Appendix D
COUNTING GEOMETRIC CORRECTIONS IN GAMMA-RADIATION MEASUREMENTS

D.1 INTRODUCTION

The gamma-ray full energy peak efficiency is sensitive to the following counting geometry factors: (1) source-to-detector distance; (2) physical form of the source (gas, liquid, solid); and (3) size and shape of the source or source container (point source, filter papers of various sizes, charcoal cartridge, liquid bottles of various sizes, gas vials of various sizes, Marinelli beakers of various sizes). For most accurate results, the source to be measured must duplicate, as closely as possible, the calibration standards in all aspects. If this is not practical, appropriate corrections must be determined and applied. The preparation of standard sources for various geometric calibration, and the methods of determining geometric correction factors are described in this procedure.

D.2 STANDARD SOURCES

  1. NIST or certified point source containing appropriate standard radioactivities.

  2. NIST or certified liquid source containing appropriate standard radioactivities.

  3. Some standard sources in various forms may be commercially available from the NIST, Analytics, Inc., and other vendors.

D.3 CALIBRATION FOR LIQUID SAMPLES

  1. Use a calibrated micropipette for transferring the standard solution.

  2. Carefully weigh a dry and clean micropipette in an analytical balance to the nearest 0.1 mg.



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Radiochemistry in Nuclear Power Reactors Appendix D COUNTING GEOMETRIC CORRECTIONS IN GAMMA-RADIATION MEASUREMENTS D.1 INTRODUCTION The gamma-ray full energy peak efficiency is sensitive to the following counting geometry factors: (1) source-to-detector distance; (2) physical form of the source (gas, liquid, solid); and (3) size and shape of the source or source container (point source, filter papers of various sizes, charcoal cartridge, liquid bottles of various sizes, gas vials of various sizes, Marinelli beakers of various sizes). For most accurate results, the source to be measured must duplicate, as closely as possible, the calibration standards in all aspects. If this is not practical, appropriate corrections must be determined and applied. The preparation of standard sources for various geometric calibration, and the methods of determining geometric correction factors are described in this procedure. D.2 STANDARD SOURCES NIST or certified point source containing appropriate standard radioactivities. NIST or certified liquid source containing appropriate standard radioactivities. Some standard sources in various forms may be commercially available from the NIST, Analytics, Inc., and other vendors. D.3 CALIBRATION FOR LIQUID SAMPLES Use a calibrated micropipette for transferring the standard solution. Carefully weigh a dry and clean micropipette in an analytical balance to the nearest 0.1 mg.

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Radiochemistry in Nuclear Power Reactors After pipetting the standard solution to the mark, weigh again and calculate the exact volume of standard solution in the pipette. Transfer the standard solution into 1M HCl solution contained in the calibrating sample container. Rinse the pipette with the solution a few times, and the solution is thoroughly mixed. Perform radioactivity measurements, as described in Appendix C, at various distances from the detector. According to the procedure described in Appendix C, gamma-ray efficiency calibration curves can be obtained for each sample counted at various distances. Alternatively, a standard point source is counted at the same positions, and the ratios of activities in the point source to that in the liquid standard solution can be calculated. The geometric correction factors (relative to the point source) can be obtained for the gamma rays in various sample sizes counted at various positions. D.4 CALIBRATION OF FILTER PAPERS Two popular sizes of filters, 47 mm and 1-in. diameters, are generally used in radiochemical measurements. It should be noted that the actual area on the filter covered by solid samples are significantly smaller than the filter. D.4.1 Preparation of Standard Disc Sources By using a calibrated micropipette (preferred 100 or 200 μL) carefully pipet the standard solution to the mark. On a filter paper, draw a cycle to be wetted by the standard solution. Place the paper on a sheet of plastic wrap.

