Collection of Radon Daughters on Filter Media Anthony Busigin, Antoon W. van der Vooren, and Colin R. Phillips* Department of Chemical Engineering and Applied Chemistry, University of Toronto, Toronto, Ontario M5S 1A4, Canada Filters commonly used in the collection of radon daughters are compared in terms of surface collection and overall collection efficiency. a spectroscopy is used to determine the magnitude of penetration of unattached and attached radon daughters to determine the surface collection efficiency of 20 different filters. Collection efficiency was found to be effectively 100%in all filters. However, surface collection varies widely, with implications for recoil loss and for measurement of a spectra. Radon-222, which is the immediate decay product of radium-226, is the heaviest member of the noble gas family and the only member of the uranium decay series which is a gas a t ordinary temperatures. Being a gas, radon readily diffuses through rock and soil and is found in radiologically significant concentrations in underground mines, caves, and even some house basements. Radon has a half-life of 3.82 days and decays to the isotopes of polonium, lead, and bismuth. These so-called radon daughters are born as single atoms that attach readily to surfaces, including aerosol particles, as a result of which they may be deposited in the lung upon inhalation. In an atmosphere of radon gas, the main radiation dose arises from the inhalation and deposition of radon daughters. Air sampling for radon daughters involves the collection of particulate airborne radioactivity on a filter. In order to determine air concentrations, the collection mechanisms of radon daughter aerosols on filter media must be known. Two factors must be considered: first, the collection efficiency of a filter (the ratio of the radioactivity that remains on or in a filter to the total radioactivity incident upon it), and, second, the extent of penetration of the radioactive aerosol into the filter. The importance of determining both the collection efficiency of the filter and the self-absorption of a particles within the filter has long been recognized in the measurement of airborne radon daughters. As early as 1952,Alercio and Harley ( I ) estimated the absorption of a particles by filter paper to be about 30%for uranium mine dust. Tsivoglou ( 2 )found that about 60% of airborne daughters could not be detected due to the combined effects of collection efficiency and penetration into the filter medium. Lindeken ( 3 ) was the first to conduct a detailed study on the relative effects of filter collection efficiency and self-absorption. He compared a number of filters with a Millipore AA (0.8-pm pore size) filter and assumed that the Millipore filter exhibited ideal behavior (100%collection efficiency and all particles were retained on the surface). In his test, natural airborne radon daughters were sampled with a test filter backed by a Millipore AA filter. A t the same time, the same atmosphere was sampled with a ref0013-936X/80/0914-0533$01.00/0
erence Millipore AA filter. After sampling, the filters were counted simultaneously using scintillation counters, and surface collection, penetration, and burial losses calculated according to: % of activity collected on test filter surface
-
counts on test paper X 100 counts on Millipore AA reference
% of penetration through test filter
- counts on Millipore backup X 100 counts on Millipore reference
9% burial losses within filter = 100 - [% surface collection
+ % penetration]
Table I describes Lindeken’s findings. Obviously, in all but one case, penetration was small, of the order of 2%, but burial losses were often substantial. This raises a number of significant questions. Scintillation counters, although ideally energy insensitive, frequently have high thresholds and fail to detect a particles which have had their energy significantly degraded by passage through first a filter and then an air gap. Investigation of filter performance can best be performed using an energy discriminatory device such as an a spectrometer. The distinction between filter burial losses and collection efficiency was not made by several subsequent investigators. Holmgren et al. ( 4 )determined filter efficiencies by sampling the same atmosphere with a test filter and a Millipore AA reference filter, counting with an energy-insensitive a counter. By assuming that the Millipore filter had 100%collection efficiency, Holmgren et al. deduced the collection efficiency of the test filter. Many of the low collection efficiencies thus reported are undoubtedly due to burial losses. In their paper can be found a review of many similar investigations. Studies of air concentrations of radon daughters by a spectrometry have frequently been concerned with the efficiency of collection and retention on filters as it pertains to energy degradation of 01 spectra. Subsequent to the development of the original spectrometric method by Martz et al. ( 5 ) , it has been shown that errors can arise from self-absorption in filters and from recoil losses of RaB (6-8). The work of Jonassen and McLaughlin ( 7 , B ) is of particular interest because of its implications for measurement techniques. Jonassen and McLaughlin found that in a vacuum the recoil loss efficiency of RaB from Millipore filters ranged from 0.18 to 0.93. Recoil loss efficiency was defined as the fraction of RaB atoms recoiling in the outward direction which escape from the deposit after the end of sampling. The experimental
@ 1980 American Chemical Society
Volume 14, Number 5, May 1980 533
~~
Table 1. Comparison of
~
~~
1
a Air-Sampling Papers a surface coilectlon, %
penetration,
Yo
burial, %
80
2
18
3.0
70
2
28
2.8
89 38 98
2 22 2
9 40
1.5 1.6
nil
7.9
54
3
43
0.8
Hollingsworth & Vose (HV-70, 9 mil) Hollingsworth & Vose (HV-70 gauze backing, 20-mil, material in roll 3-in. wide, for moving tape air monitor) Gelman E glass fibre (25-mil) Whatman 41 Whatman Ap/A Polystyrene (30mil) Microsoban (pads of polystyrene sheets: filter was 75 mil thick)
I
I
pressure drop, in. of Hg
a Reprinted, with permission, from ref 3. Copyright 1961 American Industrial Hygiene Association.
