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According to different diameters, when the thickness of the pawnshop plate reaches the effective saturation thickness of radioactive rays, the corresponding sampling quantity is the "minimum sampling quantity" of the plate quantity, and this sampling method is the thick source method.



1 Method and Principle



The sample to be tested was slowly evaporated and concentrated, converted into sulfate, evaporated to dry, and then burned in Muffle furnace to obtain solid residue. The residue with no less than the "minimum sampling amount" was accurately weighed and evenly laid out in the measuring plate, and placed on the low background A and B measuring instrument to measure the count rate of total B, so as to calculate the concentration of total B radioactivity in the sample.



2 Analysis Procedure



2.1 Determination of instrument background



The unused and unpolluted measuring plate (67) was taken, washed and soaked in alcohol for more than 1h, removed and dried, and placed on the low background A and B measuring instrument (61) to continuously measure the total B background counting rate of the instrument for 8h-24h to determine the stability of the instrument background, and the average value was taken and expressed as the counting rate RO(S-1).



2.2 Determination of effective saturation thickness (minimum amount of plate spread) Actual measurement: Weigh 80mg 100mg 120mg 140mg 160mg 180mg 200mg and 220mg240mg reference materials (5.6) in the measurement plate, and follow the same steps as sample source preparation (7.2.4). A series of standard sources with different thicknesses were made and evenly tiled on the bottom of the measuring plate. After drying, the total B count rate of each standard source was measured on A and B measuring instrument (6.1) with low background. The self-absorption curve of B was drawn with the total net count rate of B as the ordinate and the amount of plate laying as the abscissa. When the amount of pawnshop plate reaches a certain value, the net count rate of total B will not increase with the increase of the amount of pawnshop plate. The diagonal segment and horizontal segment of the self-absorption curve are extended, and the amount of plate spread corresponding to the crossing point is the effective saturation thickness of the standard source, that is, the minimum amount of plate spread of the method.



Theoretical estimation: If it is difficult to measure the effective saturation thickness, it can be calculated directly at 0.1Amg. http://unifitpv.cn