Radioactivity measurements in the Environmental Radioactivity Measurement Laboratory (LMRE).
Evelyne Barker and Patrick Bouisset
Mediterranean mussel watch
CIESM Workshop Series n°15, Marseille, 18-20 April 2002 : 29-33 / 136 pages, Monaco.
A collection edited by Frédéric Briand.
The environmental radioactivity measurement laboratory is in charge of the determination of trace of radionuclides in environmental samples. These radionuclides are gamma emitters by direct measurement, alpha emitters (239Pu, 239+240Pn 241Am 244Cm), beta emitters (90Sr, 3H, 14C), mass analysis by ICP-MS for long lived radionuclides (uranium, Pu and other alpha emitters, ....) and stable elements. Detection apparatus are located two storeys below ground level under a 3-m thick boron concrete slab. Ventilation with filtered out air allows stabilizing radon concentration.
Gamma spectrometry is performed with detectors of different measurement configurations. These configurations allow the coverage of investigated activity levels and are adapted to different environmental matrix. These configurations are based on hyper-pure germanium coaxial n-type detectors with a relative efficiency greater than 50%:
- conventional detectors (Ortec GMX) placed in a 5 cm thick standard tend shielding, internally lined with 0.7 cm electrolytic copper sheeting,
- low level detectors (high purity) with a streamline "J”, placed in a 15 cm lead castle (13 cm standard lead + 2 cm of ultra low-level activity lead), internally lined with 0.7 cm electrolytic copper sheeting,
- low-level detectors equipped with a veto-cosmic system or with a veto-Compton system,
- a very low-level detector located in an underground laboratory (Modane) shielded by a 15 cm lead castle (10 cm standard lead + 5 cm of ultra low-level activity lead).
Gamma measurements are generally performed with a counting time of 80000 seconds. For low-level samples, counting time is 160000 to 240000 seconds. Density corrections are applied and matrix composition is taken into account below 100 keV (129I, 210Pb, 234Th, 241Am). With our high efficiency detectors, true coincidence summing corrections have to be done for some radionuclides (134CS 110Agm, 60Co, 214Bi, ... ). As an example, for 134CS these corrections are around 30% for a 17 ml container (9 mm height) and around 15% for a 380 ml container (59 mm height).
Radionuclides (241Am, 244Cm, Pu isotopes) are extracted and purified by radiochemical procedures prior to alpha measurement (Goutelard et al., 1998; Agarande et al., 2001). Purified solutions are electroplated onto stainless steel discs and then measured by alpha spectrometry with a counting time ranging from 1 to 14 days. Alpha spectrometry with 48 passivated planar silicon (PIPS) detectors.
Strontium-90 detection follows the isolation of Sr from the rest of the matrix elements. After separation of 90Sr from its daughter element, 90Y, the latter is detected by proportional counting. Counting time is 72 hours.
Tritium measurement (Fournier and Calmet, 1996, 1997) is performed on fraction water sample or a water fraction after a sample freeze-drying or a water fraction after combustion. The latter case is called organic bound tritium (OBT). Whatever the water preparation, a distillation is further performed before mixing the water fraction with a scintillant. Tritium is counted by liquid scintillation with low-level background Packard liquid scintillation counters. Counting time is 1000 minutes.
Carbon 14 is extracted by combustion under pressure (H par bomb) followed by an acetylene synthesis, and then mixed with benzene and scintillant before measurement. Counting time is 1000 minutes. Measurements are performed with low-level Quantulus liquid scintillation counters.
Stable element and long-lived radionuclides are measured by ICP-MS. Purification is realised with ultra pure grade reagents. Measurements are performed with an HR ICP-MS axiom single collector (VG elemental), equipped with a double-focusing magnetic sector mass analyser of forward geometry. The mass spectrometer is installed in a class M 4.5 clean room.