Nuclear Instruments and Methods 212 (1983) 393-402 393 North-Holland Publishing Company

R A D O N C O N C E N T R A T I O N A N D E X H A L A T I O N M E A S U R E M E N T W I T H A

S E M I C O N D U C T O R D E T E C T O R A N D A N E L E C T R O S T A T I C P R E C I P I T A T O R

W O R K I N G I N A C L O S E D C I R C U L A T I O N S Y S T E M

M a r c i n W O J C I K a n d L i d i a M O R A W S K A

Institute of Physics, Jagellonian University, PL-30-059 Cracow, Reymonta 4, Poland

Received 16 November 1981 and in revised form 11 October 1982

An apparatus is described and a method presented for the determination of concentrations of radon emanated from solid and liquid samples.

In this method an object under consideration or a sample of air is enclosed in an hermitically sealed chamber. The air contaminated by radon and its daughters is circulated in a closed system a few times through an electrostatic precipitator mounted in the same housing as a semiconductor Si(Li) detector. The concentration of radon is determined by measuring the alpha activity of its daughters.

With the application of an electrostatic precipitator and a silicon detector it is possible to perform alpha spectrometric measurements and thus separate activities of RaA, RaC', and ThC, and to calculate 222Rn of 22°Rn concentrations.

The efficiency of RaA, RaB, RaC, ThB, ThC collection is constant, because the method involves the circulation of the air through the electrostatic precipitator several times.

The sensitivity of the apparatus is high. The minimum detectable radon concentration calculated from an activity measurement of RaA (fast method) is about 0.46 pCi/1 and when measuring the activity of RaC', (slow method) it is 0.028 pCi/l .

1. Introduct ion Nuclide

Man is now exposed to many dangers tha t are nega- tive effects of our civilisation. An increase of radioact ive isotope concent ra t ion in the env i ronment is one of the most impor tan t problems. Many efforts are made to de termine concent ra t ions of radioact ive isotopes and factors inf luencing an increase of radioactivity in the air, in water, soil, food and in bui lding materials.

A man spends about 90% of his life indoors. There- fore, the concent ra t ion of radioactive isotopes in build- ing materials and the resul tant heal th hazards are very impor t an t and complicated problems. Today natural bui lding materials of low activity such as t imber, are frequently replaced by waste materials such as slag. In such materials the concent ra t ion of 226Ra is much higher than in natural bui lding materials. 226Ra is an isotope f rom the 23Su decay series. 226Ra decays into the noble gas 222Rn. Atoms of 222Rn diffuse through pores of a mater ial and emana te into the air. 222Rn decaying through a lpha emission begins a series of its short-l ived daughters. Part of this process takes place in the h u m a n respiratory system, increasing the exposure of the popu- lat ion to the a lpha particles. Due to the presence of u ran ium and its daughters in bui lding materials, the total gamma ray exposure of the h u m a n popula t ion also increases. The decay schemes of 222Rn and 22°Rn into their short-l ived daughters are shown in fig. 1.

0 1 6 7 - 5 0 8 7 / 8 3 / 0 0 0 0 - 0 0 0 0 / $ 0 3 . 0 0 © 1983 Nor th -Hol l and

~Rn ~

a6po2qe

azPb214

83Bi214

s4Po TM

szPb21°

Tit2

3.82d

3.0Stair

26.Sntr

FJ.7min

1.6 d0~.

22y

Historical name

Nuoli~e Tvz HistoricQI nome Nuclide

Tn ssRn z2(: I"

ThA 6~Po 2~

1" T h B 8pb m

Th C a3Bi zlz f L ' ~2oa 1.1~ Th C" Th C' 8~Po zlz

- - ThD ,pb ~z

Fig. 1. 222p.n and 22°p.n decay schemes.

Nuclide Tlr2

asAt m 2s

1"rl 1.32rain

r.2

51.5s

0.158s

10.64h

60.5rnir

3x10-TS

394 M. W6jcik, L. Morawska / Radon concentration

Water used for all domestic purposes may be an additional source of radon, especially when water is collected from wells surrounded by soils rich in 226Ra {1].

All the processes described above also refer to 224 Ra whose increased concentration is often observed in building materials. 224Ra decays into 22°Rn, a noble gas by emitting an alpha particles. Because of its short. half-life (51.5 s) 22°Rn may emanate from the soil or a building material only in small quantities.

