Source term identification of environmental radioactive Pu/U particles by their characterization...

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Spectrochimica Acta Part B

Source term identification of environmental radioactive Pu/U particles

by their characterization with non-destructive spectrochemical

analytical techniques

M. Erikssona,T, J. Osanb, J. Jernstrfma, D. Wegrzynekc,d, R. Simone, E. Chinea-Canoc,

A. Markowiczc,d, S. Bamfordc, G. Tamborinia, S. Tfrfkb, G. Falkenbergf, A. Alseczb,H. Dahlgaardg, P. Wobrauschekh, C. Strelih, N. Zoegerh, M. Bettia,T

aEuropean Commission, Joint Research Centre, Institute for Transuranium Elements, P.O. Box 2340, D-76125 Karlsruhe, GermanybKFKI Atomic Energy Research Institute, P.O. Box 49, H-1525 Budapest, Hungary

cAgency’s Laboratories Seibersdorf, IAEA, A-1400 Vienna, AustriadFaculty of Physics and Nuclear Techniques, University of Mining and Metallurgy, 30-059 Krakow, Poland

eForschungsgruppe Synchrotronstrahlung, FZK Research Centre, D-76021 Karlsruhe, GermanyfHamburger Synchrotronstrahlungslabor HASYLAB at Deutsches Elektronen-Synchrotron DESY, Notkestr. 85, 22607 Hamburg, Germany

gRisø National Laboratory, Dk-4000 Roskilde, DenmarkhAtominstitut der Osterreischen Universitaten, Technische Universitat Wien, Stationallee 2, A-1020 Vienna, Austria

Received 19 November 2004; accepted 15 February 2005

Available online 7 April 2005

Abstract

Six radioactive particles stemming from Thule area (NW-Greenland) were investigated by gamma-ray and L X-ray spectrometry based on

radioactive disintegration, scanning electron microscopy coupled with energy-dispersive and wavelength-dispersive X-ray spectrometer,

synchrotron radiation based techniques as microscopic X-ray fluorescence, microscopic X-ray absorption near-edge structure (A-XANES) aswell as combined X-ray absorption and fluorescence microtomography. Additionally, one particle from Mururoa atoll was examined by

microtomography. From the results obtained, it was found out that the U and Pu were mixed in the particles. The U/Pu intensity ratios in the

Thule particles varied between 0.05 and 0.36. The results from the microtomography showed that U/Pu ratio was not homogeneously

distributed. The 241Am/238 +239 +240Pu activity ratios varied between 0.13 and 0.17, indicating that the particles originate from different

source terms. The oxidation states of U and Pu as determined by A-XANES showed that U(IV) is the preponderant species and for Pu, two

types of particles could be evidenced. One set had about 90% Pu(IV) while in the other the ratio Pu(IV)/Pu(VI) was about one third.

D 2005 Elsevier B.V. All rights reserved.

Keywords: Spectrochemistry; Synchrotron radiation techniques; Environmental radioactive microparticles

1. Introduction

In the last 10 years, it has repeatedly been observed that

nuclear or other radioactive material has been released into

the environment or illegally possesses. This as a conse-

0584-8547/$ - see front matter D 2005 Elsevier B.V. All rights reserved.

doi:10.1016/j.sab.2005.02.023

T Corresponding authors.

E-mail addresses: [email protected] (M. Eriksson)8

[email protected] (M. Betti).

quence of nuclear accidents and the inadvertent destruction

of devices containing nuclear warheads; nuclear weapons

tests; illegal dumping of nuclear scrap or waste; releases of

traces of radioisotopes from declared or clandestine activ-

ities; orphaned radioactive sources; diverted nuclear mate-

rial; illicit trafficking of nuclear or other radioactive

material. In investigating such incidents, questions arise

regarding their impact on the environment and human health

as well as the intended use, the origin and, when applicable,

the smuggling route for the dispersed material.

60 (2005) 455–469

M. Eriksson et al. / Spectrochimica Acta Part B 60 (2005) 455–469456

In the majority of the abovementioned release scenarios,

the radioactive material is dispersed into the environment as

discrete particles. The relatively few large sized of these

radioactive particles, also called hot particles, carry the

majority of the released activity [1,2] and act like radio-

active point sources in the environment. Therefore, it is very

important to study their environmental impact in terms of

mobility, weathering and corrosion rates [3]. From this point

of view, the identification of the source term is necessary.

On the other hand, the source term identification is essential

also in nuclear forensic investigations. The elemental

mapping of the major constituents as well as the content

and distribution of trace elements need to be determined.

From the structure of the particle, in terms of distribution of

the chemical elements, the origin and the process used for its

production can be revealed. Moreover, information on

history of the material in terms of preferential leachability

and, therefore, transfer to the food web can be obtained. In

relation to this, the elemental speciation plays one of the

most important roles. In fact, chemical speciation as the

determinant of reactivity is critical towards understanding

and predicting the fate and transport in the environment of

radionuclides like uranium, plutonium, neptunium, ameri-

cium, strontium, that are the most common related to the

abovementioned release scenarios.

In this paper, the attention has been focussed on the

source identification of particles released as a consequence

of the inadvertent destruction of four nuclear weapons as

occurred in the Thule area (NW-Greenland) in January

1968. Details of this accident and obtained results have been

published elsewhere [1,4–8]. Moreover, one particle stem-

ming from a different release scenario, namely from

Mururoa (French Polynesia) where during 1966–1996

atmospheric and underground nuclear weapons tests were

performed, has also been investigated.