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Radiochemistry in Nuclear Power Reactors Evenly distribute the solution on the encircled area of the paper. Care must be exercised to deliver the solution in very small droplets. After the paper is air dried, cover the entire filter paper with plastic wrap. The disc source should be checked for its activity content and the uniformity of activity distribution on the surface area by the following two methods: The activity content in the disc source is calibrated by counting the source at a position 30 cm away from detector where the effect of size variation is negligible. The measured activity is compared with a certified standard point source counted at the same position. The uniformity of activity distribution is checked by counting the disc source at 1 cm from the detector. The disc source is first counted at the position slightly away from the center of the detector. Repeat counting the disc source at the same center position with the disc turned around 30° and 180°. The measured activity should be consistent within ±5%. By using the disc standard source, a series of calibration curves can be obtained at various distances from the detector. D.4.2 Calibration by a Point Source Alternatively, a standard point source is counted at the same positions, and the correction factors for a disc source (relative to a point source) can be calculated. The method (similar to that reported in Ref. 1) to determine the correction factor is described below: Using a single gamma-ray point source (e.g., Cs-137) counts from the source placed on a calibrated shelf are obtained at 0.2 cm intervals, starting from the center of the detector and proceeding to the front and back.

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Radiochemistry in Nuclear Power Reactors The data points (count rate at a distance, R, from the center) are fitted to a polynominal function: The count rate of a disc source relative to the count rate of a point source with the same amount of activity can be estimated by D.5 CALIBRATION FOR GAS SAMPLES A 14.1 mL glass vial is a standard sample container for the offgas samples. A larger size glass vial (e.g., 100 mL, or a Marinelli container) may also be used to obtain the offgas and stack gas samples, if needed. The calibration method is based on a publication given in Reference 2, in which Vermiculite is used as a “weightless” supporting material for radioactivities. An aliquot of known standard solution is diluted to an appropriate volume (~3 mL for a 14 mL vial and the volume may be proportionally increased for a larger volume container) with water in a beaker. Pour the diluted standard solution over a pre-measured amount (to fill the sample container exactly) of Vermiculite in another beaker. The empty beaker is rinsed twice with ~1 mL water and each time the rinse water is added to Vermiculite. The wet Vermiculite is dried in air, mixed gently, and transferred to the sample container. Effort should be made to have the Vermiculite containing activity homogeneously mixed and tightly packed in the container.

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Radiochemistry in Nuclear Power Reactors Wash the beaker and measure the activity that is not transferred with Vermiculite. Measure the activity at various positions from the detector, and a series of calibration curves can be obtained directly. Alternatively, a standard point source can be counted at the same positions, and a series of correction factors (relative to a point source) can be calculated. D.6 CALIBRATION FOR CHARCOAL (OR Ag-ZEOLITE) CARTRIDGES The calibration procedure is similar to that described in Section D.5, except Vermiculite is replaced with charcoal or Ag-Zeolite granular material. The calibration data obtained are applicable only for the activities homogeneously distributed in the cartridge. For an actual sample, the activity is generally not homogeneously distributed in the cartridge, and it is impractical to open the cartridge and manually mix the granular materials each time the sample is counted. Alternatively, the cartridge sample is counted twice, with one side facing up and down, and an average count rate is obtained for activity calculation. If a sample is counted at a close distance, ≤3 cm, an average count rate may differ significantly from a sample with homogeneous activity distribution. An appropriate correction may be empirically obtained by the following method: Obtain a number of actual samples with various sample flow rates and sampling duration. It is expected that the penetration of airborne activity in a cartridge may be different at different flow rates and sampling duration. Count the sample cartridge twice, with face up and down, and obtain an average count rate.

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Radiochemistry in Nuclear Power Reactors Open the cartridge, thoroughly mix the solid adsorbent, and re-pack the materials into the cartridge. Count the sample again at the same position, and compare the count rate with the previously obtained average count rate. A necessary correction factor can be calculated. D.7 REFERENCES 1. R.G.Helmer, Int. J. Appl. Rad. Isot, 34, 1105 (1983). 2. C.C.Lin, Int. J. Appl. Rad. Iso., 32, 657 (1981).