X-ray or gamma ray systems or devices – Specific application – Fluorescence
Reexamination Certificate
2001-10-30
2002-11-26
Kim, Robert H. (Department: 2882)
X-ray or gamma ray systems or devices
Specific application
Fluorescence
C356S327000, C250S393000
Reexamination Certificate
active
06487269
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to apparatus for analysing a sample and in particular to apparatus for performing energy dispersive X-ray fluorescence.
DESCRIPTION OF THE PRIOR ART
Energy Dispersive X-ray Fluorescence (EDXRF) is a powerful analytical technique that is capable of simultaneous analysis of a wide range of elements in a variety of different sample types.
EDXRF uses an X-ray spectrometer which includes an X-ray tube for generating primary X-rays which are used to expose a sample material. The spectral output from the X-ray tube usually consists of characteristic X-ray lines of the anode material superimposed on a background continuum. The sample emits fluorescence radiation characteristic of the materials within the sample and an X-ray detector is then used to detect this radiation.
However, the detector, with its associated electronics, has to process the entire X-ray spectrum emitted from the sample and the measurement efficiency decreases with increasing count rate. The primary X-rays incident on the sample material can be scattered from the sample so as to also impinge on the detector. These scattered X-rays then contribute to the overall count rate detected by the detector, thereby reducing the measurement efficiency and thus the precision of the measured readings of the sample composition. This is especially true for low atomic number samples such as organic materials or oxides where the matrix will very efficiently scatter radiation. In most cases it is possible to reach the count rate limit of state-of-the-art processing electronics using only modest power levels from conventional X-ray tubes.
When using this direct excitation technique, it is possible to remove much of the background continuum by placing a suitable absorption filter between X-ray tube and sample to improve the quality of the primary excitation. This is usually the most efficient excitation method for the analysis of major and minor concentration levels. However, at trace levels the elements of interest may represent only a small proportion of the total spectrum and other methods are used to increase this proportion to improve the precision for trace element analysis.
A number of techniques are commonly used in EDXRF to improve analysis at trace levels, including primary filtration, secondary targets, polarisation and, crystal diffraction.
In the case of secondary targets, primary radiation from the X-ray tube is used to fluoresce a secondary target formed from a pure material, such as a metal. The secondary radiation emitted by the pure material is then used to excite the sample.
Because the secondary target is formed from a pure material, the emitted secondary radiation has a very specific profile which is characteristic of the target material and which is generally different from the anode material used in the X-ray tube. Furthermore, much of the primary radiation, including the background continuum, is absorbed by the secondary target. Accordingly, when compared with the primary radiation from the tube (which has continuum and characteristic radiation determined by the tube potential and the tube anode material), the secondary radiation is dominated by essentially monochromatic emission characteristic of the secondary target that is much more intense than the scattered continuum and characteristic radiation from the X-ray tube.
However, the secondary radiation usually has a much lower intensity, making secondary target excitation much less efficient than direct excitation. Furthermore, the range of elements efficiently excited by the monochromatic emission from the secondary target is limited. It is therefore usual to have a number of different secondary targets for general multielement analysis.
When the characteristic radiation from the secondary target strikes the sample it can also be scattered into the X-ray detector and this scattered radiation has to be processed by the detection system. This restricts the maximum count rate that can be achieved from the elements of interest. Furthermore, scattered radiation which has lost some energy compared to the incident radiation (“inelastic scatter”) contributes to the background on the low energy side of the scattered characteristic peak. These effects degrade the lower limit of detection for certain elements in light matrices.
The use of polarisation in X-ray spectrometry is described in detail by R. W. Ryon, J. D. Zahrt in “Polarised Beam X-ray Fluorescence”, Handbook of X-ray Spectrometry Ed1, Chapter 10 and is the subject of U.S. Pat. No. 3,944,822. In this scenario X-rays become plane polarised after they are scattered through 90°. The two most common methods used in EDXRF to polarise the beam from the X-ray tube are Barkla scatter from a low atomic number material, and Bragg diffraction from a crystalline substance.
When the three beams from X-ray tube to polariser, polariser to sample and sample to detector are positioned in a Cartesian (xyz) geometry the polarised X-ray photons scattered from the polariser have a low probability of scattering at a right angle from the sample into the detector. However, characteristic fluorescence radiation from elements in the sample is not polarised and therefore will be detected.
In theory, this is a very effective method of eliminating background radiation. However the tight collimation required to constrain the 90° beams can result in a very low intensity of radiation reaching the detector making it difficult to obtain precise measurements.
The degree of polarisation and intensity are inversely related. Accordingly, in practice, a compromise is usually necessary and complete polarisation is sacrificed by opening up the collimation to give a reasonable count rate but still a very significant reduction in scattered radiation.
The document entitled “The comparison of three excitation modes in EDXRF”, Adv. X-ray Anal. 35 (1992), 1001-1007 by Kanngiesser et al compares different modes of excitation in EDXRF. One of the conclusions of this document was “the lower detection limits [for Barkla polarisation] were achieved in spite of the poorer peak-to-background ratios. The reason for this is the absence of the strong (Rayleigh and Compton) scattering peaks in the case of secondary excitation, which aggravate the electronics without contributing to the analysis”.
Barkla polarisers must have a low atomic number and high density to give greatest scattering efficiency and materials such as B
4
C and C (amorphous graphite) are commonly used for low energy X-rays, whilst Al
2
O
3
is used for higher energies. The scattered radiation is polychromatic and therefore suitable for multielement applications.
The choice of Bragg polarisers is more restricted because of the requirement to diffract X-rays at a Bragg angle (2&thgr;) of 90°. One of the most promising materials is highly oriented pyrolytic graphite (HOPG) which has an exceptionally high integral reflectivity. HOPG is particularly useful in combination with X-ray tube anode materials such at Rhodium (Rh) and Palladium (Pd) which are widely used in EDXRF. The L&agr; lines produced from these particular anode materials diffract at Bragg angles very close to 90° i.e. Rh L&agr; (2.696keV) at 86.5° and Pd L&agr; (2.838 keV) at 81.20. The polarised radiation diffracted from HOPG is essentially a series of monochromatic lines at multiples of the predominant first order energy. The higher order lines are weaker but extend the energy range for excitation. HOPG also acts as Barkla scatterer which increases its polarising properties. High energy Barkla scatter can be further enhanced by fixing the HOPG onto a pure Al or Al
2
O
3
substrate.
Another very important property of HOPG is that it can be formed into various shapes using special techniques described by I. G. Grigorieva and A. A. Antonov in “HOPG as a powerful X-ray Optic”, Proceedings of European Conference on EDXRS-98, 115-119 (1998). Intensities can be increased by up to an order of magnitude by using a Johann semi-focusing geometry with singly bent (cylindrical) or doub
Kiknadze Irakli
Kim Robert H.
Oxford Instruments Analytical Limited
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