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Mass exhalation rates, emanation coefficients and enrichment pattern of radon, thoron in various grain size fractions of monazite rich beach placers
Journal Pre-proof Mass exhalation rates, emanation coefficients and enrichment pattern of radon, thoron in various grain size fractions of monazite rich beach placers Primal V. Pinto, K. Sudeep Kumara, N. Karunakara PII:
S1350-4487(19)30506-2
DOI:
https://doi.org/10.1016/j.radmeas.2019.106220
Reference:
RM 106220
To appear in:
Radiation Measurements
Received Date: 10 August 2018 Revised Date:
7 September 2019
Accepted Date: 13 November 2019
Please cite this article as: Pinto, P.V., Sudeep Kumara, K., Karunakara, N., Mass exhalation rates, emanation coefficients and enrichment pattern of radon, thoron in various grain size fractions of monazite rich beach placers, Radiation Measurements, https://doi.org/10.1016/j.radmeas.2019.106220. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.
1
Mass exhalation rates, emanation coefficients and
2
enrichment pattern of radon, thoron in various grain size
3
fractions of monazite rich beach placers
4
Primal V. Pinto1, 3*, Sudeep Kumara K.2 and Karunakara N.2
5 6
¹Science program, Texas A & M University, Qatar
7 8 9
2
Centre for Advanced Research in Environmental Radioactivity (CARER),
Mangalore University, Mangalagangothri – 574199, India
10 11
3
Department of Studies in Physics, Mangalore University, Mangalagangothri-574199,
12
India
13 14
*
15
Corresponding author Email: [email protected] [email protected]
16
17
18
19
20
21 1
22 23 24
Highlights •
to 222Rn and 220Rn release in the environment.
25 26
•
•
Radon and thoron emanation coefficient was higher in 1000-500 µm grain size fractions, which contained least 226Ra and 228Ra activity (Bq kg-1).
29 30
Monazite rich samples have low emantion coefficient and are not the highest contributor of 222Rn and 220Rn to the environment.
27 28
This study throws light on the relative contribution of the monazite minerals
•
Unlike
226
Ra and
228
Ra activity concentrations,
222
Rn and
220
Rn emanation
31
coefficient did not increase with the finer grain size; rather, opposite trend was
32
observed.
33
•
The
statistically
significant
correlation
coefficient
34
dependency of 220Rn mass exhalation rate (Bq kg-1 h-1) and
35
kg-1), also between
36
activity (Bq kg-1).
222
substantiated 228
Ra activity (Bq
Rn mass exhalation rate (mBq kg-1 h-1) and
37 38
39
40
41
2
the
226
Ra
42
Abstract
43
A study on exhalation rates of
44
different grain size fractions was carried out. Four different size fractions of the sand
45
samples (1000-500 µm, 500-250 µm, 250-125 µm, and <125 µm) collected from the
46
beach placers of south west coast of India were analyzed for 226Ra and
47
concentrations
48
measurement technique using Scintillation cell based monitors were used to measure
49
the
50
appropriate model to extract the mass exhalation and emanation coefficients. The
51
222
222
Rn and
(Bq
220
kg-1)
222
Rn and
by gamma
220
Rn and their enrichment pattern in
spectrometry.
232
Automated
Th activity continuous
Rn concentrations. These measured values were then fitted to
Rn mass exhalation rate varied from 0.7 ± 0.2 mBq kg-1 h-1 to 11.0 ± 0.9 mBq kg-1
52
h-1 while that of 220Rn varied from 88.5 ± 4.8 Bq kg-1 h-1 to 3066 ± 14 Bq kg-1 h-1. The
53
higher mass exhalation rates, for both radioactive gases, were observed in finer grain
54
size fractions. However, this was in contrast to the emanation coefficient values.
55
Interestingly, higher radioactive monazite rich beach samples exhibited low
56
emanation coefficient values when compared to the low radioactive soil and sand
57
samples reported in the literature. Statistical analysis showed positive correlation,
58
with correlation coefficient R=0.84 and R=0.79 respectively between mass exhalation
59
rates of
60
size fractions.
222
Rn,
220
Rn with their respective radioactive predecessor for different grain
3
222
Rn and
220
61
Keywords:
Rn, emanation coefficient, mass exhalation, monazite, High
62
Background Radiation Area (HBRA).
63 64
1. Introduction
65
The beach placers deposits along the south-west coast of India are particularly rich in
66
monazite. It is radioactive due to the presence of naturally occurring Thorium and
67
Uranium (Mohanty et al., 2003; Shetty and Narayana, 2007). As a daughter product
68
of Uranium, the release of
69
of emanation followed by exhalation. The emanation can be defined as the escape of
70
a
71
mechanism of emanation and transport of 222Rn and
72
effect of the short half-life (55.6s) of 220Rn (Kanse et al., 2013).
222
Rn atom from a
222
Rn gas to the environment takes place by the processes
226
Ra-bearing grain into pore spaces (Sakoda et al., 2011). The 220
Rn is the same, except for the
73
222
74
The phenomenon of
75
parameters such as grain size, porosity and water content. These determine the static
76
emanation coefficient which does not change with time, so the type of soil determines
77
the general
78
controls 222Rn exhalation rate showed that the emanation fractions first decreases and
79
later reaches a constant value with the increase in grain size (Sakoda et al., 2011;
80
Barillon et al., 2005). The typical values reported for the radon emanation coefficient
222
Rn emanation and then exhalation depends on several
Rn level in the soil (Chauhan, 2011). The effect of grain size that
4
81
for dry soil is 0.1 (range 0.01- 0.5) and building material is 0.05 (ranges 0.005-0.3)
82
(Porstendorfer, 1991). A representative value of 0.2 is suggested in UNSCEAR
83
(1988) for the soil with the range 0.01-0.8. The 222Rn mass exhalation rate of 24 ± 12
84
mBq kg−1 h−1 reported in soil samples and 18.8 ± 6.4 mBq kg−1 h−1 for the black sand
85
(Sahoo et al., (2007).
86
87
Radium-226, a source of 222Rn present in rocks, soils and construction materials leads
88
to the accumulation of
89
(Galan Lopez et al., 2004). Nevertheless,
90
can easily enter into the ground water by the effect of lithostatic pressure, easily get
91
released to indoor air when used in showers, humidifiers, cooking and so on.
92
Exposure to water borne
93
222
222
Rn in poorly ventilated houses, pose potential health risk
222
222
Rn from the bedrocks containing
226
Ra
Rn may occur by ingestion (drinking water containing
Rn) and by inhalation (Toscani et al., 2001; Mahesh et. al., 2001; Primal et al., 222
94
2012). The exhalation rate of
95
sands, rocks, construction materials, uranium ores and tailings have been the subject
96
of several studies (Somashekarappa et al., 1996; Jha et al., 2000; Evangelista and
97
Pereira et al., 2002; El-Amri et al., 2003). Such studies are used to assess the
98
radiation dose to the public and to estimate local environmental
99
exhalation rate of 222Rn is one of the critical factors that influence indoor and outdoor
100
222
Rn from numerous sources such as soils, mineral
Rn concentrations (Karunakara et al., 2005b; Mayya, 2004).
5
222
Rn levels. The
101 222
Unlike
103
reported, and these are; Ganesh et al. (2008) for India, Kovacs (2010) for Hungary,
104
Harley et al. (2010) for the USA and Yasuoka et al. (2010) for Japan. Thoron
105
exhalation rates of 1.7 ± 1.2 (range 0.19–4.2 Bq m-2 s-1) and 2.4 ± 1.5 (range 0.04–6.2
106
Bq m-2 s-1) were reported for the red and yellow soil surface and the Dark red soil,
107
respectively (Hosoda et al., 2010). The 220Rn level in the environment is governed by
108
its emanation from the soil or building materials containing 232Th (Kanse et al., 2016;
109
Shimo et al., 2010; Shiroma et al., 2010). Jonas et al., (2016) have reported that the
110
220
Rn, a very few studies on
220
102
Rn in the living environments have been
Rn emanation factor could not be predicted from the 222Rn emanation factor.
111 112
Wastes and tailings from Thorium bearing ores processed for metals can potentially
113
release significant
114
shale are likely to have high Thorium content. Monazite and zircon sands have an
115
extraordinarily high concentration of Thorium. Thorium is widely distributed in
116
nature with an average concentration of 10 ppm in the earth’s crust in many
117
phosphates, silicates, carbonates and oxide minerals (Ramachandran, 2010). In
118
general, Thorium occurs in association with Uranium and Rare Earth Element (REE)
119
in diverse rock types. In India, the High natural Background Radiation Areas (HBRA)
120
are identified at certain regions of coastal Kerala and Orissa due to an elevated
121
content in monazite sands (Mishra, 1993; Nambi et al., 1994; Primal and Narayana,
220
Rn (Polednak et al., 1983). Rocks composed of granite or black
6
232
Th
122
2014). Southern coast of Brazil and China are among the other numerous locations,
123
known for high levels natural radiation (Paschoa, 2000; Wei and Sugahara, 2000; Tao
124
et al., 2012). Because of their high density, monazite minerals get concentrated in
125
sands along the coastal regions.
126 127 128
The exhalation being an essential parameter for describing the potential 220
222
Rn and
Rn release from a solid matrix, in particular in HBRA. Hence, a detailed study on
129
these aspects were carried out in the samples collected from the HBRA of the
130
coastline of Kerala such as Karunagapalli, Chavara, Neendakara, and Kovalam (Fig.
