There are diffusion sources available for calibrating radon devices. See attached.

The best calibration of a chamber is with a Lucas cell. All methods require special equipment.

en.wikipedia.org/wiki/Lucas_cell

There are several calibration chambers that will calibrate the devices for you.

www.bowser-morner.com/calispiking.html

Much will depend upon your needs. How big a system is required?

Thank you very much Dr.Joseph L Alvarez for you useful answer, thank you again for your attachments, really your contributions will help in understanding and building a good opinions about the technique.

Best regards and wishes

Yours

dr. Abd Elmoniem Ahmed Elzain

Alternatively the easyiest way is to partecipate to an  intercomparison

thank you Dr. Mariagabriella Pugliese