Xenon
Paul R.J. Saey
Vienna University of Technology, Vienna, Austria
1
2
3
4
5
6
7
8
9
10
Summary
Occurrence
Radioxenon in the Atmosphere
Separation and Analytical Characterization
Techniques
Conclusions
Glossary
End Notes
Related Articles
Abbreviations and Acronyms
References
1 SUMMARY
Radioxenon isotopes are noble gases mainly produced in nuclear fission, e.g., that of 235 U. They have two
main applications.
In nuclear medicine, 133 Xe isotopes are used for
measuring the physiological parameters of lung ventilation
and to image the lungs. They are further used in isotonic
solutions to image blood flow, particularly cerebral blood flow.
Most radioactive isotopes of this element are
produced by a nuclear fission reaction, e.g., that of 235 U,
238
U, or 239 Pu. To verify the comprehensive nuclear-test-ban
treaty (CTBT) certain radioxenon isotopes are measured by
a global network to detect clandestine (underground) nuclear
explosions that vented these gases in the atmosphere. The
relevant isotopes for this application are 131m Xe, 133m Xe,
133
Xe, and 135 Xe.
Radioxenon isotopes are currently most frequently
measured with β – γ coincidence spectrometry, highresolution γ -spectrometry, or with proportional counters but
also with gas chromatography-mass spectrometry.
2 OCCURRENCE
Xenon (Xe) is a noble gas and therefore chemically
inert in the environment. The Earths’ atmosphere contains
179
179
182
183
186
187
187
188
188
188
approximately 0.087 ppm of stable xenon. The name derives
from the Greek xenon which means ‘‘the stranger’’. Xenon
was discovered in 1898 by Sir William Ramsay and Morris
Travers in residues left after evaporating liquid air. It is a
heavy, odorless, colorless, tasteless, and nonflammable gas
with element number 54 and is around 4.5 times heavier than
air. When it is excited by an electrical discharge in a vacuum
tube, it produces a blue glow. Some principal characteristics
are presented in Table 1.
Naturally occurring xenon consists of seven stable
and two radioactive isotopes (124 Xe and 136 Xe, both with very
long half-lives). Beyond these stable and semistable forms,
34 other radioactive isotopes and meta-stable states with halflives above 0.1 s have been found. Nearly half of these are
fission products of uranium and plutonium. The major part of
radioxenon isotopes is manmade — however, the spontaneous
fission of uranium in nature produces very-low levels of
radioxenon.3,4 The stable as well as the known radioisotopes
are listed in Table 2.
Radioxenon isotopes are artificial isotopes that are
created during fission of heavy atoms, like 235 U, 238 U, or
239
Pu or during nuclear reactions, like (n,p) reactions. They
can be created in or released from, among others, nuclear
power plants (NPPs), nuclear research reactors (NRRs),
radiopharmaceutical production facilities (RPFs), or nuclear
explosions (NEs).5 A very small amount of radioxenon is
also created in the atmosphere in cosmic ray reactions with
stable xenon gas. In areas with high uranium concentrations
Radionuclides in the Environment. Edited by David A. Atwood.  2010 John Wiley & Sons, Ltd. ISBN 978-0-470-71434-8
180
RADIONUCLIDES IN THE ENVIRONMENT
Table 1 Some principal characteristics of xenon1,2
Characteristic
Molecular weight
Melting point
Boiling point
Gas density at boiling point
Gas density at STP
Atomic diameter in crystal
Critical pressure
Critical temperature
Critical volume
Solubility in water at STP
Solubility in water at 20 ◦ C
Thermal conductivity at STP
Table 2 The different xenon isotopes and their characteristics
Value
131.3 g mol−1
−111.75 ◦ C
−108.04 ◦ C
9.86 kg m−3
5.761 kg m−3
3.94 Å
57.64 atm
16.058 ◦ C
118 cm3 mol−1
203.2 ml l−1
108.1 ml l−1
5.5 mW (mK)−1
underground, spontaneous fission also creates small amounts
of radioxenon.
Around 24% of the uranium or plutonium fission
products (sum of recommended cumulative yields) are noble
gases, mainly xenon isotopes. Figure 1 and Table 3 show
the fission yield for several nuclear fission relevant nuclides:
235
U, 238 U, and 239 Pu. The radioxenon isotopes that are created
after the fission of uranium are produced directly as fission
products and indirectly as daughters of fission products with
a higher neutron number. The number of atoms produced as
fission products per fission is the independent fission yield,
whereas the direct fission yield and the sum of all atoms of
the isotope produced from the radioactive decay of the other
fission products per fission are the cumulative fission yield.
Figure 2 shows the isobaric decay chains for the
masses 131, 133, and 135, of which the radioxenon isotopes
discussed later are a part.
2.1 Common Applications
Xenon-133, with a short half-life (5.243 days) and
low-energy γ -rays (81 keV), is used for measuring the
physiological parameters of lung ventilation and for imaging
the lungs. It is also used in an isotonic solution to image
blood flow, particularly cerebral blood flow.7,8 However,
many hospitals are replacing 133 Xe with the newly developed
99m
Tc gas (half-life, 9.14 h) (see Technetium). The importance
of 133 Xe in medicine and its commercial production are,
therefore, decreasing.
