DE3217025A1 - Double-crystal spectrometer - Google Patents

Abstract

The double-crystal spectrometer is used for measuring X-rays with temporal, spectral and preferably also spatial resolution with a strong interfering radiation background as occurs, for example, in the observation of thermonuclear plasmas in the form of neutrons and hard X-rays against which the spectrometer must be shielded. For this purpose, a mechanically simple design is specified which is based on the principle that the beam path between the two crystals must form a changing but always right-angle triangle with the directions parallel or perpendicular to the crystal plane, in all angular positions of the Bragg crystals which can be rotated in familiar manner by in each case the same angle. At least two of the three corner points of the triangle must be freely displaceable on or parallel to the directions of beam entry or emergence. <IMAGE>

Description

Double crystal spectrometer

The invention relates to a double crystal spectrometer according to the The preamble of claim 1, in particular for X-ray measurement in the case of strong Interfering radiation background with temporal, spatial and spectral triggering is suitable is.

For example, in the spectroscopy of thermo-nuclear plasmas (im Temperature range up to about 20 keV) occurs in addition to a soft X-ray radiation, which if necessary, it should be observed or measured over a broad spectral range, too a background of interference radiation in the form of neutrons and / or hard X-rays that must be eliminated. The problem arises for one over one great Spectral range usable spectrometer with high resolution one mechanically easy to find structure that shields the spectrometer well against direct (e.g. through the entry opening of a protective wall) or through reflections Interfering radiation guaranteed and also without difficulty to the radiation protection the environment in each case required building structures, can be arranged where it should be easily accessible outside a main protective wall.

For radiation measurement in the energy range up to about 4 keV, with which problems can occur with regard to interference radiation and / or possible resolution is already the use of a monochromator with two crystals known to which the incoming one X-ray beam according to Bragg's equation in a direction parallel to the entry direction offset exit direction is deflected (Nuclear Instruments and Methods 152/1978, Pp. 43-51). To change the Bragg angle and consequently the wavelength to be measured by twisting both crystals without changing the exit direction and thus the Here you can observe the second crystal at the same time as the rotation so along the Move the beam exit direction that the beam always goes through the center of the crystals. So far, however, there is no satisfactory one Mechanism to control the movement of the crystals.

In a known spectrometer of the type mentioned at the beginning (Nuclear Instruments and Methods 152/1978, pp. 109-111) is the guiding mechanism that to automatically adjust the amount of displacement to the angle of rotation, essentially from a solid curve surface, which when the entire system rotates with the a crystal firmly connected rod leads. Apart from mechanical difficulties in the practical implementation of this system, it has the disadvantage that in the Angular twisting of the ray on the crystal faces moves and at the same time also the beam cross-section can change.

Another known possibility is the arrangement of one crystal on a rotatable disk, on which the other crystal on one with a micrometer screw sliding carriage is mounted (Handbuch der Physik "by S. Flügge, 1957, Pp. 104-108).

The invention is based on the object of a spectrometer that can be shielded well specify with a mechanically simple structure with which it is possible relatively quickly to drive through a large spectral range, and its guiding mechanism the Always keeps the beam in the center of the crystal face. Preferably should also be on in a simple way, a continuous spatial resolution (e.g. with regard to different Locations of the radiation source).

This object is achieved by the spectrometer characterized in claim 1 solved.

The invention is based on the assumption that to solve the stated problem at all angular positions the beam path between the two crystals with the directions parallel or perpendicular to the crystal plane a changing, but must always form a right triangle whose cathetus intersection on the The center line between the entry and exit directions of the jet. At least two of the three corner points (centers of the crystal surfaces and cathetus intersection) must be freely movable on or parallel to the entry or exit directions.

In some cases it can be useful to locate one crystal to hold and next to the other crystal, the catheter intersection can be moved to store.

This applies in particular to the one on the jet entry side Crystal, which with a stationary crystal center simply around the direction of the between The two crystals passing beam can be rotated when the spectrometer In addition to the spectral resolution, it is also intended to enable spatial resolution. Furthermore is it is often expedient to have one crystal as close as possible and under defined conditions, So to keep stationary at the radiation source.

