EP3136401A1 - Device for correcting the longitudinal error of chromatic aberration of radiation of weighted particles - Google Patents

Abstract

The invention relates to a device for correcting the longitudinal aberration of the chromatic aberration of radiation of massed particles comprising at least one lens (L), at least one detector (D) and at least one diaphragm (B) with an aperture (5) which is variable in size , The invention further relates to methods for correcting the chromatic aberration.

Description

The invention relates to a device for correcting the longitudinal aberration of the chromatic aberration of radiation of massed particles. The invention further relates to a method for correcting the longitudinal aberration of the chromatic aberration.

State of the art

It is known that radiation of different wavelengths is usually focused by lenses at different focal points. This is the longitudinal error of the chromatic aberration. Before and after the respective focal point, the rays diverge for each wavelength in a conical manner. Therefore, dots are imaged as dots only for one wavelength by a lens with chromatic aberration. For all other wavelengths blurring of the image arises. Points are displayed as slices.

Lenses typically focus only precisely one wavelength in an input aperture, lens and detector system on the detector. The wavelength dependence of the refractive index shifts the focus and thus the location of the ideal image for all other wavelengths in the observed spectrum.

Methods for correcting the chromatic aberration are known, for example the use of systems of lenses with different refractive indices in so-called achromats or apochromats.

The correction of chromatic aberration often requires a complex, aspherical geometry of the lens in imaging systems. The prior art is the correction of chromatic aberration by lens systems with multiple, spherical and aspherical lenses, which should achieve a wavelength-independent focal length of the entire lens system. Lens systems of several, partly aspherical, lenses are expensive and complex to manufacture. Besides, they are sensitive.

Also for neutron radiation, the effect of chromatic aberration occurs. An optimal resolution in imaging processes with such a radiation is when, in the case of samples having a crystalline structure, this structure is visible in the image as a lattice of punctiform interference maxima. The closer the two interference maxima can be to each other, which are distinguishable from each other after imaging by the imaging method, the better the resolution. The chromatic aberration, in particular the longitudinal error, leads to a disk-shaped enlargement of these interference maxima during the imaging process.

In particular, the neutron small angle scattering is also suitable for investigating the internal structure of samples by irradiation of mass-afflicted particles. In neutron scattering experiments, a neutron beam is directed from a source onto a sample. One possible arrangement for carrying out the neutron scattering is in Jaksch et al. (Nuclear Instruments and Methods in Physics Research A 762 (2014) 22-30 ). From the sample, the neutrons are scattered at a scattering angle on the surface of a detector.

To generate a parallel beam path of the neutron beams, a collimator between the neutron source and the sample can be used. The sample is located in the beam direction directly behind the collimator. The collimation of the rays in the collimator is usually not ideal, that is, the diameter of the neutron beam grows along the beam axis in the direction of propagation. The deviation from the ideal collimation, ie the expansion of the neutron beam perpendicular to the beam axis, is indicated by the opening angle of the collimation or short collimation angle. The image is the better, the smaller the ratio of this opening angle to the scattering angle, ie α / θ.

It is known to generate neutron pulses with neutrons of different wavelengths in neutron scattering experiments for examining the samples at one frequency. The statistical distribution of the velocities of neutrons in one such pulse follows the Maxwell distribution. After scattering on the sample, the neutrons reach the detector.

For a better focusing of the beam, it is known to use neutron lenses, inter alia, lithium fluorine compounds, magnesium fluorine compounds or other magnesium salt compounds, for example between the collimator and the sample. The lens is usually located in the beam direction immediately behind the collimator and immediately in front of the sample. The lens also typically has the same extension as the sample. It may have, among other things, a circular or square or rectangular shape. The dependence of the focal length of the lenses on the wavelength of the neutron radiation in neutron scattering experiments is over Hammouda et al. (J. Appl. Cryst. (2013) .46, 1661-1371 ) known. The effect of chromatic aberration also leads to a worsening of the resolution in the area of neutron scattering. The pixel size of detectors available today necessitates elimination of the longitudinal aberration of the chromatic aberration.

Out US 8735844 For example, there is known a device for correcting chromatic aberration of neutron radiation from an arrangement of Wolter telescope type mirror surfaces. This arrangement consists of several axially symmetric superimposed mirror layers. The neutrons emanating from the neutron source are reflected at the mirror layers and thereby directed to the sample.

In US 6765197 For example, in order to correct the chromatic aberration, a combination of Fresnel lenses arranged along the propagation axis of the beam is disclosed. By using such a combination of Fresnel lenses, the chromatic aberration can be reduced.

Similarly, in Oku et al. (Nuclear Instruments and Methods in Physics Research A 600 (2009) 100-102 ) Combinations of synchronously focusing and defocusing lenses proposed to reduce the effect of chromatic aberration.

