ITMI20101992A1 - DEVICE FOR MEASURING PROPERTIES OF A BODY BY MEANS OF RADIATION DISTRIBUTION AND RELATIVE METHOD - Google Patents

Description

Device designed to measure properties of a body by means of radiation diffusion and relative method

DESCRIPTION

Among the techniques for characterizing media at least partially transparent to light in a given range of wavelengths (or frequencies), the techniques based on the diffusion of radiation generally have the advantage of being non-invasive and allow for online monitoring. of the properties of the media.

Among the techniques that analyze the light diffused by a medium, the use of the Dynamic Light Scattering (DLS) technique makes it possible to extract different structural information of a medium, such as the size of the particles, the aggregation phenomena of particles, nucleation and crystallization processes, variations in size distribution in microemulsions (with application, for example, in systems for controlled drug release), and condensation phenomena of DNA with high molecular weights. In DLS, the sample is illuminated by a laser beam and the resulting scattered light produced by the particles fluctuates in intensity at a speed that is dependent on the particle size. From the analysis of these temporal fluctuations in intensity (dynamics) of the scattered light, which are caused by the rapid Brownian motion of the particles, it is possible to derive the correlation function of the intensities, from which the typical time constant of the thermal motion of the particles and from the latter, through the Stokes-Einstein relation, the particle size.

In the case of the measurement of the dynamic properties of the scattered radiation, the incident radiation must have sufficient coherence, a condition satisfied in the case of using a laser source of incident light which is generally assumed to be a light source with both spatial and temporal coherence.

When light hits a separating surface between two media with different refractive index, it is deflected by changing the original trajectory due to collision with the separating surface. In the case of solid particles immersed in a liquid solution, a portion of the incident light undergoes a diffusion (scattering), that is, it leaves the solution with a direction different from that of incidence.

In diluted media, for example in the case of colloidal systems with a low concentration of particles, the scattering of the incident light can be described with a good approximation through the single scattering theory, i.e. the light is scattered only once before being detected. , without being further diffused by other diffusion centers (e.g. other particles). In media with higher concentration, the contribution originated by the phenomenon of multiple scattering becomes significant or even dominant with respect to the phenomenon of single scattering, making sure that the dynamic properties detected on the basis of the scattered light are different from the real properties of the particles. In general, multiple scattering increases beam dispersion and reduces resolution.

For this reason, interest has recently turned to DLS techniques that use an interferometer with a low coherence light source. Since it is possible to measure only the scattered light component from a defined portion of the analyzed sample equal to the longitudinal coherence length, the contribution of multiple scattering in the detected signal decreases dramatically and high concentration media can be measured.

US patent 6,738,144 describes a low coherence interferometric apparatus for determining the size of particles of a known concentration in a stream of colloidal or particle suspension. The described interferometric apparatus has a low coherence light source, a beam splitter that separates the beam into a probe beam and a reference beam which are, respectively, back-reflected by the sample and by a reference mirror to be then recombined to generate an interference signal. Constructive interference occurs only if the optical path difference between the two beams is less than the coherence length of the source. From the analysis of the interference signal the path length distribution of the photons is determined and from this last the colloidal suspension is dimensionally characterized.

Patent application GB 2407 379 A discloses an apparatus for measuring the diffusion of dynamic light which uses the phase modulation interference method.

The development of coherent X-ray techniques to examine the dynamics and detailed structure of matter is currently of considerable interest. In particular, the X-ray Photo-Correlation Spectrocopy (XPCS) photonic correlation spectroscopy aims to measure the correlation function of the intensity of diffused X radiation for the analysis of density and temporal fluctuations of a sample under analysis and therefore allows to study the dynamics of statistical physics of systems. The use of this technique has been largely made possible thanks to the development of new generation synchrotron X-ray sources, typically based on the use of an undulator.

K.A. Nugent, in Advances in Physics, 1460-6976, Vol. 59, n.1 (2010), pages 1â € "99, with particular reference to pages 74-84, offers a brief overview of coherent X-ray techniques for analyzing the dynamics of matter.

M. Sutton in the article â € œA review of X-ray intensity fluctuation spectroscopyâ €, published in C.R. Physique 9 (2008), pages 76-84, is a magazine on part of the literature on the XPCS technique that uses different types of diffuse radiation.

The XPCS technique has been applied in particular to observe ordered-disorder transitions in various systems, such as crystals, colloids, liquid crystals and elastomeric polymers.

For example, Brauer S. et al. in â € œX-Ray Intensity Fluctuation Spectroscopy Observations of Critical Dynamics in Fe3Alâ €, published in Physical Review Letters, Vol. 74, n. 11 (1995), pages 2010-2013, analyzes the dynamics of the critical fluctuations of a single crystal of Fe3Al through a hard X-ray beam produced by a synchrotron source with an undulator.

Dierker S.B. et al. in â € œX-Ray Photon Correlation Spectroscopy Study of Brownian Motion of Gold Colloids in Glycerolâ €, published in Physical Review Letters, Vol.

75, n. 3 (1995), pages 449-452, describe XPCS studies on the static structure factor and the dynamic correlation function of a gold colloid dispersed in a viscous liquid of glycerol.

B.C. Price et al. in â € œCoherent Soft X-Ray Dynamic Light Scattering from Smectic A-filmsâ €, published in Physical Review Letters, Vol. 82, n. 4, pages 755-758, use dynamic light scattered at the soft X-ray Bragg peak to measure thermal fluctuations in smectic A liquid crystals.

S.G.J. Mochrie et al. in â € œDynamic of Block Copolymer Micelles Revealed by X-Ray Intensity Fluctuations Spectroscopyâ €, published in Physical Review Letters, Vol. 78, n. 7, pages 1275-1278, reports intensity fluctuation spectroscopy measurements of liquid equilibrium dynamics of polymeric micelles composed of polystyrene-polyisoprene copolymer micelles in a polystyrene homopolymer matrix.

The Applicant has observed that the measurement techniques which are based on the analysis of the backscattered light originating from the interference signal of a probe beam and a reference beam require a measurement apparatus with a relatively complex optical scheme. Furthermore, in some wavelength ranges, the backscattered signal may be weak and not guarantee measurements with sufficiently high sensitivity.

The Applicant has also noted that the measurement of the dynamic diffusion of X radiation requires a sufficiently coherent radiation investigation beam, both spatially and temporally, and therefore requires very intense radiation beams from which the beam affecting the medium to be derived can be derived. to analyze. High intensity X radiation can generally be obtained from synchrotron sources or free electron lasers (Free-Elecron Laser, FEL). However, these sources have the obvious disadvantage of being extremely expensive and not easily implemented.

The Applicant has understood that if a low coherence radiation beam with coherence inclined with respect to the direction of propagation of the beam that strikes an inhomogeneous medium to be analyzed is used, it is possible to suppress or at least significantly reduce the contribution of multiple scattering. and at the same time obtain dynamic information from the analysis of the radiation diffused by the medium.

According to one aspect, the present invention relates to a method of structural analysis of a medium by means of diffused dynamic radiation, the method comprising: irradiating a volume of a medium comprising diffusion centers with an incident beam of longitudinal coherence electromagnetic radiation short with longitudinal coherence length l along an incident propagation direction, the incident beam having an inclined coherence of an inclination angle J with respect to the incident propagation direction, and revealing the distribution of temporal fluctuations in intensity of the radiation beam scattered by the volume radiated at a detection angle a with respect to the incident propagation direction.

A beam of radiation with short longitudinal coherence propagates having a coherence region inside which the correlation function is not zero.

The angle of inclination J of the coherence of the incident beam is defined as the angle between the coherence versor defined as the perpendicular versor to a main plane identified on the main surface of the section of the coherence region which penetrates, generally transversely, the volume investigated in the medium to be analyzed, the coherence vector, in the propagation plane, being inclined by an inclination angle J with respect to the incidence propagation direction. Preferably, the step of irradiating a volume of a medium comprises: generating a beam of emission radiation with short longitudinal coherence along an emission propagation direction; produce a beam of incident radiation with inclined coherence having an incidence coherence region whose section that transversely penetrates the volume investigated in the medium to be analyzed has a second main surface which identifies a main plane and a coherence vector perpendicular to said main plane , the coherence vector being inclined by an inclination angle J with respect to the direction of incident propagation, and directing the incident radiation beam onto the medium. In preferred embodiments, the emission radiation beam has an emission coherence region which has a first main surface parallel to the equiphase surfaces along the emission propagation direction. Preferably, the step of producing an incident radiation beam comprises introducing a time delay into the emission radiation beam.

