treatment with DIC involves a vacuum stage of 5 kPa, followed by a HTST (High Temperature-Short Time) type process generally using saturated dry steam absolute pressure (usually ranged between 0.05 and 1 MPa), i.e. temperature of 60 °C to 170 °C [START_REF] Besombes | Instant controlled pressure drop extraction of lavandin essential oils: Fundamentals and experimental studies[END_REF]. The short treatment time (some dozens of seconds) includes a steam injection step (lasting less than 3 s) involving vapor condensation on the product surface (moisture content may increase by ∆W of almost 0.1 g H2O/g db, resulting in soaring heating coefficient up to tens of kW m -2 K -1 ), followed by a homogenization step of both thermal and condensed water within the product. Succeeding this roughly balance step, the pressure abruptly drops>> towards the vacuum (steam tank maintained at 3 to 5 kPa) (Allaf and Allaf, 2013).

Instantaneous-type of coupled thermomechanical transformations of DIC is: i/ an autovaporization of a certain quantity of volatile molecules, mainly water to reach the new thermodynamic equilibrium; (the moisture content undergoes a decrease ∆W of approximately -0.11 g H2O/g db), ii/ mechanical stresses within the material resulting in a new structure and texture, iii/ a cooling of the material possibly involving an overshoot beyond of the glass transition. Thus, the new often expanded structure of the matrix is preserved [START_REF] Haddad | Fruits & Vegetables Drying Combining Hot Air, DIC Technology and Microwaves[END_REF], and iv/ while preserving a perfect stop of the thermal degradation.

Following a few preliminary tests, two operating parameters of DIC treatment of sunflower seeds, namely the dry saturated steam absolute pressure (P) ranged between 0.2 and 0.7 MPa, and the treatment time (t) ranged between 15 and 85 s were selected. DIC-texturing begins by introducing into the treatment vessel a quantity of 150 -200 g of sunflower seeds at 7 ± 0.2% db (dry basis) water content. An initial vacuum makes it possible to greatly reduce the diffusion resistance between the exchange surface and the steam, thus inducing an improvement of the heat transfer.

Hence, after closing the pneumatic valve, the high-pressure steam is injected to reach the desired pressure level (and temperature). It is maintained throughout the treatment until reaching a level of thermal homogeneity and humidity within the material. An abrupt pressure-drop towards the vacuum results in an autovaporization of the water, an instantaneous cooling of the solid material and a possible more or less marked expansion of the product (Mounir et al., 2011). DIC-textured oleaginous seeds have better technological aptitude regarding cold-press extraction with higher oil yield. In order to evaluate and analyze the biochemical composition, DIC-textured seeds are often crushed before an extraction operation with n-hexane.

Extraction process: Mechanical cold-press extraction

The present research aims to study the variation of the yield of sunflower oil from the two oleic and linoleic varieties and to compare the results obtained from the treated and untreated matrices. The industrial procedure for extracting sunflower oil is based on the cold-mechanical way (press, cavitator, centrifuge systems…).

At this level, the extraction of oils is based on the mechanical action of press compression.

The pressing operation allows the oleaginous seed to liberate the oily liquid contained in the kernel cells and separate it from the residual solid called "cake".

In our laboratory-scale study, the cold-pressing device is based on single-screw extrusion which operates according to a continuous process (p500R, Anton Fries Mashinenbau GmbH, Germany). It operates at a frequency of 50 Hz, a voltage of 400 V and a power of 1.5 kW. This device comprises a stainless-steel cylindrical hopper of 35 mm diameter. In the feeding zone, the seeds are pressed and freely transported under the effect of the mono-screw towards the bottom of a fixed tube to create a high-pressure cold-compression. This fixed tube contains holes along its length through which the oil is recovered at the bottom of the press by a flow through calibrated 8 mm hole called die or nozzle, positioned at cylinder end to remove the cakes in granular forms. This separation is done in an area called the filtration zone.

Following filtration, the extracted oil was measured while the cake was stored in poly-bag at a temperature of 4 °C. The oil yield is expressed in g oil/g ddb (dry dry matter, which means a mass basis of raw material rid of water and oil) with an error estimated at 0.05%. Coldpress oil extraction was successively performed on the raw-material (untreated) and DICtextured sunflower seeds after adjusting the moisture content of each sample to 0.06 ± 0.005 g H2O/g db.

/ =

Eq. 2

Design of Experiments DoE

Experimental Protocol

From literature and previous experience of our research team, succeeding few preliminary tests, a Design of Experiments DoE was defined. In the current case, since our objective was not a simple approach of operational trends, but especially to carry out a relevant optimization study of the main operating parameters of the new industrial treatment DIC operation, we opted for a 5-level 2-parameter Box-Wilson Central Composite Design (CCD).

For DIC-texturing, the two operating parameters of the absolute value of saturated dry steam pressure (P) and the treatment time (t) were ranged from 0.2 to 0.7 MPa, and from 15 to 85 s, respectively.

The experimental responses were statistically processed using Statgraphics plus software (Statgraphics Centurion XV, StatPoint Technologies, Inc., Rockville, USA). This Response Surface Methodology (RSM) allows highlighting for each dependent variable (Y) the significance level of the DIC factors as independent variables xi (P and t) through ANOVA, pvalue (P ≤ 0.05), and Pareto Charts, as well as the general trends, response surfaces, and isoresponse curves, issued from the empirical second-order polynomial model Y versus the independent variables xi, and R 2 whose value reveals the degree of validity of this empirical model:

= + " # $% & + " # $% & ' + " " ( # ($' & & ( #)% $% + * Eq. 3
Where + is the response, β,, β,,, and β,-are the regression coefficients, ., the independent variables, ε is a random error, i and j are the factor indices.

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Assessment methods and processes

Measurement of moisture content

The water content was determined according to the AOAC method (2005) at 105 °C for 24 hours until a constant weight is obtained.

Oil extraction by Randall

Usually, solvent extraction of both whole seed and residual cake (before and after the coldpress extraction, respectively), is carried out as an initial assessment stage to measure the composition of plant-base materials. Frequently, oil solvent extraction system used is "Randall's SER 148/3 model equipment from Velp Scientifica" (Italy). This is a closed loop cycle of oil solvent extraction procedure; i/ starting by bringing to boil solvent in container using a hot plate, ii/ following by solvent vapor condensation through an adequate cooling system, in the same container to wash the product placed in porous cartridge in this container (reflux washing where the solvent regenerated by condensation, flows and washes the sample contained in the cartridge), iv/ ending by a "recovery phase" during which the majority of the solvent is removed and the extracted oil remains in the containers with traces of solvent. Finally, under Sorbonne, the contents are deposited in Pyrex tubes to evaporate and remove the remainder traces of solvent from the evaporator.