There are many methods of measuring radon con- centration and concentrations of radon daughters in the air, depending on the conditions and purposes of the measurement. These methods are reviewed [2 4]. We describe some of them here.

Ref. 4 describes the method of taking gross alpha counts from a filter paper during three measurements, the first of which takes place when sampling is in progress. The minimum detectable activity by this method is 0.05 p C i / l within a 50% standard deviation.

In the method described in ref. 5 radon daughters from the air under investigation are collected on the collecting electrode of an electrostatic precipitator. The electrode rotates around its axis, so that its surface moves through the discharge region where the collection of aerosol takes place and then the activity of the collected aerosol is measured. The efficiency of this apparatus is 0.0154 (the minimum detectable activity is not given).

A very sensitive method is one in which radon gas is extracted from the air in a liquid nitrogen trap, trans- ferred to an alpha scintillation cell, and then the alpha activity is counted [6]. The minimum detectable activity by this method is about 0.01 pCi /1 .

Using a modified double-filter method the minimum detectable activity was 0.00564 pCi/1 (relative standard deviation 50%) [7]. In the double-filter method [8] the air with radon and its daughters flows through a paper filter mounted at the intake of the decay tube. The radon daughters present in the air are collected on this filter. Air flows through the decay tube and radon daughters originating in the tube are collected on the paper filter at its outlet. Alpha activity of the outlet filter is measured and on this basis (knowing the flow rate of the air) the radon concentration in the air is calculated. The modification consists in injecting aero- sol at the periphery of the decay tube. The double-filter method without injecting aerosol is much less sensitive. A minimum detectable activity is of the order of 0.1 pCi/1 within a (35-50)% standard deviation.

In this paper we present a method for measuring all the parameters of a given material with respect to the emanation of the two isotopes of radon and for de- termining the factors that influence the rate of emana- tion.

The material may be investigated as a solid or

powdered sample. The apparatus may also be adopted for measuring radon emanation from liquids and from large surfaces of solid bodies, for example, of a concrete wall or of soil.

Due to these properties the apparatus can also be employed for continuous measurements for seismologic purposes [9].

2. Apparatus design

2.1. Emanation chamber

A schematic view of the apparatus described in this paper is shown in fig. 2.

A hermetic emanation chamber is made of duralu- minium. The volume of the chamber is 39.5 l. This enables portions of material large enough to accumulate appropriate amounts of activity to be placed in the chamber.

The material under investigation (gravel, sand, slag or water) is put into four vessels placed in the chamber one above the other. For very large samples only one vessel is used. When using four vessels circulating air is forced through all of them.

2.2. Electrostatic precipitator and detection system

The electrostatic precipitator is mounted in the same housing as the detection system. The emission electrode is of a cylindrical shape having a diameter of 15 mm, with edges on the circumference (fig. 3). It is supplied with high voltage on the order of 10 kV. The electrode is mounted in a holder made of teflon. The distance between the emission and collecting electrodes can be varied between 6 and 20 mm. The distance is chosen experimentally in order to obtain the highest efficiency of the electrostatic precipitator.

The collecting electrode is made of copper in the form of a disc 25 mm in diameter. After collection is completed the collecting electrode is pushed under a

EMANAT,ON CHAMBER ; ~ ; 0 ~ WITH FOUR VESSELS

6AS FLOER ~ RATEMET

MEMBRANE PUMP

Fig. 2. Schematic diagram of measuring device.

HIGH VOLTAGE

" - ~ CHARGE PREAMP

ELECTROSTATIC PREC1F~TATOR AND Si(Li ) DETECTOR

M. Wbjcik, L. Morawska / Radon concentration 3 9 5

AIR INTAKE

/ ~ E G ~ . ~ HIGH VOLTAGE

~ CHARGE PREAMP.

____J[L____~ s~lL=~ GETECTOR

AIR . OUTLET I

Fig. 3. Schematic diagram of electrostatic precipitator and de- tection system.

Fig. 4. Block diagram of the electronics.

semiconductor Si(Li) detector *. The diameter of the active area of the detector is 25 mm. The distance between the electrode and the detector surface is I mm.

Usually the alpha activity of RaA and RaC' is mea- sured in order to calculate the concentration of radon and its daughters in the air under investigation. Because of good energy resolution alpha particles emitted by RaA, RaC' and Th' can be registered separately.