Xenon-135 is of considerable significance in the
operation of nuclear power reactors. It is the daughter of 135 I
(half-life, 6.7 h). Because of the large neutron absorption cross
section of 135 Xe (2.65 × 106 barn for thermal neutrons — the
similar figure for 133 Xe is 190 barn), 135 Xe is converted to
stable 136 Xe during the irradiation period. After the irradiation
has ended and the neutron flux stops, 135 I keeps on decaying
and producing new 135 Xe. It therefore acts as a neutron
absorber or ‘‘poison’’ that can slow down or stop the chain
reaction after a period of operation. This was discovered in
Isotope
111
Xe
Xe
113
Xe
114
Xe
115
Xe
116
Xe
117
Xe
118
Xe
119
Xe
120
Xe
121
Xe
122
Xe
123
Xe
124
Xe
125m
Xe
125
Xe
126
Xe
127m
Xe
127
Xe
128
Xe
129m
Xe
129
Xe
130
Xe
131m
Xe
131
Xe
132
Xe
133m
Xe
133
Xe
134m
Xe
134
Xe
135m
Xe
135
Xe
136
Xe
137
Xe
138
Xe
139
Xe
140
Xe
141
Xe
142
Xe
143
Xe
144
Xe
145
Xe
146
Xe
112
Natural
abundance (%)
Half-life
Major
decay mode
n.a.
n.a.
n.a.
n.a.
n.a.
n.a.
n.a.
n.a.
n.a.
n.a.
n.a.
n.a.
n.a.
0.095
n.a.
n.a.
0.089
n.a.
n.a.
1.91
n.a.
26.4
4.07
n.a.
21.2
26.9
n.a.
n.a.
n.a.
10.4
n.a.
n.a.
8.86
n.a.
n.a.
n.a.
n.a.
n.a.
n.a.
n.a.
n.a.
n.a.
n.a.
0.74 s
2.7 s
2.74 s
10 s
18 s
59 s
61 s
3.8 min
5.8 min
40 min
40.1 min
20.1 h
2.08 h
1.6 × 1014 a
56.9 s
16.9 h
stable
69.2 s
36.4 days
stable
8.88 days
stable
stable
11.84 days
stable
stable
2.19 days
5.243 days
0.29 s
5.80 × 1022 a
15.29 min
9.14 h
3.60 × 1020 a
3.818 min
14.08 min
39.68 s
13.6 s
1.73 s
1.22 s
0.3 s
1.15 s
0.188 s
0.1 s
EC
EC
EC
EC
EC
EC
EC
EC
EC
EC
EC
EC
EC
2EC
IT
EC
—
IT
EC
—
IT
—
—
IT
—
—
IT
β−
IT
—
IT
β−
—
β−
β−
β−
β−
β−
β−
β−
β−
β−
β−
Major
daughter
nuclide
111
I
I
113
I
114
I
115
I
116
I
117
I
118
I
119
I
120
I
121
I
122
I
123
I
124
Te
125
Xe
125
I
—
127
Xe
127
I
—
129
Xe
—
—
131
Xe
—
—
133
Xe
133
Cs
134
Xe
—
135
Xe
135
Cs
—
137
Cs
138
Cs
139
Cs
140
Cs
141
Cs
142
Cs
143
Cs
144
Cs
145
Cs
146
Cs
112
the earliest nuclear reactors built by the American Manhattan
Projecta for plutonium production.
Another application of radioxenon is the measurement of 129 Xe/129 I isotopic ratios in meteorites. They are a
powerful tool for studying the age difference between the
earth and the oldest meteorites found. This gives information
on the formation of the solar system.9
Environmental radioxenon gas monitoring is a
fundamental and highly sensitive technique for the detection
of underground or underwater NEs. Of all the technologies to
XENON
Sn
Fission yield (%)
101
Sn
Sb
100
Sb
91
Te
10−1
9
22.2
I
133
Fission by fission
spectrum neutrons
80
100
235U
f
235U
HE
Te
17
17.5
131
Xe
Xe
135
Xe
I
Xe
120
140
Te
70 12.5
I
98.8 1.2
10−3
60
Sb
83
78.8
Fission by high
energy neutrons
10−2
97.1
2.9
83.4
Xe
Xe
Cs
Cs
Ba
verify the CTBTb ,10 it is, together with radionuclide particulate
monitoring, the only technique that has the potential to provide
unmistakable proof of an NE.11,12 The noble gas radioisotopes
that are useful for identifying an NE are 131m Xe, 133m Xe,
133
Xe and 135 Xe5 — they are produced in significant quantities
and have half-lives that are long enough to be measured a
considerable time after any release. Depending on the fission
material (235 U, 233 U, or 239 Pu), between 1.08 × 1016 Bq and
1.33 × 1016 Bq of 133 Xe will be created in a 1 kton NE.3
To establish a global noble gas monitoring network,
as part of the International Monitoring System (IMS) to
verify the CTBT, fully automated radioxenon measurement
systems had to be developed, as no commercial systems
were available when the treaty was opened for signature.13
Four countries, France, Russia, Sweden, and USA, all with
experience of atmospheric xenon measurements, offered
to develop such systems, which are described later. With
the Provisional Technical Secretariat (PTS) for the CTBT
Organisation (CTBTO) and the German Federal Office for
Radiation Protection (Bundesamt für Strahlenschutz, BfS),
they participate in the International Noble Gas Experiment
(INGE) project.14 These systems are now being installed at
131
133
135
Figure 2 Isobaric decay chains for the masses 131, 133, and 135
with the branching ratios (in percent) — the gray dots are metastable
states (isomers)
up to 40 worldwide locations and they send their results to the
International Data Centre (IDC) in Vienna for processing and
analysis.5
After the announced NE in North Korea in October
2006, the Swedish Defence Research Agency (FOI) could
confirm out of the ratio 133m Xe/133 Xe measured in the north
of the Republic of Korea and using longer term background
measurements that the explosion was nuclear.15,16 Air sampled
independently above the Japanese Sea after the event contained
133
Xe and 135 Xe in a ratio that also confirmed the nuclear
origin of the explosion. Also, the increased 133 Xe activity
concentration measured at the Yellowknife IMS station in
North Canada in late October 2006 was consistent with leak
scenarios assumed for a low-yield underground NE on the
Korean peninsula.17 This demonstrates the importance of two
factors: radioxenon activity concentration ratios can identify
the nuclear origin of a source if several isotopes are measured
during consecutive days or if different isotopes are found
in one or more measurements and the knowledge of the
radioxenon background can help identify such an event even
in the case where only one isotope is detected.