In the present case of sunflower seeds, various differently processed, un-textured and DICtextured seeds, and cakes were studied. Before starting the properly said operation, Velp 148/3 parameters were checked out and corrected for compatibility. In the current case, since the solvent used was n-hexane, and the plate temperature was 180 °C, Viton seals were used. A sample of 5 to 7 g of crushed sunflower was placed in the porous cartridge to receive, by washing, almost 40 ml n-hexane. This operation was carried out in 6 h. For removing the remainder traces of n-hexane solvent from the evaporator, the cartridge contents were deposited in 18 ml Pyrex tubes and put under Sorbonne with airflow heated at 65 °C for 1 h. This extraction operation is faster, ampler, and more effective than a simple maceration extraction. The extracts were weighed using an analytical high-precision balance (with an accuracy of 0.001 g) and then stored at -4 °C for biochemical analyses. Oil yields were calculated with an estimated error of ± 0.0005 g oil/g ddb.

Determination of the free fatty acids

Fatty acid separation was performed using the Agilent 19091S-43 GC Gas Chromatography method (Kyoto, Japan). It is a qualitative and quantitative analysis method that serves to separate a very complex mixture of gaseous samples.

The chromatography device comprised a HP-5MS (5% Phenyl Methyl Siloxane) capillary type column (30 m × 350 μm × 0.25 μm), a furnace with initial temperature ranged from 155 to 230 °C with a speed of 45 °C/min. The final phase reached a maximum level of 240 °C and the oven run time was 10 min for each sample.

The protocol of the fatty acid analysis was to consider for each sample 15 μl of sunflower oil mix with 20 μl of the TMH, and 800 μl of the isooctane and, after stirring for a few seconds in a vortex, it was ready for analysis. A volume 0.8 μl of the assembly was eluted and injected using a syringe in split mode and the temperature of the injector was maintained at 270 °C.

Once vaporized by the injector, the compounds were entrained in the column by hydrogen as carrier gas. Peak integration and identification of fatty acids were based on the use of the mass spectral MS database of the NIST'98 National Library (National Institute of Standards and Technology of the United States (NIST), Gaithersburg, MD, USA).

Determination of the protein content

The determination of the protein content was based on the classical DUMAS method carried out by a nitrogen/protein analyzer LECO FP-528, which consists of a total combustion of 100 mg of homogeneous crushed dried sunflower seeds (24-h oven at 110°C). A double repetition was required for analysis for each sample.

DUMAS method, which is performed under oxygen and at high temperature does not directly measure the protein content. This allows quantification of the total nitrogen that is expressed as a percentage and using an internationally recognized conversion factor of 6.25 which differs from protein to protein depending on its amino acid composition, equivalent to 0.16g of nitrogen per gram of protein is required to convert the measured nitrogen content to a protein content. It's an easy and fast analysis.

Determination of tocopherol content

The determination of the tocopherol content of the sunflower oils was performed using High Performance Liquid Chromatography HPLC (SpectraPhysics, Thermo Separation Products, USA) in 40 min. The multi-λ French standard measurement ISO 9936, 1997 implies a complete separation of the 4 forms of tocopherol α, β, γ, and δ using a fluorescence detector with 298 nm as excitation wavelength, and 344 nm as emission wavelength [START_REF] Velasco | 2-D -20-Table 5. Saturated and unsaturated fatty acid amount of the whole seed and of the cake of sunflower for Linoleic and oleic variety of different DIC treatment conditions Linoleic variety whole seed of sunflower cake of sunflower Trial[END_REF].

The protocol of the tocopherol analysis was to consider a sample of 1 g of sunflower oil, and add 1 ml of acetone in a Vial; then the whole was vortexed a few seconds and directly injected into the HPLC system for analysis. The sum of the concentrations α-, β-, γ-, and δtocopherol is used to determine the total tocopherol expressed in μg/g oil.

In the current case, the mobile phase was composed of a mixture (75:25 v/v) of acetone and methanol with a flow rate of 1 cm 3 /min, provided by the HPLC Delta Chrom SDS 030 pump. The tocopherols were separated by the reversed phase C18 column at 30°C, 250 mm * 4.6 mm, and 5 μm as particle size (Macherey-Nagel Ltd., Düren, Germany). Quantification of the results was evaluated and integrated by comparison with the retention time collected with those of the respective standard surfaces using the chromatographic station software CSW 32 (Data Apex Ltd., Prague, Czech Republic).

Results and Discussions

Effect of DIC-texturing on cold-press extraction

In the current study, since the yields are expressed in terms of g Oil/g ddb (dry dry basis), we estimated the cold-press extraction yield by subtracting the oil content of the residual cake oil content Ycake from the initial sunflower seed Ywhole-seed, both obtained by performing solvent extraction (n-hexane) using the Randall velp-148 for 6 h:

+ /0122 = + 34561)21172 -+ 89:1
Eq. 4

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To better recognize the comparative aspect of DIC processing in terms of yield enhancement, we used the ratio of improvement of oil extraction (RIE) expressed as a percentage, between DIC-textured and raw-material seeds:

;<= = + 09>7966;@AB + 09>7966;CD Eq. 5

Table 1.

Table 1 and Figure 3 show the impact of DIC-texturing under different treatment conditions on cold-press oil yields expressed in (g oil/g ddb dry dry basis) of linoleic and oleic sunflower seeds. Thus, the whole DIC-textured linoleic seeds had an increased oil content Ywhole-seeds from 0.6519 g oil/g ddb for raw non-textured seeds up to 0.7208 g oil/g ddb, which means RIE of 111% RIE. Similar behavior was observed for oleic seeds; the increasing of the oil content of DIC-textured seeds was from 0.6639 g oil/g ddb for the raw non-textured seeds up to 0.7363 g oil/g ddb (or RIE=111%) of the whole DIC-textured oleic seeds. This should be due to an increasing of availability resulting from the higher porosity and possibly the broken cell walls by DIC-texturing triggered on the matrix. For both technologic and economic reasons, industries require reaching the highest coldpress extraction yields Ypress. DIC-texturing at 0.2 MPa for 50 s resulted in dramatically increasing of the cold-press yield of linoleic of 146% RIE, which means 0.3462 against 0.5060 g oil/g ddb for non-treated seeds. Similar results were obtained for oleic seeds DICtreated at 0.5 MPa for 50 s, with RIE=132% and cold press yield of 0.5206 against 0.3950 g oil/g ddb for non-treated seeds. It is worth highlighting that DIC-texturing pretreatment allows increasing the cold press yields respect to the whole seed yields by about 20%, from 53.11% to 73.56% for linoleic variety and 12%, from 59.50% to 71.64%, for oleic variety (Table 1).

Moreover, the effects of operating parameters of DIC treatment such as saturated vapor pressure (P) and heat treatment time (t) on the yields of oil, and the contents in the cake and in the whole seeds (Ypress; Ycake; Ywhole-seeds) were statistically analyzed by the statgraphic software (Figure 3).

Table 2.

Table 3.

It is worth noticing that the standardized Pareto Chart (Figure 3.A) shows a significant effect of saturated dry steam pressure (P) and treatment time (t) on the cold-press oil yield Ypress, residual oil content Ycake, and oil content in whole seeds Ywhole-seeds with p value of 0.05. The mean absolute error (MAE) is computed as the sum of the squared error values and is the most commonly used lack-of-fit indicator in statistical fitting procedures.

