|Abstract inglese (opzionale):
The international monitoring system (IMS) has been developed to verify compliance with the comprehensive nuclear test-ban treaty (CTBT). IMS associated with radionuclide monitoring stations allow identifying underground nuclear explosions. Among the fission products created by a nuclear test, the four radioactive xenon isotopes 131mXe, 133Xe, 133mXe and 135Xe are of particular interest for the CTBT since they are produced in large amounts during an explosion, and have half-lives long enough to be detected at significant distances, even several days after the event. To monitor the emissions of xenon isotopes, different kind of instrument use either β-γ coincidence spectroscopy (ARSA, ARIX and SAUNA) by exploiting combination of plastic scintillators to detect electrons from a beta decay and from internal conversion and NaI crystals to detect gamma rays, or high-resolution γ spectroscopy by means of High Purity Germanium detectors (HPGe). These systems are required to have a very sensitive detection limit, having to reach specific values of minimum detectable activity (MDA), for instance, MDA < 1 mBq/m3 for the isotope 133Xe. To improve the sensitivity of the results obtained from the measures it is important to reduce the background and, as a result, to decrease the minimum detectable activity (MDA) values. At ENEA laboratory (ENEA Research Center of Brasimone, Italy) an anticoincidence system has been developed. The system consists of a high purity germanium coaxial detector p-type ‘Extended Range’ marketed by CANBERRA (model GX6020). The detector is installed inside a low-background shield made of old lead (150 mm) with a layer of electrolytic copper (35 mm). Two NUVIA plastic scintillators were placed above the shielding to detect coincident cosmic-ray interactions. The electronics associated with the HPGe detector is a MultiChannel Analyser (ORTEC DSPEC-50) that amplifies the signal and passes it through an analogue-to-digital converter; the information on the pulse height is then stored in the MCA and recorded into proportional channels. The software GammaVision-32 (ORTEC) has been used to display and analyze the spectra; anti-coincidence mode is activated simply by modifying settings directly from the software. Tests conducted with the anticoincidence system have shown a reduction of the Compton continuum that contributes to the spectrum background. In addition, the measured MDA values, compared to those obtained using the standard system, decreased for all the radioxenon isotopes.