  In this paper, we describe 220Rn calibration source that was developed for
liquid noble gas detectors. The key advantage of this source is that it can
provide 212Bi-212Po consecutive events, which enables us to evaluate the vertex
resolution of a detector at low energy by comparing low-energy events of 212Bi
and corresponding higher-energy alpha-rays from 212Po. Since 220Rn is a noble
gas, a hot metal getter can be used when introduced using xenon as the carrier
gas. In addition, no long-life radioactive isotopes are left behind in the
detector after the calibration is complete; this has clear advantage over the
use of 222Rn which leaves long- life radioactivity, i.e., 210Pb. Using a small
liquid xenon test chamber, we developed a system to introduce 220Rn via the
xenon carrier gas; we demonstrated the successful introduction of 6 times 10^2
220Rn atoms in our test environment.
