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Seminar Removal of Radon and its Progeny from Indoor Spaces Author: Jure Senegaˇ cnik Supervisor: Dr. Andrej Trkov December 4, 2009 Abstract Radon is radioactive gas. More than half of dose in a life of ordinary human is obtained by radon and its progeny. There are especially high concentrations of radon in closed, not ventilated rooms. Where its concentration is too high, it has to be removed somehow. In this seminar several ways of radon removal are described. One of them had been also tested at IJS.
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Page 1: Removal of Radon and its Progeny from Indoor Spacesmafija.fmf.uni-lj.si/seminar/files/2009_2010/Seminar-radon.pdf · transformation products or progeny (called also daughters) emit

Seminar

Removal of Radon and its Progeny fromIndoor Spaces

Author:Jure Senegacnik

Supervisor:Dr. Andrej Trkov

December 4, 2009

Abstract

Radon is radioactive gas. More than half of dose in a life of ordinary human is obtained by radonand its progeny. There are especially high concentrations of radon in closed, not ventilated rooms.Where its concentration is too high, it has to be removed somehow. In this seminar several ways ofradon removal are described. One of them had been also tested at IJS.

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Contents

1 Introduction 2

2 Working Level 3

3 Radon Subseries 3

4 Computation Model 4

5 The Removal of Radon and its Daughters 55.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55.2 Ventilation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55.3 Theory of the Removal - Calculation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

5.3.1 Filtering Progeny . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55.3.2 Removing Radon and its Progeny by Ventilation . . . . . . . . . . . . . . . . . . . 65.3.3 Particles on the Filter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

5.4 Radon level experiments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75.4.1 The effect of air conditioners on radon level . . . . . . . . . . . . . . . . . . . . . . 7

5.5 Experiments on filters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 95.5.1 Experiments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 95.5.2 Results at the ICJT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 115.5.3 Results at the laboratory . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 125.5.4 Gamma Spectrum of Nuclei on the filter . . . . . . . . . . . . . . . . . . . . . . . . 125.5.5 Concentration of radon . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

6 Conclusions 15

7 Appendix 167.1 Decay Series . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

7.1.1 238U series . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 167.1.2 235U series . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 177.1.3 232Th Series . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

7.2 Filters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

1

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1 Introduction

Radon is a naturally occurring, colourless, odourless, almost chemically inert and radioactive gas. Radonis the heaviest noble gas and has the highest melting point (-71◦C), boiling point (-61.8◦C), criticaltemperature (104◦C) and critical pressure (62 atm) compared to the other noble gases. Although radonis an inert gas, it does form some compounds such as clathrates and complex fluorides. There have beensome unsucessful efforts to form oxides and other halides with radon [source [1], page 1].

Radon is a transformation product from the decay of 238U , 235U and 232Th which are found in variousconcentrations in all soils and minerals. It has 3 isotopes, each originating from a different parent nuclide:

Table 1: Radon isotopes

Isotope half-life series220Rn 55.6 s 232Th219Rn 3.96 s 235U222Rn 3.82 d 238U

Why are half-lives important? All other daughters of U and Th are in solid state and remain in soils.Radon is the only one in gas state, so it can migrate out of the soil. Two of the isotopes, which have veryshort half-lives, are not very important, but 222Rn has a long enough half-life to come to the surface fromdeep beneath and accumulate in our homes, especially in basements. Radon comes into the basementsfrom the soil through the floor. It arises in the soil all the time and diffuses upwards because there is alower concentration of it in the air than in the soil. Diffusion of radon through soil and through wallsis slow, but if it comes into the air, it will quickly spread uniformly in all available space. So, if radondiffuses to the surface indoors, it will spread into some m3, but if it came to the surface outdoors, it wouldbe distributed in much bigger space. Therefore the radon concentration is much higher in closed spacesthan outdoors. So we have a high concentration of radon in the soil, a medium concentration of radon inthe houses (closed spaces built on the ground) and a low concentration of radon in the outside air. Theworst case scenarios (besides an uranium mine) are basements in old farmhouses and wine cellars wherewe have soil or wooden boards on the floor instead of concrete, thick walls and thick ceiling. Radon easilycomes into such rooms, but hardly gets out.

