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Alpha-emitters in the environment I: Natural sources
Nuclear Instruments and Methods 173 (1980) 0 North-Holland Publishing Company
ALPHA-EMITTERS
197-200
IN THE ENVlROhMENT
I: NATURAL
SOURCES
J.H. FREMLIN and F. ABU JARAD University of Birmingham,
Birmingham B15 2TT, England
The main natural a-activities arise from uranium and thorium. There are 4 X lo9 t of uranium in the ocean with perhaps 10” t in the top kilometer of the earth’s solid crust, with rather more thorium. These activities are not dangerous unless inhaled or ingested and the main risk to humans arises from the inhalation of 222Rn, a decay product of * 3 8U. Radon concentrations have been measured in a number of ordinary rooms and the emission of radium from the walls has been
measured by means of Kodak LR115 plastic detectors. Rate of change of air in each room has also been measured by releasing a small quantity of freon and observing the rate of loss. Figures suggest that in Britain radon may be responsible for around 100 deaths per year from lung cancer.
Although there is some exceedingly weak activity due to four rare-earth nuclides and to low-abundance isotopes of hafnium, osmium, platinum and lead, the overwhelming majority of natural a-emitters are derived from 238U, 232Th and 235U; the first giving rise to seven a-emitting descendants, the second to five and the third to six. All three of the long-lived parent elements are widely distributed. There are some 4 X lo9 t, or a little over lo9 C of uranium in the ocean, derived from the weathering of uraniumbearing rocks. Although thorium is commoner than uranium in the average igneous rock, the amount of thorium in the ocean is relatively trivial since it is slowly precipitated by natural processes. There must, however, be comparable amounts on the ocean bottom or in sedimentary rocks which were originally formed there. For the same reason the oceanic activities of the descendants of the thorium series and of the uranium series including and following the longlived 230Th are relatively low. Thus there are about 1.50 t of 226Ra, or 150 MCi; little more than a tenth of the 238U activity. This is still very large, however, compared to the hundred thousand or so curies of 239Pu derived from bomb tests or the few tens of thousands of curies liberated into the sea by British and French processing plants. It must be remembered also that plutonium is much less readily absorbed by the human digestive system than is radium. According to the ICRF’ only three parts in 100000 of plutonium are absorbed while a quarter of ingested radium is absorbed. The land carries even more activity. The top
In this and in the following paper no technical improvements to the track methods will be presented. Our object is to show how the methods already established can be used to solve some interesting problems in the biological field. It is also desirable to stress the fact that in applying a technique derived from physics in another field of science, the most important thing is to understand thoroughly the problems in that science and to think about the implications of the solution. In this paper we want to discuss the alpha-emitters in the environment that are derived from natural sources. It is put first because the total exposure received by human beings on this planet from the natural a-emitters is far larger than the total exposure to man-made a-emitters. Of course this does not necessarily mean that the former are more important; exposure to the man-made variety could well become very much larger locally as a result of carelessness, accident or misuse. Furthermore, the man-made materials are in principle under control-meaning that we could control their production as well as their contact with people. We do not know whether we shall do so. This is not the case for the natural a-emitters where we may hope to change only their distribution and even that to a very limited extent. As we shall see, even small changes in their distribution may be well worth achieving. Accordingly the man-made activities on which we have far more influence are worth far more discussion. Some of these will be dealt with in the next paper. 197
VII. RADON / THORON MEASUREMENTS
198
J.H. Fremlin, F. Abu Jarad /Alpha-emitters