Table 3 The cumulative fission yields in percent for six fission modes relevant to nuclear fission, induced by fission spectrum
(thermal) neutrons (f) and high-energy neutrons (14.7 MeV) (he)6
131m
Xe
Xe
133
Xe
135
Xe
133m
Fission yield
235
Uf (%)
0.05
0.19
6.72
6.6
16.8
160
Mass number
238U
239Pu
f
f
238U
239Pu
HE
HE
Figure 1 Fission yield in percent for several nuclear fission
relevant nuclides: 235 U, 238 U, and 239 Pu, for fission induced by
fission spectrum (thermal) neutrons (f) and high-energy neutrons
(14.7 MeV) (he) respectively6
Isotope
181
Fission yield
235
Uhe (%)
Fission yield
238
Uf (%)
Fission yield
238
Uhe (%)
Fission yield
239
Puf (%)
Fission yield
239
Puhe (%)
0.06
0.29
5.53
5.67
0.05
0.19
6.76
6.97
0.06
0.18
6.02
5.84
0.05
0.24
6.97
7.54
0.07
0.42
4.86
6.18
182
RADIONUCLIDES IN THE ENVIRONMENT
2.2 Production of Xenon
Xenon gas is recovered on a commercial scale by
liquefying and the fractional distillation of liquid air and is, in
general, a by-product during the production of liquid oxygen
and liquid nitrogen. It is collected in the liquid oxygen fraction,
together with krypton and other noble gases that are present in
the air. Xenon is absorbed on a silica gel at low temperature
and then separated from the other noble gases by selective
absorption and desorption from activated charcoal.2
2.3 Production of Radioxenon
The most common and efficient way (more than 95%)
to produce radioxenon isotopes, e.g., for medical applications
is by neutron irradiation of highly enriched uranium (HEU;
uranium with up to 97% of 235 U) or low-enriched uranium
(LEU; uranium with less than 20% 235 U) (see Uranium).18,19
Uranium targets (in most cases, uranium pressed
between two aluminum plates) for the production of
radioisotopes are irradiated in a nuclear reactor for
2 – 20 days with a nuclear thermal flux between 1013 and
5.1014 n cm−2 s−1 . The targets are irradiated as long as is
necessary to create enough fission products, limited by the
attainment of steady-state production and the formation of
undesirable isotope by-products. After irradiation, the decayat-rest technique is adopted for a short while to remove
short-lived isotopes, in order to reduce the total radiation. The
uranium is then base or acid dissolved in heavily shielded hot
cells.20 During the dissolution process, which takes around
1 – 2 h, all the noble gases that were created during the fission
inside the targets, or that were since formed by their precursor
decay, are drawn off and taken care of in varying ways.21
Then, the rest of the different fission products are separated
and purified.
Depending on the goal of the facility, some noble
gases are recovered and carried with helium to krypton, xenon,
and/or iodine recovery cells.22 In these cells, the gases are
frozen out with liquid nitrogen and during warming up they
are separated from each other, trapped on a molecular sieve,
and further trapped on copper clippings. Subsequently, they are
purified and shipped to the end customer. At other facilities,
the noble gases are treated as waste. These facilities send
the noble gases into charcoal traps where they pass through
slowly as they decay. When leaving one trap, depending on
their activity, the gases will flow into another trap or they will
be released into the atmosphere.23
Radioxenon isotopes are further produced in nuclear
power reactor operations. Being fission products, they are
present within the nuclear fuel rods once the reactor is started
up and the fission process is initiated. If there is a crack in one
or more of the fuel rods, the gas will leak out and enter the
ventilation system of the facility, followed by a release into
the atmosphere.
During reprocessing of nuclear fuel rods, these rods
are dissolved to separate the isotopes. If this process takes
place within a few weeks after the fuel rods were irradiated,
there still will be enough radioxenon isotopes present that
would be discharged into the atmosphere. In most facilities,
however, fuel is stored first for a few years and therefore
the radioxenon isotopes would have decayed away below
measurable activities.
