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A device for the determination of low natural 222Rn and 226Ra concentrations
NUCLEAR
INSTRUMENTS
AND M E T H O D S
165 (1979) 3 4 5 - 3 4 8 ;
(~) N O R T H - H O L L A N D
PUBLISHING
CO.
A DEVICE FOR THE DETERMINATION OF LOW NATURAL 22ZRn AND Z26Ra CONCENTRATIONS HANS GEORG SCHEIBEL, JUSTIN PORSTENDORFER and ANDREAS WlCKE
Institut flit Biophysik, Strahlenzentrum der Justus Liebig Universitiit, D-6300 Lahn-Giessen, Leihgesterner Weg 217, W. Germany Received 10 July 1978 and in revised form 18 February 1979 An alpha-scintillation radon counter and an analytical system is described, suitable for a large-scale study of 222Rn (radon) and 226Ra (radium) content of either air or other environmental samples. Good results are to be obtained applying this device to the determination of radon emanation- and exhalation-rates from building materials and the radon and radium content of water.
1. Introduction 222Rn (radon), 22°Rn (thoron) and 219Rn (actinon) are the only natural radioisotopes which are noble gases. This property allows them to emanate from the ground or from building materials and to be present in the atmosphere1). Especially in closed rooms relatively high concentrations have been f o u n d 2 - 6 ) . Exposure to the inhaled short-lived decay products of radon and thoron constitutes the main natural irradiation of the various parts of the human respiratory tract. The precursors of radon and thoron, among them 226Ra (radium) and 228Th (thorium), enter the body through diet. The metabolic behaviour of radium is similar to that of calcium, and an appreciable fraction is deposited on bone surfaces and in areas of active bone turnover7). To study the radium content of the diet and in different organs of the body, an emanation method is applied (Krebsa)) and an instrument which detects low concentrations of radon is needed. The scintillation counter used by Lucas t°) is still one of the most sensitive, but the small volume of the scintillation cell (N 0.1 1) often requires an accumulation. The adsorption of radon on charcoal as a means of extending analyses to very small concentrations has been used for many years1°). In respect of our current problems, charcoal also makes it possible to study emanation and exhalation rates from different building materials. Together with an extended use of the scintillation cells we improved that method in various details. That makes it a useful tool for many radon and radium measurements, some of which are described here in detail.
2. Description of the counting chamber, the counting system and the accumulation device The scintillation cells we used are similar to those of Lucas~°). They consist of stainless steel shells, coated inside with a silver activated zinc sulfide scintillator, a valve and a clear quartz window. Inside a light-tight tube the scintillation cell is put on a photomultiplier with a grounded photocathode. This precaution prevents the electric field influencing the counting chamber and so minimizes variations of its sensitivity. The scintillations are detected by the phototube, and the pulses amplified and counted by a scaler. Counts and counting time are printed. With an extended use of scintillation cells, an automatic
52cm cm
S T N A IL E S SS T E E L
1
i~ 0m
S T A N {L E S S S T E EL J
03cm~ ~ 5 c m
SC,NT,LLA,OR
~
Fig. 1. Scintillation chamber,
346
H . G . SCHEIBEL et al.
