  The radiation emitted from radon is a critical background in rare event
search experiments conducted at the Yemi Underground Laboratory (Yemilab) in
Jeongseon, Korea. A Radon Reduction System(RRS) has been developed and
installed in Yemilab to reduce radon concentration in the air. The RRS
primarily provides a purified air of 50 m3/h to the cleanroom used to assemble
crystal detectors in the AMoRE, a neutrinoless double beta decay search
experiment. RRS can reduce the radon level by a factor of 300, so a
high-sensitivity radon detector was required. A highly sensitive radon detector
was constructed using a 70 L chamber with a large PIN photodiode to measure
radon concentration in the purified air. The radon detector shows an excellent
resolution of 72 keV (FWHM) for 6.003 MeV alphas from 218Po decay and a
sensitivity down to 23.8 +- 2.1 mBq/m3 with a boil-off N2 gas sample. The radon
concentration level from the RRS measured by the radon detector was below 0.29
Bq/m3 with a reduction factor of about 300.
