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Measurements on the emanationcontent of ground-air
Physica VIII, no 7
MEASUREMENTS b y G. J. s I z o o ,
J u l i 1941
ON THE EMANATIONCONTENT OF GROUND-AIR P. c. S A N D E R S , L. F. C. F R I E L E a n d G. J. V A N D E R MAAS
(Communication from the Natuurkundig Laboratorium der Vrije Universiteit, Amsterdam
Summary A number of measurements concerning the emanation content of th~ soil at various places in the Netherlands are performed with the method of E 1 s t e r and G e i t e 1. The mean value comes out to 9 x 1 0 - 1 1 Curie//. Very low values (#~, I × 10-It Curie/l) were found in peaty soil and in the dunes along the North-Sea, higher ones were obtained in the diluvial sands in the centre and eastern part of the country, the highest (up to #-~ 57 x 10-11 Curie/l) in clay and loamy ground. The presence of thoron could be made certain in a number of cases. 1. S y s t e m a t i c m e a s u r e m e n t s concerning the a m o u n t of e m a n a t i o n p r e s e n t in the air c o n t a i n e d in the soil in the N e t h e r l a n d s h a v e not y e t b e e n p e r f o r m e d . Only D e y 1), who i n v e s t i g a t e d the r a d o n c o n t e n t of a t m o s p h e r i c air at A m s t e r d a m , m a d e a single m e a s u r e m e n t on the r a d i o a c t i v i t y of the ground-air. I n connection with an i n v e s t i g a t i o n p e r f o r m e d in our l a b o r a t o r y concerning the radioactiv i t y of soil a n d w a t e r 2) 3), we n o w h a v e d e t e r m i n e d the e m a n a t i o n c o n t e n t of grotind-air in a n u m b e r of places in the country. Because the a i m was to get a first s u r v e y r a t h e r t h a n to achieve high accuracy, we h a v e chosen the classic m e t h o d of E 1 s t e r a n d G e i t e 1 ~), in which the ionisation, p r o d u c e d b y the active deposit, g a t h e r e d on a n e g a t i v e electrode is t a k e n as a m e a s u r e for the e m a n a t i o n c o n t e n t of the air. I t has the a d v a n t a g e to be simple a n d to afford the possibility to detect the presence of t h o r o n as well as of radon. 2. A p o r t a b l e a p p a r a t u s was constructed, c o n t a i n i n g a double acting h a n d p o m p , a m a n o m e t e r a n d two iron vessels of 3 l each. These vessels were closed airtight b y a h a r d g u m m i stop, c a r r y i n g a - - 647
648
G. J. s I z o 0 , P. C. SANDERS, L. F. C. FRIELE AND G. J. V. D. MAAS
flat axial electrode (]ength ! 6 cm, breadth 0.5 cm). To gather the air, a small,,diving-bell" (height 15 cm, diameter 10 cm) was placed in a hole, digged in the ground with the aid of an earthdrill. The depth of this hole varied from some decimeters, in places with a high level of the groundwater, to some meters in other places. After the hole was stopped, we waited two days in order to be sure that the equilibrium between ground and air was restored. Then, through an iron pipe connected with the diving-bell, the required quantity of ~ 40 l air was pumped into the vessels. In the laboratory a negative potential of 2000 V was applied to the electrode of one of the vessels during a time of 75 minutes. This exposure always was begun at least three hours after the filling of the vessels, in order to be sure that equilibrium between radon and its decay products was established. Immediately after this, the electrode was taken out of the vessel and put into a cylindrical ionisationchamber (diameter and height 20 cm) in which again it served as an axial electrode. With an electrometer the ionisation current, due to the ~-particles emitted by the active deposit was compared with the current produced by a constant y-ray source, consisting of a q u a n t i t y of pitchblend, placed outside the chamber in an exactly reproducible position. One or two days later the air in the second vessel was investigated in the same way. In order to calibrate the ionisation chamber together with the T-ray source, one of the vessels was filled with inactive air to which a known quantity of radon was added, obtained from a standard solution, prepared by dilution of a radium solution, obtained from the P.T.R. The electrode again was exposed during 75 min and the ionisation current of the active deposit compared with that of the y-ray source. By five measurements, in which the amount of radon in the vessel was changed between 1 and 9 x 10-9 Curie, the proportionality of the activity of the deposit to the amount of radon was controlled. Also a number of measurements was made in which the amount of radon was kept constant, but the pressure of the air in the vessel was varied from 1 to 5 atm. No influence of this pressure was found. Further the electric tension on the electrode was varied between 2000 and 10000 V, but this proved to have no effect, so saturation was already obtained at 2000 V. In most of these measurements the standard radium solution was boiled out in an apparatus similar to that described by S i z o o and
