Page 32 - Summer 2024

Kr 85 is dispensed into borosilicate glass ampoules adopted to check how frequently each egg in each tray appeared as a small pulse on the recorder paper and with the gas filling. The initiation of ionization remains
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using the apparatus illustrated in Fig. 3. This illustrates in an incubator received a change of air, so that the main long pulse due to the considerable dilution in effective throughout the life of the valve .
the use of localised shielding and of a remote controlled modifications could be made to the design of the venti- the large cavity formed by the burning coal. From the (e) Sulphur Determination in Oils
valve. The ampoules are evacuated while the Kr 85 lator to increase its efficiency. Unfortunately, the gap size and area of the recorded pulses, and knowing the
remains in the storage pot, which can hold 20 curies. between the eggs and the tray above was so small that gas flow rates and capacities of the pipework, the exact Although no gamma ray is associated with the disin-
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The valves of the storage pot are then opened and the no Geiger probe at present designed in this country position and size of the leak was calculated . tegration of tritium, the latter when combined with a
gas slowly allowed to fill the ampoules to the appropri- could be placed in the correct position for recording heavy metal like Zr or Ti results in the production of
ate pressure by operating the remote control valve. The the presence of the radioactive tracer gas. (c) Static Elimination soft X-rays or brehmsstrahlung. These soft X-rays have
ampoules can then be sealed off, and the unused gas As most people are aware the build-up of static in a been utilised for determining the amount of sulphur in
Allied with ventilation experiments is the use of a
absorbed on the charcoal finger and then re-transferred radioactive gas to follow the flow of gas underground in large number of industries is a source of concern to the oils.
to the storage pot. an oil field. Using I curie of T 2, Kr 85 or CH 3T, manufacturers. In the weaving industry, for instance, For the determination of sulphur in the presence of
At present the cost of Kr 85 is E5 per curie, but technicians in America have proved that each is equally warp remaining stationary on the loom during the off- carbon and hydrogen, the X-ray wavelength of greatest
within a few years when more advanced purification effective in tracing methane in a 10 acre oil field . duty period collects dust from the atmosphere, which sensitivity is 0.5-5 A. The mass absorption coefficients
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equipment has been constructed, the cost shouId be From such information it is possible to discern how oil becomes ingrained in the material as “fog” marks. of carbon and hydrogen are practically equal for an
considerably reduced. wells are interconnected and the size of gas space below In dry atmospheres very high voltages can build up to X-ray wavelength of 0.55 A, so that variable C-H ratios
Xenon 133 sources are prepared by dissolving a the Earth's surface. The experiment on the small oil such an extent that the introduction of earth con- in the oil need not be considered. The minimum wave-
uranium slug, which has been irradiated for about 14 field was so successful that we were asked to provide nection near a source of static will produce a spark length of the X-rays from a tritium source is 0.69 A.
days, in nitric acid solution, and purifying the gases in 250 curies Kr 85 for a large scale experiment in intense enough to fire inflammable vapour. Using a 0.4 mm. thick Be window and a Geiger counter,
a high vacuum glass apparatus shielded by ¼-inch lead . Venezuela. The information gained from the use of Various methods are in operation today to counteract sulphur contents as low as 0.02% have been deter-
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Remote control solenoid valves similar to the one illus- radioactive gases in this way can save oil companies these troubles. The maintenance of high humidity in mined .
trated in Fig. 3 are employed instead of stop-cocks. many months of time and money. (f) Thickness Measurement
The gas is available in small glass ampoules or ad- mills, subjecting the appropriate source of static to a
sorbed on charcoal in a glass capillary tube, the final (b) Leak Detection high frequency discharge, and more recently the ioniza- The measurement of the absorption or scattering of
volume of a source which may be as high as 15 curies Many conventional methods of leak detection are tion of the air by the intense beta radiation from a radiations by a medium is now a well established pro-
being less than 1 cc. NTP. more sensitive than the radioactive methods. Helium radioactive isotope. Thallium 204 sources have been cedure for assessing its thickness. The procedure entails
used quite successfully for a number of years. More
Argon 37, in addition to being prepared in microcurie leak detection in fact is the most sensitive method of intense sources have been prepared using Kr 85, but having a radioactive source at one side of the medium
and a counter at the other, the drop in the number of
quantities by irradiation of argon gas in the reactor for all, but requires expensive equipment, and is not always have not been produced commercially. The chief ad- counts detected being a measure of the thickness or
a few weeks, is prepared in a more enriched form by practicable especially for experiments conducted in the vantage of the krypton source is of course the rapid density of the medium. When using beta particles it
irradiating calcium in a reactor. The reaction involves field. For instance, we were asked recently for a radio- dispersal of the source in the event of a fire. No solid
the bombardment of calcium by fast neutrons and the active gas method of estimating the size and position of contamination will remain to hinder the fire fighter or is desirable, for the best accuracy, to choose the beta
particle energy so that at least 50% are absorbed in the
ejection of an alpha particle from the calcium nucleus. a leak during underground gasification of coal experi- restoration of the factory.