sample change was introduced. The essential part of this instrument is a disc with twelve gaps (4.8 cmO) in a circle. Twelve chambers can be put on it, all covered by light-tight caps. The disc is turned step by step by a motor, controlled by a timer/scaler, so that the chambers are successively positioned directly above the phototube during the counting time. This makes it possible to make measurements in a series of chambers overnight. Counting itself begins 3 h after filling the scintillation cells, i.e. when secular equilibrium between radon and its short-lived daughters is established. Calibration is done by emanating a 226Ra standard solution. This is possible either with a small bubbler directly or with the accumulation system described later. A typical sensitivity of the scintillation chambers is 4.35_+0.13cpm/pCi. This calibration error is a sum of the calibration procedure. It includes the reproducibility of several calibrations. The average background counting rate is 0.09___0.008 cpm. The detection limit of the scintillation chambers with a counting time of 10h is then 4x10-14Ci with a confidence interval of 66.6% and a statistical error of 14%. The total relative error of one measurement is given by the sum of the relative calibration error, the relative statistical error and the relative systematic error of the special measurement procedure. The small chamber volume gives a detection limit for radon concentrations of 4 x 1 0 -13 Ci/m 3. The atmospheric radon concentrations are in the same range or lower. For the evaluation of those low concentrations an accumulation method is necessary. For accumulation we choose the method suggested by Lucas 1°) of adsorption of radon in cooled charcoal followed by desorption by heating into a very small gas volume. The results of our studies of the adsorption of radon in activated charcoal in a gas-flow system are reported elsewhere in detaill~). The most useful temperature for radon adsorption is near the condensation point of radon ( - 7 6 °C) and can easily be established by a cooling bath with dry-ice and acetone. Our device results in a total adsorption of the radon of the sucked gas. Using air, the low temperature requires drying the gas and removal of CO2. To achieve this we used an adsorption and desorption equipment, schematically shown in fig. 2. The gas is sucked through firstly a cooled (-78.5°C) water freeze-out trap with copper filaments, secondly a drying column filled with calciumchloride (CaC12) and ascarite (NaOH, for removal of CO2), and the cooled char-
ACCUMULATIONEQUIPMENT FLOWMETER
M!('
/ COPPER FILAMENTS [ 785°C )
2 ~
CHARCOAL (785 C1
M I - ~ MANOMETER t-E STOPCOCKS AI-A41NLETS NV
NEEDLE VALVE
VACUUMPUMP A3~/~J
0
•"
SCINTPLLATION
:'.
CHAMBER
CHARCOAL HEATING 400oC
Fig. 2. Accumulation equipment (schematically): M1-M4, manometers; 1-6, stopcocks, NV, needle valve; A1-A4, inlets.
coal trap. The gas-flow rate is 100 l/h. After adsorption the charcoal trap (volume V N70 ml) is evacuated, put into the oven of the desorption part and heated up to 300°C. A 20 c m 3 portion of helium from a reservoir is then added to the charcoal trap and a sigmamotor pump transfers approximately 90% of the carrier gas to the counter. This is done three or four times until pressure in the counter reaches atmospheric level. The error produced by the accumulation method is always smaller than 1% and may be neglected. The accumulation method improves the radon concentration measurement up to a factor o f 10 3 to 10 4. This leads to a detection limit with a 1 h sampling and the reported statistical conditions of 4X 10 -17 Ci/m 3.
3. Applications 3.1. 222Rn-cONCENTRATION OF AIR If radon concentrations are higher than 4 x 1 0 -13 Ci/m 3 (for instance in mines or in badly ventilated dwellings) measurement can be done with scintillation chambers without accumulation, by simply filling the evacuated chambers with the air and measuring the radon activity 3 h later. For the determination of lower radon concentration a certain volume of air is sucked into the accumulation equipment. After transferring the radon to the scintillation chamber the radon activity is measured.
