CA1135879A

CA1135879A - Diffracted beam monochromator

Info

Publication number: CA1135879A
Application number: CA000342873A
Authority: CA
Inventors: Ronald Jenkins
Original assignee: North American Philips Corp
Current assignee: Philips North America LLC
Priority date: 1979-01-05
Filing date: 1980-01-02
Publication date: 1982-11-16
Also published as: JPS55112554A; DE3000122A1; FR2446000A1; GB2040149B; GB2040149A; AU5431780A; FR2446000B1

Abstract

Abstract According to the invention a novel crystal monochromator assembly is provided which is particularly useful for monochromatizing a diffracted beam of x-radiation in which a collimator is employed which may be moved in and out of position of the diffracted beam without disturbing the monochrom-ator crystal. Additionally, a simple means is provided for adjusting the receiving slit to specimen distance while the x-ray path is energized. The monochromator assembly of the invention comprises firstly a base having first and second essentially linear portions joined together at an oblique angle, with a support member extending transversely outwardly from the base for supporting a monochromator crystal in the path of the diffracted beam.
An additional support member also extends transversely outwardly from the base and has a channel for supporting a collimator such as a parallel plate assembly in the path of the diffracted beam. The collimator is supported in the channel in such a manner that it is slidably removable from the path of the diffracted beam.

Description

27~ 1979 1 P~ 20.893 Diffracted beam monochromator.

Background of the invention.
This invention relates to an x-ray monochro mator assembly for focussing a monochromatic beam of diffracted x-radiation at a de-tector.
The diffracted beam monochromator is a par-ticularly useful attachment to the routine powder dif-fractometer due to its ability to remove scattered primary white radiation and specimen fluorescence. ~lthough, when used with suitable apertures, the band pass of the graphite 1Q crystals typically employed in these monochromators is sufficien-t to reject diffracted radiation, they are not good enough to resolve, for example CuK ~1 from Cu~ ~2.
The major advantage of graphite crystals over crystals such as LiF (200) or quartz ( 1011 ) lies in its high dif-lS fraction efficiency. This high efficiency stems from theextremely mosaic nature of pyrolytic graphite which mosaicity also is the reason for the rather wide band pass.
(See l'introduction to X-ray Spectrometry" - by Ronald Jenkins, Heyden, London, 1976, page 843~ Thus, diffracted beam monochromators employing graphite crystals are typi-cally considered as means of partial monochromati~ation.
Before the advent of the graphite crystal, monochromators were generally supplied with a LiF (200) crystal. Use of such a monochromator causes 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 dis-tribution due -to the increased axial divergence of the beam. Earlier monochro~
mators were also provided with the capability of working with different wavelengths and suitable adjustments would .~ ''` ' ~

..

~3S~

27~ 1979 2 PHA 20.893 allow their use with most of the target materials used in diffractome-try. With -the modern trend to the use of high speclfic intensity, fine-focus copper anode -tubes for most routine applications in inorganic and mineral analysis, the need for this versatili-ty wi-th its associ-ated mechanical constraints has diminished.
Although the use of the collimator results in a high y improved profile shape of the diffracted beam, still some loss in beam intensity occurs thus re-sulting in reduced counting rate efficiency when :Lt isemployed.
Since under some circumstances, good profile shape rather than counting rate efficiency is important while under other circumstances counting rate is more im-portant it is desirable to have a monochromator in whichthe collimator may be moved in and out of position in the path of the diffracted beam without changing the position of the crystal monochromator.
A special problem in the alignment of this type of diffractometer configuration is the accurate set-ting of the specimen to receiving sli-t distance, which adjustment is best done with the x-ray path energized.
Summary of the invention.
It is an object of this invention to provide a monochromator assembly in which a collimator may be readi]y moved out of the diffracted beam without disturb-ing the monochromator crystal.
It is an additional object of this invention to allow easy adjus-tment of the specimen -to receiving sli-t distance.
According to the invention a novel crys-tal monochromator assembly is provided which is partlcularly useful for monochromatizing a diffracted beam of x-radiat-ion in which a collimator is employed which may be moved in and out of posi-tion of the diffracted beam without dis-turbing the monochromator crys-tal.
Additionally, a simple means is pro~ided for adjus-ting the receiving slit to specimen distance while ~S~75~

27~ 1979 3 Pl~ 20.893 the x-ray path is energized.
The monochromator assembly ;~ the invention comprises firstly a base having a first and second essen-tially linear portions joined together at an oblique angle, with a support m~mber ex-tending transversely outwardly from the base for supporting a monochromator crystal in the path of the diffracted beam. An additional support rnember also extends transversely outwardIy from the base and has a channel for supporting a collimator such as a parallel plate assembly in the path of the diffracted beam.
The collimator is supported in the channel in such a manner that it is slidably removable from -the pa-th of the dif-fracted beam.
In an embodiment of the invention the mono-chromator 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 for the 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, according to the invention, in that the support for the receiving slit assembly is movable along the base.
In addition according to still another embo-diment 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 pos-t members which are rigidly secured to the base, the support being movable between the two post members along the baseO
In an additional preferred embodiment -the monochromator assembly of the invention includes manually releasable securing means for securing the colllmatcr in a desired position in the support provided for the colli-mator.
The manually releasable securing means ispreferably a detenting means~

.' - " , , . , , . - ~; . . , ~3S~

. .

