Page 41 - Winter 2026

appeared as a small pulse on the recorder paper and with the gas filling. The initiation of ionization remains (g) Gamma Radiography
the main long pulse due to the considerable dilution in effective throughout the life of the valve . Everyone is familiar with the general application
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the large cavity formed by the burning coal. From the of X-rays or gamma rays in the field of radiography.
size and area of the recorded pulses, and knowing the (e) Sulphur Determination in Oils The less energetic the X-rays, the more definition one
gas flow rates and capacities of the pipework, the exact Although no gamma ray is associated with the disin- can achieve when radiographing thin and moderately
position and size of the leak was calculated . tegration of tritium, the latter when combined with a low density materials. For instance, Thulium 170 emits
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heavy metal like Zr or Ti results in the production of gamma rays of only 84 KeV energy, and will provide
(c) Static Elimination soft X-rays or brehmsstrahlung. These soft X-rays have very highly defined radiographs of the bone structure
As most people are aware the build-up of static in a been utilised for determining the amount of sulphur in of the body. In order to achieve a suitable radiograph
oils.
large number of industries is a source of concern to the in the shortest possible time, not only must the strength
of the source be high, but it must be placed as near the
manufacturers. In the weaving industry, for instance, For the determination of sulphur in the presence of medium as possible, and accordingly, the source must
warp remaining stationary on the loom during the off- carbon and hydrogen, the X-ray wavelength of greatest be concentrated into the smallest volume possible. The
duty period collects dust from the atmosphere, which sensitivity is 0.5-5 A. The mass absorption coefficients
becomes ingrained in the material as “fog” marks. of carbon and hydrogen are practically equal for an chief disadvantage of Tm is the build up of energetic
brehmsstrahlung or X-rays of energies up to 1 MeV as
In dry atmospheres very high voltages can build up to X-ray wavelength of 0.55 A, so that variable C-H ratios the activity of the source is increased. This effectively
such an extent that the introduction of earth con- in the oil need not be considered. The minimum wave- reduces the definition of the subsequent radiograph. On
nection near a source of static will produce a spark length of the X-rays from a tritium source is 0.69 A. the other hand, Xenon 133 sources with a similar
intense enough to fire inflammable vapour. Using a 0.4 mm. thick Be window and a Geiger counter, gamma ray energy do not emit appreciable amounts of
Various methods are in operation today to counteract sulphur contents as low as 0.02% have been deter- these more energetic rays, so the radiograph remains
mined .
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these troubles. The maintenance of high humidity in highly defined even with higher activity sources. How-
mills, subjecting the appropriate source of static to a (f) Thickness Measurement ever, the short half life of the radioisotopes limits its
high frequency discharge, and more recently the ioniza- The measurement of the absorption or scattering of application in this field.
tion of the air by the intense beta radiation from a radiations by a medium is now a well established pro- It is very unlikely that radioisotopes will compete
radioactive isotope. Thallium 204 sources have been cedure for assessing its thickness. The procedure entails with X-ray tubes in attaining the precise exposures
used quite successfully for a number of years. More having a radioactive source at one side of the medium required in clinical radiography, but when X-ray
intense sources have been prepared using Kr 85, but and a counter at the other, the drop in the number of machines are inconvenient, for example in field condi-
have not been produced commercially. The chief ad- counts detected being a measure of the thickness or tions, the above mentioned sources could well prove
vantage of the krypton source is of course the rapid density of the medium. When using beta particles it particularly applicable.
dispersal of the source in the event of a fire. No solid is desirable, for the best accuracy, to choose the beta Fig. 5. A Krypton 85 Light Source.