Another source of atmospheric radioxenon is 239 Pu,
whose spontaneous fission yields of the masses 131, 133, and
135 range between 4 and 8%. Plutonium-239 is continuously
produced in nuclear fuel elements storage ponds from
238
239
U: 238 U (n, γ ), 239 U, and 239 U
Pu + β. Radioxenon
239
isotopes originating from Pu could therefore be present in
nuclear reactors, nuclear fuel reprocessing facilities, or badly
contained waste storages.
3 RADIOXENON IN THE ATMOSPHERE
The most common environmental radioxenon isotope
is 133 Xe. Its half-life of 5.243 days is ideal for environmental
detection systems since it is not much accumulated in the
atmosphere, it is not washed out by precipitation, and it
remains long enough to be detectable after atmospheric
transportation to a monitoring station. This isotope is,
therefore, typically detected in various environmental samples,
originating from fission in different kinds of nuclear facilities.
The worldwide environmental background for the
longer lived noble gas isotope 85 Kr was well defined in
the 1990s. The background of the shorter lived radioxenon
isotopes, however, was not known accurately in the late
1990s and early 2000 because of nonavailability of global
and well-resolved timely data and their regional variation. To
distinguish globally a civilian radioxenon release from nuclear
facilities with the signal from a possible NE was a complicated
issue. Recent long-term environmental measurements of
radioxenon isotopes measured down to very low levels and at
high-time resolution have shown that they are all lognormally
distributed. It was shown that, e.g., in Europe, there is an
increase in 133 Xe activity concentration between 2000 and
2008, which can be attributed to an increase in the production
of radiopharmaceutical isotopes (in which radioxenon gases
are, in most cases, a waste product) during that period.13,24
It has further been shown in recent studies that a part
of the low background present in the northern hemisphere as
well as most extreme values measured are not attributed to
NPPs, as believed in the 1990s,11,14,25–27 but to the releases
from a very few large radiopharmaceutical isotope production
facilities.20,23,28,29 In these facilities, xenon radioisotopes are
a by-product created during the dissolution of the irradiated
uranium targets for the production of 99 Mo. It was shown
that during the production of radioisotopes for pharmaceutical
purposes, a significant amount of radioxenon gases is released
into the atmosphere. Such an individual release is likely
to be 100 – 10 000 times higher than typical releases from
183
XENON
4 SEPARATION AND ANALYTICAL
CHARACTERIZATION TECHNIQUES
Being a noble gas and having a very low
concentration in the atmosphere, it is demanding to separate
xenon from environmental air. Once xenon is separated, the
radioxenon isotopes can be measured in different ways, as
described below.
4.1 Atmospheric Radioxenon Sampling and
Measurements in the 1940s–70s
The first measurements of environmental radioactive
xenon reported in the literature took place in 1944 as a
part of the Manhattan Project intelligence efforts. It was the
idea of Luis Walter Alvarez (later a Nobel Prize laureate) to
sample gas above Nazi Germany and try to find 133 Xe traces
of any possible nuclear fission activities performed there.32
The gases were trapped on cooled activated charcoal in the
239Pu
235U
explosion
Target irradiation: 48 h
explosion
Target irradiation: 220 h
Discrimination line
t=0
103
135Xe/ 133Xe
Dissolution after
36 h of cooling
t=0
101
Irradiation stop
t = 48 h
10−1
Irradiation
stop t = 220 h
10−3
10−2
10−1
100
101
133mXe/ 133Xe
104
t=0
t=0
t = 1 day
102
135Xe/ 133Xe
a single NPP. These facilities release in routine operations
between 200 and 315 times per year, whereas NPP releases
are dominated by a puff once or twice per year. Literature
indicates that all NPPs worldwide release about 0.74 × 1015
Bq of 133 Xe per year.30 In Ref. 23 it has been shown
that the three largest radiopharmaceutical isotope production
facilities alone release in total 11 × 1015 Bq of 133 Xe per
year. It can, therefore, be concluded that these few large
facilities are the major contributors to the global radioxenon
background.
A good method to distinguish a radioxenon
measurement originating from an NPP from an NE was
developed by Kalinowski et al.31 If three or all four relevant
isotopes are measured, they can be plotted as the following
ratios: 135 Xe/133 Xe versus 133m Xe/133 Xe, or 135 Xe/133 Xe versus
133m
Xe/131m Xe as shown in Figure 3. This figure also shows
how the ratios of two different NE scenarios (235 U and 239 Pu
(see Uranium; Plutonium)) move over time (from upper right
to lower left). The indicated ratios of RPF releases show that
for short irradiation of the uranium target, feeding the RPF,
and a late separation (more than a day) of the ‘‘explosion’’xenon from its precursors produces very similar ratios. This
is natural as a short irradiation very much resembles an NE,
which, in turn, can be seen as a very short irradiation. It
should be noted, however, that in most environmental samples
in areas where nuclear facilities are present, most samples
contain only one or two different radioxenon isotopes.