According to a further aspect, the present invention relates to an apparatus for the structural analysis of a medium comprising diffusion centers by means of diffused dynamic radiation, in which a volume of the medium is irradiated by an incident radiation beam, the apparatus comprising: an electromagnetic radiation generation system adapted to produce an incident radiation beam with short longitudinal coherence with a longitudinal coherence length along an incident propagation direction, the incident beam having coherence inclined by an angle of inclination J with respect to direction of incidence propagation, and a detection system adapted to receive a beam of radiation diffused by the medium at a collection aperture centered around a detection angle a with respect to the direction of incidence propagation, the detection system being adapted to detect the distribution of temporal fluctuation of beam intensity d i radiation diffused by the radiated volume.

According to some preferred embodiments of the present invention, the method and the apparatus make it possible to detect the dynamic radiation diffused by a disordered medium at angles different from the direction of incidence of the beam or from the backscatter direction and in general allow to carry out the measurement of the dynamic radiation diffused at a detection angle suitable for obtaining structural information on the medium.

Further characteristics and advantages of the invention will result from the following detailed description made with reference to examples of embodiment of the invention given without limitation and to the attached figures in which:

Figure 1A conceptually illustrates a coherence region in a propagation plane for a coherence region radiation beam having a principal surface extending parallel to the equiphase surfaces of the radiation beam along the direction of propagation.

Figure 1B conceptually illustrates a coherence region in a propagation plane for a radiation beam with coherence inclined with respect to the equiphase surfaces of the radiation beam (i.e. coherence vector inclined with respect to the direction of propagation).

Figure 2 is a block diagram that represents a characterization apparatus owned by a medium through the analysis of the scattered radiation.

Figures 3a-3d schematically illustrate the mechanism of diffusion of the radiation from a medium which comprises diffusion centers in the case of coherence vector of the incident radiation parallel to the direction of propagation of the incident beam.

Figures 4a-4d schematically illustrate the mechanism of diffusion of the radiation from a medium which comprises diffusion centers in the case of the coherence vector of the radiation inclined with respect to the direction of propagation of the incident beam.

Figure 5 is a block diagram which represents a characterization apparatus owned by a medium by means of the radiation diffused by the medium itself, according to an embodiment of the present invention.

Figure 6 schematically illustrates a coherence orientation system, according to an embodiment of the present invention.

Figure 7 schematically illustrates a structural analysis apparatus of a medium according to an embodiment of the present invention, in particular applicable to visible light.

Figure 8 reports the correlation function as a function of time (ms) calculated by the scattered light from a sample containing calibrated polystyrene nano-spheres of 150 nm in diameter for different conditions of coherence and detected at a collection angle of 90 °.

Figure 9 schematically illustrates a medium-property characterization apparatus according to a further embodiment of the present invention, in particular applicable to X-radiation.

For longitudinal coherence, also referred to as temporal coherence in the case of waves with stationary statistical properties, we mean the coherence of the beam along a direction of propagation, that is the ability of the wave field to cause interference with a replica of itself. a subsequent instant of time. A perfectly monochromatic source emits radiation with complete longitudinal coherence, while the radiation emitted by a real source, not perfectly monochromatic, has a degree of longitudinal coherence which can be represented by the longitudinal coherence length, lc, which in practice defines a length within the which the beam can be considered coherent, i.e. phase relations are maintained along the propagation direction. For a point source that emits radiation at a central wavelength l and with a spectral width Dl, the relation holds:

l 2

=. (1)

2 D l

The coherence transversal to the direction of propagation, for which a transversal coherence length, lt is specified, is indicative of the ability of a wave field to cause interference at two different points in space along a direction parallel to the equiphase surfaces, i.e. the wave fronts of the electromagnetic field.

The longitudinal coherence length in a low coherence radiation beam can be identified from a coherence region of the propagating beam. The spatial distribution of the coherence regions of a wave field depends on various factors, including the nature of the source and the distance of propagation from it. By way of example, the light emitted by an incandescent filament of a tungsten bulb, or in general by a thermal source, at a relatively large distance, has spherical wave fronts (or equiphase surfaces), centered in the source itself, when it can be considered point-like with respect to the distance at which the coherence is observed. Within the region of coherence the correlation function is not zero, ie the interference / superposition is constructive. Since generally the correlation function has a maximum to go down to lower values, the coherence region can be defined, for some purposes of the present description, as a region, in the space affected by the propagating beam, inside which the correlation function has a value not less than a predetermined threshold value, which only by way of example can be equal to 1 / e in the case of Gaussian beams. The geometric shape of the coherence region in space (or, otherwise expressed, the shape of the Fourier spectrum) of the light emitted by a real source is generally complex. The volume of the coherence region, when projected into a plane that contains a direction of propagation, indicated below as a propagation plane, can be represented as a first approximation, and for the purposes of this description, by a ring with asymmetrical geometric shape in the two directions perpendicular and parallel with respect to the direction of propagation considered. For example, a 2 mm long incandescent filament produces locally, on the millimeter scale, coherent regions that are enlarged in the direction perpendicular to the direction of propagation and with a geometric shape of a ring delimited by plane sections of two ellipsoidal surfaces, which will be indicated, only for the sake of brevity, as coherence regions of ellipsoidal shape.

This approximation of the shape of the coherence regions can be considered to be valid in many cases of interest in the field of optics. Figure 1A schematizes the section of a coherence region 14 in the shape of a prolate ellipsoid in a propagation plane, which is also the ellipsoid symmetry plane, along a main direction of propagation of the beam (x axis), which is assumed to be the direction of incidence along the optical path between the source and the medium to be analyzed. The minor axis 13 of the coherence region 14 is perpendicular to the direction of propagation, i.e. it lies along the y axis. The section of the coherence region of the portion of the propagating beam that radiates a volume of the medium under analysis, the incident beam, i.e. the section of the coherence region present in the investigated volume of the medium where the secondary diffusion waves are generated, identifies a coherence layer with a thickness equal to 1l along the direction of incidence and which moves over time along the same direction. In the case of an ellipsoidal coherence region, the section of the coherence layer in the plane that contains the main direction of propagation and the minor axis of the ellipsoid is illustrated in a purely conceptual way with the reference number 12 in the figure 1A.

As schematically indicated in panel R of Figure 1A, the coherence layer 12 has a main surface which identifies a first plane 19 in which the coherence layer has greater extension and a second plane 15 in which the coherence layer has a smaller extension than corresponds to the length of longitudinal coherence, lc. A coherence vector unit 16 is defined as the vector perpendicular to the first main plane 19 (and in the particular case illustrated, parallel to the second main plane). In the case of figure 1A, the coherence vector 16 is parallel to the direction of incidence defined along the x axis, or, defined in another way, the main surface of greatest extension of the coherence layer extends parallel to the equiphase surfaces of the beam that propagates along the direction of incidence.

The concept of coherence layer and coherence vector unit is more generally applicable also in the case of the coherence regions which, when projected in a propagation plane, do not have the shape of an approximately two-dimensional ring between two sections of ellipsoidal surfaces, as in case illustrated in figure 1A. Even in a more general case, it is possible to define a coherence layer, starting from a direction of propagation, as the section of the coherence volume of the portion of the incident beam that invests the medium under analysis, in which the coherence layer presents a main surface, which typically crosses the medium, and a coherence vector perpendicular to the main plane of the main surface.

In some preferred embodiments, the source is adapted to emit radiation which, at a relatively large distance from the size of the source, has a coherence region in the shape of a prolate ellipsoid with a major axis along the direction of propagation. However, other embodiments may contemplate a configuration in which the incident beam has coherence regions in the shape of an oblate ellipsoid with a minor axis along the direction of propagation, or another shape that presents an asymmetry in the two directions perpendicular and parallel with respect to the direction of propagation.

In the present description, the terms â € œregions of coherenceâ € or â € œregions of coherenceâ € will be used, meaning that the radiation propagates along one direction and at different instants the coherence region will be in different positions in space.

Figure 2 is a schematic representation of an apparatus 10 for analyzing the properties of a medium by measuring the light scattered by the medium. The apparatus comprises a radiation source 20 suitable for generating a beam of electromagnetic radiation, for example a beam of light in the visible spectrum, with low coherence, more specifically with short longitudinal coherence with longitudinal coherence length lc. The radiation beam emitted by the source has an incidence propagation direction 21 and coherence regions with a coherence layer 18 which defines a coherence vector 23 parallel to said propagation direction. The emitted radiation beam is optically coupled to a medium 17 to be analyzed which comprises diffusion centers, such as a colloidal system, for example a suspension of solid particles immersed in a solution.

