close

Вход

Забыли?

вход по аккаунту

?

j.jenvrad.2018.07.022

код для вставкиСкачать
Journal of Environmental Radioactivity 192 (2018) 434–439
Contents lists available at ScienceDirect
Journal of Environmental Radioactivity
journal homepage: www.elsevier.com/locate/jenvrad
The development of a digital gamma-gamma coincidence/anticoincidence
spectrometer and its applications to monitor low-level atmospheric
22
Na/7Be activity ratios in Resolute Bay, Canada
T
Weihua Zhang∗, Kenneth Lam, Kurt Ungar
Radiation Protection Bureau of Health Canada, 775 Brookfield Road, Ottawa, Ontario, K1A 1C1, Canada
A R T I C LE I N FO
A B S T R A C T
Keywords:
Activity ratio of 7Be/22Na
Aerosols
Air mass exchange between stratosphere and
troposphere
Using a previously developed digital gamma-gamma coincidence/anticoincidence spectrometer, daily aerosol
samples collected at Resolute Bay, Canada (74.71°N, 94.97°W) from May 2016 to April 2017 were analysed for
activity concentrations of 22Na and 7Be. The spectrometer design allows a more selective measurement of 22Na
with a significant background reduction by gamma-gamma coincidence events processing. It has been demonstrated that the improved spectrometer provides a more sensitive and effective way to quantify trace amounts of
22
Na and 7Be with a critical limit of 3 mBq and 5 Bq respectively for a 20 h counting. The 7Be/22Na ratio data set
obtained in this study showed significant annual variation, which has a distinct spring (typically from February
to May) maximum and winter (from September to February of next calendar year) minimum, which suggest that
it could be used as a radiochronometer for studying the atmospheric processes. The 7Be/22Na ratios are most
likely connected to deep stratosphere-to-troposphere transport (STT) exchange events where air with a higher
7
Be/22Na ratio originates from downward flow from stratosphere to the troposphere. The aerosols with lower
7
Be/22Na ratios located between two oscillation peaks may have longer residence time. The correlations between
7
Be and 22Na activity concentration were the high during these time periods. Compared with other studies based
on weekly collected aerosol samples, the techniques greatly improve the temporal resolution of 7Be/22Na data
set that will be able to provide more detailed information to study various atmospheric phenomena.
1. Introduction
When a nuclear weapon is detonated, radioactive noble gas and
lighter particles may penetrate high into the stratosphere, and remain
there for months and even years. Stratospheric circulation and diffusion
will spread this material around the world (U.S. Arms Control and
Disarmament Agency, 1975). 7Be (half-life, 53.2 d) and 22Na (half-life,
2.6 y), both naturally occurring radionuclides of cosmogenic origin, are
also transferred to the earth surface through the processes of intrusions
of stratospheric air masses into the troposphere followed by dry or wet
deposition. It is known that 22Na can also be produced by atmospheric
nuclear weapon tests in the early 1960s. A study by Tokuyama and
Igarashi (1998) indicated the annual deposition rate of 22Na became
significantly smaller than those observed during the first half the 1960s.
From then on, after more than 20 years day the 22Na observations
should be dominated by cosmogenic origin. These two cosmogenic
radionuclides, simultaneously generated in the low stratosphere and
upper troposphere, have similar behaviors after production. Even
∗
though they are not in the CTBT (Comprehensive Nuclear-Test-Ban
Treaty) relevant radionuclide list, they are of interest to the CTBT
verification regime because their activities and activity ratios can be
used as a tracer and radiochronometer for the stratosphere-to-troposphere downward air mass exchange if the current 22Na measurement
limitations are overcome.
The analysis of 22Na and 7Be collected in daily air-filter samples has
typically been based on a measurement done with a single HPGe detector using their respective 1274.5 keV and 477.6 keV gamma-rays.
There are several practical problems associated with 22Na measurement
by this method. In addition to very low 22Na activity concentration
(about four orders of magnitude lower than that of 7Be), other important problems include the low measurement efficiency, the losses of
counts due to cascade summing and the background from Compton
scatter from 208Tl and 40K. This decreases sensitivity and degrades the
detection limit of the 1274.5 keV peak, restricting measurement of 22Na
in particular. Thus, 22Na quantification based on daily aerosol samples
is often difficult.
Corresponding author.
E-mail address: weihua.zhang@canada.ca (W. Zhang).
https://doi.org/10.1016/j.jenvrad.2018.07.022
Received 21 June 2018; Received in revised form 23 July 2018; Accepted 25 July 2018
0265-931X/ Crown Copyright © 2018 Published by Elsevier Ltd. All rights reserved.
