This document discusses the use of epithermal neutron activation analysis and Compton suppression to determine heavy metals in Arctic air filters. Key findings include: - Barium, potassium, silver, strontium, and zinc were determined in Arctic air filters from Alert, Canada using epithermal neutron activation analysis and Compton suppression. - Detection limits for barium, potassium, silver, strontium, and zinc were 5, 7, 0.01, 1 and 0.9 μg, respectively in 1/16 of a Whatman air filter. - Longer counting times would be needed to consistently achieve uncertainties less than 20% for barium, strontium and zinc. - Silver

1. Further investigation of epithermal neutron activation analysis
in the determination of heavy metals in the Arctic atmosphere
Richard Lara1 • Sheldon Landsberger1
Received: 22 June 2015
Ó Akade´miai Kiado´, Budapest, Hungary 2015
Abstract We have further explored the use of epithermal
NAA with Compton suppression to establish the presence
of barium, potassium, strontium and zinc in Arctic air fil-
ters. Silver was determined using cyclic epithermal NAA in
air filters. For one-sixteenth of a Whatman air filter
detection limits for barium, potassium, silver strontium,
and zinc were 5, 7, 0.01, 1 and 0.9 lg, respectively. Results
revealed that longer counting times for the activation
products of barium, strontium and zinc would be needed to
consistently achieve uncertainties less than 20 %.
Keywords Arctic Á Air pollution Á Epithermal NAA Á
Compton suppression
Introduction
The determination of trace elements of natural and
anthropogenic sources in the Arctic has been on-going for
more than three decades both for the study of environ-
mental impact and source-receptor modeling [1–7]. The
two main analytical methods capable of determining a wide
range of elements at the low levels typically found in the
Arctic atmosphere are neutron activation analysis (NAA)
and inductively coupled plasma-mass spectrometry. Parti-
cle induced x-ray emission (PIXE) is also a technique often
employed to determine several elements including sulphur
and lead, but has poor sensitivities for the higher Z ele-
ments such as indium, antimony, silver, rare-earth ele-
ments, etc. Since the early 1990’s both at the University of
Illinois and University of Texas techniques in thermal and
epithermal NAA with and without Compton suppression
have been judiciously employed to dramatically improve
the sensitivities of elements not typically determined by
routine activation analysis [8–12]. In recent years we have
employed a cyclic epithermal NAA system to determine
silver in various geological matrices [13]. Currently our
main analytical efforts are to improve sensitivities of ele-
ments that are typically not determined in the Arctic
atmosphere so as to add to the data base of a large study
conducted by Air Quality Research Division of Environ-
ment Canada. The main goal of this research is to inves-
tigate which other elements can be reliably determined
using short- and medium-lived NAA products and not rely
on very time consuming long irradiations and long count-
ing times of the long-lived radionuclides.
Special considerations for NAA of Arctic air filters
Typically major sources of high dead times and high
Compton continuum in NAA of environmental specimens
arise from the presence of aluminum, chlorine, sodium,
bromine and manganese for short-lived radionuclides and
sodium and bromine for medium-lived ones. The Arctic
atmosphere, because of the ocean environment, is unusually
enriched in chlorine, bromine and sodium giving rise to ele-
vated radioactivity through the following reactions: 37
Cl(n,c)
38
Cl (t1/2 = 38.3 min), 79
Br(n,c) 80
Br (t1/2 = 18.8 min),
81
Br(n,c) 82
Br (t1/2 = 35.4 h), and 23
Na(n,c) 24
Na, (t1/2 =
15 h). Using typical thermal neutrons makes the analysis of
& Sheldon Landsberger
s.landsberger@mail.utexas.edu
1
Nuclear Engineering Teaching Laboratory, University of
Texas at Austin, Pickle Research Campus R-9000, Austin,
TX 78712, USA
123
J Radioanal Nucl Chem
DOI 10.1007/s10967-015-4524-4

