Page 37 - Winter 2026

figure. A 50 mc. source of Kr 85 gives a gamma dose ties of hydrogen in the molecular or combined state, in dispersion before a sufficiently high tolerance level
of 7 mr/hour at 10 cms., so one can stand 100 cms. adhering to the walls. Tritium can very quickly is reached.
from such an unshielded source practically indefinitely. exchange with such hydrogen, causing contamination of
The half-thickness of lead for this energy of gamma- the counter and inability to regain the normal back- It is recommended, however, that source stores should
rays is 3.3 gm/cm , i.e. 0.3 cm. thickness of lead will ground count. During the past few years various be well ventilated to prevent a build up of concentra-
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reduce the gamma dose rate by half. Consequently, techniques have been evolved for counteracting this tion of the radioisotopes, in the event of a leakage.
¼-inch lead will reduce the dose rate to about 4 mr/hr contamination effect, such as converting the tritium to PREPARATION, HANDLING AND COST
at 10 crns. a saturated organic gas and using this as the counter (a) Tritium
Xenon 133 emits a much less energetic gamma-ray gas filling. Detection devices or monitors are invariably Tritium is formed by irradiating lithium compounds
than Kr 85. Although a 100 mc. unshielded source of large capacity ionization chambers into which tritium in thermal reactors, and extracting and purifying the
Xe 133 provides a gamma dose of 500 mr/hour at 10 vapour can diffuse with laboratory air, or be pumped gas at high temperatures using high vacuum glass
crns., the half thickness of lead is only 0.28 gm/cm or through by means of a simple venturi pump operated apparatus 1, 3 . Multi-curie quantities can be handled
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0.03 cm. The dose rate from 100 mc. would accord- from a compressed air supply. Standard ß/ monitors with ease in fume cupboards connected to a substantial
ingly be reduced to 4 mr/hr at 10 crns. with less than can be modified to operate on the latter principle. air extract system, which will prevent appreciable dif-
+inch lead. 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,
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(a) Half Life Argon 37 is not available in quantities more than a because of the very soft X-rays emitted. The present cost of tritium is £2-10-0 per curie plus
Both tritium and Kr 85 have conveniently long half few millicuries, but in any case during the disintegration a handling charge, and a rebate of 10% for any order
lives. The considerably shorter half life of Xenon 133 only low energy electrons and X-rays are emitted, so (e) Health Hazards over 10 curies.
does however limit its usefulness as well as the economy that virtually complete protection is afforded by stand- (b) Krypton 85 and Xenon 133
of producing it, although the availability of multi-curie ard glass tubing, i.e. 1 mm. thickness. Since gases are dispersed so rapidly in air, the release Both these gases are fission products. The former
sources compensates for the shorter half life in certain of any of them into the atmosphere will constitute a will shortly be available in multi-curie quantities as a
respects. (c) Specific activity health hazard for only a short time depending on the by-product of plutonium production, while the latter,
vicinity of the operator to the source. None of the
(b) Energies of Emitted Particles and Shielding The tritium sold by Harwell is greater than 95% gases decays to a radioactive daughter product, conse- because of its short half-life, requires special irradiation
of uranium.
No gamma ray is emitted during the disintegration pure, the impurity being hydrogen and small traces of quently no solid contamination will accrue: Although
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of tritium, and the energy of the beta particle is so Helium the decay product. The specific activity is 2.58 the maximum permissible levels quoted In Table II Krypton 85 is present only to a few parts per million
small that complete protection from the particles can curies per cc. NTP of pure tritium gas. appear relatively low, that due to tritium is based upon in a mixture of gases, which is mainly oxygen and
be accomplished by less than one tenth of a mm. of tritium in the form of tritiated water which, when nitrogen. The purification entails liquefaction and dis-
Krypton 85 is available at an abundance of 3-4%, so
glass. When the gas is combined with a fairly heavy that 1 curie of the gas occupies a volume of about 20 breathed into the lungs, is completely metabolised. It tillation under reflux to remove the less condensible
metal, it can be detected by the brehmsstrahlung cc. NTP. Inactive isotopes of krypton are formed by is, however, released from the body with a biological gases, followed by a charcoal chromatographic method
emitted from the source. These X-rays are caused by half life of a few days, which can also be reduced by to separate krypton from inert xenon. The final product
the deceleration of the beta particles as they pass decay during the fission of uranium, which reduces the increasing the turn-over of water through the body. is pure krypton containing a trace of xenon.