Thus, the empirical second-order regression models of Ypress, Ycake, and Ywhole-seeds versus the DIC processing parameters were computed. For linoleic variety, they had regression coefficients R 2 of 83.66%, 87.78%, and 90.62%, respectively, and MAE of 0.01395, 0.01313, and 0.00308, respectively (Table2). Similar study was performed in oil extraction from the oleic variety. They had regression coefficients R 2 of 69.62%, 67.02% and 91.14%, respectively, and the MAE were 0.03575, 0.03692, and 0.00395, respectively (Table 3).

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Impact of DIC on the quality of the final sunflower seed cake

Despite the great positive incidence of DIC-texturing on technological aptitudes of cold-oil pressing, it was noteworthy to study the impact of such DIC-texturing on the quality of both extracted oil and cake of sunflower. The specificity of great enhancement of oil yields by DICtexturing would not be so interesting without the perfect preservation of the main compounds and active molecules of sunflower seeds. Indeed, the conventional heating of oilseeds often leads to an increase in oil yields, but it also causes a great deterioration in the quality of both extracted oil and cake. As HTST (high temperature/short time) treatment, DIC often allows the concerned material to preserve its quality. Therefore, it was essential to carry out a detailed study of the impact of DIC-texturing on the composition of cake in proteins, and the oil in fatty acids and the availability of tocopherol.

Evolution of protein content of the sunflower whole seeds and press-cake vs DIC treatment conditions.

Table 4.

Figure 4.

The analyses of the total proteins were performed by the classical DUMAS method in the cases of the whole sunflower seeds and cakes at different conditions of DIC treatments, in comparison with the untreated seeds for both linoleic and oleic varieties.

Table 4 illustrates the results of this study and shows that the total protein content had no (or weak) change between the DIC treated and untreated samples; Figure 4 expresses a significantly absence of any effect.

Evolution of fatty acid contents of sunflower oil versus DIC treatment conditions

The composition on saturated and unsaturated fatty acids of the whole sunflower seeds and meal, mainly includes palmitic acid (C16:0), stearic acid (C18:0), oleic acid (C18:1), and linoleic acid (C18:2) of the two linoleic and oleic varieties are presented in Table 5.

The GC (Gas Chromatography) analyses proves that, whatever the processing conditions were, DIC texturing triggered no significant change in the fatty acid composition compared to the non-textured (RM) seeds. On the other hand, this observation was confirmed by the statistical analyses through Pareto Charts which reveal the analysis of the variance (ANOVA). Figure 5 highlights that, whatever the fatty acid compounds were, there was a significantly no effect for both whole seeds and cake of the two linoleic and oleic varieties.

Table 5.

Figure 5.

Tocopherol content of sunflower oil versus DIC treatment conditions

Table 6 presents the contents of total tocopherol and its forms (β, γ and α) of sunflower seed oils of the two linoleic and oleic varieties. These values were assessed by HPLC high performance liquid chromatography, for differently DIC textured and non-textured (RM) processing conditions.

The weak variation in tocopherol contents in both seed oil and cake, indicates that there was no change between the DIC-textured and non-textured samples. Moreover, the statistical analyses represented by Pareto Charts show a significantly no effect of the DIC parameters of the two linoleic and oleic varieties. This confirms that there was no degradation of the tocopherol contents (Figure 6).

Table 6.

Figure 6.

Conclusion

The great interest of industry manufacturing the sunflower oil in increasing the yields while preserving the quality of both oil and cake, as well as defining a new deodorization process was the basis of our investigation on the effect of texturing by instant controlled pressuredrop (DIC) on the press extraction of sunflower oil, respectively. DIC-texturing has allowed great impact on cold-press oil yields, reaching an increasing press-oil yields by about 46% and 32%, for linoleic and oleic varieties, respectively. Since DIC is a HTST (High Temperature/Short Time) operation, it normally results in the conservation of the biochemical composition of the material and a total preservation of the biochemical composition of both final products; oil and cake. The multi-criteria optimization of the DIC operating parameters, namely the saturated dry steam pressure (P) and the processing time (t), using a 5-level Central Composite Design (CCD) of Experimental Design (DoE) and prominently statistical analyses of the experimental results, highlighted the direct significant effect of the operating parameters P and t on the DIC performance. The highest yield of cold-press oil extraction has corresponded to 50 s of treatment time, with 0.2 and 0.5 MPa as saturated dry steam pressure, for linoleic and oleic varieties, respectively.

Thus, the Instant Controlled Pressure-Drop DIC texturation as a means of possible modification of the raw material structure appears getting great impact on increasing yields of the cold-press extraction of oil, on preserving the quality of both oil and residual cake with no change between the DIC textured and non-textured samples.

Reference

Al

  Figure 1.

  Figure 2.

  Figure 3.

Figure captions Figure 1 .

 captions1 Figure captionsFigure 1. Applied protocol of sunflower seed Figure 2. Instant controlled pressure-drop (DIC) laboratory-scale unit.

Figure 3 .

 3 Figure 3. Effect of Instant Controlled Pressure-Drop DIC parameters on oil yields by cold-press extraction, cake oil concentration and whole oil concentration of differently DIC-textured sunflower seeds: A) Standardized Pareto Chart; B) Separated direct effects and C) Estimated response area.

Figure 4 .

 4 Figure 4. Standardized Pareto charts of Instant Controlled Pressure-Drop DIC pretreatment for protein content of the sunflower whole seeds and cake for linoleic and oleic varieties versus DIC processing parameters P (saturated dry steam) and t (processing time)..

Figure 5 .

 5 Figure 5. Standardized Pareto Charts of DoE of Instant Controlled Pressure-Drop DIC for fatty acid amounts of sunflower whole seeds and cakes for linoleic varieties versus DIC processing parameters P (saturated dry steam) and t (processing time)..

Figure 6 .

 6 Figure 6. Standardized Pareto Charts for Tocopherol contents of whole sunflower seeds for linoleic and oleic variety versus Instant Controlled Pressure-Drop DIC processing parameters P (saturated dry steam) and t (processing time).

Figure 1 .

 1 Figure 1. Applied protocol of sunflower seed

Figure 2 .

 2 Figure 2. Instant controlled pressure-drop (DIC) laboratory-scale unit.

Figure 3 .

 3 Figure 3. Effect of Instant Controlled Pressure-Drop DIC parameters on oil yields by cold-press extraction, cake oil concentration and whole oil concentration of differently DIC-textured sunflower seeds: A) Standardized Pareto Chart; B) Separated direct effects and C) Estimated response area.

Figure 4 .

 4 Figure 4. Standardized Pareto charts of Instant Controlled Pressure-Drop DIC pretreatment for protein content of the sunflower whole seeds and cake for linoleic and oleic varieties versus DIC processing parameters P (saturated dry steam) and t (processing time).

Figure 5 .

 5 Figure 5. Standardized Pareto Charts of DoE of Instant Controlled Pressure-Drop DIC for fatty acid amounts of sunflower whole seeds and cakes for linoleic varieties versus DIC processing parameters P (saturated dry steam) and t (processing time).

