Cosmic ray background suppression

The background noise in a high purity Ge gamma spectrometer is caused by traces of radioisotopes incorporated in materials of the instrument itself and by cosmic rays.
When materials have been properly selected for their low activity, cosmics are the major source of noise.
Muons when they interact with lead castle walls, create neutrons.
Their interactions inside the chamber are producing gammas. Detected by the Germanium, they contribute in increasing the noise level.
To decrease this noise background, the usual strategy is to install low level activity experiments deep underground in mines or tunnels (Frejus tunnel in Europe).

Anticoincidence shield

Flat detectors shown on the picture are arranged outside and around the lead castle. Their efficiency is weak for neutrons but high for cosmics.
When an event is detected simultaneously inside the castle and outside by plastic scintillators it means that it was created by a cosmic. Hence it is rejected.

Noise background improvement

In usual laboratories a factor of 4 to 5 is readily achieved in the noise background reduction in the 500 keV to 2.7 MeV energy range and 5 to 10 in the very useful (environmental studies) energy range below 500 keV.
In water or foods radioactivity monitoring, the use of these detectors drastically improves the speed of detection for a given limit, by reduction of the noise level.

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