Low amplitude noise component.

The figure shows the composition of the low amplitude noise
which depends on the tube and type of preamplifier used.

Bkack: He020 (RIKEN-Diff)
Red: He044 (UPC-Diff)
Green: He057 (UPC-Unip)
Blue: He068 (ORNL1-Unip)
Pink: He132 (ORNL2-Unip) 

(all the tubes of the same type and preamp type behave alike)

We can see up to three different structures.
The noise in the ORNL tubes extends to higher energies (200keV)
and are the cause for the overlap of noise with neutron signals,
but other wise the shape is similar to RIKEN tubes.
The noise for UPC above 50kev is quite small.
One can also see a small difference between differential and unipolar PA
in the case of UPC tubes (red and green)