Insufficient thermalization effects on determining fission-track ages  

作　　者：PANG JianZhang ZHENG DeWen WAN JingLin LI DaMing ZHANG PeiZhen YANG Jing 

机构地区：[1]State Key Laboratory of Earthquake Dynamics, Institute of Geology, China Earthquake Administration

出　　处：《Science China Earth Sciences》2013年第7期1233-1241,共9页中国科学（地球科学英文版）

基　　金：supported by State Key Laboratory of Earthquake Dynamics (Grant No.LED2009A05);National Natural Science Foundation of China (Grant No.41030317)

摘　　要：Fission-Track Dating (FTD) has been developed as a useful technique for geological studies. Parent elements are measured by counting 235 U tracks induced by thermal neutrons. If insufficient thermalization occurs, fission of 238 U and 232 Th 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 232 Th/ 238 U ratios of samples and standards is the main factor to the experiment results. The results will not be affected if the 232 Th/ 238 U value of samples is equal to the standard samples. However, if the 232 Th/ 238 U ratio is larger than that of the standards, the results will be smaller than actual ages. Comparatively, the ages will be more than expected if the 232 Th/ 238 U 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 ζ value and using the monitor of standard glass SRM612 and CN5 together.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.

关 键 词：neutron thermalization fission track dating 232Th/23SU ratio 

分 类 号：O571.43[理学—粒子物理与原子核物理] P597.3[理学—物理]