Figure 6 .(

 6 Figure 6. Standardized Pareto Charts for Tocopherol contents of whole sunflower seeds for linoleic and oleic variety versus Instant Controlled Pressure-Drop DIC processing parameters P (saturated dry steam) and t (processing time).

  Steel Contacts on Tribo Properties in Presence of Sunflower Oil as a Bio Lubricant. Procedia Eng., INTERNATIONAL TRIBOLOGY CONFERENCE MALAYSIA 2013 68, 550-557. https://doi.org/10.1016/j.proeng.2013.12.220 Albitar, N., Mounir, S., Besombes, C., Allaf, K., 2011. Improving the Drying of Onion Using the

	Instant Controlled Pressure Drop Technology. Dry. Technol. 29, 993-1001.
	https://doi.org/10.1080/07373937.2010.507912
	Allaf, T., Allaf, K., 2013. Instant Controlled Pressure Drop (D.I.C.) in Food Processing: From
	Fundamental to Industrial Applications. Springer Science & Business Media.

Mahmud, K.A.H., Zulkifli, N.W.M., Masjuki, H.H., Varman, M., Kalam, M.A., Mobarak, H.M., Imran, A., Shahir, S.A., 2013. Working

Temperature Effect of A-C: H/A-C: H and Steel/Allaf, T., Fine, F., Tomao, V., Nguyen, C., Ginies, C., Chemat, F., 2014. Impact of instant controlled pressure drop pre-treatment on solvent extraction of edible oil from rapeseed seeds. OCL 21, A301. https://doi.org/10.1051/ocl/2014002

Table captions Table 1 .

 captions1 Cold-pressYpress, solvent extracted Residual cake oil Ycake, and total whole-seed oil yields Ywhole-seeds, expressed in g Oil/g db (dry basis: material mass devoid from water content) and g Oil/g ddb (dry dry basis: material mass devoid from water and oil contents) of the linoleic and oleic variety for untreated and differently DIC-treated sunflower seeds.

Table 2 .

 2 The empirical models retained from the second-order regression of linoleic variety

Table 3 .

 3 The empirical models retained from the second-order regression of oleic variety

Table 4 .

 4 Protein contents of whole seeds and cake of sunflower versus different DIC treatment conditions.

Table 5 .

 5 Saturated and unsaturated fatty acid amount of the whole seed and of the cake of sunflower for Linoleic and oleic variety of different DIC treatment conditions

Table 6 .

 6 Tocopherol contents of the sunflower oil for the Linoleic and Oleic varieties of different Instant Controlled Pressure-Drop DIC treatment conditions.

Table 1 .

 1

	569
	Linoleic variety Run no P(MPa) t (s) g Oil/g db g Oil/g ddb RIE% g Oil/g db g Oil/g ddb RIE% g Oil/g db g Oil/g ddb RIE% Ypress Ycake Ywhole seeds	Ypress/Ywhole-seeds
	RM	-	-	0.2096	0.3462	100% 0.2341	0.3057	100% 0.3946	0.6519	100%	53.11%
	DIC 1	0.5	50	0.2147	0.3649	105% 0.2508	0.3348	110% 0.4117	0.6997	107%	52.16%
	DIC 2	0.7	50	0.2527	0.4317	125% 0.2167	0.2767	91%	0.4146	0.7084	109%	60.94%
	DIC 3	0.5	85	0.2382	0.4057	117% 0.2292	0.2974	97%	0.4128	0.7030	108%	57.70%
	DIC 4	0.5	50	0.2002	0.3418	99%	0.2676	0.3654	120% 0.4142	0.7071	108%	48.33%
	DIC 5	0.6	74.7 0.2488	0.4281	124% 0.2264	0.2927	96%	0.4189	0.7208	111%	59.40%
	DIC 6	0.6	25.3 0.2125	0.3571	103% 0.2443	0.3233	106% 0.4049	0.6804	104%	52.48%
	DIC 7	0.5	50	0.2632	0.4471	129% 0.2011	0.2517	82%	0.4114	0.6989	107%	63.98%
	DIC 8	0.3	25.3 0.2689	0.4522	131% 0.1866	0.2294	75%	0.4053	0.6816	105%	66.34%
	DIC 9	0.3	74.7 0.2622	0.4411	127% 0.1944	0.2413	79%	0.4056	0.6824	105%	64.64%
	DIC 10	0.5	50	0.2572	0.4346	126% 0.2033	0.2552	83%	0.4082	0.6898	106%	63.01%
	DIC 11	0.2	50	0.2998	0.5060	146% 0.1539	0.1819	60%	0.4076	0.6879	106%	73.56%
	DIC 12	0.5	15	0.2733	0.4569	132% 0.1769	0.2149	70%	0.4019	0.6718	103%	68.01%
	DIC 13	0.5	50	0.2564	0.4342	125% 0.2058	0.2591	85%	0.4094	0.6933	106%	62.62%
	Oleic variety Run no P(MPa) t (s) g Oil/g db g Oil/g ddb RIE% g Oil/g db g Oil/g ddb RIE% g Oil/g db g Oil/g ddb RIE% Ypress Ycake Ywhole-seeds	Ypress/Ywhole-seeds
	RM	-	-	0.2374	0.395005 100% 0.2119 0.2688745 100% 0.3990 0.6638795 100%	59.50%
	DIC 1	0.5	50	0.2436 0.4175983 106% 0.2288 0.2966805 110% 0.4167 0.7142788 108%	58.46%
	DIC 2	0.7	50	0.1266 0.2183322 55%	0.3361 0.5062509 188% 0.4201 0.7245832 109%	30.13%
	DIC 3	0.5	85	0.2989 0.5189654 131% 0.1785 0.2172855 81%	0.4240 0.7362508 111%	70.49%
	DIC 4	0.5	50	0.3015 0.5206118 132% 0.1709 0.2061271 77%	0.4209 0.7267389 109%	71.64%
	DIC 5	0.6	75	0.2908 0.5028691 127% 0.1846	0.226392 84%	0.4217 0.7292611 110%	68.96%
	DIC 6	0.6	25	0.2683 0.4539809 115% 0.1923 0.2380834 89%	0.4090 0.6920643 104%	65.60%
	DIC 7	0.5	50	0.265	0.454888 115% 0.2074 0.2616705 97%	0.4174 0.7165584 108%	63.48%
	DIC 8	0.3	25	0.2668 0.4530433 115% 0.1968 0.2450199 91%	0.4111 0.6980632 105%	64.90%
	DIC 9	0.3	75	0.2802 0.4789306 121% 0.1872	0.230315 86%	0.4149 0.7092456 107%	67.53%
	DIC 10	0.5	50	0.2814 0.4840479 123%	0.191	0.2360939 88%	0.4187 0.7201419 108%	67.22%
	DIC 11	0.2	50	0.2649 0.4468739 113% 0.1936 0.2400794 89%	0.4072 0.6869533 103%	65.05%
	DIC 12	0.5	15	0.2648 0.4484239 114% 0.1968 0.2450199 91%	0.4095 0.6934439 104%	64.67%
	DIC 13	0.5	50	0.2865 0.4932339 125% 0.1859 0.2283503 85%	0.4191 0.7215842 109%	68.35%

Table 4 .