The presence of a longitudinal coherence of length lc, and preferably of a transverse coherence, causes the secondary radiation originating in a given portion of the irradiated medium to interfere with the secondary radiation originating from a different portion of it, giving rise to a fluctuation intensity of scattered radiation that can be detected along a given direction of detection. From the analysis of the fluctuation in intensity of the scattered radiation it is possible to obtain the dimensional characterization of the medium containing a distribution of variations in optical density (i.e. variations of the refractive index in case the source is a light source) or more in general variations of electron density in the medium comparable with the wavelength of the incident radiation.

In case the low coherence radiation beam hits a disordered system, that is to say with an unordered optical / electronic density distribution, such as for example a solid-solid or solid-liquid colloidal system, the diffused radiation is in the form of a distribution of random fluctuations (â € œspecklesâ €) of intensity, the fluctuations being caused by the correlation between the different diffusion centers. If the spatial distribution of the disordered system varies with time, the distribution of the â € œspecklesâ € varies in correspondence over time. Therefore, information on the dynamics of a disordered system can be derived from the analysis of the temporal correlation of the speckle distribution which is represented by a temporal correlation function.

Figures 3a-3d illustrate the diffusion mechanism from a medium which includes a plurality of diffusion centers in the case illustrated in Figure 1, in which the coherence vector is parallel to the direction of propagation of the incident beam or, expressed in another way , the coherence layer has a main surface of greater extension parallel to the equiphase surface of the beam along said direction of propagation. With reference in particular to Figure 3a, a short longitudinal coherence radiation beam 30 with coherence length lcsi propagates along a direction represented in the figure by the reference axis x. The coherence layer inside the radiation beam, illustrated in a purely conceptual way with the reference number 31, has a main surface of greater extension perpendicular to the direction of incidence of the beam 30 and therefore a coherence vector 31â € ™ parallel to the direction of incidence.

The incident beam 30 with coherence layer 31 penetrates a sample to be analyzed, such as a cell 25 which contains a medium at least partially transparent to the incident radiation and which comprises a plurality of diffusion centers. Only to simplify the following description, it is considered that the medium comprises, at least in the volume invested by the coherence region 31, three diffusion centers, for example three particles, 24a-24c arranged along a vertical arrangement with respect to the direction of propagation of the incident beam. When the section of the coherence region of the incoming radiation affects the scattering centers, i.e. coherence layer 31, (figure 3b), each of them generates spherical secondary waves coherent with the incident wave in a coherence region of longitudinal length lc. The secondary waves propagate concentrically from each diffusion center 24a, 24b and 24c with the respective coherence regions 26a, 26b and 26c in phase with each other. As a first approximation, the scattered radiation in the volume of the medium hit by the incoming radiation is the superposition of the contributions coming from each scattering center, e.g. broadcasting centers 24a-24c. In the superposition region of the coherence regions 26a-26c of the secondary waves there is an interference between the fields diffused by each diffusion center. Since the secondary waves are generated by the different scattering centers at the same instant and propagate concentrically in phase in the respective coherence regions, there is interference (1) in a direction of propagation of the scattered beam which coincides with the direction of the incident beam ( along the x axis) or at least in a small angular interval (i.e. smaller than the diffraction aperture of the incident beam) around the incident direction, or (2) in a direction opposite to the direction of incidence, i.e. in the direction of retro-diffusion.

Figure 3c depicts an instant following the generation of the secondary waves, in which the coherence regions 26a-26c have propagated defining a first overlap or interference region 28 in the direction of transmission of the beam and a second overlapping region 29 in the direction backscatter. The secondary waves propagate outside the medium 25 forming a beam of diffused radiation 27 in correspondence with the superposition region which can be detected along the propagation direction (figure 3d). Conversely, in the direction orthogonal to the propagation of the beam (y axis), the coherent regions 26a-26c coming from the different centers do not overlap, and therefore do not give rise to interference. More generally, in directions other than the direction of propagation of the incident and back-scattering radiation, the dynamic signal is absent or greatly reduced.

With reference again to figure 2, in the case of coherence vector of the incident radiation parallel to the direction of incidence of the beam, the measurement of the dynamic diffusion is carried out by a detection system 22 suitable for detecting the scattered radiation 21â € ™ from the medium 17 , the detection system being arranged along the direction of propagation of the incident beam.

The Applicant observed that, since the incident radiation that is not diffused by the diffusion centers inside the investigated volume is transmitted through the medium and emerges from it along the direction of incidence, the radiation diffused along the direction in forward is detected together with the transmitted radiation. Since the transmitted radiation generally constitutes the most significant component of the detected signal, it may be difficult to extract from the signal collected by the detection system the component relating only to the contribution of the scattered radiation with a sufficiently high signal / noise ratio. The signal detected along the back-scattering direction, although free from the contribution of the transmitted radiation, can be weak with a signal-to-noise ratio that is not good due to the nature of the incident radiation and / or the structural characteristics of the scattering centers at the inside of the material.

The Applicant has understood that if an incident radiation beam with short longitudinal coherence has a coherence region arranged along an inclined direction with respect to the incidence propagation direction, there is a time delay with which the coherence region affects the different diffusion centers. included in the investigated volume of the medium and that this time delay can produce a superposition of the coherence regions of the secondary waves along a direction different from the axis identified by the direction of propagation. In this way, it is possible to detect the radiation scattered by the analyzed medium along different directions and therefore in conditions that may be more favorable to the analysis of the detected signal.

The mechanism of radiation diffusion in the case of an incident beam with a coherence region inclined with respect to the direction of the incident beam is illustrated in Figures 4a-4d. An incident radiation beam 35 impacts along a direction of incidence along the x axis on the cell 25 which comprises a medium with a plurality of scattering centers 24a-24c. The incident beam has a coherence region which defines a coherence layer 36 having a first main surface of greater extension and a second main surface of smaller extension of width equal to the longitudinal coherence length lc. On the first main surface a main plane is identified from which the coherence vector 37 is defined, perpendicular to this plane, which is arranged in a direction inclined at an angle J with respect to the direction of incidence x (figure 4a).

By convention, and without any limiting purpose, the coherence vector is defined with a direction in the positive half plane of the propagation direction. With reference to the figure, the positive half plane is the half plane with a positive x axis in the (x, y) propagation plane. According to some embodiments, the angle of inclination J comprises the additional angle values to J, i.e. (180 ° -J).

In the present context, reference will be made to a radiation beam with inclined coherence, meaning a radiation beam that propagates along a direction of propagation with inclined (i.e. not parallel) coherence regions of an angle of inclination with respect to the equiphase surfaces of the beam along said direction of propagation. The angle of inclination of the coherence region of the beam is defined as the angle between the coherence versor defined as the perpendicular versor to a principal plane identified on the main surface of the section of the coherence region that transversely penetrates the investigated volume in the medium to be analyzed and which, in the most interesting cases, is the surface of greatest extension of the section of the coherence volume of the portion of the incident beam present in the investigated volume.

Due to the inclination of an angle J with respect to the direction of propagation, the diffusion centers 24a-24c are not invested at the same time by the coherence layer 36, but in a temporal sequence. In particular and with reference to Figure 4b, the first center to be hit by the coherence region is the diffusion center 24a, at a first subsequent instant the center 24b and at a second subsequent instant greater than the first instant the center 24c.

The incidence of the radiation of the beam hitting each scattering center 24a, 24b and 24c gives rise to a respective secondary spherical (diffuse) wave that propagates concentrically with the corresponding coherence regions 32a, 32b and 32c in phase with each other ( figure 4c). Due to the fact that the secondary sources of spherical waves (i.e. the diffusion centers) originate the waves at different instants, at each instant, the coherence regions of the secondary waves have different diameters and overlap each other at a given angle a with respect to to the direction of propagation of the beam (x axis). In particular, in the situation exemplified in figures 4a-4d, the coherence regions 32a-32c overlap in the downward orthogonal direction with respect to the propagation direction of the incident beam (along the y axis, a = 90 °) originating a beam of scattered radiation 34 along this direction. The radiation beam can be detected along the y axis in a detection area indicated in the (x, y) plane with the dashed rectangle 33 in figure 4d, which represents the propagation of the coherence regions of the secondary waves to a successive instant of that represented in figure 4c.