MEASUREMENTS ON THE EMANATIONCONTENT OF GROUND-AIR
649
K o e n e 3). In an other one, the air in a closed circuit was forced to bubble through the solution, heated to 60°C, until it was saturated with radon. Then the air was let into the evacuated vessel, after which inactive air was added. The agreement between the results of both methods was satisfactory. The decay curve of the active deposit of radon is given in fig. 1. The drawn curve A was obtained by interpolation between the curves given by M e y e r and S c h w e i d 1 e r 5) for various times of exposure, the points represent our measurements. The curve B in this figure shows the decay of the active deposit, obtained when thoron was brought into the vessel, instead of radon. When the exposure began thoron and ThA had already disappeared. The decay curve proved to be exponential with the half life value of ThC, 55 min. From this it m a y be concluded that only those decay products which are formed during the exposure are gathered on the electrode, for else the decay curve should have shown an initial rise due to the formation of ThC atoms from the ThB on the electrode.
~
2o
_ _...~.'~-~
~'~
~--.,
~
A( ~ " ~ 20
0 -
40
60
80
100
120rain.
TIHI:"
Fig. 1. D e c a y of t h e ~ - a c t i v i t y of t h e a c t i v e d e p o s i t s of r a d o n ( c u r v e A ) , t h o r o n ( c u r v e B) a n d of m i x t u r e s of b o t h . T h e p e r c e n t a g e s r e f e r t o t h e f r a c t i o n of t h e zero a c t i v i t y d u e t o R n .
One might suppose that the extraction of about 40 1 air from the soil would distore the equilibrium in the ground and would cause atmospheric air to flow into the ground. To control this, the two
650
G. J. SlZO0, P. C. SANDERS, L. F. C. FRIELE AND G. J. V. D. MAAS
vessels were filled immediately after each other and the air in each of them was investigated, the time interval between both measurements being as short as possible. If atmospheric air had penetrated into the soil during the pumping, the air from the second vessel should have had a lower emanation content as that from the first. No such difference was found however. 3. The method affords two possibilities for determining the proportion of thoron to radon. a. If both radon and thoron are present in the air, the decay curve of the active deposit will be a superposition of the curves A and B, shown in fig. 1. A number of such composed curves are drawn in this figure between A and B. The percentages refer to the fraction of the total zero activity which is due to radon. One remarks that though these curves show a marked difference in steepness at the beginning and at the end, they are nearly parallel between 10 and 60 minutes after the end of the exposure. Therefore very accurate measurements extended over a 10ng time are required, in order to be able to analyse the decay curve with some certaincy. We have tried this method in a number of cases, but reliable quantitative results could not be obtained with it. b. Very soon after the vessel is filled thoron and T h A will have disappeared. Let then N1 resp. N2 be the numbers of radon and T h B atoms, and their decay constants kl and X2. The exposure m a y begin at the time t ahd last until t + t', and the activity of the active deposit m a y be measured at t + t' + t". This activity then can be represented by A = al(t', t") N , e-~lt + a2(t', t") N 2 e-~,t. The relative values of a 1 and a t can be calculated from the given values of t' and t", the decay constants, the ionisation powers of the decay products which come into account, and the proportions of the ionisationchamber. Therefore if A is measured for various, at least two, values of t, the proportion N1/N2 can be found. To try this method we made six measurements on the ground-alr from the laboratory-garden, in which t was varied from 0 to 82 hours, t' and t" constantly being 75 resp. 10 min. In fig. 2 the values of A are plotted against t. The drawn curve corresponds to the formula given above, with alN1/a2N 2 ---- 1.14. For at/a 2 we calculated 0.23, so that N~/N2 came out to be 5.1.