131
1). This study is an effort to evaluate the contribution of monazite rich sand to the
132
elevated levels of
133
emanation coefficient and the effect of grain size on the release of
134
from the solid matrix to the surroundings. The study area has a significant resident
135
population exposed to higher levels of
136
Ramachandran and Sahoo, 2009; Mayya et al., 2012).
226
Ra and
228
Ra concentrations,
222
Rn and
220
Rn exhalation,
222
Rn and
220
Rn
220
Rn (Chougaonkar et al., 2004;
137
138
2. Materials and methods
139
Sand samples were collected from the HBRA beach placers were oven dried at 110ºC
140
till the constant dry weight was obtained. The dried samples were sieved into four
141
different grain size fractions; viz. 1000-500 µm, 500-250 µm, 250-125 µm, and <125
142
µm. The sieved samples were subjected to gamma spectrometry (section 2.1) to 7
226
Ra and
232
143
determine
Th activity concentrations. The methods adopted for the
144
exhalation measurements are discussed in sections 2.2 and 2.3.
145
146
2.1 Gamma-ray spectrometry
147
The samples were counted using a high-resolution, co-axial n-type high purity HPGe
148
detector (model GR 4021 Canberra USA) with 42% relative efficiency. This facility
149
was available at Centre for Advanced Research in Environmental Radioactivity
150
(CARER), Mangalore University. The efficiency calibration of the detector was
151
performed using IAEA quality assurance reference materials: RG U-238, RG Th-232,
152
RG K-1, and SOIL-6. The samples were taken in 300 ml air tight polypropylene box.
153
The standard materials and samples were taken in containers of the same size and
154
type so that the detection geometry remained the same. The samples of different grain
155
sizes were counted long enough (30,000 s) to reduce the counting error. The activity
156
concentrations of 226Ra, 232Th, and 40K in various samples were determined using the
157
Gennie-2000 spectrum analyses software. The activity of 40K was evaluated from the
158
1461 keV photopeak, the activity of 226Ra from the weighted mean of the activities of
159
three photopeaks of
160
determined from the photopeaks of
161
background counts and applying the compton correction (Karunakara et al., 2005a,
162
Karunakara et al., 2013, Karunakara et al., 2014a, Yashodhara et al., 2011). The
214
Bi (609.3, 1120.4, and 1764.5 keV), and that of 228
8
228
Ra was
Ac (911.2 keV) after subtracting the
163
Minimum Detection Levels (MDL) for the gamma spectrometry system used in the
164
present study were 0.9, 1.2, and 4.06 Bq kg-1 for 226Ra, 232Th, and 40K respectively at
165
95% confidence level.
166
167
2.2. Radon exhalation measurement
168
Mass exhalation rate measurement requires continuous online
169
this was performed using a Scintillation Radon Monitor (SRM) developed by
170
Radiological Physics and Advisory Division (RP & AD), Bhabha Atomic Research
171
Centre (BARC). It is an integrated microprocessor-based system consisting of a
172
collection chamber, a conventional ZnS:Ag scintillator detector and a counting set up.
173
The SRM records
174
temperature and humidity values. In the present study it was operated in 60 min cycle.
175
Experiments were carried in an air-conditioned room so that a uniform ambient
176
temperature and humidity condition is maintained for all the measurements. In order
177
to obtain the results under a unified experimental conditions, all the samples were
178
dried at 1100C to obtain a constant weight and the dried samples were used for
179
measurements.
222
222
Rn monitoring and
Rn concentration for each measurement along with ambient
180 181
A known quantity of dried sample was taken in a leak proof container having inlet
182
and outlet valves. The outlet of container was connected to the inlet of the SRM
9
183
through an air pump. The outlet of the SRM was connected to the inlet of the sample
184
container. A pump was used to circulate the air between sample container volume and
185
detector. The SRM monitor consists of a “progeny filter” and “thoron discriminator”
186
to eliminate
187
progeny formed inside the cell were continuously counted (60 mins cycle) and are
188
converted to
189
smart algorithm. Since the technology is purely based on direct scintillation counting
190
of an alpha particle with build-up/decay corrections achieved through software, the
191
measurements were not affected by humidity or trace gases (Sahoo et al., 2007;
192
Gaware et al., 2011). The
193
a saturation value. The SRM has a sensitivity factor of 1.2 counts h-1/Bq m-3 and an
194
upper detection limit of 50 MBq m-3. The minimum detection limit of SRM is 14
195
Bq/m3 at 95% confidence with 1 hr counting. Fig. 2 shows the arrangements for the
196
222
222
Rn progenies and
222
220
Rn. The alpha scintillations from
222
Rn and its
Rn activity concentration (Bqm-3) by an inbuilt microcontroller based
222
Rn growth was monitored till the concentration attained
Rn mass exhalation measurement using SRM. 222
197
The measured
Rn concentration, C(t), at time t is fitted to appropriate model to
198
extract the 222Rn exhalation from the sample as under (Sahoo et al., 2007),
C (t ) =
199
[
]
JmM 1 − e − λ e t + C 0 e − λ e t …. (1) V λe
200
where,
201
C0 is the 222Rn concentration (Bq m-3) in the chamber volume at t = 0,
10
202
M is the total mass of the dry sample (kg),
203
Jm is the mass exhalation rate (Bq kg-1 h-1),
204
V is the effective volume (m3) (i.e. volume of the container + volume of the
205
scintillation cell of the monitor -volume occupied by the sample),
206
λe is the effective decay constant for
222
Rn, which is the sum of the leak rate (if 222
Rn (h-1). It must be noted that
207
existing) and the radioactive decay constant of
208
current method automatically takes care of the leaks present in the system by
209
considering an effective leak rate constant.
210
t is the measurement time (h), and the Jm mass exhalation was determined from
211
equation 1.
212
213
2.3. Thoron exhalation measurement
214
The thoron monitor consists of two Scintillation Cells (Lucas cells) which are coupled
215
to separate photomultiplier tubes and associated pulse preamplifier and scalar. When
216
this monitor is operated in “thoron measurement mode” it excludes 222Rn interference
217
in the measurements. The detector has a sensitivity of 0.8 counts h-1/Bq m-3, minimum
218
detection limit of 15 Bq m-3, and can be used to measure concentration upto 100,000
219
Bq m-3 at 95 % confidence level. The
220
in Fig. 3 and it consists of air tight sample holder connected to a pump in a closed
221
loop. The uniformity of
220
Rn exhalation measurement setup is shown
220
Rn concentration was achieved by forced air mixing 11
222
through the external pump. The
223
state concentration of
224
al., 2011),
220
Rn exhalation was determined from the steady-
220
Rn inside chamber using the following equation (Gaware et
CT=Jm.M /V λT …..(2)
225 226
where,
227
CT is steady-state concentration of 220Rn in chamber (Bq m-3),
228
Jm is the mass exhalation (Bq kg-1 h-1),
229
V is effective volume (volume of sample chamber + volume of scintillation cell)
230
(m3),
231
M is the mass of the sample (kg), and
232
λT is radioactive decay constant for 220Rn (45.36 h-1).
233
The 220Rn leak rate effect is negligible when compared to radioactive decay and hence
234
λT overwhelmingly considered as the effective decay constant for the system.
235 236
2.4. Calibration and quality assurance
237 222
Rn and
220
238
The performance evaluation of the
239
frequent inter-comparison exercise was carried out at Radiological Physics and
240
Advisory division, BARC under controlled conditions in a chamber. Experiments
241
were also performed to compare the measurements of Scintillation Radon Monitors
242
against the commercially available system AlphaGuard. A fair agreement was seen
243
between, SRM and AlphaGuard (Karunakara et al., 2014b, Sudeep et al., 2012, 12
Rn monitors used in this study and
244
Sudeep et al., 2014). Similarly, in the case of thoron monitors, the measurement
245
results were validated by comparing them with those obtained using RAD-7
246
(manufactured by Durridge) monitor, which was calibrated by the manufacturer
247
against a primary standard. The 220Rn monitor was tested upto a concentration 2 MBq
248
m-3, the variations were within 3%. Regular performance evaluation and frequent
249
inter-comparison exercise were carried out to maintain the quality of the
250
measurements.
251 252
Several measurements were carried out to establish a reliable value for the instrument
253
background. The
254
300 min), in 60 min cycles before starting each experiment. The average background
255
value of
256
220
222
222
Rn monitor background was measured for a long duration (upto
Rn concentration was observed to be 10.9 ± 2.8 Bq m-3. In the case of
Rn monitor, the instrument background was measured for 240 min, in cycles of 15
257
min, and the average background value of
258
be 10.2 ± 4.3 Bq m-3.
220
Rn concentration was observed to
259 260
2.5. Radon and Thoron emanation coefficient
261
The emanation coefficient (f) or the release of
262
calculated as (Sahoo et al., 2007),
263 264 265
where,
222
f=Jm/Qλ ……(3)
13
Rn and
220
Rn from the grains is
266
Jm is the mass exhalation,
267
Q is the 226Ra content (Bq kg-1) and
268
λ is the decay constant of 222Rn (h-1) (Sakoda et al., 2010; Kanse et al., 2016).