The disadvantages of these proposals can be seen in the high cost and the number of process steps for the preparation of the respective systems, as well as in the Error-prone due to the combination of a plurality of finely tuned components within the systems.

Object of the invention

The object of the invention is to provide a device of simple construction for correcting the longitudinal aberration of the chromatic aberration of mass-laden particles of radiation. Another object of the invention is to provide a method for correcting this longitudinal error.

Solution of the task

The object is achieved with the device according to claim 1 and the method according to the independent claim. Advantageous embodiments for this purpose will become apparent from the respective claims related thereto.

Description of the invention

The device for correcting the longitudinal aberration of the chromatic aberration of mass-exposed particles comprises at least one lens, at least one detector and at least one aperture with an aperture which is variable in size. By this is meant that a shutter is used whose aperture is changeable while radiation of massed particles passes. Optionally, the device may comprise at least one collimator for the parallel alignment of the mass of massed particles. This can be arranged along the beam axis behind the diaphragm.

Advantageously, the aperture during the passage of the radiation can be chosen so small that radiation which leads to the undesirable longitudinal aberration of the chromatic aberration, is absorbed at the edge of the aperture and can not pass through them in the direction of the sample and the detector. This advantageously causes the longitudinal error, which is caused by the different wavelengths of the radiation, to be corrected.

Along the beam axis, the aperture, the lens and the detector can thus be arranged one after the other within the device starting from the source. A collimator for parallel alignment of the mass of massed particles may be disposed between the source and the lens, source and sample are not part of the device according to the invention.

The device according to the invention may comprise at least one aperture disc, so that the aperture is changeable. But there are also many other embodiments possible. A variable aperture can, for example, also be effected by components arranged in a right angle at right angles, each having a rectangular cross section, wherein the components can be moved towards one another. Also suitable for this purpose are components which have a surface with a sawtooth profile and, for example, can be guided along components with a smooth surface.

An advantageous device is characterized in that the aperture has a border, which is formed by four discs. The disks are arranged along the beam axis at a same first distance. The centers of the discs are arranged at a same second distance from the beam axis. The beam axis is in this case the axis of symmetry of the device along which the radiation propagates. The connecting lines between the beam axis and the center points are perpendicular to each other. For each slice, the distance to the center takes on different values along the edge of the slice. The border of the aperture is here to denote the entirety of the edges of the surfaces through which the radiation passes, in the planes perpendicular to the beam direction, in which the aperture disks are arranged.

In an alternative embodiment of the device, the diaphragm comprises the opening of an iris diaphragm.

Due to the different values of the distance between the edge and the center, it is advantageously possible to easily achieve a change in the diaphragm opening by rotating the four disks.

A further advantageous device is characterized in that the discs each comprise an edge which has the shape of a spiral along a turn, wherein a straight edge between the edge point of the maximum and minimum distance from the center of the respective disc extends.

Due to the different distances between the edge and the center can be achieved in a simple manner, a change in the aperture by rotating the discs.

In this embodiment, this can be adapted to the velocity distribution of a neutron pulse which passes through the aperture, so that the longitudinal aberration of the chromatic aberration is corrected.

In an advantageous device, the edge of the maximum aperture is formed by the rectilinear edges of four panes in the form of a square. For example, if neutron guides used inter alia to direct neutron radiation from the neutron source have a square cross-section, the shape of the aperture corresponds to the cross-section of these neutron guides.

A high mechanical stability is advantageously effected inter alia by the fact that the aperture can be changed by a simple rotation of the at least one disc. Furthermore, even in advantageous embodiments of the device, a small number of discs is sufficient in comparison with other forms of diaphragms, such as, for example, the iris diaphragm. In addition, the discs do not touch each other in the beam direction, so that, for example, wear effects due to friction do not occur or to a lesser extent.

A high mechanical stability is for example advantageous if the aperture is opened periodically and closed again. This is the case with neutron scattering experiments where the neutron pulses are typically generated at a given frequency. This frequency is typically in the double-digit heart area. Among other things, the mechanical stability of arrays of rotating disks is well established and reliable Chopper known. The stability must be at least so good that the aperture can be opened and closed in time with the neutron source.

The diaphragm advantageously consists of a material which absorbs the neutrons, for example of materials containing boron carbide or lithium or gadolinium.

The method for correcting the chromatic aberration in a neutron scattering experiment is carried out with a device which comprises at least one lens, at least one detector and at least one aperture. For the respective range of wavelengths of the radiation used, the size of the aperture is chosen so that the longitudinal aberration of the chromatic aberration is corrected. Optionally, the device may comprise at least one collimator for the parallel alignment of the mass of massed particles. This can be arranged along the beam axis behind the diaphragm.

This means that during the experiment the aperture is actively matched to the wavelengths of the neutrons. Advantageously, this reduces the chromatic aberration of the unwanted wavelengths.

