BS 6932:1988 pdf free download.the effective focal spot size of mini-focus and micro-focus X-ray tubes used for industrial radiography.
1 Scope
BS 6932 describes a method for measuring effective focal spot sizes within the range 5 1m to 300 jim of X-ray tubes operating at 50 kV to 200 kV.
NOTE I The same procedure may be used at higher kilovoltages but the accuracy of measurement may be poorer. NOTE 2 For focal spot sizes greater than 300 jim. the pinhole and other methods detailed in BS 6530:1984 may usually be applied.
NOTE 3 The titles of the publications referred to in BS 6932 are listed on the inside back cover.
2 Definitions
For the purposes of this British Standard the
definitions given in BS 3683-3 apply.
3 Test object
The test object shall consist of an accurately machined high precision spherical ball of tungsten carbide, having a diameter of 5 mm.
The ball shall be mounted on a thin aluminium alloy support plate having a thickness of approximately 0.5 mm by means of adhesive which does not extend beyond the periphery of the ball, To minimize extraneous scattered radiation, the support plate carrying the ball shall be mounted across a circular aperture of approximately 12 mm diameter in a lead sheet having a thickness of approximately 3 mm, in such a manner that the ball is located in the centre of the aperture.
In order to obtain the necessary projective magnification value, the mounting plate shall he of a size that enables the test object to be positioned very close to the target of the X-ray tube.
4 Procedure
4.1 Select the required kilovolt and milliamp settings and adjust the X-ray equipment to obtain the best focus in the centre of the X-ray field.
NOTE This can be conveniently achieved by means of a series of very fine wire meshes (commonly known as TEM grids) and an image intensifier system.
Record the readings on the control.
4.2 Place the test object in the X-ray beam, the tungsten carbide ball being as nearly as possible in the centre of the X-ray field. Adjust the distance of the test object and the imaging plane to provide approximately x 20 projective magnification (see Figure 1).
4.3 Take a radiograph of the test object on fine grain radiographic film so that the image of the ball is approximately 100 mm in diameter. Ensure that the set-up of the test object and film is such that the scattered radiation reaching the fihi is kept to a minimum.
NOTE 1 When practicable a focus.to.film distance (f.f.d.) of 1 m should be used but, with some equipments. it may be necessary to increase this distance in order to obtain the required projective magnification. For guidance, the exposure times required on fine grain film at an f.f.d. of I m will be approximately 50 mA s
at60kVand 15mAat 150kV.
After normal processing of the radiograph, the background density of the film shall be
approximately 1.0 to 1.5.
NOTE 2 There is evidence that. the focal spots of some
micro-focus X-ray sets may be slightly unstable, therefore:
a) the focusto-film distance should be adjusted so that the exposure time is not less than 1 mm;
b) if available. a shutter should be used, so that the X-ray set can be adjusted to its required performance with a delay of about 30 s before the exposure is made.
4.4 Radially scan the image of the edge of the ball on the film with a suitable microdensitometer using a linear magnification, N, of about x 20 to produce a trace similar to that shown in Figure 2 at the edge of the image of the ball. The trace obtained is shown in idealized form in Figure 3 and the method of
measurement is as follows.
Measure the height H between the maximum and minimum densities close to the edge trace as shown and draw horizontal lines at 0.25H and 0.75ff to cut the density trace at A and B. The straight line, AB, is then projected to cut the maximum and minimum density lines at C and D. The horizontal distance
between C and D, that is ED, is then a measure of the focal spot diameter, s, using the formula
where
M is the projection magnification used in taking the radiograph;
N is the linear magnification of the microdensitometer.
NOTE Mcan be obtained by measurement of the diameter of the image on the radiograph, knowing the true diameter of the ball.
4.5 Measure the maximum and minimum values of s and the values parallel and perpendicular to the axis of the X-ray tube. Record also the angular
relationship between the maximum and minimum values and the X-ray tube axis. With a rod-anode X-ray tube having a panoramic radiation beam, obtain a minimum of three images of the test object ball, spaced around the panoramic field.
5 Presentation of results
The results shall be presented as a table of s-values together with the angle to the X-ray tube axis at
which the measurement was made, for a series of X-ray tube voltages and tube currents, to cover the working range of the X-ray equipment.