Developing the measurement method of Sr-90 using AMS
Prof. Ming He1, Dr. qingzhang Zhao1, Dr. wenhui Zhang1, jiancheng Liu1, Pan Hu1, hongyu He1, Dr. Kangning Li1
1China Insttitute Of Atomic Energy, Beijing, China
90Sr is the major fission produce of uranium and plutonium and is interest in the environmental and health areas. In order to carry out 90Sr measurement with AMS, the methods of 90Sr measurement have been carried out at CIAE.
SrF2 is selected as target material, the procedures of preparation of SrF2 have been developed, The SrF2 sample mixed with five times its weight of PbF2 was pressed into holders and molecular negative ion of SrF3- is used for extracting from ion source.
SrF3- with beam current of about 200nA is extracted from the ion source. After passing through electrostatic analyzer and injection magnet, SrF3- is send to tandem accelerator which worked at terminal voltage of 11MV. Carbon foil is used as stripper at the terminal. A 90∘double focusing high energy analyzing magnet is used to selects 90Sr9+ with a the stripping probability of 25%. By means of the switching magnet, the particles were transported further to the Q3D spectrometer which have three dipolar magnets. In order to suppress the isobaric interference of 90Zr, the E-Q3D method is used. The ΔE-Q3D system consists of the Q3D magnetic spectrometer with an absorber placed at its entrance, which is used to absorb part of the ions’ energy, and a four-anode gas ionization chamber at the center of its focal plane. When isobars with the same energy pass through the absorber, their energy losses are different, then the Q3D magnetic spectrometer can separate isobars due to their different residual energies. Silicon nitride (Si3N4) membranes with a thickness of 1μm was used as the absorber and the ions of 90Sr20+ with a stripper possibility of 25% were selected by Q3D spectrometer. The results show that the optimum magnetic field value of 90Sr is 25 Gs larger than that of 90Zr, indicating that the residual energy of 90Sr is about 0.86% higher than that of 90Zr. According to the size of ionization chamber entrance window and the ion beam spot, the interference background of 90Zr can be reduced by 6 times after setting the Q3D magnetic field to the optimal magnetic field of 90Zr.
After separating with Q3D magnetic spectrometry, the 90Sr and part of 90Zr enter the gas ionization chamber. The chamber is used to further identify 90Sr and 90Zr by using their different energy loss. Four anodes and a cathode are used to identify 90Sr and 90Zr. A suppression factor of 6000 for 90Zr was obtained with the detector alone by application of appropriate gates on these five signals. By implementing the techniques described above, the Sensitivity of 90 Sr/Sr= 9.2×10exp(-13) has been obtained.
This work was supported by the National Natural Science Foundations of China (NSFC) under Grant No. 11875326 and the Continuous Basic Scientific Research Project(WDJC-2019-04).
Born in 1969, Doctor degree, Professor ,
the leader of AMS laboratory at CIAE .
Developed the miniaturized AMS systems at CIAE, and carried out multi-fields applications with AMS systems.