The environmental radioactive particles are generally

embedded in a bulk matrix, therefore, it is necessary to

isolate them and perform specific single grain analysis to

provide unbiased results. The analytical approach has been

to perform non-destructive measurements in order to reveal

their chemical composition. One of the advantages of using

non-destructive methods is the direct analysis of the

samples. Thus avoiding problems of cross-contamination,

particularly important for U at the ng levels, as contained in

the particles here studied.

Characteristic L X-rays and gamma rays from the

radioactive disintegrations were measured. This provides

information on the specific radionuclide composition of the

radioactive particles. Before analyzing the particles by

microscopic X-ray fluorescence (A-XRF), they have been

examined by scanning electron microscopy (SEM) com-

bined with energy-dispersive X-ray (EDX) or wavelength-

dispersive X-ray (WDX) spectrometry. This is of particular

interest when the particles are supposed to contain a mixture

of U and Pu, revealing the surface and to some extent

subsurface composition. Utilizing A-XRF at a synchrotron

radiation (SR) facility, information on the elemental

composition of the particles has been obtained. In particular,

by using a beam size of 2.5 Am, the fine structure of the

material was observed. The elemental distributions of U and

Pu in a particle were also studied with combined X-ray

absorption and fluorescence microtomography. Microscopic

X-ray absorption near-edge structure (A-XANES) has beenexploited to study the oxidation states of uranium and

plutonium in U/Pu mixed particles. This technique has

largely been applied by several authors in order to study the

oxidation states of uranium in individual particles bearing

uranium [9–13]. However, even though studies by XANES

on the oxidation states of plutonium aqueous ions have been

systematically performed [14,15], the plutonium oxidation

states in single grain analysis of environmental released

radioactive particles have been studied for the first time in

the present investigation.

2. Material and methods

2.1. Sample treatment

The hot particles were separated from sediment samples.

The samples originate from three different sampling

campaigns (in: 1968, 1979 and 1997) at the Thule accidental

site (NW-Greenland). The sediments were sliced on site and

stored frozen until the arrival at the laboratory, where they

were freeze-dried. Since then the sediments have been

stored in plastic boxes, and the material has been exposed to

normal atmosphere during the storing. The particle stem-

ming from Mururoa nuclear test site was separated from a

coral sediment.

2.2. Instrumentation

2.2.1. Beta Camera

The Beta Camera is a detector for on-line digital

detection of charged particles (a-, h-particles and conver-

sion electrons) and low energy X-rays from thin samples.

The sample is mounted in close contact to a thin scintillator

(the scintillator is on top of an image photon detector, IDE).

The Beta Camera has better sensitivity than normal auto-

radiographic films and has a spatial resolution of ~0.5 mm

[16]. This technique is much faster compared to film

autoradiography and does not need processing before the

radionuclide/radioactive particle distribution on the sample

is determined. This device has been proven to be a useful

tool to localize and identify hot particles released to the

environment [14,17].

2.2.2. Gamma- and X-ray spectrometry based on radio-

active disintegration

All the particles were analyzed by a point source

calibrated n-doped planar HP(Ge)-detector with Be-window

(EG and G ORTWC). The detector has an energy resolution

M. Eriksson et al. / Spectrochimica Acta Part B 60 (2005) 455–469 457

of 530 eV (FWHM, full width at half maximum) at 122 keV.

The detector was cooled with liquid N2 and provided with a

voltage of �2000 V from a high voltage supply (EG and G

Ortec 659). The pulses obtained from the HP(Ge)-detector

were magnified and shaped by a spectroscopy amplifier

(Canberra 2021) and analyzed in the multichannel buffer

(EG and G Ortec, Ethernim 919E). The spectra were

acquired with the software GammaVision32 (EG and G

Ortec). The detector is placed in a well-ventilated room and

is shielded with 5 cm of lead with low content of 210Pb.

2.2.3. Scanning electron microscopy

All the particles were analyzed by Personal SEM

(ASPEXk Instruments) scanning electron microscope with

an energy dispersive X-ray detector (EDX) attached. The

EDX system uses a 10 mm2 Si(Li) detector with an energy

resolution of 135 eV. The analyses were carried out with an

emission current of 30 AA at an acceleration voltage of 20

kV and the probe to sample distance was adjusted between

16 and 18 mm.

One particle was additionally analyzed in a Tescan Vega

TS 5130MM SEM with a wavelength dispersive X-ray

(WDX) analyzer. The WDX spectrometer was a INCAWave

(Oxford instruments) system using an INCA Energy+soft-

ware. For the analysis, a PET-crystal was used with an

energy resolution of 20 eV. The analyses were carried out

with an acceleration voltage of 30 kV and a beam current of

1.29 nA.

2.2.4. X-ray fluorescence microprobe

The A-XRF measurements were carried out at the X-ray

fluorescence microprobe station of the FLUO-TOPO Beam-

line of the ANKA synchrotron facility (Karlsruhe, Ger-

many) [18]. A monochromatic beam with photon energy

23.2F0.3 keV and a photon flux of ~1012 ph s�1 mm�2

was focussed by a mono-capillary. The resulting microbeam

had an intensity of ~4�109 ph s�1 and a diameter of 20

Am, resulting in a footprint of 30�20 Am2 on the sample

due to the 458 geometry. A collimated Si(Li) detector

[resolution: 133 eV (FWHM) at 5.9 keV] was placed in a

right angle to the incoming beam in order to minimize the

inelastic scattering going in to the detector. On top of the

detector collimator, a 20 Am Al foil was attached to prevent

the low energy X-rays to reach the detector and thereby

reduce the deadtime. Adjusting the detector–sample dis-

tance by moving the detector also controlled the deadtime.

