Insufficient thermalization effects on determining fission-track ages

• Published in: Science China. Earth sciences 2013-07, Vol.56 (7), p.1233-1241
• Main Authors: Pang, JianZhang, Zheng, DeWen, Wan, JingLin, Li, DaMing, Zhang, PeiZhen, Yang, Jing
• Format: Article
• Language: English
• Subjects: Age; Earth and Environmental Science; Earth science; Earth Sciences; Geophysics; Glass; Heterogeneity; Irradiation; Meteorology; Monitors; Neutrons; Reactors; Research Paper; Thermal neutrons; 中子诱发; 标准样品; 横向不均匀性; 测量误差; 热中子反应堆; 热化; 照射位置; 裂变径迹
• ISSN: 1674-7313; 1869-1897
• DOI: 10.1007/s11430-012-4471-7

Fission-Track Dating （FTD） has been developed as a useful technique for geological studies. Parent elements are measured by counting 235U tracks induced by thermal neutrons. If insufficient thermalization occurs, fission of 238U and 232Th will be induced, and further measurement error will be introduced. Therefore, whether the neutrons are well thermalized or not will affect the FTD results. Due to requirement of safe operation, the 101 reactor was terminated in 2007. By using the 492 reactor as the new thermal neutron reactor, our present paper will attempt to study the feasibility and the potential influence on FTD. By irradiating monitor glass SRM612 and CN5 in pairs, we will study the thermalization situation of the 492 reactor. Irradiated data show that thermal neutrons are not evenly distributed either in horizontal or in vertical dimension. Especially, horizontal heterogeneity is obvious. But we discovered that proper irradiation position in the reactor can meet the requirement of FTD. Under the current irradiation condition, we calculated and assessed the insufficient thermalization effects on determining fission-track ages. We found that the difference between the 232Th]238U ratios of samples and standards is the main factor to the experiment results. The results will not be affected if the 232Th/238U value of samples is equal to the standard samples. However, if the 232Th]238U ratio is larger than that of the standards, the results will be smaller than actual ages. Comparatively, the ag- es will be more than expected if the 232Th]238U ratio is less. Therefore, to reduce the irradiation error, we suggest either locating the position of irradiation strictly, or minimizing the influence of lateral heterogeneity by reducing the amount of each sample package. Additionally, accuracy of the experimental results can be improved by increasing standard samples to adjust value and using the monitor of standard glass SRM612 and CN5 together.