kilometer or so of the earth’s crust contains an average of two parts per million of uranium and rather more thorium, with huge variations from place to place; some granites may contain 20 ppm of uranium and of course uranium ores may contain up to several thousands of parts per million, while the monazite sands of India contain several percent of thorium over large areas. The earth’s crust, therefore, contains over 1012 t of uranium and the top meter alone of the earth’s crust, through which most of our drinking water has passed at some time in its passage from the atmosphere to our insides, contains around 100 MCi of uranium, together with 100 MCi of the more soluble 226Ra and the volatile 222Rn. The radium may be taken up to a significant extent by plants; brazil nuts in particular have achieved a concentration far higher than that of the ground water in which the trees are growing. Now a-emitters are dangerous to human beings only if inhaled or ingested. It would be of no consequence if their concentrations in the ground or in the sea were a thousand times their present levels. At present one would need to drink 50000 t of seawater to get the maximum permissible body burden of 226Ra. Some people drink a lot of seawater when bathing but are not put off bathing by fear of a-particles. The only serious effect in human beings arises from the gaseous “‘Rrr and 220Rn and their a-emitting descendents from the uranium and thorium series respectively, the former with a half-life of 3.8 d usually being far more important than the latter which has a half-life of only 56 s and which is therefore unlikely to get far from its point of origin. Significant quantities of “‘Rn are liberated into the atmosphere from uranium ores close to the surface. The average level of this in open country far from major bodies of ore is 0.05 pCi/l or less. This may be responsible for a few deaths per year from lung cancer in the whole world but that is not very many and there is absolutely nothing we can do about it. The decay of this radon and the precipitation of its decay products by rain must lead to the deposition of 2 ‘aPb and 210Po to an average level of 0.1 Ci/km’. This might be worth investigation, but I do not think that we need to worry about it very much. In a city the situation is quite different. Every million tons of coal burnt will distribute a few hundred millicuries of uranium, and of each of its prod-
in the environment
I
ucts, into the environment though for the open air this is still small compared to the usual level indoors. Every building made of stone, brick or even mud will liberate significant amounts of radon into the air inside. This last source is genuinely important and we can discuss the biological effect quantitatively if we know how much radon and its products are inhaled. One 6 MeV o-particle delivers an energy of lo-l2 J. Since 1 Rad is 10e2 J/kg, 10” a-particles will give a dose of 1 Rad to 1 kg of living tissue. Taking the value of 20 as recommended by the ICRP for the quality factor, 5 X lo* a-particles will deliver a dose of 1 Rem to 1 kg, or 3.5 X 10” o-particles can deliver a whole-body dose of 1 Rem to a 70 kg standard man. A whole-body dose of 1 Rem can be expected to add a risk of one in 10000 of producing a cancer in the later life of the individual; this is equivalent toL the risk of smoking 150 cigarettes. Very little work has been done on this, although pioneer preliminary measurements were made by Collinson and Haque (1965) (Haque, incidentally, came from Bangladesh) and some further studies have been made since 1972. A particularly thorough study has been done by Cliff (1977) over the last four years. He has found that the average dose to the cells lining the lungs and bronchi indoors is in the region of f-2 Rem/y. Some work has also been done in California but we have no details of this. We have now started work in Birmingham and we want to describe some experiments being done which should usefully extend the previous investigations elsewhere. Here at last the relevance to a Conference on SSNTDs of our paper will appear. Our research has been made possible only by the extreme sensitivity, simplicity and cheapness of the track technique. Cellulose nitrate (Kodak type LR-115) has been used as a detector. This will register a-particles with energies between 0.5 and 4.5 MeV when etched in 6.25 M sodium hydroxide solution at 60°C for two hours, which gives us the best combination of sensitivity and background. At first we tried the peelable material with which we could use a spark counter. The only material of this type then available was coated with lithium fluoride for neutron detection, which gave us a very high background and inadequately repeatable results. We have, therefore, used the unpeelable type and counted tracks visually on a screen projection to avoid the need for a microscope. Counting is not a serious problem as there is little point in aiming at an accuracy better than 10%. If
