1. Introduction

The Radon Daughter Monitor (RDM) measures also the Potential Alpha
Energy Concentration (PAEC) of thoron (Rn-220) decay products. It uses
continuous sampling and alpha spectroscopy. This method does not need any
calibration chamber. It is similar to methods controlling such a chamber.

The evaluation of thoron PAEC uses the 8.8 MeV alpha radiation of
Po-212. There is no influence of the thoron reading by Rn-222 progenies
(maximum alpha energy of

Because of the used technique the separation of different thoron decay
products is not possible. That is, the instrument does not give individual
results for Pb-212 and Bi-212. Additionally it cannot estimate the Rn-220 gas
concentration as it does for Rn-222.

2. Evaluation algorithm

The activity of the nuclides at the surface of the sampling filter
increases during sampling. Therefore also the calibration factor of the
instrument changes continuously. The is
taken into account by the evaluation software. During a measurement the RDM
simulates collection and decay of the radioactivity on the sampling filter of
the instrument. That is, it solves the differential equations describing this
process in real time using the actual, measured air flow rates. The simulation
calculates an alpha count for a predefined fixed PAEC. The registered alpha
counts of the Po-212 activity at the filter is compared with the simulated
alpha counts. The actual PEAC is calculated from both counts and the predefined
PAEC.

The tables in the appendix show the calculated filter activities and
alpha counts. The tables are created by exactly the same algorithm as used in
the RDM.

3. Assumptions

The calibration factor of the RDM is the ratio of alpha counts and the
Potential Alpha Energy Concentration in the air. It is influenced by the
sampling time, the air flow rate, and the mixture of thoron decay products.
There exist two main decay products in the environmental air: Pb-212 and
Bi-212. The concentration ratio R = [Bi-212]/[Pb-212] depends on the age of the
air and their wall deposition rates. That is, in living rooms the ratio depends
on the ventilation rate and the plate out at the walls and the furniture.

The concentration ratio is in the order of R = 0.5. The radioactive equilibrium (R = 1) is
unrealistic for buildings.

For relative short sampling times (up to 6 h) the concentration ratio
influences the reading of the RDM significantly. For longer sampling times the
reading is independent from the mixture ratio.

The standard software of the RDM assumes a ratio R = 0. Therefore it
overestimates the PAEC for short sampling times. This is done for radiation
protection purposes. The instrument displays never less than the real PAEC. On
request the RDM uses any other ratio. The most realistic results are given for
R = 0.5.

Table 1 shows the expected and the maximum systematic deviations. The
expected deviation is the reading at R = 0 compared with R = 0.5. The maximum
deviation is the reading at R = 0 compared with R = 1. The data are taken from
the tables in the appendix.

Table 1 |
||

Expected and maximum errors of the RDM for counting intervals of 2 h | ||

Counting interval | Expected deviation | Maximum deviation |

0 h - 2 h | 1.9 | 2.8 |

4 h - 6 h | 1.2 | 1.3 |

10 h - 12 h | 1.05 | 1.1 |

22 h - 24 h | 1.01 | 1.03 |

46 h - 48 h | 1.00 | 1.01 |

4. Lower detection limit

The tables of the appendix include also information about
the lower detection limit for thoron progeny. The tables are calculated for
typical operating conditions of the RDM.

The lower detection limit depends on the user-selected counting time
interval and the time of counting after the start of a measurement. The
examples in table 2 are calculated for 10 registered alpha counts during the
counting time interval. This corresponds with a mean error (standard deviation)

Table 2 |
||

Lower detection limit of the RDM for thoron progeny | ||

Counting interval | Working level | MeV / l |

0 h - 2 h | 0.001 | 150 |

0 h - 4 h | 0.0002 | 25 |

4 h - 6 h | 0.0001 | 15 |

10 h - 12 h | 0.00006 | 7 |

22 h - 24 h | 0.00003 | 4 |

46 h - 48 h | 0.00003 | 4 |

5. Time resolution

Sometimes the concentrations of radon (Rn-222) and thoron (Rn-220) in
buildings vary dramatically from one day to the next. Therefore a minimum
measuring time of several days is recommended. For such a long period the RDM
suits best.

The half life time of Pb-212 is 10.6 h. Therefore the time resolution
for thoron progeny is somewhere between 10 h and 20 h. Short pulses of PEAC are
not directly recognized by the instrument. But they are taken into account
precisely in the long-term mean values.

Also much shorter measuring times than 10 h are possible because the
evaluation software adjusts its calibration factor dynamically. All actual
calibration factors of the instrument are outputted on request.