Bremsstrahlung-Based Imaging and Assays of
Radioactive, Mixed and Hazardous Waste
J. KWOFIE1, D.P. WELLS1, F.A. SELIM1, F. HARMON1, S.P. DUTTAGUPTA2, J.L. JONES3,
T.WHITE3, and T. RONEY3.
1
Idaho State University, Idaho Accelerator Center, Campus Box 8263, Pocatello, ID 83209
2
Boise State University, Electrical Engineering Dept., Campus Box 2075, ID 83725
3
Idaho National Engineering and Environmental Laboratory, P.O. Box1625 - 2802, Idaho Falls, ID 83415
Abstract. A new nondestructive accelerator based x-ray fluorescence (AXRF) approach has been developed to
identify heavy metals in large-volume samples. Such samples are an important part of the process and waste
streams of U.S Department of Energy sites, as well as other industries such as mining and milling. Distributions of
heavy metal impurities in these process and waste samples can range from homogeneous to highly inhomogeneous,
and non-destructive assays and imaging that can address both are urgently needed. Our approach is based on using
high-energy, pulsed bremsstrahlung beams (3-6.5 MeV) from small electron accelerators to produce K-shell atomic
fluorescence x-rays. In addition we exploit pair-production, Compton scattering and x-ray transmission
measurements from these beams to probe locations of high density and high atomic number. The excellent
penetrability of these beams allows assays and images for soil-like samples at least 15 g/cm2 thick, with elemental
impurities of atomic number greater than approximately 50. Fluorescence yield of a variety of targets was
measured as a function of impurity atomic number, impurity homogeneity, and sample thickness. We report on
actual and potential detection limits of heavy metal impurities in a soil matrix for a variety of samples, and on the
potential for imaging, using AXRF and these related probes.
vadose zone transport models via bench-scale
soil column measurements.
Conventional X-ray fluorescence
(XRF) uses low-energy electron beams,
ranging from a few tens of keV to a few
hundred keV, to produce XRF directly, or to
produce bremsstrahlung X-ray beams which,
in turn, produce XRF [2]. These beams are
then used to probe thin samples for trace
concentrations of heavy metals. Samples
must be thin because of the strong attenuation
of low-energy X-rays. If, instead, one uses
higher energy x-rays and gamma-rays (which
we refer to interchangeably) from the
bremsstrahlung of 4-6 MeV electron
accelerators, their greater penetrability allows
substantially larger sample sizes for assay
and imaging. Accelerator-based X-ray
fluorescence
(AXRF)
exploits
the
penetrability of these gamma-rays to probe
INTRODUCTION:
Non-destructive assay and other nondestructive evaluation (NDA/E) methods of
measuring hazardous and radioactive wastes,
particularly heavy metals, are important
throughout the U.S. Department of Energy
(DOE) complex because of the large-scale
environmental contamination at DOE sites.
Such methods are also extremely important
for other governmental and industrial
applications [1]. Such non-destructive
measurements play key roles in the assay and
imaging of wastes and waste packages, and in
situ analyses of environmental contamination.
Such samples and packages are often highly
inhomogeneous, in which case measurements
must be able to detect and quantify heavy
metal “lumps”. NDA/E measurements also
play a critical role in ex situ analysis and
bench-marking of model systems, such as
CP680, Application of Accelerators in Research and Industry: 17th Int'l. Conference, edited by J. L. Duggan and I. L. Morgan
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Fifth, the spatial resolution and detection
limits of this probe are readily controlled by
beam collimation and detector array
segmentation and the specific requirements of
particular applications can be addressed by
custom design and fabrication of x-ray
detectors. Lastly, the penetrability of this
technique can be extended, with an increase
in electron beam energy, by using (γ, n)
reactions to produce nuclear isomers and
measure the characteristic gamma-rays of
their subsequent decays.
more deeply into samples than has been done
previously. It uses the resulting K-shell Xrays, which range from 10 to 120 keV in
energy, along with positron annihilation
gamma-rays (511 keV), Compton-scattered
gammas and X-ray transmission, to image
and assay samples. Note that since the cross
sections for both Compton scattering and pair
production are proportional to the square of
atomic number, they both have some
sensitivity to elemental content.
Also,
because of the high penetrability of gammarays, AXRF is essentially limited by the
requirement that a reasonable fraction of the
fluorescent X-rays, or other useful radiations,
must exit the sample, while conventional
XRF is also limited by the low penetrability
of the primary X-ray or electron beam.
In addition to the great penetrability
of gamma-rays, there are numerous
additional advantages to using the
bremsstrahlung radiation that is copiously
produced from 4-6 MeV electron linear
accelerators. First one can exploit both
atomic-origin K-shell x-rays, which are in the
10-120 keV energy range, and positron
annihilation gammas (511 keV) from pair
production in the sample to gain
complementary information about the
material under analysis [3].
Second,
Compton scattering of bremsstrahlung x-rays,
and x-ray transmission measurements,
provide additional ”levers” with which to
extract images and other information about
the sample. Third, the accelerator technology
is well understood, reliable and available offthe-shelf; the development of “cabinet safe”
accelerator technology, where users can work
in close proximity to an operating accelerator,
greatly expands the potential field
applications of this technology [4]. Fourth,
this probe requires no radioactive materials
and, because the photons are well below
neutron emission threshold for almost all
materials, there is no significant radioactivity
induced in the interrogated object or material.
