Development of the LAFARA underground
laboratory of Ferrières, French Pyrénées
The LAFARA underground laboratory was created in 2007 and is among the three underground laboratories existing in France. Two gamma spectrometers are now placed in the tunnel of Ferrières, French Pyrénées. The germanium detectors are protected by 85 m of rock, which allow us to reduce the background associated with the cosmic rays and thus, to quantify the ultra-low levels of radioactivity present in our environmental samples. Analyses are performed for scientists at LEGOS and at the Observatoire Midi Pyrénéees (OMP, Toulouse) as well as for scientists from other laboratories in France or abroad.
The use of low background gamma-ray spectrometry has increased in the past 20 years. The detection of low radioactivity levels is indeed needed in various fields, including environmental sciences, fundamental physics, control of drinking water and food, surveillance of nuclear activity etc… Regarding applications in earth sciences, both anthropogenic and natural radionuclides can be used as tracers in the marine and terrestrial environments. The decay of the radionuclides also provides the opportunity to use them as chronometers and dating tools. A large variety of half-lives allows us to cover a wide range of time scales (e.g. from hour to thousands years). However, the levels of radioactivity present in our environmental samples are often small (diluted media). Their analysis thus requires sensitive detectors and a very low background. Among the prerequisite required to conduct low background gamma-ray spectrometry, one can list the following points: (i) the construction materials of the detector and shield need to be carefully selected, (ii) the radon activity in the air near the detector needs to be as low as possible and (iii) the flux of cosmic muons should be reduced by placing the detectors in underground conditions. The rock cover above the detector thus constitutes a passive shield for cosmic radiation. The LAFARA underground laboratory located in the tunnel of Ferrières in the French Pyrénées was thus created in 2007.
Description of the facility
Two germanium spectrometers are currently running at the underground laboratory of Ferrières. The first spectrometer is a well-type, high efficiency, high purity germanium detector manufactured using selected materials. The volume of the germanium crystal is 280 cm3 (equivalent to 1.5 kg of germanium). The diameter and depth of the well are 15 mm and 50 mm, respectively. The second detector is a recent spectrometer designed by ORTEC/AMETEK that we purchased in year 2011 (FEDER funds). This detector is a semi-planar, p-point contact, detector (PROFILE-FX series). The high purity germanium crystal has a diameter of 85 mm and a thickness of 33.2 mm, with a volume of 183 cm3 (0.97 kg of germanium).
The LAFARA underground laboratory is specially designed to analyze samples automatically. This is because (i) the two germanium spectrometers are located at 100 km from our home laboratory in Toulouse and (ii) no technician stays there permanently. The spectrometers are thus equipped with autosamplers and systems that can fill liquid nitrogen automatically. Additionally, specific equipment is installed in the underground laboratory to ensure the remote functioning of the detectors at a distance (webcam ; connexion to the computer and instruments through VNC). The description of the facility can be found in van Beek et al.(2013).
Since June 2008, the underground laboratory of Ferrières is part of the European CELLAR network (Collaboration of European Low- level underground LAboRatories). This is due to the quality of the background achieved in the LAFARA laboratory. LAFARA is part of the PANGEE platform of geochemical analyses at OMP, Toulouse. Analyses are thus performed for scientists at OMP but also from other laboratories, in France or abroad. A large variety of samples is analyzed: water samples, marine particles, sediment cores (lakes, ocean), aerosols, lichens, various materials etc…
Fig. 1: Comparison of the integral background counting rate (40-2700 keV) obtained with the two detectors located at the underground laboratory of Ferrières (LAFARA) with the backgrounds obtained in different above-ground and shallow underground laboratories.
Fig. 2 : Aerosols collected on top of Pic du Midi (French Pyrénées) were analyzed at the LAFARA underground laboratory. We could quantify the 131I, 134Cs and137Cs activities that were released by the Fukushima nuclear accident.
The very low background achieved in our laboratory allows us to analyze samples with very low radioactivity levels. The analysis of the 226Ra and 228Ra activities in seawater that we use to trace the water masses and to quantify mixing processes in the ocean is thus possible, despite the low Ra concentration in the ocean (in the order of fg L-1 for 226Ra and ag L-1 for 228Ra). As another example, we were among the few laboratories that reported the activities of radionuclides released by the Fukushima nuclear accident (Japan) and determined in samples collected in France. We could thus quantify the low activities of 131I, 134Cs and 137Cs present in aerosol samples collected on top of Pic du Midi in the French Pyrénées (Evrard et al., 2012).
- Evrard O., van Beek P., Gateuille D., Pont V., Lefèvre I., Lansard B., P. Bonté, 2012. Evidence of the radioactive fallout in France due to the Fukushima nuclear accident, Journal of Environmental Radioactivity 114, Volume spécial Fukushima, 54-60.
- van Beek P., Souhaut M., Lansard B., Bourquin M., Reyss J-L., Jean P., von Ballmoos P., 2013. LAFARA : A new underground laboratory in the French Pyrénées for low-background gamma spectrometry, Journal of Environmental Radioactivity 116, 152-158.
Participants: Pieter van Beek, Marc Souhaut
LEGOS : Laboratoire d'Etudes en Géophysique et Océanographie Spatiales (Toulouse)
IRAP: Institut de Recherche en Astrophysique et Planétologie.
Fundings: LEGOS, UPS, OMP, IRD, FEDER, INSU