Gamma source close to ORNL2 tube

In order to investigate the origin of the background
component in ORNL tubes extending up to 200keV
we place a strong 137Cs source (D393) in the hole
of He136 close to the tube.

Control measurement (no source): 161012_1200_bck_control.root
With source: 161012_1229_137CsHe136.root

We observe that the induced signals are in the same energy region
as the 3rd noise component. The increase of the rate of signals
with the source is a factor 60.

See figures.

This hints to the origin of this component as due to an increased sensitivity
to gamma-rays (more  material?) or internal radioactivity (gamma,beta) 
of ORNL tubes.