Energy windows of single channel analyzers are set up for alpha particles emitted by RaA (and ThC), RaC' and ThcC'. Similarly three scalers register alpha par- ticles emitted by these isotopes. A fourth scaler registers pulses generated by the system for dead-time measure- ment. The dead-time of the electronic system is the overall time in which the output gates are closed by electromagnetic disturbances.

3. Measurement

A portion of material under investigation is placed in an emanation chamber. Before each measurement 100 l of radon-free air (aged for about two weeks) is pumped through the chamber in order to clean it of radon and its daughters. The chamber is then hermetically closed and growth of activity emanated by the material begins.

Depending on the material, the measurement starts a few hours or a few days after the chamber is closed. About half an hour before the measurement, an amount of aerosol from a special aerosol generator is introduced into the chamber. During this time radioactive balance between 222Rn and RaA attached to the particles of aerosol is achieved. While measuring RaC' the aerosol is being introduced to the chamber about three hours before a measurement.

After the high voltage that supplies the emission electrode is switched on, the circulation of the air from the chamber in the closed system through the electro- static precipitator begins. The air flow is forced by a membrane pump with a flow rate regulator. When col- lection is completed, the high voltage is switched off and the collecting electrode is pushed under a semicon- ductor detector. At the same moment the measurement of the activity of radon daughters collected on the electrode is begun.

2.3. Electronics Fig. 4 shows a block diagram of the electronics. A

standard spectrometric channel of electronics cooper- ates with the semiconductor detector.

In this apparatus a special RF pick-up unit is used. The RF pick-up unit is an electromagnetic disturbance rejection device and thus is of primary importance in the low-level counting with semiconductor detectors [10]. It is a very sensitive wide-band amplifier with a low noise input. An antenna is connected to the input of the threshold amplitude discriminator. At the output of the RF pick-up unit a logic pulse appears whenever any electromagnetic disturbances are detected. Then the pulse causes the output gate to close.

* The semiconductor detectors were produced in the Depart- ment of Physical Electronics at the Jagellonian University, Cracow.

4. Processes occurring in the chamber and on the collect- ing electrode

4.1. Radon concentration in the chamber

The increase in radon concentration in the chamber starts at the moment it is closed and continues to the moment when radioactive balance in the chamber is achieved. The concentration of radon in the chamber when radioactive balance is attained depends on radon concentration in the pores of the investigated material, the emanation rate and the volume of the chamber.

Radon concentration in a chamber of volume, V, as a function of time is given by eq. (1):

d N R n ( t ) SQ( NRn(t ) ) dt XR,NRn(t ) + ~ - 1 Np ' (1)

396 M. Wrjcik, L. Morawska / Radon concentration

where NRn( t ) is the concentration of radon in the chamber, Np is the concentration of radon in the pores of an investigated material, V is the volume of the chamber, ~kRn is the decay constant of 222Rn, t is the time measured since the moment of closing the cham- ber, S is the surface of the investigated material and Q is the emanation rate at time t = 0. Q is defined as the number of radon atoms emanated from unit surface of the material per unit time. The second term on the right hand side of eq. ( l) expresses the emanation rate's decrease with the increase of radon concentration in the chamber. Increasing radon concentration in the cham- ber counteracts the emanation of radon from the material.

The solution of eq. (1) is given as:

NRn(t ) = QS

Qs X (1 - e q:,R,+ ~ l ) (2)

The measurement of radon concentration may start at any moment after closing the chamber, not neces- sarily after the radioactive balance is achieved. How- ever, if the radon emanation rate from the material under investigation is low and the radon concentration in the chamber is low, the time interval between closing the chamber and the measurement should be longer as might be required to achieve radioactive balance.

4. 2. A ctivity concentration during pumping in the chamber and on the collecting electrode

the chamber: f o r 0 < t < t I - - , v = 0 for t j < t < t 2 v ~ 0 where t 1 is the time since the beginning of pumping, t 2 is the time since the end of pumping and t ' = t - t~'.

In this mathematical model we assume that the col- lection efficiency equals 100%. In practice this parame- ter differs from 100% and the last terms on the right hand side in eqs. (3a-c) and the first terms on the right hand side in eqs. (4a-c) should be multiplied by its value.

The solutions of eqs. (3a-c) are given in the appen- dix (see eqs. (3a'-c')].

These solutions show that the final value of the radon daughter concentration in the air depends mainly on the flow rate and is different for each radon daughter.