The slits in the beamline and absorbers of the exciting beam

were adjusted in order to reach optimal photon flux for the

measurements. The elemental maps and the Pu/U-ratios

were determined from the intensities of U and Pu-La X-ray

lines. Elemental maps were produced with a step size of 10

Am, in both horizontal and vertical direction of the sample,

with a spectral acquisition real time of 20–30 s at each

pixel. At the pixel of each recorded map where the

maximum U-La X-ray intensity was observed, a second

acquisition was done to determine the Pu/U-ratio. The data

acquisition times were chosen so that the precision of the

measurements was better than 1.5% for both elements of

interest. The spectra were deconvoluted with a software

program called AXIL [19] using non-linear least-squares

fitting. The energies and relative intensities of the L and M

line series of plutonium have been added to the X-ray

library of AXIL, based on a database of X-ray transition

energies [20].

2.2.4.1. Microspatial elemental distribution measurements

with a 2.5 lm X-ray beam. In order to obtain a better spatial

resolution for the study of the microstructure of single

particles, the X-ray beam has been focussed with a new type

of compound refractive X-ray lens [21] down to a beam size

of a few micrometers. The photon energy was set to 18.7

keV in order to obtain ideal focus conditions of the lens and

to excite Pu-L3 and U-L3 edges. By knife-edge scanning of

a 0.5 Am thin Ni/Fe structure (IRMM 301 standard), the

beam size was measured as 2.5�2.5 Am2.

2.2.5. Combined X-ray absorption and fluorescence

microtomography

The X-ray microtomography spectrometer designed by

the IAEA Laboratories (Seibersdorf, Austria) has been set

up at the FLUO-TOPO beamline at ANKA (Fig. 1). A

monochromatic synchrotron beam of an energy equal to

18.8 keV, provided by a multilayer monochromator has

been used for performing three dimensional (3D) tomo-

graphic scans of individual hot particles. The measure-

ments were performed in pencil-beam geometry.

Absorption and X-ray fluorescence projections have been

collected simultaneously.

The primary beam intensity was monitored with an

ionization chamber installed between the beamline slits and

the compound refractive lens (CRL). The lens focused the

beam down to dimensions of 7.5 Am horizontal by 3.6 Amvertical FWHM. The beam dimensions were obtained by

performing two perpendicular line scans over a 4 Am thick

tungsten wire and subtracting the wire thickness from the

profiles shown in Fig. 2. A fast silicon drift detector (SDD)

allowing acquisition at high count rates was placed behind

the sample to collect photons transmitted through the

sample. The Si(Li) semiconductor detector, for detection

of X-ray fluorescence photons, was mounted in the orbital

plane at 908 to the incoming beam. The samples were

mounted on a XYZu translation/rotation stage, allowing to

perform lateral scans of the sample as well as a sample

rotation by 3608, necessary for fluorescence tomography.

The data acquisition system consisted of 3 multichannel

analyzers for collecting transmitted photons, fluorescence

photons and monitoring the incident beam. Acquisition time

was set to 0.15 s and 0.10 s per pixel for Si(Li) and SDD,

respectively.

The particles analyzed in the tomographic setup were

glued on top of a thin graphite pin supported by micro-

manipulation under a light microscope.

Fig. 1. A picture of the X-ray microtomography spectrometer setup at the ANKA FLUO-TOPO beamline.


2.2.6. Microscopic X-ray absorption near-edge structure

spectrometry

The measurements were performed at the microfluor-

escence beamline L at the HASYLAB synchrotron facility

(Hamburg, Germany) [22]. The geometry of the exper-

imental setup was similar to that in A-XRF experiments at

ANKA, but the A-XANES measurements required a tunable

monochromatic beam with higher energy resolution. The

white beam of a bending magnet was monochromatized by

a Si(111) double monochromator. A polycapillary half-lens

(X-ray Optical Systems) was employed for focusing a beam

of 1�1 mm2 down to a spot size of 15 Am diameter. With

Fig. 2. Beam profiles obtained in the microtomography in

an energy resolution of around 10�4, a photon flux of ~109

ph s�1 in the focused beam of 15 Am diameter could be

reached. The absorption spectra were recorded in fluores-

cent mode, tuning the excitation energy near the L3

absorption edge of U and Pu (17167 and 18060 eV,

respectively) by stepping the Si(111) monochromator, while

recording the La fluorescent yield of the element of interest

using an energy-dispersive GRESHAM Si(Li) detector. The

used energy step size varied from 0.5 (edge region) to 2 eV

(more than 50 eV above the edge). The measuring time for

each energy point varied from 5 s to 20 s depending on the

concentration of the element of interest. Although the

vestigation by scanning a 4 Am thick tungsten wire.


oxidation state of the element could be estimated based on

the shift in core electron binding energies, the real position

of the absorption edge is masked with the strong white line

observed in the U and Pu-L3 XANES spectra. For this

reason, a least-squares fitting method developed originally

for arsenic K-edge absorption spectra [23] was used for

processing U and Pu-L3 XANES spectra. Analytical

functions were used to model the different fine structures

in the spectrum. Oxidation states (IV) and (VI) were

considered for the fitting, allowing the determination of

the ratio of the two oxidation states of U or Pu in the

particles. The energies and intensities of the white line and

multiple scattering peak, as well as the parameter of the

arctangent step for oxidation states (IV) and (VI) were

determined using the uranium and plutonium standard

spectra separately. The ratios of the different oxidation state

forms in the particles were calculated using these parameters

fixed in the fitting function. For uranium, particulate UO2

and U3O8 standards were measured. In addition to the

measured standard spectra, fluorescence mode standard

spectra of U(IV) and U(VI) measured by others at NSLS

[24] were also used for the spectrum processing. For

plutonium, Pu(IV) oxalate and Pu(VI) hydroxide particles

in house prepared and tested were used as standards.

