P-1b-3
Concentration Ratio of Radon Progeny in Air
Tsuneo Kobayashi
Department of Physics, School of Medicine, Fukushima Medical University, Fukushima 960-1295, Japan
INTRODUCTION
Investigations have been made on the concentration ratio of radon progeny in air. Data have been
acquired intermittently since 1988 using alpha spectroscopic method around the author's office that is located in
the northeastern part of Japan. Clarifying the behavior of radon progeny is an issue of wide importance to
radiation protection, predicting earthquakes, etc. Let RABC = ECRn(218Po)/{ECRn(214Pb) + ECRn(214Bi)}; the
concentration ratio, RABC, is relevant to the stability of the air. Statistical and time series analysis for outdoor
data indicates several interesting results: (1) Air is more stable in the morning than in the afternoon. (2) Air is
more stable in summer/autumn than in winter/spring. (3) In spite of no significant correlation between RABC and
wind speed, power spectrum of RABC is similar to that of wind speed; wind speed affects radon progeny ratio
after a certain time delay? Analysis for indoor air is still in progress.
MATERIALS AND METHOD
Intermittent measurement of 222Rn progeny employed a filtration method with alpha spectroscopy,
using membrane filters (pore size, 0.8 µm) and a self-contained alpha spectrometer with a silicon-surface barrier
semiconductor (active area: 900mm2) [1]; counting efficiency for alpha particles was 0.125. After 10 minutes
sampling (20 L/min) of the progeny in air, the activity on the filter was detected for 50 minutes. By using an
evacuated chamber, 212Po peak (8.78 MeV) was clearly separated.
The alpha spectrum for progeny measurement was divided into three regions: 218Po region (4.6 to 6.2
214
MeV), Po region (6.2 to 7.9 MeV), and 212Po region (7.9 to 9.3 MeV). Interference among these three
regions was negligible, especially because of few counts of 212Po region. Equilibrium equivalent concentration
of radon progeny, ECRn, was calculated from 218Po, 214Pb and 214Bi concentrations. Lower detection limit for
ECRn was 0.07 Bq/m3.
Measurements were made for indoor and outdoor air every day except holidays; measurements at the
identical times are essentially indispensable for time series analysis. From the difference of measurement time,
the present data are separated into three series; Ser.1: 9:15 (1988-04-04 to 1993-07-09), Ser.2: 15:00 (1993-07-19
to 1997-04-30), and Ser.3: 9:00 (1997-05-01 to 1999-01-28). Outdoor air was sampled on the fourth floor of an
emergency staircase. Indoor air was sampled around the author's office, which is on the fourth floor of a fivestoried building. The college building was built in 1988.
RESULTS
Basic statistics
Table 1 shows basic statistics for Ser.1 and Ser.2. Table 1 also lists conventional ratios, RB=CB/CA,
and RC=CC/CA, where CA, CB and CC denote the concentration of 218Po, 214Pb and 214Bi, respectively. As can be
seen from Table 1, RB and RC often show unreasonable values of greater than unity. Comparing results of Ser.1
and Ser.2, the mean value of RABC was lower in the morning, indicating that outdoor air was more stable in the
morning than in the afternoon.
Table 1: Basic statistics
Ser.1
Mean
S.D.
Min.
Max.
#
Ser.2
Mean
S.D.
Min.
Max.
#
ECRn
ECRn
7.67
787
3.35
2.20
.37
14.88
1247
2.16
.98
.17
RB
RB
7.64
748
.83
.69
.03
5.53
1221
.83
.88
.01
RC
RC
7.44
755
.81
.64
.02
7.07
1224
.81
.73
.01
1.95
777
RABC
.20
.12
.03
1.27
1236
RABC
.22
.16
.02
Log-normality of the statistical distribution
Figures 1 and 2 show statistical distribution for Ser.1 (morning) and Ser.2 (afternoon), respectively.
Log-normal distribution is one of the well-know behavior of radon progeny; Fig.1 and Fig.2 seem to indicate
log-normality. To examine the log-normality, Lilliefors test was made for logarithm of outdoor data every one
year. Table 2 shows the results, where "Year 1988" means data from April 1988 to March 1999 and circled data
passed the examination (significance exceeded 0.01). RABC passed the test 5 times for 9 years, while radon
progeny passed 8 times. No variables passed examination for normal distribution.
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P-1b-3
Fig. 1: Statistical distribution for Ser.1(9:15).
Year
Time
#
218
Po
214
Pb
214
Bi
ECRn
RB
RC
RABC
1988
9:15
209
1989
9:15
237
O
O
O
O
O
O
Fig. 2: Statistical distribution for Ser.2(15:00).
