Selection of Filter Media in Alpha Air Monitors for Emergency ...

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Selection of Filter Media in Alpha Air Monitors for Emergency Environmental Monitoring
N. Kinouchi 1 , T. Oishi 1 , H. Noguchi 1 , S. Kato 1 and M. Ishizawa 2
1 Japan Atomic Energy Research Institute, Tokai-mura, Ibaraki-ken, Japan
2 Institute of Radiation Measurements, Tokai-mura, Ibaraki-ken, Japan
INTRODUCTION
We have developed an alpha air monitor which is possible to measure rapidly and sensitively the concentrations of
airborne alpha-emitting particles, such as plutonium, for the environmental monitoring at an accident of nuclear
reprocessing plant. The monitor is designed to collect airborne alpha-emitting particles by drawing the ambient air
through a filter and to detect the activity by alpha spectroscopy. In order to achieve high-sensitive measurements,
selection of a suitable filter used in the monitor is considerably important (1, 2).
The most important requirement for the filter is that it has a high surface collection efficiency to obtain the
sharpness of the alpha energy spectrum. This makes it easy to distinguish the alpha-ray peak of plutonium from the
alpha spectrum of naturally occurring radon decay products in the environment. And the filter is also desired to have
low resistance of the air flow so that particles can be collected at a high flowrate.
We have examined the surface collection efficiency and pressure drop for the various filters. Test filters were
glass fiber, cellulose-glass fiber, membrane and so on. The surface collection efficiency has been evaluated by the
sharpness of alpha-ray energy peaks of thoron decay products generated in a laboratory and collected on the filters. The
pressure drops were measured at the various face velocities.
Furthermore, for the test filters which showed good performance at the above two experiments, we have examined
the effect of the radon decay products simultaneously collected on the filter by air sampling in the atmosphere.
The present report describes the experimental method and the results obtained.
EXPERIMENTAL METHOD
Types and designations of the test filters are shown in Table 1. These filters are commercially available in Japan.
The first item examined is the sharpness of alpha-ray energy peaks of thoron decay products collected on the test
filters. The experimental apparatus is shown in Figure 1. Thoron decay products ( ThC, ThC’ ) were generated in the
box which contained thorium oxide powder and collected on the test filters at a face velocity of 12 cm s –1 . Alpha
spectrum of thron decay products was measured by a silicon-semiconductor detector ( CANBERRA PD450 ) in vacuum.
The periods of sampling and counting were 7,200 and 10,000 sec, respectively. The sharpness of alpha-ray energy
peaks was evaluated by the following equation:
ε = NB / NA × 100
where, ε = sharpness of alpha-ray energy peaks (%)
NA = total alpha counts in energy region from 1.5 to 9.8 MeV (counts)
NB = total alpha counts in energy region from 5.3 to 6.8 MeV or from 8 to 9.5 MeV (counts)
The second item examined is air flow resistance of the filters. The relationship between the air flow rate and
pressure drop was measured on the test filters with an effective filter diameter of 29.5mm. The pressure drop was
measured by a mercury manometer. The face velocity was varied from 10 to 100 cm s –1 .
For the test filters which showed good performance at the above two experiments, the effect of radon decay
products collected on the filters was examined. We collected radon decay products on the test filters by sampling of
dust in the atmosphere. The sampling air flow rate and sampling period were about 90 L min –1 and 7,200 sec,
respectively. Then, alpha spectrum of radon decay products was measured by a silicon-semiconductor detector in
vacuum. The period of counting was 10,000 sec. The effect of radon decay products was evaluated by the
background counts of radon decay products in a plutonium region. The radon decay product background counts in a
plutonium energy region from 3.5 to 5.5MeV were normalized to total radon decay product background counts.
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Table 1
Summary of characteristics of the test filters
Filter Type and Designation Surface collection efficiency (%) Relative pressure drop
(sharpness of alpha-ray energy peaks)
GLASS-CELLULOSE FIBER
TOYO HE-40T ( Glass 20%, Cellulose 80%) 45 0.17
TOYO HE-40TA ( Glass 30%, Cellulose 70%) 56 0.20
GLASS FIBER
TOYO GB-100R 65 0.26
PALL-GELMAN T60A20 56 0.11
WHATMAN GF/F 94 1.41
SILICA FIBER
TOYO QR100 76 0.48
MEMBRANE (CELLULOSE ACETATE TYPE)
MILLIPORE AA (0.8 µm pore size) 94 1
TOYO A080A (0.8 µm pore size) 95 1.15
TOYO A100A (1.0 µm pore size) 90 0.76
MILLIPORE SMWP (5.0 µm pore size) 79 0.42
MEMBRANE (PTFE TYPE)
TOYO T080A (0.8 µm pore size) 97 2.11
TOYO T100A (1.0 µm pore size) 98 1.45
TOYO T300A (3.0 µm pore size) 87 0.50
MEMBRANE (PTFE with backing TYPE)
TOYO J100A (1.0 µm pore size) 99 0.63
MILLIPORE FSLW (3.0 µm pore size) 99 0.31
SUMITOMO LWP-300-75 (3.0 µm pore size) 95 0.28
PREFILTER FOR MEMBRANE FILTER
TOYO Y008A 98 1.80
TOYO Y020A 82 0.83
TOYO Y100A 69 0.27
MILLIPORE AW19 91 0.54
Room air dust
Prefilter
Flow meter
Test filter
Air
Thoron decay
products generator
Air pump
Chamber
Air pump
Figure 1. Experimental apparatus for the examination of the surface collection efficiency.
