Measurement of Radon in Indoor Air A Laboratory Exercise Daniel M. Downey and Glenn Slmolunas James Madison University, Harrisonburg. VA 22807 Although undoubtedly present in dwelling atmospheres from time immemorial, radon has recently received a great deal of attention in the ~ u b l i cDress due to Environmental Protection Agency (EPA) r e p o h branding the gas and its radioactive progeny as a leading cause of lung cancer ( I ) . Suggestions that more than 1-2 million U S , homes may exceed the EPA action level of 4 pCi/L (2)' have elicited a demand by many Americans for analyses of this gas in their dwellings. Since radon is inert, quantitative analyses for this element are radiometric, i.e., based on measurement of radiation (3).A simple means of determination, carbon adsorption and gamma ray counting, is described in the experiment that follows because it not only demonstrates an analysis of social consequence that can be performed by students, but also because i t can be used to teach the principles of air sampling, gamma ray spectroscopy, nuclear decay, and radioactive equilibrium. The procedure is derived from an EPA method (4) and makes use of simple, inexpensive materials and equipment. The experiment is designed such that students will he able to collect real samples in their residences or other locations, then return thesamples for measurement during a three-hour laboratory period. Theory Radon is a decay product of naturally occurring uranium and represents one step in the 14-step chain that ultimately produces stable lead. This series, known also as the uranium or '$471 2" decay chain originates fromZ3W(half-life = 4.47 X 109 years), which is widespread in geologic formations. A series of decays leads to [nuclide (half-life)] 226Ra(1.6 X lo3 yr), then to the gas, 222Rn(3.82 dl. Radon decays to 218Po (3.05 min). and 214Bi m i d... the ,~ then ~ to214Ph ~ ~(26.8 . (19.7 , . ~min)~ latter two radionuclides being ray emitters. Several more stens ulrimatelv oroduce stable 2wPb. The comolete decay scdeme may be"fdund in most standard nuclear chemistrv textbooks (5). 1; a closed system that originally contains a pure parent radionuclide, radioactive daughters will "grow" until the This is termed rate of decay matches the rateof radioactive esuilibrium and has been more fully described elsewhere (6); Of consequence in this experiment is that radon emits only alpha particles in its decay, which are difficult to measure. Thus the gamma emitting daughters,
+
~~
~~
~
~~
amm ma
'
A picocurie is an amount of radioactive material corresponding to 2.22 disintegrations per minute. The action level of 4 pCilL was
chosen because this represents the lowest level that is conveniently attalnable by exercising radon removal procedures (2). The nuclear counting electronics are fully described in ref 11. An inexpensive adsorption dish could be made by cutting a 1-lb coffee can down to 50 mm height and using the plastic lld for the cover. "1 radlation sources used In thls experiment are at low levels and present no health hazard when conventional safety practices are followed. No NRC license is needed to possess or use the radlation sources described in this experiment. A balance should be used that can welgh 300 g or more with twodecimal-place accuracy. 1042
Journal of Chemical Education
214Pb and 214Bi,are generally measured to determine the radon concentration when these progeny are in equilibrium with the 222Rnparent. The EPA method (4, 7)involving radon adsorption into charcoal beds employs simple gamma ray spectrometry of the emissions from the decay products noted above. However, there are several limiting factors. Since radon can be desorbed as well as adsorbed. there are sienificant errors when long-term exposures are'used. ~ikewise,results must be corrected for radioactive decav, water adsorotion, and the detector efficiency. However, i i experimentLare carefully controlled, results can be reasonably accurate.
The detector system used for this work was a 3- X 3-in. NaI(T1) detector powered by a Canberra Mdl. 3002 HV power supply with simsl led from the PMT hase into a Harshaw NA-17 amolifier.The e&nlified simal was then led into either a Csnherra series 35 Multi~~channel Analyzer or a Harshaw NC-22 single channel analyzer with differential and integral settings, an NT-27 timer, and an NS-12 scaler. The NaI(T1) detector was shielded with 4 in. of lead. A lowcost counting system can be substituted that consists of a 2. X 2-in. NaI(T1) detector with a Nucleus Mdl. 2010 amplifier-analyzer and Mdl. 500 scaler-timer with a concomitant decrease in precision and sensiti~ity.~ ~~~~~~~~~~~~~~~
~~~~~~~
~
~
~
Meterleis Charcoal adsorption dishes were made by adding 15 g of 4 X 12 mesh charcoal (MCB CX646) to a crystallizing dish (Corning 3140) of 100 mm (4 in.) diameter and 50 mm (2 in.) depth. A iron screen (Fisher 16mesh)was cutandfittedto hold the charcoal bed in place. The dish was placed in a 120 OC oven for 3 h to dry the charcoal. This heating also serves to remove any previously sorbed radon (7).The dish was cooled in a N2-purgeddesiccator and aged for 24 h to allow any radon daughter activity to decay. A top portion for the dish, the cover of a Petri dish, was fitted with a cut piece of foam packing material to hold the iron screen in place when attached. The foam also served to eliminate emntv heads~aceahove the charcoal bed. Cpon removal from the desiccator, the.top and lower portions of the dish were immed~atelyjoined and sealed with vinyl tape.' Radlum/Radon Standard' A srsndsrd rsdon sample was prepared with an EPAXertified 22fRasolution.A 5-mL solution containing 23.8 nCiZzRRa (Environmental Proteaion Agency, Las Vegss, KV. 702.128-2135) was slurried into a charcoal adsorption dish prepared in the same way as dishes used for sample collections,,The standard dish was then placed in a 120 OC oven overnight, cooled, and sealed with tape and epoxy.Thestandard was thenstoredfor 30daya to allowradioactive eouilibrium to be established. It was counted in the rezion of 210 to 720 keV (8s described below, to determine detector efiiciency. The efficiency vslue was the observed counts per minute for the standard dish divided by the true disintegration rare 012.38 X 10' dpm.