 4 Protein contents of whole seeds and cake of sunflower versus different DIC treatment conditions.

	578
		run n°.	linoleic variety whole seeds sunflower cake
		DIC 1	17.87	22.91
		DIC 2	18.11	22.56
		DIC 3	17.31	22.22
		DIC 4	17.71	23.36
		DIC 5	17.61	23.65
		DIC 6	18.01	22.53
		DIC 7	18.51	23.81
		DIC 8	20.47	25.91
		DIC 9	17.55	25.46
		DIC 10	18.80	25.03
		DIC 11	17.77	25.66
		DIC 12	17.67	25.91
		DIC 13	17.71	23.22
		<RM>	18.06 ± 0.80	25.59 ± 0.66
		<DIC>	18.08 ± 0.82	24.02 ± 1.38
	579	<RM>: Average of Raw Material samples
		<DIC>: Average of DIC treatments

Table 6 .

 6 Tocopherol contents of the sunflower oil for the Linoleic and Oleic varieties of different InstantControlled Pressure-Drop DIC treatment conditions.

	Linoleic variety

-17-


M.-L Dauvergne

S Gallin-Martel

G Marcatilli

Thiamova

G Simpson

S Curtoni

D Dauvergne

M.-L Gallin-Martel

S Marcatilli

G Thiamova

Performance of four digital algorithms for γ-γ timing with LaBr_3(Ce) scintillators

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

Introduction

Fast scintillator detectors are used for γγ timing and have a wide range of applications, including lifetime measurements of excited nuclear states [START_REF] Mach | A method for picosecond lifetime measurements for neutron-rich nuclei: (1) outline of the method[END_REF], medical positron-emission tomography (PET) and range monitoring in hadron therapy [START_REF] Krimmer | Prompt-gamma monitoring in hadrontherapy: A review[END_REF]. These scintillation crystals have the properties of modest energy resolution and fast decay times (ns), and are constructed often with the aim of optimising the time rather than energy resolution of the system. Recent progress in the fabrication of lanthanide-halide crystals, such as LaBr 3 (Ce), with an energy resolution of ∼3 % at 662 keV and time resolution as good as 98(2) ps for ∼1.2 MeV photons [START_REF] Vedia | Performance evaluation of novel labr3(ce) scintillator geometries for fast-timing applications[END_REF] has given renewed interest in the use and development scintillator detectors, resulting in, for example, the construction of the FATIMA array [START_REF] Roberts | A labr3: Ce fast-timing array for despec at fair[END_REF][START_REF] Fraile | Technical design report for the despec fast timing array[END_REF]. The decay time of LaBr 3 (Ce) is 16 ns and 63000 photons are emitted per MeV of energy absorbed. This compares with a decay time of 0.7 ns and 1800 photons/MeV for the fast component of the commonly used BaF 2 . These scintillator detectors can be used to measure nuclear-state lifetimes in the 10sof-ps-to-ns time range [START_REF] Mach | A method for picosecond lifetime measurements for neutron-rich nuclei: (1) outline of the method[END_REF][START_REF] Régis | The generalized centroid difference method for picosecond sensitive determination of lifetimes of nuclear excited states using large fast-timing arrays[END_REF][START_REF] Régis | Germanium-gated fast timing of excited states in fission fragments using the exill & fatima spectrometer[END_REF]. To date analogue signal processing chains have been almost exclusively used in applications which require the very best time resolution.

In principle digital acquisition systems, with very high-speed sampling, should allow equivalent, or even improved, timing performance over analogue ones, as signal processing can reduce jitter and fixed-frequency noise and bespoke algorithms can be developed for a particular detection system. Recently, time resolutions approaching, and matching, the best ones achieved with analogue electronics have been obtained with LaBr 3 (Ce) detectors, using digitizers with sampling frequencies of 0.5, 4 and 5 GS/s by applying digital timing algorithms [START_REF] Warburton | New Algorithms for Improved Digital Pulse Arrival Timing With Sub-GSps ADCs[END_REF][START_REF] Nakhostin | Digital processing of signals from labr3:ce scintillation detectors[END_REF][START_REF] Sánchez-Tembleque | Digital strategies for time and energy measurement for ultra fast scintillators[END_REF]. An improved time resolution over analogue systems was earlier obtained with Ge detectors using digital pulse-shape analysis [START_REF] Crespi | Hpge detectors timing using pulse shape analysis techniques[END_REF]. Furthermore, digital acquisition systems have other advantages over analogue ones including (potentially) lower cost per channel, fewer modules and timing stamping of individual hits, allowing offline event reconstruction. Any algorithms used for pulse-shape analysis should ideally be simple and efficient enough to be implemented on fieldprogrammable gate arrays (FPGAs), allowing real-time processing. Pulse-shape analysis has already been used to perform, for example, α/γ discrimination in LaBr 3 (Ce) crystals [START_REF] Zeng | Quantitative analysis and efficiency study of psd methods for a labr3:ce detector[END_REF] and neutron/γ selection in liquid scintillators [START_REF] Aspinall | The empirical characterization of organic liquid scintillation detectors by the normalized average of digitized pulse shapes[END_REF].

The purely statistical resolving time of a pair of detectors is given by δt

= σ √ n
where n is the number of events and σ is the width parameter of the Gaussian function describing the distribution. Hence an experiment using detectors with twice worse time resolution will require 4 times the number of counts to achieve the same statistical precision. Therefore there is strong motivation to develop timing algorithms suitable for use with digital acquisition systems which have performances equivalent to, or better than, the best analogue pulse-processing electronics.

Improved timing resolution is also of interest for clinical PET applications, which would allow lower injected patient doses. Although the γ-ray detectors used in PET applications are much smaller than the crystals used for nuclear excited-state lifetime measurements, equivalent pulse-processing techniques are used to extract timing information. Clinical PET time resolutions better than 100 ps would allow some of the artefacts affecting tomographic reconstruction to be removed for devices with partial angular coverage [START_REF] Lecoq | Pushing the limits in time-of-flight pet imaging[END_REF]. For resolutions of 10 ps time-consuming image reconstruction techniques would not be required, as true real-time 3-D image information would be available [START_REF] Lecoq | Pushing the limits in time-of-flight pet imaging[END_REF]. In the case of prompt-γ timing for particle therapy, transitions with energies typically in the range 3-6 MeV are measured, for which better time resolution is expected than at 511 keV [START_REF] Krimmer | Prompt-gamma monitoring in hadrontherapy: A review[END_REF].

With this in mind we have measured the time resolution of a pair of fast LaBr 3 (Ce) scintillator detectors when applying four different algorithms to extract timing information. These algorithms were a leading-edge discrimination of the raw detector pulse and also following a cubic polynomial fit to the rising slope, a digital constant-fraction discriminator and an ideal-pulse autocovariance function. The first and third pulse-processing algorithms are equivalent to analogue fast-signal treatment schemes. These algorithms differ from other fast filters often implemented on commercial digital acquisitions systems for use with Ge detectors, such as trapezoidal ones. These fast-filters have the aim of distinguishing real low-energy signals from noise. Limited tests of these algorithms within the present work gave degraded timing performance in comparison with the ones used below, however this does not exclude that a well-tuned algorithm of this type, with its inherent noise filtering, may give improved results in the future. The experiments were performed with a digital oscilloscope running at a sampling frequency of 40 GS/s, with a 4 GHz bandwidth and 10-bit vertical resolution. The effect of varying the sampling frequency was also studied.