269 270
3. Results and discussion
271
3.1. Radon exhalation rate
272
The
273
Neendakara, and Kovalam for different grain size fractions are shown in Fig. 4-7. The
274
influence of the grain size on
275
concentration was lowest in 1000 µm and increased in the subsequent grain size
276
fractions of 500- 250 µm, 250-125 µm, and <125 µm respectively. The highest
277
concentration upto 957 ± 11 Bq m-3 was obtained in <125 µm grains of Chavara
278
samples. The
279
sampling stations are listed in the Table 1. Although, there were variations in mass
280
exhalation values among different sampling stations, a similar trend was observed in
281
the concentration of 222Rn with respect to the grain size fractions.
222
Rn build-up curve observed in the sand of Karunagapalli, Chavara,
222
222
Rn concentration was very significant. The
222
Rn
Rn mass exhalation rates for the sample collected from all four
282 283
Grain sizes of <125 µm showed higher 222Rn mass exhalation rates ranging from 4.8
284
± 1.3 to 11.0 ± 0.9 mBq kg-1 h-1, whereas, 1000-500 µm had the lowest values,
285
ranging from 0.7 ± 0.2 to 1.9 ± 0.1 mBq kg-1 h-1 (Table 1). The
286
1000-500 µm grain sizes (ranging from 9.3 ± 0.8 to 51.8 ± 1.8 Bq kg-1) when
14
226
Ra was lower in
287
compared to the other smaller grain sizes. The 1000-500 µm grain sizes are mostly
288
the quartz, the largest component of the sand, and that it is originally not radioactive
289
(Sakoda et al., 2010). The higher
290
< 125 µm grain is because of its larger specific surface area and hence acquire more
291
radium ions by adsorption during the weathering process. The decrease of
292
subsequent fractions indicates the selective enrichment of radioactive monazite
293
minerals in the grain sizes (Primal et al., 2014). Correlation between
294
exhalation rate (mBq kg-1 h-1) and the
295
significant correlation was observed between
296
correlation coefficient of R= 0.84.
226
Ra (2395 ± 10 Bq kg-1 to 6923 ± 24 Bq kg-1) in
226
226
Ra in
222
Rn mass
Ra activity (Bq kg-1) was studied. A 222
Rn and
226
Ra concentration with a
297 298
Even though 226Ra content in the samples was much higher than world average value
299
of 45 Bq kg-1 (UNSCEAR, 2000). However, the
300
(Table 1) were found to be low for all the grain sizes when compared to the
301
representative value of 0.13 (range 0.00-0.40) for rock, 0.20 (range 0.00-0.80) for soil
302
and 0.03 (range 0.00-0.25) for minerals (Sakoda et al., 2011). This can be explained
303
based on the single grain model which was developed assuming uniform distribution
304
of
226
222
Rn significantly (Morawska and Philip, 1993; Sakoda et al., 2011). Moreover, the
305
222
Rn emanation coefficient values
Ra in the grain. According to this model grains size >10 µm do not emanate
306
low emanation fraction may be due to the short alpha recoil range of
307
The
222
Rn recoil range in sand is ~35 nm (Semkow, 1990). The 15
222
Rn in solids.
222
Rn atom gets
308
trapped inside the grain itself in large grain sizes, and are unavailable for release
309
unless internal surface area is developed due to chemical erosion, weathering or
310
intensive fracturing on a microscopic pathways created by radiation damage (Garver
311
and Baskaran, 2004). The grain sizes analysed in this study are much larger than the
312
alpha recoil from the decay of
313
outer 20 nm of a grain, implying that on a spherical grain of <40 nm diameter all the
314
222
226
Ra resulting in the direct release of
222
Rn from the
Rn will escape the mineral grain (Kigoshi, 1971).
315 316
Nevertheless, its been reported by Bossew, (2003) that 222Rn emanation in wet soil is
317
twice high than that for the dry soil and the
318
migrate out of the sample if the pore space is filled with water. The lower value of
319
emanation coefficient is also possibly due to the negligible moisture content in the
320
sand as the samples were oven dried before the analysis. Some of the reported values
321
on low
322
dry monazite samples of grain sizes between 100-200 µm and with
323
concentration of (223 ± 11)×102 Bq kg-1 , exhibited emanation fraction as low as
324
3.0×10-4 (Rama and Moore, 1984). Another study reported by Garver et al., (2004)
325
showed a emanation fraction of 5.3 ×10-3 for Uraninite samples with
326
concentration as high as (5829 ± 69)× 103 Bq kg-1.
222
Rn emanation for a high values on
327
16
222
Rn atom has a greater probabilty to
226
Ra are listed in Table 2. A study on 226
Ra activity
226
Ra
222
328
The interesting finding of this study is that the
329
increase with the decrease in grain size, rather it was higher in 1000-500 µm grains.
330
Therefore it appears that
331
predominantly quartz. However, the 226Ra content in this grain size was lower when
332
compared to the smaller grain size fractions and therefore it is not the highest
333
contributor of the 222Rn in the environment.
222
Rn emanation coefficient did not
Rn gets released better in 1000-500 µm, which are
334
335
3.2. Thoron exhalation rate
336
The
337
228
220
Rn is a daughter product of
228
Ra, the analysed samples showed very high
Ra activity (Table 3). The 220Rn concentration (Bq m-3) in four different grain size
338
fractions in samples from Karunagapalli, Chavara, Neendakara, and Kovalam are
339
shown in Fig. 8-11. It followed the similar trend as that of
340
m-3), i e., higher
341
subsequent grain fractions. The
342
equation 2 to get the mass exhalation rate (Bq kg-1h-1). Correlation between
343
mass exhalation rate (mBq kg-1 h-1) and the
344
this yielded a positively significant correlation coefficient of R= 0.79.
220
222
Rn concentration (Bq
Rn concentration (Bq m-3) in finer grain size and lower values in 220
Rn concentration (Bq m-3) values were fitted into
228
220
Rn
Ra activity (Bq kg-1) was studied and
345
346
The 220Rn mass exhalation (Bq kg-1h-1) and emanation coefficient are listed in table 3.
347
The mass exhalation was higher in Chavara samples when compared to other 17
348
locations. It is interesting to note that 220Rn emanation was increasing with increasing
349
grain sizes. In spite of
350
emanation coefficient is slightly higher when compared to that of
351
important to recollect the fact that emanation depends not only on grain size but also
352
solid density, fractal nature, etc. (Skoda et al., 2011). Apart from this, the distribution
353
of 228Ra and 226Ra in grains most likely to be inhomogeneous and it may not possible
354
to predict the
355 356
220
222
Rn and
220
Rn having similar recoil energies, the
220
Rn emanation coefficient from that of
222
222
220
Rn
Rn. It is
Rn. A very few reports on
Rn emanation coefficients are available in the literature though. The reported value
in beach placer samples varied from 4.0×10-4–3.9×10-2 (Kanse et al., 2016).
357 358
4. Conclusions
359
This study has evaluated the relative contribution of the monazite minerals to
360
and 220Rn release in the environment. The 222Rn and 220Rn mass exhalation (mBq kg-1
361
h-1) measurements have helped to understand the enrichment pattern in finer and
362
subsequent grain size fractions. The emanation coefficients of
363
increased from finer grain size to larger grains (1000-500 µm), perhaps due to the
364
better ability of the quartz to release these gases although these 1000-500 µm grain
365
sizes had lower
366
The lower values of emanation coeffients, when compared to the soil and sand
367
samples of normal background radiation areas, can be attributed to the lower recoil
368
range of these atoms. Most importantly, inspite of
226
Ra and
222
Rn and
222
Rn
220
Rn
228
Ra content when compared to the smaller grain sizes.
18
228
Ra activity in monazite placer
369
deposit being exceptionally higher the emanation coefficient is much smaller. The
370
statistically significant correlation coefficient substantiated the dependency of
371
on 228Ra and 222Rn on 226Ra
372
Acknowledgment
373
The authors are thankful to the Radiological Physics and Advisory Division (RP &
374
AD), Bhabha Atomic Research Centre, India for providing the training and resources.
375
One of the authors is grateful to the University Grant Commission (UGC)
376
Government of India for providing the financial support (RFSMS).
377
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220
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378 379
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546
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547
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548 549 550
27
551
List of figures
552
Fig. 1. Map shows the sampling stations.
553
Fig. 2. Set‐up for measurement of 222Rn emanation.
554
Fig. 3. Set‐up for measurement of 220Rn emanation.
555
Fig. 4. Radon build up in sample from Karunagapalli for 1000-500 µm, 500- 250 µm,
556 557 558 559 560 561 562 563 564
250-125 µm, and <125 µm grain size fractions. Fig. 5. Radon build up in sample from Chavara for 1000-500 µm, 500- 250 µm, 250125 µm, and <125 µm grain size fractions. Fig. 6. Radon build up in sample from Neendakara for 1000-500 µm, 500- 250 µm, 250-125 µm, and <125 µm grain size fractions. Fig. 7. Radon build up in sample from Kovalam for 1000-500 µm, 500- 250 µm, 250-125 µm, and <125 µm grain size fractions. Fig. 8. Thoron concentration in various grain sizes (samples from Karunagapalli region).
565
Fig. 9. Thoron concentration in various grain sizes (samples from Chavara region)
566
Fig. 10. Thoron concentration in various grain sizes (samples from Neendakara
567
region).