222Rn AND
347
226Ra C O N C E N T R A T I O N S
3.2. 222Rn EXHALATION Radon diffusion from the ground is the source for the radon and radon daughters of the atmosphere. Additionally, inside closed dwellings radon exhalation from building materials is responsible for relatively high radon concentrations. Radon exhalation from ground has been measured by Pearson et al. ~2) and Megumi and Mamuro~3). The lowest detectable Rn-exhalation rates are not mentioned exactly by these authors, but the exhalation rates they measured are in a range greater than 1000 pCi/m2h. Measurement of radon exhalation of walls is more difficult, since the exhalation rate of walls is about ten to a hundred times lower. For this purpose we used an exhalation chamber which has a thin charcoal layer between a PVC-plate and a wire gauze. For the measurement the exhalation chamber is put on the test surface, so that the charcoal layer is parallel to the surface to adsorb the exhaled radon. Radon activity measurement of this charcoal layer gives the exhalation rate. Therefore the charcoal layer is poured into a glass tube (without contact with air), heated to 300°C and the desorbed radon is pumped with N2 carrier gas into the accumulation equipment. The detection limit of this method is caused by the dimensions of the test surface, the sampling time and the detection limit of the scintillation chambers. If the exhalation chamber rests 1 h on the test area (0.011 m2), our device gives the value 4 pCi/m2h, with a confidence interval of 66.6% and a statistical error of 25%. Counting lasted 3 h. The error of the procedure is about 2 %, caused by small inaccuracies of the test area and so on. There is no other method useful for field measurements of the small exhalation rates of building walls. 3.3. EMANATION METHODS Another important application is the measurement of radium and radon concentration of liquids by a radon emanation method9). This method is also useful for radium determination of solutions and soluble substances. 3.3.1. t u r n content of water Natural water usually contains dissolved 226Ra as well as 222Rn which, however, almost never are in radioactive equilibrium. Generally the radon content essentially exceeds that of radium because soil and rocks release radon more easily into water than radium. Therefore radium and radon contents have
to be measured separately. The water sampling with regard to radon concentration measurement has to be done without any loss of radon. The containers we used are glass bottles with a volume of about 2.8 1. The bottles are closed with PVC-screw caps punched by two glass tubes, one of which dips to the bottom forming an emanation bottle. For emanation a nitrogen carrier gas flow is bubbled through the water. The emanation efficiency is 98% for emanating with 301 nitrogen in 20 min. The detection limit for the determination of the radon content of water is about 2 × 10 -14 Ci/i, with a confidence interval of 66%, a statistical error of 24% and a counting time of 3 h. The error of the measurement procedure is about 5%, caused by small errors in the determination of the flow rate and water volume, and of the emanation efficiency.
3.3.2. 226Ra content of water To determine the 226Ra content of water the same method and the same containers are used. The radium content of water is measured 30 days after filling the bottle in radioactive equilibrium between 222Rn and 226Ra, by measuring the radon content of the water sample. A few cm 3 nitric acid is added to prevent adsorption of radium ions or atoms at the walls of the bottles. Immediately after filling, a great volume of air or nitrogen is pumped through the water to emanate the sample completely. For testing this method, distilled water is stored in the same way and the radon content is measured 30 days later. Our results show that there is no additional radon source (for instance the walls of the bottles). The detection limit of radium content measurement and the error of the procedure equals that of radon content measurement. The lowest detectable radium content of water is 2× 10 -14 Ci/1, which equals 2× 10-14 g Ra/1. Another method due to Shahin and Koba114) using an ion exchanger, on which 226Ra from a water sample is sorbed and then PVC
~\\\\\\\\\\\\\\\\\\\I H r-,.\\\\\\\\\\\\\\\\\\\l I~:: " / : ! CHARCOAL~':.~"! : !'i.':'. ; " "i .~ RUBBER ~--.,~ ~ ' ~ ' ~ . . . . ~'-- . . . . . . .
111111 :: : ~ - -]~,~
SILVERNET
EXHALATION CHAMBER F : 5.5cm x 20crn
Fig. 3. Exhalation chamber: length 20 cm, width 5.5 crn.
348
a.G.
S C H E I B E L et al.
the radon emanated, reaches the same detection limit, but is more complicated. Therefore, the procedure errors are higher. A chemical method]5), using coprecipitation of 226Ra with barium as the sulphate to concentrate the 226Ra and then emanating with a small gas flow into scintillation chambers directly, has a worse detection limit; as the barium sulphate is not pure enough, background is increased. The experimental effort for this method is comparable.
4. Conclusion The measurement of 222Rn with a scintillation chamber is simple and cheap (compared with ionisation chambers) but has high accuracy. The use of the accumulation method gives a very low detection limit for 222Rn in a carrier gas. This gives the possibility for solving 222Rn problems like 222Rn exhalation of building walls, etc. which have not able to be done till now. Besides the reported applications it is possible to work on other questions of 222Rn and 226Ra measurement. Examples are the determination of 222Rn concentration in breath or other gases. Using the emanation technique it is possible to evaluate the 226Ra content of solutions, or commonly, of all soluble materials. Here must be mentioned the 226Ra content measurement of biological materials such as food or ashed organs.
