The 222Rn standard system established at IFIN-HH, Romania

The paper presents the 222Rn Standard System realized at the Radionuclide Metrology Laboratory from IFIN-HH, Romania. It contains a Pylon solid 226Ra source, and a glass circuit for circulation and recovery of 222Rn in glass ampoules, at the 77 K temperature. The radon can be recovered both in ampoules with liquid scintillator (LS), for absolute standardization of 222Rn by liquid scintillation counting (LSC), and in evacuated ampoules as gas, simultaneously or by the transfer of radon from gas into LS ampoules, in order to establish the traceability chain. The absolute standardization consists in the registration of the double coincidence counting rate in LS, due to the whole sequence of short life components of the 222Rn chain, in equilibrium conditions. The main correction applied to the measurement results is due to the decay of 214Po during the extendable dead time of the system. The following procedure was applied to take it into account. The value of the base duration of the dead time was precisely measured and used for the data corrections. The measurements of each source were repeated during a period of several days, and the decay curve parameters were calculated. If the dead time value and the correction formulae, presented throughout the paper, were adequate, then the re-determination of the 222Rn half life, T1/2=(3.8232±0.0008) d, using the decay curve of the corrected counting rates should be correct. On the other hand, the effective dead time values obtained by parallel calculations were compared. The paper presents the results obtained in the standardization of several sources, by LSC, and the evaluation of the uncertainties associated to the method.