contamination will remain to hinder the fire fighter or particle energy so that at least 50% are absorbed in the (h) Atomic Light Sources Although lamps have been prepared in laboratory
restoration of the factory. thickness to be measured. The half thickness of the The emission of light from phosphors bombarded by conditions which can be seen some 500 or more yards
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The gas can be very effectively sealed into a brass Kr 85 beta particles is about 40 mg/cm , which means atomic particles is a familiar application to all those away, and have sufficient intensity to make newsprint
container previously thoroughly leak tested. The win- that the technique is most sensitive with about 1.5 mm. people possessing watches or alarm clocks with lumin- legible 2-3 feet away in the dark, they will never com-
dow through which the beta particles penetrate would aluminium or 0.5 mm. iron, etc. ous dials. A phosphor is a particular type of solid pete with electric light for brightness. Consequently,
be 0.001 inch thickness nickel foil soft soldered to the Beta particle back scatter gauges are used to measure capable of converting absorbed energy, which may the applications are limited to dark room markers,
brass. The gamma dose rate will, of course, limit the the thickness of coating of a paint or metal plating on come from ultra violet rays or the bombarding particles markers in mines, ammunition dumps, ships and aero-
size of source one can produce without heavy metal a metal base. The principle relies upon the fact that mentioned above, into light rather than into heat, which planes, certain low intensity signalling lights on rail-
ways, short distance marine buoys, etc. They require
shielding. Another type of static eliminator is the ion the beta particles can be scattered back from the plated occurs with all other types of organic or inorganic com- no maintenance, no power sources and consequently no
blower, whereby air can be blown fairly rapidly past an material and the base metal, the proportion scattered pounds. By increasing the number of bombarding cables, are completely safe in an inflammable atmos-
intense beta source. This method avoids the direct back being proportional to where Z is the atomic particles the amount of light or the brightness of the phere and long lasting. Since the gases are compara-
beta radiation to which operators would be subjected number of the surface layer. A saturation value is source increases. Unfortunately, the very energetic and tively inert, the inadvertent breakage of a source will
if they were careless in leaving a source exposed when achieved when the thickness is increased to about 3 comparatively massive particles do have a deleterious
not in use. half thicknesses, which in the case of aluminium as the effect on the phosphor crystals, so as to decrease the constitute little danger if in fairly large rooms or in the
open air the gas is allowed to disperse quickly'. A
amount of light emitted. Over a period of a few
Shielding from the direct beta radiation is provided coating and using the beta particles from Kr 85, would months the brightness of the luminous paint will drop typical atomic lamp is illustrated in Fig. 5.
by the air blower, and could be increased without limit- be 4.5 mm. thickness. With thin coatings, a response by 50% or more. When the bombarding particle is
ing the usefulness of the source. The krypton can be on the counter is achieved which is intermediate be- replaced by a beta particle this destructive effect is not CONCLUSION
sealed in thin walled metal tubing so that effectively tween that for the base metal and the coating. It is so noticeable, and considerably brighter sources can be There is every indication that radioactive gases will
all the betas are utilised, which is not the case with a usual to use two counters, one of which responds to a produced which only decrease their light intensity as play a very important part in the future programme of
source containing one metal window. zero coating, so that an unbalanced current is obtained the activity decays away. radioisotopes in industry. Krypton 85, in particular, is
which is practically proportional to the coating thick- a waste product from reactors and as more civil react-
Both tritium and krypton 85 are two radioisotopes
(d) Ionization in Valves ness over a limited range. which are particularly useful for such an application. ors are constructed so the availability of the radioisitope
Most cold cathode valves require the presence of ions When the atomic numbers of the base metal and Both possess reasonably long half lives, so that the light increases and the cost decreases. In addition, the gas
is inert and therefore not retained by the body when
in the gas space if they are to operate effectively in the coating are similar, the sensitivity is naturally much intensity will only drop by one half in 10 years, to a inhaled. Since stringent precautions are always adopted
quickest possible time, i.e. the electric discharge device reduced. Although Kr 85 sources, which can be pre- quarter in 20 years, to one eighth in 30 years, and so on. to seal all types of radioisotopes, used in industry, there
must have a stable striking voltage. Ultra violet light pared as for static elimination, have not yet been used The atomic lights are prepared from metal, glass or will be no more difficulty in sealing a gas compared to
will initiate a sufficient number of free electrons to in this field commercially, it is possible to measure Perspex in the form of cups or bulbs, with a thin layer a solid. A source containing an inert gas will have
prevent statistical time lags before the device will oper- thickness down to 0.75 microns of tin on iron within of phosphor lining the containers. A window of suit- the additional advantage of not constituting any form
ate, but many valves are operated in complete darkness 0.05 microns. able glass or Perspex allows the light to penetrate from of contamination hazard in the unfortunate event of a
and accordingly require some additional source of The same technique with equivalent sensitivity can a hermetically sealed lamp filled with the radioactive source being damaged by unqualified personnel or by
ionization. This can be effectively accomplished by be employed using the X-rays from a tritium-titanium gas. fire.
introducing a few microcuries of tritium or krypton 85 source.
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