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Radon Daughters - Physical data and measuring units

As usual the radon (Rn-222) gas concentrations unit in air is Bequerel per cubic meter. The decay of radon results in a decay chain with the short-living radioactive nuclides Po-218, Pb-214, Bi-214 and Po-214. In general Po-214 is not specified. Because of its short halflife the concentration of Po-214 is in practice equal to the concentration of Bi-214.

Decay chain of Rn-222

Decay products concentration often is described with a special unit: The Potential Alpha Energy Concentration (PAEC). This unit origins from uranium ore mining and quantifies the biological influence of a decay products mixture. It describes the resulting a-Energy of all decay products in a distinct volume until their total decay. Common units are: MeV/l or Working Level (1 WL = 130000 MeV/l).

Data of radon decay products
Nuclide Po-218 Pb-214 Bi-214 Po-214
Halflife 3.05 min 26.8 min 19.7 min 1.6·10-4 sec
Decay a b b a
a-Energy 6.00 MeV 0 0 7.68 MeV
pot. a-Energy/atom 13.68 MeV 7.68 MeV 7.68 MeV 7.68 MeV
Atoms/Bq 264 2320 1710 0.23·10-3
pot. a-Energy/Bq 3612 MeV 17820 MeV 13130 MeV 1.8·10-3 MeV
pot. a-Energy/100pCi 13350 MeV 65890 MeV 48460 MeV 6.6·10-3 MeV

In the case of equilibrium between radon and its progenies exist a stationary relation between radon gas concentration and the PAEC.

13350 MeV/l + 65890 MeV/l + 48460 MeV/l = 127700 MeV/l

1.3·105 MeV/l = 1 Working Level (WL)

The PAEC often is expressed as Equivalent Equilibrium Concentration (EC or EEC) with:

1 WL = 3700 Bq/m³ (EC)

The EC value is in most cases lower than the real radon gas concentration. The relation between EC and radon gas concentration CRn is called Equilibrium Factor F.

F = EC / CRn

The value of the Equilibrium Factor is F = 0...1 - in buildings about F = 0.5 as mean value.