While the air circulates through the electrostatic pre- cipitator the concentration of radon daughters on the collecting electrode increases up to the moment when radioactive balance is achieved. The time evolution of this process is given by the system of eqs. (4a-c) [for solutions see the appendix eqs. (4a'-c')].

d NAF ( t ' ) d t ' VNA ( t ' ) + NAY(t') h A ; (4a)

dNBV(t') v N B ( v ' ) - - N ~ ( t ' ) X B + N F ( t ' ) X A : (4b) d t '

d N # ( t ' ) v N c ( r ) - NcV(t') ac + NV( t ' ) aB , (4C)

d t '

NFA(t'), NV(t ') and NcV.(t ') are the numbers of RaA, RaB and RaC atoms on the collecting electrode, NA(t' ), NB(t' ) and Nc(t ' ) are taken from eqs. (3a-c) t ' is defined as in eqs. (3).

During the circulation of the air the radon daughters are collected on the electrode of the electrostatic pre- cipitator. Consequently, the concentration of radon daughters in the air decrease until they achieve a mini- mum value. This process is described by eqs. (3a-c)

dUA( t ' ) UA(t) dt' NR"( t ' )XR"- -NA( t ' )XA +V V , (3a)

d N a ( t - ) NB( t ' ) dt ' N A ( t ' ) X A - - N . ( t ' ) > ' . - - V V , (3b)

d N c ( t ' ) N c ( t ' ) dt' NB(t ' )XB-- N c ( t ' ) X c - - V V ; (3c)

NA(t'), Na(t ' ) , N c ( t ' ) are RaA, RaB and RaC con- centrations in the chamber, NRn(t~) is the radon con- centration in the chamber at the beginning of pumping taken from eq. (2). It is assumed that the radon con- centration in the chamber is constant during the circula- tion of the air. This assumption is true because the pumping time is very short in comparison with the time between closing the chamber and the measurement. ?'A, XB, ?'C are decay constants of RaA, RaB and RaC, v is the gas flow rate, t is the time measured since closing

4. 3. Activity concentration on the collecting electrode after the end of pumping

When the pumping of the air from the chamber through the electrostatic precipitator is stopped, the only process occurring on the collecting electrode is successive radioactive decay. The concentrations of radon daughters on the electrode versus time since the end of pumping are given by the system of eqs. (5a-c) [for solutions see appendix eqs. (5a'-c')].

d NAF(t") (5a) dr"

d NvB(t") (5b) dt"

d N F ( t ' ' ) d t " (5c)

NAF(t") • A ;

NV( t " ) XB + NAF(t") ~.A;

NcV.(t")Xc + NV(t")XB;

* In these calculations there are no separate equations for the RaC' concentration in the chamber or RaC' activity on the collecting electrode. Because of the very short half-life of RaC' (1.6><: 10 -4 s) it is assumed that RaC' activity always equals RaC activity.

M. W6jcik, L. Morawska / Radon concentration 397

t " is the time measured since the end of pumping,

t" = t - 12 .

5 . D e t e r m i n a t i o n o f r a d o n c o n c e n t r a t i o n i n t h e c h a m b e r

The energy spectrum of alpha particles emitted by radon daughters collected on the collecting electrode of an electrostatic precipitator is shown in fig. 5.

Alpha particles emitted by RaA ( E , = 6 MeV) are registered in the first energy window (4.28-6.23 MeV) and particles emitted by RaC'(E~ = 7.68 MeV) are reg- istered in the second energy window (6.5-8.18 MeV).

It should be noted that a certain number of alpha particles emitted by RaC' are registered in the RaA channel. These are particles which fall on the detector surface at a very acute angle, and lose a part of their energy in the dead layer of the detector. This effect is taken into account in the ~ l lowing calculations.

