The French Institute of
Radiation Protection and Nuclear Safety (IRSN) is in charge of the
radiological surveillance of the environment. In this framework the
Laboratory of Environment Radioactivity Measurement (LMRE) measures the
radioactivity concentrations in various environmental samples:
biological matrices (aquatic moss, seaweed, milk, vegetables …), soils,
waters or aerosol filters.
Artificial radionuclides searched are in low
proportion compared to natural radionuclides: potassium 40 in biological
matrices or radon particular daughters in aerosol filters. The
significant Compton continuum induced in the gamma spectrum makes
difficult the identification and the quantification of radionuclides
present at trace level.
The use of two detectors enables to make
coincident spectrometry in order to decrease this Compton background.
This technique was developed with an existing system of the laboratory,
the anti-Compton system, composed of a germanium detector surrounded by a
NaI(Tl) scintillator. A data analysis algorithm was developed and also a
Monte Carlo calibration if radionuclides measured are not available in
standard source. Moreover a new coincident measurement system was
designed by Monte Carlo simulation, called Leda consisting of two
germanium detectors face to face surrounded by a NaI(Tl) scintillator.
This new system overcomes the limits of the anti-Compton system.
Different measurement channels improve the detection limits for all
radionuclides measured in our laboratory.