Geometric Considerations in the

Transcription

1 Geometric Considerations in the Calibration of Germanium Detectors for Analytics, Inc. examines geometric questions involved in calibrating and counting filter papers using germanium detector, gamma-ray spectrometers. Several different methods of preparing filter-paper standards are compared. Also compared, are the counting efficiencies for different size filters (active areas). One major problem encountered when calibrating a germanium detector for filter-paper counting is the production of a standard which either has a uniform planar distribution of radioactive material or one that accurately simulates a uniform planar deposition to the detector. If a 47 -mm, glass-fiber filter is completely soaked with a dilute solution of radioactive material (using about 0.7 grams of solution) and dried under a heat lamp, it is unlikely a uniform deposition will be obtained. Figure 1 is an auto-radiograph of a source prepared by drying grams of a mixed gamma-ray standard solution onto a 47-mm, glass-fiber filter. The mixed gamma-ray liquid standard contained 109Cd, 57CO, 139Ce, 203Hg, 113Sn, 137CS, 88y, and 60Co in 4 M HCI with about 100 ppm of each element as carrier. The bright crescent in the auto-radiograph shows that a significant amount of the activity has migrated to the outside edge of the soaked filter. It has been suggested that in addition to gross migration, some chromatographic separation of the chemical species present in the mixture may also occur. From the gamma-ray counting data this soaked filter does not appear to show chromatographic separation. However, another filter to be discussed later may demonstrate chromatographic separation of cobalt and ce- SIum. Figure 2 is an auto-radiograph of a 47-mm, simulated-filter standard prepared by gravimetrically depositing 197 drops of liquid, mixed gamma-ray standard on polyester tape using a computer controlled micropipetter and an x-y applicator. Each drop weighed about one milligram. The matrix was designed to keep the activity-per-unit area constant across the entire filter. As the auto-radiograph shows, some irregularities do occur in the deposition or drying due to variations in the tape or static charges. It was expected that using the small grid (2.925 mm) and the large number of drops would negate the effects of random variations in drop size and placement. After the liquid was evaporated, a glass-fiber filter was placed on top of the radioactive material and the source was sealed with a second layer of tape. This type of source simulates an active filter in a plastic or glassine bag with the radioactive material on one side of the filter. To test how well this distribution simulates a uniform planar distribution it was compared to several filters prepared by other methods, including filter standards prepared using a 385- drop and a 49-drop matrix and a simulated-filter standard (734-20) distributed by the National Institute of Standards and Technology under the USCENNIST Measurements Assurance Program for the Nuclear Power Industry. Figure 3 is an auto-radiograph of a 47-mm-diameter, simulated-filter (734-20) prepared by NIST Reprint of "From the Counting Room", Vol. 2, No

2 Radioactivity 6- Radiochemistry The Counting Room: Special Edition Figure 1 Auto-radiograph of 47-mm, soaked-filter standard. Figure 3 Auto-radiograph of a 47-mm, simulated-filter standard (734-20). Figure 2 Auto-radiograph of 197-drop, 47-mm, simulated-filter standard. Figure 4 Auto-radiograph of a 47-mm, filter standard (632-23) in June, The report of test distributed with the standard gave the following description of the preparation: "The source contains cobalt-57, zinc-65, and cesium-137, as chlorides, deposited on a simulated, 47-mm-diameter filter. The active area was simulated by deposition of approximately equal calibrated volumes of solution onto an array of 6-mm-diameter, filter-paper disks. Two concentric rings of six and 12 filter-paper disks surround a center disk, and are sandwiched between two layers of O.OO6-cm-thick polyester tape, The basis weight of the filter paper used to make the disks, as specified by the 56 Reprint of "From the Counting Room", Vol. 2, No. I, 1991

3 "Geometric Considerations in the Calibration of Germanium Detectors for Filter-Paper Counting" Table 1 47-mm, 197-drop, simulated-filter standard. Counted 2.95 mm from a 12.7% HPGe detector. manufacturer, was 85 g/m2 with a nominal thickness of 0.22 mm." Even though some of the individual disks do show a slight crescent in the auto-radiograph, the gammaray counting data shows excellent agreement with the deposited drop simulated filters at 0 cm, 2 cm, and 6 cm on several different detectors. Before discussing the counting data in detail, the production of two other filter standards will be described. Figure 4 is an auto-radiograph of , a filter containing 57 Co, 60Co, and 137 Cs produced by depositing 37 drops of the mixture on a mmthick, glass-microfiber filter in three concentric rings of 6, 12, and 18 drops surrounding a center drop. It was initially thought that the active area did not extend entirely to the outside edge of the filter (3-5 mm inside). However, the auto-radiograph shows that the activity does extend to the edge of the filter and shows a fairly uniform distribution. After examining the gamma-ray counting data, it appears that the film is more sensitive to the high-energy electrons from the 137 Cst137Ba decay and does not show the non-uniform distribution of 57 Co and 60CO. This filter may be an example of chromatographic separation. The last method of filter-standard preparation tested was "carrier dispersion:' Nine, 3-mg drops of mixed gamma-ray liquid standard were deposited onto a 47-mm, glass-fiber filter and dispersed by depositing 50 mg of concentrated carrier solution on the middle of each drop of active liquid. This amount of liquid did not completely saturate the filter. Table 1 is a comparison of the counting efficiencies obtained for the 197-drop filter, the soaked 47- mm filter, the 9-drop carrier-dispersed filter, the array of 6-mm filter disks (734-20), and the 37-drop filter (632-23). Mercury-203 was not included in the comparison since the soaked filter suffered some loss in preparation. Zinc-65 in was not included because comparison with the mixed gamma-ray standards would require coincidence summing corrections to the mixed gamma-ray efficiencies in the close-counting positions and will be the subject of a future article. The measurements presented in Table 1 were performed on 12.7% (65 cm3), p-type, coaxial, intrinsic germanium detector with 1.75 key FWHM at 1332 key. The filters were positioned on the bottom of the plastic shelf holder with only 2.95 mm of plastic between the filter and the face of the detector. All of the counting efficiencies were normalized to the efficiencies obtained with the 197- drop filter which was counted with the active material facing away from the detector. The efficiencies for the 385-drop filter and the 49-drop filter were not included since all values were within 2% of the values obtained with the 197-drop filter and showed no systematic trend. The 197-dropfilter was chosen as the best general array, since it is very difficult to keep 385 drops from running together and the 49- drop array is more sensitive to minor variations in Reprint of "From the Counting Room", Vol. 2, No.1,