For this purpose, the following considerations are essential to the invention: The focal length of the lenses used is dependent on the wavelength of the massed particles, shown here on the example of neutrons. The surface of the sample typically has a size of up to 5 cm x 5 cm. The distribution of the velocities also determines the distribution of the wavelengths of the neutrons. The smaller the wavelength, the faster the neutrons. Therefore, the smaller their wavelength, the faster they traverse a given distance, for example, between an optional collimator and the sample and between the sample and the detector. After the scattering on the sample, the neutrons with the smallest wavelength reach the detector first. Then follow the slower neutrons in the sequence of increasing wavelengths.

The scattering process during neutron scattering can be described by the scattering vector q. This is defined as the difference between the wave vector k _{i of} a wave entering the sample and the wave vector k _f one after the Scattering expiring wave: q = k _f - k _i . In particular, for elastic scattering processes, such as, inter alia, the neutron small angle scattering, the amount of the scattering vector can be written at a scattering angle 2θ as: $$q=4\pi \frac{\mathrm{sin\theta }}{\lambda }$$

The amount of the scattering vector is determined by the wavelength in the radiation and on the other hand by the scattering angles. Especially for small amounts of the scattering vector q, this means that radiation with larger scattering angles is scattered, the smaller the wavelength is. Smaller wavelengths thus correspond with larger scattering angles. For small angle neutron scattering, such small amounts of scattering vector q to typically 2 Å ^{-1 are} relevant. For the correction of the chromatic aberration of the neutron scattering in particular small amounts of the scattering vector q up to 0.01 Å ^{-1 are} relevant.

The focal length of a neutron lens with radius of curvature R and refractive index n is given by: $$f=\frac{\mathrm{<figure-callout\; id="2"\; label="R"\; filenames="imgf0001.png"\; state="\{\{state\}\}">R</figure-callout>}}{2\left(1-n\right)}$$

The refractive index is given at an atomic density p, a neutron scattering length b and a neutron wavelength λ given by: $$n=1-\frac{\mathit{<figure-callout\; id="2"\; label="\rho b"\; filenames="imgf0001.png"\; state="\{\{state\}\}">\rho b</figure-callout>}}{2\mathrm{<figure-callout\; id="2"\; label="\pi \; \lambda "\; filenames="imgf0001.png"\; state="\{\{state\}\}">\pi \; </figure-callout>}}{\lambda }^{}{}^2$$

For describing the aberration in different directions, a coordinate system is introduced whose x and y axes extend in the direction of the height and width of the detector or a detector pixel, the beam is in the positive z-direction. In addition, the following notations are introduced: L ₂ denotes the distance between the lens and the detector, L ₁ denotes the distance between the beam source and the lens, r ₁ denotes the radius of the source and r ₂ denotes the radius of the sample, which typically coincides with the radius of the lens , λ denotes the used neutron wavelength, λ ₀ denotes the wavelength of neutrons for which the rays converge on the surface of the detector in one point, Δλ denotes the error of the neutron wavelengths, which typically occurs in neutron scattering experiments, Δ x ₃ denotes the width of the detector cell in the x direction.

mit $$s\left(\mathit{\lambda },{\mathit{\lambda }}_0\right)=\left[1-{\left(\frac{\lambda }{{\lambda }_0}\right)}^2\right]+{\left(\frac{\mathrm{\Delta \lambda }}{\lambda }\right)}^2{\left(\frac{\lambda }{{\lambda }_0}\right)}^2\left[{\left(\frac{\lambda }{{\lambda }_0}\right)}^2-\frac{1}{3}\right]+\frac{1}{15}{\left(\frac{\lambda }{{\lambda }_0}\right)}^4{\left(\frac{\mathrm{\Delta \lambda }}{\lambda }\right)}^4$$

The geometric part of the variance of the spatial resolution, which is a measure of the magnification of the image by the longitudinal aberration of the chromatic aberration, is given in the x-direction by: $${\left[{\sigma }_x^2\right]}_{\mathit{geo}}={\left(\frac{L_2}{{\mathrm{<figure-callout\; id="1"\; label="L"\; filenames="imgf0001.png,imgf0003.png"\; state="\{\{state\}\}">L</figure-callout>}}_1}\right)}^2\frac{{\mathrm{<figure-callout\; id="1"\; label="r"\; filenames="imgf0001.png,imgf0003.png"\; state="\{\{state\}\}">r</figure-callout>}}_1^2}{4}+{\left(\frac{{\mathrm{<figure-callout\; id="1"\; label="L"\; filenames="imgf0001.png,imgf0003.png"\; state="\{\{state\}\}">L</figure-callout>}}_1+L_2}{{\mathrm{<figure-callout\; id="1"\; label="L"\; filenames="imgf0001.png,imgf0003.png"\; state="\{\{state\}\}">L</figure-callout>}}_1}\right)}^2\cdot S\left(\mathit{\lambda },{\mathit{\lambda }}_0\right)\frac{{\mathrm{<figure-callout\; id="2"\; label="r"\; filenames="imgf0001.png"\; state="\{\{state\}\}">r</figure-callout>}}_2^2}{4}+\frac{{\left({\mathrm{\Delta }\mathit{x}}_3\right)}^2}{12}$$