With a configuration in which the beam hits the medium with a coherence vector inclined with respect to the direction of incidence, the interference produced by the envelope of the secondary waves can originate along a direction different from the axis identified by the direction of propagation and along this direction it is possible to observe a dynamic signal of sufficiently high intensity. If J is the angle at which the coherence vector of the incident beam is inclined with respect to the incident propagation direction, i.e., inclination angle, the interference takes place, in the propagation plane, substantially at an angle 2J and a corners adjacent to it. Preferably, the angle of detection is between (2J-d) and (2J + d), where d is the diffraction angle of the volume of the medium struck by the incident beam. The opening angle for diffraction d is in general dependent on the diameter of the incident beam, in the case of detection in the far field, ie at a distance significantly greater than the size of the source. According to some preferred embodiments, the detection angle is equal to about 2J.

In three-dimensional space, the overlap between the regions of coherence of the scattered radiation also occurs outside the plane in which the direction of incidence and the coherence unit lie, i.e. plane (x, y). Therefore, although the schematic representations of the principle of the measurement technique shown in the figures refer to a propagation plane identified by the direction of propagation of the radiation and by the coherence unit vector, azimuth angles that lie outside this plane can be monitored. In fact, outside the plane considered, the directions of propagation of the radiation are all the directions that lie on the surface of a cone whose axis identifies the coherence vector unit and has a half-opening equal to J. One of the radii of the cone coincides with the direction of propagation of incidence in the propagation plan. Preferably, the azimuth angle of detection is greater than or equal to the aperture angle by diffraction of the volume of the medium struck by the incident beam and less than or equal to 2J. It should be noted that, although to simplify the description, figures 3a-3d and 4a-4d exemplify a medium comprising diffusion centers arranged vertically with respect to the direction of propagation of the incident beam, the general principle of the diffusion mechanism depends on the Orientation of the coherence regions of the incident beam is also valid for the more general case of a random arrangement of the diffusion centers inside the medium. In fact, the coherent region affects the diffusion centers in sequence, with a temporal delay proportional to their position along the direction of propagation, i.e. x axis. In this way, the delay produces a superposition of the coherence regions in directions different from the direction of propagation and, in the case illustrated in Figures 4a-4d, in the direction orthogonal to that of the incident beam.

According to some preferred embodiments, the incident radiation beam is a radiation beam in the visible or near infrared spectrum and preferably the inclination angle J between the coherence vector and the incidence direction is included from 5 ° at 85 °, even more preferably between 20 ° and 70 °. In a particularly preferred embodiment, the angle of inclination J is about 45 °. In some preferred embodiments, the incident radiation beam is a radiation beam in the wavelength spectrum ranging from about 1 to 30 nm and preferably the inclination angle between about 0.01 rad (" 0.57 °) and 10 °. At these inclination angles the detection of the signal diffused by the medium takes place at a low angle, preferably at a detection angle lower than or equal to 10 ° and greater than or equal to the opening angle by diffraction of the radiation diffused by the medium. In other embodiments, the angle of inclination is between about 10 ° and 90 °.

More generally, the minimum inclination angle depends on the relationship between the average size of the diffraction centers of the medium and the wavelength of the incident radiation and is preferably selected to be greater than the aperture angle by diffraction of the incident beam.

Figure 5 is a schematic representation of an apparatus for characterizing properties of a medium by analyzing the scattered radiation, according to an embodiment of the present invention. The radiation beam with a coherence region that defines a coherence layer 18 with a longitudinal coherence length and a coherence versor 23 parallel to the direction of propagation 21 is emitted by the source 20 and enters a coherence inclination system 45 capable of tilting of a given angle J the coherence vector of the beam incident on the system, indicated below with the coherence inclination system. The radiation beam 41 leaves the system 45 with a coherence layer 48 having a first main surface of greater extension and a second surface of lesser extension of a width equal to 1c, in which the first main surface of greater extension identifies a coherence vector 49 inclined by an angle J different from zero with respect to the direction of incidence. The radiation beam with an inclined coherence versor affects the medium 17 to be analyzed by investing a volume of the medium with a thickness of the order of magnitude lce, affecting the diffusion centers inside the volume. In general, the investigated volume includes a plurality of diffusion centers.

If the coherence region of the beam emitted by the source 20 in a propagation plane has the shape of a volumetric ring between two surfaces of a prolate ellipsoid, whose minor axis is oriented perpendicular to the direction of propagation (as illustrated in Figure 1A), at the exit of the device 45 the coherence region has a minor axis oriented at a certain angle other than 90 ° with respect to the direction of propagation considered. More generally, the section of the main surface of the coherence region of the portion of the beam that strikes the medium to be analyzed is no longer extended parallel to the equiphase surfaces. With reference to figure 1B, which illustrates a projection of the coherence region, approximated with a volumetric ring between two ellipsoidal surfaces in the propagation plane (x, y) where the x-ray propagation direction and the minor axis lie 46 of the plane of symmetry of the ellipsoids. The coherence vector unit 49 is inclined by an angle J and therefore the minor axis 46 of the ellipsoid is inclined by an angle complementary to J, (90 ° -J), with respect to the direction of incidence x. This type of inclined coherence, with coherence regions inclined with respect to the equiphase surfaces of the propagating beam (or, as defined above, with a coherence vector not parallel to the direction of propagation considered), is described, as well as by means of a length of transverse coherence and a longitudinal coherence length, from the angle of inclination J. Figure 1B indicates the coherence layer 48 of the incident beam that radiates the investigated volume.

Preferably, the radiation beam that impinges on the medium has transverse coherence with a transverse coherence length, lt, which corresponds to the transverse dimension of the beam or at least a transverse coherence length not less than the transverse dimension of the volume of the medium investigated by the beam. In case the source is not able to emit a beam with transversal coherence sufficient for the purposes of the measurement technique, the apparatus can comprise a spatial filter, such as a transmission mask with a pinhole having a dimension of the same order of magnitude of the desired transverse coherence length. For example, in the case of a spatial filter that includes a circular aperture that transmits the beam (pinhole), lt »D1, where D1 is the diameter of the aperture. Referring again to Figure 5, the scattered radiation beam 42 from the medium 17 is collected at an angle a with respect to the direction of propagation of the incident beam by a detection system 43 which includes a detector (not shown). In the example of figure 5, the angle of inclination J of the coherence vector 49 of the beam that affects the medium is about 45 ° and the detection angle a is of 2J, that is about 90 ° .

In some preferred embodiments, the collection opening of the detection system around the detection angle a is included within a cone centered at the angle a and of half-opening equal to the opening angle by diffraction d of the volume of the medium hit by the incident radiation beam. In some embodiments, angles outside the cone can reduce the amplitude of the signal. The intensity of the scattered radiation collected by the detector is electronically processed in order to determine the time correlation function of the intensity fluctuation by solving the direction of diffusion of the radiation using one of the known methods, which include homodyne and heterodyne techniques. From the time correlation function it is possible to obtain information on the inhomogeneity of the medium, i.e. variation in optical density, such as the size of the particles that make up the scattering centers. Preferably, and in particular if the detection system comprises a single channel detector, such as a photodiode, and the medium under analysis is a disordered medium, the system comprises an optical device adapted to select a single speckle. In a different embodiment, the detection system comprises a multiple channel detector, such as a CCD (Charge Coupled Device) sensor, the analysis of the correlation function is preceded by the processing of the detected signal in order to extract from it a single speckle, i.e., in order to proceed with the analysis of the fluctuation of a single region of coherence.

The coherence orienting device 45 of a short longitudinal coherence beam comprises at least one spectral dispersion element, that is, an element that spatially deviates different wavelengths within the beam that affects it. The inclination angle is caused by the angular spectral dispersion introduced by the dispersive element. More generally, the spectral dispersion element introduces a time delay between the spectral components of the beam.

An example of embodiment of a system suitable for inclining coherence is shown in Figure 6. The system 45 comprises as a dispersive element a diffraction grating operating by reflection 50 on which a radiation beam 21 has a coherence region with a layer of coherence 18 which identifies a coherence vector (not shown in the figure) parallel to the direction of propagation of the beam itself (J = 0). A diffraction grating by reflection can be used for light radiation at wavelengths in the visible and near infrared (e.g. 380-1400 nm). The diffraction grating introduces an angular dispersion into the incident beam 21 giving rise to diffracted beams of radiation of spectral order m (m = 0,1,2, â € ¦) emerging from the same incidence side of the grating. The incident and diffracted beams follow the known relationships of diffraction:

siny -sing = m × <l> (2)

d

where g and y (not shown in the figure) are the angles between the normal to the grating plane (i.e. the diffraction plane) and, respectively, the incident and diffracted beams, d is the grating pitch and m the spectral order of diffraction.