References z) H. Israel, Die natiirliche Radioactivitiit in Boden, Wasser und Luft; Beitriige zur Physik der Atmosphiire, Bd 30, S. 177 (1958). 2) A. T6th and J. Feher, Effective Ra-226 content of some Hungarian building materials, Report KFKI-78-8 (Jan. 1976). 3) N. Jonassen, Health Phys. 29 (1975) 216. 4) F. Steinh~usler and E. Pohl, The concentration of Rn-222, Rn-220 and their daughters in the air, Proc. 2nd Europ. Congr. Int. Rad. Prot. Ass., Budapest (1973) pp. 397-400. 5) A.C. George, Indoor and outdoor measurements of natural radon and radon daughter decay products in New York City air, in Proc. Natural radiation environment 11, Houston (1972). 6) E.M. Krisiuk, S.J. Tarasov, V.P. Shamov, N.J. Shalak, E.P. Lisachenko and L. P. Gomelsky, A study of radioactivity of building materials, Leningrad Research Institute of Radiation Hygene, Ministry of Public Health of RSFSR, Leningrad (1971). 7) R.D. Evans, Health Phys. 27 (1974) 497. 8) A. Krebs, Fundamenta Radiologica 89 (1939) 129. 9) E. Pohl and I. Pohl-R0hling, Health Phys. 31 (1976) 343. 10) H.F. Lucas, Rev. Sci. Instr. 28, no. 9 (1957) 680. 11) H.G. Scheibel, J. Porstend6ffer and A. Wicke, Radon adsorption in a gas flow by activated charcoal, Rev. Sci. Instr., to be published (1979). 12) j.R. Pearson, D.H. Rimbey and G.E. Jones, J. Appl. Meteorol. 4 : 3 (1965) 349. 13) K. Megumi and T. Mamuro, J. Geophys. Res. 77 no. 7 (I972). ]4) M. Shahin, 1. Kobal et al., Sorption-emanation method for the determination of Ra-226 in water, AED-conf-72-277-002, Bucharest (1972). 15) Radiochemical methodology for drinking water: radium-226 j n drinking water, Report EPA-600:4-75-008, Cincinnati, Ohio (Sept 1975).
INSTRUMENTS
AND M E T H O D S
165 (1979) 3 4 5 - 3 4 8 ;
(~) N O R T H - H O L L A N D
PUBLISHING
CO.
A DEVICE FOR THE DETERMINATION OF LOW NATURAL 22ZRn AND Z26Ra CONCENTRATIONS HANS GEORG SCHEIBEL, JUSTIN PORSTENDORFER and ANDREAS WlCKE
Institut flit Biophysik, Strahlenzentrum der Justus Liebig Universitiit, D-6300 Lahn-Giessen, Leihgesterner Weg 217, W. Germany Received 10 July 1978 and in revised form 18 February 1979 An alpha-scintillation radon counter and an analytical system is described, suitable for a large-scale study of 222Rn (radon) and 226Ra (radium) content of either air or other environmental samples. Good results are to be obtained applying this device to the determination of radon emanation- and exhalation-rates from building materials and the radon and radium content of water.
1. Introduction 222Rn (radon), 22°Rn (thoron) and 219Rn (actinon) are the only natural radioisotopes which are noble gases. This property allows them to emanate from the ground or from building materials and to be present in the atmosphere1). Especially in closed rooms relatively high concentrations have been f o u n d 2 - 6 ) . Exposure to the inhaled short-lived decay products of radon and thoron constitutes the main natural irradiation of the various parts of the human respiratory tract. The precursors of radon and thoron, among them 226Ra (radium) and 228Th (thorium), enter the body through diet. The metabolic behaviour of radium is similar to that of calcium, and an appreciable fraction is deposited on bone surfaces and in areas of active bone turnover7). To study the radium content of the diet and in different organs of the body, an emanation method is applied (Krebsa)) and an instrument which detects low concentrations of radon is needed. The scintillation counter used by Lucas t°) is still one of the most sensitive, but the small volume of the scintillation cell (N 0.1 1) often requires an accumulation. The adsorption of radon on charcoal as a means of extending analyses to very small concentrations has been used for many years1°). In respect of our current problems, charcoal also makes it possible to study emanation and exhalation rates from different building materials. Together with an extended use of the scintillation cells we improved that method in various details. That makes it a useful tool for many radon and radium measurements, some of which are described here in detail.
