GSG-7 Occupational Radiation Protection - Framework for Occupational Radiation Protection - Dosimetric Quantities - Quantities for monitoring short lived progeny of radon (222Rn)

Quantities for monitoring short lived progeny of radon (222Rn)

2.60.

The dose to the lung arises almost entirely from the short lived progeny of ²²²Rn, rather than from²²²Rn itself (see para. 5.45). The short lived progeny are unlikely to be in equilibrium with the parent radionuclide. Therefore, for purposes of radiation protection, special quantities are used for expressing the concentration of ²²²Rn progeny in air and the resulting exposure due to inhalation.

Potential alpha energy

2.61.

The potential alpha energy ε_p of a single atom of a short lived ²²²Rn progeny radionuclide is the total alpha energy emitted by that atom during complete decay from ²²²Rn to ²¹⁰Pb.

2.62.

The potential alpha energy emitted by 1 Bq of a radionuclide, rather than by a single atom, is given by:

where λ is the decay constant (in reciprocal seconds) and t is the half-life of the radionuclide (in seconds). The relevant values for the short lived progeny of ²²²Rn are given in Table 1.

TABLE. 1. Potential Alpha Energies of Short Lived ²²²Rn Progeny

Radionuclide	Half-life	Alpha energy (J)	Yield (%)	Potential alpha energy
Radionuclide	Half-life	Alpha energy (J)	Yield (%)	Per atom ε_p (J)	Per unit activity ε_p/λ (J/Bq)
Po-218	3.098 min	0.962 × 10⁻¹²	100	2.19 × 10⁻¹²	0.588 × 10⁻⁹
Pb-214	26.8 min	Nil (beta emitter)	-	1.23 × 10⁻¹²	2.85 × 10⁻⁹
Bi-214	19.9 min	Nil (beta emitter)	-	1.23 × 10⁻¹²	2.12 × 10⁻⁹
Po-214	164.3 μs	1.23 × 10⁻¹²	100	1.23 × 10⁻¹²	3 × 10⁻⁹

Source: 2014 data from the NuDat Database (see www.nndc.bnl.gov/nudat2).

Potential alpha energy concentration

2.63.

When considering exposure situations involving ²²²Rn progeny, it is usual to express the total potential alpha energy as an energy concentration in air (in joules per cubic metre). This is referred to as the potential alpha energy concentration. For any mixture of short lived ²²²Rn progeny in air, the contribution of each radionuclide to the potential alpha energy concentration is its potential alpha energy per unit activity (ε_p/λ) as given in Table 1 multiplied by its activity concentration c. The total potential alpha energy concentration is then the sum of these individual contributions:

2.64.

It can be deduced from Table 1 (simply by adding the values in the right hand column) that if all the progeny were to be in equilibrium with the parent ²²²Rn at a concentration of 1 Bq/m³, the potential alpha energy concentration of the mixture would be 5.56 × 10⁻⁹ J/m³.

2.65.

In practice, the progeny will rarely, if ever, be in equilibrium, and the potential alpha energy concentration will, therefore, be some fraction of the equilibrium value. This fraction is called the equilibrium factor F:

2.66

By way of example, consider a non-equilibrium mixture of ²²²Rn and its progeny, in which the individual radionuclide activity concentrations are 100 Bq/m³ for²²²Rn, 75 Bq/m³ for ²¹⁸Po, 50 Bq/m³ for ²¹⁴Pb and 25 Bq/m³ for each of ²¹⁴Po and ²¹⁴Bi. From Table 1, the potential alpha energy concentration of the mixture is:

2.67.

If the mixture were in equilibrium, all radioisotopes of the decay series would have an activity concentration of 100 Bq/m3 and the potential alpha energy concentration, in accordance with para. 2.64, would be:

The equilibrium factor of the mixture is therefore:

Potential alpha energy exposure⁷

2.68.

The exposure of an individual to ²²²Rn progeny (P_RnP) is determined by multiplying the potential alpha energy concentration (in joules per cubic metre) by the exposure period (in hours). The exposure is therefore expressed in units of joule hours per cubic metre. Since the potential alpha energy concentration will generally vary during the exposure period, the exposure should be calculated as an integral over time:

where τ is the period of exposure. The exposure period is usually calculated over the course of one year. It is common to adopt a default annual exposure period of 2000 h for workplaces. It should be borne in mind that the adoption of this default value may lead to a conservative estimate of the annual exposure.

Equilibrium equivalent concentration and equilibrium equivalent exposure

2.69.

There is an alternative way of referring to the concentration of ²²²Rn progeny in air. If the ²²²Rn gas concentration (in becquerels per cubic metre) is multiplied by the equilibrium factor F, the resulting quantity is called the equilibrium equivalent concentration (EEC) of the ²²²Rn parent (also expressed in units of becquerels per cubic metre). The EEC can be regarded as the concentration of ²²²Rn in equilibrium with its progeny that would give the same potential alpha energy concentration as the actual non-equilibrium mixture. It can be determined from para. 2.64 that the numerical relationship between the potential alpha energy concentration and the EEC is as follows:

In the same way, exposure due to ²²²Rn progeny can be expressed as the equilibrium equivalent exposure, in units of becquerel hours per cubic metre:

The choice between potential alpha energy exposure and equilibrium equivalent exposure is not important, since these two quantities are simply related by a constant factor of 5.56 × 10⁻⁹ J/Bq.

²²²Radon gas concentration as a surrogate for exposure due to ²²²Rn progeny

2.70.

In many situations involving exposure due to ²²²Rn progeny, the measurement process can be simplified considerably by using the time weighted average ²²²Rn gas concentration in air (in units of becquerels per cubic metre) as a surrogate for potential alpha energy. For instance, measurements in a large number of buildings over an extended time period are best made using passive track etch devices that detect ²²²Rn. Such devices are small, simple, robust and inexpensive. When adopting this approach, an appropriate value for the equilibrium factor F should be assumed. The use of a default value of 0.4 is usually adequate for this purpose. It has been found that most values of F in indoor air are within 30% of this value. However, workplaces such as underground mines or water treatment facilities can have significantly lower F values. The potential alpha energy exposure is then given by:

Potential alpha energy exposure (J·h·m^-3) = ²²²Rn concentration x 5.56 x 10^-9 x 0.4 x T (18)

where T is the exposure period (h). By using a default annual exposure period of 2000 h for workplaces, this formula gives a potential alpha energy exposure of 4.45 × 10⁻⁶ J·h·m⁻³ for a²²²Rn concentration of 1 Bq/m³.