568
Fig. 11. Thoron concentration in various grain sizes (samples from Kovalam region)
569
Fig. 12. Correlation between
570
222
Rn mass exhalation rate (mBq kg-1 h-1) and
activity (Bq kg-1).
28
226
Ra
571 572
Fig 13. Correlation between
220
Rn mass exhalation rate (Bq kg-1 h-1) and
228
Ra
activity (Bq kg-1).
573 574
List of tables
575
Table 1
576
Radium-226 (Bq kg-1),
577
coefficient for various grain size fractions for samples collected from Karunagapalli
578
(KI), Chavara (CH), Neendakara (N), and Kovalam (KM) regions.
222
Rn mass exhalation (mBq kg-1 h-1), and
222
Rn emanation
579 580
Table 2
581
Comparison of
582
concentrations in the dry samples.
222
Rn emanation fractions for a high
226
Ra (Bq kg-1) activity
583 584
Table 3
585
Radium-228 (Bq kg-1),
586
coefficient for various grain size for samples collected from Karunagapalli (KI),
587
Chavara (CH), Neendakara (N), and Kovalam (KM) regions.
220
Rn mass exhalation (Bq kg-1h-1), and
588 589 590 591 592 593 594 29
220
Rn emanation
595 596 597 598
599 600
Fig. 1. Map shows the sampling stations.
601 602 603 30
604 605 606 607 608
609
610
Fig. 2. Set‐up for measurement of 222Rn emanation.
611
612
613
614
615
616 31
617
618
619
620 621
Fig. 3. Set‐up for measurement of 220Rn emanation.
622
32
623 624 625 626 627
628 629
Fig. 4. Radon build up in sample from Karunagapalli for 1000-500 µm, 500- 250
630
µm, 250-125 µm, and <125 µm grain size fractions.
631 632 633
33
634 635 636 637 638 639
640 641
Fig. 5. Radon build up in sample from Chavara for 1000-500 µm, 500- 250 µm,
642
250-125 µm and <125 µm grain size fractions.
643 644
34
645 646 647 648 649 650
651 652
Fig. 6. Radon build up in sample from Neendakara for 1000-500 µm, 500- 250 µm,
653
250-125 µm and <125 µm grain size fractions.
654 655 656 657 658 659 35
660 661 662 663
664 665
Fig. 7. Radon build up in sample from Kovalam for 1000-500 µm, 500- 250 µm,
666
250-125 µm and <125 µm grain size fractions.
667 668 669 670 671
36
672 673 674 675
676 677
Fig. 8. Thoron concentration in various grain sizes (samples from Karunagapalli
678
region).
679 680 681 682 683
37
684 685 686 687 688
689 690
Fig. 9. Thoron concentration in various grain sizes (sample from Chavara region).
691 692 693 694 695 696 697 38
698 699 700 701
702 703
Fig. 10. Thoron concentration in various grain sizes (samples from Neendakara
704
region).
705 706 707 708 709
39
710 711 712 713 714 715
716 717
Fig. 11. Thoron concentration in various grain sizes (samples from Kovalam region).
718 719 720 721 40
722 723 724 725 726 727 728 729
730 731
Fig. 12. Correlation between 222Rn mass exhalation rate (mBq kg-1 h-1) and 226Ra
732
activity (Bq kg-1).
733 734 735 736 41
737 738 739 740 741 742
743
744
Fig. 13. Correlation between 220Rn mass exhalation rate (Bq kg-1 h-1) and 228Ra
745
activity (Bq kg-1).
746
747
42
748 749 750 751 752 753 754
Table 1
755
Radium-226 (Bq kg-1), 222Rn mass exhalation (mBq kg-1h-1), and 222Rn emanation
756
coefficient for various grain size fractions for samples collected from Karunagapalli
757
(KI), Chavara (CH), Neendakara (N), and Kovalam (KM) regions.
758 759 222
Sample size 226 Ra (µm) with (Bq kg-1) Location ID KI <125 2395 ± 10 KI 250-125 328 ± 4 KI 500-250 82.8 ± 6.0 KI 1000-500 19.2 ± 1.1 CH <125 6816 ± 36 CH 250-125 1269 ± 11 CH 500-250 299 ± 2 CH 1000-500 44.6 ± 2.4 N <125 6923 ± 24 N 250-125 1394 ± 9 N 500-250 653 ± 6 N 1000-500 9.3 ± 0.8 KM <125 6146 ± 19 KM 250-125 224 ± 13 KM 500-250 29.9 ± 1.3 KM 1000-500 51.8 ± 1.8
Rn mass exhalation (mBq kg-1 h-1) 6.9 ± 0.9 3.9 ± 1.1 1.5 ± 0.4 0.7 ± 0.2 9.8 ± 1.1 3.1 ± 0.9 2.2 ± 0.7 1.0 ± 0.1 4.8 ± 1.3 2.9 ± 1.0 2.7 ± 1.3 1.9 ± 0.1 11.0 ± 0.9 3.5 ± 0.6 1.0 ± 0.2 0.8 ± 0.1
760 761 762 763 43
222
Rn emanation coefficient (40 ± 5)×10-5 (16 ± 4)×10-4 (24 ± 6)×10-4 (45 ± 11)×10-4 (20 ± 2) ×10-5 (30 ± 9)×10-5 (10 ± 3)×10-4 (30 ± 4)×10-4 (10 ± 3)×10-5 (3 ± 1)×10-4 (6 ± 3)×10-4 (26 ± 3)×10-3 (20 ± 2)×10-5 (20 ± 4)×10-4 (21 ± 3)×10-4 (33 ± 5)×10-4
764 765 766 767
Table 2
768
Comparison of
769
concentrations in the dry samples.
222
Rn emanation fractions for a high
226
Ra (Bq kg-1) activity
770
Samples
226
Ra (Bq kg-1)
222
Rn emanation fraction
References
Monazite grain size of 100-200 µm Zircon
(22.3 ± 1.1) × 103
3.0 ×10-4
(3.8 ± 2.9) × 103
1.0 ×10-4
Uraninite<63µm (Canada) Monazite <63 µm
(58.3 ± 0.7)× 105
5.3 ×10-3
(17.6 ± 0.4) ×103
(20.5 ± 0.3)×10-3
Zircon+Monazite 100-200 µm
(26.5 ± 1.3) ×103
5.0 ×10-4
Rama and Moore, 1984 Rama and Moore, 1990 Garver and Baskaran, 2004 Garver and Baskaran, 2004 Rama and Moore, 1984
Monazite grain size 125-1000 µm
9.3 ± 0.8 - 6923 ± 24 (range)
(10.0 ± 2.8)×10-5 (26.0 ± 2.7)×10-3 (range)
771
772 773 774 775 776
44
Present Study
777 778 779 780 781
Table 3
782
Radium-228 (Bq kg-1),
783
coefficient for various grain size of Karunagapalli (KI), Chavara (CH), Neendakara
784
(N) and Kovalam (KM).
220
Rn mass exhalation (Bq kg-1h-1), and
220
Rn emanation
785
Sample size (µm) with Location ID KI <125 KI 250-125 KI 500-250 KI 1000-500 CH <125 CH 250-125 CH 500-250 CH 1000-500 N <125 N 250-125 N 500-250 N 1000-500 KM <125 KM 250-125 KM 500-250 KM 1000-500
220
228
Ra (Bq kg-1) 11279 ± 39 793 ± 9 186 ± 2 44.6 ± 1.8 (44.38 ± 0.21) ×103 6224 ± 40 1189 ± 8 212 ± 4 (41.77 ± 0.12) ×103 5994 ± 35 1893 ± 17 35.0 ± 1.9 34731 ± 92 (2.75 ± 0.14) ×103 265 ± 5 107 ± 4
Rn mass exhalation (Bq kg-1 h-1) 965 ± 3 669 ± 7 354 ± 4 94.0 ± 3.8 3066 ± 14 2456 ± 16 441 ± 3 191 ± 4 1877 ± 5 770 ± 5 494 ± 4 88.5 ± 4.8 2076 ± 6 1597 ± 83 251 ± 5 101 ± 3
45
220
786
Rn emanation 787 coefficient (200 ± 1)×10-5 -4 (190 ± 2)×10788 (420 ± 5)×10-4 (46 ± 2) ×10-3 (200 ± 1)×10-5 (90 ± 1)×10-4 (80 ± 1)×10-4 (200 ± 4)×10-4 (100 ± 3)×10-5 (300 ± 2)×10-5 (60 ± 1)×10-4 (56 ± 3)×10-3 (1000 ± 3)×10-6 (13 ± 1)×10-3 (210 ± 4)×10-4 (210 ± 7)×10-4
HIGHLIGHTS •
This study throws light on the relative contribution of the monazite minerals to 222Rn and 220
•
Rn release in the environment.
Monazite rich samples have low emantion coefficient and are not the highest contributor of 222Rn and 220Rn to the environment.
•
Radon and thoron emanation coefficient was higher in 1000-500 µm grain size fractions, which contained least 226Ra and 228Ra activity (Bq kg-1).
•
Unlike 226Ra and 228Ra activity concentrations, 222Rn and 220Rn emanation coefficient did not increase with the finer grain size; rather, opposite trend was observed.
