Research Of Virtual Detection Efficiency Calibration Of Detector Of γ Internal Contamination In Lungs

After the nuclear accident, radioactive aerosols easily inhaled into the lungs and deposited. Therefore, in vitro detection of Gamma rays emitted from radionuclides can be informed of the type of inhaled radionuclides in the human body. And the activity of inhaled radionuclides in lungs can be calculated by the detection efficiency. Combined with knowledge of radiation dosimetry, the human body irradiated dose could be estimated. According to the irradiated dose, appropriate therapeutic measures would be taken.Efficiency calibration is an important work to ensure the measurement results are accurate and reliable. Accuracy efficiency calibration will determine the reliability of the measurement results. Standard source calibration method is the simplest and most accurate efficiency calibration method. The standard source should have single energy or some energies which are separately wide enough. Because of the radioactive aerosol was evenly distributed in the lungs, and then the lungs can be seen as a volume source. Therefore, the standard source used in efficiency calibration work should have the same or similar shape to lungs.With the development of computer technology, the efficiency calibration technology by the using of Monte Carlo software to build a virtual model of the human body has been mature. MCNP code provides a variety of geometric description methods; you can create high-purity germanium detector and human body models. Medical imaging technology such as Computed tomography imaging provides technical support to establish voxel phantoms which can be used in MCNP code. Voxel phantom is consisted of a large number of small voxels, as the voxel phantom is very close to the real human body structure, the voxel phantoms can be used for precise efficiency calibration.This article relies on the subtask of Gamma radioactive internal contamination detection in lungs which supported by Special Program of Major Instruments of the Ministry of Science and Technology of China. MCNP code was used for the research of the virtual efficiency calibration of Gamma radioactive internal contamination in lungs, and the following research works have been carried out. (1). Establishment of HPGe detector model. Because of its high energy resolution, HPGe detector can well resolve the species of inhaled radionuclides. However, due to the dead layer, the detection capability to low-energy Gamma-rays of HPGe detector is limited. The HPGe detector Manufacturers often offers the dead layer thickness which cannot be directly used to establish the detector model by MCNP code. In order to establish an accurate HPGe detector model, both experiment and simulation should be taken. A standard152Eu point source was used in the experiment. The Full Width at Half Maximums (FWHM) were chosen from three total-energy peaks which are121.78keV,778.9keV and1408keV gamma rays’, and the channel width of the energy spectrum was calculated. According to the FWHM and channel width, parameters of the GEB and En in MCNP input file can be determined. As the result of experiment, the detection efficiency to121.78keV Gamma-ray is8.7694×10-5. By adjusting the thickness of the dead layer is MCNP simulation, the detection efficiency to121.78keV Gamma-ray is8.78×10-5when the dead layer thickness set to1.7mm, and it’s very close to the experiment data, thus the detector can be considered that its dead layer thickness is1.7mm.(2). Model establishment of Gamma radioactive internal contamination in lungs. KTMAN-2voxel phantom is build by the human body CT image. Consisted elements and tissue density make reference to ICRR Therefore, this model is very similar to real human body. In the establishment of Gamma radioactive internal contamination in the KTMAN-2voxel lungs, entire lungs were set as the starting point of emission Gamma photon, and their emission directions were isotropic within4π solid angle.9typical gamma rays were chosen from eight common radionuclides, their energies were0.09886MeV,0.10531MeV,0.12206MeV,0.36448MeV,0.6617MeV,1.1732MeV,1.3325MeV,1.4608MeV and1.836MeV. In order to reduce the relative estimation error of simulation results, the number of particles in the sample is set to1.3×108.(3). Design of efficiency calibration model. When photons were emitted from lungs, they will interact with the body tissue, due to the different densities of each body tissue, the results of the detection efficiency in vitro detection of different positions are different. In order to know the best way to detect the effect, this detector chooses eight positions including upper right chest, lower right, upper left, lower left, and the back of the upper right, lower right, upper left, lower left. Simulate detection efficiency of the detector at each position for each energy Gamma rays by using MCNP code. From the simulation results, we can see that it gets the maximum detection efficiency in the upper left chest. In the low energy part, detection efficiency increases with Gamma ray energy increasing. When Gamma ray energy increases to a certain extent, the detection efficiency decreases with Gamma ray energy increasing.