4 Radioactivity 6- Radiochemist7)' The Counting Room: Special Edition Table 2 47-mm, 197-drop, simulated-filter standard. Counted 2 cm from a 12.7% HPGe detector. Table 3 47-mm, 197-drop, simulated-filter standard. Counted 2.95 mm from a 19.4% HPGe detector. drop size and placement. All of the deposited-drop filters show excellent agreement with the NIST filter The soaked filter (radiographed in Figure 1) was counted in the bottom counting position and gave considerably lower counting efficiencies than any of the other filters except The nine-drop, carrier-dispersed filter is closer in counting efficiency to the deposited-drop filters than the soaked filter but still shows significant differences at low energy. The detector chosen for these measurements was expected to show large variations between the different distributions since the active diameter of the detector is only 42.2 mm and the radiograph of the soaked filter showed a concentration of activity in the 46- to 47-mm region of the filter. Additional geometric considerations in calibrating for filter-paper counting are the determination of the active area of the samples and the effect of positioning the filter with activity toward the detector or away from the detector. Table 1 includes a comparison of a 44-mm, deposited-drop filter with the 47-mm, deposited-drop filter. Most air-filter holders mask off 1-3 mm of the filter during sampling, thus decreasing the actual diameter of the radioactive sample being measured. The efficiency data shows a 3-7% difference between the 44-mm and the 47-mm filter depending on the energy of the gamma ray measured. The last column of Table 1 shows a comparison between the 47-mm, 197-drop filter counted with the activity toward the detector, divided by the efficiency obtained with the activity away from the detector. The differences between activity toward/away are in the range of 2-4% for these 0.6- to 0.7 -mm thick, single filters. All the effects discussed so far could add together. If the results from two labs were compared, where one lab calibrated with a 47-mm, soaked-filter standard and the other calibrated with a 44-mm, deposited -drop standard (counted with activity toward the detector), differences of 30% in the final results could be obtained from geometric effects alone. The filter standards in this study simulate single-air filters (0.6- to 0.7-mm thick) counted in plastic bags. Single-air filters, counted in petri dishes or metal planchettes, and all composite filters should be compared to standards designed to simulate their particular geometry. Table 2 is a comparison of filter standards counted 2 cm from the 12.7% germanium detector. As expected the differences between the different distributions decreased with increasing distance from the detector. At 6 cm from this detector the differences in counting efficiencies between the depositeddrop filters, and , decreased to less than 2%. 58 Reprint of "From the Counting Room", Vol. 2, No.1, 1991

5 "Geometric Considerations in the Calibration of Germanium Detectors for Filter-Paper Counting" Table mm, 197-drop, simulated- filter standard. Counted 2.95 mm from a 13.80/0 HPGe detector. Table 3 compares filter standards counted on the bottom of the plastic shelf holder (2.95 mm) using a 19.4% (84 cm3) p-type, coaxial, intrinsic germanium detector having a 50-mm diameter. The differences in efficiency between the various distributions are lower for this larger detector. The 197-drop filter and the soaked filter were counted on a third detector to measure the effect of detector diameter. Table 4 is the comparison of the 197-drop filter and the soaked filter on shelf 0 using a 13.8% (69-cm3) p-type, coaxial, intrinsic germanium detector with a SO.7-mm diameter. As expected, the results for the SO-mm detector and the SO.7-mm detector are very close to each other in spite of the large difference in relative efficiency. Unfortunately, other important data such as the exact detector-to-window distance and the thickness of the surface dead layer was not available for all three detectors. In order to perform accurate determinations of low-level radioactivity in filter samples using a germanium gamma-ray spectrometer, uniform planar standards accurately simulating the counting geometry must be used. Samples must be reproducibly positioned and counted in the same orientation as the standards. These factors are most important when counting close to the face of a germanium detector. R&R Reprint of "Prom the Counting Room", Vol. 2, No.1, 1991