•
The statistically significant correlation coefficient substantiated the dependency of mass exhalation rate (Bq kg-1 h-1) and
228
Ra activity (Bq kg-1), also between
exhalation rate (mBq kg-1 h-1) and 226Ra activity (Bq kg-1).
222
220
Rn
Rn mass
S1350-4487(19)30506-2
DOI:
https://doi.org/10.1016/j.radmeas.2019.106220
Reference:
RM 106220
To appear in:
Radiation Measurements
Received Date: 10 August 2018 Revised Date:
7 September 2019
Accepted Date: 13 November 2019
Please cite this article as: Pinto, P.V., Sudeep Kumara, K., Karunakara, N., Mass exhalation rates, emanation coefficients and enrichment pattern of radon, thoron in various grain size fractions of monazite rich beach placers, Radiation Measurements, https://doi.org/10.1016/j.radmeas.2019.106220. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.
1
Mass exhalation rates, emanation coefficients and
2
enrichment pattern of radon, thoron in various grain size
3
fractions of monazite rich beach placers
4
Primal V. Pinto1, 3*, Sudeep Kumara K.2 and Karunakara N.2
5 6
¹Science program, Texas A & M University, Qatar
7 8 9
2
Centre for Advanced Research in Environmental Radioactivity (CARER),
Mangalore University, Mangalagangothri – 574199, India
10 11
3
Department of Studies in Physics, Mangalore University, Mangalagangothri-574199,
12
India
13 14
*
15
Corresponding author Email: [email protected] [email protected]
16
17
18
19
20
21 1
22 23 24
Highlights •
to 222Rn and 220Rn release in the environment.
25 26
•
•
Radon and thoron emanation coefficient was higher in 1000-500 µm grain size fractions, which contained least 226Ra and 228Ra activity (Bq kg-1).
29 30
Monazite rich samples have low emantion coefficient and are not the highest contributor of 222Rn and 220Rn to the environment.
27 28
This study throws light on the relative contribution of the monazite minerals
•
Unlike
226
Ra and
228
Ra activity concentrations,
222
Rn and
220
Rn emanation
31
coefficient did not increase with the finer grain size; rather, opposite trend was
32
observed.
33
•
The
statistically
significant
correlation
coefficient
34
dependency of 220Rn mass exhalation rate (Bq kg-1 h-1) and
35
kg-1), also between
36
activity (Bq kg-1).
222
substantiated 228
Ra activity (Bq
Rn mass exhalation rate (mBq kg-1 h-1) and
37 38
39
40
41
2
the
226
Ra
42
Abstract
43
A study on exhalation rates of
44
different grain size fractions was carried out. Four different size fractions of the sand
45
samples (1000-500 µm, 500-250 µm, 250-125 µm, and <125 µm) collected from the
46
beach placers of south west coast of India were analyzed for 226Ra and
47
concentrations
48
measurement technique using Scintillation cell based monitors were used to measure
49
the
50
appropriate model to extract the mass exhalation and emanation coefficients. The
51
222
222
Rn and
(Bq
220
kg-1)
222
Rn and
by gamma
220
Rn and their enrichment pattern in
spectrometry.
232
Automated
Th activity continuous
Rn concentrations. These measured values were then fitted to
Rn mass exhalation rate varied from 0.7 ± 0.2 mBq kg-1 h-1 to 11.0 ± 0.9 mBq kg-1
52
h-1 while that of 220Rn varied from 88.5 ± 4.8 Bq kg-1 h-1 to 3066 ± 14 Bq kg-1 h-1. The
53
higher mass exhalation rates, for both radioactive gases, were observed in finer grain
54
size fractions. However, this was in contrast to the emanation coefficient values.
55
Interestingly, higher radioactive monazite rich beach samples exhibited low
56
emanation coefficient values when compared to the low radioactive soil and sand
57
samples reported in the literature. Statistical analysis showed positive correlation,
58
with correlation coefficient R=0.84 and R=0.79 respectively between mass exhalation
59
rates of
60
size fractions.
222
Rn,
220
Rn with their respective radioactive predecessor for different grain
3
222
Rn and
220
61
Keywords:
Rn, emanation coefficient, mass exhalation, monazite, High
62
Background Radiation Area (HBRA).
63 64
1. Introduction
65
The beach placers deposits along the south-west coast of India are particularly rich in
66
monazite. It is radioactive due to the presence of naturally occurring Thorium and
67
Uranium (Mohanty et al., 2003; Shetty and Narayana, 2007). As a daughter product
68
of Uranium, the release of
69
of emanation followed by exhalation. The emanation can be defined as the escape of
70
a
71
mechanism of emanation and transport of 222Rn and
72
effect of the short half-life (55.6s) of 220Rn (Kanse et al., 2013).
222
Rn atom from a
222
Rn gas to the environment takes place by the processes
226
Ra-bearing grain into pore spaces (Sakoda et al., 2011). The 220
Rn is the same, except for the
73
222
74
The phenomenon of
75
parameters such as grain size, porosity and water content. These determine the static
76
emanation coefficient which does not change with time, so the type of soil determines
77
the general
78
controls 222Rn exhalation rate showed that the emanation fractions first decreases and
79
later reaches a constant value with the increase in grain size (Sakoda et al., 2011;
80
Barillon et al., 2005). The typical values reported for the radon emanation coefficient
222
Rn emanation and then exhalation depends on several
Rn level in the soil (Chauhan, 2011). The effect of grain size that
4
81
for dry soil is 0.1 (range 0.01- 0.5) and building material is 0.05 (ranges 0.005-0.3)
82
(Porstendorfer, 1991). A representative value of 0.2 is suggested in UNSCEAR
83
(1988) for the soil with the range 0.01-0.8. The 222Rn mass exhalation rate of 24 ± 12
84
mBq kg−1 h−1 reported in soil samples and 18.8 ± 6.4 mBq kg−1 h−1 for the black sand
85
(Sahoo et al., (2007).
86
87
Radium-226, a source of 222Rn present in rocks, soils and construction materials leads
88
to the accumulation of
89
(Galan Lopez et al., 2004). Nevertheless,
90
can easily enter into the ground water by the effect of lithostatic pressure, easily get
91
released to indoor air when used in showers, humidifiers, cooking and so on.
92
Exposure to water borne
93
222
222
Rn in poorly ventilated houses, pose potential health risk
222
222
Rn from the bedrocks containing
226
Ra
Rn may occur by ingestion (drinking water containing
Rn) and by inhalation (Toscani et al., 2001; Mahesh et. al., 2001; Primal et al., 222
94
2012). The exhalation rate of
95
sands, rocks, construction materials, uranium ores and tailings have been the subject
96
of several studies (Somashekarappa et al., 1996; Jha et al., 2000; Evangelista and
97
Pereira et al., 2002; El-Amri et al., 2003). Such studies are used to assess the
98
radiation dose to the public and to estimate local environmental
99
exhalation rate of 222Rn is one of the critical factors that influence indoor and outdoor
100
222
Rn from numerous sources such as soils, mineral
Rn concentrations (Karunakara et al., 2005b; Mayya, 2004).
5
222
Rn levels. The
101 222
Unlike
103
reported, and these are; Ganesh et al. (2008) for India, Kovacs (2010) for Hungary,
104
Harley et al. (2010) for the USA and Yasuoka et al. (2010) for Japan. Thoron
105
exhalation rates of 1.7 ± 1.2 (range 0.19–4.2 Bq m-2 s-1) and 2.4 ± 1.5 (range 0.04–6.2
106
Bq m-2 s-1) were reported for the red and yellow soil surface and the Dark red soil,
107
respectively (Hosoda et al., 2010). The 220Rn level in the environment is governed by
108
its emanation from the soil or building materials containing 232Th (Kanse et al., 2016;
109
Shimo et al., 2010; Shiroma et al., 2010). Jonas et al., (2016) have reported that the
110
220
Rn, a very few studies on
220
102
Rn in the living environments have been
Rn emanation factor could not be predicted from the 222Rn emanation factor.
111 112
Wastes and tailings from Thorium bearing ores processed for metals can potentially
113
release significant
114
shale are likely to have high Thorium content. Monazite and zircon sands have an
115
extraordinarily high concentration of Thorium. Thorium is widely distributed in
116
nature with an average concentration of 10 ppm in the earth’s crust in many
117
phosphates, silicates, carbonates and oxide minerals (Ramachandran, 2010). In
118
general, Thorium occurs in association with Uranium and Rare Earth Element (REE)
119
in diverse rock types. In India, the High natural Background Radiation Areas (HBRA)
120
are identified at certain regions of coastal Kerala and Orissa due to an elevated
121
content in monazite sands (Mishra, 1993; Nambi et al., 1994; Primal and Narayana,
220
Rn (Polednak et al., 1983). Rocks composed of granite or black
6
232
Th
122
2014). Southern coast of Brazil and China are among the other numerous locations,
123
known for high levels natural radiation (Paschoa, 2000; Wei and Sugahara, 2000; Tao
124
et al., 2012). Because of their high density, monazite minerals get concentrated in
125
sands along the coastal regions.
126 127 128
The exhalation being an essential parameter for describing the potential 220
222
Rn and
Rn release from a solid matrix, in particular in HBRA. Hence, a detailed study on
129
these aspects were carried out in the samples collected from the HBRA of the
130
coastline of Kerala such as Karunagapalli, Chavara, Neendakara, and Kovalam (Fig.