Procedure The instructor may choose to have atudenta make their own charcoal adsorption dishes or may have them prepared prior to issuance. After weighing,jthe dishes should be opened in a room suspected of radon contamination (such as the basement of an old dormitory or chemistry building) where there islittle air flow and no likelihoodof tampering. The time of opening should be noted, and the dishes
Table 1. Gamma Ray Energies (Ref 7)
Table 2.
Water Gain Versus Callbratlon Factor tor a Two-Day Exposure Water gain (g)
CF
0 0.500 1.00 1.50 2.00 2.50 3.00 3.50 4.00 4.50 5.00 5.50 6.00 6.50 7.00
0.106 0.103 1.101 0.0991 0.0972 0.0953 0.0935 0.0917 0.0899 0.0882 0.0865 0.0848 0.0832 0.0815 0.0800 n nlRA
7
5n
C T.ECF.DF
200
400
600
800
GBmma rav soectrum 01 radon dauahters of Ra-226 standard. Channel number value^ are senmgs on the 10-1.m voltage potent ometer used to ad "st tne
lover leveldlscr!mlnatw Upper levsldmr~mmtorsenmg r A% 011.11energy window above lower level discriminator net counts per minute, C, are obtained by subtracting the 10-min background from the 10-min gross counts and dividing by 10. The 48-h exposure time corresponds to 2880 min.Phe detector efficiency is obtained from the 2"Ra standard explained above. The calibration factor, CF, is dependent on charcoal particle size, humidity, surface area, and temperature. The mass of water gained is a ready measure of humidity and can he used to judge the radon adsorption rate. The value for CF may then be found by interpolation from the data in Table 2, which were obtained for charcoal adsorption dishes constructed in the prescribed fashion and exposed to standard radon atmosphere of varying humidity for 48 h. The decay factor,DF, compensates for the loss of radon through radioactive decay. The decay equation for radon is given by
Data Reduction The concentration of radon in picocuries per Liter of air, R, is found by (7) =
0 0
CHANNEL NUMBER
should be resealed 48 h later6 The dishes should be aged at least 3 h prior to counting.7 The exposed dishes should be reweighed and the increase in mass due to water adsorption calculated by difference. After learning the operation of the nuclear counting equipment, the students should set up their instruments and adjust amplifiers such that gamma spectra in the approximate range of 0-1000 keV can he measured. This is accomplished by having the students place a L3'Cs check source (The Nucleus, Inc.) near the detector, set the single channel analyzer lower level discriminator (LLD) to6.4 V, set the integral to 0.1 V, and gradually increase the amplifier gain until significant counts above background are registered. The gamma ray spectrum of ' W s then can be obtained by varying the LLD a t 0.1 increments from 7.0 V down to zero. while maintsinine the inteeral at 0.1 V. A plot of LLD aettinl: versus net cmmts gives the '-.Cr/ ".'"Ra gamma spectrum. A similar appruach may then be used to ohtarn the spectra ofC5Znand ",Ba sources. The literature gamma ray energies (8) (see table) of these sources may be plotted versus observed LLD settings to obtain a calibration curve for later use. The spectrum of radon daughters obtained by the above procedure is given in the figure. Three peaks, the 296.2- and 352.0-kcV eamma ram of 214Pband the 609.3-ke\' rav 01' "Ri. are tvnirallv .. . ;sed for the determination. From the nrevi&lv obtained calihratloncurve.earhstudent should select LLD and upper level disrrimi. nstor (ULD) settings that correspond to 271)and 720 keV, reapertively. Ten-minute counts for background and samples are then obtained for this region.