Table II. Recoil Loss Efficiency of RaB for Millipore Filters a filter types
nominal pore diam, Nrn
recoil loss efficiency (fractional)
vc
0.1 & 0.008 0.3 f 0.02
0.933 0.514 0.371
PH HA AA RA a
ss
0.45 f 0.02 0.8 f 0.05 1.2 f 0.3 3.0 f 0.9
Reprinted, with
permission, from ref
0.288 0.268
DISTANCE
0.179 8. Copyright 1976 Pergamon
Press
Ltd.
values of the recoil loss efficiency for Millipore filters as determined by Jonassen and McLaughlin (8)are given in Table 11. Permanent recoil loss of RaB during sampling is unlikely since the short range of recoiling RaB in air (0.15 mm) is sufficiently small that the probability of redeposition is high. After the end of sampling, and in particular during a counting in a vacuum, recoil losses can be permanent, with negligible redeposition. I t should be noted that the recoil loss efficiency of RaB from a filter depends on the nature of the filter (pore size, structure), the size distribution of the radioactive aerosol, and the flow rate employed. Jonassen and McLaughlin did not accurately specify all these parameters in their experiment. The recoil loss efficiencies measured by Jonassen and McLaughlin are of considerable significance in the measurement of radon daughters collected on a filter. If the counting of filter a activity is performed in a vacuum, recoil losses can be considered permanent, but the overall error during counting is reduced (or even cancelled) by the contamination of the detector. If several samples are counted successively, such errors become significant since the (detector) contamination from the first sample will add counts to subsequent samples. Recoil losses of RaB after sampling and during transfer of the filter into the counting apparatus are unavoidable. Consequently, counting should commence as quickly as practicable after the end of sampling. If a scintillation or other counter not requiring a vacuum is employed to count filter a activity, recoil losses of RaB during counting may or may not result in redeposition or in contamination of the detector. Since the recoil range of RaB in air is quite short (0.15 mm), recoiling atoms may leave a filter but not be able to reach the detector. Being essentially “unattached”, they would very probably diffuse quickly either back to the filter surface or to the detector surface. They could, however, attach themselves to aerosol particles and be carried 534
Environmental Science & Technology
(nr)
Figure 1. The
results of transport factor measurements for recoiling RaB atoms in air as determined by Ermilov et al. (9)
out of the vicinity of the detector by convective air movement. The transport of recoiling RaB in air as it pertains to the contamination of detectors has been investigated by Ermilov et al. (9). Ermilov et al. defined a transport factor k for recoiling RaB as the ratio of the number of atoms transported a distance h from the source to the number of disintegrations. Measurement of this quantity was performed by depositing radon daughter products by diffusion onto the polished surface of a piece of aluminum located in a radon atmosphere. The aluminum bearing the deposit was removed from the radon atmosphere and set up a distance h from a polished target plate. RaB recoil atoms were transported to the target plate through the air by diffusion. After 5 min of exposure, the aluminum and target plates were counted by two identical detectors consisting of CsI(T1) scintillators 0.2 mm thick. Differentiation between RaA and RaC’ a disintegrations was accomplished by pulse height analysis. The results of transport factor measurements for recoiling RaB atoms in air as determined by Ermilov et al. (9) are shown in Figure 1.In Figure 1,the points labeled 1correspond to the value of the transport factor for air for RaB atoms that escaped from the surface of a filter. This value of k , considerably less than for a polished surface, was explained by the different surface structure of the filter. a recoil detachment of particles from filters has been found to result in anomalously high penetrations of a active aerosols through high-efficiency filter media, in addition to producing the recoil loss effect. Filter retention efficiencies considerably lower than the expected 99.97% for ordinary particulate matter have been reported for 212Pb,253Es,238Pu,and 239Pu particulates (10, 1 1 ) . McDowell et al. (11) demonstrated that a-emitting aerosols do indeed penetrate filters more effectively than p- or y-active aerosols. This was attributed to the fact that a-active particles are repeatedly dislodged from positions within a filter and, in the presence of air flow, become repeatedly re-entrained. This phenomenon is not of great significance in the measurement of radon daughters. In