Measurement of alpha activity of RaA and RaC' can usually be started about 7 s after the pumping has ended. Due to the half-life of RaA the time of measure- ment is 10 min if the "fast method" is applied. In this case the number of counts in the first and second channels are N 1 and N2:

N] = 10B, + NRaC, + W['"=lOminuFA(t")XAdt" (6) " t " = 0

It " = I0 m i n

N2= 10B2+ N~(t")Xcdt"-NRac, . (7) 0 2 v t , , _ O

In eqs. (6) and (7) o: is the total efficiency of the apparatus. This parameter is defined as the ratio of a number of registered alpha particles to all alpha par- ticles which would be registered if the air flow rate is infinite, collection efficiency is 100%, registration ef- ficiency is 100% and if there is no deposition on the walls. NV(t2) and NV(t2) are the respective numbers of

RaA and RaC atoms on the electrode at the end of pumping. These values are calculated on the basis of eqs. (5a) and (5c') (see appendix). B 1 and B 2 are the backgrounds in the first channel and in the second channel determined in a separate measurement (B] equals 0.025 cpm and B 2 = 0.004 cpm). NRa c, is the number of particles emitted by RaC' and registered in the first channel during 10 min of the measurement. In order to determine NRa c, it is necessary to know the ratio of the number of alpha particles emitted by RaC' and registered in the second energy window to the number of those alpha particles emitted by RaC' which are registered in the first energy window. For chosen energy windows the value of this ratio, k, is constant.

t ' t " = 10 ra in F tt t/ N~(t ) X c d t --NRa c,

k (8) N R a C ,

In our experiment, k, is equal to 11.8. This value was determined by the registration of alpha particles in the first and in the second energy window during the time interval between 30 and 120 min after the end of pumping. Because the half-life of RaA is short, all the atoms of RaA have decayed, and the only particles registered during this time interval (except the back- ground) in both energy windows are the particles emitted by RaC'. The value NRa C, equals:

N 2 - 10B2 (9) NR~C' k

Substituting this value in eq. (6) and using eq. (5a') one derives the value N~(t2):

NAY(t2 ) _ [ U ] - 1 0 B ] - ( N 2 + lOB2)/k]l.l15 (10) b)

The numerical factor in eq. (10) originates from the solution of eq. (5a') for the time interval 0 -10 min.

The number of RaA atoms on the collecting elec-

20C

150 U3 I--- Z

O 100

50

,.-I,

RoC' E= =7.68

I

RoA E= =5.99 ~'. RoB Ejmo, =l.03MeV - - - -

, , , '; ~. .; '-_-~-. . .~ • :... . . .v~._-.:i.." ! " , '," - . 10 z0 3o 40 so 60 70 no 90 100 110 lz0 60 I~0 is0 1~0 ~0 ~0 ~0 z00

C H A N N E L N U M B E R

=peak is visible offer 5-10h of measurement

C' E==8.78

Fig. 5. The energy spectrum of alpha and beta particles emitted by 222Rn daughters, within RaA, RaC' and ThC' energy channels.

398 M. Wb;;lrik, L. Morawku / Radon concentratum

trode at the end of pumping is proportional to the radon concentration in the chamber [see appendix eq. (4a’)]. For t’ = 7 min

1 -epY”A+;‘=(),~~~

and the eq. (4a’) could be written in the form:

starts three hours after the end of pumping. and is

performed for about thirty hours. In the first energy window alpha particles emitted by ThC (E, = 6.1 MeV) are registered and in the third one, the width of which is (8.22-8.84 MeV). particles emitted by ThC’ (EC? = 8.78 MeV) are registered.

6. Efficiency of the apparatus where t’ is the time measured since the beginning of

pumping. If the determination of radon concentration in the

chamber is based on RaC’ activity measurement (slow method), the measurement is performed for 150 min. By then 96% of all radon daughters collected on the elec- trode have decayed. The count number in the second

channel is:

Nz = 150B, + w’ /

‘,‘== ‘50 m’nNcJ( r”)X,dt” - Nk,,.., ,“S”

(12)

where w’ is the total efficiency of the apparatus for RaC’ measurement. Substituting for N’,,,, (Nk,c. = (N; - 150B2)/k):

+a r”=‘5”“““N~(tff)h~dt”=(N;- 1508,)1.09.

! 1 + ; = I .09

1 (13)

In the case of radioactive balance between RaA and its daughters we can write:

/ r”= Iso m’“NF( t”)X,dt” = 16.25A’,F( rz), (14)

,‘,=O

[see appendix eq. (5c’)] and eq. (13) takes the form:

(N;- 150B,)1.09= 16.25NL(t,)w’ (15)

then

NC(t*) = (N; - 150B,)1.09

16.25~’ (16)

Substituting this value of NI( r2) into eq. (1 l), the radon concentration in the chamber is determined.

