The concentration of thoron in France is estimated to be one-tenth that of radon and is therefore a significant public health issue. To meet the need for traceability of measurements of this natural thorium descendant, a new primary standard instrument has been developed and validated at the LNHB to calibrate the instruments for measuring thoron volume activity in air.

Achievements:
– Creation of an adapted thoron measurement system (short half-life: 55.8 s)
– Complete study of the nuclear properties of radon, thoron and their progeny
– Description of charge distribution of thoron progenies nano-particles and their transport in air

Major results:
Analysis of the alpha spectra obtained made it possible to accurately qualify the volumic activity of a thoron atmosphere with an associated 1% relative standard uncertainty. The portable device thus made it possible to calibrate the thoron atmosphere produced in the Baccara facility, the experimental radon chamber of the IRSN Aerosol Physics and Metrology Laboratory (LPMA) dedicated to radon metrology studies.
Comparisons with ENEA (Italy) in the MetroNORM European Metrology Project

REFERENCES:
This work has been supported by the European Metrology Research Programme (EMRP), JRP-Contract IND57 MetroNORM.
– Sabot B., Pierre S., Michielsen N., Bondiguel S., Cassette P., (2015), Development of a primary thoron activity standard for the calibration of thoron measurement instruments, Radiat Prot Dosimetry 167 (1-3), 70-74.
https://doi.org/10.1093/rpd/ncv221
– Sabot B., Pierre S., Michielsen N., Bondiguel S., Cassette P., (2016), A new thoron atmosphere reference measurement system, Appl Radiat Isot. 109, 205-209.
http://dx.doi.org/10.1016/j.apradiso.2015.11.055
– Sabot B., (2015), Calibration of thoron (²²⁰Rn) activity concentration monitors, PhD Thesis,
http://www.theses.fr/2015SACLS122