With $$s\left(\mathit{\lambda },{\mathit{\lambda }}_0\right)=\left[1-{\left(\frac{\lambda }{{\lambda }_0}\right)}^2\right]+{\left(\frac{\mathrm{<figure-callout\; id="2"\; label="\Delta \lambda \; \lambda "\; filenames="imgf0001.png"\; state="\{\{state\}\}">\Delta \lambda \; </figure-callout>}}{\lambda }\right)}^2{\left(\frac{\lambda }{{\lambda }_0}\right)}^2\left[{\left(\frac{\lambda }{{\lambda }_0}\right)}^2-\frac{1}{3}\right]+\frac{1}{15}{\left(\frac{\lambda }{{\lambda }_0}\right)}^4{\left(\frac{\mathrm{<figure-callout\; id="4"\; label="\Delta \lambda \; \lambda "\; filenames="imgf0001.png"\; state="\{\{state\}\}">\Delta \lambda \; </figure-callout>}}{\lambda }\right)}^4$$

For the y-direction, the corresponding equation results by replacing the y-coordinate with the x-coordinate. Within the scope of the invention, it has been recognized that the width of the variance depends on the radius of the source or on the aperture through which the neutron beam passes before it enters the collimator.

It has further been recognized that after determining the parameters with which the neutron scattering is performed, the radius r _{1 of} the aperture can be calculated from equation (4) on the condition that the size of the image of an area which is the size of a pixel of the Detector has, on the detector surface is equal to this pixel size of the detector. This means that in equation (4) the value of the variance [σ _x ] _geo must be set equal to the value of the pixel width of the detector. The radius of the aperture as a function of the wavelength, ie r ₁ (λ), is then determined from equation (4). In this case, approximately [σ _x ] _geo ≈ [σ _y ] _geo can advantageously be assumed, ie the variances in the x and y directions are equal.

The iris discs are fully or partially rotated as a neutron pulse passes. The neutrons with the smallest wavelength first pass through the aperture. Then follow the neutrons with increasing wavelength. Each rotational adjustment of the discs during the rotation, which can be measured for example by a rotation angle, corresponds to a Wavelength of the neutrons. The diaphragm is correspondingly stable in order to meet the requirement according to the invention during the process.

For the following concept essential to the invention, the edge point is considered in each case of a disc which lies on the edge of this disc on the connecting line between the center of the disc and the center of the diaphragm aperture. In this context, D denotes the distance between the center of a disc and the center of the aperture. At each wavelength, during rotation, the distance between this edge point of each slice is given to the center of that slice as: d (λ) = D -r ₁ (λ). This determines the shape of the area of the edge of the aperture disc which is used to change the aperture upon passage of the neutrons. If the disks are completely rotated during the passage of the neutrons, the entire course of the edge of the disk is determined. Advantageously, the area of the aperture perpendicular to the beam direction can be assumed to be approximately circular.

Advantageously, the dependence of this distance d (λ) can be approximated as linear. It then suffices to determine the values of r ₁ (λ) for the largest and smallest wavelengths used. All other values of r ₁ (λ) then result from the linear interpolation between these extreme values.

The rotation of the discs can be done, for example, by a conventional gear on rotation axes, which are arranged through openings around the centers of the diaphragm discs. Synchronous rotation of the discs may be effected, inter alia, using toothed belt drives or gears, or synchronization of multiple motors by electrical circuitry, for example, with TTL pulses.

It has been recognized that the size of the aperture must be adjusted so that the size of the image of an area having the size of a pixel of the detector on the detector surface is at most equal to that pixel size of the detector. Then the aberration is due to the chromatic aberration no longer measurable the detector. This is also the case with modern detectors where the height and width of the detector pixels are in the range of millimeters.

It has been recognized within the scope of the invention that the maximum possible intensity of the neutron radiation, that is to say the maximum possible number of neutrons per time per area, then advantageously passes through the lens while the longitudinal error is suppressed by chromatic aberration.

This is advantageous, for example, over the possibility of correcting the aberrations due to the different wavelength of the neutrons through a very small, fixed aperture (infinitely small for a complete elimination of aberrations). The smaller the aperture is selected, the more intensity of radiation is lost for imaging. By means of the method according to the invention, it is advantageously effected that the maximum possible intensity of the neutron radiation, that is to say the maximum possible number of neutrons per time per area, passes through the lens with simultaneous suppression of the longitudinal error due to chromatic aberration.

