Development of a primary thoron activity standard for the calibration of thoron measurement instruments.

The LNHB and IRSN are working on a reference atmosphere for thoron ((220)Rn) instrument calibration. The LNHB, as the national metrology institute for activity measurement in France, has to create a new thoron reference standard in order to estimate with accuracy the thoron concentration of a reference atmosphere. The measurement system presented in this paper is based on a reference volume using an alpha detector, which is able to measure thoron and its decay products to define the thoron concentration of a thoron reference atmosphere. This paper presents the first results with this new system using a well-known radon ((222)Rn) atmosphere and a thoron ((220)Rn) atmosphere.