For m = 0, a reflection grating acts as a mirror producing a reflected beam 57 having a coherence layer 58 with a coherence vector still parallel to its direction of propagation. For diffraction of higher order than zero order, m = 1.2, â € ¦, according to known relationships in the field of radiation diffraction, the angular dispersion, which is a function of the diffraction angle, which in turn depends from the incident wavelength, it causes a time delay in the diffracted beam. The time delay introduced by the angular dispersion gives rise to an inclination angle J of the coherence unit vector. Therefore, the diffracted beam 51 with order m¹0 which propagates along a direction of propagation has a coherence layer 54 whose coherence vector is inclined by an angle J with respect to said direction of propagation.

The angle of inclination J between the coherence vector and the direction of propagation of the diffracted beam depends on various factors such as the pitch of the grating, the angle of incidence g of the beam and the angle of deviation y of the diffracted beam and can be selected by acting on one or more of these parameters. For example, the angle of inclination can be varied by varying the angle of incidence of the beam on the grating. From purely geometric considerations, the angle of inclination can be calculated through the relation:

g

tanJ = tan y - <sin>. (3)

cos y

The radiation coherence orientation system 45 is preferably configured so as to extract the first order diffracted beam. Preferably, the system 45 comprises a first converging lens 52 and a second converging lens 53, the diffraction grating being positioned in the front focus (focal distance f) of the first converging lens and the second converging lens being positioned at a distance 2f with respect to the first lens so as to form the image of the diffraction grating 50 on a determined image plane 59 and select the diffracted beam of the first order of diffraction (m = 1). The diffracted beam 56 of first order radiation with coherence layers 55 inclined with respect to its direction of propagation (inverted by 90 ° from the passage through the pair of lenses) leaves the system 45 to be directed to the medium to be analyzed. The optical system formed by the two lenses creates an image equal in intensity and phase of the field near the lattice inside the investigated medium. Preferably, the medium is positioned at a distance approximately equal to the focal distance f of the second converging lens, so that the components of the scattered radiation beam at different wavelengths overlap. Within the investigated volume, the secondary wave radiation beams have coherence regions with a longitudinal coherence length that increases with the distance from the overlap area between the scattered radiation beams.

Other embodiments of the coherence orientation system 45 of Figure 6 may comprise different devices for the selection of the diffraction order of the diffracted radiation. For example, the system 45 can comprise a converging lens, in which the grating is positioned in the front focus of the lens which converges the first order diffracted beam in a specific plane outside the system 45, the medium to be analyzed being positioned in such a way that this plane conjugated to the lattice crosses it. It is to be understood that the system can be configured in such a way as to release diffracted beams of the order higher than the first.

In another embodiment, the coherence orientation system comprises a diffraction grating, operating by transmission that can be used in case of incident radiation in the visible spectral region (380-760 nm) or soft X-rays ( e.g., 1-20 nm).

In a further embodiment, the coherence orientation system comprises a dispersive prism which introduces a progressively increasing time delay transversely to the incident beam and therefore an inclination of the coherence regions. Without wishing to be bound by any particular theory, relations governing the time delay in a laser pulse of the ps are described for example in Zs 'Group velocity dispersion in prisms and its application to pulse compression and traveling-wave excitation'. Bor and B. Rácz, published in Optics Communications (1985), vol. 54, n. 3, page 165. A system comprising a dispersive prism can be used in apparatuses that analyze media by means of incident radiation in the spectral region of the visible or of X-rays, both hard and soft.

In a different embodiment, the coherence orientation system comprises a diffractive device with a multilayer structure that operates as a Bragg reflection device, such as for example a Si / Mo multilayer structure typically used for diffraction in the spectral region that includes the soft X-rays and the extreme ultraviolet field, e.g. 1-30 nm. The multilayer structure can be configured in such a way that the reflection occurs in a symmetrical or non-symmetrical way, that is to say it can be configured with its external surface respectively parallel or not parallel to the planes that generate the Bragg reflection.

A still further embodiment is directed to a system suitable for tilting the coherence regions which includes a diffractive element which comprises a two- or three-dimensional crystal lattice, such as a photonic crystal. For example, a silicon single crystal can be used in an apparatus that analyzes a medium by means of radiation in the spectral region that includes hard X-rays in the wavelength range of about 0.1-1 nm. The crystal lattice can be configured in such a way that the reflection occurs in a symmetrical or non-symmetrical way, that is to say the lattice can be configured with its external surface respectively parallel or not parallel to the planes that generate the Bragg diffraction.

In some embodiments, the system adapted to incline the regions of coherence of radiation in the spectrum of wavelengths from 0.1 nm to 1400 nm includes at least one spectral dispersion element consisting of a diffractive and / or refractive element. Although the preferred embodiments are directed to a method and an apparatus in which the investigation beam is selected in a spectrum of wavelengths suitable for the characterization of disordered systems and / or dynamic information on order-disorder transitions or symmetry changes, the present invention does not exclude that the incident radiation the medium to be analyzed is a beam of electrons, for example with wavelengths of the order of 0.1 nm. In this case, a possible application is the characterization of crystallographic classes by means of the analysis of the radiation diffused by an ordered medium, such as a solid with a crystalline structure.

Figure 7 schematically illustrates a medium property characterization apparatus according to an embodiment of the present invention. The apparatus of the embodiment of figure 7 is particularly suitable for the characterization of a colloidal system such as a colloidal suspension immersed in a liquid medium through the dynamic diffusion of visible light. The analysis apparatus 60 comprises a source 61 which includes a diode laser 62, electrically connected to a power supply 63 which supplies an operating current lower than the threshold value for the laser effect (i.e. power supply below the threshold). Preferably, in the case of visible light, the diode laser is a semiconductor laser, in which the front face is treated with an anti-reflective coating. For example, the diode laser is a commercial laser SAL-660-25 manufactured by Sacher Lasertechnik typically used for external cavity with central wavelength of the emission band of 660 nm, half-height bandwidth (FWHM) of 8 nm, when kept below threshold, and output power of the order of 0.2 mW. The beam exiting the laser diode is collimated by a collimating optical element, e.g. an aspherical lens 64, so as to obtain a collimated light beam 67 with a diameter of about 1 mm. When operated below the threshold, the laser indicated in the example produces a beam with coherence regions of two-dimensional shape with an elliptical ring having a longitudinal coherence length, lc, of about 20 mm and a transversal coherence length, lt, downstream of the collimator device, equal to approximately the diameter of the beam (i.e., single transverse mode). The beam emitted by the source of the above example, when collimated by the collimating optical element, can maintain the coherence properties up to a distance of a few meters from the collimating optical element. The coherence vector of the beam emitted by the source is parallel to the direction of propagation.

In a different embodiment, the source 61 can comprise a superluminescent LED diode, for example a commercial superluminescent diode module produced by Superlum Diodes Ldt. and which operates in the visible spectrum.

If the source is not capable of generating a beam of coherent radiation in the transverse direction, the incident radiation can be made spatially coherent by means of spatial filtering, for example by passing the beam emitted by the source through a through opening (â € œpinholeâ €, not shown) of dimensions of the same order as the desired transverse coherence length. The light beam 67 impacts a coherence orientation system 65 for low coherence radiation. The system 65 is for example of the type described with reference to Figure 6 and comprises a diffraction grating 66 operating in reflection, for example a reflecting layer with a grating pitch of 600 lines per millimeter and angled (blazed) of 17.5 ° . The position of the grating is adjusted so that the angle of deviation between the beam 67 affecting the grating and the resulting beam diffracted from the grating, g + y in equations (2) and (3), is approximately 115 °. The diffracted beam has coherence regions with a longitudinal coherence length equal to the coherence length of the beam emitted by the source but with a coherence versor inclined with respect to the direction of propagation of about 45 °. The first order of the diffracted light beam 70 is collected by a pair of converging lenses 68 and 69, each having a focal distance f = 15 cm, and directed as a beam 71 towards the medium to be analyzed. The medium (not shown in the figure), for example a suspension of colloids in water, is contained in a cell 72 with a quadrangular section, for example a square section with parallel flat faces, transparent in glass and with an optical path of 1 cm. The direction of incidence of the radiation beam is perpendicular to the incident face of cell 72. The incident beam 71 strikes a volume of the medium having a thickness of the same order of magnitude as the longitudinal coherence length of the beam (ie about 20 mm). The light scattered by the scattering centers (i.e. particles) in the investigated volume is collected at a detection angle a with respect to the direction of incidence. In the apparatus of figure 7, a detection system 80 is positioned in such a way that the detector is able to collect the scattered light at an angle a of about 90 °. Preferably, the detector has an angular opening not greater than the opening angle by diffraction of the diffraction center contained in the volume of the medium radiated by the beam.

