Latitude Surveys of Time-Delay Histograms from a Semi-Leaded Neutron Monitor during 2018-2020

P. Yakum*,1, W. Nuntiyakul 1, S. Khamphakdee 1, A. Seripienlert 2, D. Ruffolo 3, A. Saiz 3, P. Evenson 4, P.-S. Mangeard 4, P. Jiang 5, P. Chuanraksasat 2, C. Banglieng 6, and K. Munakata 7
1 Department of Physics and Materials Science, Faculty of Science, Chiang Mai University, Chiang Mai 50200, Thailand
2 National Astronomical Research Institute of Thailand (NARIT), Chiang Mai 50180, Thailand
3 Department of Physics, Faculty of Science, Mahidol University, Bangkok 10400, Thailand
4 Bartol Research Institute and Department of Physics and Astronomy, University of Delaware, Newark, DE 19716, USA
5 Polar Research Institute of China, Pudong, Shanghai 200136, China
6 Department of Physics, Faculty of Science and Technology, Rajamangala University of Technology Thanyaburi, Pathum Thani 12110, Thailand
7 Physics Department, Shinshu University, Matsumoto, Nagano 390-8621, Japan

We have developed a portable neutron monitor with three counters for latitude surveys to investigate cosmic ray spectral variations. This “Changvan” uses the NM64 design, except that the middle counter lacks the lead producer, so we call this a “semi-leaded” neutron monitor. The Changvan was operated on two voyages of the Chinese icebreaker Xue Long between China and Antarctica in the 2018 and 2019. The standard measurement during a latitude survey is the count rate as a function of geomagnetic cutoff rigidity, i.e., the response function of the total count rate. Repeated measurements with the same detector over different solar cycle phases provide precise information about cosmic ray spectral variation. In addition, we have tested two techniques to track spectral variation, which have or could be implemented at fixed stations. First, histograms of the time delay between successive neutron counts determine the leader fraction, previously used to monitor short-term and solar-cycle spectral variations. Second, the count rate ratio of unleaded vs. leaded counters varies enormously with geomagnetic cutoff rigidity, indicating sensitivity to the cosmic ray spectrum. This may have advantage relative to using a “bare” counter in that this “unleaded” counter is shielded from the environment by the reflector and has a higher count rate due to the adjacent lead. We report measurement of the response function of the count rate and the leader fraction of the unleaded and leaded counters during these two latitude surveys. Partially supported by grant RTA6280002 from Thailand Science Resesarch and Innovation.

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