ABSTRACT A novel concept for detection of thermal neutrons based on lanthanide halide nanocrystals containing gadolinium, an element with by far the highest thermal ne utron capture cross section among all stable isotopes, is presented. Colloidal synthesis of GdF 3 nanocrystals, GdF 3 nanocrystals doped with Ce, and LaF 3 nanocrystals doped with Gd is reported. The nanocrystals were characterized by transmission electron microscopy (TEM), energy-dispersive X-ray spectroscopy (EDS), dynamic light scattering (DLS) analysis, and steady state UV-VIS optical absorption and photoluminescence spectroscopy. Neutron detection has been confirmed in experiments with Gd-containing nanocrystalline material irradiated with 252 Cf neutron source. Keywords: Radiation detectors; Neutron detectors; Nanoscintillators; Lanthanum fluoride; Gadolinium fluoride; Gadolinium doping; Cerium doping 1. INTRODUCTION Colloidal nanocrystals (NCs) have attracted tremendous interest over the last few years for a wide range of biomedical, biochemical sensing, and optoelectronic applications. So far, however, their potential has largely eluded the nuclear detection community. In contrast to wide exploitation of quantum confinement effects in optoelectronic and electronic devices, the physics and technology of inorganic scintillators is still primarily limited to bulk materials. Yet, compared to currently used large-size single crystals or scintillating particle s of bulk micrometer size, nanocrystals offer the prospect of significantly im proved performance. Large single-crystal inorganic scintillators have high output efficiency, but are very fragile, expensive to grow, and the size of high-quality crystals is limited. Particulate scintilla ting semiconductors of micrometer size could bring scalability and robustness to the field of inorganic scintillators. Their use, however, is limited by their low solubility in organic and polymeric matrices, and when prepared in inorganic matrices, such as sol-gel, they produce an optically opaque gel, which significantly reduces scintillation output. One important approach to overcoming these limitations could be using suspensions or composites of nanocrystalline materials. Due to their small size, NCs are expected to have better solubility in organic and polymeric matrices, and to cause much less scattering when loaded into inorganic sol-gel or porous host materials, which should result in higher effi ciency of the scintillator. NCs of known and novel scintillation materials will allow for production of large robust nanocomposites with a variety of shapes and sizes. Colloidal NCs are being researched for a wi de range of biomedical applications [Osi ski 2006], [Osi ski 2007], [Osi ski 2008], [Osi ski 2009], [Osiski 2010], to produce fast efficient phosphors for light emitting diodes [Matsui 2005], [Li 2007], and as active element in photovoltaic devices [Klimov 2006]. Detection of nuclear radiation by its conversion to UV or visible light can be another attractive application of colloidal NCs. With scintillating nanoparticles, novel radiation-based cancer treatments may become possible [Chen 2006]. There have been very few published preliminary studies of radiation response of nanocomposites based on colloidal NCs. Lithiated (