Table 4 summarizes the typical order of magnitude of
radioxenon release from different nuclear facilities.23 It should
be noted that facility releases, although given in Bq d−1 , do
not imply that emissions happen every day, as they of course
depend on local work schedules. For NPPs, the releases are
often correlated with revision periods.
t = 2 days
Dissolution after
36 h of cooling
100
t = 5 days
10−2
Irradiation
stop t = 220 h
10−4
Irradiation stop
t = 48 h
t = 10 days
10−6
100
101
102
103
133m
Xe/
104
105
131m
Xe
Figure 3 Xenon isotopic ratio plots for two RPF feed irradiation
times (48 and 220 h) and two types of explosions (235 U and 239 Pu).
The upper plot uses three isotopes (excluding 131m Xe) and the lower
one all four. Such plots follow the ratio dependence from zero time
(sample out of reactor and explosion time respectively — upper right
starting point of all curves). They are normally useful up to around
a week. The explosion curves are shown for two cases: immediate
precursor separation (dashed lines) and no precursor separation (full
lines). Other separation times fall in between but after 1 or 2 days
they are quite close to the ‘‘no-separation’’ line. For the isotope
production, a specific separation line is used and there the lines go
from full to dashed
Table 4 Order of magnitude of releases of radioxenon at different
nuclear facilities24
Type of release
Typical order of magnitude
of 133 Xe release
Hospitals
Nuclear power plants
Radiopharmaceutical facilities
1 kt nuclear explosion underground
1 kt nuclear explosion atmospheric
∼ 106 Bq d−1
∼109 Bq d−1
∼109 to ∼1013 Bq d−1
0 to ∼1015 Bq
∼1016 Bq
184
RADIONUCLIDES IN THE ENVIRONMENT
Most xenon isotopes, in general, can be identified
using gas chromatography-mass spectrometry (GC-MS),
which is, however, both time and cost intensive.
The important radioxenon isotopes for environmental
monitoring and for NE verification (131m Xe, 133m Xe, 133 Xe, and
135
Xe), all emit photons (X-rays and/or γ -rays) in coincidence
with β- or conversion electrons (see Table 5). The β-spectrum
has a continuum (defined by its maximum energy) from the
β-decay of 133 Xe and 135 Xe and defined peaks from the
monoenergetic conversion electrons from 133m Xe and 131m Xe,
which are immediately followed by X-rays.
X-rays are in the 30-keV range and have a total
branching ratio of about 50%, except for 135 Xe, which has
just a 5% X-ray branch. The strongest associated conversion
electrons in coincidence with the X-rays are 129.4, 198.7,
45.0, and 213.8 keV for 131m Xe, 133m Xe, 133 Xe, and 135 Xe,
respectively (see Figure 4). Other strong coincident decay
modes are the 346-keV endpoint energy β-decay of 133 Xe
in association with an 81.0-keV γ -decay, and the 901-keV
endpoint energy β-decay in 135 Xe, which is followed by a
249.8-keV γ -ray (see Table 5).
131m
Xe
)
3.9
16
131
Xe
(2.
)
133m
0%
V
ke
T1/2 = 11.934 d
.0%
3.2
Xe
Ec = 129.4 keV (61.0%)
X-rays (av) 30.41 keV
(54.2%)
133
23
133m
133
Xe
V
ke
(10
T1/2 = 2.19 d
Ec = 198.7 keV (64.0%)
X-rays (av) 30.41 keV
56.5%)
135
135
b T1/2 = 5.243 d
ma
x =
)
34
.0%
133
(9 6.4
38
Xe
9.2 k
V(
% eV
e
k
)
.0
Ec = 45.0 keV
81
(55.1%)
133
Cs
X-rays (av) 31.64 keV
(49.7%)
T1/2 = 9.14 h
Xe
keV
.0 )
01
= 9 96.0%
(
4.2 Current Measurement Methods for Radioxenon
Isotopes
131m
b max
bomb bay of Douglas A-26 airplanes. Back at the laboratory,
these charcoal traps were heated to extract the gases. Xenon
was separated out using its boiling point. The radioactive
measurement of the radioxenon gas was then performed with
a standard Geiger – Müller counter.33
During the late 1950s, high-pressure gas-sampling
systems, collecting gas in stainless steel spheres, in most
cases, mounted in the bomb bays of airplanes, were developed
for the US Defence Atomic Support Agency by the Air
Force Technical Applications Centre (AFTAC) to evaluate
worldwide fallout from NEs.34–36 In the laboratories, the gas
was then analyzed using Geiger – Müller counters and later
proportional counters, sodium iodide (NaI) detectors, and
more recent high-resolution high-purity germanium (HPGe)
detectors. From the 1960s on, it is also reported that in Sweden,
Germany, and the Soviet Union radioxenon measurements
were performed to identify signals of NEs.
)
9.8
131m
Xe
Xe
133
Xe
135
Xe
133m
(a)
(b)
Half-life
Energy X-ray (keV)
(Kα1 and Kα2 )
11.84 days
2.19 days
5.243 days
9.14 h
29.62
29.62
30.80
30.80
k
24
Ec = 213.8 keV (5.7%)
X-rays (av) 31.64 keV
(5.2%)
Figure 4 The strongest decay modes for
and 135 Xe5
131m
Xe,
135
Cs
133m
Xe,
133
Xe,
Several new measurement methods are in place or are
under development to measure the radioxenon isotopes. They
are based on short sampling periods (8, 12, or 24 h) and highsensitive radioactive measurements (β – gated γ -coincidence,
high-resolution γ -spectroscopy, and proportional counting)
that can measure environmental 133 Xe as low as 0.1 mBq m−3 .