The height and width of the detector pixels can range from 50 μm to a few centimeters for neutron scattering experiments. Typical widths or heights in the range of 3 mm - 8 mm . The maximum diameter of the aperture typically assumes values of 20 mm - 30 mm . The diaphragm effects correction of the longitudinal aberration of the chromatic aberration, especially at high wavelengths, typically in the range of 10 Å-15 Å. The area of the aperture for this case in this wavelength range is typically a few square centimeters.

The reciprocal of the frequency at which neutron pulses are generated may typically be 72 milliseconds. This is the time interval in which the aperture is changed by turning the aperture disks from the maximum aperture to the minimum aperture.

In an advantageous embodiment of the method, the aperture is reduced continuously or stepwise starting from the largest opening. Advantageous Thus, each time a pulse of neutrons passes through the shutter is closed so that the radiation causing the longitudinal aberration of the chromatic aberration is absorbed at the edge of the shutter.

Since in a neutron pulse the neutrons with small wavelengths first pass through the aperture and then the neutrons with increasing wavelength, the aperture is advantageously initially selected to be maximally large for small wavelengths and then reduced continuously or stepwise.

The intensity of the radiation is greatest at the smallest wavelengths used. Therefore, the intensity distribution of the spectrum is optimally utilized and aberrations due to chromatic aberration at high wavelengths of neutrons, typically in the range of 10 Å, 11 Å, 12 Å, 13 Å, 14 Å, 15 Å, are avoided.

In the case of neutron scattering experiments, the intensity is greatest at small wavelengths, typically in the range of 2Å - 5Å, and decreases for longer wavelengths. The neutron lenses usually interact with neutrons of longer wavelengths, so that no aberrations caused by the chromatic aberration for neutrons of small wavelengths. Furthermore, the small wavelength neutrons are scattered into large scattering angles. This can be seen inter alia from equation (1), in particular for small amounts of the scattering vector q with q <1 Å ^-1 . Therefore, the aberration given by the ratio of the opening angle of the collimation to the scattering angle α / θ is negligible for the small-wavelength neutron. Only the radiation of neutrons of large wavelengths must be further focused by a lens. In addition, due to the quadratic dependence for smaller wavelengths, the refractive index is also much closer to 1 than to longer wavelengths, ie n → 1 for λ → 0. This can be seen for example from equation (3). This means, inter alia, that the smaller the wavelength of the respective neutrons, the smaller the effect of the lens becomes.

Since the number of particles decreases sharply to larger wavelengths, typically greater than 15 Å, in particular greater than 17 Å, the aperture is advantageously no longer reduced in this wavelength range. It will be for a still usable Particle flow maintain a minimum aperture. This minimum aperture is typically an area of a few square millimeters, typically 2 mm x 2 mm . The region of longer wavelengths is usually above 7.5 Å, in particular in the range of 10 Å-15 Å. Since the intensity of the radiation decreases approximately with the wavelength λ according to λ ^-4 , measurements of radiation with wavelengths beyond 15 Å are typically only possible with high intensity loss.

In addition, in the detector elements located in the central area of the detector surface, the signals of the first 10 milliseconds can be erased. Since the fast neutrons are scattered into the outer elements of the detector, the signals of the first 10 milliseconds come only from a noise signal, such as particles that hit the detector surface, but not from the incident on the detector surface neutron pulse.

In an advantageous embodiment of the method, the range of wavelengths of the radiation is limited before passing through the aperture and the lens.

This limitation ensures that radiation hits the aperture in the desired range of wavelengths. In the case of neutron scattering, this also removes particles from earlier or later neutron pulses, for example particularly slow particles from an earlier neutron pulse or particularly fast particles from a later neutron pulse. As a result, the individual neutron pulses have, inter alia, a comparable number of neutrons.

For this purpose, advantageously at least one chopper, in particular a T0 chopper, can be used. This consists for example of a rotor with rotor blades, which block the neutron beam. The rotor blades have openings or gaps through which the neutron beam can pass. The rotor can advantageously rotate at the same frequency with which the neutron pulses are generated. Advantageously, a plurality of along the beam axis at a distance arranged choppers are used, which rotate synchronously. The openings or gaps of the rotor blades of the various choppers can be arranged rotated against each other, as known from the prior art.

Advantageously, the time interval in which the aperture is changed by rotating the aperture discs from the maximum aperture to the minimum aperture and back to the maximum aperture, is determined by the reciprocal of the frequency at which the neutron pulses are emitted from the neutron source. The maximum aperture is then advantageously set at the same frequency at which the neutron pulses are generated in the neutron source.

In a continuous neutron source, the time interval in which the aperture is changed by rotating the aperture disks from the maximum aperture to the minimum aperture is advantageously determined by the reciprocal of the rotational frequency of the T0 chopper.

