  Precise radon measurements are a requirement for various applications,
ranging from radiation protection over environmental studies to material
screening campaigns for rare-event searches. All of them ultimately depend on
the availability of calibration sources with a known and stable radon emanation
rate. A new approach to produce clean and dry radon sources by implantation of
$^{226}$Ra ions into stainless steel has been investigated. In a proof of
principle study, two stainless steel plates have been implanted in
collaboration with the ISOLDE facility located at CERN. We present results from
a complete characterization of the sources. Each sample provides a radon
emanation rate of about 2 Bq, which has been measured using electrostatic radon
monitors as well as miniaturized proportional counters. Additional measurements
using HPGe and alpha spectrometry as well as measurements of the radon
emanation rate at low temperatures were carried out.
