IAEA SAFETY GUIDES - gnssn - International Atomic Energy Agency

This publication is no longer validPlease see http://www-ns.iaea.org/standards/to identify the source(s) of the contamination. Samples should be collected atbranches in the air distribution system and at any places where mined out areasmay connect to the primary airway. Leaking bulkheads can be a major source ofcontamination.4.4. THORON (220Rn) AND THORON DAUGHTERSSampling strategies for thoron and thoron daughters are essentially the sameas those for radon and radon daughters, but the widely different half-lives of theradionuclides necessitate changes in the techniques employed. The very short halflifeof thoron means that measurements with ionization chambers or scintillation cellsmust be performed immediately after sample collection. However, in most cases thepresence of thoron is inferred from measurements of daughter concentrations mademany hours after sample collection. The accurate determination of the thorondaughter concentrations requires a counting delay until the short-lived radon daughtershave decayed to negligible levels (generally three hours). Details are given inSection 5. Several of the detection methods used for radon daughters can be modifiedby changing the delay times and counting times to give acceptable measurements ofthoron daughters. These modifications will affect the work cycle of the person takingthe sample, possibly changing the number of samples that can be processed in a shiftand changing the person’s optimized sampling route through the mine. However, thebasic theory, relating sampling frequency to the coefficient of variation of the results,remains unchanged.Because of their different half-lives, thoron and its daughters respond differentlyto ventilation changes. The very short half-life of thoron means that appreciableventilation changes have virtually no effect on its concentration. However, thorondaughter concentrations are significantly more affected than radon daughter concentrations.These characteristics should be kept in mind when sampling results are usedto trace the sources of high concentrations and to assess the effects of ventilationchanges.The presence of thoron daughters will influence the results of radon daughtermeasurements, resulting in overestimates of radon daughter concentrations. Whenthorium is present at appreciable levels, the presence of thoron and its daughtersshould be checked for to assess the magnitude of the resulting error.Personal dosimeters commercially available for monitoring exposure to radondaughters can also measure exposure to thoron daughters. The use of these devicesis subject to the same limitations as is their use for radon and radon daughters.Except in the case of a thorium mine or mill, thoron daughter exposure is generallymuch lower than radon daughter exposure and is unlikely to require individualpersonal dosimetry.20