The detection system comprises a converging lens 73, for example with a focal length of 2 cm, which sends the light collected in an optical fiber 78, the input terminal portion of which is optically coupled to a lens with a graduated refractive index (lens GRIN) 74 with a length of 2.5 mm and a pitch of 0.25 to couple the numerical aperture of the light collected by the lens with the numerical aperture of the optical fiber and select a single speckle in the speckle distribution of the light diffused by the system colloidal. The light leaving the optical fiber is sent to a detector 77 which comprises an avalanche photodiode which returns an electrical signal at the output. In another embodiment, the detection system comprises a photomultiplier tube.

The dynamic portion of the electrical signal leaving the detector contains information on temporal fluctuations in intensity of the light scattered by the medium (i.e. dynamic signal). The electrical signal coming out of the detector is sent to a correlator device 76, for example an electronic correlator, which analyzes the dynamic portion of the signal and from it calculates the temporal correlation function of the intensity of the scattered light, with methods per se © known. For example, the output signal from the optical fiber is sent to an avalanche photodiode included in a commercial instrument for analyzing the dynamic diffusion of light, Brookhaven ZetaPlus from Brookhaven Instruments, which includes a digital electronic correlator capable of calculating the correlation function in times ranging from the order of the ms to the second.

Finally, the correlator device can be interfaced with a processing unit (e.g. Central Processing Unit, CPU), for example a PC 75, which acquires the time correlation function processed by the correlator device and analyzes it. For example, the analysis includes the interpolation (fitting) of the correlation function with a mathematical function consisting of the sum of a plurality of decreasing exponentials and having as interpolation parameters the amplitude and decay time of each exponential . The amplitude and the decay time of each exponential allow to trace the diameter and density of the colloids in suspension and in general of the diffusion centers included in the medium.

In the analysis apparatus 60 of the present embodiment a homodyne detection scheme is used, ie the scattered light beam hits the detector directly. However, the present embodiment can contemplate a detection system with a heterodyne pattern, in which a part of the beam incident on the medium to be analyzed is added to the scattered beam.

In some embodiments, the detection system comprises a multiple channel detector such as a CCD sensor.

Figure 8 shows the time correlation function, C (t), of the scattered light from a sample containing 150 nm diameter calibrated polystyrene nano-spheres as a function of time t (ms) for different coherence conditions. The scattered light is detected at a detection angle of approximately 90 °. The curve with solid line is the correlation function derived from light diffused from the sample irradiated by coherent laser light (i.e., with a longitudinal coherence length of a few mm) at 660 nm and follows the typical law of an exponential decay, characteristic of motion Colloid Brownian. The contrast is close to 30%. The other two curves represent the correlation function calculated by scattered light from the sample irradiated by short longitudinal coherence light, with a central wavelength of 660 nm and a longitudinal coherence length of about 17 mm. In the curve with a dashed line, the incident light has a coherence versor parallel to the direction of incidence on the sample, while in the curve with a dotted line the incident light has a coherence region with a coherence unit vector inclined at 45 °. In the case of a coherence vectoror not inclined with respect to the direction of propagation of the beam, the curve is substantially flat, due to the fact that the longitudinal coherence length is shorter than the differences in the length of the optical path between the beams scattered by the different regions of the sample. By tilting the coherence, the correlation function is clearly visible, with a contrast of about 13%, and is such as to allow to evaluate the decay times and / or amplitudes from the exponential decay of the curve and therefore to derive structural information of the medium , such as the diameter of the colloids contained in the colloidal suspension.

The measurement of the dynamic diffusion of light in the visible spectrum according to the present invention can be used, for example, for the measurement of the diameters of colloids in colloidal systems. Colloids of larger diameters, e.g. from 800 nm to about 1 mm, can be studied with incident light in the near infrared spectrum, if the liquid media of the colloidal suspension are not opaque to infrared. By way of example, the colloidal systems that can be analyzed include colloids composed of polymers, solids or liquids, in particular glues and paints; micelles and liposomes composed of lipids, of interest for the production of drugs or parapharmacy products; powders of micrometric or nanometric dimensions, interesting both as products (cements, cosmetics) and for the environmental monitoring of polluting elements (toxic nanoparticles, PM10, asbestos dust). In general, the technique allows the monitoring of production processes, in which it is necessary to control step by step the distribution of particle sizes, both in the disintegration phase (for example, grinding) and in the growth phase (for example, growth of particles with controlled diameter).

Preferably, the light source for the investigation of the medium is capable of emitting radiation with a longitudinal coherence length of less than 1 mm, more preferably less than 100 mm. However, it is noted that the selection of the short longitudinal coherence radiation with an appropriate longitudinal coherence length depends on the average size of the scattering centers to be investigated. It is observed that the use of a radiation beam with inclined coherence makes it possible to measure dynamic scattering even with radiation having a longitudinal coherence length smaller than the optical path difference of the radiation scattered by the different areas of the investigated volume of the medium, in the direction of revelation.

The use of incident light with short longitudinal coherence cancels or drastically reduces the contribution of multiple scattering in the detected signal. This fact is particularly advantageous in the case of analysis of turbid samples or with a high concentration of particles, in which the multiple diffusion prevents a correct evaluation of the diameters or the concentration of the particles.

The Applicant has understood that the measurement technique according to the general principles of the present invention can be used to measure structural characteristics of the order of about 1-30 nm, or even lower than nm, in media with variation of electron density, analyzing the diffusion of the X radiation, in general including in the spectrum the wavelengths typically indicated as extreme ultraviolet (UV), e.g. 20-30 nm. Structural characterizations by means of X-radiation generally require that the investigation beam possesses adequate longitudinal coherence, i.e. which affects the medium with almost monochromatic radiation. Typically, to obtain an adequate length of longitudinal coherence in the X radiation emitted by a source, it is necessary to use a monochromator which, however, drastically reduces, even by three orders of magnitude, the power of the radiation useful for the investigation and hence the sensitivity of the measurements that use diffused radiation. With the advent of synchrotron radiation sources, characterized by high intensity, it was possible to use X-ray radiation with a sufficient degree of coherence to conduct dynamic measurements on diffusion. However, synchrotron sources have the obvious disadvantage of being extremely expensive and not easily implemented.

Figure 9 schematically illustrates an embodiment of the present invention. The apparatus of the embodiment of figure 9 is particularly suitable for the characterization of the structure of a medium through the diffusion of soft X-rays, wavelength between about 1 nm and 20 nm. The apparatus 90 comprises a source 91 of electromagnetic X-ray radiation. For example, the source comprises an X-ray microfocus 92 tube, with an emission area of diameter of 10 mm, which emits radiation of â € œbremsstrahlungâ €, with maximum emission power at a central wavelength of 1 nm and bandwidth of the order of 50% of the peak length. This source can therefore be defined, for the purposes of some embodiments of the present invention, a source of incoherent radiation both longitudinally and transversely.

The radiation emitted by the source 91 passes through a coherence orientation system 93 which includes a spectral dispersion element. The spectral dispersion element is a diffrative that comprises a first mask 97, positioned for example at a distance L of about 10 cm from the X-ray tube, which includes an X-ray opaque 96 metal sheet (for example of platinum ), on which a transparent region is made to the transmission of X-rays. For example, the transparent region includes a hole or through opening 95 (pin-hole), for example a hole with a diameter of 12 mm made in the metal, and a grid 98 of parallel lamellae of material opaque to the radiation, this grid being superimposed on the hole.