131
1). This study is an effort to evaluate the contribution of monazite rich sand to the
132
elevated levels of
133
emanation coefficient and the effect of grain size on the release of
134
from the solid matrix to the surroundings. The study area has a significant resident
135
population exposed to higher levels of
136
Ramachandran and Sahoo, 2009; Mayya et al., 2012).
226
Ra and
228
Ra concentrations,
222
Rn and
220
Rn exhalation,
222
Rn and
220
Rn
220
Rn (Chougaonkar et al., 2004;
137
138
2. Materials and methods
139
Sand samples were collected from the HBRA beach placers were oven dried at 110ºC
140
till the constant dry weight was obtained. The dried samples were sieved into four
141
different grain size fractions; viz. 1000-500 µm, 500-250 µm, 250-125 µm, and <125
142
µm. The sieved samples were subjected to gamma spectrometry (section 2.1) to 7
226
Ra and
232
143
determine
Th activity concentrations. The methods adopted for the
144
exhalation measurements are discussed in sections 2.2 and 2.3.
145
146
2.1 Gamma-ray spectrometry
147
The samples were counted using a high-resolution, co-axial n-type high purity HPGe
148
detector (model GR 4021 Canberra USA) with 42% relative efficiency. This facility
149
was available at Centre for Advanced Research in Environmental Radioactivity
150
(CARER), Mangalore University. The efficiency calibration of the detector was
151
performed using IAEA quality assurance reference materials: RG U-238, RG Th-232,
152
RG K-1, and SOIL-6. The samples were taken in 300 ml air tight polypropylene box.
153
The standard materials and samples were taken in containers of the same size and
154
type so that the detection geometry remained the same. The samples of different grain
155
sizes were counted long enough (30,000 s) to reduce the counting error. The activity
156
concentrations of 226Ra, 232Th, and 40K in various samples were determined using the
157
Gennie-2000 spectrum analyses software. The activity of 40K was evaluated from the
158
1461 keV photopeak, the activity of 226Ra from the weighted mean of the activities of
159
three photopeaks of
160
determined from the photopeaks of
161
background counts and applying the compton correction (Karunakara et al., 2005a,
162
Karunakara et al., 2013, Karunakara et al., 2014a, Yashodhara et al., 2011). The
214
Bi (609.3, 1120.4, and 1764.5 keV), and that of 228
8
228
Ra was
Ac (911.2 keV) after subtracting the
163
Minimum Detection Levels (MDL) for the gamma spectrometry system used in the
164
present study were 0.9, 1.2, and 4.06 Bq kg-1 for 226Ra, 232Th, and 40K respectively at
165
95% confidence level.
166
167
2.2. Radon exhalation measurement
168
Mass exhalation rate measurement requires continuous online
169
this was performed using a Scintillation Radon Monitor (SRM) developed by
170
Radiological Physics and Advisory Division (RP & AD), Bhabha Atomic Research
171
Centre (BARC). It is an integrated microprocessor-based system consisting of a
172
collection chamber, a conventional ZnS:Ag scintillator detector and a counting set up.
173
The SRM records
174
temperature and humidity values. In the present study it was operated in 60 min cycle.
175
Experiments were carried in an air-conditioned room so that a uniform ambient
176
temperature and humidity condition is maintained for all the measurements. In order
177
to obtain the results under a unified experimental conditions, all the samples were
178
dried at 1100C to obtain a constant weight and the dried samples were used for
179
measurements.
222
222
Rn monitoring and
Rn concentration for each measurement along with ambient
180 181
A known quantity of dried sample was taken in a leak proof container having inlet
182
and outlet valves. The outlet of container was connected to the inlet of the SRM
9
183
through an air pump. The outlet of the SRM was connected to the inlet of the sample
184
container. A pump was used to circulate the air between sample container volume and
185
detector. The SRM monitor consists of a “progeny filter” and “thoron discriminator”
186
to eliminate
187
progeny formed inside the cell were continuously counted (60 mins cycle) and are
188
converted to
189
smart algorithm. Since the technology is purely based on direct scintillation counting
190
of an alpha particle with build-up/decay corrections achieved through software, the
191
measurements were not affected by humidity or trace gases (Sahoo et al., 2007;
192
Gaware et al., 2011). The
193
a saturation value. The SRM has a sensitivity factor of 1.2 counts h-1/Bq m-3 and an
194
upper detection limit of 50 MBq m-3. The minimum detection limit of SRM is 14
195
Bq/m3 at 95% confidence with 1 hr counting. Fig. 2 shows the arrangements for the
196
222
222
Rn progenies and
222
220
Rn. The alpha scintillations from
222
Rn and its
Rn activity concentration (Bqm-3) by an inbuilt microcontroller based
222
Rn growth was monitored till the concentration attained
Rn mass exhalation measurement using SRM. 222
197
The measured
Rn concentration, C(t), at time t is fitted to appropriate model to
198
extract the 222Rn exhalation from the sample as under (Sahoo et al., 2007),
C (t ) =
199
[
]
JmM 1 − e − λ e t + C 0 e − λ e t …. (1) V λe
200
where,
201
C0 is the 222Rn concentration (Bq m-3) in the chamber volume at t = 0,
10
202
M is the total mass of the dry sample (kg),
203
Jm is the mass exhalation rate (Bq kg-1 h-1),
204
V is the effective volume (m3) (i.e. volume of the container + volume of the
205
scintillation cell of the monitor -volume occupied by the sample),
206
λe is the effective decay constant for
222
Rn, which is the sum of the leak rate (if 222
Rn (h-1). It must be noted that
207
existing) and the radioactive decay constant of
208
current method automatically takes care of the leaks present in the system by
209
considering an effective leak rate constant.
210
t is the measurement time (h), and the Jm mass exhalation was determined from
211
equation 1.
212
213
2.3. Thoron exhalation measurement
214
The thoron monitor consists of two Scintillation Cells (Lucas cells) which are coupled
215
to separate photomultiplier tubes and associated pulse preamplifier and scalar. When
216
this monitor is operated in “thoron measurement mode” it excludes 222Rn interference
217
in the measurements. The detector has a sensitivity of 0.8 counts h-1/Bq m-3, minimum
218
detection limit of 15 Bq m-3, and can be used to measure concentration upto 100,000
219
Bq m-3 at 95 % confidence level. The
220
in Fig. 3 and it consists of air tight sample holder connected to a pump in a closed
221
loop. The uniformity of
220
Rn exhalation measurement setup is shown
220
Rn concentration was achieved by forced air mixing 11
222
through the external pump. The
223
state concentration of
224
al., 2011),
220
Rn exhalation was determined from the steady-
220
Rn inside chamber using the following equation (Gaware et
CT=Jm.M /V λT …..(2)
225 226
where,
227
CT is steady-state concentration of 220Rn in chamber (Bq m-3),
228
Jm is the mass exhalation (Bq kg-1 h-1),
229
V is effective volume (volume of sample chamber + volume of scintillation cell)
230
(m3),
231
M is the mass of the sample (kg), and
232
λT is radioactive decay constant for 220Rn (45.36 h-1).
233
The 220Rn leak rate effect is negligible when compared to radioactive decay and hence
234
λT overwhelmingly considered as the effective decay constant for the system.
235 236
2.4. Calibration and quality assurance
237 222
Rn and
220
238
The performance evaluation of the
239
frequent inter-comparison exercise was carried out at Radiological Physics and
240
Advisory division, BARC under controlled conditions in a chamber. Experiments
241
were also performed to compare the measurements of Scintillation Radon Monitors
242
against the commercially available system AlphaGuard. A fair agreement was seen
243
between, SRM and AlphaGuard (Karunakara et al., 2014b, Sudeep et al., 2012, 12
Rn monitors used in this study and
244
Sudeep et al., 2014). Similarly, in the case of thoron monitors, the measurement
245
results were validated by comparing them with those obtained using RAD-7
246
(manufactured by Durridge) monitor, which was calibrated by the manufacturer
247
against a primary standard. The 220Rn monitor was tested upto a concentration 2 MBq
248
m-3, the variations were within 3%. Regular performance evaluation and frequent
249
inter-comparison exercise were carried out to maintain the quality of the
250
measurements.
251 252
Several measurements were carried out to establish a reliable value for the instrument
253
background. The
254
300 min), in 60 min cycles before starting each experiment. The average background
255
value of
256
220
222
222
Rn monitor background was measured for a long duration (upto
Rn concentration was observed to be 10.9 ± 2.8 Bq m-3. In the case of
Rn monitor, the instrument background was measured for 240 min, in cycles of 15
257
min, and the average background value of
258
be 10.2 ± 4.3 Bq m-3.
220
Rn concentration was observed to
259 260
2.5. Radon and Thoron emanation coefficient
261
The emanation coefficient (f) or the release of
262
calculated as (Sahoo et al., 2007),
263 264 265
where,
222
f=Jm/Qλ ……(3)
13
Rn and
220
Rn from the grains is
266
Jm is the mass exhalation,
267
Q is the 226Ra content (Bq kg-1) and
268
λ is the decay constant of 222Rn (h-1) (Sakoda et al., 2010; Kanse et al., 2016).
