Novel Mixed-Field Neutron and Gamma Detector Development for Rock Formation Evaluation

Nuclear well logging employs high activity neutron sources downhole to bombard rock
formations and measure the induced response. Common neutron sources in use
include Americium-Beryllium (AmBe) and Californium-252, typically with activities
in the giga and terabecquerel range. The presence of these high activity sources across
logging sites all over the world is a concern for nuclear security and safeguarding, and
a potential replacement is available in Deuterium-Tritium Pulsed Neutron Generators
(D-T PNGs). These sources can be turned off when not in use, and also offer
additional timing information due to their pulsed nature. This could provide
improved detector capabilities when coupled with fast timing and position sensitive
detectors. In this thesis, a low-cost, modular system is proposed that can measure the
flux of thermal neutrons and gammas at various distances from the pulsed source.
The detector modules consist of in-house manufactured plastic scintillator coupled to
BN:ZnS(Ag) thermal neutron converter foils. Many detector designs are simulated for
feasibility in GEANT4 optical simulations, and software tools were developed to
predict readout pulses and evaluate expected figures of merit for PSD based on
changing geometry and optical characteristics of detectors.
These mixed-field
detectors show good figure of merit for neutron-gamma discrimination at low-cost,
allowing construction of positional and temporal distributions of detected neutrons
and gammas when used as part of a multi-detector prototype tool. A prototype
detector was constructed and tested within the University of Sheffield Neutron
Facility with both radioisotope and DT neutron sources. Pulse shape discrimination
algorithms were explored, along with pile-up mitigation techniques for the case of DT
PNGs. The detector was finally tested using a mock rock test bench, with aims to
test detector response to changes in surrounding moderator.