A neutron spectrometer using nested moderators

: The design, simulation results and measurements of a new neutron energy spectrometer are presented. The device, which may be called NNS, for Nested Neutron Spectrometer, works under the same principles as a Bonner Sphere Spectrometer (BSS) System, i.e. whereby a thermal neutron detector is surrounded by a polyethylene moderator. However, the moderator is cylindrical in shape. The different thicknesses of moderator are created by inserting one cylinder into another, much like nested Russian dolls. This design results in a much lighter instrument that is also easier to use in the field. Simulations and measurements show that, despite its shape, the device can be made to offer a near angular isotropic response to neutrons and that unfolded neutron spectra are in agreement with those obtained with a more traditional BSS.