Nonlinear Study of Elemental Measurements by PGNAA Method

Prompt gamma neutron activation analysis (PGNAA) is a kind of high sensitivity to nondestructive testing technology. Application of the technology of on-line PGNAA analyzer is widely used in industrial control fields such as cement, coal, which has advantages of fast response and high precision of analysis. The thickness of the material on the belt is often not constant due to the change of industrial field production. When the material thickness changes, the measurement results of the on-line analyzer will deviate. In order to eliminate the measurement deviation of materials with different thickness measured by online analyzer, the reason of deviation is studied through theoretical analysis and Monte Carlo simulation, and the correction model is sought. First, through the theoretical analysis, according to the calculation formula of gamma ray detector characteristics, mainly due to the self-shielding effect of neutron and self-attenuation gamma rays of the material inside, the neutron field distribution and gamma receiving efficiency change, then lead to feature gamma counting nonlinear increase with the increase of thickness. Then, on-line PGNAA analyzer device model is built by using the Monte Carlo software (MCNP), and the changes of characteristic ray count of Al₂O₃, SiO₂, CaO, Fe and cement raw materials with thickness are simulated respectively. Simulation and experimental results show that the characteristics of gamma radiation intensity is not linear with the increase of the thickness of the material. The on-line analyzer measurement error increases by linear calibration method, the average relative error is more than 10%. Finally, taking cement on-line analyzer as an example, the modified model is sought. According to the relationship between the characteristic ray count of each element of cement raw material and the material thickness, a polynomial thickness correction model suitable for measuring cement raw material is established. Compared with linear calibration, the measurement error of the modified analyzer is reduced by more than 30%. The modified method can be applied to PGNAA detection of lump materials such as coal, sintering materials and bauxite.