Experimental Setup

The time resolution of γγ coincidences detected in a pair of scintillator detectors was studied in order to determine pulse-processing algorithm perfor- trace. The oscilloscope ran in an "AND" mode where traces were captured only if triggers on both signals fired within a time window of a few nanoseconds. The high voltage was set to ∼-1100 V so that pulses with energies up to ∼1.5 MeV could be recorded on the oscilloscope. This voltage is slightly lower than the -1200 V used in [START_REF] Vedia | Performance evaluation of novel labr3(ce) scintillator geometries for fast-timing applications[END_REF], which was found to be optimal with the same detectors.

The limited vertical acceptance range of the oscilloscope (1 V) meant that the optimal voltage could not be used. However, in [START_REF] Régis | The time-walk of analog constant fraction discriminators using very fast scintillator detectors with linear and non-linear energy response[END_REF] the difference in resolution between LaBr 3 (Ce) detectors operating at -1100 V and -1300 V was found to be ∼15 ps. Hence the use of a slightly lower than optimal voltage is expected to result in only a small degradation in time resolution (<15 ps).

Data taken with a 264-kBq 60 Co source were used to find the best resolution of each algorithm at energies of ∼1.2 MeV. This source first β -decays and then emits a cascade of two γ rays of energy 1173. Use of these cascades would add appreciable widths to time spectra and are unsuitable for this study. However the 344.3-keV transition is coincident with five transitions covering an energy range of 367.8-1299.1 keV. For four of these γγ coincidences the intermediate state is the 344.3-keV one, with a lifetime of 46.7 ps [START_REF] Régis | The mirror symmetric centroid difference method for picosecond lifetime measurements via coincidences using very fast labr3(ce) scintillator detectors[END_REF]. The fifth one is the 367.8-344.3-keV coincidence which is part of a triple-γ cascade with a mean lifetime of 57.2 ps. Therefore for this set of γγ cascades mean lifetimes of a similar ∼50 ps are present, allowing a qualitative comparison of the performance of each algorithm as a function of the γ-ray energy coincident with a 344.3-keV transition. The performance of these algorithms at energies of around 511 keV is relevant to PET applications [START_REF] Ahmed | Characterisation of silicon photomultipliers for time-of-flight pet[END_REF], though the crystals used in the current study are optimised for lifetime measurements of excited nuclear states.

Algorithms and Results with 60 Co

Before applying any timing algorithms the first procedure was to extract the average value of the pulse baseline. This quantity varies from pulse to pulse and was obtained simply by finding the average of the first 300 sample points. These all lie at times earlier that the first sample point registering the interaction of a γ ray, as can be seen in Fig. 1. This shows a typical anode pulse from one of the were reflected across the x-axis so that all values are positive. This was done to simplify algorithm implementation. The energy of the deposited γ-ray signal was found using a simple running integration algorithm, which sums all sample points found above the baseline. An energy resolution (FWHM) of 3.4(1) % was measured at 1332.5 keV, the same as reported in [START_REF] Nakhostin | Digital processing of signals from labr3:ce scintillation detectors[END_REF][START_REF] Sánchez-Tembleque | Digital strategies for time and energy measurement for ultra fast scintillators[END_REF].

If the energies of both pulses were found to fall within ±20 keV of the individual photopeak energies of interest then timing algorithms were applied to the event. These algorithms are listed below. Examples of gated energy spectra measured using the 60 Co source are shown in Fig. 2. Here one observes only the other transition of the γγ cascade and Compton background.

In order to quantify the timing performance of each algorithm when analysing γγ events, differences between the trigger times of each detector were plot- ted. In all cases the full-width half-maximum (FWHM) value of the assumed Gaussian time distribution is used to define the resolution. As the detectors are essentially identical, then the measured FWHM can be divided by √ 2 to obtain the resolution of each individual detector, to a good approximation. This allows a comparison with results reported in the literature for each detector type, for example those of [START_REF] Vedia | Performance evaluation of novel labr3(ce) scintillator geometries for fast-timing applications[END_REF]. Identical γ-ray gates were set when processing the data with each algorithm.

Leading edge with a 60 Co source

The leading-edge algorithm produces a trigger-time marker when the pulse trace first crosses a set threshold. For the data taken with the 60 Co source, the FWHM was measured as a function of the threshold energy. Use of interpolation between the sample points did not improve the time resolution. This agrees with the conclusions of [START_REF] Warburton | New Algorithms for Improved Digital Pulse Arrival Timing With Sub-GSps ADCs[END_REF] for high sampling frequencies. There it was reasoned that the higher density of sampling points means that the difference between the actual detector pulse and a linear interpolation between any two sample points becomes negligible.

An example time spectrum obtained with the LaBr 3 (Ce) detectors is shown in Fig. 3. The results obtained are shown in Fig. 4 where the change in FWHM is shown as a function of the threshold value. The maximum pulse height is around 35000 (arbitrary units) and the best FWHM values are obtained with the threshold set at 10-20 % of this maximum. A similar behaviour is found in analogue leading-edge modules and in previous studies using digital leadingedge algorithms (15 %) [START_REF] Ahmed | Characterisation of silicon photomultipliers for time-of-flight pet[END_REF]. One observes that there is a regular degradation of the FWHM with increasing threshold values. Threshold values below ∼5 % of the pulse height produced spurious peaks due to noise. The best resolution obtained with this algorithm was 177(4) ps. accurately reproduce the rising edge of the detector pulses, in line with the results of [START_REF] Bardelli | Time measurements by means of digital sampling techniques: a study case of 100ps fwhm time resolution with a 100msample/s, 12bit digitizer[END_REF]. A leading edge threshold was then applied to the fit function describing each individual pulse, producing a reference time. Fits were applied across rising slopes varying from 10 %-90 %, 5 %-90 %, 5 %-50 % and 5 %-30 % of the pulse peak height. The best results were obtained for fits over the range 5 %-90 % and 5 %-50 % of pulse height, where equivalent time resolutions, within errors [143(3) ps], were obtained. The threshold parameter was also varied until the smallest FHWM was found. A value close to the one in Sec. 3.1 was optimal. The results are shown in Fig. 5. One notes that it is possible to apply this algorithm with a lower threshold value than for the raw signals of Sec. 3.1, due to the absence of high-frequency noise on the fitted polynomial function.

Cubic Polynomial

The digital leading-edge algorithm of Sec. 3.1 and the cubic polynomial slope fit, followed by a leading edge trigger, used here trigger on signals in a very sim-ilar manner. A comparison of the results obtained by each allows an estimation of the influence of higher frequency noise harmonics on the signal time resolution, because high-frequency noise is smoothed out in the fit analysis. As noise contributions add in quadrature, then one obtains FWHM highf req =104(3) ps for the high-frequency noise component, a significant amount.