The grid 98, for example with a grating pitch of 100 nm and a coverage ratio of 50%, associated with the region transparent to the transmission of X-rays acts as a diffraction grating that operates in transmission and introduces an angular dispersion in the beam that passes through it. originating diffracted beams of radiation of spectral order m (m = 0.1, ..) which emerge from the side opposite to that of incidence on the grating. For m> 0, the angular dispersion causes an inclination of the coherence regions by an inclination angle J of the transmitted diffracted beams. The first mask 97, and in particular the diffractive element created by the grid 98 arranged in correspondence with the opening of the first mask, therefore has mainly two functions: it increases the length of longitudinal coherence and inclines the regions of coherence of the radiation. Preferably, the first mask is adapted to produce spatial filtering in order to obtain transverse spatial coherence. It is observed that a pin-hole, such as the hole 95 included in the first mask, operates as a spatial filter by introducing a transversal coherence of transversal coherence length corresponding to the size of the opening.

The beam diffracted with non-zero order by the device 93 has coherence regions with a coherence layer inclined by an angle J with respect to the direction of propagation indicated in the figure with the arrow 111 and passes through a second mask 101 arranged at a distance of about 2 mm from the first mask 97. The second mask 101 is adapted to select the first order of diffraction (m = ± 1) and comprises a radiation opaque metal sheet 103 which includes a pinhole 102. For a pinhole with a diameter of about 10 mm the first order of diffraction of the radiation is transmitted at an angle of inclination J of about 0.01 radian, equal to the diffraction angle of the grating, with respect to the beam 112 transmitted by the mask (m = 0). Moving away from the first mask, the diffracted beams progressively separate and consequently the length of longitudinal coherence increases. With reference to the above example, at the output of the second mask 101, a beam of spectral width, Dl, of about 10% with respect to the peak wavelength is selected, which corresponds to a longitudinal coherence with coherence length of about 10 times the central wavelength of the radiation emitted by the source, e.g. 10 nm for an emission wavelength of 1 nm and Dl = 0.1 nm. The radiation beam leaving the second mask 101, which has a longitudinal coherence of length and a transversal coherence of about 12 mm, radiates a medium 100 to be analyzed which comprises a plurality of diffusion centers. For example, the medium consists of a compound approximately 10 mm thick along the direction of incidence formed by an elastomer loaded with palladium colloids of 5 nm in diameter. The diffusion centers originate a beam of diffused radiation that emerges from the medium in theory in every direction of space (depending in practice on the configuration and structure of the medium). In the case of colloids of the order of a few nm, such as the palladium colloids indicated above, which are therefore significantly larger than the incident wavelength (1 nm), the diffusion takes place mainly in the forward direction. It may therefore be advantageous to configure the measuring apparatus so that it collects the scattered radiation at a low angle, for example at about a = 0.02 rad (a = 2J), in order to detect a signal of relatively high intensity. A detection of radiation scattered from a medium at small angles is generally indicated as small-angle X-ray scattering (SAXS). In a preferred embodiment, the detection angle of the scattered radiation is between 0.01 rad (»0.57 °) and 10 °.

In the case of analysis of a medium that includes a disordered system, the radiation diffused by the medium preferably passes through a third mask 107, arranged downstream of the medium to be analyzed, which comprises a sheet 104 of opaque material to the radiation with pinhole 105, for example a ... € ™ 10 mm wide opening. The third mask is designed to select a single speckle produced by the radiation diffused by the medium at a determined angle, for example of about 0.01 radians, with respect to the transmitted beam. The scattered radiation passing through the third mask 107 is detected by a detector device 110 which includes a photomultiplier tube sensitive to soft X-rays 106. The dynamic portion of the electrical signal leaving the photomultiplier contains information on temporal fluctuations in intensity of the light scattered by the medium (i.e. dynamic signal). The electrical signal output from the photomultiplier is sent to a correlator device 108, for example a digital electronic correlator which analyzes the dynamic portion of the signal and from it calculates the temporal correlation function of the intensity of the scattered light, with methods per se © known.

Finally, the correlator device can be interfaced to a CPU, for example a PC 109, which acquires the time correlation function processed by the correlator device and analyzes it.

It is noted that when in disordered systems the optical path differences in the waves diffracted by the different colloids are of the order of 100 nm, therefore much greater than the coherence length of the order of 10 nm, in the absence of the inclination of the coherence region, the dynamic signal would be absent.

If the medium is a medium with an ordered structure, for example a single crystal, a single region of coherence, i.e. the speckle, coincides with the diffraction distribution of the diffracted beam of a given order. In this case, the apparatus of figure 9 may not include the third mask 107, with a consequent increase in power of the beam that affects the medium.

In some preferred embodiments, the apparatus of figure 9, or at least the portion of the apparatus from the source to the detector, is kept in vacuum, preferably at a pressure lower than 1x10 <-3> atm, more preferably 1x10 <-4> atm, or at low pressure in a helium atmosphere.

According to some preferred embodiments in which the incident radiation is an X-ray beam, the length of longitudinal coherence, taken in the plane of incidence of the medium to be analyzed, is equal to or greater than 5 times the central wavelength of the incident beam. In practice, in some cases of interest, in which a commercial source is used, such as an X-ray tube, a beam incident in the medium is obtained with adequate power to allow the analysis of the radiation scattered by disordered media for coherence lengths longitudinal equal to or less than 10 times the central length of the incident beam. However, this value must not be taken as a limiting value.

According to a preferred aspect, the present invention makes it possible to use an investigation beam in the X-ray emission spectrum of relatively high power, without losses due to the monochromator. In the case of incoherent sources both longitudinally and transversely, it is preferable to filter the beam emitted by the source through at least one coherence filter able to select a transversal mode of the beam and to produce a beam with short longitudinal coherence. According to some embodiments, the apparatus comprises a first and a second coherence filter, each comprising a respective first and second pinhole arranged in sequence along the direction of propagation of the beam. According to some embodiments, and assuming pinholes of circular through aperture, the diameters of the first and second pinholes D1 and D2 of the coherence filter are selected according to the following relationship, known per se:

<l> c = <D> 1

<l> (4)

(D2-D1)

According to some embodiments, the transverse coherence length is selected by setting the value of D1, i.e. lt = D1. According to a known relationship, D1 is selected to be equal to LÃ — l / s, where L is the distance, along the propagation direction, between the source and the first pinhole and s is the diameter of the source. In the apparatus of figure 9, the first coherence filter comprises the mask 97 and the second coherence filter comprises the mask 101. In the embodiment of figure 9, the coherence orientation system is positioned in correspondence with the first filter of coherence, since the pinhole of the first filter acts both as a spatial and temporal filter of the radiation that passes through it and as a transparent region which together with the grid 98 forms the diffraction grating in transmission. However, it is to be understood that other embodiments may comprise a coherence orientation system at the second coherence filter or downstream of the first or second coherence filter.

It is noted that although the transmission through a pinhole typically used for the selection of the transverse and longitudinal modes causes a significant loss of the intensity of the emitted radiation, this loss is still less than that which would occur if an additional filtering in length were necessary. wave or a more selective filtering, to obtain a â € œcompleteâ € longitudinal coherence.

The measurement technique of the present invention allows to make dynamic diffusion measurements with apparatuses that use experimental devices at relative low cost. However, the application of the measurement technique according to the present invention to investigation beams produced by a synchrotron is not excluded, for example in the case of wanting to significantly increase the power of the beam. The dynamic measurement of X-ray diffusion according to the general principles of the present invention allows to analyze important properties of the media in various fields of industrial interest. A possible application is found in the field of crystallography, in which the analysis technique illustrated above allows to study the dynamics of phase transitions, both solid-liquid and solid-solid, and in particular symmetry changes or order-disorder transitions. In particular, in the field of metallurgy, static diffractometers are often traditionally used to determine the characteristics of metal alloys in preparation, such as their composition or structural changes caused by heat treatments (e.g. hardening, annealing). The present invention makes it possible to obtain dynamic information on the production of metals or metal alloys by using a measurement technique that is compatible both in terms of cost and size with industrial metal production processes. For example, the measurement technique makes it possible to analyze metal compositions in proximity to critical points, and thus obtain information on the time required for the procedures or on the expected life span of a product obtained.

The study of transitions in crystals of industrial interest, such as piezoelectric crystals, or ferroelectric perowskites, of the static or dynamic disorder in macromolecular crystals, such as protein crystals, are further possible fields of application of the present invention with X-ray investigation radiation. intermediates between colloids and single molecules can be analyzed with ultraviolet rays, in the case of materials that are at least partially transparent to them. Furthermore, applications of dynamic X-ray scattering include the analysis of the composition and formulation of liquid crystals and the study of very turbid colloidal systems, in which the phenomenon of multiple scattering also takes place with low coherence visible light beams or systems colloids that are too absorbent in visible light.