In the advantageous embodiment of the device with four mutually perpendicular aperture discs with a spiral edge, the maximum aperture is bordered by the four straight edges of the aperture disks. The synchronous rotation of the discs, the aperture is minimized starting from the maximum aperture. When reaching the full angle, ie a rotation through 360 °, there is a sudden transition from the minimum to the maximum aperture.

Advantageously, the respective closing of the diaphragm opening after the generation of a neutron pulse is only delayed after the lapse of a time interval. This time interval advantageously results as the quotient of the distance between the neutron source and the diaphragm on the one hand and, on the other hand, the speed of the fastest neutron in the respective neutron pulse which is to pass through the diaphragm aperture. Thereafter, the shutter is started only after the fastest neutrons of a pulse have passed through the diaphragm opening in the direction of the sample. This advantageously has the effect that the aperture is changed so that the neutrons with such wavelengths, that a longitudinal aberration of the chromatic aberration is caused to be absorbed at the edge of the aperture.

Advantageously rotate in the device of the chopper or with the same frequency with which the aperture is opened at maximum, and with which the neutron source emits the neutrons.

The shutter mechanism of the diaphragm advantageously operates isochronously with the neutron source and the time of flight of the fastest neutrons in the respective neutron pulse correspondingly out of phase with the neutron source.

Advantageously rotate in the device of the chopper or with the same frequency with which the aperture is opened at maximum, and with which the neutron source emits the neutrons.

Suppose the neutron source emits neutron pulses at a frequency of 14 Hz. The duration of the emission of a single neutron pulse may be 2.4 milliseconds. In this time interval, neutrons of different wavelengths or velocities are emitted from the neutron source, the statistical distribution of the velocities in the neutron pulse following the Maxwell distribution. After emission from the neutron source, the neutrons move in the neutron pulse along the beam axis in the direction of the diaphragm or lens. For the alignment of the neutron pulse, known neutron guides can be used.

In an advantageous embodiment, the four diaphragm discs rotate with a spiral edge synchronously with a same such sense of rotation that the aperture is reduced, for example, levorotatory. The synchronous rotation of the diaphragm discs can be done via a conventional gear, for example via a toothed belt transmission or gears. In this case, the disks are rotated by 360 ° in a first time interval which corresponds to the reciprocal of the frequency with which the neutron source emits neutron pulses. This value is 71.4 milliseconds in this example. Each time the neutron source emits a neutron pulse, the aperture disks have rotated 360 °. The rotation The slices are isochronous with the time intervals between the start times of the emissions of the neutron pulses from the neutron source.

In an advantageous embodiment, the closing of the diaphragm opening, starting from the maximum diaphragm opening, is started only after a second time interval, when the fastest neutrons of the neutron pulse have reached the diaphragm opening. The closing of the aperture thus occurs out of phase with the emission of neutron pulses from the neutron source. The second time interval, which describes this time delay, advantageously results as a quotient of the distance between the neutron source and the diaphragm on the one hand and, on the other hand, the speed of the fastest neutron in the respective neutron pulse.

The aperture disks are rotated synchronously continuously, wherein a rotation of 360 ° in the first time interval and from the maximum aperture to the emission of the neutron pulses by the second time interval is offset in time. The fastest neutrons from the neutron pulse pass through the aperture when this is maximum. Then, the slower neutrons pass through the sequence of increasing wavelengths, while the aperture is continuously reduced by rotating the aperture disks to a minimum aperture. When the minimum aperture is reached, the neutrons of a pulse have passed through the aperture. Now causes the further rotation of the aperture disks, that at the end of the first time interval, the maximum aperture for the fastest neutron of the next neutron pulse is set again. This advantageously causes the neutrons with such wavelengths that a longitudinal aberration of the chromatic aberration is caused to be absorbed at the edge of the aperture.

After the massed particles, in particular neutrons, have passed through the aperture, they are focused by at least one lens whose longitudinal aberration of the chromatic aberration is corrected by the use of the aperture, and then strike the surface of at least one detector.

Preferably, in the device according to the invention, the diaphragm is thus arranged in front of the lens and these in turn are arranged in front of the detector.

When structures of samples are examined using this method, these samples are typically located along the beam axis behind the lens. The neutrons are then scattered by the lens on the sample after focusing, and then strike the detector surface.

To generate a parallel beam path of the neutron beams, a collimator between the neutron source and the sample can be used. The sample is located along the beam axis immediately behind the collimator.

The range of wavelengths of radiation may be limited prior to passing through the aperture and the lens by using known choppers. Advantageously, the choppers can rotate 360 ° in the first time interval. The rotation is isochronous with the emission of neutron pulses from the neutron source. Advantageously, the rotation of the chopper can take place offset in time to the emission of the neutron pulses from the neutron source.