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Count (nomalized)
1.0
0.8
0.6
0.4
0.2
0.0
GB-100R
Count (nomalized)
1.0
0.8
0.6
0.4
0.2
0.0
FSLW
2000 4000 6000 8000 10000
Energy (keV)
2000 4000 6000 8000 10000
Energy (keV)
Figure 2.
Alpha spectrum of thoron decay products collected on GB-100R and FSLW.
T080A
GF/F
AA
Pressure drop ( mmHg )
10 2
10 1
AW19
FSLW
GB-100R
HE-40T
10 0
10 1 10 2
Face velocity (cm s -1 )
Figure 3.
Relationship of pressure drop to face velocity for various test filters.
RESULTS and DISCUSSION
Figure 2 shows the examples of alpha spectrum of thoron decay products collected on GB-100R (Toyo Roshi Co.
Ltd.) and FSLW ( Millipore Corporation ), respectively. The sharpness of alpha-ray energy peaks obtained for test
filters is shown in Table 1.
Figure 3 shows the relationship of pressure drop to face velocity for some test filters. The relative pressure drop
defined by a ratio of the pressure drop for the filter of interest to that for AA ( Millipore Corporation ) at the face velocity
50 cm s –1 is also shown in the right column of Table 1.
All types of fiber filters, except GF/F (Whatman LabSales ) , show poor surface collection efficiencies. GF/F has
a high surface collection efficiency, but this filter is not suitable for alpha monitoring because of high pressure drop.
All types of membrane filter show high surface collection efficiencies. Small pore membrane filters have a tendency
toward high pressure drop. Of membrane filters which have the same 1.0 µm pore size, J100A is superior to the other
filters, A100A and T100A, because J100A has a lower pressure drop and a similar surface collection efficiency to that of
A100A and T100A. FSLW is a PTFE (polytetrafluoroethylene) membrane filter with polypropylene backing and
LWP-300-75 ( Sumitomo Electric industries Ltd. ) is a PTFE membrane filter with polyethylene baking. Both of these
filters show a similar performance to that of J100A. In addition, of prefilters, AW19 has relatively high surface
collection efficiency in spite of low pressure drop.
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Table 2 Relative background counts of radon decay products in a plutonium region obtained for the test filters
Filter Type and Designation
Relative background counts of radon decay products
in a plutonium region
MEMBRANE( PTFE with backing TYPE)
MILLIPORE FSLW (3.0 µm pore size) 0.006 ± 0.002
PREFILTER FOR MEMBRANE FILTER
MILLIPORE AW19 (5.0 µm pore size) 0.04 ± 0.005
On the basis of these results, we examined background counts in a plutonium energy region about two filters. We
selected FSLW and AW19 out of PTFE membrane filters with backing and prefilters. The relative background counts
of radon decay products obtained for the test filters are shown in Table 2. This also showed that PTFE membrane filters
with backing such as FSLW is suitable for alpha monitoring because of lower background counts.
SUMMARY
Some characteristics of commercial air filters were studied so as to select the suitable filter media in an alpha air
monitor for emergency environmental monitoring .
Cellulose acetate membrane filters and PTFE membrane filters without baking showed high surface collection
efficiency, but these filters are not suitable for alpha air sampling because of high pressure drop. It was found that the
PTFE membrane filter with backing had a high surface collection efficiency and low pressure drop. Further research on
the particle collection efficiency for the PTFE membrane filters with baking of various pore sizes is required in order to
make a choice of the optimum pore size.
(This research was conducted under Science Technology Agency Contract. )
REFERENCES
1. L.B.Lockhart,Jr., R.L.Patterson,Jr. and W.L.Anderson, Characteristics of Air Filter Media Used for Monitoring
Airborne Radioactivity. NRL Report 6054, U.S. Naval Research Laboratory, Washington D.C., USA (1964)
2. M.D.Hoover and G.J.Newton, Update on Selection and Use of Filter Media in Continuous Air Monitors for Alpha-
Emitting Radionuclides. in Inhalation Toxicology Research Institute Annual Report, LMF-138, Albuquerque,
USA (1992)
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