| 1-131 is a fission product,which is the main characteristic nuclide released to the environment when a nuclear accident occurs in a nuclear power plant.At the same time,it is also one of the most widely used nuclides in nuclear medicine applications.When 1-131 is released into the air,radioactive aerosols are formed and exist in the form of gaseous iodine and particulate iodine.1-131 is easily absorbed by human thyroid gland,and it will do harm to people if it exceeds a certain amount.Therefore,the measurement of 1-131 concentration in aerosols is an important part of radiation environmental monitoring.It plays an important role in the early warning of nuclear accidents,accident status and public dose assessment of accidents,and is also the main project of radiation environment measurement in nuclear medicine application sites.Among them,gaseous iodine needs to be collected by iodine box containing activated carbon,which is usually measured by gamma-ray spectrometry.The concentration of I-131 collected in the iodine box varies with the depth according to the negative exponential law,which is not the usual uniform distribution.At present,gamma-ray spectroscopy mainly uses uniformly distributed volume standard sources for efficiency calibration.It is difficult to prepare the non-uniformly distributed experimental sources.The geometrical distribution of the standard sources is not consistent with the actual measured samples,resulting in large errors in the measurement results.Therefore,it is necessary to study and solve the error correction method based on uniform volume source efficiency calibration.In this paper,the correction of measurement error of 1-131 based on uniform volume source efficiency scale is studied by combining theoretical simulation with experimental measurement.(1)Monte Carlo software MCNP5 is used to simulate the detection efficiency of the body source on GR5021,BE5030 and GC4019 detectors;the detection efficiency of the body source on three detectors is measured by experiment,and the error between the simulation results of MCNP5 and the experimental results is compared.(2)Using MCNP5 to simulate the detection efficiency simulation results of three detectors for non-uniformly distributed body sources under the negative exponential distribution parameter a=0.23,a method to realize non-uniformly distributed body sources based on controlling the measurement time of detectors is proposed,and the experimental verification of non-uniformly distributed body sources is indirectly realized by using this method,and the errors between the simulation results of MCNP5 and the experimental results are compared.The detection efficiency of homogeneous source and the simulation efficiency of homogeneous and inhomogeneous source simulated by MCNP5 are measured by experiments.The correction coefficients of I-131 efficiency calibration using homogeneous source as inhomogeneous source in three detectors are given.(3)The GR5021 detector is used to simulate the detection efficiency of I-131 under different exponential distribution parameters.The average positive and negative measurement results are compared with those obtained by using the detection efficiency of uniformly distributed I-131 as efficiency calibration.(4)In the nuclear medicine department of a third-class A hospital,the concentration of I-131 in different places and the distribution of I-131 in different forms were obtained by using the efficiency scale measurement method of the uneven distribution I-13 1 obtained.Through this paper,It can be concluded that the error between the detection efficiency of theoretical analog body source on different detectors and that of experimental measurement is generally between 1%and 15%.Based on the method of controlling the measuring time of the detector,the relative errors of the ratio of the detection efficiency of uneven and uniformly distributed body sources and the ratio of the detection efficiency of MCNP5 simulated uneven and uniformly distributed body sources are between 1%and 7%when the two distribution parameters are 0.23 and 0.1 8.According to the measured detection efficiency value of uniform volume source and the simulation efficiency value of MCNP5 simulated non-uniform and uniformly distributed volume source,The efficiency calibration values of GR5021,BE5030 and GC4019 detectors are multiplied by correction coefficients 1.05,1.05 and 1.05respectively when the uniformly distributed standard source is used to calibrate the non-uniformly distributed I-131.Through the simulation analysis of mcnp5,it is obtained that when the detection efficiency of uniformly distributed 1-131 is used to measure the forward and reverse placement of iodine box,the error is less than 0.5%when the average value is taken as the measurement result of non-uniformly distributed I-131.By measuring and comparing the concentration of particulate I-131 and gaseous I-131 in the nuclear medicine department,it is concluded that the concentration range of gaseous 1-131 in the nuclear medicine department is 5.6E-O1Bq/m3～1.2E+02Bq/m3,the concentration range of granular 1-131 is 1.0E-01 Bq/m3～8.9E+00Bq/m3,and the ratio of gaseous to granular I-131 in the medicine room and ward is between 0.053 and 0.659.The content of gaseous 1-131 is higher than that of granular 1-131,and the ratio varies greatly at different locations in nuclear medicine. |
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