Journal of Environmental Radioactivity 192 (2018) 434–439
W. Zhang et al.
Fig. 1. Scatter plots showing the correlation between 7Be activity concentrations determined by the HPGe detector and the developed BGO spectrometer.
being relatively low in other seasons. It may indicate that the stratosphere-to-troposphere downward exchange events affect this area more
frequently during spring and winter.
Extended aerosol sampling intervals, such as weekly or monthly,
were used to increase sensitivity of 22Na measurement (Błazej and
Mietelski, 2014; Steinmann et al., 2013; Leppanen et al., 2012;
Jasiulionisa and Wershofen, 2005). Recently a technique based on
spectral summation of sequential daily high-volume aerosol samples
was developed for low-level 22Na measurement (Hoffman et al., 2017).
However, it would be more desirable to directly measure 22Na in daily
high volume aerosol samples to get a better temporal resolution.
To improve 22Na detection limit, a prototype gamma-gamma coincidence spectrometer was developed using list-mode data acquisition
techniques by Zhang et al. (2011a). An initial evaluation of quantitative
coincidence counting was performed based on four certified 22Na point
sources and weekly aerosol samples, which corroborated the feasibility
of this approach. This prototype has been further improved in several
aspects, such as a rebuilt bismuth germinate scintillators (BGO) detector system with lower background materials, a lead cabinet for
background reduction, an automatic sample changer that can hold up to
50 aerosol samples and supply them to the spectrometer for counting
for given time intervals, an open source software to extract the recorded
coincidence/anticoincidence events and histogram spectrum respectively, a new peak search and fit algorithm to locate the anticoincidence
peaks at 477.6 keV and the coincidence peak at 511.0 keV in each
spectrum and to calculate the net peak areas for 7Be and 22Na quantification (Zhang et al., 2014).
The spectrometer design allows a more selective measurement of
22
Na with a significant background reduction by gamma-gamma coincidence events processing. It has been demonstrated that this improved
spectrometer provides a more sensitive and effective way to quantify
trace amounts of 22Na and 7Be with a critical limit of 3 mBq and 5 Bq
respectively for a 20 h counting interval (Zhang et al., 2014). The use of
a list-mode data acquisition technique enabled simultaneous determination of 22Na and 7Be activity concentrations using a single measurement by coincidence and anticoincidence mode, respectively.
Using the spectrometer developed in previous study (Zhang et al.,
2014), the aerosol samples collected at Resolute Bay, NU, Canada
(74.71°N, 94.97°W) airborne particulate monitoring station from May
2016 to April 2017 were counted. The activity concentrations of 22Na
and 7Be were analysed. Based on the results from the Resolute Bay
station, the study confirms that the seasonal distribution of activity
ratios of 7Be/22Na has a significant peak in spring and winter, while
2. Gamma-gamma coincidence/anticoincidence spectrometer and
aerosol sampling
The gamma-gamma coincidence/anticoincidence spectrometer
consists of two BGO detectors and an XIA LLC Digital Gamma Finder
(DGF)/Pixie-4 software and card package. A sample is positioned between the two BGO detectors for measurement. This orientation is
optimized for maximum coincidence counting efficiency for the two
511.0 keV annihilation photons produced following the decay of the
positron-emitting nuclide 22Na. It also allows more selective 22Na
measurement and significant background reduction by gamma-gamma
coincidence counting. Using the list-mode data acquisition technique,
7
Be analysis was also developed by introducing gamma-gamma anticoincidence counting mode. Through this list-mode software-based anticoincidence/coincidence spectrometry, optimal determinations of 7Be
and 22Na activities were possible in a single measurement (Zhang et al.,
2014).
The CTBT particulate monitoring stations are equipped with aerosol
sampler named as “Grey Owl” (Zhang et al., 2011b). The sampler draws
air through 2500 cm2 filters at a flow rate approximately of 700 m3 per
hours and a daily sample volume around 16 000 m3. These daily collected air-filter samples are shipped to RPB laboratory in Ottawa for
gamma-emitting artificial and natural radionuclides analysis. In the
laboratory, the filters are pressed into a 50 mm diameter and 4 mm
height puck by means of a hydraulic press, and then counted for 6 h by
an HPGe detector (Canberra high purity n-type germanium BEGE5030).
The activity of 7Be was quantified for each sample, but no 22Na detections were observed by 6 h counting times using the HPGe detector.
