Operating Manual

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X-ray image of an X-ray crawler in pipe at weld location
18
Special
radiographic applications
There are many special applications of radiography in NDT. This chapter describes a limited
number of different examples to illustrate this diversity. Apart from the use of radiation in
image forming radiography, it is also used in, for instance, measuring instruments such as
metal alloy analysing instruments (Positive Material Identification, PMI) and humidity
detection in insulation of thermally insulated pipping. This type of non-image forming
instruments and applications are outside the scope of this book.
18.1 Measuring the effective focal spot
The effective focal spot size is an important feature of an X-ray tube and is specified by the
manufacturer. In general it can be said; “the smaller the better”. As focal spot size is a criti-
cal exposure parameter (see section 11.1) for a particular application, the accuracy of the
manufacturer’s information is of vital importance.
Since 1999, EN 12543-1 requires a standardised method which, however, does not have the
general support of suppliers, as it requires expensive instrumentation and is
time-consuming. The EN-method, suitable for effective foci >0.2 mm, involves scanning the
X-ray tube radiation beam with a scintillation counter through a double collimator with an
extremely small opening of 10 £gm. The resulting intensity values are then represented in
a three-dimensional (isometric) diagram from which the effective focal spot can be deduced.
EN 12543-1 replaces the (older) less accurate IEC 336 procedure. The values based on
EN 12543 are often still reported together with data based on the IEC 336
procedure, for example: “3.5 mm (EN 12543) / 2.2 (IEC 336)". The numbers based
on the IEC refer to a look-up table from which the focal dimensions expressed in mm
can be derived. Those dimensions are given with a wide tolerance. In fact the IEC values
were too inaccurate to calculate image sharpness, being the main reason to develop the EN-
procedure. Manufacturers of X-ray tubes usually apply the so called Sténopé pinhole tech-
nique (camera obscura) to determine focal spot size.
The X-ray tube projects its focus through a very small hole (pinhole) in a lead plate onto a
film. The lead plate is positioned exactly halfway between focus and film. To prevent
scattered radiation, the hole is sometimes made in a tungsten plug which forms part of the
lead plate. After development, the effective focal spot size can be measured on the film, with
the aid of a magnifying glass. The latter method, still allowed and accepted by EN, results in
marginally smaller effective focal spot sizes.
Establishing the effective focal spot size of a panoramic X-ray tube is considerably more
complicated. To circumvent this, it is therefore recommended to just make a radiograph of
the object - pipe or vessel weld - with the right IQI’s and check the results for compliance
with the quality requirements specified.