2. many elements such as copper, arsenic, antimony, indium,
vanadium, etc. virtually unattainable or with very poor
detection limits and high uncertainties. Our philosophy has
been to expand the number of elements one can determine in
theArcticairfilterswithoutrelyingonlong-livedNAAwhich
involves long irradiations, decay and counting times. For
instance a project which involves *700 samples would need
some 3500 h of counting just for long-lived radionuclides. To
that end we have further explored epithermal NAA with and
without Compton suppression to determine elements that are
amenable to these two methodologies to strengthen source-
receptor modeling.We reportforthefirst time the elements of
barium, potassium, strontium, silver and zinc in Arctic air
filters from Alert, Canada using these methodologies in a
systematic approach which significantly adds to the dataset of
this 35-year continuing study.
Epithermal NAA and Compton suppression
The theory of epithermal neutron activation analysis is well
understood [14] and likewise, techniques in Compton
suppression NAA for air filters and other sample matrices
are well established in our laboratory and details have
appeared elsewhere [15]. To better understand the reason
why these methods were chosen we have outlined the
characteristics of the reactions which are shown in Table 1.
For barium, silver, strontium and zinc the ratio of the
resonance integral/thermal cross section are relatively
strong and the interfering isotopes from 24
Na, 28
Al, 38
Cl
and 80
Br have values close to one [16]. So for example the
determination of silver is 16 times more sensitive than
using thermal neutrons alone. However, other factors can
also lead to better detection limits. For example, the added
characteristic of Compton suppression signifies that
gamma-rays in coincidence with each other are also sup-
pressed. This means that the strong 1368.4 and 2754.0 keV
photons and 1642.0 and 2167.4 keV photons belonging to
24
Na and 38
Cl, respectively, are reduced in intensity as
well. While 81
Br has a resonance integral/thermal cross
section of 18.5 it also has a multitude of gamma-rays in
coincidence and therefore many of the strong photons such
as 554.4 keV and 776.5 keV belonging to 82
Br are also
significantly reduced.
Experimental
For this study sixty-five air filter samples including blanks
were received from the Air Quality Research Division of
Environment Canada. The air filters are collected at Alert in
the Canadian side of the Arctic Circle, on a weekly basis with
about 14,000 m3
of air collected. One half of the 1/8 strips of
the Whatman 40 filters with dimensions of 20.2 cm 9
27.9 cm (8 inches 9 11 inches) were placed into poly-
ethylene vials and irradiated in the TRIGA MARK II reactor
with epithermal neutrons with a flux of *2.5 9 1011
n cm-2
s-1
. A total of eight filters were sequentially irra-
diated for 10 min and allowed to decay for 10 min to
determine 139
Ba with a counting time of 15 min using a
Compton suppression system. Afterwards 5–8 irradiated
samples were placed in a sample changer and counted 6–8 h
later for 1.5 h to determine 69m
Zn, 87m
Sr and 42
K. Silver was
determined using epithermal cyclic activation analysis using
the remaining half of the filter. Three cycles of a 60 s irra-
diation, 10 s delay and 30 s counting time was done. An
overview of the cyclic activation system is shown in Fig. 1.
Calibration was done using certified NIST traceable
elemental solutions from Inorganic Ventures for barium,
potassium, silver, strontium and zinc. Quality control was
done using NIST 1632 coal, 1632c coal, 1633 flyash and
Table 1 Nuclear characteristics
of elements
Element Reaction Resonance integral/thermal
cross section Ic
rc
Half-life Gamma-ray
(keV)
Barium 138
Ba(n,c)139
Ba 9.6 1.4 h 165.3
Potassium 41
K(n,c)42
K 0.96 12.8 h 1524.0
Silver 109
Ag(n,c)110
Ag 16.0 25.6 s 657.8
Strontium 86
Sr(n,c)87m
Sr 5.7 2.8 h 388.5
Zinc 68
Zn(n,c)69m
Zn 3.3 13.8 h 438.9
Fig. 1 Overview of cyclic system with sample holder
J Radioanal Nucl Chem
123

3. Fig. 2 Normal and Compton suppressed spectra of Arctic air filter for medium-lived NAA
Fig. 3 Normal and Compton suppressed expanded spectra of Arctic air filter for medium-lived NAA showing the 388.5 keV and 438.9 gamma
rays for87m
Sr and69m
Zn, respectively
J Radioanal Nucl Chem
123

4. 1648a diet. For silver Canadian Certified Reference Mate-
rials Project (CCRMP) zinc–lead–tin–silver and concen-
trated zinc ore standards were used for quality control. All
results were within 5–10 % of the certified values. To
emphasize the difference between normal and Compton
suppression Fig. 2 shows vast improvement in the peak to
background metric, while a more detailed Fig. 3 shows how
indistinguishable the peaks are from the background as
compared to the Compton suppressed spectrum. Figure 4
shows the 110
Ag peak from cyclic epithermal activation
analysis. An example of the silver results is depicted in Fig. 5
using weekly air volumes of approximately 14,000 m3
. The
air back-trajectory HYSPLIT model by the NOAA reveals
the source of silver coming from the northern Canada, rather
than the usual wintertime Eurasian sources.
Results
The results are presented for each element in terms of
concentration values, uncertainty in the counting statistics
and detection limits.
Barium
The majority barium concentrations for the 1/16 strip of
the Whatman filter were below the detection limit of 5 lg.
In only five samples barium was detectable with typical
uncertainty in the counting statistics of about 25 %. It is
clear that a different counting regime is needed to improve
the detection, perhaps from 15 min to 1 h.
Fig. 4 Cyclic activation analysis spectrum for Arctic air filter
Fig. 5 Airborne silver
concentrations in Alert, Canada
J Radioanal Nucl Chem
123

5. Potassium
Typical detection limits for potassium were about 7 lg
with the vast majority of the results being above this value.
Uncertainty in the counting statistics ranged from 3 to
15 %, depending on the concentration of potassium.
Silver
Since silver has a high resonance integral/thermal cross
section of 15.6, the detection limit was about 0.01 lg.
Typical uncertainties were about 5–10 %. However, 20 %
of the samples had concentration above this value.
Strontium
Detection limits for strontium were about 1 lg but only
about 1/3 of the samples were above this limit. Typical
uncertainties in the counting statistics were 10–40 %, again
depending on the concentrations. A shorter decay time of
6–10 h is probably needed since the half-life of88m
Sr is
1.4 h.
Zinc
Typical detection limits for zinc were about 0.9 lg. Zinc
was detected with typical uncertainties of 10–30 %.
Conclusions
We have successfully demonstrated that the elements of
potassium, silver and zinc can be added to the list of ele-
ments that can be determined in the Arctic atmosphere at
Alert, Canada. Silver and zinc can be considered to be
important anthropogenic markers while potassium is a
source of fire burning in the summer months. Our prelim-
inary data has shown that potassium concentrations in the
summer months are increased significantly on certain days.
While strontium and barium have the potential to be rou-
tinely determined shorter decay times and longer counting
times are needed, respectively to obtain more reliable
results with better detection limits. Elevated silver con-
centrations reported for the first time in Alert, Canada
reveal the source is in the summer time arising from
northern Canada.
Acknowledgments We are grateful to the Air Quality Research
Division of Environment Canada for the air filter samples.
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