Development and calibration of a method for direct measurement of ²²⁰Rn (thoron) activity concentration

The interest in the measurement of thoron (²²⁰Rn) activity concentration in air has recently increased, with the attention for the development of standards useful for the calibration of measuring instruments. Due to its short half-life (55.8 s), consolidated techniques for the realization and the use of controlled atmospheres of radon (²²²Rn) are not effective in the case of thoron. New adequate methodology are required. A method for the measurement of the thoron reference activity based on the direct detection of the alpha particles produced by the decay of ²²⁰Rn, from natural samples containing ²³²Th, is here proposed. The possibility of observing an acceptable spectrum is entrusted to the realization of measurement chambers small enough to reduce as much as possible the energy loss of the alpha particles before they reach the Silicon detector. Such a chamber was realized and used with a known thoron atmosphere in a controlled environment. The experimental results are compared with those obtained from the simulation with the Monte Carlo method: (i) the alpha spectra coming directly from thoron atoms exhibit similar shape with a linear trend plus an exponential trend on the left side of the ²²⁰Rn peak, depending from the distance between source and detector; (ii) the corresponding ²²⁰Rn detection efficiency values are compatible considering the uncertainties. An investigation was conducted to study the shape of the ²²⁰Rn spectra vs the different volume of chambers, through Monte Carlo simulations. The results show: (i) the linear part of the spectra shape goes to reduce with the increasing of the height of the chamber; (ii) the ²²⁰Rn detection efficiency decreases with the increasing of the height of the chamber.