J.H. Fremlin, F. Abu Jarad /Alpha-emitters
one is prepared to count 10000 tracks it is far more valuable to count 100 tracks on each of the samples derived from 100 houses between which variation far greater than 10% occurs, than to count 10000 tracks to get a 1% value for a single house. This of course is typical of applied physics, in which one should always consider not just the obtaining of maximum accuracy in each individual measurement but should try to optimise the gain of information by a careful balance between number of experiments performed and the accuracy of each experiment. We are using two different methods for radon measurements. The first employs an air pump to suck a known volume of air through a paper filter 25 mm in diameter. We use Whatman GF/A, of thickness 0.25 mm and particle retention to 0.6 pm, the air filtration efficiency above this size being 99.999%. Parallel to and directly in front of this filter paper, at a distance of about 2 mm, is placed a 25 mm disc of LR-115, where it will be struck by almost all of the o-particles leaving the nearer face of the filter. The detection efficiency is increased by interposing an aluminium foil 12 or 24 pm in thickness in front of the detector to reduce the energies of the a-particles to the registrable range. This is necessary as the chief activity collected on the filter paper is not radon itself but one of its daughters, an isotope of polonium, bismuth or lead. Any of these would be quickly adsorbed on to dust particles and hence stopped by the filter. Only two of these are likely to be observed: *r*Po (3.05 min; 6.00 MeV) and *r4Po (164 ~_ls;7.7 MeV). The *14Pu decay follows the two successive unobservable fl decays of ‘14Pb and *14Bi with halflives of 27 and 20 mm respectively. Neither of these will give etchable tracks unless some of their energy is removed before striking the plastic. We had not originally thought that this would be necessary as we expected the filter to act as a thick source, In one of the earlier experiments, however, it was noticed that a larger o-count was registered from the down-stream face of the filter showing that most of the activity had been trapped in a layer of the filter thin compared with the a-particle range. This was a useful observation, not only enabling us to improve our efficiency by introducing the aluminium absorber but showing the high efficiency of even a thin layer of the filter. Preliminary work in the radiochemistry basement of our own Department of Physics gave us a quite embarrassing number of tracks in a quarter of an hour’s operation and by comparing samples from
in the environment I
199
several points we located a place where a radium compound had been spilt nearly twenty years before, although we had believed that it had been thoroughly cleaned up at the time. This also gave us a source strong enough to count a surface-barrier detector which gave us a good energy spectrum. This showed us as expected the cy decays of 21BP~ and 214Po together with a smaller peak from “*PO evidently derived from **‘Rn in the thorium decay series. The very sharp peaks corresponding to the full cr energy with quite small tails to an energy perhaps i MeV lower demonstrated very clearly the small penetration into the filter paper of the active particles. This preliminary work was folIowe’d up by a series of measurements in various houses belonging to co-operative members of our academic and technical staff. We are exceedingly grateful to these as we must have been a considerable nuisance. The actual working period during which known volumes of air were sucked through the filter was only about 45 min, but before this could be done the room involved had to be kept closed for three hours in order that the *“Rn and its decay products should reach an equilibrium concentration, and afterwards we had to measure the rate of ventilation in these equilibrium conditions. This was achieved by liberating a small known quantity of freon and getting it well mixed with the air of the room with the help of a fan. At intervals during the following hour or so samples of air were collected for analysis by gas chromatography, from the result of which the number of air changes per hour could be determined. In parallel with this measurement we obtained a much larger quantity of data by a different technique. In this samples of LR-115 were placed in a small impervious vessel sealed to typical parts of the walls. Half Coca-Cola tins stuck on with Blue Tack were used for this. In each can one sample of LR-115 would be near the wall and would thus be affected both by a-particles from the emergent radon and its descendants and from uranium and other non-volatile emitters still in the upper layers of the wall. A second sample was hidden by a diaphragm from the wall but was freely reached by the emergent radon. This would reach an equilibrium concentration after a week or so and hence, knowing the geometry of the system, something approaching an absolute measure of the emergent radon could be obtained. If the diaphragm was placed close to the wall, the inner detector gave us the direct Q emission from the wall itself while the outer one gave us only the decay of VII. RADON / THORON MEASUREMENTS
