Page 84 - ILLS Annual Report 2018

TECHNICAL AND COMPUTING DEVELOPMENTS
Fluorinated nanodiamonds as a unique neutron reflector
With the pronounced, worldwide trend of increasing the range of useful neutrons towards smaller energies—driven
in particular by large-scale structure diffractometers, reflectometers, time-of- flight and spin-echo techniques as well as by particle physics—progress is limited by the low fluxes of the less energetic part of the cold neutron spectrum. One fundamental reason lies behind this
drop in flux: regardless of the choice
of materials for neutron reflectors, they are all composed of atoms. Atoms in solids and liquids are separated by distances of a few tenths of nm. If the neutron wavelength is larger than that, neutrons are weakly scattered by atoms/ nuclei and the diffraction is limited by
the inter-plane distances available; thus, neutrons pass through a reflector with low interaction with its material and are lost.
AUTHORS
V. Nesvizhevsky (ILL) ARTICLE FROM
Carbon 130 (2018) 799—doi: 10.1016/j.carbon.2018.01.086
REFERENCES
[1] V.V. Nesvizhevsky, Phys. Atom. Nucl. 65 (2002) 400
[2] P.J. DeCarli and J.C. Jamieson, Science 133 (1961) 1821
[3] E.V. Lychagin, A.Yu. Muzychka, V.V. Nesvizhevsky, G. Pignol,
K.V. Protasov and A.V. Strelkov, Phys. Lett. B 679 (2009) 186
[4] V.V. Nesvizhevsky, U. Köster, M. Dubois, N. Batisse, L. Frezet,
A.B. Bosak, L. Gines and O. Williams, Carbon 130 (2018) 799
To construct efficient slow-neutron reflectors, currently there is no alternative other than to mimic conventional reflectors by replacing atoms/nuclei with nanoparticles of low- absorbing, highly scattering materials—thereby changing the characteristic length scale and consequently the neutron wavelength corresponding to efficient neutron reflection [1]. Nanodiamonds [2] are an obvious choice for the material of such reflectors, as the cross section of absorption in carbon is exceptionally low, the coherent scattering length is very high and the volume density is higher than that for other carbon materials. The characteristic sizes of available nanodiamonds are found to be in the range of optimum theoretical values. In addition, the reflectivity of available nanodiamonds has been assessed to be much higher than that of any alternative reflector [3]; however, it remains low for neutron velocities above 160 m/s, mainly because of the high content of hydrogen impurities.
We overcome this principal difficulty by using fluorinated nanodiamonds [4]. We performed a detailed analysis
of samples of this material using several complementary techniques, as follows. Using X-ray diffraction, we found that the diamond cores (sp3) of nanoparticles remain unaffected upon fluorination, while the sp2 carbon disappears almost completely from the nanoparticle shells. A combination of Raman scattering and infrared absorption qualitatively confirms the disappearance of sp2 carbon, carbon-hydrogen bonds, oxygen-hydrogen and carbonyl-containing functional groups. Analysis of
the multinuclear solid-state NMR data essentially confirms the results of vibrational spectroscopy, while being more quantitative. The ultimate mean of hydrogen content is determined by prompt-γ analysis. This shows the hydrogen content in nanodiamonds to be drastically reduced by
the fluorination, reaching a level 35−60 times lower
than that before fluorination. Neutron activation analysis reveals impurities, which become radioactive in high neutron fluxes, as well as impurities that strongly absorb neutrons. Their content can be reduced by the purification of nanodiamonds in strong acids at high temperature; however, the degree of purification remains to be improved in the future.
ANNUAL REPORT 2018