MEASUREMENTS ON THE EMANATIONCONTENT OF GROUND-AIR
651
Of course such series of measurements can only be carried out if the air is gathered in the immediate neighbourhood of the laboratory. Therefore, as was stated above, we always filled two vessels at the same time, which were investigated with an interval of one or two days. Because of the possible error in the two measurements of A the value of NJN2 calculated from them cannot be very accurate. Besides this it is hardly possible to draw from this proportion a quantitative conclusion as to the proportion of thoron to radon in the ground-air. During the pumping the equilibrium between thoron and ThB will certainly have been disturbed. The number of ThB atoms m a y have been diminished by adsorption on the walls of the tubes through which they have to pass before entering the vessel. Also the ground itself can act as a filter. This was clearly shown by another measurement on the ground-air of the laboratory garden, pumped from a divingbell, which had been put in place three months before. Practically no thoron was found this time, though the amount of radon had not changed. The bell was digged out and we stated that for the greater part it had been filled with moist sand by a rise of the level of the groundwater. Evidently the sand had acted as filter for the solid decay products. I0 8
6."2.....
t,
0
\
\.
IS ,-TIME
i
30
l 45
!
60
! 75
90 Hours
Fig. 2. A c t i v i t y of the active deposit of ground air, plotted against the time, elapsed between the p u m p i n g a n d the exposure.
For these reasons in the report of our final measurements, which is given in table I, we have restricted ourselves to note the presence of thoron, in those cases in which it could be made certain by one of the both methods described. 4. The results of the measurements on the emanationcontent of
652
G. J. SIZOO, P. C. S A N D E R S , L. F. C. F R I E L E A N D G. J . V. D. MAAS
the ground-air in several places in the Netherlands are given in table I. TABLE I E m a n a t i o n c o n t e n t of ground-air
No.
Place
Groningen
Radonequivalent in 10-n Curie//
Short indica- L e v e l o f Depth of tion of the the the diThoron nature of the ground- ving-bell soil water in m.
O. 1
peat, marshy
high
0,2
5-11-'40
+
peat until 0.7 m; deeper clay
high
0.5
15-10-'40
+
clay and peat
high
0.5
19- 9-'40 26- 9-'40
Onnerpolder
Sloten (near
0.8
Amsterdam) Zuiderakerpolder
De Bilt Meteor. Inst.
2.1 1.8 -
Heemstede
Date
-
-
t
-
-
2.9 2.1
+
dunes, sand
low
2.3
waterworks
12- 7-'40 28- 8-'39
's- Gravenkage
2.7
+
dunes, sand
low
1.5
23-12-'40
1.3
sand
high
1.2
25-1 I-'40
2.6
sand
ra t he r high
1.1
16-12-'40
sand sand
low low
2 1.5
11-10-'40 18-11-'40
sand
low
2 4.5
?- 7 : 3 9
waterworks
Zeist Zeisterbosch
Groningen waterworks
Hilversum a) b) 9 Ede waterworks
7.5 8.3
+ +
10 17
I0 A peldoorn waterworks
7.9
sand
low
1.4
7-11-'40
Nijmegen
5.8
sand
low
1.3
29- I 1-'40
sand, bogore
low
1.5
11-1 I-'40
6.0
sea-clay
high
0.5
29-10-'40
14 A lphen aid Rijn waterworks
15.6
river-clay
ra t he r low
1.2
21-10-'40
15 Tiel
57
river-clay and loam
low
1.0
3-12-'40
II
Scheidingsweg 12 Enschede waterworks 13 Beemster Purmerend
21
+
The activities are given in ,,radon-equivalent", by which is mear~t the radon content which would have caused the same activity of the
MEASUREMENTS ON THE EMANATIONCONTENT OF GROUND-AIR
653
active deposit. In the second column the ,+ sign means that the presence of thoron is certain. The level of the groundwater is called high if the divingbell is placed very near above it (1 ~ 2 decimeters) ; low if the level was much lower than the depth of the boring. The individual figures of course m a y have been influenced by accidental meteriological circumstances and by the condition of the soil, Still measurements made on one place at different dates, even with long intervals (see no. 4) proved that the reproducibility was sufficient to draw some general conclusions from the results. Also places with some km distance but with the same kind of soil gave nearly the same acitivity (no. 8a and b). The lowest values were obtained from peaty soil with a high level of the groundwater (no. 1 to 3). The emanationcontent of the air from the dunes along the North-Sea (no. 4 and 5) is