There is also an another source of radon indoors. The uranium in the bricks which the building ismade of. Radon therefore comes into the rooms also from walls.

The Earth is very old and all the decay products in a chain are in secular equilibrium (their activitiesare the same, and do not change with time), so we have a natural perpetual source of radon. Radon isinert, so it is unlikely to get into the body by inhalation or ingestion, therefore the radiotoxicity of radonitself is low. Once radon accumulates in our homes, it undergoes radioactive transformation1. Resultingproducts are not gases anymore. They are in solid state and become attached to dust particles due totheir electrostatic charge. Radon and these particles are then inhaled. Because transformation productsof radon are electrically charged, they readily deposit in the lung and since they have half-lives in theorder of minutes, their transformation energy is almost certain to be deposited in the lung tissue. Radontransformation products or progeny (called also daughters) emit alpha particles (helium nuclei) withenergies ranging from 6 to 7.69 MeV. Because of the high charge and their mass, the energy is deliveredin a huge jolt to the cellular structure of the surface of the bronchi and the lung, which damages andkills these cells. The body can replace damaged cells, but if a damaged cell replicates, then this cellulardefects can lead to lung cancer. First notes about the lung cancer are more than 300 years old. Minersin Germany and Czechoslovakia died of mysterious disease. It is known nowadays that that disease wasthe lung cancer [source [1]]. Nowadays people still die of consequences of being exposed to radon and itsprogeny. The United States Environmental protection Agency (EPA) estimated that 20000 deaths occurannually from radon-induced lung cancer.[source [4]]

An average human gets more than half of his dose in the lifetime from radon. If concentration ofradon were decreased, the doses to humans would be lower. This seminar is focused on the effects offiltering radon’s progeny away. If we have a secular equilibrium among radon and his four daughters(which is established after a couple of hours), we can theoretically reduce the activity of the air by 80 %(best case scenario). The efficiency of a filter can be measured by measuring its activity. The higher itis, the more progeny is on it and we can say, that it is more efficient. This conducted research was ontesting the filters to determine which one has the best efficiency and what kind of filters are best for thisjob.

1Decay series are represented in appendix 7.1.

2

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2 Working Level

The unit used widely to describe the potential alpha energy concentration of radon progeny is the workinglevel (WL). It carries this name, because that is the highest estimated level of radon concentration in theair that it’s still safe to work in such atmosphere 40 hours per week and not suffer biological damage.The concept of WL was first introduced in 1955 by representatives of several uranium mining states. Itwas decided that a standard should be based on a single unit, rather than specification of the individualconcentrations for each progeny. They proposed that a safe level at which to work (a working level)would be a 100 pCi/L (= 3700 Bq/m3) of radon and each daughter. This standard can be also stated as1.3 · 105 MeV/l of potential alpha energy. But how many atoms is that? Well, this can be calculated byequation 1:

N =A

λ= Nτ =

N · T1/2

ln 2(1)

And now we can get the highest estimated safe number densities of atoms. Numbers of atoms per 3700Bq/m3 are represented in table 2:[source [1], pages 20-26]

Table 2: Decay Characteristics Relating to the Determination of the Definition of a Working Level for222Rn Series [source [1], p. 22].

Radionuclide T1/2 n [atoms/l]222Rn 3.823 d 1.76 ·106

218Po 3.05 min 977214Pb 26.8 min 8590214Bi 19.7 min 6310214Po 164 µs 0.0009

3 Radon Subseries

The radioactive transformation of radon and its radioactive products is a practical example of series decay.As shown in figure 1, 222Rn undergoes transformation by alpha emission to produce 218Po, which in turnemits 6.0MeV alpha particles with a half-life of 3.11 min to 214Pb. Beta-particle emissions from 214Pb(t1/2 = 26.8 min) and 214Bi (t1/2 = 19.9 min) produce 214Po, which quickly (t1/2 = 164 µs) produces anend product 210Pb by emission of 7.69 MeV alpha particles. The 210Pb is relatively stable with a half-life22.3 years. Let us put these data in a table for a greater clarity:

Table 3: Radon and its progeny[source [2]Element Symbol Mass Radiation Half-lifeRadon 222

86 Rn 222 α 3.82 dPolonium 218

84 Po 218 α 3.11 minLead 214

82 Pb 214 β−, γ 26.8 minBismuth 214

83 Bi 214 β−, γ 19.9 minPolonium 214

84 Po 214 α 164 µsLead 210

82 Pb 210 β−, γ 22.3 yBismuth 210

83 Bi 210 β− 5.01 dPolonium 210

84 Po 210 α 138.4 dLead 206

82 Pb 206 stable ∞

By β− decay we mean that a neutron in a nucleus is transformed into a proton and an electron. Theelectron is then emitted and called a β particle. α radiation means that a nucleus decays by emittinga 4He nucleus, which is also called an α particle. Note that only the part between 222

86 Rn and 21082 Pb is

important for us. 21082 Pb has too long half-life to decay further in our lungs in large quantities.

3

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Figure 1: Radon subseries - decay series from 222Rn to 210Pb [source [2], p. 206]

4 Computation Model

Let us look now at the equations for decay in table 3. The number of nuclei which decay per unit timeis proportional to their abundance. This is described by the equation below:

dN1

dt= −λ1N1 (2)

where λ1 is the decay constant - the reciprocal value of decay time (time in which the number of nucleiis decreased by factor e (≈ 2.71)). Second nuclides in a series are produced by the decay of the firstnuclides, but they themselves also decay:

dN2

dt= λ1N1 − λ2N2 (3)

and so on:dNidt

= λi−1Ni−1 − λiNi (4)

We have N01 nuclei of element 1 at the beginning and no others. The solution for the first and the n-th

element is:

N1(t) = N01 exp(−λ1t) (5)

Ni(t) = N01

i−1∏l=1

λl

i∑j=1

exp(−λjt)∏ik=1(λk − λj)

(6)

with provision that k runs over all integers from 1 to i, except2 j. If we want activities instead of thenumber of nuclei, we just have to multiply them by their λ. When after some time an equilibrium isreached, the time derivatives are zero from equation 4 it follows that λi−1Ni−1−λiNi = 0, which impliesthat the activities of the daughter and its precursor are the same and do not change with time. This iscalled secular equilibrium.[2]

2the denominator is therefore (λ1 − λj)(λ2 − λj)...(λj−1 − λj)(λj+1 − λj)...(λi−1 − λj)(λi − λj). There is no (λj − λj)term. There are i such fractions. The most inner loop is k, then j and finally i. Index l is not nested.

4

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5 The Removal of Radon and its Daughters

5.1 Introduction

The level of airborne activity of short-lived radon daughter products in indoor air may be controlledby various means, like filtration and ventilation. Ventilation with radon-free air is a very effective way,because non-filtered air in the houses has always higher concentrations of radon than the air outdoors.But ventilation unfortunately makes the heating more expensive, especially in winter. Internal filtration,on the other hand, where the air is circulated through a filtration system, will not affect the radon level,but may reduce the level of the daughter activities. Filtration may, however, also change the aerosolcomposition of the air, and increase the fraction of daughter products in the unattached state. [source[3]] Other ways are also basement pressurisation and basement sealing. [source [4]] There was researchon radon concentration dependence with air conditioning on or off in 1996 in UK. They figured outthat concentration of radon’s progeny is four times higher when air conditioning is off then whilst it ison.[source [5]] (Assuming every air conditioner has an air filter.)

5.2 Ventilation

The most effective means of reducing the indoor concentration of radon and its airborne, short-livedprogeny is ventilation. If the air is replaced e.g. once per hour with radon-free air, radon concentrationin the room can at the most reach 0.75% of the unventilated value and the daughter products even lessthan that. The problem is, that the ventilation is not always practical or acceptable. We should findalternatives for such cases, e.g. filtration. When air is drawn through a filter (mechanical or electrostatic),radon will pass unhindered, because it is an inert gas.

