Page 31 - Summer 2024

ties of hydrogen in the molecular or combined state, in dispersion before a sufficiently high tolerance level
adhering to the walls. Tritium can very quickly is reached.
exchange with such hydrogen, causing contamination of It is recommended, however, that source stores should
the counter and inability to regain the normal back- be well ventilated to prevent a build up of concentra-
ground count. During the past few years various tion of the radioisotopes, in the event of a leakage.
techniques have been evolved for counteracting this
contamination effect, such as converting the tritium to PREPARATION, HANDLING AND COST
a saturated organic gas and using this as the counter (a) Tritium
gas filling. Detection devices or monitors are invariably Tritium is formed by irradiating lithium compounds
large capacity ionization chambers into which tritium in thermal reactors, and extracting and purifying the
vapour can diffuse with laboratory air, or be pumped gas at high temperatures using high vacuum glass
through by means of a simple venturi pump operated apparatus 1, 3 . Multi-curie quantities can be handled
from a compressed air supply. Standard ß/ monitors with ease in fume cupboards connected to a substantial
can be modified to operate on the latter principle. air extract system, which will prevent appreciable dif-
Kr 85 and Xe 133 can also be assayed by incorpora- fusion of the gas into the laboratory, should accidental
ting them as part of the counter filling, or detected by breakage of apparatus occur.
gamma dose through the containing material. Standard Providing the outlets of vacuum pumps are connected
calibrated equipment for the detection of the isotopes to tubing leading out of the building and reasonable
in laboratory air are not available, but the use of a thin care is adopted in glass blowing operations, and the
glass walled Geiger probe can be employed for the cleaning of stop-cocks, quantities up to 10 millicuries of
purpose. Such probes, whose glass wall thickness is tritium can be handled at a time without resorting to
about 10-20 mg/cm , can detect a concentration of much the use of fume cupboards. The remaining high
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less than 1 µc/cc. air. vacuum equipment can be conventional in all respects.
The assay and detection of A is similar to tritium, The present cost of tritium is £2-10-0 per curie plus
37
because of the very soft X-rays emitted. a handling charge, and a rebate of 10% for any order
over 10 curies.
(e) Health Hazards
Since gases are dispersed so rapidly in air, the release (b) Krypton 85 and Xenon 133
of any of them into the atmosphere will constitute a Both these gases are fission products. The former
health hazard for only a short time depending on the will shortly be available in multi-curie quantities as a
vicinity of the operator to the source. None of the by-product of plutonium production, while the latter,
gases decays to a radioactive daughter product, conse- because of its short half-life, requires special irradiation
quently no solid contamination will accrue: Although of uranium.
the maximum permissible levels quoted In Table II Krypton 85 is present only to a few parts per million
appear relatively low, that due to tritium is based upon in a mixture of gases, which is mainly oxygen and Fig. 2. A simple Krypton 85 Source Filling System.
tritium in the form of tritiated water which, when nitrogen. The purification entails liquefaction and dis-
breathed into the lungs, is completely metabolised. It tillation under reflux to remove the less condensible
is, however, released from the body with a biological gases, followed by a charcoal chromatographic method
half life of a few days, which can also be reduced by to separate krypton from inert xenon. The final product
increasing the turn-over of water through the body. is pure krypton containing a trace of xenon.
Only a fraction of one per cent. of tritium as the Handling of millicurie quantities of Kr 85 can be
hydrogen molecule is metabolised by being converted carried out in conventional high vacuum glass appara-
to water on passage through the lungs. Since detecting tus, with localised shielding where the concentration of
equipment is unable to differentiate between HT and the gas is expected to be greatest. In order to reduce
HTO, and the presence of a flame or hot metal in the the beta particle penetration it is advisable to ensure
vicinity of a tritium leak will result in some conversion that the glass tubing less than 1 mm wall thickness is
of HT to HTO, most operators work at the lower m.p.l. not used. Fig. 2 illustrates a simple apparatus for filling
level for safety reasons. sources from a stock of Kr 85 which may be as much as
The other radioactive gases are completely inert, and 1 curie. The charcoal storage trap is filled with sufficient
in many respects, less hazardous than tritium. None activated charcoal to ensure that the partial pressure of
gas above the charcoal at room temperature is less than
arc metabolised in the body, and the m.p.l. quoted is one atmosphere. On cooling with liquid air, the kryp-
based upon a man standing in an infinite cloud of the ton is completely adsorbed. Consequently, after filling
gas, i.e. at the centre of a sphere of radius 8 feet which a source, the unused material can all be re-transferred
is the maximum range of a Kr 85 beta particle. Even to the storage pot. In the event of only dry ice being
the clothes on one's body can afford some protection available as a refrigerant, the partial pressure above the
from the beta rays of both Kr and Xe. charcoal when cooled to -80°C may be a few mm
In addition, the gases are invariably sealed in con- mercury. Any gas remaining in the dead space of the
tainers at sub-atmospheric pressure, which means that, system can then be salvaged by isolating the stock
in the event of a leak occurring, air will first enter the vessel and adsorbing the remaining krypton onto the
container and leakage out will be so slow as to result charcoal finger also cooled to -80°C. Fig. 3. Kr 85 Dispensing Apparatus.
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