Time Associated Coincidence Method To Measure The Enrichment Of Uranium Components

Uranium is major material of nuclear components, and the research of the radiation detection method for nuclear components is an important part of nuclear disarmament verification. The spontaneous fission rate of the plutonium is relatively high and can be measured by passive method; however the spontaneous fission of uranium rate is low so that we have to measure uranium components by active method.As an active method, Time correlation coincidence can be used in non-destructive analysis of fissile material, and it's an important means of uranium components verification. Based on this method Oak Ridge National Laboratory has developed Nuclear Materials Identification System (NMIS) for verification of components containing fissile material. On this method there are some simulation results in international publications, but only very few experimental results. And there is no related paper published yet in domestic.This paper describes the experiments conducted on the base of time correlation coincidence measurements and performed with uranium metal castings of similar quality (about 8 kg) and shape (Hemispherical shell) but in different enrichments. The neutron sources are a 5 Cf fast fission chamber and a timing and directionally tagged DT generator. By analyzing the characteristic of time correlation coincidences counts which are between source-detector and detector-detector, many group of special parameters can be obtained to characterize the fission signal. These parameters are very sensitive to changes of enrichment, and we can distinguish uranium metal castings of different enrichments with them for the purpose of verification.Preliminary simulation has been carried out in the paper. Simulation on basic conditions with MCNP shows the time correlation coincidences counts, and the distribution of the neutron and gamma rays signal is analyzed. The result of simulation proves that the experimental program is feasible.In the experiment with the 252Cf fast fission chamber neutron source, this paper measure and analyze the characteristic of time correlation coincidences counts which are between source-detector. First of all with the basis of many comparative experiments, the suitable layout of configuration and the measurement conditions are designed. At the same time distinguish the neutron and gamma rays signal by rise time method. Then carefully analyze the background and effective part in the time correlation signal and put forward a feasible method of data processing. With the signature parameter R12 we can effectively distinguish uranium metal castings of different enrichments and the sensitivity of this ratio is 19.4% per kg of 235U. The sensitivity coefficient is defined asΔR/Δm/R where m is the uranium mass in kilograms and R is the average ratio as a function of mass.In the experiment with the DT accelerator neutron source, this paper measure and analyze the characteristic of time correlation coincidences counts which are between source-detector and detector -detector. Carry out exploratory experiments with the experience of 252Cf experiment. With the signature parameter R12 and R23 we can effectively distinguish uranium metal castings of different enrichments and the sensitivity of this ratio is 12.09% and 16.88%. This experimental result shows the feasibility of timing tagged DT generator.This paper analyzes the characteristics of the time correlation signal C12(τ) and C23(τ). R23 is more sensitive than R12 and has excellent SNR. But the count rate of C23(τ) is too low and a strong neutron source is needed.This paper analyzes the advantages and disadvantages of the two kind of neutron source, then put forward a clear technical requirements for development of DT neutron tube. The 252Cf fast fission chamber neutron source has better distinguished results and higher SNR, but the DT accelerator neutron source has shorter measurement time and can measure C23(τ). The DT neutron tube will have a great advantage in practice.