New Structured Scintillators for Neutron Radiography

Abstract

We report on the development of novel neutron scintillators fabricated in microcolumnar formats using the physical vapour deposition (PVD) method. Such structures mitigate the conventional trade-off between spatial resolution and detection efficiency by channelling the scintillation light towards the detector while minimizing lateral spread in the film. Consequently, high resolution and high contrast neutron images can be acquired in a time efficient manner. In this paper, we discuss methods and characterization for scintillator films made from three distinct compositions, Thallium (Tl) or Europium (Eu) doped Lithium CesiumIodide (Li3Cs2I5:Tl,Eu, referred to as LCI), Tl or Eudoped Lithium Sodium Iodide (LixNa1-xI:Tl,Eu, referred to as LNI), and Cerium (Ce)-doped Gadolinium Iodide (GdI3:Ce, referred to as GDI).LCI and LNI scintillators are derived from the well-known CsI and NaI scintillators by the incorporation of 6Li into their lattice. Based on our measurements reported here, LCI/LNI scintillators have shown to exhibit bright emissions, fast, sub-microsecond decay, and an ability to effectively discriminate between neutron and gamma interactions using pulse shape (PSD) and/or pulse height (PHD) discrimination. LCI has a density of 4.5g/cm3, a measured peak emission wavelength of 460nm (doped with Eu), and a light yield of ∼50,000 photons/thermal neutron. LNI has a density of 3.6g/cm3, an emission peak measured at 420nm, and a light yield of ∼100,000 photons/thermal neutron.The recently discovered GDI exhibits excellent scintillation properties including a bright emission of up to 5,000 photons/thermal neutron interaction, 550nm green emission, a rise time of ∼0.5ns and a primary decay time of ∼38ns (Glodo et al., 2006). Its high thermal neutron cross-section of ∼255kb makes it an attractive candidate for neutron detection and imaging. Although it has high density of 5.2 gm/cm3 and effective atomic number of 57, its gamma sensitivity can be minimized by lowering the film thickness and its neutron sensitivity can be maximized through the use of enriched Gd.The fabrication of micro-structured films of these materials using an evaporation technique permits the cost-effective volume synthesis of high-quality neutron scintillators over large areas (20cm x 20cm) in short time. In addition, the vapour deposition permits stoichiometry and dopant control not possible using conventional crystal growth.