The samples collected from May 2016 to April 2017 (over a one-year
period) were re-counted by the BGO spectrometer for 20 h, rendering
22
Na detectable in 256 of the 365 samples.
3. Results and discussion
Using the peak search and fit algorithm developed in this study and
gamma-gamma anticoincidence counting mode, the aerosol samples
435
Journal of Environmental Radioactivity 192 (2018) 434–439
W. Zhang et al.
were analysed for 7Be. The activity concentrations determined are
plotted in Fig. 1 together with the values determined by the HPGe
detector. The results showed strong correlation (r = 0.92) between the
results obtained by the HPGe detector and the gamma-gamma anticoincidence counting, indicating good accuracy and reproducibility of
new algorithm at peak search and fit routine.
It is obvious that the activities obtained by the two methods are
evenly distributed on both sides of the correlation line. Within the
measurement uncertainty, agreement between the two methods is
good. It should be pointed out that there are points that are distant from
the correlation line. These outliers are due to variability in the BGO
spectrometer measurement. The delay from the end of aerosol sample
collection time to the start of coincidence/anticoincidence measurement was varied from 2 to 7 month depending on the sample availability. After several months decay, the activity levels of 7Be in the
sample became too low to be counted with good counting statistics,
which caused large uncertainties and significant deviations with HPGe
determined results. Thus, the 7Be activity concentrations determined by
HPGe detector were used in this study.
Out of 365 daily ground level aerosol samples collected, 22Na activity has been quantified in 256 samples at the radionuclide monitoring station of Resolute Bay. Fig. 2 shows the observed activity
concentrations of 7Be and 22Na in aerosols collected daily from Resolute
Bay between May 2016 and April 2017 as the day of calendar year. As
shown in Fig. 2, the 7Be data is largely dispersed through the year,
however an annual cycle can still be seen. The activity concentration of
7
Be rose beginning in March with peak values typically observed in
April. In July and August, the activity concentration reached the lowest
values.
Compared with 7Be, different seasonal variation patterns were observed for 22Na. The activity concentrations for 22Na are lower with a
mean value of 0.31 μBq/m3 in cold seasons (from September to May of
next calendar year). In warm seasons (from June to August) the observed concentrations are about 4 times higher with a mean value of 1.2
μBq/m3. The recorded values correspond with the results reported by
similar studies (Błazej and Mietelski, 2014; Steinmann et al., 2013). The
seasonal patterns of 22Na concentrations at Resolute Bay in the Canadian Arctic possibly suggest that during spring and early summer 22Na
is primarily from the stratosphere as the air mass exchanged more actively between stratosphere and troposphere (Hoffman et al., 2018).
The higher 22Na activity concentration in summer months could be
caused by repeated sampling of the original spring and early summer
stratospheric air injections, or an enhanced aquatic 22Na flux from the
warm air blown from the Arctic Ocean during periods when the sea ice
has melted.
The 7Be/22Na activity ratios were calculated using the observed 7Be
Fig. 2. Activity concentration of 7Be and
January 1st).
22
and 22Na activity concentrations between May 2016 and April 2017.
Fig. 3 shows histograms of the observed 7Be and 22Na activities and the
7
Be/22Na ratios. The data are all asymmetric and log-normally distributed. About 80% of the activity concentrations of 7Be and 22Na
ranged from 0.74 to 2.8 mBq/m3, and from 2.2 × 10−4 to 8.8 × 10−4
mBq/m3 respectively. The mean activity concentration (1.5 mBq/m3) of
the 7Be is about four orders of magnitude higher than that of the 22Na
(4 × 10−4 mBq/m3). The 7Be/22Na activity ratios show similar rightskewed distribution shape. The data ranges from 58 to 24 000, but 90%
are ranged from 870 to 9802 mBq/m3 with a mean value of 4118 mBq/
m3. Very similar distributions have been reported in the study of
Leppanen et al. (2012). The full-year sets of sample collected on daily
basis from Resolute Bay were analysed using the BGO spectrometer.
The observed activity ratios for 7Be/22Na were plotted versus the day of
calendar year in Fig. 4.
