General information

The thoron concentration can be measured analogue to the radon concentration. For the analysis, the Po-216 peak is used. Due to the short half-life of Rn-222 of only 55 sec, the equilibrium between the exhalating Rn-220 atoms and the positive charged Po-216 is adjust after some minutes. Therefore the intensity of the peak should be constant in all intervals.

Typical sample spectrum

Calculation

During the n-th. interval, the thoron concentration (Bq/m³) inside the measuring sphere is:

c_n = I_n / (TeV) with

I_n: measured counts per interval n,
T: length of interval (s),
e: efficiency of measurement device (counts per decay),
V: Volume of the measurement sphere (m³)

The exhalation rate (Bq/m²s) can be determined to:

F = lVc / A with

l: decay constant of thoron (1/s),
c: mean concentration (Bq/m³) inside the measurement volume (V),
A: the exhalating area (m²)

Measurements with an calibrated thoron source and high voltage (some kV) between the measuring volume and the surface barrier detector showed the same efficiency as with radon.