X-Ray analysis apparatus having an adjustable stray radiation slit

In an X-ray analysis apparatus, there is provided between a test specimen (5) or an analyzing crystal and a detector (9) a stray radiation slit (15) that can be adjusted in dependence on the goniometer angle. By an optimum adjustment of this slit in correspondence with the variation in the goniometer angle it can be achieved that the detector will always see the same portion of the surface of the test sample or analyzing crystal. Especially, the adjustment of the slit is coupled to the adjustment of an automatic divergence slit (11) so that the portion of the surface which is irradiated, remains unaltered. Especially for small goniometer angles, that is to say for the analysis of a substance in which there is a large distance between crystal planes, a considerably improved signal-to-noise ratio in the measurement signal is thus obtained.

The invention relates to an X-ray analysis apparatus including an X-ray 
source, a goniometer with an angle-adjusting mechanism and an X-ray 
detector. 
Such an X-ray analysis apparatus in the form of an X-ray diffractometer is 
known from U.S. Pat. No. 3,852,594. The apparatus described therein 
comprises a goniometer with an angle-adjusting mechanism for orienting a 
test sample, to which an angular adjustment for an entrance slit for the 
X-ray beam, sometimes called a divergence slit, is coupled so that the 
same portion of the surface of the sample is irradiated throughout a range 
of incidence angles to be used during a measurement. 
In such an apparatus, especially at smaller angles of incidence, a 
detrimental effect is produced by background radiation, as a result of 
which the signal-to-noise ratio of the measurement is adversely affected. 
The invention has for its object to provide an X-ray analysis apparatus, in 
which, especially for the range of smaller angles of incidence, an 
improvement in the signal-to-noise ratio of the measurement signal is 
effected. According to the invention, an X-ray analysis apparatus of the 
kind mentioned in the opening paragraph is therefore characterized in that 
between the goniometer and the detector there is provided a stray 
radiation slit that can be adjusted in correspondence with the angular 
adjustment of the goniometer. Here and hereinafter a goniometer is assumed 
to be an angle-adjusting mechanism either for the sample stage of an X-ray 
diffractometer or for the analysing crystal of an X-ray crystal 
spectrometer. 
By adjusting the stray radiation slit in accordance with the adjustment of 
the divergence slit it can be achieved that the detector will always see 
substantially the same part of the surface of the test sample through the 
stray radiation slit throughout the entire range of angles of incidence. 
In a practical construction, this part of the surface will be adjusted to 
correspond to that part of the surface which is directly irradiated. At 
smaller values of the incidence angle, consequently a considerably smaller 
amount of the surface will be seen than with a fixed slit which will be 
adjusted so that at large angles of incidence the whole of the irradiated 
part of the surface can be seen. Consequently, especially at smaller 
angles of incidence, a considerable improvement in the signal-to-noise 
ratio of the measurement signal is provided, as a result of which 
especially in this operational region, measurements can be made with 
greater accuracy and with greater sensitivity. 
In a preferred embodiment according to the invention, an adjustment 
mechanism for the goniometer ensures directly by means of a mechanical 
coupling, an angular adjustment of the stray radiation slit and preferably 
also directly an angular adjustment of the divergence slit. As a result, 
an optimum signal-to-noise ratio is attained for each value of the 
incidence angle. The coupling mechanism can then be constructed in 
accordance with the mechanism described in U.S. Pat. No. 3,852,594 and 
which could take the form of a dual mechanism. In an X-ray diffractometer, 
it is favourable to arrange the two slits at about the same distance from 
the sample. The slits and the adjustment mechanisms for the two slits can 
then be made to have substantially the same form of construction. 
In a further preferred embodiment, both adjustments are correlated with the 
angular adjustment of the goniometer controlled from the central control 
unit. The advantage is then obtained that it is possible to depart from 
the geometrically fixed proportions. For a given range of incidence 
angles, for example, a greater or a lesser improvement can be provided 
with respect to other ranges. For example, the effective slit width of the 
stray radiation slit may be varied non-linearly with the effective slit 
width of the divergence slit. For desired ranges, for example, the noise 
can then be reduced to an even greater extent, albeit at the expense of 
some signal strength, or the dynamic range of the output signal can be 
reduced.