0.5 r I

A single measurement allows one to determine (Nan(t) the radon concentration in the chamber. While determining Nan(t) for two different values of t one can obtain values of Nr andQ . S [see eq. (2)].

The time of measurement is short in comparison with the half-life of ThC, and the concentration of **‘Rn in the chamber is about one order of magnitude lower than the concentration of ***Rn. Consequently in our calculations alpha particles emitted by ThC and ThC’ and registered in the RaA and RaC’ channels are not taken into account. The errors resulting from this are negligible.

e I I I 1 I I I I I ,I 10 20 30 LO 50 60 70 60 90 100

FLOW RATE (1 mid J

In order to determine ‘*‘Rn concentration the mea- Fig. 6. The dependznce of the total theoretical and expermen-

surement of alpha activity on the collecting electrode tal efficiency on the flow rate.

The total efficiency of the apparatus depends on the collection efficiency of the electrostatic precipitator, on the registration efficiency and on losses resulting from the deposition of radon daughters on the walls of the chamber, the electrostatic precipitator and in the con- nection pipes.

Parameters influencing the efficiency of the appara- tus are: the value of the electrostatic precipitator cur- rent intensity, the flow rate of the air. the distance between the emission and the collecting electrode, the length of pumping time and the aerosol concentration

in the chamber. The air from the chamber with radon daughters

attached to the aerosol particle is pumped in a closed system through the electrostatic precipitator. A fraction of the total amount of radon daughters is collected on the collecting electrode. The rest returns to the chamber. Some fraction of the total number of aerosol particles with radon daughters attached to them is deposited on the walls of the pipes passing through the system of connection pipes between the electrostatic precipitator and the chamber. This process greatly decreases the efficiency of the apparatus.

Fiow rate is a parameter of great significance as far as the efficiency of the apparatus is concerned. If the flow rate was infinite. all the radon daughters from the chamber would be instantly collected on the collecting electrode. In practice the flow rate is finite: that is why

M. W6jcik, L, Morawska / Radon concentration 399

0.388

0.3

I.L

05

V- ~ o.~

I

flow rate 311-min I

P U M P I N G T I M E ( rn in )

Lu

I .0 .1024

LIJ

o.o5

t ~

Fig. 7. The dependence of the total theoretical and experimen- tal efficiency on pumping time.

0.1

Z

0.08 tJ- u- w 0.06

~0.0~

o.o~

w

for each measurement current intensity was maximum possible (still without spark discharge )

"b 1~ ~o 2'5 D I S T A N C E [ ram)

Fig. 9. The dependence of the total efficiency on the distance between the emission and collecting electrode.

par t of the total n u m b e r of radon daughters always remains in the chamber .

The curves for the total theoretical and experimental efficiency versus flow rate are shown in fig. 6.

The theoretical efficiency was calculated on the basis of eq. (11), assuming that the collection efficiency equals 100%, the registrat ion efficiency 50% (27r geometry), and there are no losses on wall deposit ion.

The experimental efficiency was measured using a s tandard source of radon, the activity of which was 1083 pCi _+ 3% (a s tandard solution of RaCI 2 is in radioactive ba lance with radon).

It can be seen from fig. 6 that the experimental efficiency, as opposed to the theoretical efficiency, de- creases with an increase in air flow rate above 30 1/min. At this flow rate the kinetic energy of the aerosol particles is high enough to enable some of them to pass the electrode wi thout being collected on it. The total exper imental efficiency does not change for flow rates between 15 and 31 1/min, achieving the max imum value of about 0.1.

Fig. 7 shows the dependence of the total theoretical and experimental efficiency on the length of pumping time. The theoretical efficiency is calculated on the basis of eq. (11), for a flow rate 31 1 /min and assuming a 100% collection efficiency, a 50% registrat ion efficiency

0.1 t,u

~- 0.08 U- LU

.~ 0.06

Z

_~ 0.04

o. 0 .02 distance between the etectrodes 6mm

s'o ~6o 1~o 2ha 2'so 3'00 3~o CURRENT INTENSITY [,uA}

Fig. 8. The dependence of the experimental efficiency on the intensity of the electrostatic precipitator current.

(2~ geometry) and no deposit ion on the walls. The shape of the experimental efficiency curve is in

this case similar to the shape of the theoretical curve. After four to five min of pumping, radioactive balance on the collecting electrode is achieved and there is no need to cont inue pumping.

