Page 85 - ILLS Annual Report 2018

MODERNISATION PROGRAMMES AND TECHNICAL DEVELOPMENTS
82-83
Figure 1
The left insert illustrates the diffusion of slow neutrons in the powder
of the fluorinated nanodiamonds. On the right, the neutron albedo
is shown as a function of neutron velocity for: (i) raw nanodiamonds with a thickness of 3 cm (solid red line, data from [3]); (ii) the albedo extrapolated from the same raw-nanodiamond layer (dashed red line); (iii) a simulated albedo from an ‘ideal’, infinitely thick layer of impurity- free optimal-size diamond nanoparticles (dotted green line)—fluorinated nanodiamonds are expected to provide the albedo between these two lines, approaching the ‘ideal’ line; (iv) cut-off velocities for graphite (dashed-dotted blue line). The incident neutron flux is assumed to be isotropic. These estimations are preliminary and should be confirmed in the future by direct measurements.
The most important effect of the fluorination of nanodiamonds is the removal of hydrogen and sp2 carbon, which respectively decreases the absorption
of neutrons and increases their scattering. Using this information, we evaluated the albedo of slow neutrons from reflectors consisting of this powder. In figure 1, we present the reflectivity of reflectors of various types as a function of neutron velocity, and comment on the quality of this new reflection in the figure caption.
In reference [4] we proposed a new class of reflectors, based on designed fluorinated nanodiamonds, that can provide a continuous reflectivity curve with high albedo, thus minimising the existing ‘leak’ of neutrons through
the so-called reflectivity gap. This high diffusive and quasi-specular reflection will dramatically improve the performance of neutron sources, the efficiency of neutron delivery and thus the fluxes of slow neutrons in neutron instruments. It might also allow a new generation of neutron sources and experiments to be designed.
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