Modern neutron tools use a He-3 thermal neutron detector. Older tools sometimes used BF-3 or scintillation detectors configured to detect the capture gamma ray, but these are now rare. There is yet another neutron detector scheme using a special Li based scintillation crystal, but it is seldom seen in real world logging tools. So, to make a long story short, the majority of neutron tools use a He-3 detector.
He-3 detectors produce a puny pulse, so a typical tool has a very high impedance, high gain amplifier on the front end to get the pulse up to a level where it can be discriminated and further processed. The HV, usually 1200 or so volts, must be pretty clean or the ripple will swamp the pulses. This high gain high impedance input is why the regulation for neutron tools cannot be prone to oscillation or generate much noise.
The Reuter-Stokes division of GE is the byword producer of He-3 detectors. The Reuter-Stokes helium-3 filled proportional counter (RS-P4-0806-207) is the most common thermal neutron detector for porosity measurement. Built to withstand the harshest of environments, the GE He-3 detector incorporates more than 30 years of experience in developing sensitive, accurate and reliable instruments for rugged, high temperature applications. In addition to oil and gas exploration, the sensor is designed for use in mineral exploration and industrial counting applications.
Anyway, most neutron tools are problematic about oscillating when out of the pressure housing. This necessitates using a cylindrical metal shield over the He-3 detector and over at least part of the neutron preamp board in most tools when on the bench being worked on. This of course complicates making adjustments when the tool is fired up. He-3 detectors put out a small and very fast pulse, so I always assumed the problem was simply endemic with the very high impedance and very high gain preamp circuitry (they either use a JFet first stage or a fast high gain very high impedance op amp).
Now for the interesting thing: Woody at Tek-Co says in their tools using our regulator, they no longer need the shield and the thing will not oscillate. He attributes this miraculous behavior to our regulator being so quiet.
"I've got one in and it's working fine. Very stable, better than the Victoreen actually, and quieter. Our application is not downhole, but in radiation monitors (called 'Chipmunks') which were designed in the late 70's, early 80's by Fermi National Lab."