In other cases it may be more beneficial to change the wavelength only the two crystals are moved while the catheter intersection is fixed is limited by shielding, e.g. with regard to the smallest possible Shift range of the ray path between the two crystals. The range of displacement in this case remains relatively small, especially if the Bragg angle is symmetrical to / 4 changes approximately between 30 ° and 60 °.

Since in the system described here, the beam is inevitably in the Crystal center can be held and with an angle adjustment also its cross-section does not change, the invention also has the advantage of optimal utilization of the available crystal surface and consequently maximum radiation intensity for each set wavelength. Accordingly, one comes for reasons of intensity (desired number of photons) predetermined beam cross-section with a relatively small homogeneous crystal, which is important because certain types of crystal with become very costly as they grow in size.

The invention is explained in more detail with the aid of the drawing.

The figures show: FIG. 1 a schematic representation of the shielded Arrangement of the spectrometer according to a first embodiment on a concrete wall for example a plasma test facility; Figure 2 shows the beam path and the positions of the two crystals for three different Bragg angles in the embodiment the spectrometer according to Figure 1; and FIG. 3 shows a test model for a second Embodiment of the invention.

The spectrometer described here is suitable for. B. for broadband Impurity measurements during the discharge phases (DD and DT) of a plasma-physical Fusion device like the "Joint European Torus" (JET), which is a massive, more as a 2 m thick concrete wall 1 with an opening 2 through which the X-ray beam 3 entry. Outside the wall 1, the spectrometer is accessible in or on the illustrated Parts of a shield 4 are arranged, which are extended to the outside by a further wall 5 with an opening 6 for the exiting jet 7 is limited. The beam 7 arrives through a collimator 8 into a detector 9, at which it is purposefully can be a proportional counter. The distance between the walls 1 and 5 can be 2 m, for example.

The two crystals A, B of the spectrometer are arranged so that the incoming ray 3 to be measured from the center of the crystal A to the center of the Crystal B and from this in the exit direction offset parallel to the entry direction (Beam 7) is deflected. This is also the case when changing the Bragg angle and consequently the wavelength of the measured beam both crystals around the same Rotated amount and at the same time along the entry and exit directions by appropriate Routes can be moved to the positions indicated at A 'or B'. Next to The double crystal principle has the advantage of a constant detector position simple shielding of the detector 9 against especially in the active phase of the fusion device Neutrons and interfering rays entering the spectrometer through opening 2.

The detector can come from the point of impact, which is expediently equipped with paraffin 10 of the neutrons are isolated by special concrete. In addition, the angle adjustment with simultaneous displacement of both crystals, a very extensive one, as shown Shielding of the path of rays between crystals A, B against reflection rays possible, namely through the parts of the shield protruding into the web on both sides 4, which is only a narrow one in the middle of the path between the two crystals Leave gap free and funnel-like to the possible positions of the crystals to open. The necessary guides for the crystals can be used in this case to the jet plane (Drawing plane in Fig. 1) parallel planes are offset.

Adjusting two separate crystals requires high mechanical precision, but on the other hand allows deliberate alignment errors to be suppressed of higher-order interference rays or to compensate for thermal effects.

The crystals are assembled as shown schematically in Figure 2 is shown. At least one, but both in the example according to FIG. 1 Crystals A and B are with their pivot points along one of the entry and exit directions of the beam parallel guide C or D mounted displaceably. During the crystal A is connected to a straight guide E parallel to the direction of its plane, is the crystal B with a direction perpendicular to this direction, i.e. the crystal plane straight guide F connected. The guides E and F are in their respective longitudinal directions slidably mounted and connected to each other so that they are always perpendicular to each other stand. Under these conditions, the directions form over the entire adjustment range of guides E and F with the respective beam direction between the two crystals A, B is always a right-angled triangle, the shape of which is constantly changing, but its cathetus intersection 0 is always on the center line M between the crystal centers lies.

The various tours can be, for. B. in the form of rails or the like or as rods arranged in plain bearings (FIG. 3).

