Abstract: The calibration methods for the radioactive Kr and Xe gases produce the key data for the judgment of nuclear fission reaction, whose accurate measurement has always been a difficult problem in operation for a long time. In order to obtain the accuracy, it is very important to calibrate the efficiencies of these gas sources, especially for proficiency test exercise of the laboratory, to analyze the CTBT samples of radioactive xenon, which are used to judge the nuclear test of the country and the measurement system. The relative measurement method has not realized in the experimental calibration, and the Monte Carlo method has large uncertainty also. Therefore, a new measurement method and experimental technology is needed. In order to avoid the above shortcomings, we need to develop a source-less efficient calibration method based on the virtual point source (VPS). In the past, it was suggested that for point sources placed on the symmetry axis, a Ge(Li) or an HPGe cylindrical detector can be changed to an virtual point detector (VPD), where all -ray interactions are considered to occur. This is not a real physical model but only a mathematical description. Aiming at the VPD, we put forward an innovative approach and define the concept of VPS. But, the concept is introduced in a volume source. In this concept, it is assumed that the total photons emission has occurred in a distance within the source described, and it is from the whole source to an imaginary point. If there is really a point located on the symmetry axis of the detector, whose efficiency is similar to that of the whole real volume source, the geometrical considerations used in calibrations of the source will be much simpler. The calibration process of the VPS is: firstly, a standard point source is placed at different position on the symmetry axis to obtain its full-energy peak efficiency. Secondly, the relationship between the height and the efficiency can be established. The position of the VPS can be deduced according to the full-energy peak efficiency of the volume source. Finally, a standard point source, instead of a volume source, is placed at the virtual point position to finish the efficiency calibration work. In this study, the LabSOCS software is used to simulate the detection efficiencies including different volumes of gas source and point source at different points on the symmetry axis. According to the calculated data, a function relationship between the volume of gas source and the virtual point source position is established. It has been proved theoretically that the volume of gas source and the virtual point location have a good linear relationship. This provides a new way in theory to solve the virtual source calibration technology. The VPS efficiency calibration technology is very important in the field of verification of nuclear test-ban, nuclear emergency measurement and environmental radioactivity measurement.

Keywords: 
• gas source /
• LabSOCS /
• efficiency calibration /
• virtual point source