The figure depicts an apparatus for measuring x-ray diffraction (XRD) and/or x-ray fluorescence (XRF) in a specimen of material. The specimen could be, for example, a standard XRD powder sample of a mineral, the elemental composition of which one seeks to identify. It is common practice to characterize samples in terms of both XRD and XRF, but heretofore, it has been necessary to use separate XRD and XRF apparatuses.
The apparatus (see figure) includes a standard x-ray emitter - preferably one that generates x-rays at a number of photon energies. The x-ray beam from the source passes through a collimator, and the collimated beam passes through a band-pass filter, so that the sample is irradiated by a collimated, monochromatic beam.
The irradiated specimen emits mainly two kinds of x-rays: (1) primary (diffraction) photons at the incident photon energy; and (2) secondary (fluorescence) photons, which have lower energies. The photons emitted by the specimen travel to a charge-coupled device (CCD) containing a two-dimensional array of pixels, wherein the photons are detected. In general, the CCD pixel outputs depend on both the fluorescence spectrum and on the diffraction pattern projected onto the array of pixels. By suitable choice of the mode of operation, one can extract diffraction or fluorescence information from the CCD pixel outputs, as explained below.
Typical CCDs rival traditional Si(Li) detectors with regard to energy resolution and sensitivity in the energy range of 0.2 to 10 keV. Taking advantage of this characteristic, the CCD can be operated in a photon-counting mode; the CCD can be interrogated at such short intervals that in each successive interrogation, the output comprises indications of the energies of individual photons incident on single pixels. Taking further advantage of this characteristic, the CCD pixel outputs can be processed to select only those signals in a desired photon-energy range. The CCD pixel outputs are processed partly in a controller and partly in a microprocessor connected to a display unit.
The apparatus can be operated in any of several distinct modes, of which four are described below:
- The sample is irradiated with a monochromatic beam and only the diffraction pattern is of interest. To discriminate against fluorescence photons, the CCD and the processing circuitry are operated so that only photons at the primary beam energy are counted in computing the diffraction pattern.
- The sample is irradiated with a monochromatic beam and both diffraction and fluorescence are of interest. The CCD and processing circuitry are operated to measure both the primary and lower-energy photons. The diffraction pattern is extracted from the signals at the primary photon energy, while the fluorescence spectrum is extracted from the signals at lower photon energies.
- The irradiating beam is monochromatic and only the fluorescence spectrum is of interest. In this mode, CCD outputs at the primary beam energy are rejected, and only the lower-energy signals are used in calculating the fluorescence spectrum.
- The x-ray beam is polychromatic (a band-pass filter is not used), and only a particular diffraction pattern is of interest. In this mode, the CCD and processing circuits are operated to detect only diffraction at selected multiple beam energies and thus to discriminate against photons at all other energies. This mode is useful for diffraction experiments in which there is a need for fine adjustment of the x-ray beam energies to avoid strong absorption in the samples.
This work was done by David F. Blake, Charles Bryson, and Friedemann Freund of Ames Research Center.
This invention has been patented by NASA (U.S. Patent No. 5,491,738). Inquiries concerning nonexclusive or exclusive license for its commercial development should be addressed to
the Patent Counsel
Ames Research Center
Refer to ARC-12043.