Study On Ams Measurement Method Of²³⁶U And Its Isotope Ratios In Trace Uranium

236U (T1/2=2.342(3)×107y)is trace in nature. The main236U obtained by U235capturing a neutron.235U(n,γ)236U. Nineteen fifties nuclear activities began to discharge236U of synthetic into the environment. Now above99%of mobile236U library in environment are artificially generated. Banned nuclear testing, a new energy of nuclear energy are widely used in the world with its safety, clean and other characteristics. Uranium is a kind of important fissile material, it is widely used in many fields, such as nuclear reactor, nuclear power plant. With sensitive trace U particles of nuclear in micron or submicron order, it deposited on the surface or around the nuclear facilities, or formed to aerosol suspension by long time in the atmosphere and spread to the distant of the environments, the collecting of these samples, determining the ratio of the236U and its isotope, which has very important significance for the characters, revealing present and past nuclear activities of the nuclear facilities, monitoring the safety operation of the nuclear reactor and so on. Analysis of isotopic abundance ratio of trace uranium is a common used methods including mass spectrometry, spectroscopy, fluorescence method. And with the advantages of little sample and high precision, analysis of mass spectrometry has became the most common method of analysis.At present, the development of analysis techniques are mainly three methods, such as combined fission track with thermal surface ionization mass spectrometry (FT+TIMS), combined scanning electron microscope with the same ionization mass spectrometry (SEM+SIMS) and accelerator mass spectrometry (AMS). However, due to the background of the presence of molecule interfering at measurement process, it is more difficult to determine the isotopic composition of the firstly two methods for234U,236U and others at low abundance.While the accelerator mass spectrometry (AMS) method has the advantages of less sample consumption, short measurement time, the higher sensitivity measurement, at recent years, with the continuous development of AMS technology and particle detection method, the samples of the uranium content in ng～pg order has become possible for AMS measurement accuracy.In the China Institute of Atomic Energy accelerator mass spectrometry group has created the uranium content in the traditional measuring method of236U-AMS in mg order, the work based on this study, AMS measurement methods of the uranium content were studied in the order of ng in the sample ratio of the236U and its isotope. (1)The main study of this work included three aspects of the sample preparation, the AMS measurement and the test method. Sample preparation:we compared the effect of AMS measurement of the samples which were made by the method of spot and the co-precipitation, determining the best method of the sample preparation, the method is reliable and practical. The series of the carrier of the iron powder of the uranyl nitrate of standard laboratory sample was prepared using236U/238U in10-1,10-2,10-3,10-4order, and established the process of sample preparation about the uranium content in the n g order.(2)The AMS measurement:it took the study of the beam analog transmission for samples of trace uranium, the best method were determined about the analog transmission; it were separately measured with a semiconductor detector on the ions of the234U,235U,236U and238U in AMS system terminal, avoiding the system error introduced by the two sets system of detection for measuring uranium isotopes (conventional method is to measure the238U using the Faraday cup in the beam of the low energy terminal and to measure the ion of the36U using semiconductor detector at the system terminal); for the samples of high content of the238U or235U, using the utilization of the particle beam attenuation system on AMS system attenuate the beam to meet the requirements of the strength of particle beam for the detector of semiconductor; comparing the AMS measurement effect of alternating measurement and scanning measurement method, optimizing the scheme of the measurement, AMS method were preliminarily established by the ratios of sample of236U and its isotope in the content of the uranium at the order of ng;(3) Testing method:Through the measurement of the series of standard sample, results show that measurement of236U/234U and236U/235U values are basically consistent with the nominal value, and the background of the238U is high in the carrier of high pure iron powder, limiting the accurate measurement of the samples (236U/238U-AMS) of Trace Uranium, so this paper used the measured background value of iron powder to correct the count rate of the sample of the238U, the modified abundance ratio is basically consistent with the nominal values showing a good linear relationship.This study firstly established the AMS method for measuring the ratios of236U and its isotope of ultra trace amounts of uranium, the method not only broadens the scope of application of AMS, but also laid a solid experimental foundation for the AMS measurement of granular samples in nuclear safeguards.