Referring to a diffractometer according to the invention, FIG. 1 shows an 
X-ray source 1, a test sample 5 arranged with a goniometer 3 and a 
detector 9 provided with a detector slit 7. Between the source 1 and the 
sample 5 there is located a rotatable divergence slit 11 with an adjusting 
mechanism 12. As described in U.S. Pat. No. 3,852,594, the rotation of 
this slit is made to correspond with the rotation of the test sample which 
is to be carried out by means of the goniometer for measurement purposes. 
Thus, it is achieved that an X-ray beam 13 emitted by the source will 
irradiate the same amount of the surface of the test sample even when the 
goniometer angle .theta. is varied. Between the test sample and the 
detector slit 7 there is present in known diffractometers a fixed stray 
radiation slit not shown in the Figures. This slit determines for the 
detector an aperture angle .alpha., as a result of which for each 
goniometer angle at least the whole of the irradiated portion of the 
surface of the test sample will remain visible. The width of the slit is 
then chosen in the usual manner so that at large goniometer angles the, 
then apparently extensive, irradiated surface of the test sample is seen 
by the detector. At decreasing values of the goniometer angle .theta., a 
gradually increasing part of the remainder of the surface of the test 
sample will become visible to the detector in addition to the directly 
irradiated portion of the surface. As a result, a comparatively large 
quantity of stray radiation can reach the detector, which will adversely 
affect the signal-to-noise ratio in the measurement signal. When in this 
arrangement the stray radiation slit is also made adjustable, which in 
principle limits the angular aperture, and which for all values of the 
goniometer angle .theta. makes visible to the detector only the directly 
irradiated portion of the surface, a considerable improvement is obtained. 
A stray radiation slit 15, which according to the invention is provided 
with an adjusting mechanism 16, may be adjusted, for example, completely 
in accordance with the arrangement used to adjust the divergence slit 11 
in the known apparatus. Starting from the angular displacement means of 
the goniometer, the two slits are then adjusted synchronously by means of 
the direct mechanical coupling 18. This adjustment need not be a rotation, 
but may alternatively be provided by opening the slits to a greater or 
lesser extent or, albeit rather less conveniently, by displacing the slits 
along the direction of the main ray of the X-ray beam. If the two 
adjustments are not to be coupled directly in operation, both slits can be 
adjusted in correspondence with the goniometer adjustment from a central 
control unit 17 according to a selected program. In order to reduce the 
dynamic range in the signal to be detected, a small variation in the 
irradiated amount of the surface of the test sample can be effected for 
this purpose. 
The position of the stray radiation slit 15 can be chosen almost at will. 
The advantage of making the distances from the test sample to the 
divergence slit 11 and to the stray radiation slit 15 the same, has 
already been pointed out. A position of the stray radiation slit close to 
the test sample would result in a greater margin of adjustment due to the 
fact that the width of the beam is greater there. A position close to the 
detector, in which case an alternative position between the detector slit 
and the detector would be permissible, can result in better screening from 
stray radiation, but would require more accurate adjustment. Corresponding 
considerations apply to the positioning of a stray radiation slit with 
respect to an analysing crystal in an X-ray spectrometer. 
In FIG. 2, the relative intensity I as a function of the goniometer angle 
2.theta. is shown by a graph. Measurement results from an apparatus 
according to the prior art are represented by a curve a and measurement 
results from an apparatus according to the invention are represented by a 
curve b, both curves being drawn, of source, to the same scale. 
Especially for goniometer angles 2.theta. up to about 60.degree., the 
advantage will be clearly apparent. As a result, the analysis of test 
samples, especially of substances having a comparatively large d-value, 
that is to say a large relative distance between crystal planes, has 
become considerably more accurate whereas the mere presence of these very 
substances could only be indicated by other prior methods with 
comparatively great difficulty due to the low signal-to-noise ratio that 
can be attained in analysis thereby.