Correlation between thermal neutrons and soil moisture measured by ENDA

Hadrons are the "skeleton" of extensive air shower (EAS). They possess favorable information concerning composition and energy of cosmic ray. Thermal neutrons generated by the EAS hadrons in the ground as well as charged particles in EAS front plane can be detected by Electron-Neutron detector (EN-detector). A prototype of EN-Detector Array (ENDA), ENDA-16 was built at Large High Altitude Air Shower Observatory (LHAASO) to test its performance of detection of cosmic ray composition and energy spectrum. It has been proved in former work that there is a decrease of thermal neutrons detected in rainy season. For quantitative evaluation of influence of soil moisture on thermal neutrons, at the center of ENDA-16, five soil moisture meters are installed to record soil moisture. Negative correlation between thermal neutron counting rate and soil moisture is obtained. Moreover, it is demonstrated that a soil depth 0.5 m over the soil moisture sensor is enough for monitoring negative correlation between thermal neutron counting rate and soil moisture. The results provide us a method to correct the experimental data during the rainy season so as to reduce systematic uncertainty of thermal neutron measurement in the ENDA experiment.