Table 111. Filter Collection Efficiencies pore size, liiter
0.4 0.8 1.o 3.0 5.0 8.0
Nuclepore
Millipore
Gelman Metricel Whatman
Ctm
PHWP HAWP AAWP RAWP SSWP BSWP BDWP FHUP AW06 Awl9 FHLP WSWP THWP GA-8 GN-6 DM-800GD GFIA
0.3 0.45 0.80 1.2 3.0 2.0 0.6 0.45 0.5 2 0.5 3 0.45 0.2 0.45 0.8
collection efficiency, YO unattached attached radon radon daughters daughters a
99.99 99.7 99.3 98.0 96.2 94.8 99.99 99.99 99.99 99.99 99.99 99.99 99.99 99.99 99.99 99.99 99.99 99.99 99.99 99.99 99.99 99.99 99.99
11 7 99.99 99.99 99.99 99.96 99.93 95.10 99.90 99.88 99.97 99.97 99.94 99.95 99.90 99.99 99.99 99.99 99.99
Aerosol geometric mean diameter was 0.05 pm. All filters were 47 mm diameter and the sampling flow rate was 3.6 L/min.
most practical situations, due to the overwhelming abundance of aerosol particles relative to radon daughters, it is improbable that more than one radon daughter is attached to the same aerosol particle. Holmgren et al. ( 4 )performed repeated experiments to determine if re-entrainment of radon daughters occurs and concluded that, during the course of sampling, the retention of captured activity by a filter is essentially 100%.
Experimental S t u d y of the Collection and Retention of Radon Daughters on Filter Media There is no consensus in the literature as to what filters, or even types of filters, are most suitable for collection of radon daughters. The main criteria by which filters can be compared and evaluated are as follows: collection efficiency (the ratio of the radioactivity that remains on or in a filter to the total radioactivity incident upon it) the extent of penetration of the radioactive aerosol into the filter the magnitude of the recoil loss effect filter resistance to flow filter handling characteristics. It was not possible in this investigation to look a t all the filters currently available for air sampling. Nonetheless, many different types of filters were studied and some comparisons made. The extent of penetration of a radioactive aerosol into each filter was determined by an cy spectrometric technique. Filters exhibiting the best surface collection characteristics were identified by the sharpness of the cy energy spectrum of the collected activity. The magnitude of the recoil loss effect for different filte‘rs was not determined. It was, however, verified by several measurements that the magnitude of this effect is of the order reported by Jonassen and McLaughlin ( 7 , 8 ) . The collection of unattached-as distinct from at-
tached-radon daughters on filters can be expected to be quite different. In the experiments with attached radon daughters, the carrier aerosol was produced by a zinc oxide aerosol generator and had a geometric mean diameter of 0.05 pm. Since this work was performed with high radon concentrations and low dust loading, self-absorption of CY particles by zinc oxide was not a problem. In actual field samples, with heavy filter loading, self-absorption may be significant. The collection efficiencies of various filters were determined using scintillation counters. In contrast to previous investigations, the threshold of the counters was set as low as possible, to give an cy detection threshold of about 2 MeV. The background counting rate of about 15 cpm was still negligible compared to the activity of most samples. The collection efficiency of each filter was determined by sampling an atmosphere of radon daughters. Following sampling, both the filter being tested and a back-up filter were counted simultaneously in the identical scintillation counters. Owing to the low threshold of these counters and additional spectroscopic measurements, it was determined that these measurements did not suffer from the usual “burial loss” problem. The collection efficiencies of the filters studied are shown in Tables I1 and 111. From the data in Table I11 it can be seen that the collection efficiency of most common air filters was essentially 100%for both attached and unattached radon daughters. Only Nuclepore filters of large pore size allowed significant penetration of the radioactive aerosol. This was expected from their structure. The major part of this work consisted of a study of the penetration of aerosol particles into filters. This was accomplished by collecting samples of airborne radon daughters on various filters and determining, in a vacuum, the energy spectrum of the emitted cy particles. Filters with superior surface collection characteristics could be identified since they resulted in sharp RaA and RaC’ cy energy peaks. The CY energy spectra obtained using two membrane filters under different conditions are shown in Figures 2 and 3 for illustration of different degrees of surface collection. Volume 14, Number 5, May 1980