Claims (30)

CLAIMS 1. A method of structural analysis of a medium using dynamic diffused radiation, the method comprising: radiating a volume of a medium comprising scattering centers with an incident beam of short longitudinal coherence electromagnetic radiation with a longitudinal coherence length l along an incident propagation direction, the incident beam having a coherence inclined by an angle of inclination J with respect to the direction of incidence propagation, and reveal the distribution of temporal fluctuations in intensity of the radiation beam scattered by the radiated volume at a detection angle a with respect to the incidence propagation direction. 2. The method according to claim 1, in which the detection angle is between (2J-d) and (2J + d), where d is the diffraction angle of the volume radiated by the beam of incident radiation. 3. The method according to claim 1, in which the detection angle is 2J. 4. The method according to claim 1, in which the detecting step comprises receiving the beam of radiation scattered by the medium at a detection angle greater than or equal to the aperture angle by diffraction of the volume of the medium struck by the incident beam and included inside a half-opening collection cone equal to the angle of inclination J. The method according to any one of the preceding claims, in which the step of irradiating a volume of a medium comprises: generating a beam of emission radiation with short longitudinal coherence along an emission propagation direction; produce a beam of incident radiation with inclined coherence having an incidence coherence region whose section which transversely penetrates the volume investigated in the medium to be analyzed has a main surface which identifies a main plane and a coherence vector perpendicular to said main plane, the coherence unit vector being inclined by an inclination angle J with respect to the incidence propagation direction, e direct the incident radiation beam on the medium. The method according to claim 5, wherein the step of producing an incident radiation beam with an inclined coherence region comprises introducing a time delay in the emission radiation beam. The method according to claim 6, wherein introducing a time delay in the emission radiation beam comprises directing the emission radiation beam to at least one spectral scattering element and angularly separating the emission radiation beam into distinct spectral components for medium of the spectral dispersion element. The method according to claim 5, wherein the step of producing an incident radiation beam with an inclined coherence region comprises: diffract the emission radiation beam by angularly separating the emission radiation beam into distinct diffracted beams, each having a diffraction order and select a diffracted beam with non-zero diffraction order, and the step of directing the incident radiation beam onto the medium comprises directing the selected diffracted beam onto the medium. The method according to any one of the preceding claims, in which the detecting step comprises generating a signal representative of the distribution of the detected intensity temporal fluctuations. The method according to claim 9 which further comprises: process the signal representative of the distribution of the detected intensity temporal fluctuations by correlating the detected intensity as a function of time to produce a temporal correlation function of the detected intensity at the detection angle a, and determine at least one parameter indicative of the structure of the medium from the analysis of the time correlation function. 11. The method according to any one of the preceding claims, in which the radiation beam is a radiation beam centered at a wavelength between 1 nm and 30 nm, preferably between 1 nm and 20 nm. The method according to claim 11, wherein the angle of inclination J is equal to or greater than 0.01 rad and equal to or less than 90 °, and preferably between 0.01 rad and 10 °. 13. The method according to any one of claims 1 to 10, in which the radiation beam is a beam of light centered at a wavelength ranging from 380 nm to 1400 nm, preferably ranging from 380 nm to 760 nm . 14. The method according to claim 13, in which the angle of inclination J is comprised between 5 ° and 85 °, preferably comprised between 20 ° and 70 °. 15. An apparatus for the structural analysis of a medium comprising diffusion centers by means of dynamic diffused radiation, in which a volume of the medium is irradiated by an incident radiation beam, the apparatus comprising: an electromagnetic radiation generation system adapted to produce an incident radiation beam with short longitudinal coherence with a longitudinal coherence length l along an incident propagation direction, the incident beam having coherence inclined by an angle of inclination J with respect to the direction of propagation of incidence, e a detection system adapted to receive a beam of radiation diffused by the medium at a collection aperture centered around a detection angle a with respect to the direction of incidence propagation, the detection system being able to detect the distribution of temporal fluctuation of intensity of the radiation beam diffused by the radiated volume. 16. The apparatus according to claim 15, in which the collecting aperture is centered at a detection angle a equal to about 2J, preferably the aperture angle by diffraction of the volume radiated by the incident radiation beam it has an amplitude equal to 2d, where d is the angle of aperture by diffraction of the volume radiated by the incident radiation beam. 17. The apparatus of claims 15 or 16, in which the radiation generation system of the incident beam comprises: a source adapted to emit a short longitudinal coherence emission radiation beam along an emission propagation direction, the emission beam having a coherence region having a first main surface parallel to the equiphase surfaces along the emission propagation direction, And a coherence orientation system optically coupled to the source, the system being adapted to orient the coherence region so as to produce an incident beam with a coherence region inclined by an angle of inclination J with respect to the incidence propagation direction. 18. The apparatus of claim 17, wherein the coherence orientation system comprises a spectral scattering element configured to introduce a time delay in the emission radiation beam. 19. The apparatus of any one of claims 15 to 18, in which the detection system comprises at least one detector adapted to detect the distribution of temporal fluctuation of intensity of the radiation beam scattered by the radiated volume and to generate a signal representative of the distribution of temporal fluctuations of detected intensity. 20. The apparatus of claim 19 which further comprises an analysis system suitable for receiving and processing the signal representative of the distribution of the detected intensity temporal fluctuations by correlating the detected intensity as a function of time to produce a temporal correlation function of the Intensity measured at at least one angle included inside the collection opening. 21. The apparatus of claim 20, in which the analysis system comprises an electronic correlator adapted to correlate the detected intensity as a function of time to produce a time correlation function of the detected intensity at at least an angle included at Inside the collection opening, the correlator being connected to a processing unit capable of analyzing the temporal correlation function and determining at least one parameter indicative of the structure of the medium. The apparatus of any one of claims 15 to 21, in which the incident radiation beam has transverse coherence with a transverse coherence length lt equal to at least the transverse dimension of the incident radiation beam. 23. The apparatus according to any one of claims 17-18 and from 19 to 22, when dependent on claim 17, in which the source is capable of generating a beam of light centered at a wavelength comprised from 380 nm to 1400 nm, preferably comprised from 380 nm to 760 nm. 24. The apparatus of claim 23, in which the coherence orientation system includes: a diffraction grating optically coupled to the source and configured to separate the light beam emitted by the source into distinct diffracted beams and an optical device adapted to select a diffracted beam of non-zero diffraction order which forms the incident radiation beam. 25. The apparatus of claim 24, in which the diffraction grating is arranged in a diffraction plane and is adapted to receive the emission radiation beam emitted by the source at an angle of incidence with respect to the normal to said plane diffraction and to produce a spectral component of non-zero diffraction order at a diffraction angle with respect to the normal to said diffraction plane, the angle of inclination of the coherence being determined on the basis of the incidence and diffraction angles of the diffraction grating . 26. The apparatus according to claim 15, wherein the radiation generation system of the incident radiation beam comprises: a source (91) adapted to generate a radiation beam; a first coherence filter (97) optically coupled to the source and adapted to produce a first radiation beam with short longitudinal coherence and coherence region which has a first main surface parallel to the equiphase surfaces along the emission propagation direction, and a coherence orientation system (93) arranged downstream of the first coherence filter and optically coupled to it, the system being able to orient the coherence region of the emitted radiation beam with respect to the direction of propagation so as to produce a radiation beam with coherence inclined by an angle of inclination J with respect to the direction of incidence propagation. 27. The apparatus according to claim 26 which further comprises a second coherence filter (101) optically coupled to the first coherence filter (97) and arranged downstream thereof, the second spatial filter being adapted to produce a second beam of radiation with short longitudinal coherence of length of longitudinal coherence lc, the second radiation beam forming the incident radiation beam on the medium. 28. The apparatus according to claims 26 or 27, in which the coherence orientation system (93) comprises a spectral dispersion element (98) configured to introduce a time delay in the beam which affects said spectral dispersion element, the spectral dispersion element comprising a diffraction grating (95, 98) operating in transmission and configured to separate the radiation beam emitted by the source into distinct diffracted beams and to transmit at least one diffracted beam of non-zero diffraction order. 29. The apparatus according to claim 28, when dependent on claim 27, in which the diffraction grating comprises a pinhole (95) and a grid (98) comprising a plurality of radiation opaque regions, said grid being arranged at of the pinhole so that the pinhole of the diffraction grating acts as the first coherence filter (97). 30. The apparatus according to any one of claims 26 to 29, in which the source is adapted to emit an X-ray emission beam centered at a wavelength between 1 nm and 20 nm.