GB2040149A - Diffracted X-ray beam monochromator - Google Patents

Abstract

An X-ray beam monochromator comprises a monochromator crystal mounted in a housing 16 supported on posts 7 fixed to a base plate 1,and provided with an output slit and detector coupler 17. A further support 5 projects into the housing 16 and is provided with a channel into which a Soller collimator plate 6 can be inserted and releasably held by a detent mechanism. The crystal may be graphite, which provides a high diffraction efficiency compared with e.g. LiF or quartz, but without input collimation cannot provide sufficient resolution to reject closely adjacent source frequencies e.g. Cu Ka2 when using Cu Ka1. The presence of a collimator, however, reduces the overall sensitivity and the arrangement enables the collimator to be readily located or removed depending on whether a good beam profile or good sensitivity is required. <IMAGE>

Description

SPECIFICATION Diffracted X-ray beam monochromator This invention relates to an x-ray monochromator assemblyforfocussing a monochromatic beam of diffracted x-radiation at a detector.

The diffracted beam monochromator is a particu larly useful attachment to the routine powder diffractometer due to its ability to remove scattered primary white radiation and specimen fluorescence.

Although, when used with suitable apertures, the band pass of graphite crystals typically employed in these monoch romators is sufficient to reject diffracted radiation, they are not good enough to resolve, for example CuK a, from CuK a2. The major advantage of graphite crystals over crystals such as LiF (200) or quartz (1011) lies in its high diffraction efficiency. This high efficiency stems from the extremely mosaic nature of pyrolytic graphite which mosaic nature is also the reason for the rather wide band pass. (See "introduction to X-ray Spectrometry" - by Ronald Jenkins, Heyden, London, 1976, page 84). Thus, diffracted beam monochromators employing graphite crystals are typically considered as means of partial monochromatization.

Before the advent of the graphite crystal, monochromators were generally supplied with a LiF (200) crystal. Use of such a monochromatorcauses a loss of some 80% of the intensity of the characteristic diffracted beam and consequently every attempt was made to reduce further loss of efficiency of the device. As an example, no collimator was employed between specimen and detector. The lack of such collimation causes some deterioration of the diffracted beam profile distribution due to the increased axial divergence of the beam. Earlier monochromators were also provided with the capability of working with different wavelengths and suitable adjustments would allow their use with most of the target materials used in diffractometry.

With the modern trend to the use of high specific intensity, fine-focus copper anode tubes for most routine applications in inorganic and mineral analysis, the need for this versatility with its associated mechanical constraints has diminished.

Although the use of the collimator results in a highly improved profile shape of the diffracted beam, some loss in beam intensity nevertheless occurs thus resulting in reduced counting rate effi ciencywhen it is employed.

Since under some circumstances, good profile shape rather than counting rate efficiency is important while under other circumstances counting rate is more important, it is desirable to have a monochromator in which the collimator may be moved in and out of position in the path of the diffracted beam without changing the position of the crystal mono ch romator.

A special problem in the alignment of this type of diffractometer configuration is the accurate setting of the specimen to receiving slit distance, which adjustment is best done with the x-ray path energized.

It is an object of this invention to provide a monochromator assembly in which a collimator may be readily moved out of the diffracted beam without disturbing the monochromator crystal.

It is an additional object of this invention to allow easy adjustment of the specimen to receiving slit distance.

According to the invention there is provided a focussing crystal monochromator assembly for receving polychromatic x-radiation and focussing the resultant monochromatized x-radiation at a detector comprising: a base having first and second substantially linear portions forming an oblique angle therebetween; a first support member extending transversely outwardly from said base for fixedly supporting a monochromator crystal in the path of said polychromatic x-radiation; a second support member extending transversely outwardly from said base and having a channel therein for supporting a collimator, for limiting the axial divergence of the polychromatic x-radiation, slidably removable from the path of said polychromatic x-radiation.

The collimator can be a parallel plate assembly arranged in the path of the diffracted beam.

In an embodiment of the invention the monochromator assembly also comprises a support member for a receiving slit assembly which support member also extends transversely outwardly from said base and similarly to the support forthe collimator has a channel from which the receiving slit assembly is slidably removable from the path of the diffracted beam.

This embodiment may be modified, in that the support for the receiving slit assembly is movable along the base.

In addition according to another embodiment of the invention a single support member, movable along the base, supports both the receiving slit assembly and the collimator.

In a preferred embodiment of the invention the movable support is positioned between two post members which are rigidly secured to the base, the support being movable between the two post members along the base.

In an additional preferred embodiment the monochromator assembly of the invention includes manually releasable securing means for securing the collimator in a desired position in the support provided for the collimator.

The manually releasable securing means is preferably a detenting means.

An embodiment of the invention will now be described, by way of example, with reference to the accompanying drawings, of which: Fig. 1 is a schematic view of the optics of a monochromator assembly embodying the invention, Fig. 2 is a perspective view of the monochromator assembly embodying the invention as shown in Figure 1, attached to the 20 arm of a goniometer, and Fig. 3 is a perspective view of a preferred support for a collimator assembly and a receiving slit assembly utilized in the monochromator assembly of Fig.

2.

The following description of the preferred embod imentofthe invention is made with reference to the Figures.

As shown in Fig. 1 polychromatic x-radiation 20 containing for example CuK radiation is directed by means of a receiving slit and the slits of a collimator 6 such as a Soller collimator supported by a housing 21, onto a monochromator crystal 22 of graphite which is curved to follow a circle 25 in the plane of Figure 1 and whereby a single wavelength such as CuK radiation is diffracted to pass through a slit 23 onto a detector 24 such as a scintillation detector.

