GS20® glass has been adopted as the technology of choice by top tier oilfield service companies for downhole neutron-porosity logging tools, due to its ability to withstand high temperature and high shock environments. This characteristic is essential when porosity measurements are performed during drilling (LWD/MWD) and the tools are subjected to the high levels of vibration and heat generated by the drilling activity.
Neutron-porosity logging tools contain a neutron emitting source, typically an americium Beryllium (Am-Be) or Plutonium Beryllium (Pu-Be) source, or an electronic neutron generator producing fast neutrons, which travel across the formation surrounding the well. The fast neutrons, with energy ranges from 4 MeV to 14 MeV, are slowed down and thermalized by collisions with hydrogen nuclei present in the pores of the rock. Some of these thermal neutrons are then scattered back towards the tool. Here, the back-scattered neutrons are counted by two scintillator detectors placed at different distances from the source (near and far) to compensate for the borehole effects. This configuration is often referred to as a compensated logging tool.
The 6Li isotope in the GS20® offers a 940 barn cross section for thermal neutron capture via the following reaction:
n + 6Li → α + 3H + 4.78 MeV
The secondary particles of this reaction lose their energy within the surrounding scintillator material near the interaction point and this gives rise to the emission of light. This generates approximately 6,000 photons for each neutron absorbed by the GS20® glass. The photons emitted by the GS20® glass are subsequently converted into electrical signal by a photomultiplier tube, to give a characteristic thermal neutron peak at Gamma Equivalent Energy (GEE) a little above 1.6 MeV at room temperature. The GS20® glass of one of the detectors is often shaped as a thin-walled cylinder to lower the gamma sensitivity. This design enables to obtain an acceptable neutron gamma discrimination up to temperatures of approximately 150°-175°C, despite the shift of the neutron GEE peak towards lower energies.
Scintacor and its predecessors have been involved with glass scintillators including lithium-6 (6Li) enriched products since 1958. Back then, as Levy West Laboratories, we started manufacturing these glasses with various compositions based on the work by Anderson et al. Today Scintacor is the owner of the GS20® trademark and a trusted partner of many leading research organizations and commercial customers that have chosen GS20® and the companion formulations for their unique properties, mechanical robustness, temperature range of use -200°C to 250°C and resistance to all organic and inorganic solvents except hydrofluoric acid.