3. Results and discussion

3.1. Separation of individual hot particles and identification

by a Beta Camera

In order to obtain single grain hot particles from bulk

sediment samples, a sample splitting technique was used.

The technique is based on the decay of the plutonium

isotope 241Pu (TO=14.35 y), which is present in the

weapons material, to the gamma emitting daughter 241Am

(TO=432.2 y, gamma line: 59.54 keV, intensity: 35.9%),

measurable by gamma spectrometry. When 241Am activity

is identified, the sample is split into two equal halves and

measured again. The part with the activity is again divided

and measured. After about 20 sample splitting, starting with

about 150 g of bulk material, only a few grains (~Ag)remain. This technique is explained in detail elsewhere [17].

Only one single radioactive particle remains after a

complete sample splitting of the bulk material. The longest

working time spent on the sample splitting (the particle with

the lowest activity, ~2.5 Bq) was around 4 h. The single

radioactive particle was attached to an adhesive carbon tape.

To identify and locate the particle position on the tape, the

Beta Camera was used. Since the Beta Camera is also a

light-sensitive device, and there is need for a position

reference system on the tape, five holes were punched out in

a special pattern. The holes were superimposed by light on

the image obtained from the acquisition of the hot particle.

For all the particles in the present study, the acquisition time

on the Beta Camera was less than 30 min. This is a very

short time compared to the more common autoradiographic

film technique, which usually takes weeks before an image

is produced.

3.2. Gamma and X-ray spectrometry

The differences between the XRF and the characteristic

X-rays occurring in the disintegration process of a radio-

active nuclides is that in XRF the excitation is stimulated

(by photoelectric interaction) and the atoms emit the

characteristic X-rays. In the disintegration of one radioactive

nuclide, there is a competition between gamma and internal

conversion electron emission of the excess decay energy of

the daughter nuclide. Each radioactive nuclide has its own

emission yield, and by analyzing the gamma and low energy

X-ray spectra isotopic and nuclide information can be

determined, whereas from the XRF analysis only elemental

information can be gained. Emission of a conversion

electron will lead either to characteristic X-ray or Auger

electron emissions. For elements with a high atomic

number, like Pu and Am, the probability of emitting

characteristic X-rays is much higher than that of Auger

electron emission. The particles investigated in this study

contained Pu, Am and U, where the contribution of U to the

total radioactivity is very small, even though the mass of235U exceeds the mass of 239Pu (mass ratio 239Pu/235U ~1/

2.9, equalling activity ratio 239Pu/235U ~10,000). As for241Am, the relationship is opposite, i.e. the activity ratio239Pu/241Am will be 1 for a mass ratio 239Pu/241Am=56.

The main activity of the particles, however, originates from

the Pu-isotopes: 238,239,240,241Pu. Since 241Pu only emit

negligible numbers of characteristic X-rays in its disintegra-

tion process (about 4–5 orders of magnitude less than for238,239,240Pu), it is possible to determine the 241Am and238,239,240Pu activities by analyzing the X-ray spectra. The

four L X-ray peak regions in the spectra, P1, P2, P3 and P4(Fig. 3a), are a linear combination of the contribution from

each disintegration of the four nuclides, 238,239,240Pu and241Am, consequently a set of equations can be derived like

in Fig. 3b. Assuming that the efficiency for each peak region

is constant, the peak area can be divided by its peak region

efficiency (Pi / ei =PVi). In Fig. 3a, the key lines occurring

from the 241Am and 238,239,240Pu disintegration processes

are marked with red and grey/blue colors, respectively. As

can be seen, each peak region, P1–P4, consists of more than

one L X-ray line. The total probability of X-ray emission per

disintegration (activity of nuclide i, Ai) for a particular

emission energy for the nuclide i in one peak region, is

denoted with: gnuclide ipeak. The equation system can be

written in a matrix form (P=H A) and there will be one

unique solution if the H matrix can be inverted (A=H�1 P).

However, the uncertainties of the determined activities with

the used spectrometric setup are relatively large, and for

some spectra, the solution of matrix operation gave negative

activities for one or more of the radionuclides. This is due to

poor statistics, the fluctuation of the background during the

10 11 12 13 14 15 16 17 18 19 20 21 22

Energy [keV]

102

103

104

105

2

345

7

2

345

7

2

345

7

2

345

7

2

Cou

nts

Zoom

10 20 30 40 50 60101

102

103

104

24

24

24

2

P1

P2 P3

P4

a)

b)

Fig. 3. (a) Spectrum from a Thule particle on a n-doped HP(Ge) detector with Be window. The spectrum contains gamma and L X-ray lines from 241Am (red

lines) and Pu isotopes (grey lines are L X-ray intensities assuming activity: 19.5% 240Pu, 79.5% 239Pu and 1% 238Pu). Blue lines indicate gamma lines from239Pu disintegration. (b) The L X-ray matrix calculation for the determination of the radionuclide activities in the spectra.



acquisitions, the uncertainties on the intensities-data of the

X-ray transitions and possible self-absorption processes.

