Study of the periodicity of daily ambient gamma ray rates in the mid-Korean Peninsula

A study of the ambient gamma ray exposure rate is an essential part of underground astro-particle and astro-nuclear physics experiments. The daily variation of the ambient gamma ray exposure rate has been measured at a gamma-ray monitoring post in the mid-Korean peninsula from 2003 to 2006 by using a sinusoidal function with two free parameters, an amplitude and a phase, as a function of the time. The shifted phase indicates that the best-fitted sine curve starts 3 hours and 7 minutes delayed from 0:00 o'clock. The maximum and the minimum ambient gamma exposure rates appear at 9:07 am and at 9:07 pm in the best-fitted sinusoidal curve for the daily variation of ambient gamma ray rate. Also, we will provide some information for the daily gamma ray exposure rate.