Hi Kwon, For comparison, the background radiation level in Berkeley, California, near the UC Berkeley campus is about 150 mrem. This number was measured around 1989/1990 during the decommissioning of the campus TRIGA research reactor. Radoslav Radev Office of Radiation Safety UC Berkeley, CA 94720 (510) 642-6167 e-mail: rado@uclink2.berkeley.edu