269 270
3. Results and discussion
271
3.1. Radon exhalation rate
272
The
273
Neendakara, and Kovalam for different grain size fractions are shown in Fig. 4-7. The
274
influence of the grain size on
275
concentration was lowest in 1000 µm and increased in the subsequent grain size
276
fractions of 500- 250 µm, 250-125 µm, and <125 µm respectively. The highest
277
concentration upto 957 ± 11 Bq m-3 was obtained in <125 µm grains of Chavara
278
samples. The
279
sampling stations are listed in the Table 1. Although, there were variations in mass
280
exhalation values among different sampling stations, a similar trend was observed in
281
the concentration of 222Rn with respect to the grain size fractions.
222
Rn build-up curve observed in the sand of Karunagapalli, Chavara,
222
222
Rn concentration was very significant. The
222
Rn
Rn mass exhalation rates for the sample collected from all four
282 283
Grain sizes of <125 µm showed higher 222Rn mass exhalation rates ranging from 4.8
284
± 1.3 to 11.0 ± 0.9 mBq kg-1 h-1, whereas, 1000-500 µm had the lowest values,
285
ranging from 0.7 ± 0.2 to 1.9 ± 0.1 mBq kg-1 h-1 (Table 1). The
286
1000-500 µm grain sizes (ranging from 9.3 ± 0.8 to 51.8 ± 1.8 Bq kg-1) when
14
226
Ra was lower in
287
compared to the other smaller grain sizes. The 1000-500 µm grain sizes are mostly
288
the quartz, the largest component of the sand, and that it is originally not radioactive
289
(Sakoda et al., 2010). The higher
290
< 125 µm grain is because of its larger specific surface area and hence acquire more
291
radium ions by adsorption during the weathering process. The decrease of
292
subsequent fractions indicates the selective enrichment of radioactive monazite
293
minerals in the grain sizes (Primal et al., 2014). Correlation between
294
exhalation rate (mBq kg-1 h-1) and the
295
significant correlation was observed between
296
correlation coefficient of R= 0.84.
226
Ra (2395 ± 10 Bq kg-1 to 6923 ± 24 Bq kg-1) in
226
226
Ra in
222
Rn mass
Ra activity (Bq kg-1) was studied. A 222
Rn and
226
Ra concentration with a
297 298
Even though 226Ra content in the samples was much higher than world average value
299
of 45 Bq kg-1 (UNSCEAR, 2000). However, the
300
(Table 1) were found to be low for all the grain sizes when compared to the
301
representative value of 0.13 (range 0.00-0.40) for rock, 0.20 (range 0.00-0.80) for soil
302
and 0.03 (range 0.00-0.25) for minerals (Sakoda et al., 2011). This can be explained
303
based on the single grain model which was developed assuming uniform distribution
304
of
226
222
Rn significantly (Morawska and Philip, 1993; Sakoda et al., 2011). Moreover, the
305
222
Rn emanation coefficient values
Ra in the grain. According to this model grains size >10 µm do not emanate
306
low emanation fraction may be due to the short alpha recoil range of
307
The
222
Rn recoil range in sand is ~35 nm (Semkow, 1990). The 15
222
Rn in solids.
222
Rn atom gets
308
trapped inside the grain itself in large grain sizes, and are unavailable for release
309
unless internal surface area is developed due to chemical erosion, weathering or
310
intensive fracturing on a microscopic pathways created by radiation damage (Garver
311
and Baskaran, 2004). The grain sizes analysed in this study are much larger than the
312
alpha recoil from the decay of
313
outer 20 nm of a grain, implying that on a spherical grain of <40 nm diameter all the
314
222
226
Ra resulting in the direct release of
222
Rn from the
Rn will escape the mineral grain (Kigoshi, 1971).
315 316
Nevertheless, its been reported by Bossew, (2003) that 222Rn emanation in wet soil is
317
twice high than that for the dry soil and the
318
migrate out of the sample if the pore space is filled with water. The lower value of
319
emanation coefficient is also possibly due to the negligible moisture content in the
320
sand as the samples were oven dried before the analysis. Some of the reported values
321
on low
322
dry monazite samples of grain sizes between 100-200 µm and with
323
concentration of (223 ± 11)×102 Bq kg-1 , exhibited emanation fraction as low as
324
3.0×10-4 (Rama and Moore, 1984). Another study reported by Garver et al., (2004)
325
showed a emanation fraction of 5.3 ×10-3 for Uraninite samples with
326
concentration as high as (5829 ± 69)× 103 Bq kg-1.
222
Rn emanation for a high values on
327
16
222
Rn atom has a greater probabilty to
226
Ra are listed in Table 2. A study on 226
Ra activity
226
Ra
222
328
The interesting finding of this study is that the
329
increase with the decrease in grain size, rather it was higher in 1000-500 µm grains.
330
Therefore it appears that
331
predominantly quartz. However, the 226Ra content in this grain size was lower when
332
compared to the smaller grain size fractions and therefore it is not the highest
333
contributor of the 222Rn in the environment.
222
Rn emanation coefficient did not
Rn gets released better in 1000-500 µm, which are
334
335
3.2. Thoron exhalation rate
336
The
337
228
220
Rn is a daughter product of
228
Ra, the analysed samples showed very high
Ra activity (Table 3). The 220Rn concentration (Bq m-3) in four different grain size
338
fractions in samples from Karunagapalli, Chavara, Neendakara, and Kovalam are
339
shown in Fig. 8-11. It followed the similar trend as that of
340
m-3), i e., higher
341
subsequent grain fractions. The
342
equation 2 to get the mass exhalation rate (Bq kg-1h-1). Correlation between
343
mass exhalation rate (mBq kg-1 h-1) and the
344
this yielded a positively significant correlation coefficient of R= 0.79.
220
222
Rn concentration (Bq
Rn concentration (Bq m-3) in finer grain size and lower values in 220
Rn concentration (Bq m-3) values were fitted into
228
220
Rn
Ra activity (Bq kg-1) was studied and
345
346
The 220Rn mass exhalation (Bq kg-1h-1) and emanation coefficient are listed in table 3.
347
The mass exhalation was higher in Chavara samples when compared to other 17
348
locations. It is interesting to note that 220Rn emanation was increasing with increasing
349
grain sizes. In spite of
350
emanation coefficient is slightly higher when compared to that of
351
important to recollect the fact that emanation depends not only on grain size but also
352
solid density, fractal nature, etc. (Skoda et al., 2011). Apart from this, the distribution
353
of 228Ra and 226Ra in grains most likely to be inhomogeneous and it may not possible
354
to predict the
355 356
220
222
Rn and
220
Rn having similar recoil energies, the
220
Rn emanation coefficient from that of
222
222
220
Rn
Rn. It is
Rn. A very few reports on
Rn emanation coefficients are available in the literature though. The reported value
in beach placer samples varied from 4.0×10-4–3.9×10-2 (Kanse et al., 2016).
357 358
4. Conclusions
359
This study has evaluated the relative contribution of the monazite minerals to
360
and 220Rn release in the environment. The 222Rn and 220Rn mass exhalation (mBq kg-1
361
h-1) measurements have helped to understand the enrichment pattern in finer and
362
subsequent grain size fractions. The emanation coefficients of
363
increased from finer grain size to larger grains (1000-500 µm), perhaps due to the
364
better ability of the quartz to release these gases although these 1000-500 µm grain
365
sizes had lower
366
The lower values of emanation coeffients, when compared to the soil and sand
367
samples of normal background radiation areas, can be attributed to the lower recoil
368
range of these atoms. Most importantly, inspite of
226
Ra and
222
Rn and
222
Rn
220
Rn
228
Ra content when compared to the smaller grain sizes.
18
228
Ra activity in monazite placer
369
deposit being exceptionally higher the emanation coefficient is much smaller. The
370
statistically significant correlation coefficient substantiated the dependency of
371
on 228Ra and 222Rn on 226Ra
372
Acknowledgment
373
The authors are thankful to the Radiological Physics and Advisory Division (RP &
374
AD), Bhabha Atomic Research Centre, India for providing the training and resources.
375
One of the authors is grateful to the University Grant Commission (UGC)
376
Government of India for providing the financial support (RFSMS).
377
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220
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378 379
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541
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546
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547
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548 549 550
27
551
List of figures
552
Fig. 1. Map shows the sampling stations.
553
Fig. 2. Set‐up for measurement of 222Rn emanation.
554
Fig. 3. Set‐up for measurement of 220Rn emanation.
555
Fig. 4. Radon build up in sample from Karunagapalli for 1000-500 µm, 500- 250 µm,
556 557 558 559 560 561 562 563 564
250-125 µm, and <125 µm grain size fractions. Fig. 5. Radon build up in sample from Chavara for 1000-500 µm, 500- 250 µm, 250125 µm, and <125 µm grain size fractions. Fig. 6. Radon build up in sample from Neendakara for 1000-500 µm, 500- 250 µm, 250-125 µm, and <125 µm grain size fractions. Fig. 7. Radon build up in sample from Kovalam for 1000-500 µm, 500- 250 µm, 250-125 µm, and <125 µm grain size fractions. Fig. 8. Thoron concentration in various grain sizes (samples from Karunagapalli region).
565
Fig. 9. Thoron concentration in various grain sizes (samples from Chavara region)
566
Fig. 10. Thoron concentration in various grain sizes (samples from Neendakara
567
region).
568
Fig. 11. Thoron concentration in various grain sizes (samples from Kovalam region)
569
Fig. 12. Correlation between
570
222
Rn mass exhalation rate (mBq kg-1 h-1) and
activity (Bq kg-1).
