FIFTH CANADIAN CONFERENCE ON NONDESTRUCTIVE ... - IAEA

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The disadvantages of neutron diffraction are few but significant. Sufficiently
intense neutron beams are available only from nuclear fission reactors or from
spallation reactions in the target of a charged particle accelerator; neutron
beams and neutron spectrometers are both costly and few in number throughout
the world. With the NRU reactor at Chalk River and its associated neutron
spectrometers, we have equipment in Canada that is as good as any in the world
for these kinds of experiments.
The paper is arranged as follows. The basis of the neutron technique is
described in section II, while section III contains two examples of recent
neutron experiments at Chalk River, viz. the measurement of residual strain in
overrolled Zr 2.5 wt% Nb pressure tubes and in bent Incoloy-800 steam generator
tube. A number of other applications are mentioned.
2. NEUTRON DIFFRACTION TECHNIQUES
A typical neutron spectrometer mounted at a fission reactor can select a beam
of neutrons with any desired wavelength in the range » 1-5 Â, that can be
directed onto the samples. Figure 1 shows such a spectrometer mounted at the
L3 beam facility of the NRU reactor, Chalk River. Typically about 10 neutrons
cm" s~ impinge on the sample, and the intensity of powder diffraction is of
the order 10-100 counts/sec when the diffracting volume is of order one cm .
The spectrometer, because of stringent shielding requirements, weighs several
tons and can accommodate samples of up to about a ton on the sample table. The
maximum cross-sectional area of the neutron beam is 50 mm * 50 mm; smaller
sizes are obtained by suitable collimators that cut down the beam with absorbing
masks. The neutron beam size in any experiment is chosen to match the
spatial scale of the feature under investigation. For example in the experiment
on Incoloy-800 steam generator tubing there were marked changes in strain
in one dimension over a distance of order 1 mm, so the masks were chosen to be
1 mm wide and 10 mm long. The angular resolution is easily altered to suit the
demands of the measurement, by appropriate choice of wavelength and collimation
of the incident and diffracted beams. For crystallographic texture measurements,
low resolution is required to integrate over the appropriate Bragg
peaks; on the other hand, the highest available resolution, of order 0.01% is
often desirable for residual strain measurements. Typically the time taken to
characterize the texture for a particular set of crystallographic planes is
about a day, whereas the determination of the angle of diffraction 29j,kji in a -
strain measurement takes a few hours. The interplanar spacings of the crystal
lattice constitute miniature internal strain gauges. The angle of diffraction
is related to the corresponding spacing, dhkJi, by Bragg's law.
X = 2