Constant Fraction with a 60 Co source

The constant fraction (CFD) algorithm used in the analysis of the detector signals is the digital equivalent of the ones used in analogue modules. An input signal is duplicated, inverted and delayed. The original signal is then attenuated by a fraction f and the zero-crossing of the sum of these two signals corresponds to the reference time. The CFD algorithm is written as

CF D[i] = f × V [i] -V [i -delay] ( 1
)
where f is the fraction of the attenuated signal, V [i] is the pulse height of the sample at bin number i and delay is the time by which the duplicated signal is retarded [START_REF] Fallu-Labruyere | Time resolution studies using digital constant fraction discrimination[END_REF]. This algorithm was tested over a range of values of f , from ∼0.2 to 0.4, corresponding to the ones used in analogue modules. Values of f below 0.15 could not be applied due to noise. The results are shown in Fig. 6. Similarly the delay parameter was varied until an optimal result was obtained. The best delay values are close to the peak rise time, again in line with the settings of an analogue CFD. The performance of the algorithm is relatively insensitive to changes in delay times over a large range of values, as shown in Fig. 7. A time resolution of 178(4) ps was the best one obtained with this algorithm.

Autocovariance with averaged pulse-shapes with a 60 Co source

For each pair of applied energy gates pulses in each detector were summed and then averaged. This produced approximately "ideal" pulses , almost noise free, though still retaining any subtle systematic inflexions inherent to each detector pulse.

Once the set of average pulses had been obtained then each pulse in a given detector, falling within the range of the energy gate, was then compared to it. This was done by calculating the variance between the rising slope of a pulse of a given event and the average one, within the same height interval, using Welford's algorithm [START_REF] Ling | Comparison of several algorithms for computing sample means and variances[END_REF]. The event pulse was then shifted by one sample and the variance calculated again. Once the variance had been calculated across a set range of shifts, the minimum variance was obtained, allowing the "lag" between the individual pulse and the average one to be determined. The lag value can then be used to determine a trigger time. It is worth noting that this algorithm has no threshold dependence.

The vertical range over which the variance was calculated was changed and the optimal one was found to be 5-20 % of the pulse height, giving a resolution of 155(3) ps. This is shown in Fig. 8, where resolution is plotted as a function of analysed pulse height. Equivalent results were obtained when comparing 20 channels of the event pulse to the average one, once a low-energy threshold was crossed. This latter method is less computationally intensive.

The method described here is practically identical to the "Mean PMT pulse model" used by Aykac et al. to analyse pulses from LSO cyrstals [START_REF] Aykac | Timing performance comparison of digital methods in positron emission tomography[END_REF]. We note that a procedure with a similar philosophy has been used for an entirely different γ-ray spectroscopy application. The shapes of pulses recorded from the segmented outer contacts of 36-fold AGATA Ge detectors are compared to those found in a library of measured interactions [START_REF] Akkoyun | Agata-advanced gamma tracking array[END_REF]. This allows the interaction position of γ rays to be found with a precision of a few mm in a large-volume Ge detector.

60 Co Source Results Summary and Comparison with Literature

A summary of the results obtained with each algorithm is shown in Table 1.

As ∼1000 γγ coincidences were analysed, then the statistical contribution to the error ( σ √ n ) is around 2 ps. These results can be compared to the time resolutions reported in the literature using analogue pulse-processing electronics. A study of the performance of conical LaBr 3 (Ce) detectors, with the same dimensions as used here, reported a FWHM value of 110(3) ps with a 60 Co source [START_REF] Vedia | Performance evaluation of novel labr3(ce) scintillator geometries for fast-timing applications[END_REF]. recent study using a 16-bit 5 GS/s digitizer with the same LaBr 3 (Ce) detectors a time resolution of 106(1) ps was reported, using the same type of radioactive source and a timing algorithm developed using machine learning [START_REF] Sánchez-Tembleque | Digital strategies for time and energy measurement for ultra fast scintillators[END_REF]. This result surpasses all the ones obtained in the present work. In [START_REF] Warburton | New Algorithms for Improved Digital Pulse Arrival Timing With Sub-GSps ADCs[END_REF] a spline interpolation with a sinc function of the pulse rising slope using a 14-bit, 0.5 GS/s digitizer gave results equivalent to those reported using analogue electronics [START_REF] Vedia | Performance evaluation of novel labr3(ce) scintillator geometries for fast-timing applications[END_REF] for 1"×1" LaBr 3 (Ce) crystals [97 ps versus 98 [START_REF] Krimmer | Prompt-gamma monitoring in hadrontherapy: A review[END_REF] ps]. This shows that a higher sampling frequency does not necessarily lead to improved time resolution if the vertical resolution is low and these points are discussed below.

Sampling Frequency

The performance of each algorithm was tested as a function of sampling frequency, using the data taken with the 60 Co source. The results are shown in Fig. 9. Optimal parameters at sampling rates of 40 GS/s, reported in previous sections were used throughout. Unsurprisingly the best performance is found at a sampling frequency of 40 GS/s, though little degradation of the FWHM is seen at 20 GS/s. At sampling frequencies of 4 or 5 GS/s, the resolution is typically 30-60 % worse. The results obtained in the present work differ from the conclusions of Aykac et al. [START_REF] Aykac | Timing performance comparison of digital methods in positron emission tomography[END_REF] who studied the performance of small LSO detectors for PET applications. There they found that optimal digital algorithm performance was already attained at a sampling rate of 4 GS/s. Similarly Warburton and Henning [START_REF] Warburton | New Algorithms for Improved Digital Pulse Arrival Timing With Sub-GSps ADCs[END_REF] and Nakhostin et al. [START_REF] Sánchez-Tembleque | Digital strategies for time and energy measurement for ultra fast scintillators[END_REF] were able to obtain results equivalent to the best ones achieved with analogue systems, though at lower frequencies of 0.5 and 4 GS/s using digitizers with 14 and 16 bits. This points towards vertical resolution being a more important parameter than sampling speed in the GS/s domain for LaBr resolution (number of bits) must be high enough that the quantization error is below the electronic noise of the signal [START_REF] Bardelli | Time measurements by means of digital sampling techniques: a study case of 100ps fwhm time resolution with a 100msample/s, 12bit digitizer[END_REF].

152 Eu data

The evolution of the peak FWHM as a function of energy, measured with the 152 Eu source is shown in Fig. 10. Here one gate was set on the 344.3-keV γ-decay of 152 Gd and the peak FWHM was measured when the second gate was set at other photopeak energies (367.8, 411.1, 778.9, 1089.7 and 1299.1 keV).

There were around 1000 counts in each coincidence time spectrum. The 344.3-1089.7-keV data points were removed from the fit as they were found to be systematically higher than the trend lines, likely due to contamination with the 1085.9-keV transition in 152 Sm.

In Fig. 10 experiments aiming to study the lifetimes of excited states in the 10s-of-ps to ns time range.