As shown in Fig. 4, the ratios show significant variability on a daily
basis, but annually, a clear pattern in the 7Be/22Na ratio can be seen
where higher ratios are observed during winter or spring and lower
ratios during summer or autumn. In order to reduce the “noise”, 7-point
moving average was applied. During winter and spring months (February–June), the 7Be/22Na ratio show strong oscillations, which may
indicate that a STT exchange event be possibly occurring. During
summer months (June–September), those oscillation peaks have been
significantly reduced, but still observable. After September the STT
events become infrequent. A high 7Be/22Na ratio indicates a high
stratosphere or upper troposphere origin or a shorter residence time of
the air mass in the stratosphere. The presented frequency distributions
for 7Be/22Na activity concentration ratio could possibly be explained as
the results of an STT cycle, which starts with a spring injection of
cosmogenic nuclides from the stratosphere in February, followed by a
period of reduced air mass mixing during summer (June–September),
and finally almost no air mass mixing from September until February of
the following year. This finding is overall similar to the corresponding
results from Finland (Leppanen et al., 2012), Switzerland (Steinmann
et al., 2013) and Poland (Błazej and Mietelski, 2014). However, compared with these literature observations, there are some seasonal shifts
or delays between these air mass vertical mixing events, which could be
understood as 7Be/22Na ratio variability at a synoptic scale from different monitoring stations. Additionally, more detailed information has
been provided in this study about the frequency distribution of troposphere air mass mixes with ground-level air in the daily time series.
The above discussion on the air masses vertical mixing events is
further substantiated by constructing backward trajectories calculations
(Fig. 5a–c) using the HYSPLIT model of the National Oceanographic
and Atmospheric Administration (NOAA) for the maxima of the
7
Be/22Na ratio (24046), its preceding minima (4309) and its following
Na in ground level air in Resolute Bay between May 2016 and April 2017 for daily collected aerosols samples (day 1,
436
Journal of Environmental Radioactivity 192 (2018) 434–439
W. Zhang et al.
Fig. 3. Histograms show asymmetric and log-normal distributions of 7Be and
Fig. 4. The 7Be and
22
22
Na activities and 7Be/22Na ratios.
Na ratio in daily aerosol samples calculated using the activity concentration of Fig. 3 (day 1, January 1st).
Fig. 5. The representative seven-day backward trajectories of air masses over Resolute Bay on the day of (a) 2016-03-18 19:06, (b) 2016-03-23 19:14, and (c) 201603-28 19:21 (The backward trajectories were obtained from http; //www.arl/noaa.gov/ready).
437
Journal of Environmental Radioactivity 192 (2018) 434–439
W. Zhang et al.
Fig. 6. Correlation plots for 7Be and
22
Na activity concentration in consecutive 7 or 8 day time intervals at different time periods.
change the absolute 7Be and 22Na concentrations, but not the relative
amounts. Thus, the constant ratio results in linear correlations between
7
Be and 22Na activity concentrations. The observations presented here
again confirm the explanation of 7Be/22Na ratio annual variation based
on vertical atmospheric air mass mixing cycle.
minima (6889) on the day 82 (2016-03-23 19:14), 77 (2016-03-18
19:06), and 87 (2016-03-28 19:21) respectively. Seven-day backward
trajectories were considered for the study since the mean residence time
of 7Be attached aerosols in the atmosphere is 5–9 days (Winkler et al.,
1998). From Fig. 5a–c, it can be seen that the air mass had its origins
predominantly in the north of Arctic on these three days, but on the day
82 (2016-03-23) the trajectories starting height was 2500 m AGL,
which was the highest compared with its preceding and following days.
The 7Be and 22Na have the same cosmogenic origin. High correlations are expected for their relative activity concentrations assuming a
constant rate of production for both of them. In fact, no correlation
(squared Pearson factor r2 = 0.0063) was found between the 7Be and
22
Na activity concentration from all data gathered in this study. If the
correlation analysis is conducted in smaller time scale (i.e. 7 or 8
consecutive days), a short period of correlations exists. As shown in
Fig. 6, during these time periods, there is a significant positive or negative correlation between 7Be and 22Na activity concentrations. The
squared Pearson factor (r2) are all higher than 0.48. These time periods
are corresponding to squared frame areas, as displayed in Fig. 4. It is
interesting to note that these time periods are all located in the valley
between two oscillation peaks. The highest 7Be/22Na ratio values are a
result of more air from the upper troposphere and the stratosphere
mixing with ground-level air. In contrast, with the lower 7Be/22Na ratios, the aerosols have a longer residence time that allows more 7Be to
decay than 22Na, thus decreasing the 7Be/22Na ratio. Assuming that
both 7Be and 22Na aerosols behave in a similar manner in the atmosphere, this means that atmospheric processes (i.e. wet scavenging) can
4. Conclusions
With the development of coincidence/anticoincidence gamma
spectroscopy techniques, it is possible to directly measure 22Na in daily
high volume aerosol samples. Compared with the studies based on
weekly collected aerosol samples, the techniques greatly improve the
temporal resolution of 7Be/22Na data set that will be able to provide
more detailed information to study various atmospheric phenomena.