EXPERIMENTAL METHODS:
Targets were irradiated with bremsstrahlung
beams from three different pulsed electron
LINACs. Electron LINACs of 4 MeV, 6
MeV and 18 MeV were used. LINACs were
pulsed at repetition rates ranging from 100
Hz to 300 Hz with pulse lengths ranging from
2 to 3 µs. Bremsstrahlung beams were
doubly collimated to a beam diameter (at
target) of 2.54 cm (see Figure 1).
X-ray targets for these proof-ofprinciple measurements were of two types.
The first type which were intended to address
detection limits of homogeneously mixed
heavy metals in a soil matrix, were prepared
by uniformly mixing 99.9% pure heavy
elements with soil. The eleven heavy metal
impurities ranged from tin (Z=50) to uranium
(Z=92). Impurity levels were 1% by mass.
These cylindrical targets were prepared in
two sizes, either 2.5 cm diameter, or 6.4 cm
diameter. Each of these targets was then
irradiated with a 2.5 cm diameter collimated
bremsstrahlung beam. These beams were
incident upon the sides of the cylindrical
targets. The second type of target was a 15
cm diameter cylindrical target of sand with
uranium and mercury vials placed within.
This was placed on a precision translation
and rotation table for elemental imaging
experiments.
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with IC the known impurity content (ppm), SB
(counts) the standard deviation of background
in the vicinity of the XRF peaks, and Y
(counts) is the XRF yield for the impurity in
question.
Characteristic X-rays were detected
with a shielded beryllium-window highpurity germanium detector, placed at back
angles with respect to the direction of the
bremsstrahlung beam, to minimize the large
forward background from primary and
secondary photo-electron reactions in the
samples. The 511 keV annihilation gammas
from pair production were detected with a
high-purity germanium detector (HPGe),
placed at 90 o to maximize solid angle. The
Transmission detector was a 2.5cmX2.5cm
NaI(Tl) detector.
Further experimental
details can be found in reference 3.
80
Counts
60
20
HIGH-PURITY
GERMANIUM
DETECTOR
CONCRETE WALL
40
0
0
50
100
150
200
Energy keV
FIGURE 2.
The spectrum for tin mixed
homogeneously in sand (1% by mass) is shown.
SAMPLE
LINAC
COLLIMATORS
BERYLLIUM-WINDOW
HIGH-PURITY
GERMANIUM
DETECTOR
1000
Detection Limit (ppm)
800
FIGURE 1. Shown is a generic schematic of the
AXRF set-up.
Not shown is the transmission
measurement detector.
600
400
200
0
RESULTS:
50
60
70
80
90
100
Atomic number (Z)
FIGURE 3. Detection limits versus atomic
The primary results of these experiments are
the detection limits of homogeneously mixed
heavy metal impurities in a soil-type matrix,
such as ores and wastes, and the potential for
elemental-specific imaging of the distribution
of heavy metals.
A typical spectrum for measuring
detection limits of heavy metals is shown in
Figure 2, where a tin spectrum is displayed.
Detection limits versus atomic number are
shown in Figure 3. Here DL (ppm by mass) is
defined by [2]:
DL = 2 IC SB / Y,
2.5cm
6.4cm
number. These could be improved by approximately
three orders of magnitude by using a continuous
electron beam.
Lastly, we demonstrated the potential
of AXRF for element-specific imaging. XRF
and 511 keV yield are plotted as a function of
translated location in Figure 4, where
transmission data are overlaid with these xray scattering data. The strong correlations
between all of these data convincingly
demonstrate the imaging potential of these
probes. Also, given the high penetrability of
(1)
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511 keV gammas, and 6 MeV bremsstrahlung
spectrum beams, we expect that this scheme
will work on full-scale waste packages.
CONCLUSIONS:
We have demonstrated AXRF as a viable
assay and imaging tool for the heavy metal
portion of radioactive, hazardous and mixed
waste. Detection limits, which are currently
≈ 200 ppm, can be reduced by approximately
2 orders of magnitude (see Eq. 1) with
continuous electron beams. This is because
the duty-factor of our pulsed linacs, which is
the fraction of the time that these accelerators
are “on”, is approximately 10-4. In addition,
if one exploits the more penetrating 511 keV
gammas, Compton scattered x-rays, and
transmission measurements, it appears that
one can elementally (or nearly so) image fullscale waste packages.
ACKNOWLEDGEMENTS:
FIGURE 4: A typical spectrum from the HPGe
detector showing XRF peaks at low x-ray energy, the
511 keV peak, and the Compton scattering yield in
between these two.
This work has been supported by the Inland Northwest
Research Alliance under contract ISU 001.
REFERENCES:
[1] US Environmental Protection Agency, An X-ray
fluorescence survey of lead contaminated residential
soils in Leadville, Colorado: a case study,
Environmental Monitoring Systems Laboratory Office
of Research and Development, EPA/600-R93/073, March 1993.
2400
2000
511keV yield
U XRF yield
Transmission beam
Hg XRF yield
Counts
1600
1200
[2] R. Jenkins, X-Ray Fluorescence Spectrometry,
John Willey Sons, 1999.
800
400
0
150
200
250
300
350
[3] F. Selim, D.P. Wells, J. Kwofie et al., Proceedings
of the 50th Annual Dever X-ray Conference,
Steamboat Springs, Co., 2000.
400
Translation Distance
[4] D.P. Wells et al., Cabinet Safe Study of 1-8 MeV
Electron Accelerators, Nuclear Instrumentation and
Methods in Physics Research. A463, 118 (2001).
FIGURE 5: XRF yield of uranium and mercury as
a function of translated location are shown. Also
shown are 511 kev yield and transmission data.
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