5.3 Theory of the Removal - Calculation

If we use equations 2 to 4 from section 4 and correct boundary conditions, we can figure out, whatactivities and how many particles are we going to have as a function of time. The first importantquestion is: “Which is more efficient: filtering or ventilation”. Numerical calculations were made usingWolfram Mathematica:

5.3.1 Filtering Progeny

Let us look first at filtration. We can only remove radon’s progeny by filtering. Radon cannot be removedthat way. Radon is decaying every second. If we remove its progeny, new will be born. But how fast?The only thing, that affects that, is the half-life of radon’s progeny. Half-life of radon affects only thefinal level of progeny but not the time of reappearance. We can consider radon activity term N1λ1 inequation 3 as a constant. So, the equation is exactly the same as for charging a capacitor:

N(t) =λ1N1γ

λ2[1− exp(−λ2t)] (7)

where number 1 means radon and number 2 means its first daughter. γ is a constant which representsefficiency of the filter. Equation 7 is also applicable for the accumulation of radon’s progeny on filters. Thedifferential equation is the same in both cases (but not the boundary conditions!). We have a constantflow of incoming radioactive particles. The decay rate (activity) of these particles is proportional to theirnumber density(equation 8). When these effects are matched, we get an equilibrium (λN = φ1 ⇒ dN

dt =0).

dN

dt= −λN + φ1 (8)

where φ1 is the rate at which radon is decaying, and τ the average decay time of all progeny. Note thatequations 7 and 8 are only an approximation. We have in fact four daughters, so we need to solve asystem of Bates equations and get a solution like equation 6, which describes what is happening after allprogeny is removed and filtering turned off. Boundary conditions for particles on the filter are differentand so is the solution. Equations 2-4 were solved numerically (they could be solved also analytically, butequations are very long and mistakes can appear) and graphs were plotted, which are depicted in figures(2-6).

We can see that secular equilibrium is soon established again (within two or three hours!). asfasfdsaas asf asf saf as fasf df af afa fadsf dsafsad fsad fas saf asf asdf asf sadf asf asf afdsa fsa fasf asf ads fsafas fs..

3The activity of 214Bi is the same as of 214Po

5

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(a) Daughters (b) Activity

Figure 2: The return of radon’s daughters and their activity3versus time after being removed

5.3.2 Removing Radon and its Progeny by Ventilation

If we remove radon AND its progeny from air by ventilating a room, much more time is needed for radonand progeny to establish the old state again. As said before: the time, in which the equilibrium state

(a) Concentration of daughters (b) Activities are the same for radon and all daughters

Figure 3: The return of radon’s daughters and activity4versus time after ventilating the room. Thenumber of 214Po nuclei is not represented, because it’s much lower than the others.

is reestablished, depends only of the half-life of the nuclide, whose equilibrium is considered. We havehere radon, which has a half life of 3.82 days, which is much longer than that of its progeny, which issome minutes. We are able to see from graphs, that radon is the one, whose number of particles increasesslowly and consequently the number of daughters increases at the same rate. Number of progeny cannotincrease faster than the number of radon atoms.

4The activity of radon and all daughters is almost the same.

6

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(a) After filtering (b) After ventilating

Figure 4: A comparison between filtering an venting. It can be seen, that there is a significant differencewhether we remove only progeny or if we remove also radon. [source [7]]

5.3.3 Particles on the Filter

When air is filtered, dust particles and also radon’s progeny stay on the filter. Filter’s activity is increasinguntil the decay balances loading of radioactive particles on the filter. The decay is an ordinary radioactiveexponential decay and loading is usually with constant speed. The power of filtering machine usually doesnot change with time. Calculations were made to show, how is the filter filled by particles versus time andhow it is then decontaminated versus time (shown in the figure 5). The equation for the accumulation ofthe first daughter is similar to 8, but we have different boundary conditions here compared to those insection 4. Equation 6 is not correct for this problem. It may be possible to derive it analytically, but Irather used a numerical solution.

(a) Daughters (b) Activity

Figure 5: Accumulation of radioactive particles and activity5on the filter and their decay. Filtering isturned on for the first 300 minutes

5.4 Radon level experiments

There had been made a lot of research on removing radon and/or its progeny. One of the experiments isdescribed in folowing lines.