Up to 1 mc can be prepared from 2 gm. Ca irradiated ments. In the process, air or oxygen is pumped at high thickness to be measured. The half thickness of the
Kr 85 beta particles is about 40 mg/cm , which means
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for 6 weeks, at a flux of 10 ns/cm /sec. velocity through a long pipe leading into a coal seam. The gas can be very effectively sealed into a brass that the technique is most sensitive with about 1.5 mm.
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By melting the calcium under vacuum the argon is At the end of the pipe the coal is fired and the resultant container previously thoroughly leak tested. The win- aluminium or 0.5 mm. iron, etc.
released and purified from traces of quantities of gases from the combustion returns along the air space dow through which the beta particles penetrate would
between the ingoing pipe and the seam, entering another
occluded gases which are gettered by the molten and pipe before returning to the surface (Fig. 4).* be 0.001 inch thickness nickel foil soft soldered to the Beta particle back scatter gauges are used to measure
evaporated calcium. The yield can be calculated by brass. The gamma dose rate will, of course, limit the the thickness of coating of a paint or metal plating on
removing a minute aliquot and counting it in a gas size of source one can produce without heavy metal a metal base. The principle relies upon the fact that
counter. The radioisotope is supplied in a glass ampoule, shielding. Another type of static eliminator is the ion the beta particles can be scattered back from the plated
or absorbed on charcoal also in a glass ampoule. The blower, whereby air can be blown fairly rapidly past an material and the base metal, the proportion scattered
cost is E12 per mc. intense beta source. This method avoids the direct back being proportional to where Z is the atomic
beta radiation to which operators would be subjected
INDUSTRIAL APPLICATIONS if they were careless in leaving a source exposed when number of the surface layer. A saturation value is
achieved when the thickness is increased to about 3
(a) Ventilation Experiments not in use. half thicknesses, which in the case of aluminium as the
The siting of plenum fans and air inlets in factories Shielding from the direct beta radiation is provided coating and using the beta particles from Kr 85, would
and other buildings is an important factor of modern by the air blower, and could be increased without limit- be 4.5 mm. thickness. With thin coatings, a response
building. In order to ensure that the air is frequently ing the usefulness of the source. The krypton can be on the counter is achieved which is intermediate be-
changed in all parts of the building and that equipment sealed in thin walled metal tubing so that effectively tween that for the base metal and the coating. It is
can be positioned correctly to prevent the accumulation all the betas are utilised, which is not the case with a usual to use two counters, one of which responds to a
of pockets of stagnant air, something can be added to zero coating, so that an unbalanced current is obtained
the incoming air to follow its course through the build- source containing one metal window. which is practically proportional to the coating thick-
ing. One method would be the introduction of smoke ness over a limited range.
or some other gas which can be checked by detectors (d) Ionization in Valves
at various positions. Most cold cathode valves require the presence of ions When the atomic numbers of the base metal and
Much the simpler technique is the introduction of a Fig. 4. Leak detection in Underground Gasification in the gas space if they are to operate effectively in the coating are similar, the sensitivity is naturally much
few millicuries of an inert gas which will disappear System using Krypton 85. quickest possible time, i.e. the electric discharge device reduced. Although Kr 85 sources, which can be pre-
through the normal extract and leave no trace. Geiger must have a stable striking voltage. Ultra violet light pared as for static elimination, have not yet been used
counters positioned in the suspected parts of the build- When 100 mc Kr 85 was injected into the ingoing air will initiate a sufficient number of free electrons to in this field commercially, it is possible to measure
thickness down to 0.75 microns of tin on iron within
ing will provide a rapid means of checking the efficiency stream, a Geiger counter fitted into the return pipe prevent statistical time lags before the device will oper-
of the extract system. quite easily differentiated between a small leak which ate, but many valves are operated in complete darkness 0.05 microns.
There are naturally limitations to the technique. For and accordingly require some additional source of The same technique with equivalent sensitivity can
instance, it was anticipated that the method could be *Kr 85 is now being used to determine leaks in transistors ionization. This can be effectively accomplished by be employed using the X-rays from a tritium-titanium
down to
cc/s.
introducing a few microcuries of tritium or krypton 85 source.
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