Fig. 8 shows the dependence of the experimental efficiency on the current intensi ty of the electrostatic precipitator. It can be seen that the experimental ef- ficiency increases with increasing current intensity. Due to the spark discharge it is impossible to increase cur- rent intensity above 350/~A.

The experimental dependence of the total efficiency of the appara tus on the distance between the emission and collecting electrode is shown in fig. 9. The effi- ciency increases with a decrease of this distance but it is not possible to decrease it below 6 mm because of the cons t ruct ion of the electrostatic precipitator.

The total efficiency of the appara tus depends on the aerosol concent ra t ion in the chamber. The diffusion coefficient of a free a tom of a radon daughter is much larger than the diffusion coefficient of an aerosol par- ticle with an a tom of a radon daughter at tached to it. As a result of this, free atoms of radon daughters are deposi ted much faster on the walls of the chamber than those at tached to aerosol particles. Losses for wall de-

0.1

--- 0.08

m 0.06

0.04

~: 0.02

without adding aeroso~ to the chamber befor measurement 1,2.3.4 aerosol added to the chamber befor measurement 5.6.7

flow rote 26.61 rain I pumping time ? r a i n

1 2 3 4 5 6

NUMBER OF A MEASUREMENT time inte~a[ between the meosure~ts 30rain

Fig. 10. The dependence of the total experimental efficiency on the aerosol conditions in the chamber.

400 M. W6jcik, L. Morawska / Radon concentration

position of radon daughters are the lowest when the aerosol concentration in the chamber is high enough to enable all radon daughters to be attached to aerosol particles. The aerosol we used in our experiments was cigarette smoke.

A distinct result showing the influence of aerosol introduced into the chamber before the measurement was obtained by the following experiment. A sample of a building material - light cube shaped concrete - was enclosed in the chamber. Some time after closing the chamber, a series of measurements were performed at 30 min intervals. Results are shown in fig. 10. Before measurements 1, 2, 3 and 4 no aerosol was introduced into the chamber. The total experimental efficiency was lower for succeeding measurements because of the ex- haustion of natural aerosol present in the air of the chamber. Before measurements 5, 6, and 7 aerosol was introduced into the chamber. Immediately the total efficiency increased and remained a constant within the limits of error.

The best parameters chosen for the operation of our apparatus are:

Flow rate 26.6 1/min Time of pumping 7 min Electrostatic precipitator

current intensity 250/~A Distance between the electrodes 6 ram.

The total efficiency of the apparatus when RaA is measured is 0.10. The efficiency has the same value for RaG' measurements.

7. Discussion

The apparatus described in this paper and the method of measurement applied make it possible to choose one of several variants of measurement, depending on the nature of the problem. We can determine, within a required sensitivity, the radon concentration in the air under investigation (free air or air enclosed in the chamber), the emanation rate or the influence, for ex- ample, of temperature, pressure, humidity and condi- tions of the surface of the material on these parameters.

The great advantages of the apparatus are: its small size, simplicity and the possibility of repeated measure- ments of 222Rn concentration in the same air sample

with only short intervals between the measurements. During these intervals the radioactive balance between 222Rn and RaA is established. The intervals have to be longer, if the concentration of 222Rn is calculated from RaC' measurements. In such a case the sensitivity of this method is higher.

The application of a semiconductor Si(Li) detector with a large surface makes it possible to record alpha particles emitted by RaA, RaC' and ThC' in three separate energy channels. This is the reason why the background of the apparatus is very small.

The application of an electrostatic precipitator on the collecting electrode of which daughters of radon are collected gives us the possibility to obtain a thin source for spectrometric measurements in which no selfabsorp- tion of alpha particles occurs.

By locating the electrostatic precipitator in the same housing as the detector the time interval between the end of pumping and the beginning of measurement is only a few seconds. This is of importance because of the short half-life of RaA.

The application of a closed system in which the air is circulated makes the efficiency of the electrostatic pre- cipitator independent of the air flow rate fluctuations and of the fluctuations in the collection efficiency of aerosol particles with radon daughters attached to them. Due to the construction of the apparatus the collection efficiency is high and constant. The efficiency of this apparatus is 0.1. The theoretical efficiency cannot be greater than 50%, because the measurement of alpha activity of radon daughters collected on the electrode is performed in the 2~r geometry.