Advantageously, the sample is arranged as close as possible behind the lens. This advantageously has the effect that part of the radiation is scattered by the sample and that the information about properties of the sample in the scattering image is clearest. Here you can work at a small wavelength with the maximum aperture.

Ausführunsgbeispiel:

• Figur 1: Stellung der Blendenscheiben bei maximaler Blendenöffnung (a) und bei geschlossenem Zustand der Blende (b).
• Figur 2: Aufbau zur Durchführung eines Neutronenstreuexperiments, in dem die erfindungsgemäße Vorrichtung verwendet wird.
• Figur 3: Intensität der gestreuten Strahlung von Neutronen als Funktion des Betrags des Streuvektors q für verschiedene Einstellungen der Blendenöffnung.

Hereinafter, a concrete embodiment with neutrons as the bulked particles is carried out to the invention. This example is illustrative of the invention and is not intended to be limiting.

• FIG. 1 : Position of the aperture discs at maximum aperture (a) and when the aperture (b) is closed.
• FIG. 2 : Construction for carrying out a neutron scattering experiment, in which the device according to the invention is used.
• FIG. 3 : Intensity of the scattered radiation of neutrons as a function of the amount of the scattering vector q for different settings of the aperture.

The following text describes the design for apertures to correct chromatic aberration in mass-afflicted particles, here neutrons.

The aperture used is in FIG. 1 shown. Alternatively, an iris diaphragm is possible. The aperture 5 has a border, which is formed by four discs 1, 2, 3, 4, which are arranged along the beam axis at a same first distance, wherein the discs each have a center. The center of a disk is here indicated as 7 representative of the centers of all disks. The discs may advantageously have an opening in their middle.

The connecting lines between the beam axis and the center points are perpendicular to each other. For each slice, due to the spiral shape of the slice, the distance to the center along the edge takes on different values.

The disks each comprise an edge which is in the form of a spiral along a winding, with a rectilinear edge extending between the edge point of the maximum and minimum distance from the center 7 of the respective disk. The straight edge of a disc is representative of the rectilinear Edges of all discs labeled 6. The edge of a disk is here called 8 representative of the edge of all disks.

The discs are rotated synchronously in a rotational direction such that the aperture is reduced starting from the maximum aperture. In the FIG. 1 this is done by a rotation with left orientation. This is shown in the figure by an arrow over disc 2. For reasons of clarity, the direction of rotation is shown only for disc 2, but all other discs are rotated synchronously with disc 2 with the same direction of rotation.

The aperture B is introduced in a neutron scattering in the beam path such that by rotation of the discs 1, 2, 3, 4, the aperture, through which the neutrons fly, is adapted in size.

The preferred position here is the longest possible collimation distance of the structure, in neutron instruments about 20 m. The lens L is mounted directly in front of the sample P.

The disks 1, 2, 3, 4 must be constructed of a material which absorbs the bulked particles to function as an effective diaphragm, for example boron carbide for neutrons, metals / conductive materials for electrons. The curvature of the curve of the edge is to be chosen in a way that the opening profile for each wavelength optimum imaging properties are achieved. The imaging properties are optimal in the experiment when the image of a point in the size of a detector pixel when imaged on the detector does not exceed the size of that pixel. Thus, a maximum intensity can be achieved at the same time with the best possible resolution. The height and width of the detector pixels was in the range of a few centimeters for detectors previously used in neutron scattering experiments. Meanwhile, detectors are typically used in which the height and width of the detector pixels in the range of a few millimeters. This leads to a higher requirement for the resolution of the imaging methods.

In neutron scattering experiments with the investigation of interference patterns, an additional consideration is added: small wavelengths (fast particles) become imaged at large angles and vice versa. Since lenses are mainly used to improve the resolution at small angles, the aperture for fast particles can be fully opened here and then closed. Since most neutron sources have a Maxwell distribution with a maximum of neutron flux at 2-5 Å wavelength, the high flux at these wavelengths can be exploited. At the larger wavelengths, which are mapped to smaller angles, the shutter is then closed to achieve the optimum low-angle imaging characteristics. On reaching a certain wavelength, the flow becomes so low and the resolution is only insignificantly improved, so that a minimal opening is advantageous.

The necessary prerequisite for the sensible use of the diaphragm system or lens system is either a pulsed source or a T0 chopper, so that a particle packet is optimally cut for each cycle of the diaphragm. The same considerations as for neutrons can be applied to scattering using all mass-bearing particles, for example electrons. This particularly concerns systems in which pulses of particles are generated and used for imaging processes.

In FIG. 2 a device according to the invention for correcting chromatic aberration is shown for neutron scattering experiments, in which the use of the diaphragm system brings an advantage. In this case, the diaphragm B is arranged 20 m in front of the sample P and the lens L directly in front of the sample P. The lens diameter L corresponds approximately to the sample diameter P. The detector D can be optimally positioned depending on the resolution. The optimum position of the detector is determined by the desired angle coverage by the scattering angle.