Therefore, the gamma and X-ray spectra were only used to

determine the 241Am and the total 238 + 239 + 240Pu activities.

This is possible for the particles investigated here, since it is

known that the debris from Thule consists only of two

sources of unique Pu isotopic composition [1,25,26].

Knowing that, for these two different sources, denoted as

bHigh ratioQ and bLow ratioQ, the relative activity for238Pu,239Pu and240Pu is 1.56; 81.49; 16.95 and of 0.99;

90.01; 9.00%, respectively, the total 238 + 239 + 240Pu activity

could be calculated. In addition, the 241Am activities were

determined from the 59.54 keV gamma line. By using this

determined activity, each X-ray peak in the spectra consists

only of one undetermined component, i.e. the activity of the

bHigh ratioQ or of the bLow ratioQ. In Table 1, the

determined 241Am and 238+ 239 + 240Pu activities are reported.

The differences in the determined activities for the bHighratioQ and the bLow ratioQ are small, as expected. The241Am/238 + 239 + 240Pu activity ratios vary from 0.13 to 0.17,

which is in accordance with other studies of sediment from

the Thule area [1,27].

A similar approach has been used to determine the Pu

isotopic ratios in bulk Thule samples [25,28,29]. A radio-

chemical separation of Pu from the bulk sample was

performed and the 238Pu and 239+ 240Pu activities were

determined by alpha spectrometry. With this information,

the 240Pu/239Pu activity ratio is obtained from the X-ray

spectrometric data.

3.3. SEM

All the particles were analyzed by SEM with an EDX

detector attached. The morphology, size and the surface

elemental composition of the particles were studied. The

secondary (SE) and back-scattered electron (BE) images

also served as guiding maps when relocation of the particles

was done at the SR-facilities ANKA and HASYLAB. The

SEM used has a maximum acceleration voltage of 20 kV,

which did not allow the efficient excitation of U and Pu-L

shells. In the EDX spectra the M X-ray peaks of U and Pu

were not resolved due to the strong overlap between the Ma

peaks of Pu and the Mh peaks of U. Absolute determination

of the Pu/U element ratio could not be done, as standard

mixed Pu/U particles would be required to calibrate the

Table 1

Particle size as obtained by SEM

Thule particle ID.

(sample year)

Particle

size (Am)

Activity241Am (Bq)

238+

bHig

Thu68-3 (1968) 24 4.18 26.5

Thu68-5 (1968) 42 9.05 53.7

Thu68-6 (1968) 37 2.42 16.5

975374-5 (1997) 15 0.31 2.0

241Am activity and Pu isotope activity in high and low ratio source term as obtai

Elemental ratios as derived from the A-XRF measurements at ANKA.

pulse height spectra in combination with deconvolution of

the spectra or a Monte Carlo simulation of the source–

sample–detector system [30].

From the BE images with the EDX spectra in Fig. 4

(left column), it can be seen that both U and Pu are present

on the surface of all the particles. In Table 1, measured

average sizes of the high atomic number areas (appear as

white areas in the BE images, due to the atomic number

contrast formed by back-scattered electrons) are presented.

The particle sizes ranged from 15 to 42 Am. As can be

seen, some of the particles are embedded or to some extent

coated. These coatings were analyzed with EDX and

contained elements typical in sediment material, i.e. Si, Al,

Mg and Fe. Several spots on the high atomic number areas

of the particles were analyzed with EDX. All the analyses

showed that U and Pu co-exist on the whole high atomic

number surface. From the U and Pu M X-ray peak

intensities variations between different particles were

observed; however, the magnitude of the variations could

not be determined with this system.

In addition, one particle (Thu79-3) was measured with a

SEM equipped with a WDX analyzer. The difference

between the two systems is that EDX uses a Si(Li) detector

with a multichannel analyzer (MCA), while WDX is based

on the diffraction of the produced characteristic X-rays in a

crystal lattice and the X-ray quanta are detected by a

proportional counter. WDX measures only one energy

(wavelength) at a time, leading to a better resolution but a

lower efficiency. However, the low efficiency leading to low

count-rate can be compensated to some extent by the use of a

higher beam current from the filament. With the WDX

system used, it was possible to determine the Pu/U elemental

ratios from the M X-ray lines without doing deconvolution

on the spectra. This was done by correcting the area for the U

Ma and Pu Mh peaks (non overlapping peaks) with the

emission probabilities.

In Fig. 5 (upper row), the BE image of the particle, along

with SEM-EDX elemental maps of C, Si and U is

illustrated. As can be seen, this particle is constituted of

many small fragments/inclusions. Carbon is the main

constituent of the particle, indicating that it was formed

during the accident, probably from burning of rubber or

similar materials. When the weapons were disintegrated by

conventional explosion, fragments from them were trapped

in this piece of burning material. Five spots/inclusions of

240 + 240Pu

h ratioQ (Bq)

230+ 240+ 240Pu

bLow ratioQ (Bq)Pu/U

(La–intensity ratio)

29.0 0.30

58.9 0.32

18.1 0.22

2 2.21 0.36

ned by gamma and L X-ray measurements.

Fig. 4. Left column: BE images of the Thule hot particles with the corresponding EDX-spectra; middle column: uranium intensity maps; right column:

plutonium intensity maps. The intensity maps are corresponding to the La X-ray lines. The unit of the intensity color scale is in counts per sec; the units of the

x–y axis are in Am.


