International
Tables for
Crystallography
Volume C
Mathematical, physical and chemical tables
Edited by E. Prince

International Tables for Crystallography (2006). Vol. C, ch. 7.3, pp. 646-648

Section 7.3.3.4. Films

P. Converta and P. Chieuxa

aInstitut Laue–Langevin, Avenue des Martyrs, BP 156X, F-38042 Grenoble CEDEX, France

7.3.3.4. Films

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Films are classed as position-sensitive detectors. Two types of neutron converter are used in neutron film-detection processes. In the case of the scintillation film system, a light-sensitive film is pressed close to one or between two plastic 6LiF/ZnS(Ag) scintillator screens (Thomas, 1972[link]). In the case of the Gd-foil converter, the conversion electrons are emitted isotropically, with a main energy peak at 72 keV, and collected by an X-ray film in close contact with the converter (Baruchel, Malgrange & Schlenker, 1983[link]).

In addition to the advantages given by the film technique in itself (simplicity, low price, direct picture, etc.), neutron photographic methods give the best spatial resolution. However, the resolution is inversely related to the detector efficiency and thickness. A good compromise appears to be a thickness of 0.25 mm for a plastic scintillator [i.e. a capture efficiency of about 12% and a resolution of 0.1 mm for a one-screen converter at λ =1 Å; see the size of the ionization volumes in a scintillator, Fig. 7.3.3.3(a)[link]]. Here, however, as in light scattering, the optical density depends on the exposure time as well as on the incoming flux (Schwartzschild effect), which necessitates a calibration (Hohlwein, 1983[link]). For a natural Gd-foil converter, an optimum thickness is 0.025 mm, giving a resolution of 0.020 mm. The Gd-foil film detector is one order of magnitude less efficient than the scintillator, but, as in electron microscopy, the optical density is nearly proportional to the exposure. This explains the use of the Gd foil in neutron-diffraction topography.

If we take into account the possible inhomogeneity of the converter and the difficulties related to the film (homogeneity, development, and photodensitometry), an accuracy of 5 to 10% is achievable in the intensity measurements under good conditions.

Owing to the differences in the processes, neutron photographic techniques are much more efficient than those for X-rays. In the case of the plastic scintillator, the gain is about 103, which compensates for the much lower neutron fluxes.

References

Baruchel, J., Malgrange, C. & Schlenker, M. (1983). Neutron diffraction topography: using position-sensitive photographic detection to investigate defects and domains in single crystals. Position-sensitive detection of thermal neutrons, edited by P. Convert & J. B. Forsyth, pp. 400–406. London: Academic Press.
Hohlwein, D. (1983). Photographic methods in neutron scattering. Position-sensitive detection of thermal neutrons, edited by P. Convert & J. B. Forsyth, pp. 379–390. London: Academic Press.
Thomas, P. (1972). Production of sensitive converter screens for thermal neutron diffraction patterns. J. Appl. Cryst. 5, 373–374.








































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