The structure of a preferred embodiment of the monochromator assembly of the invention, the optics of which are illustrated in Fig. 1 are as follows: As shown in Figs. 2 and 3, a base 1 formed of two substantially linear sections 2 and 3 joined together at an oblique angle, is attached to the 2 0 arm of a goniometer4 by an attachment which is not shown.

A support 5 for a collimator, one form of which is a parallel assembly 6 such as a Soller collimator, extends traversely outwardly from the base 1 and is positioned between two upward posts 7, permanently fixed to the base 1 of the monochromator assembly, by a screw 8 and is movable between the posts 7 by rotation of the screw 8. The support 5 is locked in position by a movable clip 9.

The collimator 6, the lower surface of which 10 has a detent receiving notch 11, is slid into a channel 12 of the support 5 and held in place through a spring loaded detented ball held in position by a screw only the head 13 of which is shown.

A receiving slit assembly 15 is held in place, in a similar fashion, in a channel 14 in support 5.

A monochromator crystal, not shown, is positioned in a support housing 16, which also extends transversely outwardly from the base 1, and serves to monochromatize polychromatic x-radiation coming through the collimator 6. Monochromatized x-radiation then passes via a detector coupler 17 to a detectorwhich is not shown in Figure 2.

In order to evaluate the performance of the monochromator assembly of the invention a series of measurements were made on an a-SiO2 (Novaculite, Arkansas Stone) specimen. As a monochromator crystal, there was employed a pyrolytic graphite sheet 18 x lOx 1 mm, Union Carbide Grade zya, bent to a radius of 223.5 mm.

As a collimator there was employed a Soller collimator comprising molybdenum foils spaced at 0.5 mm and having a total length equal to 5 mm.

The specimen was irradiated with x-radiation from a fine focus copper anode tube, 45kV 40 mA. As the detector a scintillation detector and pulse height selection was employed.

Slow scans were made over the (100) reflection to establish profile distribution and absolute intensity similar scans were made over the quartz quintuplet (212), (203), (301) to establish resolution and intensity. Measurements were made with and without the monochromator. When the monochromatorwas employed measurements were made both with and without the Soller collimator.

Table 1, which follows, shows the absolute count rates obtained on the a-SiO2 (100) reflection under various conditions. As will be seen from the table the use of the monochromator gives count rates comparable to that obtained with the beta-filter, that is about 20% less when the monochromator is used with the Soller collimator, and about 20% more when the monochromator is used without the Soller collimator.

TABLE COMPARISON OF PEAK IN TENSION ON a-SiO2 (100) REFLECTION WITH AND WITHOUTMONOCHROMA TOR.

a) No monochromator, no -filter 47,000c/s b) No monochromator, with P4ilter 23,000c/s c) With monochromator, no Soller collimator 28,500c/s d) With monochromator, Soller collimator in position 1 7,800c/s All measurements were done with fine focus copper anode tube, 45 kV 40 mA, and a scintillation detector with pulse height selection.

The effect of the removable Soller collimator on the profile shape is shown in Table 2. In this table measurements were made at 50, 30 and 10% of the peak intensity maximum. In each instance, the measurements were made to low and high angle sides of the 2 0 value corresponding to the peak intensity maximum.

As shown in the Table 2 the collimator has no significant effect on the high angle side of the profile shape. However, when the collimator is not employed there is a profile distortion on the low angle side which varies from a factor of about 1.1 at the 50% intensity point to 1.5 at 10%, in other words at the base of the profile.

TABLE 2.

PROFILE MEASUREMENTS ON THE a-SiO2 (100) REFLECTION.

Low Angle Side High Angle Side With Without With Without Collimator Collimator Collimator Collimator 50% 13mm 15mm 7mm 7mm 30% 19mm 29mm 9mm 9mm 10% 36mm 50mm 13mm 13mm It will be apparent that modifications of the apparatus shown can be made without departing from the scope of the invention as defined by the following claims.

Claims (12)

1. A focussing crystal monochromator assembly for receiving polychromatic x-radiation and focussing the resultant monochromatized x-radiation at a detector comprising: a base having first and second substantially linear portions forming an oblique angle therebetween; a first support member extending transversely outwardly from said base for fixedly supporting a monochromatorcrystal in the path of said polychromatic x-radiation; a second support member extending transversely outwardly from said base and having a channel therein for supporting a collimator, for limiting the axial divergence of the polychromatic x-radiation, slidably removable from the path of said polychromatic x-radiation.

2. The monochromator assembly of claim 1 wherein the collimator is a parallel plate assembly.

3. The monoch romator assembly of claim 2, wherein said support member for said plate assembly comprises manually releasable securing means for securing said collimator plate assembly at a desired position in said channel.

4. The monochromator assembly of claim 3 wherein said manually releasable securing means comprises at least one detenting means.

5. The monochromator assembly of claim 4 wherein said detenting means is a spring loaded detented ball.

6. The monochromator assembly of any one of claims 1 to 5 wherein a support member for a receiving slit assembly extends transversely outwardly from said base and has a channel for supporting a receiving slit assembly slidably removable from the path of said x-radiation.

7. The monochromator assembly of Claim 6 wherein the support member for the receiving slit assembly is movable along said base.

8. The monochromator assembly of Claim 7 wherein a single movable support member is employed for both the receiving slit assembly and the collimator.

9. The monoch romator assembly of Claim 7 or Claim 8 wherein said movable support member is positioned between two post members rigidly secured to said base and is movable from one to the other of said post members along said base.

10. The monochromator assembly of Claim 9 wherein the movable support member is secured to said post members by fastening means extending through said post members and said movable member.

11. The monochromator assembly as claimed in any one of the preceding claims wherein the monochromator crystal is formed of graphite.

12. A focussing crystal monochromator assembly for receiving polychromatic X-radiation and focussing the resultant monochromatised X-radiation at a detector, substantially as herein described with reference to the accompanying drawings.