Y. Bourlat et al., MEASUREMENT OF LOW-LEVEL RADIOACTIVITY IN THE MODANE UNDERGROUND LABORATORY, Nuclear instruments & methods in physics research. Section A, Accelerators, spectrometers, detectors and associated equipment, 339(1-2), 1994, pp. 309-317
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.