200
J.H. Fremlin, F. Abu Jarad /Alpha-emitters
radon in the outer part of the tin plus the decay of “sPo and 214Po deposited on the inner walls of the tin. This method is also being applied to a number of building materials; bricks, stone, etc. and finally is being used to test the efficiency of various types of paint or other covering that might be expected to stop or reduce the emission of radon. We found that for a series of ordinary red clay house bricks the ratio between outer and inner detectors was similar, showing a similar degree of porosity and hence of escape probability for the radon. The behaviour of bricks made from crushed stone was however quite unexpectedly different. Here the ratio of outer to inner detector count was several times higher, showing a much greater radon escape. An extremely simple test consisting of sucking at the corner of each brick by mouth showed that the crushed stone bricks were indeed far more porous than the clay ones. This investigation may soon be quite important in Britain since we are being presented with proposals for a major programme of thermal insulation of houses to save energy. Good thermal insulation necessarily means cutting down draughts and ventilation. Even in rooms with no chimney this may mean an increase in radon concentration by a factor of two, and in rooms which used to use an open fire, either coal or gas, could mean a factor of nearly ten. If we take the present average annual lung dose in Britain from this source as already in the region of a Rem, equivalent to perhaps 15 mRem to the whole body,
in the environment
1
and hence averaged over our 56 million population, radon may already be responsible for something approaching a hundred lung-cancer deaths each year. A successful campaign for better thermal insulation of houses might increase this by a factor of two or more. Although warmer houses may well save far more than 200 lives a year in a damp and chilly country such as Britain, we cannot disregard the probable extra 100 lives which might be lost from lung cancer as a result of decreased ventilation. A final and important point is that this study could give us an experimental numerical limit to the effect of small doses to large populations. Lung cancer is not very common among non-smokers and if we could determine the approximate population dose of a-particles to the lungs of the British city population, together with the statistics of lung cancer, especially among housewives who spend most of their time in the same environment, we could at least give an upper limit to the effect of doses of the order of 1 Rem a year to the lung alone. This could then give us more confidence in the big extrapolation from the effects of much higher doses to the whole body that are the present main sources of data from which the ICRP derives its figures. Atomic scientists come in for a lot of criticism these days. We hope that our application of the new track techniques to the hitherto neglected radiation from the environment will lead to the eventual saving of a great many more lives than the civilian nuclear industry is ever likely to destroy.
ALPHA-EMITTERS
197-200
IN THE ENVlROhMENT
I: NATURAL
SOURCES
J.H. FREMLIN and F. ABU JARAD University of Birmingham,
Birmingham B15 2TT, England
The main natural a-activities arise from uranium and thorium. There are 4 X lo9 t of uranium in the ocean with perhaps 10” t in the top kilometer of the earth’s solid crust, with rather more thorium. These activities are not dangerous unless inhaled or ingested and the main risk to humans arises from the inhalation of 222Rn, a decay product of * 3 8U. Radon concentrations have been measured in a number of ordinary rooms and the emission of radium from the walls has been
measured by means of Kodak LR115 plastic detectors. Rate of change of air in each room has also been measured by releasing a small quantity of freon and observing the rate of loss. Figures suggest that in Britain radon may be responsible for around 100 deaths per year from lung cancer.
