# Siam Physics Congress 2015

20-22 May 2015
Asia/Bangkok timezone
The Centennial Celebration of General Relativity Theory and 80 Years of Thai Physics Graduate

## Variations in the Neutron Time Delay Distribution at the Princess Sirindhorn Neutron Monitor

21 May 2015, 13:30
15m
White Lotus ()

### White Lotus

Oral presentation Astronomy, Astrophysics, and Cosmology

### Speaker

Dr Alejandro Sáiz (Department of Physics, Faculty of Science, Mahidol University, Bangkok 10400 Thailand)

### Description

The Princess Sirindhorn Neutron Monitor (PSNM) has operated since late 2007 at the summit of Doi Inthanon, Thailand's highest mountain, in Chiang Mai province. PSNM records the flux of galactic cosmic rays (GCRs) impinging on Earth's atmosphere, as well as the GCR flux variations with time due to solar activity. PSNM has the world's highest cosmic ray vertical cutoff rigidity (momentum per charge) for a fixed station, 16.8 GV. The recorded count rate comes from the detection of nuclear disintegrations of $^{10}$B in BF$_3$-filled proportional counters, mostly due to interactions with neutrons evaporated from the Pb producer when hit by a cosmic-ray-generated atmospheric shower particle (usually a neutron). In addition to monitoring the count rate, the PSNM electronics can record neutron time delays (the time between one count and the next) in each counter. An analysis of time delay histograms shows an exponential tail at long times (> 1 ms) that can be interpreted in terms of chance coincidences, i.e., counts associated with independent atmospheric particles. Shorter time delays, however, are dominated by counts originating from the same interaction between a Pb nucleus and an atmospheric particle. This component contains information about the energy distribution of atmospheric shower particles and, thus, about the GCR energy spectrum as well, and its variations with time are of interest. In this work we analyze neutron time delay histograms from PSNM and derive the leader fraction $L$, i.e., the fraction of neutron counts not associated with a previous count in the same counter from the same nuclear interaction. We report on variations of $L$ with time, their dependence on atmospheric depth and water vapor pressure, and show that, after correcting for these environmental effects, the remaining variations are of interplanetary origin. This represents a qualitatively new type of observation in the 60-year history of neutron monitor science.

### Primary author

Dr Alejandro Sáiz (Department of Physics, Faculty of Science, Mahidol University, Bangkok 10400 Thailand)

### Co-authors

Burin Asavapibhop (Department of Physics, Faculty of Science, Chulalongkorn University, Bangkok 10330, Thailand) Chanruangrit Channok (1.Department of Physics, Faculty of Science, Ubon Ratchathani University, Warin Chamrap, Ubon Ratchathani 34190, Thailand, 2.Rajamangala University of Technology Isan, Nakhon Ratchasima, 30000, Thailand) Chawewan Chaiwattana (Department of Physics, Faculty of Science, Ubon Ratchathani University, Warin Chamrap, Ubon Ratchathani 34190, Thailand) Prof. David Ruffolo (Department of Physics, Faculty of Science, Mahidol University, Bangkok 10400 Thailand) John M. Clem (Bartol Research Institute, University of Delaware, Newark, DE 19716) John W. Bieber (Bartol Research Institute, University of Delaware, Newark, DE 19716) Kazuoki Munakata (Department of Physics, Shinshu University, Matsumoto 390- 8621, Japan) Manit Rujiwarodom (Department of Physics, Faculty of Science, Chulalongkorn University, Bangkok 10330, Thailand) Nattapong Kamyan (Department of Physics, Faculty of Science, Mahidol University, Bangkok 10400 Thailand) Nipon Gasiprong (Department of Physics, Faculty of Science, Ubon Ratchathani University, Warin Chamrap, Ubon Ratchathani 34190, Thailand) Paisan Tooprakai (Department of Physics, Faculty of Science, Chulalongkorn University, Bangkok 10330, Thailand) Paul Evenson (Bartol Research Institute, University of Delaware, Newark, DE 19716) Dr Pierre-Simon MANGEARD (1.National Astronomical Research Institute of Thailand, Chiang Mai, 50200 Thailand 2.Department of Physics, Faculty of Science, Mahidol University, Bangkok 10400 Thailand) Supon Sumran (Department of Physics, Faculty of Science, Ubon Ratchathani University, Warin Chamrap, Ubon Ratchathani 34190, Thailand) Suttiwat Madlee (Department of Physics, Faculty of Science, Mahidol University, Bangkok 10400 Thailand) Tanin Nutaro (Department of Physics, Faculty of Science, Ubon Ratchathani University, Warin Chamrap, Ubon Ratchathani 34190, Thailand)

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