International
Tables for
Crystallography
Volume H
Powder diffraction
Edited by C. J. Gilmore, J. A. Kaduk and H. Schenk

International Tables for Crystallography (2018). Vol. H, ch. 3.3, pp. 265-266

Section 3.3.3.2. The neutron pulse shape

R. B. Von Dreelea*

aAdvanced Photon Source, Argonne National Laboratory, 9700 S. Cass Avenue, Argonne, IL 60439–4814, USA
Correspondence e-mail: vondreele@anl.gov

3.3.3.2. The neutron pulse shape

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The neutron pulse shape depends on the mode of production. Early studies (Buras & Holas, 1968[link]; Turberfield, 1970[link]) used one or more choppers to define a polychromatic pulse from a reactor source, resulting in essentially Gaussian powder peak profiles whose FWHM (ΓG) is nearly constant (B ≃ 0):[{\Gamma }_{G}^{2}=A+Bd^2,\eqno(3.3.18)]so that the Rietveld technique can easily be used (e.g. Worlton et al., 1976[link]). Unfortunately, this approach gave very low intensities and relatively low resolution powder patterns.

A more useful approach uses a spallation source to produce the pulsed neutron beam. Neutrons are produced when a high-energy proton beam (>500 MeV) strikes a heavy metal target (usually W, U or liquid Hg) via a spallation process (Carpenter et al., 1984[link]). These very high energy neutrons strike small containers of moderating material (usually H2O, liquid CH4 or liquid H2) which then comprise the neutron source seen by the powder diffraction instrument. The entire target/moderator system is encased in a neutron-reflective material (usually Be) to enhance the neutron flux and then further encased in a biological shield. Each moderator may be encased on the sides away from the instrument (e.g. powder diffractometer) in a thin neutron absorber (e.g. Cd or Gd) and may also contain an inner absorber layer (`poison') to sharpen the resulting pulse of thermal neutrons. These sources produce a polychromatic neutron beam that is rich in both thermal (<300 meV) and epithermal (>300 meV) neutrons. The proton pulses can have a very short duration (∼200 ns) (from a `short-pulse' source, e.g. ISIS, Rutherford Laboratory, UK or LANSCE, Los Alamos National Laboratory, USA) or a much longer duration (>500 ns) (a `long-pulse' source, e.g. SNS, Oak Ridge National Laboratory, USA or ESS, European Spallation Source, Sweden); the pulse repetition rate at these sources is 10–60 Hz. These characteristics are largely dictated by the proton accelerator and neutron source design. The resulting neutron pulse results from complex down-scattering and thermalization processes in the whole target/moderator assembly; it may be further shaped by choppers, particularly for long-pulse sources, to give what is seen at the powder diffractometer.

Consequently, the neutron pulse structure from these sources has a complex and asymmetric shape, usually characterized by a very sharp rise and a slower decay, both of which are dependent on the neutron wavelength. The resulting powder diffraction peak profile (Fig. 3.3.3[link]) is then the convolution [equation (3.3.2)[link]] of this pulse shape (Gλ) with symmetric functions (GI) arising from beamline components (e.g. slits and choppers) and the sample characteristics (GS).

[Figure 3.3.3]

Figure 3.3.3 | top | pdf |

The observed and calculated Ni 222 diffraction line profile from the Back Scattering Spectrometer, Harwell Laboratory, Chilton, UK. The curves A and B are computed from the two terms in equation (3.3.19)[link] and curve C is the sum (from Von Dreele et al., 1982[link]).

References

Buras, B. & Holas, A. (1968). Nukleonika, 13, 591–620.Google Scholar
Carpenter, J. M., Lander, G. H. & Windsor, C. G. (1984). Instrumentation at pulsed neutron sources. Rev. Sci. Instrum. 55, 1019–1043.Google Scholar
Turberfield, K. C. (1970). Time-of-flight neutron diffractometry. In Thermal Neutron Diffraction, edited by B. T. M. Willis. Oxford University Press.Google Scholar
Worlton, J. G., Jorgensen, J. D., Beyerlein, R. A. & Decker, D. L. (1976). Multicomponent profile refinement of time-of-flight neutron diffraction data. Nucl. Instrum. Methods, 137, 331–337.Google Scholar








































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