From left to right along the beam axis, a thin and a wide collimator with choppers C at 11.5, 14.5 and 21.5 m, the position of the sample P and the detector tube D can be seen. A thin collimator K " _1" with a length of 8 m is positioned in front of the first chopper C, while the wide collimator K " _2" follows with a length of 12 m behind it.

The total distance between detector D and sample P is 20 m. In order to improve the resolution, the diaphragm system B can be arranged 20 m in front of the sample, the lens L is then located directly in front of the sample P and the detector D in the detector tank on the right.

The frequency with which the neutron pulses are generated in the neutron source is 14 Hz in this exemplary embodiment. The time duration with which the individual pulses are generated in the neutron source is 2.8 milliseconds. The length of time that the neutron pulses pass through the aperture is about the inverse of the frequency of 14 Hz at about 72 milliseconds. In this time interval, the diaphragm B is reduced from the largest aperture opening to the minimum aperture and then increased again to the maximum aperture. The height and width of the pixels in this version are each 3 mm. The size of the sample is 1 cm × 1 cm . The choppers limit the wavelengths of the neutron radiation that reaches the sample, typically to a range of 2Å to 15Å. The time interval between the generation of the neutron pulse and the closing of the aperture is typically 2-3 milliseconds here. This time interval is determined by the time it takes for the fastest neutrons to pass from the neutron source to the aperture.

A simulation showing the improvement through the aperture is in FIG. 3 to see. The improvement of the resolution by the aperture system is shown by a simulation of three delta peaks per logarithmic decade at SKADI. With a reasonable choice of the aperture as a function of the time of flight of the neutrons, the resolution can be significantly improved (data curve 32 against data curve 31). The simulation was performed for the same number of neutrons for all instrument settings.

The normalized intensity of the scattered radiation of neutrons is shown as a function of the magnitude of the scattering vector q, which is plotted logarithmically. Each delta peak or environment of a maximum of one of the illustrated data curves would be infinitely sharp or narrow at ideal resolution. The widening of the peaks reveals the resolution, the narrower the peak, the better better is the resolution. Shown here are three delta peaks per logarithmic decade.

The aperture is maximally open for small wavelengths with λ 7.5 Å or large amounts of the scattering vector q and is then closed further with increasing values of the wavelengths or smaller amounts of the scattering vector q.

The number of neutrons in the simulation calculations was 10 ⁹ / ( s cm ² ).

The data curve 31 shows the case that no lens and a small aperture of 2 mm × 2 mm are used. The data curve 32 simulates the case of using a lens with a changed aperture depending on the wavelength. The narrowness or "sharpness" of the peaks, and thus the resolution, is significantly more pronounced than in the data curve 31, in particular at values q < 0.01 Å ^-1 .

Claims (11)

Apparatus for correcting the longitudinal aberration of the chromatic aberration of massaged-mass radiation, comprising at least one lens (L), at least one detector (D) and at least one diaphragm (B) with an aperture (5) which varies in size as a function of the wavelength the radiation passing through this aperture is changeable. Device according to claim 1, characterized in that the diaphragm (B) is formed by 1, 2, 3 or more, preferably 4, discs or by an iris diaphragm. Device according to one of claims 1 to 2, characterized in that the discs (1, 2, 3, 4) each comprise a rim (8) which has the shape of a spiral along a turn, with a rectilinear edge (6) between the edge point of the maximum and minimum distance from the center (7) of the respective disc extends. Device according to one of Claims 1 to 3, in which the discs (1, 2, 3, 4) comprise a neutron-absorbing material, in particular boron carbide. Device according to one of claims 1 to 4, characterized in that the device comprises at least one collimator. A method of correcting the longitudinal aberration of the chromatic aberration of massaged-mass radiation with a device comprising at least one lens (L), at least one detector (D) and at least one aperture (B), the aperture varying as a function of the wavelength of the massed particles becomes. A method according to claim 6, characterized in that for the respective range of the wavelengths of the radiation used, the size of the aperture is chosen so that the longitudinal aberration of the chromatic aberration is corrected. Method according to one of claims 6 to 7, characterized in that the size of the aperture is chosen so that the size of the image of a surface having the size of a pixel of the detector on the detector surface is at most equal to this pixel size of the detector. Method according to one of claims 6 to 8, characterized in that the aperture is reduced continuously or stepwise starting from the largest aperture, each time a pulse of neutrons passes. Method according to one of claims 6 to 9, characterized in that the range of wavelengths of the radiation is limited before passing through the aperture and the lens, in particular by at least one chopper and more preferably by at least one T0 chopper. Method according to one of claims 6 to 10, characterized by the choice of neutrons as mass-afflicted particles.

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