535
Table IV. Performance index for Various Filters
filler
Nuclepore
pore size, wm
0.4 0.8 1.o 3.0
5.0 Millipore
Gelman
Whatman
PHWP HAWP SSWP BSWP BDWP AW06 Awl9 FHLP WSWP WHWP GA-8 GN-6 DM-800GD GFIA
8.0 0.3 0.45 3.0 2.0 0.6
0.5 2.0 0.5 3.0
0.5 0.2 0.45 0.8
performance index X l o 3 unattached attached radon radon daughters daughters
108 72 67 51 44 40 104 147 61 44 61 60 56 60 77 68 95 100 89 71
83 35 35 62 24 23 70 74 59 74 41 62 68 50 43 49
In order to rank the filters according to penetration of aerosol particles, a performance index, PI, was defined. The performance index is the ratio of the height of the RaC' peak (in counts) to its area in total counts. Results for the filters studied are summarized in Table IV. From Table IV, it can be seen that unattached radon daughters are collected more on the surface of filters than attached radon daughters. Small pore size filters are preferable to large pore size filters. The anomalous results obtained with some of the filters are due a t least in part to the different structure of the filters. In any case, the results should be treated as only a qualitative indication of surface collection, since the precision of the results does not justify otherwise. Additional considerations in filter comparisons are ease of filter handling, retention of radon daughters after sampling, moisture and humidity effects, and pressure drop during sampling. These, however, depend on the specific application and were therefore not evaluated here.
Conclusions The importance of understanding the collection and retention of radon daughters on filters has long been recognized. In this work, previous work is critically reviewed. Measurements of the collection characteristics of many common air filters were made using an cy spectroscopic technique. The important findings are as follows: I t is important that a distinction be made between the collection efficiency of a filter and its self-absorption characteristics toward cy particles from the penetrating radioactive aerosol. Scintillation counters, although ideally energy insensitive, frequently have high thresholds and fail to detect cy particles which have had their energy significantly degraded by passage through first a filter and then an air gap. Investigations of filters can best be performed using an energy-sensitive device such as an cy spectrometer. Permanent recoil loss of RaB from filters during sampling is unlikely since the short range of recoiling RaB in air is sufficiently small that the probability of redeposition is high. After the end of sampling, and in particular during a counting in a vacuum, recoil losses can be permanent with redeposition 536
Environmental Science & Technology
c
800
2001
being negligible. Filters exhibiting the best surface collection suffer from the highest recoil losses of RaB (as high as 93100% of the outward recoiling atoms). It is unclear exactly what effect this may have on measurement errors. Most common filters used for air sampling have a collection efficiency of greater than 99.9%. Penetration of the aerosol into the filters varies significantly from filter to filter. As expected, small pore size filters exhibit the best surface collection. Also, unattached radon daughters deposit more readily on the surfaces of filters than attached radon daughters. The cy spectra of radon daughters collected on various filters are indicative of filter surface collection characteristics, and can be used as a guide to filter selection.
Acknowledgment The scientific liaison of Dr. H. Stocker is gratefully acknowledged. Literature Cited (1) Alercio, J. S., Harley, J. H., Nucleonics, 10(11), 87 (1952). (2) Tsivoglou, E. C., Ayer, H. E., Holaday, D. A., Nucleonics, 11(9), 4 (1953). (3) Lindeken, C. L., J . A m . Ind. H3g. Assoc., 22(4), 232 (1961). (4) Holmeren. R. M.. Wagner. W. W.. Llovd. R. D., Pendleton. R. C., H e a l t h h y s . , 22, 297-1300 (1977). ( 5 ) Martz, D. E., Halleman, D. F., McCurdy, D. E., Schiager, K. J., Health Phys., 17,131 (1969). (6) Jonassen, N., Hayes, E. I., Health Phys., 27,310-3 (1974). (7) Jonassen, N., McLaughlin, J. P., Health Phys., 30, 234-8 (1976). (8) Jonassen, N., McLaughlin, J. P., J . Aerosol Sci., 7, 141-9 (1976). 19) Ermilov. A. P.. Klinov. V. V.. Labushkin, V. G., Trans. Atomnaya Energ, 25(1), 61-2 (1968). (10) Ryan. M. T., Shrable, K. W., Chabot, G., Health Phys , 29,798 (197i). (11) McDowell, W. I., Sealey, F. G., Ryan, M. T., Health Phys., 32, 445 (1977). ~
Received for review August 10, 1979. Accepted November 26, 1979. This u)ork u a s supported by a contract from the Atomic Energy Control Board of Canada.
