MEASUREMENT OF LOW-LEVEL RADIOACTIVITY IN THE MODANE UNDERGROUND LABORATORY

A coaxial high-purity n-type Ge detector system has been installed in the Modane underground laboratory for low-level monitoring of environmental samples. This laboratory is located near the mid-point of the thirteen-kilometer Frejus road tunnel at the French-Italian border. The facility is shielded from cosmic radiation by 1700 m of rock (equivalent to 4400 m of water), thus cutting the background level by a factor of approximately 70 compared with the operating conditions of an equivalent system in. low-background counting room of the usual type. A high-purity germanium detector is used with a sensitive volume of 207 cm(3), 52% relative efficiency and FWHM resolution of 2.1 keV at 1332 keV. The detector is shielded by 15 cm of old lead and the detector hood and sample (up to 500 cm(3) in volume) are protected by an electrolytic copper shield to eliminate the influence of radon; the integrated count rate within the 20-2000 keV energy interval is 1.7 x 10(-2) s(-1). The detector system has been used to monitor atmospheric gamma radioactivity, especially Cs-137 and (CS)-C-134. Radioactive aerosol particles are collected from the surface atmosphere onto cellulose filters, and these samples are stored in batches corresponding to a six-month period, before being calcined and dissolved in acid. The filtered air volume is approximately 450000 m(3). Very low annual Cs-137 concentrations between 1.5 x 10(-7) and 2.0 x 10(-7) Bq/m(3) were measured, while no Cs-134 was detected in 1991 at Tahiti or Mururoa (less than 4 x 10(-8) Bq/m(3)). A Cs-134 concentration of 7 x 10(-8) Bq/m(3) was measured at Montlhery, near Paris, in 1991.