With the system in T9 and a Keithley Source Meter supplying HV to the detector.

The source meter supplies 200V and measures the current .... 

I noticed that the current value was not stable... varying by +/-0.5uA. In conversation with Marcello i learnt that normally much more stable than this. 

I have carried out the following changes.

added a 22nF X7R capacitor at the detector end of the Kapton cable from the HV connector pin  ( G/R ) to the ground plane of the Kapton.

added a 14k resistor in series with the HV supply +ve signal before it connects to the adaptor board.

isolated the G/R connection on the second Kapton that would normally be connected to the HV on the detector.

The HV now is a lot quieter +/-0.01uA.

Next i disconnected the HV -ve input from ground at the adaptor board. This resulted in all the FEE64 being equally noisy!!

Next I connected a 22nF X7R capacitor between the HV negative and ground at the adaptor. This resulted in all FEE64 becoming equally calm. 

The FWHM of the spectra is not good at 250 to 300 but the fact that all FEE64 are equal now is interesting.

I will update this post with pictures.

 Update:- 

This change was not stable and repeatable. I changed back to the original connections and things haven't gone back. 

So power down and leave for today.... It doesn't make sense to me ... but then that's nothing new :-)

It appears that something is heating/charging up during the day and the noise becomes swamped by this effect ????