An important parameter in fast-timing measurements is the prompt-response difference (PRD) [START_REF] Régis | The generalized centroid difference method for picosecond sensitive determination of lifetimes of nuclear excited states using large fast-timing arrays[END_REF][START_REF] Régis | Germanium-gated fast timing of excited states in fission fragments using the exill & fatima spectrometer[END_REF][START_REF] Régis | The mirror symmetric centroid difference method for picosecond lifetime measurements via coincidences using very fast labr3(ce) scintillator detectors[END_REF]. This function is used to determine the zero-time position as a function of energy and it depends on the settings of the analogue discriminator used. The "walk" of this zero-time position typically changes by a few hundred picoseconds over an energy range of 100 keV to 1.5 MeV. Uncertainties in the PRD generally limit the precision of high-statistics fasttiming measurements, hence the interest in obtaining PRD functions which are as flat as possible [START_REF] Régis | Reduced time walk to below 50 ps using the multiplexed-start and multiplexed-stop fast-timing technique with labr3(ce) detectors[END_REF][START_REF] Régis | A simple procedure for -lifetime measurements using multi-element fast-timing arrays[END_REF]. Despite the 3.1 × 10 6 coincident events recorded with the 152 Eu source there were insufficient statistics to reliably determine the PRDs of the all digital algorithms tested here.

Conclusion

The time resolutions obtained with four different timing algorithms have The autocovariance function was found to have the best performance for coincidences in the energy range 344.3-1299.1-keV. This may be because the cubic polynomial slope fit algorithm used relies on a leading-edge threshold trigger, with settings optimised for 60 Co lines. The performance of this algorithm may therefore degrade when applied to lower-energy γ-rays. Reducing the sampling frequency to 20 GS/s was found to only slightly degrade the time resolution of these algorithms. The results obtained in the present work are a few tens-ofpicoseconds worse than those obtained using a 16-bit, 5 GS/s digitizer module [106(1) ps] [START_REF] Sánchez-Tembleque | Digital strategies for time and energy measurement for ultra fast scintillators[END_REF], demonstrating that high vertical resolution is more important than sampling speed in the GS/s domain when using these detectors.

  mance. A pair of truncated-cone shaped LaBr 3 (Ce) scintillator detectors were used to detect γ rays emitted from 60 Co and 152 Eu radioactive sources. The detectors were placed ∼2.5 cm from the source and at 90 • to each other, to minimise Compton scattering. The LaBr 3 (Ce) crystals were 38 mm long and 38-mm wide at the base. Their exact dimensions are reported in [3]. These crystals were mounted on Hamamatsu R9779 photomultiplier tubes (PMTs). The anode output signal of the PMT base was connected directly to the oscilloscope and the dynode output was terminated with a 50-Ω resistor. The oscilloscope was a LeCroy HDO9404 model with 10-bit vertical resolution and a vertical range of 1 V. It ran at a sampling speed of 40 GS/s. Each digitized trace was 4096 samples (102.4 ns) long, enough to contain all of the LaBr 3 (Ce) signal

2 -

 2 and 1332.5-keV. The halflife of the intermediate state is 0.7 ps, negligible compared to the time resolution of the γ-ray detectors. Some 3 × 10 5 coincident traces were captured. A 27.0-kBq 152 Eu source was used to determine how the different algorithms perform for γ-ray transitions across a wider energy range. A total of 3.1 × 10 6 coincidence traces were recorded. Around 20 intense γ rays are emitted by this source, which form γγ cascades distributed in two nuclei, 152 Gd and 152 Sm [16]. These cover an energy range of 121.8 to 1299.1 keV, however several lifetimes of the intermediate states are in the 100-ps-to-ns time range.

LaBr 3 (

 3 Ce) detectors captured by the oscilloscope. The data points presented

Figure 1 :

 1 Figure 1: Example of an anode pulse captured by the 40 GS/s oscilloscope. The data points shown are the ones analysed and these are the raw ones reflected about the x-axis, as explained in the text. The zoomed inset allows the noise present in the baseline and at the start of the rising pulse to be observed.

Figure 2 :

 2 Figure 2: Energy spectra obtained with the LaBr 3 (Ce) detectors when energy gates are set on one transition and then the other of the cascasde.

Figure 3 :Figure 4 :

 34 Figure 3: Example time spectrum measured with the leading edge algorithm and a 60 Co source.

Figure 5 :

 5 Figure 5: Full width half maximum (FWHM) of time signals obtained with a cubic slope-fit algorithm as a function of the threshold value for a single detector, de-convoluted from the measured distribution.

Figure 6 :

 6 Figure6: Full width half maximum (FWHM) of a digital constant-fraction algorithm as a function of the fraction of the attenuated signal for a single detector, de-convoluted from the measured distribution. A delay of 6.25 ns was used.

Figure 7 :

 7 Figure 7: Full width half maximum (FWHM) of a digital constant-fraction algorithm as a function of the delay time of the duplicated signal for a single detector, de-convoluted from the measured distribution. A fraction of f =0.18 was used.

Figure 8 :

 8 Figure8: Full width half maximum (FWHM) of a digital autocovariance algorithm as a function of upper limit of the peak signal voltage for a single detector, de-convoluted from the measured distribution.

3 (

 3 Ce) detectors. More formally the vertical

Figure 9 :

 9 Figure 9: Full width half maxima (FWHM) of time peaks extracted with different digital algorithms as a function of sampling frequency for a single detector, de-convoluted from the measured distribution.

  the autocovariance function is seen to have the best resolution across the range of energies studied. This energy range is typical of fast-timing

Figure 10 :

 10 Figure 10: Measured peak full width half maximum (FWHM) values as a function of transitions found in coincidence with the 344.3-keV γ ray emitted following the decay of a 152 Eu source.

  been measured with a pair of 38-mm long, 38-mm wide truncated-cone shaped LaBr 3 (Ce) fast-timing detectors using a 10-bit, 40 GS/s oscilloscope. The time resolution obtained with a cubic polynomial slope fit leading-edge algorithm gave the best result with a 60 Co source [143(3) ps], though this is 30(1) % worse than values achieved using analogue pulse-processing electronics. The performance of the autocovariance function was slightly inferior [155(3) ps], but better than the leading edge [177(4) ps] and constant-fraction [178(4) ps] algorithms.

  , G. Bosson and Ch. Vescovi of the LPSC Grenoble are thanked for helpful discussions. Financial support for this project was provided by NuPNET/IN2P3. The IKP Cologne, NIPNE Bucharest and the ILL Grenoble are thanked for the loan of equipment. The authors (DD, MLGM, SM and SC) acknowledge support from LabEx PRIMES (ANR-11-LABX-0063), INCa Physique-Cancer (CLaRyS-UFT project) and the FP7 MEDINET-ENSAR2 framework.

Table 1 :

 1 Summary of best values of FWHM achieved for each algorithm with a 60 Co source and a 10-bit 40 GS/s oscilloscope.

	Leading Edge Leading Edge Slope Fit	CFD	Autocovariance
	177(4) ps	143(3) ps	178(4) ps	155(3) ps