X-ray or gamma ray systems or devices – Specific application – Fluorescence
Reexamination Certificate
2000-12-21
2002-05-21
Church, Craig E. (Department: 2882)
X-ray or gamma ray systems or devices
Specific application
Fluorescence
Reexamination Certificate
active
06393093
ABSTRACT:
The invention relates to an X-ray analysis apparatus which includes:
a sample location for accomodating a sample to be analyzed,
supply means for conducting analyzing X-rays to the sample,
an energy dispersive detector which is provided with a detector surface for the detection of X-rays generated in the sample and is arranged relative to the sample in such a manner that the detector receives the X-rays from the sample at a comparatively large solid angle,
the supply means including at least one X-ray conducting capillary which is inserted through an opening in the detector surface.
An apparatus of this kind is known from the published German patent application No. 197 24 660 A1.
Generally speaking, X-ray analysis of materials has two techniques available for the detection of the X-rays generated in the sample to be examined, that is, energy dispersive detection (“Energy Dispersive X-ray Detection or EDX”) and wavelength dispersive detection (“Wavelength Dispersive X-ray Detection or WDX”). Each of these detection techniques has its specific advantages and drawbacks as will be described in detail hereinafter.
For each photon absorbed in the detector an energy dispersive detector supplies a current pulse whose charge contents are equal to the energy of the photon. These current pulses can be electronically selected in respect of charge contents, so that for all current pulses the number of current pulses of a given charge contents (i.e. the intensity) is determined in one measuring period in dependence on the charge contents (i.e. the energy of the photon). Because the energy of a photon of X-rays is inversely proportional to the wavelength of this radiation, the intensity of the X-rays incident on the detector is thus determined as a function of the wavelength. This type of detector includes, for example the known Si(Li) detector. Even though this detector has a rather favorable signal-to-noise ratio in comparison with other energy dispersive detectors (such as a gas-filled detector), this ratio is still comparatively large for small charge contents (i.e. long wavelengths). This is due to the fact that the spread in the charge contents Q for one given photon energy is proportional to Q; this effect, therefore, increases towards a low Q. This means that in practice X-rays excited by elements having an atomic number lower than 11 cannot be measured, or only with great difficulty, by means of an energy dispersive detector. (For this problem see also “Principles and Practice of X-ray Spectrometric Analysis”, 2
nd
ed. by Eugene P. Bertin, Plenum Press, New York-London, chapter 6, paragraph 4.)
Each photon in a detector of the wavelength dispersive type is converted into an electric pulse whose pulse amplitude and/or charge contents are irrelevant to the energy resolution. Thus, this detector determines exclusively the number of photons. Such a detector is formed, for example by an assembly which successively consists of a Soller slit, an analysis crystal and an X-ray count tube. The Soller slit selects the radiation of the desired direction from the beam emanating from the sample; this radiation is subsequently incident on the analysis crystal. In conformity with the known Bragg relation, this crystal reflects only approximately one wavelength, that is, the wavelength that fits the angle of incidence (and a close vicinity thereof, for example, 0.25°) of the selected radiation. The entire desired interval of angles of incidence is traversed by rotating the analysis crystal during the measurement, so that the associated interval of wavelengths is also traversed. The relationship between the radiation intensity (being proportional to the count rate of the count tube) and the wavelength is thus established. Because the radiation applied to the analyzer crystal must be exactly parallel, the crystal is preceded by a collimator, for example a Soller slit. The parallelization of the radiation emanating from the sample strongly reduces its intensity.
As appears from the foregoing, it is a drawback of the WDX detection method that it requires a comparatively complex analysis device and that, because of the reduced X-ray intensity on the detector, the measuring times in such a device are comparatively long. In comparison with WDX an EDX detection method offers the advantage that the construction of the analysis device may be comparatively simple and that comparatively short measuring times are possible. It is a drawback of such a detection method, however, that it enables only a comparatively low maximum count rate only, that is, approximately ten times lower than that of a WDX detector. This comparatively low maximum count rate for a EDX detectors is mainly due to the electronic reading out of the detector.
The cited German patent application describes an X-ray analysis apparatus in which an EDX detection method is used. The analyzing X-rays are applied to the sample in the known apparatus by means of an X-ray conducting capillary which is inserted through an opening in the detector surface. The capillary is aimed at the sample to be analyzed which is arranged so that its surface extends parallel to the detector surface of the energy dispersive detector. Because an X-ray conducting capillary has a small cross-section, small areas of the sample can be selectively irradiated by means of such a capillary, so that a suitable position resolution is achieved on the sample. Furthermore, the detector is arranged near the sample so that the detector receives the X-rays from the sample at a large solid angle. As a result, practically all X-rays emanating from the sample are detected by the detector, so that an as low as possible intensity of the analyzing X-rays suffices. This is of importance notably in the case of samples which are susceptible to radiation damage, for example integrated electronic circuits.
The known poor energy resolution of the known EDX detectors constitutes a further drawback of such detectors. By way of illustration it can be stated that the energy resolution of a conventional EDX detector is of the order of magnitude of 120 eV whereas that of a WDX detector is of the order of magnitude of 30 eV. This is a pronounced drawback notably for chemical elements having a low atomic number (for example, lower than the atomic number 14). This is because such elements have a characteristic radiation of low energy, that is, of the order of magnitude of from 400 eV to 1500 eV. In this energy range said poor resolution may readily give rise to overlap with the characteristic radiation of heavier elements, for example, the M lines of elements having an atomic number 50 or higher, or, for example, the L lines of copper. This impedes the measurement of spectral X-ray lines of said light elements in this range.
The described drawbacks of the known EDX detectors are of importance notably for the execution of measurements on integrated electronic circuits in which frequently light elements such as boron, nitrogen, oxygen, fluorine and aluminium have to be measured. The metals copper and tungsten are frequently present in such environments and their characteristic M radiation could make measurements impossible in practical circumstances.
It is an object of the invention to provide an X-ray analysis apparatus of the kind set forth which is suitable for the measurement of low-energetic X-rays with a suitable energy resolution while preserving the described advantages (such as a suitable position resolution on the sample, a comparatively simple construction and a low radiation load for the sample). To this end, the apparatus according to the invention is characterized in that the energy dispersive detector is constructed as an X-ray sensitive charged coupled device array. The invention is based on the recognition of the fact that nowadays CCD arrays are available that are suitable for use with visible light and also for the detection of soft X-rays. Such CCD arrays have an energy resolution of the order of magnitude of 90 eV; this represents a distinct improvement in comparison with the resolution
Dirken Marinus Willem
Jans Johannes Cornelis
Van Den Hoogenhof Waltherus Wilhelmus
Church Craig E.
Koninklijke Philips Electronics , N.V.
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