Although there is some exceedingly weak activity due to four rare-earth nuclides and to low-abundance isotopes of hafnium, osmium, platinum and lead, the overwhelming majority of natural a-emitters are derived from 238U, 232Th and 235U; the first giving rise to seven a-emitting descendants, the second to five and the third to six. All three of the long-lived parent elements are widely distributed. There are some 4 X lo9 t, or a little over lo9 C of uranium in the ocean, derived from the weathering of uraniumbearing rocks. Although thorium is commoner than uranium in the average igneous rock, the amount of thorium in the ocean is relatively trivial since it is slowly precipitated by natural processes. There must, however, be comparable amounts on the ocean bottom or in sedimentary rocks which were originally formed there. For the same reason the oceanic activities of the descendants of the thorium series and of the uranium series including and following the longlived 230Th are relatively low. Thus there are about 1.50 t of 226Ra, or 150 MCi; little more than a tenth of the 238U activity. This is still very large, however, compared to the hundred thousand or so curies of 239Pu derived from bomb tests or the few tens of thousands of curies liberated into the sea by British and French processing plants. It must be remembered also that plutonium is much less readily absorbed by the human digestive system than is radium. According to the ICRF’ only three parts in 100000 of plutonium are absorbed while a quarter of ingested radium is absorbed. The land carries even more activity. The top
In this and in the following paper no technical improvements to the track methods will be presented. Our object is to show how the methods already established can be used to solve some interesting problems in the biological field. It is also desirable to stress the fact that in applying a technique derived from physics in another field of science, the most important thing is to understand thoroughly the problems in that science and to think about the implications of the solution. In this paper we want to discuss the alpha-emitters in the environment that are derived from natural sources. It is put first because the total exposure received by human beings on this planet from the natural a-emitters is far larger than the total exposure to man-made a-emitters. Of course this does not necessarily mean that the former are more important; exposure to the man-made variety could well become very much larger locally as a result of carelessness, accident or misuse. Furthermore, the man-made materials are in principle under control-meaning that we could control their production as well as their contact with people. We do not know whether we shall do so. This is not the case for the natural a-emitters where we may hope to change only their distribution and even that to a very limited extent. As we shall see, even small changes in their distribution may be well worth achieving. Accordingly the man-made activities on which we have far more influence are worth far more discussion. Some of these will be dealt with in the next paper. 197
VII. RADON / THORON MEASUREMENTS
198
J.H. Fremlin, F. Abu Jarad /Alpha-emitters
kilometer or so of the earth’s crust contains an average of two parts per million of uranium and rather more thorium, with huge variations from place to place; some granites may contain 20 ppm of uranium and of course uranium ores may contain up to several thousands of parts per million, while the monazite sands of India contain several percent of thorium over large areas. The earth’s crust, therefore, contains over 1012 t of uranium and the top meter alone of the earth’s crust, through which most of our drinking water has passed at some time in its passage from the atmosphere to our insides, contains around 100 MCi of uranium, together with 100 MCi of the more soluble 226Ra and the volatile 222Rn. The radium may be taken up to a significant extent by plants; brazil nuts in particular have achieved a concentration far higher than that of the ground water in which the trees are growing. Now a-emitters are dangerous to human beings only if inhaled or ingested. It would be of no consequence if their concentrations in the ground or in the sea were a thousand times their present levels. At present one would need to drink 50000 t of seawater to get the maximum permissible body burden of 226Ra. Some people drink a lot of seawater when bathing but are not put off bathing by fear of a-particles. The only serious effect in human beings arises from the gaseous “‘Rrr and 220Rn and their a-emitting descendents from the uranium and thorium series respectively, the former with a half-life of 3.8 d usually being far more important than the latter which has a half-life of only 56 s and which is therefore unlikely to get far from its point of origin. Significant quantities of “‘Rn are liberated into the atmosphere from uranium ores close to the surface. The average level of this in open country far from major bodies of ore is 0.05 pCi/l or less. This may be responsible for a few deaths per year from lung cancer in the whole world but that is not very many and there is absolutely nothing we can do about it. The decay of this radon and the precipitation of its decay products by rain must lead to the deposition of 2 ‘aPb and 210Po to an average level of 0.1 Ci/km’. This might be worth investigation, but I do not think that we need to worry about it very much. In a city the situation is quite different. Every million tons of coal burnt will distribute a few hundred millicuries of uranium, and of each of its prod-