also small'. Measurement 6 refers to a place, in which the soil consisted of sand, but which was not far from the border between sand and peat, and in which the level of the groundwater was high. Distinctly higher values were obtained from places in the diluvial sandgrounds in the centre (no. 8 to 10) and eastern part of the country (no. 11 and 12). The agreement with the values, obtained by I s r a ~ 1 and S a 1d u k a s 6) who found l0 ~ 15 × l0 -11 Curie// in diluvial sand at Potsdam, is good. In their measurements on the radon content of water S i z o o and K o e n e 2) also found much higher values in these regions than in the dunes. A relative high emanation content of the air was also found in clay and loamy ground (no. 13 to 15). The measurement of K o e n e 2) shows that the activity of this type of soil is some three or four times higher than that of the diluvial sandgrounds. The emanation contents do not show such a distinct difference, though the highest of all values was indeed found in the clay ground at Tiel (no. 15). Here a layer of loam was present under clay, and the diving-bellwas placed just beneath this layer. It may be that the ,,breathing-out" of the emanation into the atmosphere is diminished by this layer. On the other hand a high level of the groundwater seems to be unfavourable for the emanation content of the ground-air. The mean of all va.lues is 9 × 10-" Curie/l, which is about half the mean value derived by H e s s 7) from the results of observers in other countries. Received June 9th, 1941.
654
MEASUREMENTS ON T H E E M A N A T I O N C O N T E N T OF GROUND-AI
REFERENCES I) 2) 3) 4) 5) 6) 7)
J . L . L . D e y, dissertation, Amsterdam, 1938. G . J . S i z o o andC. P. K o e n e , PhysicaS, 215,1938. C. P: K o e n e , dissertation Vrije Universiteit, Amsterdam, 1938. J. E l s t e r and It. G e i t e l , Phys. Z. o 590, 1901; 3, 305, 574, 1902. S t . M e y e r and E. S c h w e i d l e r , Radioaktivit~t, Teubner 1927, p. 439. H. I s r a ~1 and J. S a 1 d u k a s, Meteorol. Z. 56, 39, 1939. V . F . H e s s, Electr. Leitf. der Atm., Vieweg 1926.
MEASUREMENTS b y G. J. s I z o o ,
J u l i 1941
ON THE EMANATIONCONTENT OF GROUND-AIR P. c. S A N D E R S , L. F. C. F R I E L E a n d G. J. V A N D E R MAAS
(Communication from the Natuurkundig Laboratorium der Vrije Universiteit, Amsterdam
Summary A number of measurements concerning the emanation content of th~ soil at various places in the Netherlands are performed with the method of E 1 s t e r and G e i t e 1. The mean value comes out to 9 x 1 0 - 1 1 Curie//. Very low values (#~, I × 10-It Curie/l) were found in peaty soil and in the dunes along the North-Sea, higher ones were obtained in the diluvial sands in the centre and eastern part of the country, the highest (up to #-~ 57 x 10-11 Curie/l) in clay and loamy ground. The presence of thoron could be made certain in a number of cases. 1. S y s t e m a t i c m e a s u r e m e n t s concerning the a m o u n t of e m a n a t i o n p r e s e n t in the air c o n t a i n e d in the soil in the N e t h e r l a n d s h a v e not y e t b e e n p e r f o r m e d . Only D e y 1), who i n v e s t i g a t e d the r a d o n c o n t e n t of a t m o s p h e r i c air at A m s t e r d a m , m a d e a single m e a s u r e m e n t on the r a d i o a c t i v i t y of the ground-air. I n connection with an i n v e s t i g a t i o n p e r f o r m e d in our l a b o r a t o r y concerning the radioactiv i t y of soil a n d w a t e r 2) 3), we n o w h a v e d e t e r m i n e d the e m a n a t i o n c o n t e n t of grotind-air in a n u m b e r of places in the country. Because the a i m was to get a first s u r v e y r a t h e r t h a n to achieve high accuracy, we h a v e chosen the classic m e t h o d of E 1 s t e r a n d G e i t e 1 ~), in which the ionisation, p r o d u c e d b y the active deposit, g a t h e r e d on a n e g a t i v e electrode is t a k e n as a m e a s u r e for the e m a n a t i o n c o n t e n t of the air. I t has the a d v a n t a g e to be simple a n d to afford the possibility to detect the presence of t h o r o n as well as of radon. 2. A p o r t a b l e a p p a r a t u s was constructed, c o n t a i n i n g a double acting h a n d p o m p , a m a n o m e t e r a n d two iron vessels of 3 l each. These vessels were closed airtight b y a h a r d g u m m i stop, c a r r y i n g a - - 647