0

5

10

15

20

20 40 60 80 100 120

20

40

60

80

U/Pu-intensity ratio Thu79-3

0

1000

2000

3000

4000

20 40 60 80 100 120

20

40

60

80

Pu-intensity Thu79-3

0

5000

10000

15000

20 40 60 80 100 120

20

40

60

80

U-intensity Thu79-3

C Si U Ma U Mb

Fig. 5. Upper row: BE image of a Thule hot particle and SEM elemental map of the particle area investigated by A-XRF (100�130 Am2). Bottom row: U and

Pu-La intensity map produced with a 2.5 Am beam A-XRF. The unit of the intensity color scale is in counts per sec; the units of the x–y axis are in Am.


this particle were analyzed with the respect on the Pu/U

elemental ratio. In Fig. 6, a typical WDX spectrum can be

seen from these analyses. Superimposed to the WDX

spectrum is the corresponding EDX spectrum, for compar-

ison of the different resolutions of the systems. The

determined Pu/U elemental ratios were from 0.09 to 0.43

in these five spots. The large variations in the ratio are

probably due to the fact that the inclusions originated from

different parts of the weapons rather than to an effect of their

preferential leaching.

3.4. l-XRF

From the elemental maps of U and Pu as reported in

Fig. 4 (middle and right column), it can be seen that the

two elements are homogeneously distributed in the

UMα

2.5

Skalenbereich ED 1757 cts Skalenbereich WD 244.0 cts/s Cursor: 3.337

3

Fig. 6. Example of one SEM-WDX spectrum collected from a U/Pu particle. The S

comparison. (For interpretation of the references to colour in this figure legend, t

particles. In Fig. 7, the A-XRF spectra using a 23.2 keV

monochromatic beam for the four particles can be seen. In

the spectra from the particle 975274-5, there are peaks

originating from lead. However, lead does not originate

from the particle but from the carbon tape support. As can

be seen, for the other three particles where lead free carbon

supports were used, no lead was detected. Since the Pu/U-

La intensities are only corrected for the detector deadtime

and not for physical properties of the photon transport in

the particles, these ratios cannot be considered as concen-

tration ratios. As reported in Table 1, the Pu/U intensity

ratios vary between 0.22 and 0.36. This variation may

reflect several phenomena. A preferential leaching of one

element with respect to the other one; the origin of the

particles from different parts of one of the involved nuclear

weapons; or that the particles stem from a different source

PuMβ

3.5 4

keV

EM–EDX spectrum of the same particle is superimposed in yellow color for

he reader is referred to the web version of this article.)

0 5 10 15 20 25

0.01

0.10

1.00

10.00

100.00

CP

S

Thu68-3

U, Pu

U

Pu U

U, Pu

PuU, Pu

Fe

0 5 10 15 20 25

0.01

0.10

1.00

10.00

100.00 Thu68-5

U, Pu

U

Pu UU, Pu

Pu

U, Pu

0 5 10 15 20 25

0.10

1.00

10.00

100.00

CP

S

Thu68-6

Energy [keV]

U, Pu

U

PuU

U, Pu

PuU, Pu

0 5 10 15 20 25

0.01

0.10

1.00

10.00

100.00

CP

S

975374-5

Fe

Pb Pb

U

Pu UU, Pu

PuU, Pu

CP

S

Energy [keV] Energy [keV]

Energy [keV]

Fig. 7. A-XRF spectra collected at ANKA using 23 keV monochromatic excitation at the maximum uranium intensity locations of the four particles, Thu68-3,

Thu68-5, Thu68-6 and 975374-5.


term. Detailed investigation on the Pu isotopic ratios by

mass-spectrometry, complementary to those below

described as microspatial elemental distribution and micro-

tomography, must be performed before concluding which

of the abovementioned causes is the most probable.

3.4.1. Microspatial elemental distribution measurements

with a 2.5 lm XRF beam

The same particle analyzed with the SEM–WDX system

was analyzed using a 2.5 Am A-XRF beam at ANKA. The

particle, with the size of 350�250 Am2, was scanned over

an area of 130�100 Am2 with step sizes of 1.25 Am in both

z and y directions and 3 s per point as measuring time.

Fig. 8. Left: reconstructed distribution of elements in U/Pu-rich particle recovered

(green) are coated with iron (red) rich sediment matrix. Right: distribution of pluto

As can be seen in Fig. 5 (bottom row), the U and Pu-

La intensity maps correspond fairly well, however, in the

U/Pu intensity ratio there are variations. Some regions

have an U/Pu intensity ratio as high as 20 (Pu/U 0.05).

This means that the inclusions in the particle originate

from different parts of the weapons or from different

weapons, rather than due to an effect of leaching, as the

inclusions have been more or less isolated from the

aquatic environment in this large particle. A-XRF measure-

ments with 20 Am beam (Section 3.4 A-XRF) would have

never revealed the fine structure of the particle due to the

blowQ spatial resolution. When analyzing individual Pu/U

particles with the respect of source term identification and

from sea sediment, Thule, Greenland. The plutonium (blue) and uranium

nium (blue) and strontium (yellow) in a particle collected at Mururoa Atoll.

Fig. 9. Cumulative X-ray spectrum obtained by summing 15360 individual spectra collected during A-tomographic scanning of the U/Pu-rich Thule particle.

