RADIOACTIVITY OF THE ATMOSPHERE 
(lower limit of measurement approximately 2 < 107” 
C em™). For further details see Israél [29]. 
Induction Method. If a charged body is exposed to 
air containing Rn, its surface acquires a certain activ- 
ity, which becomes greatest with a negative charge. Be- 
cause of this, it is apparent that the resultant products, 
principally RaA, must be positively charged. Accord- 
ing to Heckmann [18], the RaA atoms are in fact posi- 
tively charged immediately after their formation. 
Hence, they would be useful for an indirect quantita- 
tive An measurement, if they did not soon lose part of 
their charge by interaction with the ions in the atmos- 
phere. Furthermore, the other by-products also carry, 
at least in part, electric charges. Finally, it must be 
considered that these charged mductions move under 
the influence of the electric field of the atmosphere. 
All these possibilities of error greatly reduce the re- 
lability of the mdirect method of measurement. This 
method supplies only relative values which, when ob- 
tamed under similar working conditions, are more or 
less comparable with one another. However, the rela- 
tive values can be correlated with the direct-emission 
measurements only with very great uncertainty. Even 
the various attempts at applyimg corrections, made by 
Salpeter [49] and Curie [13] did not succeed in elimi- 
nating this uncertainty. Only Hve’s and Aliverti’s mod- 
ification of the procedure (see below) leads to reliable 
quantitative results. 
The oldest type of a practical technique for carrying 
out such measurements is that in which a wire is kept 
at a high negative potential, exposed for several hours, 
and then wound on a spool and examined in an ioniza- 
tion chamber. The resultant rate of discharge, expressed 
in volts per hour divided by the length of the wire 
(in meters), is known as the “activation number.” An 
extensive improvement of this technique has been in- 
troduced by Swann [56], who segregated the contribu- 
tions of various types of inductions by observing the 
decrease in wire activity with time. Gerdien [22] and 
Bauer and Swann [4, 5] use an aspiration process for 
collection. Eve [20] exposes the collecting wire in a 
large closed vessel, 16 m* in volume, and compares the 
resultant activity with that obtained under otherwise 
identical conditions, but with a known quantity of 
Rn in the vessel. In Aliverti’s method [1, 2], all induc- 
tions contained in the atmosphere are deposited in a 
manner similar to that of electrostatic precipitators. 
The quantitative values for Rn and Tn can be obtained 
from the discharge curves. 
Results 
From the foregoing discussion, it is apparent that 
the activation numbers can give only an approximate 
picture of actual conditions. The values given in Table 
Ill represent averages derived from a large number of 
individual tests. 
If we disregard the uncertainties involved, it is ap- 
parent from the values given in Table III that the 
lowest values are found over the oceans, increasing 
toward the shore, and the highest values in high moun- 
tains with strongly emanating igneous rocks. They 
157 
give clear evidence that the radioactive admixtures 
get into the atmosphere exclusively from the conti- 
nents. 
The right-hand column of Table III gives mean 
values of the Rn content after improved induction 
measurements (aspiration method); they probably are 
much too small, particularly over the continents. 
Tasie III. Average VaLurs RESULTING FROM 
MEASUREMENTS OF INDUCTIONS 
| Rn content after 
, | Activation F F 
Loc: | ind 
ocation number In Wee ea 
—-- —| 
Oceans eer sete he.: sweat approx. 10 1.9 
COMMEND. oo0c0cc0rre00ec | #Pprex. 40 35 
High Mountains 
ANI OSS soe Ae ox SRR RR approx. 100 
| e 
Direct Rn measurements from various parts of the 
world are available in large number; a summary of 
such measurements is presented in Table IV. It will 
be seen from Table IV that the mean Rn content of the 
atmosphere near the surface over continents amounts 
to about 100-120 * 10~* C em ™® (omitting the larger 
values for the high mountains (Innsbruck) and the 
smaller ones for high altitudes), and over oceans to 
about 1-2 < 1078 C em”. Accordingly, one liter of 
air over the continent contains about 2000 atoms of Rn, 
over the oceans about 30 atoms. Thus, there can no 
longer be any doubt that the atmospheric Rn content 
is of purely continental origin; Bongards’ assumption 
[12] that the atmospheric Rn is of cosmic origin can 
therefore be discarded. The same conclusion follows 
from the decrease of the Rn content with height (see 
below). 
As far as the Th- and Ac-products of the atmosphere 
are concerned, data are more scarce. On the whole, it 
can be stated that (1) even over the continents, Tn, 
together with its disintegration products, contributes 
to the entire ionization probably less than, or at best 
just as much as, Rn with its inductions [44], and (2) 
An with its derivatives contributes hardly more than 
3 per cent towards the formation of atmospheric ions. 
The exhalation of Rn (Rn emission of the ground), 
according to the results obtained so far, is of the order 
of approximately 40 * 1078 C em sec! (Table V). 
The exhalation seems to have a single diurnal period 
with a maximum in the early forenoon [39], but its 
variation is strongly modified by meteorological factors 
[35, 61, 63]. The seasonal variation has a maximum in 
late summer [61]. Precipitation reduces the exhalation 
considerably, and solar radiation and an increase in 
temperature raise it; falling atmospheric pressure causes 
an increase of exhalation, rising pressure a decrease. 
Frost decreases it very sharply [63] and can stop it 
entirely [11]. 
Variations of the Atmospheric Radon Content. Over 
flat country and valleys the diurnal curves show un- 
equivocally a single period with a maximum during 
the night (toward morning) and a minimum during 
the afternoon [9, 39]. In mountainous country the 
High Cordilleras ........ 450-500 