through the electron field surrounding the nucleus of abundance of the radioactive isotope to 7%. In addi- Only a fraction of one per cent. of tritium as the Handling of millicurie quantities of Kr 85 can be
tion, during the dissolution of uranium to release the
the heavy element, and have a continuous energy spec- gas, sufficient atmospheric krypton is unavoidably hydrogen molecule is metabolised by being converted carried out in conventional high vacuum glass appara-
trum similar to the beta spectrum, so that the maximum to water on passage through the lungs. Since detecting tus, with localised shielding where the concentration of
energy of the X-rays is 18 keV. The X-rays are more present in the oxygen used which reduces the abund- equipment is unable to differentiate between HT and the gas is expected to be greatest. In order to reduce
penetrating than the beta particles, and can accordingly ance still further to 3-4%. No means are available yet HTO, and the presence of a flame or hot metal in the the beta particle penetration it is advisable to ensure
for enriching the radioisotope.
be detected through a thin walled end window counter vicinity of a tritium leak will result in some conversion that the glass tubing less than 1 mm wall thickness is
a few inches from the source. The specific activity of Xe 133 is very much higher of HT to HTO, most operators work at the lower m.p.l. not used. Fig. 2 illustrates a simple apparatus for filling
Krypton 85 emits a much more energetic beta so that it is possible to produce a 20 curie source in a level for safety reasons. sources from a stock of Kr 85 which may be as much as
particle, 0.67 MeV (Max), with a maximum range of volume much less than 1 cc. Argon 37, prepared by The other radioactive gases are completely inert, and 1 curie. The charcoal storage trap is filled with sufficient
about 1.5 mm. in glass. Thus a few mm. thickness of the irradiation of Calcium, is also available at very in many respects, less hazardous than tritium. None activated charcoal to ensure that the partial pressure of
glass or perspex and a much thinner sheet of a heavier high specific activity. arc metabolised in the body, and the m.p.l. quoted is gas above the charcoal at room temperature is less than
metal will afford complete protection from the beta based upon a man standing in an infinite cloud of the one atmosphere. On cooling with liquid air, the kryp-
particles. Unfortunately, the radioisotope does emit a (d) Detection and Assay gas, i.e. at the centre of a sphere of radius 8 feet which ton is completely adsorbed. Consequently, after filling
small proportion of gamma rays, which are in coinci- is the maximum range of a Kr 85 beta particle. Even a source, the unused material can all be re-transferred
dence with 0.5% of the disintegrations, so that for the As mentioned above, the energy of the beta particles the clothes on one's body can afford some protection to the storage pot. In the event of only dry ice being
more intense sources some form of heavy metal shield- emitted by tritium is so low that they do not penetrate from the beta rays of both Kr and Xe. available as a refrigerant, the partial pressure above the
ing is required. The maximum permissible level for very thin glass. Detection of the radioisotope can charcoal when cooled to -80°C may be a few mm
gamma radiation for scientists and technicians using accordingly only be accomplished by incorporating the In addition, the gases are invariably sealed in con- mercury. Any gas remaining in the dead space of the
radioactive isotopes is 0.3 rads in one week and an gas as part of the gas filling of a Geiger counter, pro- tainers at sub-atmospheric pressure, which means that, system can then be salvaged by isolating the stock
average level of 0.1 rad/week. The permissible level portional counter or ionization chamber. Most count- in the event of a leak occurring, air will first enter the vessel and adsorbing the remaining krypton onto the
for exposure of the general public is 1/10 of the latter ers, despite careful evacuation, still retain trace quanti- container and leakage out will be so slow as to result charcoal finger also cooled to -80°C.

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