  Radon and radon daughters pose significant backgrounds to rare-event
searching experiments. Activated carbon, which has very strong adsorption
capacity for radon, can be used for radon removal and radon enrichment. The
internal $^{226}$Ra concentration ultimately limits its radon enrichment
ability. In order to measure the intrinsic background and study the radon
adsorption capability of Saratech activated carbon at various temperatures, a
radon-emanation measurement system with a high-sensitivity radon detector and
an adsorption-performance research-system have been developed. In this paper, a
0.71~mBq/m$^3$ high-sensitivity radon detector and measurement details of the
radon-adsorption capability of Saratech activated carbon at low temperature
will be presented.
