RADIOACTIVITY OF SOIL GASES 67 
Second Reading After 
First Reading (Time Change of Sensitivity 
Station of Discharge of 4 (Time of Discharge of 
Scale Divisions) 4 Scale Divisions) 
A 10 © Sec. 24 «Sec. 
B Bas 8.5 
C 2 Bos 
D (Very wet hole) >40 
D 3-4 10 
E 21 50 
F 6 13 
G 4505 85 
The check is very good (the ratio between readings being of the order 
of 2), especially since the error in measuring some of the shortest time 
intervals is appreciable. 
However, consistent results are not always obtained. For example, 
in dry cracked ground the readings are increased after a rain has closed 
up the cracks. After a rain not only is there a possibility that a better 
seal can be obtained around the casing and that the gases in the 
ground can not diffuse into the atmosphere so freely, but also, as is 
well known, dry radioactive salts tend to retain the radon produced 
by their decay. The radon can be released by dissolving the radio- 
Lfsec é 
e 8 
Ber got BISCHARGE , Div, 
' 2 3 6 7 8 8 
4 5 
DISTANCE, MILES 
Fic. 2.—Typical line survey in Gulf Coast region. 
active salts, and thus it may be that radon is set free in dry ground 
after a rain by this process of dissolving dry radioactive salts. How- 
ever, if the ground becomes too wet, it becomes difficult to extract 
gas as the ground pores are filled with water for the most part. There 
are a number of factors which influence the radon content of the soil 
gases, and since these have been discussed at length by others, it 
seems hardly worth while to discuss them further. 
A typical line survey of the Gulf Coast region is seen in Figure 2, 
in which the rate of discharge of the electroscope in divisions per 
second is plotted in the ordinate direction and the distance in miles 
is plotted in the abscissa direction. It will be seen that the radon con- 
tent of the soil gases varies in a rather erratic fashion from point to 
point even though there is no apparent change in soil, except of a 
local nature. The curves in Figure 3 represent two line surveys in a 
589 