The individual spectra originated from a 3D region surrounding the U/Pu-rich volume of the particle.


leachability, the use of the larger (20 Am) and the smaller

(2.5 Am) spot size is a complementary. The large beam

provides the average elemental ratio in the particles and the

small beam the possible heterogeneity in the elemental

distribution.

As can also be seen in Fig. 5, between the U X-ray map

produced with a SEM–EDX and the A-XRF map, there is no

good correlation for the scanned area. This is due to the fact

that the SR-XRF penetrates into the depth of the large

particle while the SEM–EDX only maps the surface and to

some extent in the subsurface. Underlying objects in the

particle that are excited by the SR X-ray beam will be

incorrectly positioned in the produced image, as the

measurements are performed with a 458 angle. To solve

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

-40 -20 0 20 40

Energ

No

rmal

ized

ab

sorp

tio

n

Fig. 10. The U A-XANES spectra of the particles Thu68-

this problem, confocal three dimensional (3D) measure-

ments [31,32] in hyper plans of the particle or micro-

tomography measurements would be needed.

3.5. Microtomography

The elemental distribution in two particles was inves-

tigated: in a U/Pu-rich particle recovered from the sea

sediment near Thule, NW-Greenland, and in a Pu-rich

particle embedded in a coral matrix, collected from the

surface soil in the Mururoa Atoll. The particles were

scanned in pencil-beam geometry [33,34]. For each particle,

64 area projections, separated by 5.6258, consisting of 64

(horizontal) by 10 (vertical) pixels were collected simulta-

60 80 100 120 140

y (eV)

UO2

U(VI)

Thu68-3

Thu68-5

Thu68-6

975374-5

U3O8

3, Thu68-5, Thu68-6, 975374-5 and the standards.

Table 2

Results of least squares fitting of the U-L3 A-XANES spectra of four individual U/Pu particles collected from Thule

Thule particle ID. U(IV) (%) U(VI) (%) Iw(IV) Iw(VI) rw Is(IV) Is(VI) rs RMS

Thu68-3 91 9 0.73 0.07 6.97 0.09 0.05 6.36 0.00011

Thu68-5 91 9 0.78 0.08 7.12 0.09 0.05 6.34 0.00011

Thu68-6 100 0 0.91 0.00 6.28 0.10 0.00 5.71 0.00019

975374-5 75 25 0.75 0.25 7.10 0.07 0.02 5.62 0.00014

Iw and rw: intensity and width of the white line, Is and rs: height and width of the multiple scattering peak, RMS: root-mean-square. Indices (IV) and (VI)

stand for the oxidation state.


neously in X-ray fluorescence and X-ray absorption modes.

The horizontal beam spacing were 19 Am and 11 Am for the

Mururoa and the Thule particles, respectively. The corre-

sponding vertical beam spacings were 42 Am and 20 Am.

The 3D distributions of Pu, U and the matrix elements (Sr

for the Mururoa particle and Fe for the Thule particle) were

reconstructed (see Fig. 8). The individual slices of the

tomographic reconstruction were obtained using a filtered

backprojection algorithm [33]. A preliminary method of

absorption effect correction was applied [35]. In this

method, the data collected by the X-ray fluorescence

detector, the XRF sinograms, were corrected for absorption

effects using the X-ray absorption sinograms collected with

the SDD placed behind the analyzed sample. The recon-

structed slices were processed by a 3D rendering software

AMIRA [36] to obtain 3D element distribution models, as

shown in Fig. 8. From the reconstructed data, it can be

noticed that the Thule particle, recovered from the sea

sediment, has been partially coated with Fe-rich sediment

matrix (marked by a red color in Fig. 8) during its long

residence (29 y) in the seabed. It can also be seen in the Fig.

8 that uranium (green) and plutonium (blue) elemental

distribution is not even in the particles. The Mururoa

particle, recovered from the surface of the coral atoll, was

found to be attached to the coral matrix (represented by Sr,

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

-20 0 20

Energy

No

rmal

ized

ab

sorp

tio

n

Fig. 11. Example of least-squares fitting of U-L3 A-XANES

yellow color in Fig. 8) only from one side. It suggests that

the merging of plutonium with the matrix material occurred

during the nuclear test explosion, the source term of this

particle.

For the Thule particle, a cumulative X-ray spectrum has

been calculated by adding 15360 individual spectra from a

3D region surrounding the Pu/U rich volume. The cumu-

lative spectrum is shown in Fig. 9. The observed intensity

ratio of Pu-La to U-La was equal to 0.23, which can be

considered as the average ratio in the analyzed particle. This

ratio is similar to that obtained from the other Thule

particles analyzed in the A-XRF experiments.

3.6. l-XANES

In order to study the reactivity of the particles in terms of

mobility, leachability, weathering and transport/transfer of

radionuclides into the environment and from the environ-

ment to the humans, the determination of their oxidation

states is essential to predict fate and transport. XANES is an

established technique for the determination of oxidation

states and local chemical environment, even at relatively

low concentrations. As already asserted, studies have

systematically been performed for actinides aqueous ions

[14,15] and it has been recently exploited also for U at

40 60 80

(eV)

Experiment

Approximation

Peaks

Edge

spectra of the 975374-5 particle collected from Thule.

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

-40 -20 0 20 40 60 80 100 120 140

Energy (eV)

No

rmal

ized

ab

sorp

tio

n

Pu(IV)

Pu(VI)

Thu68-3

Thu68-5

Thu68-6

975374-5

Fig. 12. The Pu A-XANES spectra of the particles Thu68-3, Thu68-5, Thu68-6, 975374-5 and the standards.


particles level [9–13]. The particles separated from the

Thule sediments were investigated by A-XANES as for U

and Pu oxidation states. For the first time, a systematical

study of Pu oxidation states in environmental released

particles has been performed.