27~ 1979 ~ PHA 20.893 Description of the drawings.
Fig. 1 is a schematic view of the optics of the monochromator assembly of the invention, Fig. 2 is a perspec-tive view of the monochro-mator assembly of th0 invention attached to 2 0 arm of agoniometer, 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.
Descr:iption o~ the preferred embodiment.
The fo:llowing description of the preferrecl embodiment of the invention is made with reference to the ; Figures.
As shown in Fig. 1 polychromatic x-radiation conta~ning for example CuK ~ radiation is directed by means of a receiving sli-t and the slits o~ a collimator such as a Soller collimator to a monochromator crys-tal where a single wavelength such as Cu~ o~ radiation is dif- -fracted to a detector such as a scintillation detector.
The structure of a preferred embodim0nt of the monochromator assembly of the invention, -the op-tics of which are disclosed in Fig. 1 are as follows:
As shown in ~igs. 2 and 3, a base 1 formed of two essen-tially linear sections 2 and 3 is joined together at an oblique angle and is attached to the ~Oarm of a goniometer 4 by an;a-ttachment which is not shown.
Support 5 for a collimator, one form of which is a parallel plate assembly 6 such as a Soller collimator, extends tra-versely outwardly from the base 1 and is positioned betwe0n two upward pos-ts 7, perman0ntly fixed to base 1 of the monochromator assembly, by screw 8 and is movable between the posts 7 by rotation of the screw 8. The support 5 is locked in position by movable clip 9.
The collimator 6, the low0r surface of which 10 has a detent receiving notch 11, is slid into channel 12 of 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.

1~3~

27~ 197~ 5 PHA 20.8~3 A recei~ing slit assembly 14 is held in place, in a similar fashion, in channel 15 in support 5.
A monochromator crystal, not sho~n, is posi~
tioned in suppor-t 16 which also extends transversely out-wardly from base I and serves to monochromatize polychro-matic x-radiation coming through the collimator 6. Mono-chromatized x-radiation then passes via detector coupler 17 to a detector which is no-t shown.
In order to evaluate the performance of the i l monochromator assembly o~ the invention a series o~
measurements were made on an ~-SiO2 (Novaculite, Arkansas Stone) specimen. As a monochromator crystal, there was employed a pyrolytic graphite sheet 18 x 10 x 1mm, Union Carbide Grade zya, bent to a radius of 223.5 mm.
As a collima-tor there was~ employed a Soller collimator comprising molybdenum ~oils spaced at 005 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 de-tector a scintillation detector and pulse height select-ion was employed.
Slow cans were made over the (100) reflection to es-tablish profile distribution and absolute intensity plus over the quart~ quintuplet (212)~ (203), (301) -to , ~5 establish resolution and intensity. Measurements were made with and withou-t the monochromator. When the monochromator was employed the measurements were made both and without the Soller collimator.
Table 1, which follows, shows the absolute count rates obtained on the CL -SiO2 (100) reflection under various condi-tions. As will be seen from the table the use of the monochromator gives count rates compar~ble to that obtained with -the beta-filter, tha-t is abou-t 20% less when the monochromator is used with the Soller collimator, and about 20% more when the monochromator is used with the Soller collimator.

'--~

~35~7~

27~ 1979 6 P~ 20.893 TABLE I.
COMPA~o3LD` ~ r~l~2~NSION ON ~ -Si02 (100) REFLECTION
WITH AND WITHOUT MONOCHROMATOR.

a) No monochromator, no 0 filter 47,oooc/s b) No monochromator, with ~ --fil-ter 23,000c/s c) With monochromator, no Soller collimator 28,500c/s d) With monochromator, Soller collimator in posi-tion 17,800c/s .i tO
All measurements done with flne focus copper anode tube~
4s kV 40 mA.
Scintillation detec-tor with pulqe helght selec-tion.
. .
15 ~ The effect of the removable Soller collimator on the pro~ile shape is shown in Table 2. In this table measurements were made at 50, 30 and 10% of the peak iIl-tensity~ maxim-um. In each instance, the measurements were made to low and high angle sides of the 2 ~ value corre-sponding to the peak intensity ma~imum.
As shown in the Table 2 the collimator has no signi~icant e~fect on the high angle side of the profile shape. However, when the collimator is not emploved there is a profile distortion on the Iow angle side which varies from a factor of about 1.1 at the 50/0 intensity point to 1.5 a-t 10%~ in other words at the base of the profile.

TABLE 2.
PROFILE MEAS REMENTS ON THE ~-SiO2 ~ .
Low Angle Side High Angle Side With Without With Without Collimator Collimator Collimator Collimator 50/0 13 mm l5 mm 7 mm 7 mm 30% 19 mm 29 mm 9 mm 9 mm 10% 36 mm 50 mm 13 mm 13 mm 1135i~7~

27~ 1979 7 PHA 20.893 It will be apparent that many modifications of the appara-tus shown can be made wi.thout departing from the scope of the invention as definecl by the following claims.

,.~ 10 : :15 :

:
:

:
: : : 25 : :: :

: : 30 : : :
:
.

: 35 . .

:
,. .. .

Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

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 objique angle therebetween;
a first support member extending transversely outwardly from said base for fixedly supporting a monochromator crystal in the path of said poly-chromatic 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-radi-ation.

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

3. The monochromator 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 claim 2 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 the polychromatic 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 monochromator assembly of claim 7 wherein the support member for the receiving slit assembly 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 ex-tending through said post members and said movable member.