  In order to reduce backgrounds from radon-daughter plate-out onto detector
surfaces, an ultra-low-radon cleanroom is being commissioned at the South
Dakota School of Mines and Technology. An improved vacuum-swing-adsorption
radon mitigation system and cleanroom build upon a previous design implemented
at Syracuse University that achieved radon levels of
$\sim$0.2$\,$Bq$\,$m$^{-3}$. This improved system will employ a better pump and
larger carbon beds feeding a redesigned cleanroom with an internal HVAC unit
and aged water for humidification. With the rebuilt (original) radon mitigation
system, the new low-radon cleanroom has already achieved a $>$$\,$300$\times$
reduction from an input activity of $58.6\pm0.7$$\,$Bq$\,$m$^{-3}$ to a
cleanroom activity of $0.13\pm0.06$$\,$Bq$\,$m$^{-3}$.