648
G. J. s I z o 0 , P. C. SANDERS, L. F. C. FRIELE AND G. J. V. D. MAAS
flat axial electrode (]ength ! 6 cm, breadth 0.5 cm). To gather the air, a small,,diving-bell" (height 15 cm, diameter 10 cm) was placed in a hole, digged in the ground with the aid of an earthdrill. The depth of this hole varied from some decimeters, in places with a high level of the groundwater, to some meters in other places. After the hole was stopped, we waited two days in order to be sure that the equilibrium between ground and air was restored. Then, through an iron pipe connected with the diving-bell, the required quantity of ~ 40 l air was pumped into the vessels. In the laboratory a negative potential of 2000 V was applied to the electrode of one of the vessels during a time of 75 minutes. This exposure always was begun at least three hours after the filling of the vessels, in order to be sure that equilibrium between radon and its decay products was established. Immediately after this, the electrode was taken out of the vessel and put into a cylindrical ionisationchamber (diameter and height 20 cm) in which again it served as an axial electrode. With an electrometer the ionisation current, due to the ~-particles emitted by the active deposit was compared with the current produced by a constant y-ray source, consisting of a q u a n t i t y of pitchblend, placed outside the chamber in an exactly reproducible position. One or two days later the air in the second vessel was investigated in the same way. In order to calibrate the ionisation chamber together with the T-ray source, one of the vessels was filled with inactive air to which a known quantity of radon was added, obtained from a standard solution, prepared by dilution of a radium solution, obtained from the P.T.R. The electrode again was exposed during 75 min and the ionisation current of the active deposit compared with that of the y-ray source. By five measurements, in which the amount of radon in the vessel was changed between 1 and 9 x 10-9 Curie, the proportionality of the activity of the deposit to the amount of radon was controlled. Also a number of measurements was made in which the amount of radon was kept constant, but the pressure of the air in the vessel was varied from 1 to 5 atm. No influence of this pressure was found. Further the electric tension on the electrode was varied between 2000 and 10000 V, but this proved to have no effect, so saturation was already obtained at 2000 V. In most of these measurements the standard radium solution was boiled out in an apparatus similar to that described by S i z o o and
MEASUREMENTS ON THE EMANATIONCONTENT OF GROUND-AIR
649
K o e n e 3). In an other one, the air in a closed circuit was forced to bubble through the solution, heated to 60°C, until it was saturated with radon. Then the air was let into the evacuated vessel, after which inactive air was added. The agreement between the results of both methods was satisfactory. The decay curve of the active deposit of radon is given in fig. 1. The drawn curve A was obtained by interpolation between the curves given by M e y e r and S c h w e i d 1 e r 5) for various times of exposure, the points represent our measurements. The curve B in this figure shows the decay of the active deposit, obtained when thoron was brought into the vessel, instead of radon. When the exposure began thoron and ThA had already disappeared. The decay curve proved to be exponential with the half life value of ThC, 55 min. From this it m a y be concluded that only those decay products which are formed during the exposure are gathered on the electrode, for else the decay curve should have shown an initial rise due to the formation of ThC atoms from the ThB on the electrode.
~
2o
_ _...~.'~-~
~'~
~--.,
~
A( ~ " ~ 20
0 -
40
60
80
100
120rain.
TIHI:"
Fig. 1. D e c a y of t h e ~ - a c t i v i t y of t h e a c t i v e d e p o s i t s of r a d o n ( c u r v e A ) , t h o r o n ( c u r v e B) a n d of m i x t u r e s of b o t h . T h e p e r c e n t a g e s r e f e r t o t h e f r a c t i o n of t h e zero a c t i v i t y d u e t o R n .