5.4.1 The effect of air conditioners on radon level

This research had been made in a hospital in Northamptonshire, UK. Radon and progeny level had beenmeasured in an operating room, which was pressurised and had an air conditioner which exchanged air

5The activity of 214Bi is the same as that of 214Po

7

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Figure 6: The α and the β activity of the filter which filters 300 minutes and is then shut off

with exterior about 15-20 times per hour. The room was 5 m long, by 2 m wide, by 2.5 m high. Theresults of this research show that radon level was about four times lower when air conditioning was onthen whilst it was off. The progeny level was over five times lower than whilst it was off. These resultsare shown in figures 7 and 8

Figure 7: Radon level in an operating chamber in a hospital

8

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Figure 8: Radon level in an operating chamber in a hospital during the weekend

Note that radon level is higher during nights and weakends when doctors don’t work and thereforethe air conditioning is off. [source [5]]

5.5 Experiments on filters

5.5.1 Experiments

This research has been mostly done on filters to determine, which one is the best and what are thedifferences between them. I tested all the filters in two different rooms shown in figure 9:

(a) The laboratory KF17 (6.8m X 4.8m X 2.8m)

.(b) The ICJT (29m X 17m X 4.2m)

Figure 9: Rooms in which experiments took place

Two most significant differences between the lab and the ICJT are, that the ICJT is a much biggerroom and contains more radon and also higher concentrations of it because its construction properties.The following hardware was used:

1. A CoMo-300 Contamination Monitor

2. A Berthold alpha, beta, gamma ionisation chamber

3. A vacuum cleaner

4. A fan

5. An anemometer

6. A gamma spectrometer

9

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(a) CoMo-300

.(b) Anemometer

Figure 10: Hardware

Figure 11: The experiment

The experiment, shown in figure 11 was the fol-lowing: I removed the tube of the vacuum cleanerand put a net where air is sucked into the vacuumcleaner. I used a scotch tape to affix it. I put afilter on the net at each measurement, so that theair went through the filter. Filters had differentdensity and air permeability. The vacuum cleanerwas then turned on to filter the air for an hour ortwo. I turned it off every couple of minutes andmeasured the activity of the filter. Then I put thefilter back and continued. The air speed was mea-sured by mounting a tube (d = 5cm) with a holeonto the vacuum cleaner. I put an anemometer inthe hole. The tube was mounted by scotch tape,so that the air was not able to blow past the tube.

10

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5.5.2 Results at the ICJT

Table 4: Filters, their final activities and how fast were they achieved at the ICJT

filter αmax[cps] τα [min] βmax[cps] τβ [min] β/α v[ms ] βmax/v7-Kirby 1500 19 9407 66 6,27 18 5222025m 1353 26,6 5914 35,6 4,37 19 3116-DM 418 48,5 5785 36 13,84 18 3211-Micro 1100 41,3 5606 34,8 5,1 13 431VA600-G10 557 62,3 4685 37,6 8,41 13 36011-FACKU 1158 46,9 4111 29,3 3,55 11 37310-NAPA 1173 40 3876 30,2 3,3 17 2286309 946 89,3 3689 42,2 3,9 20 1846329 709 75 3613 40,6 5,1 20 1802040 787 81,1 3210 43,6 4,08 20 1602020b 681 54,9 3081 35,9 4,52 18 1711525 645 61,2 2917 43,5 4,52 20 1454-FACKM 585 48,1 2676 37,2 4,57 20 1338-KKC 423 70,7 2628 61,6 6,21 11,5 2280515 703 55,5 2525 35,2 3,59 21 1209-KKB 381 59 2391 70 6,28 17,5 1362-SUPIS 656 39 2368 23,7 3,61 18 1313 545 50,7 2017 35,9 3,7 5,6 3605-GELMAN 863 51,6 1755 32,7 2,03 3,2 548

(a) Alpha activity (b) Beta activity

Figure 12: Measured results at ICJT: Contamination of filters versus time

11

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5.5.3 Results at the laboratory

Table 5: Filters, their final activities and how fast were they achieved at the laboratoryαamax [cps] τα [min] βmax [cps] τβ [min] β/α v [m/s] β/v

