摘要
影响^(40)Ar/^(39)Ar高精度定年的测试因素主要包括质谱仪的稳定性、质谱仪质量歧视效应,以及小信号离子束测试的准确性等。采用ArgusⅥ多接收稀有气体质谱仪获得高精度^(40)Ar/^(39)Ar定年数据,分别用1012Ω高阻放大器的法拉第杯和电子倍增器(CDD)对小信号离子束36 Ar进行测试对比,发现CDD的测试精度有较大的优势,当离子束信号强度低于5 000cps时,CDD的测试精度比配备1012Ω放大倍数的法拉第杯提高了1个数量级。对电子倍增器效率进行不同时段的连续测定,发现其测试效率存在波动且缺乏规律性,多次测量数据的相对标准偏差达5.3‰,因此需进行日常性的校正,以避免引起较大的测试误差;利用空气标准对仪器进行2个月60次循环测试,获得质量歧视因子d1amu为1.009±0.002(2σ),说明仪器测试性能稳定,对不同气量的空气标准进行测量时,仪器不存在由于气体压力不同而引起的质量歧视效应。通过测试实际样品,CDD可以提高低含量36 Ar样品^(40)Ar/^(39)Ar年龄的测试精度;对于混有一定比例的大气氩,但放射成因40 Ar比例较低的样品,CDD效率的微小变化会对年龄值计算产生显著的影响。证明了电子倍增器效率的日常性校正对于高精度^(40)Ar/^(39)Ar定年的重要性。
During the process of measurement of ^40Ar/^39Ar dating,the high precision of ages is limited partial by the stability of mass spectrometer,mass discrimination and the precision of the measurement of small ion beam.This study mainly described the advan-tages of ArgusⅥ multi-collector mass spectrometer in application of obtaining accurate high-precision ^40Ar/^39Ar dating data.Through comparison of the measurement results of small ion beam36 Ar collected by Faraday cup with 1012Ωamplifier and compact discrete dynode（CDD）respectively,it was found that the precision could be improved up to one order of magnitude if the signal size was smaller than 5 000 cps by CDD.The efficiency of CDD fluctuates lacked regularity as a result of environmental change and self-consuming,whose relative standard error was as high as 5.3‰ during continuous measurement,indicating that CDD should be calibrated frequently to avoid the obvious error of data.The air standard was determined for 60 cycles during 2 months with the mass discrimination factor d1 amuof 1.009±0.002（2σ）,indicating that the instrument was very stable,and different sizes of sample had little effect on the mass fractionation bias correction.Finally,two typical samples were chosen for ^40Ar/^39Ar dating measurement,the results showed that the CDD could improve the precision of the dating data for the sample which included extremely low36 Ar.Meanwhile,the slight change of efficiency of the CDD could greatly change the ^40Ar/^39Ar calculation age when the sample included a certain atmospheric 40 Ar but lack of radiogenic 40 Ar,proving the importance of routine calibration of the CDD for the accurate high-precision ^40Ar/^39Ar dating.
作者
刘汉彬
李军杰
张佳
金贵善
张建锋
韩娟
石晓
IU Han-bin;LI Jun-jie;ZHANG Jia;JIN Gui-shan;ZHANG Jian-feng;HAN Juan;SHI Xiao(Beijing Research Institute of Uranium Geology,Beijing 100029,China)
出处
《质谱学报》
EI
CAS
CSCD
北大核心
2018年第4期407-415,共9页
Journal of Chinese Mass Spectrometry Society
基金
中核集团龙腾项目(LTC-1605)资助