One might suppose that the extraction of about 40 1 air from the soil would distore the equilibrium in the ground and would cause atmospheric air to flow into the ground. To control this, the two
650
G. J. SlZO0, P. C. SANDERS, L. F. C. FRIELE AND G. J. V. D. MAAS
vessels were filled immediately after each other and the air in each of them was investigated, the time interval between both measurements being as short as possible. If atmospheric air had penetrated into the soil during the pumping, the air from the second vessel should have had a lower emanation content as that from the first. No such difference was found however. 3. The method affords two possibilities for determining the proportion of thoron to radon. a. If both radon and thoron are present in the air, the decay curve of the active deposit will be a superposition of the curves A and B, shown in fig. 1. A number of such composed curves are drawn in this figure between A and B. The percentages refer to the fraction of the total zero activity which is due to radon. One remarks that though these curves show a marked difference in steepness at the beginning and at the end, they are nearly parallel between 10 and 60 minutes after the end of the exposure. Therefore very accurate measurements extended over a 10ng time are required, in order to be able to analyse the decay curve with some certaincy. We have tried this method in a number of cases, but reliable quantitative results could not be obtained with it. b. Very soon after the vessel is filled thoron and T h A will have disappeared. Let then N1 resp. N2 be the numbers of radon and T h B atoms, and their decay constants kl and X2. The exposure m a y begin at the time t ahd last until t + t', and the activity of the active deposit m a y be measured at t + t' + t". This activity then can be represented by A = al(t', t") N , e-~lt + a2(t', t") N 2 e-~,t. The relative values of a 1 and a t can be calculated from the given values of t' and t", the decay constants, the ionisation powers of the decay products which come into account, and the proportions of the ionisationchamber. Therefore if A is measured for various, at least two, values of t, the proportion N1/N2 can be found. To try this method we made six measurements on the ground-alr from the laboratory-garden, in which t was varied from 0 to 82 hours, t' and t" constantly being 75 resp. 10 min. In fig. 2 the values of A are plotted against t. The drawn curve corresponds to the formula given above, with alN1/a2N 2 ---- 1.14. For at/a 2 we calculated 0.23, so that N~/N2 came out to be 5.1.
MEASUREMENTS ON THE EMANATIONCONTENT OF GROUND-AIR
651
Of course such series of measurements can only be carried out if the air is gathered in the immediate neighbourhood of the laboratory. Therefore, as was stated above, we always filled two vessels at the same time, which were investigated with an interval of one or two days. Because of the possible error in the two measurements of A the value of NJN2 calculated from them cannot be very accurate. Besides this it is hardly possible to draw from this proportion a quantitative conclusion as to the proportion of thoron to radon in the ground-air. During the pumping the equilibrium between thoron and ThB will certainly have been disturbed. The number of ThB atoms m a y have been diminished by adsorption on the walls of the tubes through which they have to pass before entering the vessel. Also the ground itself can act as a filter. This was clearly shown by another measurement on the ground-air of the laboratory garden, pumped from a divingbell, which had been put in place three months before. Practically no thoron was found this time, though the amount of radon had not changed. The bell was digged out and we stated that for the greater part it had been filled with moist sand by a rise of the level of the groundwater. Evidently the sand had acted as filter for the solid decay products. I0 8
6."2.....
t,
0
\
\.
IS ,-TIME
i
30
l 45
!
60
! 75
90 Hours
Fig. 2. A c t i v i t y of the active deposit of ground air, plotted against the time, elapsed between the p u m p i n g a n d the exposure.
For these reasons in the report of our final measurements, which is given in table I, we have restricted ourselves to note the presence of thoron, in those cases in which it could be made certain by one of the both methods described. 4. The results of the measurements on the emanationcontent of
652
G. J. SIZOO, P. C. S A N D E R S , L. F. C. F R I E L E A N D G. J . V. D. MAAS
the ground-air in several places in the Netherlands are given in table I. TABLE I E m a n a t i o n c o n t e n t of ground-air
No.
Place
Groningen
Radonequivalent in 10-n Curie//
Short indica- L e v e l o f Depth of tion of the the the diThoron nature of the ground- ving-bell soil water in m.
O. 1
peat, marshy
high
0,2
5-11-'40
+
peat until 0.7 m; deeper clay
high
0.5
15-10-'40
+
clay and peat
high
0.5
19- 9-'40 26- 9-'40
Onnerpolder
Sloten (near
0.8
Amsterdam) Zuiderakerpolder
De Bilt Meteor. Inst.
