Radon measurement facility

Radon set-up

Radon, a naturally occurring radioactive gas, produces radioactive isotopes of lead, bismuth and polonium. It is present in the environment at concentrations that vary from region to region. It is thus one of the main factors of exposition to ionizing radiation for populations. 222Rn is identified as the second risk factor for lung cancer, so on-site measurements are needed to identify risk areas. This monitoring is done using a variety of commercial measuring devices that need to be calibrated. To ensure metrological traceability of radon and its progeny, LNHB has developed a primary standard of 222Rn.

Cold finger outside the device

This 222Rn standard consists of an alpha spectrometry of a frozen radon source using the Defined Solid Angle method. The installation is a set of secondary vacuum tubes and valves (10-4 Pa) divided into three distinct parts.
The first part consists of a 226Ra source producing 222Rn. It is connected to a pumping assembly and allows the production of 222Rn quantities ranging from 100 Bq to 3 MBq depending on the chosen accumulation time.

Metal container in liquid nitrogen

This 222Rn migrates to the second part of the set-up which is a measuring chamber on the surface of a cold finger whose temperature is stabilized at 80 K (-193.15 °C). The source thus produced is a 222Rn frozen disk which is measured by the ASD method with an alpha detector with an intrinsic yield of 100%.

Once this measurement is completed, the cold finger is warmed up and radon will migrate to the third part of the device: a metal container bathed in liquid nitrogen. When the container is closed, the activity contained in the standard is known with a relative standard uncertainty of 0.3%.

This method has proven its robustness through several comparisons with other ionizing radiation metrology laboratories around the world. It remains today the most accurate and best defined method for the production of 222Rn primary standard.

REFERENCES:
B. Sabot, S. Pierre, P. Cassette, An absolute radon 222 activity measurement system at LNE-LNHB, Applied Radiation and Isotopes 118 (2016) 167–174.

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