The 7Be/22Na ratio data set obtained in this study from Resolute Bay
between May 2016 and April 2017 showed significant annual variation,
which has a distinct spring (typically from February to May) maximum
and winter (from September to February of next calendar year)
minimum. These results suggest that the daily 7Be/22Na ratio data set
could be used as a radiochronometer for studying the atmospheric
processes. The 7Be/22Na ratios are most likely connected to atmospheric STT exchange events where air with a higher 7Be/22Na ratio
originates from downward flow from stratosphere to the troposphere.
The aerosols with lower 7Be/22Na ratios located between two oscillation peaks may have longer residence time. The correlations between
7
Be and 22Na activity concentration were the high during these time
periods.
438
Journal of Environmental Radioactivity 192 (2018) 434–439
W. Zhang et al.
Appendix A. Supplementary data
phenomena. J. Atmos. Sol. Terr. Phys. 74, 164–180.
Steinmann, P., Zeller, M., Beuret, P., Ferreri, G., Estier, S., 2013. Cosmogenic 7Be and
22
Na in ground level air in Switzerland (1994–2011). J. Environ. Radioact. 124,
68–73.
Tokuyama, H., Igarashi, S., 1998. Seasonal variation in the environmental background
level of cosmic-ray-produced 22Na at Fukui City, Japan. J. Environ. Radioact. 38 (2),
147–161.
U.S. Arms Control and Disarmament Agency, 1975. Worldwide effects of nuclear war.
http://www.atomicarchive.com/Docs/Effects/wenw_index.shtml.
Winkler, R., Dietl, F., Frank, G., Tschiersch, J., 1998. Temporal variation of 7Be and 210Pb
size distributions in ambient aerosol. Atmos. Environ. 32 (6), 983–991.
Zhang, W., Yi, J., Mekarski, P., Hoffman, I., Ungar, K., Leppanen, A.-P., 2011a. A system
for low-level the cosmogenic 22Na radionuclide measurement by gamma–gamma
coincidence method using BGO detectors. J. Radioanal. Nucl. Chem. https://doi.org/
10.1007/s10967-010-0758-3.
Zhang, W., Bean, M., Benotto, M., Cheung, J., Ungar, K., Ahier, B., 2011b. Development of
a new aerosol monitoring system and its application in Fukushima nuclear accident
related aerosol radioactivity measurement at the CTBT radionuclide station in Sidney
of Canada. J. Environ. Radioact. 102, 1065–1069.
Zhang, W., Ungar, K., Stukel, M., Mekarski, P., 2014. A gamma-gamma coincidence/
anticoincidence spectrometer for low-level cosmogenic 22Na/7Be activity ratio measurement. J. Environ. Radioact. 130, 1–6.
Supplementary data related to this article can be found at https://
doi.org/10.1016/j.jenvrad.2018.07.022.
References
Błazej, S., Mietelski, J.W., 2014. Cosmogenic 22Na, 7Be and terrestrial 137Cs, 40K radionuclides in ground level air samples collected weekly in Krakow (Poland) over years
2003-2006. J. Radioanal. Nucl. Chem. 300 (2), 747–756.
Hoffman, I., Lewis, B., Chan, P., Ungar, K., 2017. Analysis of 22Na using a spectral
summation technique on high volume aerosol samples. J. Environ. Radioact.
169–170, 151–158.
Hoffman, I., Lewis, B., Chan, P., 2018. Circulation of cosmogenic 22Na using the global
monitoring network of the Comprehensive Nuclear-Test-Ban Treaty Organization
(CTBTO). J. Environ. Radioact. 187, 8–15.
Jasiulionisa, R., Wershofen, H., 2005. A study of the vertical diffusion of the cosmogenic
radionuclides 7Be and 22Na in the atmosphere. J. Environ. Radioact. 79, 157–169.
Leppanen, A.-P., Usoskin, I.G., Kovaltsov, G.A., Paatero, J., 2012. Cosmogenic 7Be and
22
Na in Finland: production, observed periodicities and the connection to climatic
439
Документ
Категория
Без категории
Просмотров
1
Размер файла
2 087 Кб
Теги
022, 2018, jenvrad
1/--страниц
Пожаловаться на содержимое документа