1-MICRO - - 123 20 - 13 9,42-SUPIS - - 56 55 - 18 3,13 - - 54 15 - 5,6 9,64-FACKM - - 87 58 - 20 4,45-GELMAN 49 74 112 27 2,3 3,5 326-DM - - 191 56 - 11 177-KIRBY 84 27 325 22 3,9 18 188-KKC 4,9 20 82 22 16 16 5,19-KKB 17 94 158 57 9,3 20 7,910-NAPA 47 14 325 18 6,9 16 2011-FACKU 81 63 380 34 4,7 11 346349 18 42 130 21 7,2 20 6,56309 22 38 187 25 8,5 20 9,40514 16 58 91 24 5,7 21 4,32020b 22 48 158 38 7,2 18 8,81525 35 20 250 21 7,1 20 132040 22 28 180 21 8,2 20 92025m6 36 28 240 19 6,7 19 13GD10 19 24 240 19 13 13 19

(a) Alpha activity (b) Beta activity

Figure 13: Measured results at ICJT: Contamination of filters versus time

We can see that some filters are much more efficient than others. Results at the ICJT and at the laboratorysuggest approximately the same order of quality of filters (from the best to the worst). There can bealso seen from the results that there was much more radon and progeny at ICJT. Also, measurement ofradon level confirmed this (figure 16). Saturation times of filters are all about the same (the same orderof magnitude) and also in accordance with the theory. See table 7.

5.5.4 Gamma Spectrum of Nuclei on the filter

Filter was put onto the gamma spectrometer after filtering was finished. In the output we found mainly214Pb and 214Bi as predicted. They are the only daughters in the 222Rn series that emit gamma rays.The interesting thing is, that also some photons of daughters of 220Rn from 232Th series were found.Not many, but values were markedly higher than the noise, so we can say for sure that filtering removesalso 220Rn progeny. 212Pb has much longer half-life (11 hours) so the contribution of 220Rn progenyincreases6 with time, because the 222Rn progeny decays much faster. This is not background for sure,because, radiation of elements of the 220Rn series decreases exponentially. If this was background, it

6Note the appearance of line at 583.1 keV in the figure 14b, which is not present in 14a because of the noise and becauseother lines are much higher

12

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would remain the same all the time. There was MUCH more 222Rn daughters than 220Rn daughters, soI am going to describe only results on 214Pb and 214Bi. I can’t tell the exact proportion between 222Rndaughters and 220Rn daughters because there are too many gamma peaks and many of them barelyvisible. The proportion between heights of the highest peaks of each isotope has been measured, buteven the two highest lines of 214Bi were not in the same proportion as expected from data tables, so theresults in figure 15 are not very reliable. There is one or two orders of magnitude more 222Rn daughtersthan 220Rn daughters.

The gamma spectrum of nuclei on the filter does not change much with time. The only difference isthat some lines of the 220Rn daughters become visible after long time. 214Bi has about 100 gamma linesand 214Pb also many. The most significant ones can be seen in figure 14.

(a) Gamma spectrum right after filtering is finished (b) Spectrum of gamma rays accumulated from right after endof filtering until the next day

Figure 14: Gamma spectrum of the filter

Another thing worth mentioning is that the quotient between number of Pb and Bi nuclei is decreasingwith time. This is shown in figure 15b. Bismuth has a shorter half-life than lead, therefore it decaysfaster. But lead transforms into bismuth when it decays. There would be less bismuth than lead sooneror later, if it did not arise from lead. But it does. An equilibrium which follows from solving equation 4is reached in the infinity:

limt→∞

APb(t)ABi(t)

= 1− λPbλBi

= 0, 257 (9)

Graphs in figure 15b were calculated by the highest bismuth peak (609 keV) and the highest lead peak(352 keV) (see figure 14). About 40 % of 214Pb and 214Bi emit gamma rays at these energies. Almostthe same initial activity of Pb and Bi in figure 15 was predicted in figure 5b.

(a) Normalised activity of Pb and Bi (b) Proportion between normalised Pb and Bi activity

Figure 15: Pb and Bi on the filter. Activities were normalised to initial activity of lead. Bismuthactivity line is theoretic (where it should be) and dots are the results of our measurements. Activity wasnormalised because the spectrometer was not calibrated.

