EPJ Web of Conferences 35, 01002 (2012)
DOI: 10.1051/epjconf/20123501002
C Owned by the authors, published by EDP Sciences, 2012
Actinides, accelerators and erosion
S.G. Tims1, a and L.K. Fifield1
1
Department of Nuclear Physics, Research School of Physics and Engineering, Australian National University, Canberra,
ACT 0200, Australia
Abstract. Fallout isotopes can be used as artificial tracers of soil erosion and sediment accumulation. The
most commonly used isotope to date has been 137Cs. Concentrations of 137Cs are, however, significantly lower
in the Southern Hemisphere, and furthermore have now declined to 35% of original values due to radioactive
decay. As a consequence the future utility of 137Cs is limited in Australia, with many erosion applications
becoming untenable within the next 20 years, and there is a need to replace it with another tracer. Plutonium
could fill this role, and has the advantages that there were six times as many atoms of Pu as of 137Cs in fallout,
and any loss to decay has been negligible due to the long half-lives of the plutonium isotopes. Uranium-236 is
another long-lived fallout isotope with significant potential for exploitation as a tracer of soil and sediment
movement. Uranium is expected to be more mobile in soils than plutonium (or caesium), and hence the 236U/Pu
ratio will vary with soil depth, and so could provide an independent measure of the amount of soil loss. In this
paper we discuss accelerator based ultra-sensitive measurements of plutonium and 236U isotopes and their
advantages over 137Cs as tracers of soil erosion and sediment movement.
1 Introduction
Soil erosion in Australia is recognized as a major ongoing
issue, and its mitigation by sustainable land management
practices will be an ongoing process into the future. The
effects of climate, for instance, will directly impact soil
stability, and in traditional agricultural areas will need to
be managed alongside the increasing pressure to provide
enough food for our expanding population. The need to
produce more food will require changes in farming
operations and is also likely to force expansion of
agriculture into areas not currently used. It is likely that
this will be into areas with soils of poorer structure and
less resistance to erosion. The ability to estimate decadalscale erosion losses in association with these changes in
land management practice will therefore be important in
the coming years.
Radio-isotopic tracers, particularly fallout 137Cs, have
been used for many years to study soil and sediment
movement, and in conjunction with modelling, provide a
means to assess the effectiveness of individual
management practices. Assessing the effects of land
management on soil erosion is however a difficult
process, because of the spatial and time scales involved.
Significant reliance is therefore placed on modelling, and
isotopic tracers provide a valuable tool for testing and
developing such models.
a
e-mail: steve.tims@anu.edu.au
The total 137Cs activity dispersed by the atmospheric
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the order of 1000 PBq [1]. This, and the ability to
determine the 137Cs concentration readily by counting the
662 keV J-ray emitted when the 137Cs (t1/2 = 30a) decays,
have allowed significant use of 137Cs as a tracer of soil
and sediment transport that has occurred in the time since
fallout deposition. In the Southern Hemisphere however,
fallout levels were significantly lower compared to those
in the north, and combined with the steady decline in
137
Cs activity as a result of radioactive decay, is
beginning to limit the analytical uncertainties of the
measurements. As a consequence, many 137Cs erosion
applications will become untenable within the next ~20
years. This will constrain the future utility of fallout Cs as
a tracer, and there is a real need for an equivalent
replacement.
In addition to 137Cs, the plutonium isotopes 239, 240Pu
were also distributed around the globe as a result of the
nuclear weapons tests. The total released activity was an
order of magnitude less than that of Cs, and plutonium
fallout from the tests was not widely monitored; however
the historical fallout pattern is believed to show a
structure similar to that of caesium [2]. In terms of its
suitability as a replacement for 137Cs, there is growing
evidence that Pu and Cs display similar particle-reactive
behaviour in terrestrial environments [3-5]. Furthermore,
the long half-lives of 239Pu (t1/2 = 24,110a) and 240Pu (t1/2
= 6561a) has resulted in the radioactive decay of only
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EPJ Web of Conferences
0.2% of the original fallout, and permit their use as
monitors of net soil redistribution for many decades into
the future.
background interference and improved statistical
precision. A further significant advantage is that the
240
Pu/239Pu ratio from local nuclear weapons test sites can
differ from the global average. This can permit
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to the inventory [7]. This information cannot be gained
from 137Cs measurements alone. In addition, the AMS
technique also offers the opportunity to investigate a new,
complementary tracer to plutonium for the assessment of
soil loss and movement: fallout 236U.
2 Plutonium measurements
Figure 1 shows a schematic representation of the AMS
system at the Australian National University (ANU) as
configured for Pu measurements. Soil and sediment
samples are prepared at the ANU for AMS analysis based
on the techniques described in [4]. This entails addition
of a 242Pu spike to the homogenised soil or sediment
material, and the plutonium is then leached from the
sample with hot nitric acid and purified using ion
exchange columns. The extracted material is then
dispersed in an iron-oxide matrix and pressed into AMS
sample holders.
Plutonium isotopes are currently measured with the
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the isotopes 242Pu, 240Pu and 239Pu are injected as PuOions into the 14UD accelerator in turn by adjusting the
magnetic field in the mass-analysing (Inflection) magnet
(figure 1). Details of the AMS system and measurement
methodology relevant to actinide measurements are given
in [6, 8]. The 242Pu, 240Pu and 239Pu isotopes are normally
measured for 1, 3 and 2 minutes, respectively, with the
sequence being repeated as many times as is necessary,
with three loops being typical. The isotopic ratios
239
Pu/242Pu, 240Pu/242Pu and 240Pu/239Pu are determined
from the data, and 239Pu and 240Pu concentrations in the
original sample material deduced from the known amount
of added spike. A typical 239Pu spectrum recorded with
the ANU AMS system is shown in figure 2.
