Electronics and 3He Counter Acquisition Tests During the Pulses of a D-T Neutron Generator

Nuclear logging industry uses Active Neutron Interrogation devices for geological prospection and exploitation. Modern neutron logging tools uses a pulsed neutron generator emitting fast neutrons (2.5 or 14.1 MeV) with an average emission in the 10
⁸
s
^-1
range. Some measurement techniques rely on nonfully thermalized neutrons, called epithermal neutrons, which are less sensitive to neutron absorbers than thermal neutrons. The signal due to generator neutrons backscattered towards helium 3 proportional counter is used to assess hydrogen porosity. It decays within hundreds of microseconds. Historically, epithermal neutron measurements, with a cadmium layer around the detector, suffered of insufficient counting statistics. To parry this drawback, our goal is to perform the measurement during the neutron generator burst. Therefore, the detector and acquisition chain must be able to handle a high flux of neutrons. This work investigates the behavior of
³
He counters with different gas additives, adapted either to charge or to current amplifiers, located close to a D-T neutron generator, with the same total detection efficiency we have in a logging tool. We tested instantaneous neutron emissions between 10
⁷
and 2.5∙10
⁹
s
^-1
during generator pulses with a duration ranging from 40 to 800 µs. It appears that the throughput of the acquisition chain does not only depend on the generator instantaneous emission rate, but also on burst duration. Unexpectedly the maximum output count rate 4.4∙10
⁶
s
^-1
is thus reached with short generator pulses of 40 µs length and a very high instantaneous neutron emission 8.4∙10
⁹
s
^-1
, using a 30NH15CS
³
He counter adapted to G-ADSF10 current amplifier.