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 environm ental samples. This laboratory is located near the mid-point of the th irteen-kilometer Frejus road tunnel at the French-Italian border. The facility is shielded from cosmic radiation by 1700 m of rock (equivale nt to 4400 m of water), thus cutting the background level by a factor of approximately 70 compared with the operating conditions of an equiv alent system in. low-background counting room of the usual type. A hig h-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 electrolyti c copper shield to eliminate the influence of radon; the integrated co unt 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 radioa ctivity, especially Cs-137 and (CS)-C-134. Radioactive aerosol particl es are collected from the surface atmosphere onto cellulose filters, a nd these samples are stored in batches corresponding to a six-month pe riod, before being calcined and dissolved in acid. The filtered air vo lume is approximately 450000 m(3). Very low annual Cs-137 concentratio ns 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 measure d at Montlhery, near Paris, in 1991.