It is also possible to mark point 0 along the center line M (or in the corresponding central plane parallel to D) to be displaceable as with the Exemplary embodiment of the invention explained below with reference to FIG. if but one keeps the point 0 stationary and the guides that are perpendicular to one another E and F can only be rotated around this point O (or around an axis passing through 0), the intersection of the ray R with the center line M moves in a point of interest Range of the Bragg angle e from, for example, e1 = 30 ° to e3 = 6ob only on the relatively small path, which together with the desired beam width is the minimum opening the shield 4 (Fig. 1) determined between the crystals. At 62 = 450 the Beam R deflected perpendicular to the exit direction.

For a symmetrical change in the Bragg angle 6 on both sides R desired Andean sides of / 4 (450), i.e. from a desired starting angle OO to / 2 - EO lead the crystals A and B a movement along the distance 1 = d.cot 2 e off if 0 d is the distance between the centers passing through the crystal Axes of guides C and D is. The crystals slide on the guides E and F on a length between d / (2 cos OO) and d / (2 sin iO), and the point of intersection of the beam with the center plane M moves by d 1 ß - d / 4 (cot 0o + tan 60 - 2) 2 sin 2 60 1) o o A spatial resolution of the double crystal spectrometer is achieved in that one of the crystals, e.g.

A around the axis of the ray passing between crystals A, B. R is rotatably mounted. At 0 4 the radiation sources are on a straight line in the plane which perpendicularly intersects the plane of the drawing in FIG. 2 along the guide C, at 6 / ist / 4 on the other hand on hyperbola with a large opening. The changeability of the Bragg angle Under this condition, the additional movement 5-freedom also does not impaired like the constant exit direction. When used on a shielded Fusion plasma systems can enter the different directions of entry there must be several entry openings in the protective wall.

In the experimental model shown in Fig. 3, which in a considerably On a smaller scale, the crystal B and the cathetus intersection are O (Fig. 2) along the beam exit direction or parallel to it in the plane of Center line M (Fig. 2) mounted displaceably.

In contrast, the center of the crystal A is stationary, whereby for a desired spatial resolution rotating crystal A around ray R (Fig. 2) is facilitated. The above-mentioned tours D, F and E as well as the required liche further guidance G, which is required to move the point 0, exist in the model shown made of round bars.

To move on its guide D, the crystal B sits on one Bearing block 11 through which the rod forming the guide D is slidably displaceable extends. Of the.

Bearing block 11 is rotatable about an axis perpendicular in FIG. 3 a second bearing block 12 is mounted, which in turn is slidable is mounted on the stationary guide D. Rigidly connected to the bearing block 12 is a rotatably mounted carrier 13, on which one rigidly to a bearing structure 20 for the crystal A belonging support 14 and rotatable a bearing block 15 through which slidably extending the rod-shaped guide E, are arranged. The guides E and F are rigidly connected to each other at O, namely on one Connecting part 16, which is rotatable on a sliding on the fixed guide G slidably mounted block 17 is arranged. The block supporting block 11 12 is arranged on the carrier 13 so as to be displaceable relative thereto.

The rotatable bearing block 15 is via a lever 18 with a relative connected to the support 14 along a guide rod 19 movable block 21, which, when it is moved via a further lever 22, a holder 25 of the crystal A pivoted about a bearing axis 23 in the open end of a U-shaped bracket 24 is arranged.

The bracket 24 is in turn in the support 14 and in one as well on the support 13 standing further support 26 of the bearing structure 20 so mounted, that it is used for spatial resolution, i.e. to change the beam entry direction the axis of the beam R (Fig. 2) is rotatable.

So if in the construction shown in Figure 3, the Bragg angle should be changed and for this purpose, for example by acting on the Carrier 13 the The entire system is twisted, it is misaligned the movement of the right triangle formed by the guides E and F Crystal B by the desired angle. with blocks 12 and 17 in parallel to each other on guides D and G and guides F and E in blocks 11 or 15 slide. At the same time, the rotating block 15 moves via the lever 18 the block 12 by such an amount that this over the further lever 22 to the Crystal A pivots about its bearing axis 23 into a position in which it - at perpendicular standing bracket 24 - exactly parallel to the crystal B remains. When the crystal A is steered by turning the bracket 24 in a different beam entry direction, thereby nothing changes in the direction of the ray R from A to B and consequently at the beam exit direction along the guide D.

