NEW, DOUBLE LENGTH 5-GRID ION
CHAMBER DETECTOR
The design depth of the original 3 grid ion chamber was determined by the estimated range over which an electron could be collected with the available battery voltage (45 V) in a reasonable time (~ 0.1 sec). These parameters fixed the front window to collecting grid and collecting grid to back plane distances at ~ 1.6 cm. The count rate vs voltage response plateau shown in Fig. 4 proves that these parameters result in a detector which has a flat response plateau (measured flat to 200 V) where the output is insensitive to small changes in applied voltage. The installed 45 V battery must significantly discharge (battery lifetime is typically a few years) before the detector will be on the knee of the plateau where the response may be noisy. We have constructed and tested a new 5 grid ion chamber detector which doubles the absorption distance by incorporating an additional collector grid and back plane grid with which higher energy fluorescent x-rays may be collected more efficiently. The new detector is shown in Fig. 6. All the dimensions are the same as our original 3 grid detector except for a 3.3 cm greater depth. The ion chamber/amplifier unit may be attached to the previously used sample box and slits. Absorption characteristics of 3 grid and 5 grid ion chambers. The fraction of radiation absorbed (1 - I/Io) in the ion chambers with various fill gases was calculated from tabulated absorption coefficients, I = Io exp(-(m/r)rx) where m/r was from tables, x = 3.3 and 6.6 cm for the 3 and 5 grid detectors and r = 1.784, 3.736 and 5.887 x 10-3 g/cm3 for gaseous Ar, Kr and Xe, respectively. This is shown in Fig. 5 and suggests that a significant performance gain may be obtained by doubling the ion chamber depth with any fill gas. Note the discontinuities at the Kr and Xe K edges. Sometimes these features may be used to advantage to increase the sensitivity range of the detector. In Fig. 5 the fraction of radiation absorbed in the 6.6 cm deep, 5 grid detector are compared with Ar, Kr and Xe fill gas. Note, particularly, that by choosing the appropriate fill gas nearly 50% of the incident radiation can be absorbed and measured over a very wide range of energy. The tight seal which we have achieved on these new detectors make the use of even high-priced Xenon nonprohibitive. The predicted performance characteristics were verified by testing over the complete x-ray energy range to the Te K edge at 31.8 KeV.
Filters and Calibration Foils. The sensitivity of the ion chamber is maximized by using filters chosen to absorb the scattered radiation but pass the fluorescent radiation of the element being scanned. The standard fluorescent filter set includes the elements Ti-Se, Zr and Mo mounted in aluminum frames, 10 cm x 10 cm, to fit the detector. The set includes two thicknesses, 3 and 6 absorption lengths, for each element. At the Cu edge, for example, the 3 absorption length filter would reject 95% of the scattered radiation while passing 60% of the desired Cu Ka and the 6 absorption length filter would reject 99.75% of the scattered radiation and pass 37% of the desired Cu Ka. Special filters routinely available are Sr, Y, Ag, Ru and In. Call us for other elements or for unmounted filter material. The filter set is delivered in a furniture quality wooden box for storage and/or transportation to your beam line. Sets of metal foils for the 3d, 4d and 5d elements are available. [ Basic 3-grid ion chamber detector ] [ Home ] [ Top of Page ] |
