The understanding of radon migration in the earth's crust is important
in many applications, such as radiation protection in residential bui
ldings, uranium ore prospecting, or earthquake prediction. Recent stud
ies also show correlations of radon exhalation with short-periodic roc
k stresses induced by tidal forces. The borehole radon sensor presente
d can be used to investigate subsurface radon migration in situ. It is
capable of measuring the concentration of both radon and thoron in ai
r with a time resolution of less than an hour and a detection limit of
approximately 1 Bq m(-3), even in volumes as small as 100 ml. The sen
sor is fully automated and can be calibrated in situ. (C) 1997 America
n Institute of Physics.