It goes without saying that one also has the other crystal B around the beam R could shift rotatably.

If the spectrometer described here for measurements in the O O range from 1 A to 20 A should be used al: material for the crystals A and lX e.g. 5 lulninium oxide (NaA1llol7) ot-quartz (SiO2) and tndiumantirno! (InSb) in combination with Calcid (211) -, Ge (111) - and Si (III) -crystals into consideration, which, in contrast to organic crystals, has the necessary resistance to intense Have neutron and X-rays.

Claims (8)

Double crystal spectrometer patent claims fm 0 double crystal spectrometer especially for measuring soft X-rays, with two parallel to each other arranged at the same angle with respect to the beam exit direction rotatable Bragg crystals, one of which when rotating through an angle of rotation corresponding amount is displaceable along the beam exit direction, with a Guide mechanism that automatically adjusts the amount of displacement and the angle of rotation coordinate with each other, characterized that at least one of the crystals (A, B) with its pivot point along a to Entry or exit direction of the beam parallel guide (C, D) mounted displaceably is that the one crystal (A) with a straight guide parallel to the crystal plane (E) and the second crystal (B) with a straight line perpendicular to the crystal plane Guide (F) is connected, and that these two guides (E, F) in their respective Slidably mounted in the longitudinal direction and connected to one another so that they are always stand perpendicular to each other. 2. Spectrometer according to claim 1, d a d u r c h g e -k e n n z e i c h n e t that both crystals (A, B) along one each to the direction of the beam exit parallel guide (C, D) are slidably mounted. 3. Spectrometer according to claim 1, d a d u r c h g e -k e n n z e i c h n e t that the two perpendicular guides (E, F) on a central guide (line M) parallel to the beam exit direction are stored. 4. Spectrometer according to claim 2 or 3, d a d u r c h g e k e n n shows that the two perpendicular guides (E, F) slidably connected to their respective crystal (A or B) and on their common connection (point 0) are rotatably mounted about an axis, which perpendicular to the directions of displacement, always in the middle between the inputs and Exit directions lies. 5. Spectrometer according to one of the preceding claims, d a d u r c h e k e k e n n n n n n e i n e t that at least some of the tours (D, E, F, G), are formed from rods that are each supported by a bearing block (12, 15, 11 or 17) extend that the rods that lead to the Beam exit direction Form parallel guides (D, G), are stationary and displaceable bearing blocks (12, 17), while the perpendicular guides (E, F) forming rods in their bearing blocks (11, 15) are displaceable, and that the the latter bearing blocks (11, 15) to be perpendicular to the directions of displacement Axes are rotatably mounted. 6. Spectrometer according to claim 5, d a d u r c h g e -k e n n z e i c h n e t that the bearing block (11) is one of the perpendicular guides (F) rotatable on the bearing block (12) of the one parallel to the beam exit direction Guide (D) is mounted and carries a crystal (B), and that the connecting part (16) of the perpendicular guides (E, F) rotatable on the bearing block (17) of the other guide (G) parallel to the exit direction is supported. 7. Spectrometer according to one of the preceding claims, d a d u r c h g e k e n n n z e i c h n e t that the rotatable bearing block (15) one of the one on top of the other vertical guides (E) via a mechanism formed by levers (18, 22) one of the crystals (A) twisted so that when the other crystal is adjusted (B) is adjusted by the corresponding angle. 8. Spectrometer according to claim 7, d a d u r c h g e -k e n n z e i c h n e t that a lever (18) hinged to the rotatable bearing block (15) has a Block (21) along the axis between the two crystals (A, B) Beam (R) moves, and that this block (21) via another lever (22) on a holder (25) of the one crystal (A) engages, which around a bearing axis (23) is pivotable, which in turn around the axis of the between the two crystals (A, B) running beam (R) is rotatably mounted.