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5.5.5 Concentration of radon

The Radon Monitor 1028 of the SNC group was used to measure concentration of radon in air. Resultscan be seen on figure 16:

Figure 16: Concentration of radon in air in two rooms where experiments had been made. Measurementresults are very noisy. [source [7]]

Measurements have confirmed that the concentration of radon at the ICJT was higher than in thelab. Because of noisy measurements, we need to calculate the average. It is shown in the table 6

Table 6: Radon activity per volume in the laboratory and at the ICJT.room Activity/Volume [Bqm3 ]Laboratory 70 ± 50ICJT 330 ± 80

From table 6, we can see that the concentration of radon at the ICJT is about 4 times higher thanthe concentration in the laboratory. But final activities of filters tested at ICJT are an average 25 timeshigher than the ones in the laboratory. This suggests that filter efficiency and velocity are not the onlysignificant variables.

Φv = S · v (10)Φm = S · v · ρ (11)

The γ in equation 7 is a variable representing the efficiency of filtering. It depends on mass flux(equation 11) and radon concentration. But these are not the only important values. The big differenceis likely due to the dust particles in the air. Only radionuclides which are bound to the dust particlescan be removed by filtering.

The time, which is needed to achieve maximum activity of radioactive particles on the filter was alsomeasured and was as predicted the same in both rooms and also in good agreement with the theory:

Table 7: Saturation time of filters - time in which activity approaches max value by a factor of esaturation time α βplace t[min] σt[min] t[min] σt[min]ICJT 54 18 41 12laboratory 41 23 30 15theory7 51.5 0.5 47.9 0.4

7The theoretical saturation time was calculated by fitting the data of the figure 6

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6 Conclusions

• Ventilation removes radon and its progeny. Filtering removes only progeny.

• Removing radon from indoor air by ventilation is much more efficient than filtering. After ventilationit takes much longer to establish the former high level of radon’s progeny.

• Removing progeny by filtering is reasonable when ventilation is too expensive (very cold winter),or when air conditioning or filtering is on anyway etc.

• Filters collect radon progeny during the filtering and become radioactive for a few hours.

• If a filter is used for many years, it becomes radioactive for decades because 21082 Pb has a half-life of

22.3 years.

• Gamma spectra show that filters collect 222Rn progeny and very small quantities of 220Rn progeny.This indicates that Rn from the decay chains of 238U and 234Th are both present.

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7 Appendix

7.1 Decay Series

7.1.1 238U series

Figure 17: 238U series [source [1], p. 7]

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7.1.2 235U series

Figure 18: 235U series [source [1], p. 9]

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7.1.3 232Th Series

Figure 19: 232Th series [source [1], p. 8]

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7.2 Filters

Filters which were tested are shown in the figures 20 and 21:

Figure 20: Filters 1-11 [source [7]]

Figure 21: Other filters [source [7]]

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References

[1] Cothern, Smith: Environmental Radon (Plenum press, 1987)

[2] James E. Martin: Physics for Radiation Protection (Wiley-VCH Verlag GmbH Co. KGaA, 2004)-pages 205-211 and 158-169

[3] Jonassen: Removal of Radon daughters by filtration and electric fields (Radiation Protection Dosime-try, Vol 7 No. 1-4, p.407-411, Nuclear Technology Publishing)

[4] Chih-Shan Li, Hopke: Efficacy of air cleaning systems in controlling indoor radon decay products(Health Physics, Vol 61, No. 6 (December) pp. 785-797, 1991)

[5] Marley, Denman, Phillips: Studies of Radon and Radon Progeny in Air Conditioned Rooms inHospitals (Radiation Protection Dosimetry, Vol. 76, No. 4, pp. 273-276, 1998)

[6] Jarad, Sextro: Reduction of Radon Progeny Concentration in Ordinary Room due to a MixingFan(Radiation Protection Dosimetry, Vol. 24. No. 1/4 pp. 507-511(1988)

[7] Senegacnik, Snoj, Lukek: Removing Radon Progeny by Filtering (IJS delovno porocilo 10272),Ljubljana 2009

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