2. Description of the counting chamber, the counting system and the accumulation device The scintillation cells we used are similar to those of Lucas~°). They consist of stainless steel shells, coated inside with a silver activated zinc sulfide scintillator, a valve and a clear quartz window. Inside a light-tight tube the scintillation cell is put on a photomultiplier with a grounded photocathode. This precaution prevents the electric field influencing the counting chamber and so minimizes variations of its sensitivity. The scintillations are detected by the phototube, and the pulses amplified and counted by a scaler. Counts and counting time are printed. With an extended use of scintillation cells, an automatic
52cm cm
S T N A IL E S SS T E E L
1
i~ 0m
S T A N {L E S S S T E EL J
03cm~ ~ 5 c m
SC,NT,LLA,OR
~
Fig. 1. Scintillation chamber,
346
H . G . SCHEIBEL et al.
sample change was introduced. The essential part of this instrument is a disc with twelve gaps (4.8 cmO) in a circle. Twelve chambers can be put on it, all covered by light-tight caps. The disc is turned step by step by a motor, controlled by a timer/scaler, so that the chambers are successively positioned directly above the phototube during the counting time. This makes it possible to make measurements in a series of chambers overnight. Counting itself begins 3 h after filling the scintillation cells, i.e. when secular equilibrium between radon and its short-lived daughters is established. Calibration is done by emanating a 226Ra standard solution. This is possible either with a small bubbler directly or with the accumulation system described later. A typical sensitivity of the scintillation chambers is 4.35_+0.13cpm/pCi. This calibration error is a sum of the calibration procedure. It includes the reproducibility of several calibrations. The average background counting rate is 0.09___0.008 cpm. The detection limit of the scintillation chambers with a counting time of 10h is then 4x10-14Ci with a confidence interval of 66.6% and a statistical error of 14%. The total relative error of one measurement is given by the sum of the relative calibration error, the relative statistical error and the relative systematic error of the special measurement procedure. The small chamber volume gives a detection limit for radon concentrations of 4 x 1 0 -13 Ci/m 3. The atmospheric radon concentrations are in the same range or lower. For the evaluation of those low concentrations an accumulation method is necessary. For accumulation we choose the method suggested by Lucas 1°) of adsorption of radon in cooled charcoal followed by desorption by heating into a very small gas volume. The results of our studies of the adsorption of radon in activated charcoal in a gas-flow system are reported elsewhere in detaill~). The most useful temperature for radon adsorption is near the condensation point of radon ( - 7 6 °C) and can easily be established by a cooling bath with dry-ice and acetone. Our device results in a total adsorption of the radon of the sucked gas. Using air, the low temperature requires drying the gas and removal of CO2. To achieve this we used an adsorption and desorption equipment, schematically shown in fig. 2. The gas is sucked through firstly a cooled (-78.5°C) water freeze-out trap with copper filaments, secondly a drying column filled with calciumchloride (CaC12) and ascarite (NaOH, for removal of CO2), and the cooled char-
ACCUMULATIONEQUIPMENT FLOWMETER
M!('
/ COPPER FILAMENTS [ 785°C )
2 ~
CHARCOAL (785 C1
M I - ~ MANOMETER t-E STOPCOCKS AI-A41NLETS NV
NEEDLE VALVE
VACUUMPUMP A3~/~J
0
•"
SCINTPLLATION
:'.
CHAMBER
CHARCOAL HEATING 400oC
Fig. 2. Accumulation equipment (schematically): M1-M4, manometers; 1-6, stopcocks, NV, needle valve; A1-A4, inlets.
coal trap. The gas-flow rate is 100 l/h. After adsorption the charcoal trap (volume V N70 ml) is evacuated, put into the oven of the desorption part and heated up to 300°C. A 20 c m 3 portion of helium from a reservoir is then added to the charcoal trap and a sigmamotor pump transfers approximately 90% of the carrier gas to the counter. This is done three or four times until pressure in the counter reaches atmospheric level. The error produced by the accumulation method is always smaller than 1% and may be neglected. The accumulation method improves the radon concentration measurement up to a factor o f 10 3 to 10 4. This leads to a detection limit with a 1 h sampling and the reported statistical conditions of 4X 10 -17 Ci/m 3.