Fig. 1. Essential features of the ANU AMS system for the
measurement of Pu.
The technique of Accelerator Mass Spectrometry
(AMS) counts atoms directly, rather than measuring their
radioactive decay. This is of note, because the nuclear
tests yielded over six times as many atoms of 239+240Pu as
137
Cs atoms. AMS plutonium measurements also confer a
number of advantages over caesium measurements [6],
including reduced counting times, the near absence of
Fig. 2. Typical 239Pu spectrum obtained from a soil sample. The
~20 g soil sample was collect from the top 0±5 cm of the soil
profile and yielded a 239Pu peak of over 600 counts in a 2
minute collection period.
In soils the concentrations of fallout isotopes vary
with depth as a consequence of bioturbation, mechanical
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Heavy Ion Accelerator Symposium 2012
processes that move soil grains down the profile, and also
possibly as a result of movement in solution. The shape
of the depth profile is determined by factors such as soil
type, organic matter content, porosity and by the
chemical properties of the isotopic tracer (i.e. by how
well the tracer binds to the soil particles). Caesium-137
and the plutonium isotopes bind tightly to soil particles,
and there is good evidence that the soil depth profiles of
both elements peak at or near the surface, at
approximately the same depths (figure 3) [5]. There is
also evidence that their concentrations can be well
correlated in soils and sediments collected from different
land-use types [e.g. 4, 9]. The similar behaviour of Pu
and Cs in soils, combined with the superior statistical
precision of the AMS measurements, should permit Pu
measurements to be referenced against existing 137Cs soil
inventory data. Quantification of the effects of recent
land use change, through such referenced data sets, could
provide a means to assess changes in soil redistribution
associated with modern changes in land management
practices, and will also provide new data sets with which
to test and validate soil and sediment re-distribution
models.
difference is likely to give rise to differently shaped depth
profiles ± 236U concentration depth profiles should peak
at a greater depth than those for plutonium. This may get
around a limitation of using Pu alone, i.e. that the loss of
only a thin layer of surface soil can result in the loss of
much of the Pu, which is concentrated in the top few cm
of the soil. Determination of the actual amount of soil
loss is then sensitive to the assumed depth profile of Pu
close to the surface, and is a significant source of
uncertainty. If the 236U/Pu ratio varies with depth, the
ratio in the surface soil at an eroded site could provide a
semi-independent measure of the amount of soil loss. The
ratio could also provide a means to check how well the
sampling and reference site soil characteristics match.
Furthermore, the measurement of this ratio in transported
sediment could also provide valuable information on the
average depth from which sediment has been derived by
the combination of surface wash and gullying processes.
AMS is by far the most sensitive technique with
which to measure fallout 236U in environmental samples,
and the only technique currently capable of measuring
236
U at the requisite sensitivity [11]. Furthermore,
extraction and measurement of 236U and the Pu isotopes
from the same environmental material has been
demonstrated at the levels necessary [12, 13]. There is,
however, very little data in the literature regarding
concentrations of fallout 236U in soils, and even less
regarding the use of 236U as an environmental tracer,
although first attempts have been reported recently [14,
15].
4 Summary
Fig. 3. Depth profiles for 137Cs and 239Pu in soils collected from
different land-use types. From [5].
3 Uranium-236
Comparison of 137Cs depth profiles from eroded sites
with those from undisturbed reference sites has long been
used to deduce the depth of soil material that has been
lost to erosion. This technique assumes either that the soil
characteristics at the reference site match those at the site
under investigation, or that any differences in the
characteristics are not significant factors in the loss of
soil. The use of Pu isotopes in place of 137Cs is subject to
the same assumptions.
Bomb-produced 236U also has a long half-life (t1/2 =
23.42 Ma), and potential as a complementary tracer to
plutonium for soil loss and sediment movement studies.
Uranium is expected to be more mobile in soils than
plutonium, particularly in acidic soils [10]. This
In an era where Australian agriculture is undergoing
changes in traditional management practices, and
expansion into new areas likely to be more susceptible to
erosion, AMS actinide measurements can provide a longterm solution to the declining sensitivity of 137Cs
measurements used for the assessment of soil loss and
sediment movement. The superior precision of the AMS
plutonium measurements, compared to 137Cs gammaspectroscopy data, and the opportunity for new methods
for erosion analysis using the new fallout isotope 236U,
will allow for continued improvement and validation of
soil and sediment re-distribution models.
It is noteworthy that the 240Pu/239Pu ratio, routinely
determined by the AMS analysis, can indicate the
significance of any contribution to the fallout inventory
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cannot be taken into account from 137Cs measurements
alone. This could be important in the Australian context,
as candidate areas for the expansion of agriculture
potentially fall within regions influenced by fallout from
nuclear weapons tests carried out in Australia.
There is also significant potential to make use of the
extensive database of 137Cs results that already exist, via
data sets referenced to AMS plutonium measurements.
Such data could be used to assess the efficacy of changes
in land management methods focused on minimising soil
loss.
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