28
226
Ra
571 572
Fig 13. Correlation between
220
Rn mass exhalation rate (Bq kg-1 h-1) and
228
Ra
activity (Bq kg-1).
573 574
List of tables
575
Table 1
576
Radium-226 (Bq kg-1),
577
coefficient for various grain size fractions for samples collected from Karunagapalli
578
(KI), Chavara (CH), Neendakara (N), and Kovalam (KM) regions.
222
Rn mass exhalation (mBq kg-1 h-1), and
222
Rn emanation
579 580
Table 2
581
Comparison of
582
concentrations in the dry samples.
222
Rn emanation fractions for a high
226
Ra (Bq kg-1) activity
583 584
Table 3
585
Radium-228 (Bq kg-1),
586
coefficient for various grain size for samples collected from Karunagapalli (KI),
587
Chavara (CH), Neendakara (N), and Kovalam (KM) regions.
220
Rn mass exhalation (Bq kg-1h-1), and
588 589 590 591 592 593 594 29
220
Rn emanation
595 596 597 598
599 600
Fig. 1. Map shows the sampling stations.
601 602 603 30
604 605 606 607 608
609
610
Fig. 2. Set‐up for measurement of 222Rn emanation.
611
612
613
614
615
616 31
617
618
619
620 621
Fig. 3. Set‐up for measurement of 220Rn emanation.
622
32
623 624 625 626 627
628 629
Fig. 4. Radon build up in sample from Karunagapalli for 1000-500 µm, 500- 250
630
µm, 250-125 µm, and <125 µm grain size fractions.
631 632 633
33
634 635 636 637 638 639
640 641
Fig. 5. Radon build up in sample from Chavara for 1000-500 µm, 500- 250 µm,
642
250-125 µm and <125 µm grain size fractions.
643 644
34
645 646 647 648 649 650
651 652
Fig. 6. Radon build up in sample from Neendakara for 1000-500 µm, 500- 250 µm,
653
250-125 µm and <125 µm grain size fractions.
654 655 656 657 658 659 35
660 661 662 663
664 665
Fig. 7. Radon build up in sample from Kovalam for 1000-500 µm, 500- 250 µm,
666
250-125 µm and <125 µm grain size fractions.
667 668 669 670 671
36
672 673 674 675
676 677
Fig. 8. Thoron concentration in various grain sizes (samples from Karunagapalli
678
region).
679 680 681 682 683
37
684 685 686 687 688
689 690
Fig. 9. Thoron concentration in various grain sizes (sample from Chavara region).
691 692 693 694 695 696 697 38
698 699 700 701
702 703
Fig. 10. Thoron concentration in various grain sizes (samples from Neendakara
704
region).
705 706 707 708 709
39
710 711 712 713 714 715
716 717
Fig. 11. Thoron concentration in various grain sizes (samples from Kovalam region).
718 719 720 721 40
722 723 724 725 726 727 728 729
730 731
Fig. 12. Correlation between 222Rn mass exhalation rate (mBq kg-1 h-1) and 226Ra
732
activity (Bq kg-1).
733 734 735 736 41
737 738 739 740 741 742
743
744
Fig. 13. Correlation between 220Rn mass exhalation rate (Bq kg-1 h-1) and 228Ra
745
activity (Bq kg-1).
746
747
42
748 749 750 751 752 753 754
Table 1
755
Radium-226 (Bq kg-1), 222Rn mass exhalation (mBq kg-1h-1), and 222Rn emanation
756
coefficient for various grain size fractions for samples collected from Karunagapalli
757
(KI), Chavara (CH), Neendakara (N), and Kovalam (KM) regions.
758 759 222
Sample size 226 Ra (µm) with (Bq kg-1) Location ID KI <125 2395 ± 10 KI 250-125 328 ± 4 KI 500-250 82.8 ± 6.0 KI 1000-500 19.2 ± 1.1 CH <125 6816 ± 36 CH 250-125 1269 ± 11 CH 500-250 299 ± 2 CH 1000-500 44.6 ± 2.4 N <125 6923 ± 24 N 250-125 1394 ± 9 N 500-250 653 ± 6 N 1000-500 9.3 ± 0.8 KM <125 6146 ± 19 KM 250-125 224 ± 13 KM 500-250 29.9 ± 1.3 KM 1000-500 51.8 ± 1.8
Rn mass exhalation (mBq kg-1 h-1) 6.9 ± 0.9 3.9 ± 1.1 1.5 ± 0.4 0.7 ± 0.2 9.8 ± 1.1 3.1 ± 0.9 2.2 ± 0.7 1.0 ± 0.1 4.8 ± 1.3 2.9 ± 1.0 2.7 ± 1.3 1.9 ± 0.1 11.0 ± 0.9 3.5 ± 0.6 1.0 ± 0.2 0.8 ± 0.1
760 761 762 763 43
222
Rn emanation coefficient (40 ± 5)×10-5 (16 ± 4)×10-4 (24 ± 6)×10-4 (45 ± 11)×10-4 (20 ± 2) ×10-5 (30 ± 9)×10-5 (10 ± 3)×10-4 (30 ± 4)×10-4 (10 ± 3)×10-5 (3 ± 1)×10-4 (6 ± 3)×10-4 (26 ± 3)×10-3 (20 ± 2)×10-5 (20 ± 4)×10-4 (21 ± 3)×10-4 (33 ± 5)×10-4
764 765 766 767
Table 2
768
Comparison of
769
concentrations in the dry samples.
222
Rn emanation fractions for a high
226
Ra (Bq kg-1) activity
770
Samples
226
Ra (Bq kg-1)
222
Rn emanation fraction
References
Monazite grain size of 100-200 µm Zircon
(22.3 ± 1.1) × 103
3.0 ×10-4
(3.8 ± 2.9) × 103
1.0 ×10-4
Uraninite<63µm (Canada) Monazite <63 µm
(58.3 ± 0.7)× 105
5.3 ×10-3
(17.6 ± 0.4) ×103
(20.5 ± 0.3)×10-3
Zircon+Monazite 100-200 µm
(26.5 ± 1.3) ×103
5.0 ×10-4
Rama and Moore, 1984 Rama and Moore, 1990 Garver and Baskaran, 2004 Garver and Baskaran, 2004 Rama and Moore, 1984
Monazite grain size 125-1000 µm
9.3 ± 0.8 - 6923 ± 24 (range)
(10.0 ± 2.8)×10-5 (26.0 ± 2.7)×10-3 (range)
771
772 773 774 775 776
44
Present Study
777 778 779 780 781
Table 3
782
Radium-228 (Bq kg-1),
783
coefficient for various grain size of Karunagapalli (KI), Chavara (CH), Neendakara
784
(N) and Kovalam (KM).
220
Rn mass exhalation (Bq kg-1h-1), and
220
Rn emanation
785
Sample size (µm) with Location ID KI <125 KI 250-125 KI 500-250 KI 1000-500 CH <125 CH 250-125 CH 500-250 CH 1000-500 N <125 N 250-125 N 500-250 N 1000-500 KM <125 KM 250-125 KM 500-250 KM 1000-500
220
228
Ra (Bq kg-1) 11279 ± 39 793 ± 9 186 ± 2 44.6 ± 1.8 (44.38 ± 0.21) ×103 6224 ± 40 1189 ± 8 212 ± 4 (41.77 ± 0.12) ×103 5994 ± 35 1893 ± 17 35.0 ± 1.9 34731 ± 92 (2.75 ± 0.14) ×103 265 ± 5 107 ± 4
Rn mass exhalation (Bq kg-1 h-1) 965 ± 3 669 ± 7 354 ± 4 94.0 ± 3.8 3066 ± 14 2456 ± 16 441 ± 3 191 ± 4 1877 ± 5 770 ± 5 494 ± 4 88.5 ± 4.8 2076 ± 6 1597 ± 83 251 ± 5 101 ± 3
45
220
786
Rn emanation 787 coefficient (200 ± 1)×10-5 -4 (190 ± 2)×10788 (420 ± 5)×10-4 (46 ± 2) ×10-3 (200 ± 1)×10-5 (90 ± 1)×10-4 (80 ± 1)×10-4 (200 ± 4)×10-4 (100 ± 3)×10-5 (300 ± 2)×10-5 (60 ± 1)×10-4 (56 ± 3)×10-3 (1000 ± 3)×10-6 (13 ± 1)×10-3 (210 ± 4)×10-4 (210 ± 7)×10-4
HIGHLIGHTS •
This study throws light on the relative contribution of the monazite minerals to 222Rn and 220
•
Rn release in the environment.
Monazite rich samples have low emantion coefficient and are not the highest contributor of 222Rn and 220Rn to the environment.
•
Radon and thoron emanation coefficient was higher in 1000-500 µm grain size fractions, which contained least 226Ra and 228Ra activity (Bq kg-1).
•
Unlike 226Ra and 228Ra activity concentrations, 222Rn and 220Rn emanation coefficient did not increase with the finer grain size; rather, opposite trend was observed.
•
The statistically significant correlation coefficient substantiated the dependency of mass exhalation rate (Bq kg-1 h-1) and
228
Ra activity (Bq kg-1), also between
exhalation rate (mBq kg-1 h-1) and 226Ra activity (Bq kg-1).
222
220
Rn
Rn mass