2.1 1.8 -
Heemstede
Date
-
-
t
-
-
2.9 2.1
+
dunes, sand
low
2.3
waterworks
12- 7-'40 28- 8-'39
's- Gravenkage
2.7
+
dunes, sand
low
1.5
23-12-'40
1.3
sand
high
1.2
25-1 I-'40
2.6
sand
ra t he r high
1.1
16-12-'40
sand sand
low low
2 1.5
11-10-'40 18-11-'40
sand
low
2 4.5
?- 7 : 3 9
waterworks
Zeist Zeisterbosch
Groningen waterworks
Hilversum a) b) 9 Ede waterworks
7.5 8.3
+ +
10 17
I0 A peldoorn waterworks
7.9
sand
low
1.4
7-11-'40
Nijmegen
5.8
sand
low
1.3
29- I 1-'40
sand, bogore
low
1.5
11-1 I-'40
6.0
sea-clay
high
0.5
29-10-'40
14 A lphen aid Rijn waterworks
15.6
river-clay
ra t he r low
1.2
21-10-'40
15 Tiel
57
river-clay and loam
low
1.0
3-12-'40
II
Scheidingsweg 12 Enschede waterworks 13 Beemster Purmerend
21
+
The activities are given in ,,radon-equivalent", by which is mear~t the radon content which would have caused the same activity of the
MEASUREMENTS ON THE EMANATIONCONTENT OF GROUND-AIR
653
active deposit. In the second column the ,+ sign means that the presence of thoron is certain. The level of the groundwater is called high if the divingbell is placed very near above it (1 ~ 2 decimeters) ; low if the level was much lower than the depth of the boring. The individual figures of course m a y have been influenced by accidental meteriological circumstances and by the condition of the soil, Still measurements made on one place at different dates, even with long intervals (see no. 4) proved that the reproducibility was sufficient to draw some general conclusions from the results. Also places with some km distance but with the same kind of soil gave nearly the same acitivity (no. 8a and b). The lowest values were obtained from peaty soil with a high level of the groundwater (no. 1 to 3). The emanationcontent of the air from the dunes along the North-Sea (no. 4 and 5) is also small'. Measurement 6 refers to a place, in which the soil consisted of sand, but which was not far from the border between sand and peat, and in which the level of the groundwater was high. Distinctly higher values were obtained from places in the diluvial sandgrounds in the centre (no. 8 to 10) and eastern part of the country (no. 11 and 12). The agreement with the values, obtained by I s r a ~ 1 and S a 1d u k a s 6) who found l0 ~ 15 × l0 -11 Curie// in diluvial sand at Potsdam, is good. In their measurements on the radon content of water S i z o o and K o e n e 2) also found much higher values in these regions than in the dunes. A relative high emanation content of the air was also found in clay and loamy ground (no. 13 to 15). The measurement of K o e n e 2) shows that the activity of this type of soil is some three or four times higher than that of the diluvial sandgrounds. The emanation contents do not show such a distinct difference, though the highest of all values was indeed found in the clay ground at Tiel (no. 15). Here a layer of loam was present under clay, and the diving-bellwas placed just beneath this layer. It may be that the ,,breathing-out" of the emanation into the atmosphere is diminished by this layer. On the other hand a high level of the groundwater seems to be unfavourable for the emanation content of the ground-air. The mean of all va.lues is 9 × 10-" Curie/l, which is about half the mean value derived by H e s s 7) from the results of observers in other countries. Received June 9th, 1941.
654
MEASUREMENTS ON T H E E M A N A T I O N C O N T E N T OF GROUND-AI
REFERENCES I) 2) 3) 4) 5) 6) 7)
J . L . L . D e y, dissertation, Amsterdam, 1938. G . J . S i z o o andC. P. K o e n e , PhysicaS, 215,1938. C. P: K o e n e , dissertation Vrije Universiteit, Amsterdam, 1938. J. E l s t e r and It. G e i t e l , Phys. Z. o 590, 1901; 3, 305, 574, 1902. S t . M e y e r and E. S c h w e i d l e r , Radioaktivit~t, Teubner 1927, p. 439. H. I s r a ~1 and J. S a 1 d u k a s, Meteorol. Z. 56, 39, 1939. V . F . H e s s, Electr. Leitf. der Atm., Vieweg 1926.