A study of cosmic ray spectrum in ancient time with the measurement of the time variations of cosmic rays 15000 years past

点击翻译按钮获取中文摘要

Study of secular variations of cosmic rays provides us one of the clues to investigate origin, propagation, and acceleration of cosmic rays. Also, solar activities and geomagnetic variation affect the intensity of cosmic rays incoming to the earth and hence the measurement of the time variations of cosmic rays in the past time allows to know past solar activity and past environment of the earth. Here, we measured the cosmogenic radioisotope C-14 in a piece of wood buried in a stratum at Yamagata prefecture. The stratum was layered in about 250 years for 2000 years from 13000 years past. The extraction rates of the wood samples were 25 % and 5 % for the conditions of the wood and sub-charcoal, respectively. The radiocarbon age of the sample was 13756±51 year BP with ultra-low level liquid scintillation counting system of the accuracy of 0.2 %. The radiocarbon ages for the samples of five layers with AMS were like in line as a function of the depth except a sample. The difference of the ages was approximately 1500 years for the 3 m deep. However, the age of one sample was about 1000 years younger than the estimated age from each sample of the layers of the stratum. It implies that the concentration of C-14 around 13500 year BP past changed with the time scale of 300 years. This study indicates that the measurements of radiocarbon ages are effective for the investigation of C-14 variations and hence of time variation of cosmic rays employing the accumulated layer of the stratum.