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

研究宇宙线的长期变化为我们研究宇宙线的起源、传播和加速提供了线索之一。此外，太阳活动和地磁变化会影响进入地球的宇宙射线的强度，因此，测量过去时间内宇宙线的时间变化可以了解过去的太阳活动和地球过去的环境。在这里，我们测量了山形县一块埋在地层中的木头中的宇宙放射性同位素C-14。从13000年前到2000年，这个地层大约在250年被分层。在木材和亚炭条件下，木材样品的萃取率分别为25%和5%。样品的~(13)C年龄为13756±51年BP，采用超低水平液体闪烁计数系统，准确度为0.2%。除1个样品外，AMS的5个层位样品的~(14)C年龄随深度的变化规律相似。对于3米深来说，年龄相差大约1500年。然而，其中一个样本的年龄比地层各层样本的估计年龄早了约1000年。这表明13500年前C-14的浓度随300年的时间尺度而变化。这项研究表明，放射性碳年龄的测量对于利用地层的累积层研究宇宙线的C-14变化和时间变化是有效的。

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.