Table 2. Lilliefors Test for Log-normality
1990
1991
1992
1994
9:15
9:15
9:15
15:00
225
235
240
268
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
1995
15:00
186
1996
15:00
175
1997
9:00
209
O
O
O
O
O
O
O
O
O
Seasonal variation
To see the seasonal variation, one-way layout analysis was made for four groups of data, i.e., spring
(March to May), summer (June to August), autumn (September to November), and winter (December to
February). Table 3 and Fig.1 show mean values of the four groups in three series. In Fig.1, ** and * denote
p-value of 0.01 and 0.05, respectively. RABC indicated significantly higher level in spring and winter season, in
other words, air was stable in summer and autumn.
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P-1b-3
Correlation analysis
Since Ser.3 had perfect meteorological data, correlation analysis was made for this series. Table 4
shows correlation between ratios and meteorological data. In this table, the symbols + and - denote
significantly positive and negative correlation, respectively. In the cases of + and -, every p-value was less than
0.01. Contrary to natural expectation, RABC had no correlation with wind speed. Table 5 shows correlation
between ratios and progeny concentrations. RABC had correlation with all progeny concentrations, whereas RB
and RC indicated lack of correlation with two progeny concentrations. Table 6 shows correlation between ratios
themselves.
Spring
Summer
Autumn
Winter
Table 3: Seasonal variation
Ser.1 9:15 Ser.2 15:00 Ser.3 9:00
0.215
0.254
0.199
(336)
(193)
(71)
0.189
0.207
0.194
(334)
(171)
(103)
0.191
0.195
0.174
(300)
(205)
(84)
0.211
0.247
0.262
(266)
(200)
(81)
Fig. 3: Seasonal variation of radon progeny and ratios.
ECRn
RB
RC
RABC
Table 4: Correlation with meteorological data
Tmperature
Humidity
Atm. Pressure
+
+
-
Wind speed
-
+
Table 5: Correlation with progeny concentration
218
214
214
ECRn
Po
Pb
Bi
RABC
+
RB
+
+
RC
Table 6: Correlation with ratios
RABC
RB
RC
RABC
.
RB
.
+
RC
+
.
Time series analysis
Time series analysis (spectrum analysis) was made for various variables using data from Ser.3; power
spectra were estimated with autoregressive model that is equivalent to maximum entropy method [1-4]. Fig.4
and Fig.5 show time series used for analysis. Fig.6 and Fig.7 show the results of spectrum analysis for
progeny/ratios and meteorological data, respectively. Power spectrum for RABC was most similar to that of wind
speed. One-year period, that is always remarkable for radon progeny, was not significant for RABC. Three- to
nine-day periods also appear for RABC, radon progeny, wind speed, and atmospheric pressure. These severalday periods are probably attributed to the passage of air masses. Twenty-day to forty-day peak may be
attributed to meteorological phenomena corresponding to the rotation period of the sun. Temperature indicated
no significant periodicity except dominating one-year period.
Wind speed is well known to affect the radon progeny in air. As mentioned previously, Pearson
correlation analysis indicated no significant correlation between RABC and wind speed, whereas the power
spectrum of RABC was most similar to that of wind speed. Although the wind speed directly affects radon
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P-1b-3
progeny, it would take a certain time for progeny ratio to change, say one hour or so.
of wind speed to RABC might probably appear after a certain time lag.
Fig. 4: Time series for radon progeny and ratios
using data from 1997-05-01 to 1998-01-09.
1y
100
20d
Fig. 5: Time series for meteorological data from
1997-05-01 to 1998-01-09.
10000
ECRn
9.9d
3.6d
7.1d
0
0.2
9.1d
RB
0.4
2000
2.2d
2.7d
Power spectral density
Power spectral density
0
2
43d
0
4
0
0.2
32d
2
RC
5.1d
0.4
3.0d
2.4d
0
0
0.2
0.4
Atmospheric Pressure
1y
1000
6.0d
23d
0
4000
0
0.2
0.4
Humidity
6.1d
2000
2.6d
18d 8.8d
0
0
0.1
0.2
RABC
37d
0
Temperature
1y
2.2d
0
4
In conclusion, the effect
0.4
0
0.2
5
0.4
Wind Speed
32d
2.6d
2.5d
8.0d
6.9d 4.8d 3.9d
14d
4.8d
3.6d
0
0
0.2
0.4
–1
Frequency [day ]
0
Fig. 6: Power spectra for radon progeny and ratios
using data from 1997-05-01 to 1998-01-09.
0.2
0.4
–1
Frequency [day ]
Fig. 7: Power spectra for meteorological data
from 1997-05-01 to 1998-01-09.
REFERENCES
1.
2.
3.
4.
T. Kobayashi and Y. Takaku, Intermittent Measurements of 222Rn and 220Rn Progeny in Air for Four Years.
RADIOISOTOPES , 46(9), 603-614 (1997).
H. Akaike. Annals Inst. Statist. Mathem. 21, 243-247 (1969).
M. Hino, Spectral Analysis. Asakusa Shoten, Tokyo (1977) (in Japanese).
J. P. Burg. Geophys., Oklahoma City, Okla., Oct. 31 (1967).
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