Page 28 - Summer 2024

FROM THE ARCHIVES

figure. A 50 mc. source of Kr 85 gives a gamma dose
of 7 mr/hour at 10 cms., so one can stand 100 cms.
from such an unshielded source practically indefinitely.
The half-thickness of lead for this energy of gamma-
rays is 3.3 gm/cm , i.e. 0.3 cm. thickness of lead will
2
reduce the gamma dose rate by half. Consequently,
Radioactive Gases and Their Industrial Uses ¼-inch lead will reduce the dose rate to about 4 mr/hr
at 10 crns.
by Dr. E. J. WILSON* Xenon 133 emits a much less energetic gamma-ray
(A lecture demonstration delivered at the Thirteenth Annual Electronics Exhibition and Convention than Kr 85. Although a 100 mc. unshielded source of
of the Northern Division of the Institution of Electronics). Xe 133 provides a gamma dose of 500 mr/hour at 10
The general characteristics of radioactive gases are outlined and the health hazards and general handling techniques crns., the half thickness of lead is only 0.28 gm/cm or
2
are discussed. Their industrial uses including Ventilation experiments, leak detectors, static eliminators and low 0.03 cm. The dose rate from 100 mc. would accord-
intensity light sources are considered. ingly be reduced to 4 mr/hr at 10 crns. with less than
INTRODUCTION to conduct a small ventilation experiment in a room +inch lead.
In some respects it is unfortunate that the gases most of 1000 cu. ft.
frequently used in industry do not possess radioactive
isotopes of reasonably long half life, and emitted However, in the majority of experiments with gases,
particles which can be detected with ease. For instance, it is not necessary to use a radioactive or enriched stable
no radioisotope of nitrogen or oxygen with a half life isotope of the gas in question. The addition of a radio-
longer than 10 minutes has to date been discovered. It active isotope of a different gas will be suitable for

(a) Half Life Argon 37 is not available in quantities more than a
Both tritium and Kr 85 have conveniently long half few millicuries, but in any case during the disintegration
lives. The considerably shorter half life of Xenon 133 only low energy electrons and X-rays are emitted, so
does however limit its usefulness as well as the economy that virtually complete protection is afforded by stand-
of producing it, although the availability of multi-curie ard glass tubing, i.e. 1 mm. thickness.
sources compensates for the shorter half life in certain
respects. (c) Specific activity
(b) Energies of Emitted Particles and Shielding The tritium sold by Harwell is greater than 95%
No gamma ray is emitted during the disintegration pure, the impurity being hydrogen and small traces of
3
of tritium, and the energy of the beta particle is so Helium the decay product. The specific activity is 2.58
small that complete protection from the particles can curies per cc. NTP of pure tritium gas.
be accomplished by less than one tenth of a mm. of 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
metal, it can be detected by the brehmsstrahlung cc. NTP. Inactive isotopes of krypton are formed by
emitted from the source. These X-rays are caused by
the deceleration of the beta particles as they pass decay during the fission of uranium, which reduces the
abundance of the radioactive isotope to 7%. In addi-
through the electron field surrounding the nucleus of
the heavy element, and have a continuous energy spec- tion, during the dissolution of uranium to release the
gas, sufficient atmospheric krypton is unavoidably
trum similar to the beta spectrum, so that the maximum
energy of the X-rays is 18 keV. The X-rays are more present in the oxygen used which reduces the abund-
penetrating than the beta particles, and can accordingly ance still further to 3-4%. No means are available yet
for enriching the radioisotope.
be detected through a thin walled end window counter
a few inches from the source. The specific activity of Xe 133 is very much higher
Krypton 85 emits a much more energetic beta so that it is possible to produce a 20 curie source in a
particle, 0.67 MeV (Max), with a maximum range of volume much less than 1 cc. Argon 37, prepared by
about 1.5 mm. in glass. Thus a few mm. thickness of the irradiation of Calcium, is also available at very
Fig. 1. A Radioactive Gas Handling Laboratory. glass or perspex and a much thinner sheet of a heavier high specific activity.
is obvious that the activity would have decayed to a many experiments providing any large differences in metal will afford complete protection from the beta
negligible proportion before such a short half life iso- density of the gases do not seriously affect the validity particles. Unfortunately, the radioisotope does emit a (d) Detection and Assay
tope had even been delivered from Harwell. The use of of the results. For instance Xe 133 or Kr 85 can be small proportion of gamma rays, which are in coinci-
isotope techniques for following the course of chemical used to follow the flow of air in ventilation experiments. dence with 0.5% of the disintegrations, so that for the As mentioned above, the energy of the beta particles
and physical experiments with such gases can only be more intense sources some form of heavy metal shield- emitted by tritium is so low that they do not penetrate
carried out with a source containing one of the stable Table I contains a list of the commoner gases with ing is required. The maximum permissible level for very thin glass. Detection of the radioisotope can
isotopes in an enriched form. Analyses of samples the appropriate isotope which could be used in tracer gamma radiation for scientists and technicians using accordingly only be accomplished by incorporating the
becomes more difficult, in many cases, necessitating the experiments. radioactive isotopes is 0.3 rads in one week and an gas as part of the gas filling of a Geiger counter, pro-
use of a mass spectrometer. average level of 0.1 rad/week. The permissible level portional counter or ionization chamber. Most count-
Although oxygen and nitrogen enriched in O 18 and AVAILABLE RADIOACTIVE GASEOUS for exposure of the general public is 1/10 of the latter ers, despite careful evacuation, still retain trace quanti-
N respectively, are available in this country, the high ISOTOPES
15
price forbids their use, in for instance, ventilation ex- 23
periments. The limit of detection of the enriched stable Neglecting the gaseous compounds of carbon and
isotope is such that it wo u ld require about £200 f 0 18 sulphur, only four radioactive gaseous isotopes are
o
The appropriate physical
produced in this country.
*Radiochemical Centre, Amersham. properties of these gases are given in Table II.
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