Chapter 1 – Part 2

Observation of Nature of Tubes and Quality of Rays.

The quality of the X rays produced is shewn above to depend upon the nature of the tube from which they are produced; hence it is very important to observe the nature of a tube during operation.

1. The colour in the tube, both of the fluorescent hemi- sphere in front of the antikathode and of the hemisphere behind that plane, varies according to the hardness; but this factor is too indefinite to give practical indication of other than gross differences. In a tube acting properly the hemi- sphere in front of the antikathode should show a bright apple-green fluorescence, while the hemisphere behind should be free from luminosity. In a soft tube the fluorescence is intensely green and uniform, while the gas in the tube shews a faint bluish luminosity plainly seen behind the antikathode. In a hard tube the fluorescence is thin and grey-green in tint, while irregular, flickering green spots are seen on the walls of the tube.

The penetration or penetrative power, dependent directly on the hardness, may be measured by several methods—

(a) By observing the shadow cast on a fluorescent screen by a hand interposed between that and the X-ray tube, the shadow cast by the bones approximating in density to that cast by the rest of the hand in direct ratio to the hardness of the tube. This is at the best but a rough, relative test, and is not to be recommended, as such repeated exposure of the operator’s hand may lead to a serious dermatitis.

(b) By radiometer or radio-chromometer, an instru- ment made on the same principle as the tintometer apparatus for estimating the haemoglobin of the blood. In this the power of the rays to penetrate a metal of uniform density but varying thickness is observed and compared with a standard. The instrument of Benoist consists of a thin central disc of silver surrounded by a flat ring of aluminium graded by steps in thickness from 1 to 12 millimetres. For use, hold this up so as to intercept the rays from a tube, observe the shadow cast on a fluorescent screen, and note the sector or step by which is produced a shadow of density similar to that of the standard disc in the centre. The sectors being numbered according to their thickness, the higher numbers will correspond to harder tubes. Modifications of this intrument have been introduced, some of them of value, but all are similar in principle.

(c) On the equivalent spark-gap more dependence in practical work is placed than on all other methods of indicating or estimating hardness or softness of tubes.
“When an X-ray tube is connected to an induction coil in the ordinary way, the discharge of the induced current may take place along two alternative paths, as is shewn dia- grammatically in the annexed figure (Fig. 5). Discharge by way of the X-ray tube produces the special phenomena in the tube; discharge by the other path across the gap between the two discharging points of the coil takes the form of a series of sparks, The current in discharging will always take the path of least resistance. The discharge-gap is variable at will by moving the points nearer or farther apart, and the resistance offered to discharge across these points will vary directly with the air-distance between them.

While a tube is in operation, if the points, at first far apart, be gradually approximated, a position will be reached at which discharge takes place through the air between the points in preference to passing through the tube. Conversely, if discharge is taking place across the points, and they are gradually drawn apart, then, after passing the above position, discharge will cease across the points, and will take place through the X-ray tube. This distance between the points, called the ‘ equivalent spark-gap/ or * alternative path,’ denotes the resistance and consequent nature of the tube.

In adjusting the distance between the discharge points care must be observed that the points are not allowed to come into direct contact, so as to make a closed circuit, for under such circumstances the induction-coil might be badly damaged by the heavy discharge of current permitted.

The arrangement is sometimes called a spintermeter, or spark-measurer, and the hardness of a tube is denoted by a number corresponding to the distance noted, which distance is indicated on the sliding-rod of the discharge-gap. Thus, tubes with an equivalent spark of—

1 to 2 inches are soft.
3 to 4 inches are medium.
Above 5 inches are hard.

For purposes of comparison, it is necessary that the electrodes or discharging points of the spark-gap should be uniform in form and dimensions, since the same current will discharge across the electrodes at a greater distance apart if their opposing points be sharp than if they be rounded.

More uniformity in this is observed on the Continent, but all workers in this country should also adopt the standard of two spherical endings of 1 centimetre in diameter.

To prevent perforation, when using a hard tube it is well to leave the spark-gap only a little wider than the working distance of the tube. Otherwise, if the tube becomes too hard, there is no alternative path provided, and the current, if ‘ pushed/ will pass between the electrodes of the tube through the air outside, or seek a shorter path from the kathode to the outside by piercing the glass. This ‘ perforation ‘ will destroy the vacuum, thereby rendering the tube useless, and repair is very difficult. If perforation occur, the tube will be seen to change rapidly and violet light to appear, while production of X rays entirely ceases.

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