4.2.1 β — γ Coincidence Spectrometry
The nuclear measurement component of the
Swedish SAUNA (Swedish Automatic Unit for Noble Gas
Acquisition)39 developed by the Swedish Defence Research
Institute (FOI) near Stockholm, Sweden, of the US ARSA
(Automated Radioxenon Sampler and Analyzer) developed
by Pacific Northwest National Laboratory (PNNL), Richland, USA40,41 and of the new Russian ARIX-IV (ARIX,
Analyzer of Radioactive Isotopes of Xenon) developed and
commercialized by the Khlopin Radium Institute (KRI) based
in St. Petersburg, Russian Federation, are systems based on
β – γ -coincidence spectrometry. This technique is used to
suppress the noncoincident background and to achieve high
sensitivity to the coincidence events characteristic of the
radioxenons of interest (see Table 1).
Table 5 The four radioxenon isotopes discussed in this section, their half-lives, and their most intense γ -ray and X-ray
(from Ref. 37)
Isotope
.0%
90
(
eV
Intensity(a) (%)
44.4
46.1
40.9
2.1
Energy γ -ray (keV)
Intensity (%)
163.930
233.22
80.997
249.77
1.91
8.2(b)
38.0
90.0
These values are the weighted averages of the Kα1 and Kα2 X-rays. The intensities are the sum of these two Kα lines.
From Ref. 38.
XENON
In most of these systems the xenon purification
method is similar. Environmental air is sampled with an
airflow that is larger than 0.4 m3 h−1 . The sampled air is
cleaned from aerosols, water, Rn, Ar, N2 , O2 , CO2 , and xenon
is adsorbed by activated charcoal. This is followed by thermal
desorption of the xenon into a helium or nitrogen carrier. The
stable xenon volume of the concentrated gas is quantified by
gas chromatography and via an in-line thermal conductivity
measurement.
The SAUNA-II system samples air in 12-h cycles.
Then, the collected xenon fraction is purified and concentrated
for about 7 h before it is counted with the (plastic and NaI(Tl))
β – γ coincidence detector for around 12 h. The ARIX-IV
collects air as well for 12 h, while the ARSA samples for 8 h.
The β – γ detector has a NaI crystal with a drilled
hole, where the gas flows in. The hole is coated with a
plastic scintillator layer. On top and underneath the NaI cell
is a photomultiplier to count the γ -pulses and there are two
photomultipliers at the end of the scintillator cell to count the
β-pulses. The electronic system counts the γ -, the β-, and
the coincidence pulses for around 12 h. A typical spectrum is
shown in Figure 5. The two diagrams on the right side indicate
clearly the advantage of a gated spectrum versus a nongated
spectrum: the background is a few orders of magnitude lower
when using the coincidence mode.
185
Before each sample measurement, a quality control
source (e.g., 125 Eu) enters the cell and is measured, to verify
the stability of the detector. Then a gas background of the
empty cell is measured for 11 h to count the possible memory
effect of a previous sample. Memory here refers to xenon that
has diffused into the plastic cell wall, where it will contribute
to subsequent measurements. A typical memory effect in the
current β – γ systems is some 5%. Around 2 ml of stable xenon
may be extracted per sample, depending on the system used
and a minimum detectable concentration (MDC; the minimum
concentration that, with a given risk, can be expected to be
detected by a given process) of 0.1 mBq m3 for 133 Xe in a 12-h
measurement can be reached.
4.2.2 β-Gated γ -Coincidence Spectrometry
The nuclear measurement component of the ARIX
I, II, and III systems is based on β-gated γ -coincidence
spectrometry. The system collects air in 12-h cycles, which
is then purified and concentrated for around 4 h before it
is counted with the (plastic and NaI) β-gated γ -detector
for around 18 h.42 The detectors of the ARIX I – III consist
of a low-resolution γ -detector (NaI detector) and a plastic
scintillator β-detector, which are operated in β – γ coincidence
mode. To minimize the memory effect, the plastic scintillator
500
15
400
10000
Counts
450
214Pb
10
100
350
g- energy [keV]
Nongated
Gated
1000
300
5
135Xe
10
0
250
100
200
300
400
500
g- energy [keV]
0
750
200
Nongated
Gated
150
Counts
500
133
Xe
100
250
50
0
0
0
250
131m
Xe
133m
500
750
b-energy [keV]
1000
1250
1500
0
500
1000
b - energy [keV]
1500
Xe
Figure 5 This β – γ coincidence spectrum originates from an environmental air sample collected between 7 a.m. and 3 p.m. on June 10,
2002 with an ARSA system in Charlottesville, USA. It shows the presence of (0.77 ± 0.11) mBq m−3 of 133m Xe and (4.26 ± 0.35) mBq m−3
of 133 Xe. The Region of Interest (ROI) boxes are marked in white. The color code indicates the counts5
186
RADIONUCLIDES IN THE ENVIRONMENT
used is so thin that the β-energy cannot be measured and only
β-gated γ -spectra but no γ -gated β-spectra can be recorded.