R
-
(1)
where C = net counts per minute, T = exposure time of dish in minutes, E = detector efficiency factor. CF = radon adsorption rate calibration factor rliters per minute), and OF = decay factor. The
where h is the decay constant for 222Rn.The decay constant is the probability of decay per nucleus per unit time and is used to determine the radioactivitv of a s a m ~ l efrom the number of radioactive atoms prexnt. It is given hy the In 2 11 ? ahere t , .is the half-life. The r d is thp decay trme in minutes for the period from the midpoint of the exposure to the start of the counting time. The above procedure was adapted from EPA 52015-87-005. Results and Discussion There a r e several important cases of radioactive equilibriu m demonstrated by this experiment t h a t t h e instructor may choose t o discuss. It is appropriate to have t h e students first develop a c h a r t of t h e complete decay chain from 238Ut o 206Pbusing a handbook or t h e Chart of Nuclides (10).Of particular interest is t h e decay of 226Ra t o 2Z2Rn, since this is used for t h e calibration of t h e adsorption dish. I n a case where t h e parent radionuclide is much longer lived t h a n its daughter, t h e process is described a s secular equilibrium when a steadv rate of ~ r o d u c t i o nand decav of t h e dauehter is establishei. T h e students should be tbld t o prove t h e equilibrium is attained within seven half-lives of t h e daughter (9). T h u s 30 days a r e allowed to pass before using t h e 22Qa standard for countine efficiencv determination of t h e 3.82-day 222Rn daughter. ~ k t h e r m o r et,h e students should
Shorter or longer exposure times will necessitate adlustment of the calibration factor in eq 1. Initial aging permits the establishment of radioactive equilibrium between radon and its daughters. Samples may be counted up to 10 days after closure, but best results are obtained within 25 h. Volume 65
Number 12
December 1988
1043
Drove that the activitv of the dauehter - eauals the activity of the parent in secular equilibrium. In the case of the decav of 222Rnto its d a ~ g h t e r s ~ and ~~pb 214Bi, the equilibrium i i termed transient since the parent decays with a noticeable rate. Since the determinations are made relative to the standard of the same size, counting geometry, matrix, etc., it is not essential for the students to calculate the absolute value of the activity of the daughters. However, i t is of importance to verify that the time required for transient equilibrium to be established, t,, is given by t, = -In-,'D
,',,',
,' -,
where values are the decay constants for daughter (D) and parent (P). Thus at least 3 hare allowed topass after the dish is closed. In addition, the activity of the daughters, AD,may be determined from the parent activity at any time, Ap, by
Although the above procedure was designed to employ the simplest and least expensive counting equipment, the instructor may choose to collect spectra with a Multichannel Pulse Height Analyzer when available. The visual display of eamma peaks, the intearation of peak areas, background stripping, andother feaGres of thistype of instrumentation can -areatlv enhance the students' understanding of the analysis. A significant problem with this determination is the low ratio of signal counts to background counts. Except for the
BThegamma ray is emitted from the 1.1-ps-lived daughter. "Ar,
1044
Journal of Chemical Education
most heavily radon-laden samples, a count rate of a few hundred counts ner minute should be exoected. Thus hackground will haveto be minimized with lead shielding for the best statistical results. Backaround counts principally originate from radioisotopes stored in the vicidity o f t h i d e t i c tors and natural radioactivity. A major natural background source is 40K,which emits 1460 keV gamma rays8 These are discriminated from sample counts in this experiment by setting the ULD at 720 keV, which ensures inclusion of the 609.3 keV 214Bigamma ray. However, Compton counts from the 40Kpeak will still be measured. The LLD was set at 270 keV to discriminate low energy background and electronic noise, but sacrifices the counts from the 241.9 keV 214Pb gamma ray. Nonetheless, background count rates of nearly 100 will be obtained unless the most stringent measures are taken to reduce this interference. Acknowledgment. This work was performed with equipment partially funded by National Science Foundation Grant CSI-8750124 and a James Madison University summer grant. Literature Cited 1. Time 1985,126131.72. 2. Chem. Eng News 1987. IAug. 171.22. 3. Hopke, K. Radon ond its Decoy Products; ACS Symposium Series 331: American Chemical Society: Washington, DC, 1987. 4. EPA 52015-87-W5. June 1987.(See simcohen.B. L.: Cohen, E. S . H d l h Phys. 1983, 45,501.) 5. F,iedisnder G.: KennedyJ.:MaeiaaE.: MiiierJ.Nucleor ondRodioehrmislry,3dad.: Wiley-Intorscience: NPW Ywk. 1981; 1) 8. 6. Downey D. M.; Quarantilio E. P. J . Chsm. Educ. 1984 61.71. 7. George A. C.HedlhPhya. 1986.46.867. 8. Lederer C. M.; Shirley V. S. Table ~(laotopos,7thed.; Wiley-Interscience:New Yark, 1978. 9. Ref 5. pp 19b199. 10. Chart o(Nuriide~,13th ed.; General Eleetrie. 1983. 11. Chappin. G. R.: Rydberg. J. Nuclaar Chamisfry; Pergamon: Eimsford, NY, 1980; Chapter 17. pp 366403.