The very low background of the apparatus and high efficiency of the collection and registration of alpha particles allows for an activity concentration as low as 0.46 pCi/1 of 222 Rn to be measured (standard deviation 50%) while applying a "fast method" for the determina- tion of radon concentration from the RaA measure- ment. A "slow method" of determination of radon concentration based on RaC' measurement increases the sensitivity of the apparatus, and the minimum detecta- ble concentration equals 0.028 p C i / l (standard devia- tion 50%).

The authors thank Professor Kazimir Grotowski for his interest in this investigation, and for many helpful discussions.

Appendix

A. 1. Concentrations of radon daughters in the chamber during pumping

It is assumed that at the beginning of pumping (t = t I ) the radioactive balance in the chamber is established:

NA(t,)XA = N B ( t l ) x n = N c , ( t l ) ~ c.=NRn(tl)ykR..

M. W6jcik, L. Morawska / Radon concentration 401

Eqs. (3a ' -c ' ) are the solutions of the eqs. (3a-c), see sect. 4.2.

t "h f 1 + k A - h A NA(t')=NR,(1) Rntk ~ ~ e-k^C] ;

NB(t )=NRn( t l )XR, e kat'+ hA ( l _ e k , t ' ) + kAkB kA(k B - k A )

Nc( t ' )=NRn(t l )hRn 1 kAka k A ( k B _ k A ) +

[ 1 1 ( X A X B X B ( k A - - X A ) ) XAXB + hc k c - k B 1 - k A k a k A ( k B _ k A ) kAkBkc

k A = h A + v / / V k B = X B + V / V k c = h c + v / V

] (e--kAt ' _ ekat')]

,hB(k A - h A )

k A ( k B - - k A ) ( k c - kA)

h B ( k a -- hA)

t ~ = t - - t l .

e--kAt'

e k c " + hahB \ kAkakc J ;

A.2. Activity concentration on the collecting electrode during pumping

It is assumed that the numbers of radon daughters on the electrode at the beginning of pumping (t = t I ) are equal 0:

NAY(t,) = N~( t,) = NcV( t,) = O.

Eqs. (4a ' -c ' ) are the solutions of the eqs. (4a-c), see sect. 4.2.

N ~ ( t ' ) = V N R n ( t l ) h R n ~ ( 1 - - e kAr),

NFB( t') = VNRn( tl )hRn[ B'(e-kat --e-Xat) + B"(e-kAt' --e-'\at') + B '" (1--e-Xat') ] ;

NCF.(t')=VNRn(t,)XRn(Cle kBt '+C2e kAC+C3e-kct'_C4e-XB,'_Cse-Xc,'+C6)"

1 hA k A - - h A B ' =

XB(X B - k B ) - k A k ~ ( h ~ - k B ) kA(k B - k g ) ( h a - k . )

k A - h A 1 1 h A B " -

kA(k B - ka)(h . - kA) k A ( h a - kA)

C N + B'X B Ci2 + B"X a C) hc _ ka C 2 hC _ kA C 3

( B ' + B " + B " ) X B C4= h c _ k c C 5 = 6 1 + C 2 + 6 3 - C 4 + 6 6

l [ h h. Cil k c _ k a 1 kakB k A ( k a _ k A ) ]

h B ( k A - h A ) b a h a C i 2 = Ci3

kA(k B - k A ) ( k c - kA) kAkakc

1 CI4 = X~c - Cn - CI2 - C~3"

B '" ~ + hBkAkB

C14 h c - k c

Ci3 + B '" h B c6 hc

( 3 a ' )

(3b ' )

(3c')

(4a ')

(4b ' )

(4c')

A.3. Activity concentration on the collecting electrode after the end of pumping

Concentrations of radon daughters on the electrode at the end of pumping (t = t 2 ~ t = 0) are equal NAF(t2), NF(t2) and Nff(t2) (taken from eqs. (4'-c ') .

Equations (5a ' -c ' ) are the solutions of the eqs. (5a-c), see sect. 4.2.

NFA ( t") = NAF( t2) e-;~at";

N~(t") = k I e--hAt"+ k 2 e-hat" ;

hBk2 N(F(t,,) hak l e--hAt- + e-hat" + h c - h A

NAF(t2) hA k, NAV(t2)XAxa_ hA k2=NV(t2) h a - hA

(5a ')

(5b ' )

UcF(0) X cXBk'- X A X ch"k2- h . ) e XcC'. (5c')

402 M. W6jcik, L. Morawska / Radon concentration

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