In order to study uranium oxidation states in individual

particles U-L3 A-XANES spectra were collected from the

center as well as from the edge region of each particle. The

U XANES spectra collected at the center of four U/Pu

particles sampled at Thule (Thu68-3, Thu68-5, Thu68-6,

975374-5) compared with the spectra of the standards are

shown in Fig. 10. The zero of the energy scale was set to

the inflection point of the U(IV) standard. All spectra were

processed using the least-squares fitting method described

above. Table 2 summarizes the fitting results obtained for

four individual U/Pu particles. Iw and rw denotes the

intensity and width of the white line, while Is and rs

correspond to the height and width of the multiple

scattering peak. No significant difference was encountered

between the results obtained for spectra collected from the

center and from the edge of the same particles. The error

of the fitting method is within 5% (absolute) [23]. Fig. 11

shows an example of the least squares fitting of the

XANES spectra of individual particles recorded at the U-

L3 edge. The table indicates that uranium is present in the

Table 3

Results of least squares fitting of the Pu-L3 A-XANES spectra of four individual

Thule particle ID. Pu(IV) (%) Pu(VI) (%) Iw(IV) I

Thu68-3 92 8 0.61 0

Thu68-5 97 3 0.79 0

Thu68-6 25 75 0.23 0

975374-5 33 67 0.29 0

Iw and rw: intensity and width of the white line, Is and rs: height and width of t

stand for the oxidation state.

particles mostly in the U(IV) form, the maximum ratio of

U(VI) to the total U content was obtained as 25%

indicating that the particles contained mainly the less

mobile form U(IV).

Plutonium oxidation states were studied as that of for

uranium, i.e. through recording and fitting Pu-L3 A-XANESspectra from different regions of each individual particle.

The Pu-L3 XANES spectra collected at the center of four U/

Pu particles (Thu68-3, Thu68-5, Thu68-6, 975374-5)

compared to the standard spectra are shown in Fig. 12.

The zero of the energy scale was set to the inflection point

of the Pu(IV) standard. The fitting results obtained for four

U/Pu particles are summarized in Table 3. The plutonium

oxidation state was also not significantly different in

different regions of the same particle. As an example of

the least squares fitting of the Pu-L3 XANES spectra

(Fig. 13), exactly the same particle was chosen as in Fig.

11. Based on the Pu oxidation state results in Table 3, two

kinds of particles can be distinguished. The particles Thu68-

3 and Thu68-5 contain Pu mostly (more than 90%) in the

Pu(IV) form. In the particles Thu68-6 and 975374-5,

however, the Pu(VI) form is dominant (67–75%).

From the results obtained, it is clear that the reactivity of

the particles in the environment as well as in human fluids

will be different and therefore it would be worthwhile to

U/Pu particles collected from Thule

w(VI) rw Is(IV) Is(VI) rs RMS

.05 7.13 0.06 0.03 7.47 0.00006

.03 7.11 0.10 0.00 11.40 0.00017

.69 6.99 0.01 0.10 7.20 0.00014

.57 7.15 0.05 0.06 9.11 0.00018

he multiple scattering peak, RMS: root-mean-square. Indices (IV) and (VI)

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

-20 0 20 40 60 80

Energy (eV)

No

rmal

ized

ab

sorp

tio

n

Exper iment

Approximation

Peaks

Edge

Fig. 13. Example of least-squares fitting of Pu-L3 A-XANES spectra of the 975374-5 particle collected from Thule.


study to which extent the preponderance of one or another

oxidation state will influence Pu fate and transport. In order

to obtain this information, leaching of the particles in

environmental media and human body fluids needs to be

carried out.

4. Conclusion

It has been demonstrated that by non-destructive

synchrotron radiation based techniques, combined with

SEM–EDX–WDX, gamma and X-ray spectrometry, infor-

mation on the source terms of discrete environmental

radioactive particles can be obtained. In particular, the

chemical mapping as well as fine structure composition can

be achieved by A-XRF and the 3D elemental distribution by

tomography. The results for the four particles investigated

by A-XRF showed that U and Pu are homogeneously

distributed. However, in the particles investigated with

A-tomography and the 2.5 Am A-XRF beam, heterogeneities

of the U/Pu ratio distribution were observed. XANES

providing information on the oxidation states can give

indication on the reactivity of the particles and conse-

quently, on their transport and fate in the environment and

human body fluids. The combination of all these non-

destructive techniques is a first step in providing an entire

picture on origin, route and fate of the nuclear material

released as particles.

Acknowledgement

The present research was partially supported by the IHP-

Contract HPRI-CT-2001-00140 and the GRD-Contract

G6RD-CT-2000-00345 of the European Commission. The

Janos Bolyai Research Fellowship for J. Osan is also

appreciated. The tomography work was co-funded by the

Agency’s Laboratories Seibersdorf, IAEA, Vienna, Austria.

The authors wish to thank Dr. Sven P. Nielsen for providing

us bulk sediment samples and the access to the RisbNational Laboratory (Denmark) Thule sample archive. We

would also like to thank Dr. Kaj Ljunggren, Department of

Radiation Physics, Lund University, Sweden for the use of

the Beta Camera and Dr. Christos Apostolidis and Dr.

Alfred Morgenstern, ITU, Germany for providing the

plutonium oxidation state standards.

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