3. Applications 3.1. 222Rn-cONCENTRATION OF AIR If radon concentrations are higher than 4 x 1 0 -13 Ci/m 3 (for instance in mines or in badly ventilated dwellings) measurement can be done with scintillation chambers without accumulation, by simply filling the evacuated chambers with the air and measuring the radon activity 3 h later. For the determination of lower radon concentration a certain volume of air is sucked into the accumulation equipment. After transferring the radon to the scintillation chamber the radon activity is measured.
222Rn AND
347
226Ra C O N C E N T R A T I O N S
3.2. 222Rn EXHALATION Radon diffusion from the ground is the source for the radon and radon daughters of the atmosphere. Additionally, inside closed dwellings radon exhalation from building materials is responsible for relatively high radon concentrations. Radon exhalation from ground has been measured by Pearson et al. ~2) and Megumi and Mamuro~3). The lowest detectable Rn-exhalation rates are not mentioned exactly by these authors, but the exhalation rates they measured are in a range greater than 1000 pCi/m2h. Measurement of radon exhalation of walls is more difficult, since the exhalation rate of walls is about ten to a hundred times lower. For this purpose we used an exhalation chamber which has a thin charcoal layer between a PVC-plate and a wire gauze. For the measurement the exhalation chamber is put on the test surface, so that the charcoal layer is parallel to the surface to adsorb the exhaled radon. Radon activity measurement of this charcoal layer gives the exhalation rate. Therefore the charcoal layer is poured into a glass tube (without contact with air), heated to 300°C and the desorbed radon is pumped with N2 carrier gas into the accumulation equipment. The detection limit of this method is caused by the dimensions of the test surface, the sampling time and the detection limit of the scintillation chambers. If the exhalation chamber rests 1 h on the test area (0.011 m2), our device gives the value 4 pCi/m2h, with a confidence interval of 66.6% and a statistical error of 25%. Counting lasted 3 h. The error of the procedure is about 2 %, caused by small inaccuracies of the test area and so on. There is no other method useful for field measurements of the small exhalation rates of building walls. 3.3. EMANATION METHODS Another important application is the measurement of radium and radon concentration of liquids by a radon emanation method9). This method is also useful for radium determination of solutions and soluble substances. 3.3.1. t u r n content of water Natural water usually contains dissolved 226Ra as well as 222Rn which, however, almost never are in radioactive equilibrium. Generally the radon content essentially exceeds that of radium because soil and rocks release radon more easily into water than radium. Therefore radium and radon contents have
to be measured separately. The water sampling with regard to radon concentration measurement has to be done without any loss of radon. The containers we used are glass bottles with a volume of about 2.8 1. The bottles are closed with PVC-screw caps punched by two glass tubes, one of which dips to the bottom forming an emanation bottle. For emanation a nitrogen carrier gas flow is bubbled through the water. The emanation efficiency is 98% for emanating with 301 nitrogen in 20 min. The detection limit for the determination of the radon content of water is about 2 × 10 -14 Ci/i, with a confidence interval of 66%, a statistical error of 24% and a counting time of 3 h. The error of the measurement procedure is about 5%, caused by small errors in the determination of the flow rate and water volume, and of the emanation efficiency.
