(Reprinted from Nature, Vol. 189, No. 4762, pp. 348-352,
February 4, 1961)
NATURALLY OCCURRING ALPHA-ACTIVITY OF DRINKING WATERS
By R. C. TURNER, J. M. RADLEY and Pror. W. V. MAYNEORD
Physics Department, Institute of Cancer Research : Royal Cancer Hospital, London, S.W.3
HE present article represents a continuation of
investigations into the nature and levels of
naturally occurring x-activitv in man and his environment. We have demonstrated the grat variations of
%-activitvy to be found as between different human
foods'. Following the observations on foods it was
natural to study the radioactivity of drinking waters
supplied to the population of Great Britain.
Several investigators?-* have reported measurements of the radium-226 content of potable waters in
other count.ies, but very few data have been puhlished during recent years on the activity of waters
in Britain.
Following the observations of J. J.
Thomson?early in this century on Cambridge water,
there was considerable interest in this subject, but
much of the earlier work leaves doubt as to whether
the observed values applied to radium-226 or also
included gaseous products such as radon-222, known
to exist in certain waters at considerably higher levels
of activity than its parent. In the present experiments we have measured the radiuin-226 content of
71 drinking waters available in Britain, and in
addition we report the values for radon-222 and
radium-224 (thorium .Y) when present.
In most instances the water samples have been
made available to us by the co-operation of the local
water undertakings at the request of the Ministry of
Housing and Local Government
Experimental Procedure
Tre water samples were selected from several
points of view, including the size of population supplied and the nature of the associated geological
formations. The specimens were chosen with the view
of assessing the total daily intake vf natural radioactivity throughout Britain from its water supplies.
Preliminary experiments indicated that. if polythene containers were used for the colleetion and
transport of the samples, it was unnecessary to add
either carrier or acid agent to the water at the time
of sampling. No residual «-activity could be detected
on the walls of such containers even after they had
housed specimens of relatively high activity for
several days.
A litre of each specimen of water to be measured
was evaporated to dryness in a ‘Vitreosil’ beaker and
the «-activity of the residue measured, using the «-ray
counting technique previously described’. In a few
eases where the mineral content of the water was
very low, 20 mgm. of barium chloride having known
low «-activity was added to the water before
evaporation in order to provide adequate residue.
Measurements were made on each specimen of
residue immediatelyafter evaporation andat iutervals
of a few days during the following 30 days, by which
time the radon-222 and its a-emitting daughters
polonium-218 (radium A) and polonium-214 (radium
C’) would have reached equilibrium with the radium226 present. After 30 days no further change of
a-activity of the residue could be detected in any of
the specimens. Assessment of the amount of radon222 above the level required for equilibrium with the
ri lium-226 present in each water was effected by
observations on a liquid sample 2-3 mm. deep,
sealed in contact with a zinc-cadmium sulphide
phosphorof area 93 cm.*. Adsorption byzinc sulphide
crystals of the daughter products of radon-222 present
in solution iad been previously reported by Rosholt",
who used the method to obtain quantitative yields of
lead-218 (radium B) and polonium-214 (radium C’).
It has been observed by us that, when water con-
taining dissolved radon at a level of activity many
times higher than its content of radium-226 is assembled in such an arrangement, the «-counting-rate
increases for the first hour after sealing the sample
and thereafter decreases with the half-life of radon-222
(3-8 days), until the activity has fallen to the level
represented by the content of radium-226 and other
long-lived activitv. If at any intermediate stage the
water is removed and the activity of the phosphor
itself observed, the counting-rate decreases rapidly
during the first 10 min. or sy, corresponding to decay
of polonium-218 (radiuin ‘), and thereafter falls with
a half-life of approximately 40 min. compatible with
the decay of lead-214 (radium 8) and bismuth-214
(radium C) It is thought that the solid decay products of the dissolved radon are deposited on the
surface of the zinc sulphide crystals and the «-particles
emitted by polonium-218 (radium 4) and polonium214 (radium C’) are assumed counted with 27 afficiency. So long as the water itself is present, the
observed activity is maintained with the half-life
characteristic of radon, although the radon «-particles
themselves may contributelittle to the counting-rate
since their range in the medium is only 1 or 2 per
cent of the depth of the water layer.
Fig. 1 shows the charactcristic pattern of decay of
x-activity observed in a water sample containing
radon greatly in excess of the amount required for
equilibrium with its radium-226 content.
Each
estimate of radon content has been corrected for
decay between the time of collection of the sample
and the actual measurement.
In a numberof the more active waterresidues, fast
pairs of a-particles were observed, indicating the
presence of at least the shorter-lived members of the
thorium series!®.
This conclusion was borne out by
the subsequent variation of the thorium series
activity with time, since it decreased rapidly during
the first 20 days or so following evaporation, after
which the thorium activity remained constant at a
relatively low level.
The presence of radium-224
(thorium -Y) in American potable waters has already
been reported by Stehney?, who found the levels of
radium-224 in freshly sampled waters comparable
with those observed for radium-226. Due to the short
half-life of radium-224 (3-64 days) andof its «-emitting daughter members of the thorium series, this
x-activity decreases to very low values in the 2-3
weeks subsequent to sampling. Thereis little doubt
that the decreasing thorium series activity observed