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Radon-in-gas activity standards

The source used for the preparation of radon-in-gas activity standards is solid radium sulphate in a special steel confinement, embedded in a filling facility made of special steel and evacuated before filling. This source allows Rn-222 with an activity A of about 2.2 MBq to be produced. By evacuating the facility and subsequent cooling of a suitable filling vessel with liquid nitrogen, Rn-222 can be completely transferred from the source without use of a carrier gas. By adding a carrier gas (N2) the pressure in the vessel can be increased up to ambient pressure.

Special steel cylinders with bellows-type valves (volume V = 50 cm3 or = 100 cm3) or glass flasks of different volumes varying from V = 40 cm3 to = 140 cm3 are preferably used as filling vessels. After consulting with the PTB, filling vessels may also be forwarded by the customer.

The activity of the radon gas is determined with the aid of a calibrated photon spectrometer by measuring the short-lived daughter products which are in radioactive equilibrium with Rn-222 after about 4 hours.

The relative expanded measurement uncertainty (k = 2) assigned to the activity is 2 %.

Availability: on request.