3.3.2. 226Ra content of water To determine the 226Ra content of water the same method and the same containers are used. The radium content of water is measured 30 days after filling the bottle in radioactive equilibrium between 222Rn and 226Ra, by measuring the radon content of the water sample. A few cm 3 nitric acid is added to prevent adsorption of radium ions or atoms at the walls of the bottles. Immediately after filling, a great volume of air or nitrogen is pumped through the water to emanate the sample completely. For testing this method, distilled water is stored in the same way and the radon content is measured 30 days later. Our results show that there is no additional radon source (for instance the walls of the bottles). The detection limit of radium content measurement and the error of the procedure equals that of radon content measurement. The lowest detectable radium content of water is 2× 10 -14 Ci/1, which equals 2× 10-14 g Ra/1. Another method due to Shahin and Koba114) using an ion exchanger, on which 226Ra from a water sample is sorbed and then PVC
~\\\\\\\\\\\\\\\\\\\I H r-,.\\\\\\\\\\\\\\\\\\\l I~:: " / : ! CHARCOAL~':.~"! : !'i.':'. ; " "i .~ RUBBER ~--.,~ ~ ' ~ ' ~ . . . . ~'-- . . . . . . .
111111 :: : ~ - -]~,~
SILVERNET
EXHALATION CHAMBER F : 5.5cm x 20crn
Fig. 3. Exhalation chamber: length 20 cm, width 5.5 crn.
348
a.G.
S C H E I B E L et al.
the radon emanated, reaches the same detection limit, but is more complicated. Therefore, the procedure errors are higher. A chemical method]5), using coprecipitation of 226Ra with barium as the sulphate to concentrate the 226Ra and then emanating with a small gas flow into scintillation chambers directly, has a worse detection limit; as the barium sulphate is not pure enough, background is increased. The experimental effort for this method is comparable.
4. Conclusion The measurement of 222Rn with a scintillation chamber is simple and cheap (compared with ionisation chambers) but has high accuracy. The use of the accumulation method gives a very low detection limit for 222Rn in a carrier gas. This gives the possibility for solving 222Rn problems like 222Rn exhalation of building walls, etc. which have not able to be done till now. Besides the reported applications it is possible to work on other questions of 222Rn and 226Ra measurement. Examples are the determination of 222Rn concentration in breath or other gases. Using the emanation technique it is possible to evaluate the 226Ra content of solutions, or commonly, of all soluble materials. Here must be mentioned the 226Ra content measurement of biological materials such as food or ashed organs.
References z) H. Israel, Die natiirliche Radioactivitiit in Boden, Wasser und Luft; Beitriige zur Physik der Atmosphiire, Bd 30, S. 177 (1958). 2) A. T6th and J. Feher, Effective Ra-226 content of some Hungarian building materials, Report KFKI-78-8 (Jan. 1976). 3) N. Jonassen, Health Phys. 29 (1975) 216. 4) F. Steinh~usler and E. Pohl, The concentration of Rn-222, Rn-220 and their daughters in the air, Proc. 2nd Europ. Congr. Int. Rad. Prot. Ass., Budapest (1973) pp. 397-400. 5) A.C. George, Indoor and outdoor measurements of natural radon and radon daughter decay products in New York City air, in Proc. Natural radiation environment 11, Houston (1972). 6) E.M. Krisiuk, S.J. Tarasov, V.P. Shamov, N.J. Shalak, E.P. Lisachenko and L. P. Gomelsky, A study of radioactivity of building materials, Leningrad Research Institute of Radiation Hygene, Ministry of Public Health of RSFSR, Leningrad (1971). 7) R.D. Evans, Health Phys. 27 (1974) 497. 8) A. Krebs, Fundamenta Radiologica 89 (1939) 129. 9) E. Pohl and I. Pohl-R0hling, Health Phys. 31 (1976) 343. 10) H.F. Lucas, Rev. Sci. Instr. 28, no. 9 (1957) 680. 11) H.G. Scheibel, J. Porstend6ffer and A. Wicke, Radon adsorption in a gas flow by activated charcoal, Rev. Sci. Instr., to be published (1979). 12) j.R. Pearson, D.H. Rimbey and G.E. Jones, J. Appl. Meteorol. 4 : 3 (1965) 349. 13) K. Megumi and T. Mamuro, J. Geophys. Res. 77 no. 7 (I972). ]4) M. Shahin, 1. Kobal et al., Sorption-emanation method for the determination of Ra-226 in water, AED-conf-72-277-002, Bucharest (1972). 15) Radiochemical methodology for drinking water: radium-226 j n drinking water, Report EPA-600:4-75-008, Cincinnati, Ohio (Sept 1975).