The problem was put forward by our company to use the "automatic building radioactive detector" produced by Hubei Fangyuan Measuring Instruments and Equipment Manufacturing Co., Ltd., which is the NaI γ spectrometer. The specifications provided by the instrument specification meet the requirements of GB6566-200. We carry out the inspection work according to the experimental methods and instrument manuals required by the guidelines. For the objective, fair, and scientific report of the test results, we made an evaluation of the uncertainty of the measurement results. Samples were taken from all-ceramic tiles. The particle size of the samples was not more than 60 μm. Reproducibility experiments revealed that the measurement data had poor reproducibility. Subsequently, the calibration source was calibrated with the instrument configuration. Samples and criteria were measured again. Samples were dried at 0±5° C., packed for 24 h, placed for 3 weeks, and inspected at an ambient temperature of 22 to 26° C.
The sample measurement data is shown in Table 3, and the relative error of the criterion source is shown in Table 2. At this point the instrument passed the metrological verification for 3 months (valid year).
In response to this problem, we asked the testing agencies to conduct comparison experiments. The testing instruments are all NaI γ spectrometers. The # and 2# manufacturers are the same (Beijing), and the models are different. The 3# and 4# models and manufacturers are the same (Hubei). Survey data and uncertainty are shown in the table.
Discussion of Instrument Stability The influence of lead shielding room wall thickness, size, and crystal size of sodium iodide detectors on the measurement results Table 2 gives the associated parameters of the instrument used in this experiment and the uncertainty of the source determination. The survey results also show that the measured data of 3# and 4# have a large degree of dispersion, and it is difficult to meet the requirements of the GB6566-200 criterion ≤ 20%.
Table 2 Correlation Parameters and Criteria of the Instrument Source Measurement Uncertainty Natural radionuclides such as radium-226, plutonium-232, and potassium-40 are found everywhere in the natural world. The distribution of radioactivity varies greatly in different regions and environments. Therefore, some shielding must be performed during measurement.
Studies have shown that using a low background γ-ray spectrometer to inspect the samples for radium-226, thorium-232, and potassium-40 activity, the background and detection efficiency will affect the accuracy of the survey results, the lower the background, The higher the energy resolution, the lower the lower limit of detection, which can help improve the accuracy of the instrument analysis, the smaller the dispersion of the survey results. If the thickness of the lead chamber is not enough, the number of gamma particles penetrating into the lead chamber into the sodium iodide detector increases, increasing the background of the gamma spectrometer.
The impact of the data acquisition system and stability of the measurement instrument on the measurement results From the principle of the instrument, the γ-ray spectrometer should have the function of searching for peaks and setting the region of interest to set the function of live time or real time, and the data acquisition system. There should be multiple analyzer basic functions. The γ-ray spectrometer is a relative surveying device. In the actual survey, it is impossible to scale every sample. Therefore, it is required to have good stability. In the actual survey, when the repetitive survey data was not good, the instrument was calibrated using the criterion source, but the data differed greatly between the scale values ​​of each time. Obviously, such an instrument does not have the basic function of a multi-channel analyzer and cannot meet the inspection requirements.
Evaluation of measurement uncertainty: According to JJF059-999 metrological specification, measurement uncertainty consists of multiple components. Through the analysis of the uncertainty of the radioactivity measurement results, it is known that the measurement uncertainty mainly consists of three components of the repeatability of the survey, the source of the criteria, and the stability of the instrument. In this example, the reproducibility of # and 2# surveys is good, and the difference between the measured value of the criterion source and the guideline value is less than ± 0%, and the data is reasonable. Through inspecting the sources of the 3# and 4# instruments, it was found that the difference between the results of the three nuclide surveys and the guideline value was close to or exceeded the allowable value of 20% set by the national standard GB6566-200. The uncertainty of the sample detection result is much greater than 20%, and the data is not desirable. If the instrument uncertainty provided by the measurement verification of this instrument is 3# (232Th: 5.7% (K=2), 226Ra: 6.9% (K=2), 40K: 5.8% (K=2), the measurement uncertainty The degree is in compliance with the tolerance (20%) stipulated in GB6566-200, and the data is valid.Because the measurement result is directly related to the rationality and stability of the instrument, the GB6566-200 experimental method should clearly specify the extended uncertainty it refers to. 20% is the calculation result of the criterion source measurement, or the uncertainty given by the measurement verification certificate.According to the measuring principle of the instrument, the relative criterion deviation of the criterion source measurement data is more meaningful to the measurement result, and the expanded uncertainty of the instrument measurement verification. Can not truly reflect the rationality of the instrument.
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