| º Application. X-ray absorption spectroscopy for elements P and above.
º Detection limits. 100 ppm and higher depending upon sample matrix.
º Accessories. E-yield detector, hot/cold sample holder.
In use at synchrotron laboratories world wide.
This simple detector is based upon collection and amplification of electrons created by the absorption of x-rays within the gas of an ionization chamber. The ionization gas is chosen appropriately to absorb the fluorescent x-rays and flows slowly through the detector at atmospheric pressure. The electron current is amplified by a close-coupled and shielded, variable gain, electrometer-type Op Amp (included) to a voltage and current level that may be connected directly to your Analog-to-Digital converter. Alternatively, the included amplifier may be disconnected and the ion chamber connected directly to your current amplifier (such as a Keithley) operated at
109 to 1011 gain; however, the noise level will be higher due to the noise picked up in the connecting coax cables. Since this device is operated in the current mode, it is ideal for applications on modern, high intensity beam lines.
Fig. 1 Photograph of basic ion chamber detector, amplifier power supply on the left. The ruler gives an indication of size. Actual sample detector size is 15.5 cm along the x-ray beam direction and 22 cm perpendicular to the beam, including the sample box, slits, ion chamber and amplifier. The active area of the ion chamber is 10 x 10 cm for an effective aperture of 25% of
2 .
Fig. 2 Schematic illustration of fluorescent x-ray ion chamber. By reversing the direction of the slit assembly and the sample holder position, the detector may be positioned for x-ray beams from right to left or from left to right.
As shown in Fig. 2, the sample is positioned at 45° to the incident x-ray beam within the sample box, which can be flushed with He to minimize absorption and scattered radiation. The fluorescent x-rays from the sample pass through a filter chosen to minimize scattering at the absorption edge being scanned but that will transmit the desired fluorescent radiation. The appropriate atomic number of the filter material is Z-1 for the K edge of elements from V to Br then either Z-1 or Z-2 from Kr to Ag, then Z-1, 2 or 3 from Cd to Ba. (Thanks to Dr. Steve Wasserman of Argonne National Laboratory and his "Filter" program.) For low atomic number elements, P to Ti there are no filtering elements available. The problem of choosing filters for L-edge spectroscopy is more difficult but usually can be managed with the K edges of the elements available in the standard filter set. (Other filters are available by special order.) After the filter, the fluorescent x-rays pass through a Stern-Heald slit assembly oriented so as to minimize the passage of secondary fluorescent radiation arising in the filter while maximizing passage of the desired fluorescent radiation into the chamber. In effect, the slit assembly makes the filter about 10 times more effective than without it. Notice that the slit assembly can be unscrewed from the sample box and rotated 180° and the sample holder finger fixed in the alternate position in order to reverse the beam direction through the detector. It is important that the relative positions of the sample holder, slits and x-ray beam are as shown in Fig. 2. The slant of the slits always must point in the same direction as the x-ray beam!
The ion current collecting mechanism within the ion chamber consists of a gas tight, 6 mm aluminized Mylar outer window and two inner grids of 90% transparent Ni mesh. (On special order we will furnish the detector with all Mylar collector grids, which makes it slightly more susceptible to harmonic noise induced by gas flow through the chamber.) The Mylar window and both meshes are captured at their edges between conducting rings separated from each other within the ion chamber body by Teflon insulators. Electrical contact with the supporting rings is made internally through the BNC connectors. The entrance window and back plane of the ion chamber are held at -45 V with respect to the central collecting mesh which collects the secondary electrons produced by the absorption of the fluorescent x-rays. During final assembly all detectors are carefully sealed and tested for leaks. Final checkout includes a 24 hr test with a manometer pressure of 10 cm of water. One must be very careful in applying any pressure or it will permanently bulge the front window.
Fig. 3 Example of sensitivity, 100 ppm Pt in Al2O3. (Thanks to Grayson Via and Graham George.)
Sensitivity. The primary application is for solid samples with concentrations of the element of interest between 0.01 and 1.0%, depending upon the background elements, or for liquid samples with concentrations from
10-4 to 10-1 M. Fig. 3 is an example of 100 ppm Pt in
Al2O3 measured with the 3-grid ion chamber as the detector on an in situ catalyst cell using an excellent beam line (X-10C at NSLS). The catalyst cell is illustrated farther on. The lower sensitivity limits may be increased somewhat by multiple scans; however, 10 ppm and
10-4 M are absolute limits for ideal solid samples and aqueous solutions, respectively. In A/B comparisons with the 3-grid detector and a 13 element Ge detector mounted simultaneously on the same beam line, it was found that in aqueous U solutions (measuring the
L3 edge), the 13 element detector was useful down to a concentration of
10-5 M with significantly better data than the ion chamber at
10-4 M, but the ion chamber could get better data, more quickly, above
10-3 M. The dispersive detector will always be best when there are interfering edges or when fluorescence from an element at lower energy causes a high background level that is not removed by a filter. Measurement of samples with high concentrations will cause the amplifier to saturate (an option is to unplug the included amplifier from the collector grid and switch to a Keithley at lower gain); however, the data still may be distorted due to the so-called thickness effect. The data shown in Fig. 4 proves that higher voltages than the 45 V battery (furnished) are not necessary to operate either the 3 or 5-grid detector.
Fig. 4 Measured response of either the 3 or 5 grid detector to applied collector voltage. The response was verified over counting rates from 102-105. The response was found to be flat to 200 V.
[ Basic 3-grid ion chamber detector ]
[ New, double length 5-grid ion chamber detector
]
[ electron-yield accessory ] [ Hot/cold accessory ]
[ Furnace/cryostat body with slits and ion chamber ] [
Boat type insert ]
[ Cylindrical insert with removable holder ]
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