in the environment
I
ucts, into the environment though for the open air this is still small compared to the usual level indoors. Every building made of stone, brick or even mud will liberate significant amounts of radon into the air inside. This last source is genuinely important and we can discuss the biological effect quantitatively if we know how much radon and its products are inhaled. One 6 MeV o-particle delivers an energy of lo-l2 J. Since 1 Rad is 10e2 J/kg, 10” a-particles will give a dose of 1 Rad to 1 kg of living tissue. Taking the value of 20 as recommended by the ICRP for the quality factor, 5 X lo* a-particles will deliver a dose of 1 Rem to 1 kg, or 3.5 X 10” o-particles can deliver a whole-body dose of 1 Rem to a 70 kg standard man. A whole-body dose of 1 Rem can be expected to add a risk of one in 10000 of producing a cancer in the later life of the individual; this is equivalent toL the risk of smoking 150 cigarettes. Very little work has been done on this, although pioneer preliminary measurements were made by Collinson and Haque (1965) (Haque, incidentally, came from Bangladesh) and some further studies have been made since 1972. A particularly thorough study has been done by Cliff (1977) over the last four years. He has found that the average dose to the cells lining the lungs and bronchi indoors is in the region of f-2 Rem/y. Some work has also been done in California but we have no details of this. We have now started work in Birmingham and we want to describe some experiments being done which should usefully extend the previous investigations elsewhere. Here at last the relevance to a Conference on SSNTDs of our paper will appear. Our research has been made possible only by the extreme sensitivity, simplicity and cheapness of the track technique. Cellulose nitrate (Kodak type LR-115) has been used as a detector. This will register a-particles with energies between 0.5 and 4.5 MeV when etched in 6.25 M sodium hydroxide solution at 60°C for two hours, which gives us the best combination of sensitivity and background. At first we tried the peelable material with which we could use a spark counter. The only material of this type then available was coated with lithium fluoride for neutron detection, which gave us a very high background and inadequately repeatable results. We have, therefore, used the unpeelable type and counted tracks visually on a screen projection to avoid the need for a microscope. Counting is not a serious problem as there is little point in aiming at an accuracy better than 10%. If
J.H. Fremlin, F. Abu Jarad /Alpha-emitters
one is prepared to count 10000 tracks it is far more valuable to count 100 tracks on each of the samples derived from 100 houses between which variation far greater than 10% occurs, than to count 10000 tracks to get a 1% value for a single house. This of course is typical of applied physics, in which one should always consider not just the obtaining of maximum accuracy in each individual measurement but should try to optimise the gain of information by a careful balance between number of experiments performed and the accuracy of each experiment. We are using two different methods for radon measurements. The first employs an air pump to suck a known volume of air through a paper filter 25 mm in diameter. We use Whatman GF/A, of thickness 0.25 mm and particle retention to 0.6 pm, the air filtration efficiency above this size being 99.999%. Parallel to and directly in front of this filter paper, at a distance of about 2 mm, is placed a 25 mm disc of LR-115, where it will be struck by almost all of the o-particles leaving the nearer face of the filter. The detection efficiency is increased by interposing an aluminium foil 12 or 24 pm in thickness in front of the detector to reduce the energies of the a-particles to the registrable range. This is necessary as the chief activity collected on the filter paper is not radon itself but one of its daughters, an isotope of polonium, bismuth or lead. Any of these would be quickly adsorbed on to dust particles and hence stopped by the filter. Only two of these are likely to be observed: *r*Po (3.05 min; 6.00 MeV) and *r4Po (164 ~_ls;7.7 MeV). The *14Pu decay follows the two successive unobservable fl decays of ‘14Pb and *14Bi with halflives of 27 and 20 mm respectively. Neither of these will give etchable tracks unless some of their energy is removed before striking the plastic. We had not originally thought that this would be necessary as we expected the filter to act as a thick source, In one of the earlier experiments, however, it was noticed that a larger o-count was registered from the down-stream face of the filter showing that most of the activity had been trapped in a layer of the filter thin compared with the a-particle range. This was a useful observation, not only enabling us to improve our efficiency by introducing the aluminium absorber but showing the high efficiency of even a thin layer of the filter. Preliminary work in the radiochemistry basement of our own Department of Physics gave us a quite embarrassing number of tracks in a quarter of an hour’s operation and by comparing samples from
in the environment I
199
