Capillary neutron optics for prompt-gamma activation analysis

A neutron lens has been constructed to focus cold neutrons from the exit of a Ni-58 neutron guide, which delivers a beam to the Prompt-Gamma Activation Analysis (PGAA) station at the MST Cold Neutron Research Facility. The lens compresses a neutron beam of cross section 50 mm x 45 mm onto a focal spot of diameter 0.53 mm (fwhm) with an average gain of 80 in neutron current density. PGAA measurements have been performed to demonstrate the enhanced sensitivity and detection limits for various elements and the spatial resolution in one transverse dimension. For the two test particles (a gadolinium glass bead and cadmium metal of sizes less than 0.5 mm), the gain in the gamma-count rate with the lens is a factor of 60, and the detection limit is improved by a factor of 20. The system can be used for two-dimensional mapping of samples on a sub-millimeter scale to complement other analytical techniques such as neutron depth profiling (NDP).