156 
Rn and its three short-lived decay products, RaA, 
RaB, and RaC. The current is thereby increased to 
20.0 X 10~* amp.' These two “current equivalents” are 
the basis for emanometry. 
A closed cylindrical vessel containing an insulated 
electrode is generally used as an ionization chamber. 
Between the walls of the vessel and the electrode, a 
potential of a few hundred volts is applied which causes 
the ions produced to deposit before their number 
changes appreciably by recombination (saturation cur- 
rent). As the ion concentration in the air is small, the 
measurement is usually made electrometrically by the 
charge or discharge method shown in Fig. 1. 
(a) 
Fie. 1.—Cireuit diagram for emanometrical measurements 
by the charge method (a) and by the discharge method (6). 
(b) 
Conversion of the measured data imto radon units 
requires various corrections: 
1. The ‘time correction” considers the change of 
the current equivalent due to formation of RaA, RaB, 
and RaC. Formerly, measurements were not begun 
until radioactive equilibrium had been attained be- 
tween Rn and RaA, RaB, and RaC, at the earliest 
about 3 hr after pure An had been put into the measur- 
ing vessel. This method can now be shortened by using 
the imtermediate values of the current equivalent 
(Table II). 
2. Because of the limitation of the ionization space, 
not all alpha rays can become fully effective. Great 
difficulties are encountered in the numerical computa- 
tion of the correction factor [14, 21, 54]. Therefore, the 
following empirical corrections developed by Duane 
and Laborde [16, 17] are used in most cases: 
J = K (1 — 0.517 0/V) 
J! = IK’ Gl. = 0.572 OV), 
where J and J’ represent the measured current values 
1. In this case only 50 per cent of the ionization effect of the 
alpha rays from RaA, RaB, and RaC is considered, since these 
inductions deposit on the walls. The beta and gamma radia- 
tions from RaB and RaC constitute less than 1 per cent of the 
current and are usually not considered. Furthermore, the con- 
tribution of RaD, RaH, and RaF is far below the range of 
accuracy of measurement and may be disregarded. The only 
manifestation of these substances is the slowly increasing 
“Hollution’’ of the measuring chamber which can usually be 
eliminated mechanically. 
ATMOSPHERIC ELECTRICITY 
for pure Fn or for radon in equilibrium with RaA, 
RaB, and RaC; K and K’ are the corrected current 
values; O is the surface, and V the volume of the ioni- 
zation vessel. 
3. It is difficult to attain saturation im ionization by 
alpha radiation, because a very strong initial recombi- 
nation becomes effective in the closely packed ion 
columns concomitant to their formation. In this case, 
also, theoretical determination of a satisfactory correc- 
tion factor is impossible. A generally valid empirical 
correction cannot be found either, since the saturation 
deficit is a function of the shape and size of the vessel 
and of the total quantity of ionizing substances. There- 
fore, this correction must be determined individually 
for each type of emanometer for varying quantities of 
radon. Suggestions for its practical evaluation have 
been made by Israél [81]. 
4. The effect of the atmospheric pressure and tem- 
perature on emanometrical measurements can be con- 
sidered according to Lester [86]. In general, however, 
it remains a minor error compared with the other errors 
present [6]. 
TasLe I]. Current EqutvaLents Sp ror EMANOMETRICAL 
MBEASUREMENTS* 
t ST t ST 
(min) (10-4 amp) (min) (10-4 amp) 
0 9.22 60 17.25 
2 11.03 80 18.24 
4 12.21 100 18.91 
6 12.97 150 19.78 
8 13.48 200 19.98 
10 13.80 250 20.00 
20 14.69 300 19.95 
30 15.34 400 19.80 
4U 16.00 500 19.61 
* Values of S7 are for ¢ minutes after the introduction of 
pure #n into an ionization chamber, taking into account the 
disintegration of Rn. 
These corrections may be avoided if the measure- 
ments are compared with those of known quantities of 
radon. In order to obtain such small, but well-defined 
Rn quantities, the so-called radium standards are used, 
aqueous solutions of RaCl, which can be produced 
easily [8] or which can be obtained commercially. The 
precautions to be taken when working with such test 
solutions are discussed elsewhere [8, 29]. 
Various types of emanometers, especially useful for 
atmospheric An measurements, have been developed, 
for example, by Becker [7], Messerschmidt [42, 43], 
Janitzky [383], and Israél [28, 31]. Moreover, such de- 
vices for measurements in an air stream were devel- 
oped by Israél [82] and Deij (see [16, 17]). Since the 
Rn concentration in the air is extremely small, a method 
of enriching it is often used to increase the accuracy. 
This method utilizes the very great solubility of Rn mm 
some organic substances, its ability to condense at the 
temperature of liquid air, or, most conveniently, its 
property of being adsorbed by coconut-shell charcoal 
or activated carbon. By heating the adsorbent to in- 
candescence, the Rn will again be given off completely. 
When large ionization chambers are used at high sensi- 
tivity, the ‘enrichment method” need not be used 