The spectra are, therefore, a contraction of the β-axis, i.e.,
a summation of all β-energy channels of the corresponding
γ -energy channel. The γ -peaks at 80 keV and 250 keV
contain only counts from 133 Xe and 135 Xe, respectively and
hence can be used to quantify these two isotopes separately. As
a consequence of the β-energy contraction however, the peaks
of 131m Xe, 133 Xe, and 133m Xe at 30 keV cannot be separated by
energy spectral analysis, but by performing decay rate analysis
of the two hourly preliminary spectra. The MDC for 133 Xe
reach 0.2 mBq m−3 for a 12-h measurement — the MDC’s for
the other isotopes, however, are much higher.
In early 2007, KRI decided to stop the production
of β-gated γ -coincidence detectors for their ARIX systems
and changed to β – γ coincidence spectrometry (see Section
4.2.1).
4.2.3 High-Resolution γ -Spectrometry
A system for sampling and analyzing small amounts
of radioxenon in ambient air was developed around 1980 by
the Swedish Defence Research Agency (FOI, formerly FOA).
This was a forerunner to the SAUNA system but at that time
used charcoal adsorption of the xenon gas at −80 ◦ C and
high-resolution γ -spectroscopy for nuclear detection.43 The
MDC for 133 Xe was in average around 1 mBq m−3 .
The French Commissariat à l’Énergie Atomique
(CEA) started developing the SPALAX system (Système de
Prélèvement d’air Automatique en Ligne avec l’Analyse des
radio-Xénons) in the late 1990s. This equipment continuously
samples air for 24 h per cycle. At the end of such a collection
cycle and of the final purification, the xenon gas (around
7 ml) is transferred into the counting system, which is a
p-type broad energy high-purity germanium γ -ray (HPGe)
detector for around 23 h.44 The gas sample cell is made of
low background aluminum, on top of the germanium crystal.
At some stations, the standard HPGe end cap, which has
an aluminum window, has been replaced by a carbon fiber
window to give improved X-ray transmission. The newest
versions reach an MDC of around 0.2 mBq m−3 for a 24-h
measurement of 133 Xe.
as additional gas component. This integral counting method
gives the total activity of all radioxenons but a separation of
the components can be done by decay analysis.45
Xenon-133 is the most abundant of the radioxenons
observed in environmental samples, although contributions of
131m
Xe and 135 Xe can be determined down to a few percent of
the total β-activity. The MDC for 133 Xe in routine samples is
about 1 mBq m−3 .
4.2.5 New Developments
Some portable systems that measure the four
radioxenon isotopes 131m, 133, 133m, and 135 in the
atmosphere, have been developed at Argonne National
Laboratory in collaboration with the University of Cincinnati,
but were not built commercially. These integrated systems
consist of a fluid-based concentration subsystem and a
detection subsystem, based on NaI(Tl) photon detectors
along with either gas proportional plastic scintillator or
passivated implanted planar silicon detectors to distinguish
radioxenon signature emissions and discriminate against radon
background.46,47
At the University of Coimbra, Portugal, a β – γ
coincidence system to measure the metastable isotopes 131m Xe
and 133m Xe in high resolution (1.4 keV X-ray and 25 keV
β-emission) has been developed in cooperation with the
Los Alamos National Laboratory. It is based on two gas
proportional scintillator counters and a multiwire proportional
counter with two silicon charged particle detectors, all built
in a beryllium box, which absorbs β-signals from outside the
detector.48,49
A group at the PNNL is currently developing and
evaluating a simpler detector system than the existing ones,
named PhosWatch, consisting of a CsI(Tl)/BC-404 phoswich
well detector with digital readout electronics and pulse shape
analysis algorithms implemented in a digital signal processor
on the electronics. This system uses a single phoswich detector
in which β – γ coincidences are detected by pulse shape
analysis.50 Different prototypes are currently under testing.
5 CONCLUSIONS
4.2.4 Proportional Counting
The method of proportional counting has been
used since the 1970s by the noble gas laboratory at the
German Federal Office for Radiation Protection (BfS) in
Freiburg to continuously monitor the 85 Kr and 133 Xe activity
concentrations in ground level air in a global network. The
sample collection time during routine operation is 7 days. The
total volume sampled is around 10 m3 of air. The procedures
for sampling, enrichment, and purification of the noble
gas fractions are all manual. The integral β-activity of the
samples is measured in proportional counters using methane
Most radioisotopes of xenon are anthropogenic and
created principally in nuclear fission of 235 U, 238 U, or 239 Pu.
They are released into the atmosphere from NPPs, RRs, RPFs,
reprocessing facilities, and NEs.
Xenon-133 used to be an important isotope in nuclear
medicine, however, it is being replaced by 99m Tc gas, which
has the same physiological characteristics as 133 Xe, but a much
shorter half-life, which is, of course, in favor of the patient
(see Technetium).
The current most important application is the
measurement of radioxenon isotopes in the environment to
XENON
detect nuclear test explosions, to monitor and to verify
compliance with the CTBT. These newly developed highsensitive measurement techniques can also serve other nuclear
nonproliferation applications. Several ultralow measurement
systems have been developed and improved in the last 10 years.
To perform a waterproof verification of compliance
with the CTBT, especially to detect underground NEs
from long distances and remote stations, the global
environmental radioxenon background needs to be known
and understood. This also includes the expected radioxenon
activity concentrations as the expected and calculated ratios
of these isotopes as an advanced atmospheric transport model.
