Positron Analysis Research Based On Pair Production Effect

Current positron analysis technologies can not measure the defect inside samples because of the small range of positron produced from source outside. We show in this thesis a new method of positron analysis, Photon Induced Positron Analysis (PIPA), to solve this problem. When irradiating samples with Bremsstrahlung-induced X-ray based on LINAC, pair production takes place inside the material. Analyzing the annihilation photons helps to reveal the information of micro-structure in the samples. This method also avoids the problem of (γ, n) reaction based PIPA with high costs on LINAC and difficulties on shielding.As to proof its feasibility in real operation, we studied several key issues of this method quantitively by simulation and experiments in real environment. We also set up an experimental system, and measured the spectra shape of 511 keVγ.1) Sensitivity of Positron Analysis is studied. With the application of a 7MeV LINAC, the sensitivity of materials such as steel, copper is researched experimentally.2) Spectra shape of 511keVγis measured precisely in the strong radiation field when LINAC works. By designing the shielding and colliating geometry carefully, we successfully improved the ratio signal-background of the spectra, as well as suppressing the piling up of spectra, which is caused by the large number of photons in 4μs pulse width. In addition, electromagnetic interference during LINAC’s operation was diminished, and a fine energy resolution of 1.33keV@662keV is obtained.3) With Positron Analysis on different samples carried out, the relationship between defect concentration and PIPA result is appreciable.4) The capability of measuring defect inside samples is confirmed. The depth of PIPA into samples is possibly up to 5cm.Both of simulation and experiments proved that PIPA based on pair production effect is feasible. This method of Positron Analysis is promising to have application in on-line nondestructive examination, especially in internal defect measurement and lifetime prediction of aerial metal, such as blade of aviation engine, undercarriage, etc.