FIFTH CANADIAN CONFERENCE ON NONDESTRUCTIVE ... - IAEA

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the instrument is the same for both detectors, the error arising from
misalignment of the specimen surface relative to the center of the
instrument when the instrument is rotated is eliminated (5,6). This is
particularly important for a portable instrument designed to examine
large specimens, because the instrument rotates about the specimen and
providing an accurate center of rotation can be difficult.
ADVANCES IN INSTRUMENTATION FOR X-RAY STRESS MEASUREMENT
The explosion in the development of solid state electronics in
the past 15 years has resulted in many advances which make it possible
for accurate measurements of stress to be made with a portable instrument.
The new developments include inexpensive, compact but very powerful,
computers, compact solid state high voltage supplies capable of
delivering 100-200 W and stable, accurate electronic circuits for signal
analysis. Very small X-ray tubes have been developed to complement the
solid state power supplies (Fig. 5(a)).
However, the most important advance has been the development of
small position sensitive proportional counters (7). These measure the
distribution of X-ray intensity in one dimension with a resolution of
50-60 um. Detectors as small as 105 x 50 x 50 mm, with an active length
of 50 mm (Fig. 5(b)) and weighing only = 0.6 kg are now commercially
available. A position sensitive proportional counter consists of a fine
wire centered in a chamber filled with Xe-lOJCHij or Ar-10?CHij (Fig.
6(a)). For the counters shown in Fig. 5 the wire is a 25 ym diameter
quartz filament, coated with carbon, which has a resistance of 8 kjj/mm.
The wire is at a high positive voltage relative to the walls of the chamber.
X-rays passing through the chamber ionize the gas causing electron
cascades onto the wire. The voltage pulses observed at each end of the
wire, arising from each cascade, have rise times proportional to RC,
where R is the resistance from the end of the wire to the point of the
electron cascade, and C is the capacitance between the end of the wire
and ground (Fig. 6(b)). The two voltage pulses are shaped and amplified
to produce bipolar pulses whose cross-over times are proportional to the
rise times (Fig. 6(c)). Each cross-over time is marked by a very sharp
voltage pulse, one of which is delayed a constant amount 6 (Fig. 6(d))
so that it always follows the other. Thus, their time separation T is
proportional to the position of the X-ray photon, x, in the counter.
Electronic circuitry to perform this analysis is available commercially.
DESIGN OF THE CANMET PORTABLE STRESS DIFFRACTOMETER
These new technologies have been applied to design a versatile
diffractometer capable of accurate stress measurement by either single
or multiple exposure techniques. The CANMET diffractometer will be suitable
for field use under clement conditions but with sufficient versatility
and accuracy for dedicated laboratory use.