several points we located a place where a radium compound had been spilt nearly twenty years before, although we had believed that it had been thoroughly cleaned up at the time. This also gave us a source strong enough to count a surface-barrier detector which gave us a good energy spectrum. This showed us as expected the cy decays of 21BP~ and 214Po together with a smaller peak from “*PO evidently derived from **‘Rn in the thorium decay series. The very sharp peaks corresponding to the full cr energy with quite small tails to an energy perhaps i MeV lower demonstrated very clearly the small penetration into the filter paper of the active particles. This preliminary work was folIowe’d up by a series of measurements in various houses belonging to co-operative members of our academic and technical staff. We are exceedingly grateful to these as we must have been a considerable nuisance. The actual working period during which known volumes of air were sucked through the filter was only about 45 min, but before this could be done the room involved had to be kept closed for three hours in order that the *“Rn and its decay products should reach an equilibrium concentration, and afterwards we had to measure the rate of ventilation in these equilibrium conditions. This was achieved by liberating a small known quantity of freon and getting it well mixed with the air of the room with the help of a fan. At intervals during the following hour or so samples of air were collected for analysis by gas chromatography, from the result of which the number of air changes per hour could be determined. In parallel with this measurement we obtained a much larger quantity of data by a different technique. In this samples of LR-115 were placed in a small impervious vessel sealed to typical parts of the walls. Half Coca-Cola tins stuck on with Blue Tack were used for this. In each can one sample of LR-115 would be near the wall and would thus be affected both by a-particles from the emergent radon and its descendants and from uranium and other non-volatile emitters still in the upper layers of the wall. A second sample was hidden by a diaphragm from the wall but was freely reached by the emergent radon. This would reach an equilibrium concentration after a week or so and hence, knowing the geometry of the system, something approaching an absolute measure of the emergent radon could be obtained. If the diaphragm was placed close to the wall, the inner detector gave us the direct Q emission from the wall itself while the outer one gave us only the decay of VII. RADON / THORON MEASUREMENTS
200
J.H. Fremlin, F. Abu Jarad /Alpha-emitters
radon in the outer part of the tin plus the decay of “sPo and 214Po deposited on the inner walls of the tin. This method is also being applied to a number of building materials; bricks, stone, etc. and finally is being used to test the efficiency of various types of paint or other covering that might be expected to stop or reduce the emission of radon. We found that for a series of ordinary red clay house bricks the ratio between outer and inner detectors was similar, showing a similar degree of porosity and hence of escape probability for the radon. The behaviour of bricks made from crushed stone was however quite unexpectedly different. Here the ratio of outer to inner detector count was several times higher, showing a much greater radon escape. An extremely simple test consisting of sucking at the corner of each brick by mouth showed that the crushed stone bricks were indeed far more porous than the clay ones. This investigation may soon be quite important in Britain since we are being presented with proposals for a major programme of thermal insulation of houses to save energy. Good thermal insulation necessarily means cutting down draughts and ventilation. Even in rooms with no chimney this may mean an increase in radon concentration by a factor of two, and in rooms which used to use an open fire, either coal or gas, could mean a factor of nearly ten. If we take the present average annual lung dose in Britain from this source as already in the region of a Rem, equivalent to perhaps 15 mRem to the whole body,
in the environment
1
and hence averaged over our 56 million population, radon may already be responsible for something approaching a hundred lung-cancer deaths each year. A successful campaign for better thermal insulation of houses might increase this by a factor of two or more. Although warmer houses may well save far more than 200 lives a year in a damp and chilly country such as Britain, we cannot disregard the probable extra 100 lives which might be lost from lung cancer as a result of decreased ventilation. A final and important point is that this study could give us an experimental numerical limit to the effect of small doses to large populations. Lung cancer is not very common among non-smokers and if we could determine the approximate population dose of a-particles to the lungs of the British city population, together with the statistics of lung cancer, especially among housewives who spend most of their time in the same environment, we could at least give an upper limit to the effect of doses of the order of 1 Rem a year to the lung alone. This could then give us more confidence in the big extrapolation from the effects of much higher doses to the whole body that are the present main sources of data from which the ICRP derives its figures. Atomic scientists come in for a lot of criticism these days. We hope that our application of the new track techniques to the hitherto neglected radiation from the environment will lead to the eventual saving of a great many more lives than the civilian nuclear industry is ever likely to destroy.