If three or four isotopes are measured, their ratios can give an
indication of the source, namely, where and how they were
made and in which process: e.g., during a long irradiation
of fuel rods in an NPP, in a short irradiated 235 U target for
radiopharmaceutical purposes, or during a very short reaction
in an NE.
In future projects, the theoretical releases for
different scenarios should be confirmed by performing online
measurements of all four xenon isotopes in the stack of
radiopharmaceutical facilities. Further, the releases of other
nuclear facilities such as research reactors should be studied.
Furthermore, a total reduction of the emissions also
appears possible, including the use of retention lines with
charcoal traps or other methods. A reduction of emissions
by a factor of 1000 or more is technically possible for
several of these known RPFs and would bring the releases to
the same level as in the NPPs (∼109 Bq d−1 ). The benefits
of such reductions should be accordingly communicated
to the radiopharmaceutical producing community, especially
keeping in mind that medical isotope production is predicted
to increase in the future.
6 GLOSSARY
Absorber: Any material that stops ionizing radiation. Lead,
concrete, and steel attenuate γ -rays. A thin sheet of paper or
metal will stop or absorb α-particles and most β-particles.
Intensity: Fraction of a decay event that results in the
radiation(s) (e.g., a γ -line at a specific energy or a β – γ
coincidence pair). Intensity is sometimes used to mean
abundance.
187
a very short timescale. May also refer to the detection of
other photon – electron coincidence events such as an X-ray
with a conversion electron.
β -gated γ -spectrum: A γ -spectrum, in which all photons
registered with its energy were in coincidence with an
electron irrespective of its energy.
Concentration: For example, activity per unit volume of air
(e.g., Bq m−3 ).
Electron capture: A radioactive decay process in which an
orbital electron is captured by and merges with the nucleus.
The mass number is unchanged, but the atomic number is
decreased by one as the process involves the transmutation of
one proton into a neutron.
Fission products: Radionuclides formed by the fission of
heavy elements. They are of medium atomic weight and
almost all are radioactive, for examples, 90 Sr, 133 Xe, and
137
Cs.
Fission: The splitting of a heavy nucleus into two major
parts of high kinetic energy, a few neutrons, and γ -energy.
Germanium detectors: In order for a significant absorption
of a γ -ray to take place, the material must have a high
enough absorption coefficient, which can be provided by a
material of high atomic number. It must also have a low
bandgap for conduction to occur, and must also have low
levels of impurities in order to satisfy the conduction
requirements. This leaves only a few possible options — the
two main candidates are silicon and germanium.
K-capture: The capture by an atom’s nucleus of an orbital
electron from the innermost shell (K) surrounding the
nucleus.
Scintillation counter: An instrument that detects and
measures γ -radiation by counting light flashes (scintillations)
induced by the radiation.
7 END NOTES
β-particle; β-radiation; β-ray: An electron of either
positive charge (ß+) or negative charge (ß−) that has been
emitted by an atomic nucleus or neutron in the process of a
transformation. β-particles are more penetrating than
α-particles but less than γ -rays or X-rays. May also refer to
other electron radiations, e.g., a conversion electron.
β – γ coincidence event: Nuclear decay producing both a
γ -ray and a β-particle that are registered in a detector within
a.
The Manhattan Project was the code name for the
project to develop the first atomic bombs during World War
II.
b.
The CTBT was opened for signature in 1996 and
is a key element in the nonproliferation of nuclear weapons
and a crucial basis for the pursuit of nuclear disarmament as it
bans any kind of nuclear explosion.
188
RADIONUCLIDES IN THE ENVIRONMENT
8 RELATED ARTICLES
H. Satorius, J. Schulze, and W. Weiss, J. Environ. Radioact.,
2002, 59(2), 139.
Anthropogenic Radioactivity.
12. L.-E. De Geer, ’Atmospheric Radionuclide Monitoring: A
Swedish Perspective’, in ‘Monitoring a Comprehensive Nuclear
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9 ABBREVIATIONS AND ACRONYMS
13. M. Auer, T. Kumberg, H. Sartorius, B. Wernsperger, and C.
Schlosser, Pure Appl. Geophys., 2010, 167(4), 4.
AFTAC = Air Force Technical Applications Centre; ARIX = Analyzer of Radioactive Isotopes of Xenon;
ARSA = Automated Radioxenon Sampler and Analyzer;
BfS = Bundesamt für Strahlenschutz; CEA = Commissariat
à l’Énergie Atomique; CTBT = comprehensive nucleartest-ban treaty; CTBTO = CTBT Organisation; GC-MS =
gas chromatography-mass spectrometry; HEU = highly
enriched uranium; IDC = International Data Centre; IMS =
International Monitoring System; INGE = International
Noble Gas Experiment; KRI = Khlopin Radium Institute;
LEU = low-enriched uranium; MDC = minimum detectable
concentration; NaI = sodium iodide; NE = nuclear explosion; NPP = nuclear power plant; NRR = nuclear research
reactor; PNNL = Pacific Northwest National Laboratory;
PTS = Provisional Technical Secretariat; ROI = Region of
Interest; RPF = radiopharmaceutical production facilities;
SAUNA = Swedish Automatic Unit for Noble Gas Acquisition; SPALAX = Système de Prélèvement d’air Automatique
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