Note: Descriptions are shown in the official language in which they were submitted.
CA 02347401 2001-03-05
WO 00/20847 PCT/EP99/06214
Method and apparatus for isotope-selective measurement of chemical
elements in materials
1o The present invention concerns a method and an apparatus for
isotope-selective measurement of chemical elements in materials. It can
be used in particular in relation to isotope-selective measurement of
radioactive elements, in particular uranium and plutonium, in radioactive
materials, such as for example in highly active waste glasses. The method
according to the invention and the apparatus according to the invention
can also be put to use in the context of measuring the isotope composition
of lead for the determining the age of minerals. In a further configuration
the invention also concerns the so-called remote measurement of
radioactive isotopes and elements, in particular of uranium and plutonium,
in radioactive materials, that is to say the measurement of those isotopes
and elements from great distances, in order to minimize the danger both
to human beings and equipment due to radioactivity.
In the reprocessing of depleted nuclear fuels for the recovery of
fissile material radioactive wastes are produced in the various steps in the
process. Accordingly for example the vitrification of highly active liquid
waste (HLLW) results in an end product with a non-negligible content of
uranium and plutonium. Hitherto, systematic on-line analysis of the
glasses and other waste objects has been possible only at a comparatively
high level of technical complication and expenditure if a large amount of
3o analysis data is to be acquired. In particular there is a wish on the part
of
the operators of reprocessing and vitrification installations to be able to
analyze for example both the content of plutonium and uranium and also
1
CA 02347401 2001-03-05
the corresponding isotope composition, by means of an analysis device
which is easy to handle.
DE 195 31 988 A1 discloses an apparatus which is easy to handle,
for the measurement of uranium or plutonium in radioactive materials,
having a measuring head which can be put onto the sample to be
analyzed. The known apparatus operates on the basis of pure optical
emission spectroscopy and makes it possible in a comparatively simple
manner to ascertain the content of plutonium and uranium in radioactive
materials. It will be noted however that in the context of measurement
operations with that apparatus, it is not possible to acquire further
analysis data such as for example the isotope ratio of the material being
analyzed. Further measurement operations are required for that purpose,
which represent an undesirable additional expenditure.
Accordingly the object of the present invention is to provide a
method and an apparatus for isotope-selective measurement of chemical
elements, in particular radioactive elements in materials, in particular
radioactive materials, by means of which, besides the content of chemical
elements, in particular radioactive elements, it is also possible to analyze
the corresponding isotope composition as rapidly and accurately as
possible without in that respect having to implement additional
measurement procedures.
That object is attained by a method as set forth in claim 1 and an
apparatus as set forth in claim 6. Further embodiments of the invention
are set forth in the appendant claims.
In accordance with the invention there is proposed a method of
isotope-selective measurement of chemical elements, in particular
radioactive elements, in materials, in particular radioactive materials, in
which for measurement purposes the per se known optical emission
spectroscopy method in which a plasma is generated in the form of a
sample vapor cloud by laser ablation of a sample to be analyzed is coupled
or combined with the per se known laser-induced fluorescence
2
CA 02347401 2001-03-05
spectroscopy method in which laser-induced fluorescence excitation of the
sample vapor cloud is effected.
Optical emission spectroscopy (referred to hereinafter as OES for
the sake of brevity) with laser ablation is based on the following principle:
On the sample to be investigated, an amount of sample of the order
of magnitude of Ng or less is ablated by laser bombardment and at the
same time a plasma with thermally excited atoms is generated by the high
level of laser output. The wavelength spectrum of the radiation from the
plasma is characteristic in respect of the contained elements and the
intensity of the radiation is proportional to the concentration of the
associated element. The plasma has a life of between about 200 and 400
Ns and glows for between about 20 and 50 Ns, in each case in dependence
on the ambient pressure and the irradiated laser power. Analysis of the
radiation with a spectrograph or another dispersive element affords
quantitative information about the composition of the sample. Detection
is effected with photosensitive elements such as for example
photomultipliers, photodiodes or photodiode arrays (referred to
hereinafter as PDA for the sake of brevity) and photodiode panels
(referred to hereinafter as CCD for the sake of brevity). In order to be
able to implement accurate sensitive measurements, laser power, the
measurement moment and the measurement duration as well as the
pressure, and the atmosphere at which the measurement operations take
place must be optimised. It will be appreciated that careful adjustment of
all optical elements, in particular the converging and focusing units, is the
basic prerequisite for success with the measurement operations and can
represent a major problem depending on the respective measurement
location involved.
In contrast thereto, laser-induced fluorescence spectroscopy
(hereinafter referred to as LIF for the sake of brevity) is based on the
following principle:
3
CA 02347401 2001-03-05
In LIF an optical transition of a given kind of atom in an atomized
sample vapor cloud which already in existence is selectively excited. By
irradiation with a very narrow-band laser, the line width of the laser
preferably being less than that of the transition to be excited, a kind of
atom is transformed into an excited state and the radiation intensity which
is emitted upon subsequent decay and which is proportional to the atom
concentration is measured. Detection of the fluorescence is effected as in
the case of OES using photosensitive elements such as for example
photomultipliers or in the simplest case photodiodes. In this case local
1o resolution as afforded by PDAs and CCDs is not absolutely necessary. LIF
is extremely sensitive and accurate and is usually employed for isotope-
selective measurement procedures. As in the case of OES the
measurement moment and the measurement duration as well as the
pressure and the atmosphere at which the measurement operations take
place must also be optimized in LIF. The same applies in regard to
adjustment of the necessary optical systems.
The core of the LIF method is a laser of tunable wavelength in order
to be able to find and excite the appropriate atomic transition. It is
possible for that purpose to use for example dye lasers which however are
Zo comparatively expensive and relatively complicated in terms of handling.
It is also possible to use small inexpensive diode lasers. The type of diode
laser is selected according to the desired wavelength and power as the
tunable and useable wavelength range is not as great for the individual
laser diode, as in the case of dye lasers. It is however possible to find for
most kinds of atoms transitions for which there are suitable, commercially
available diode,lasers. Diode lasers often have even a narrower line width
than the conventional dye lasers.
In accordance with the invention the two above-outlined OES and
LIF measurement methods are combined together in such a way that the
3o plasma which is generated by laser ablation in the context of OES
functions as an atomized sample vapor cloud whose existence is a
4
CA 02347401 2001-03-05
prerequisite for excitation of an optical transition in accordance with LIF.
The method according to the invention has the advantage that only a
single measurement pass is required in order on the one hand to ascertain
the total concentration of the individual elements by means of the OES
measurement procedure and on the other hand to determine the isotope
composition of the sample to be analyzed, by means of the LIF
measurement procedure.
Two different kinds of radiation occur in the context of the method
according to the invention. The first kind involves the light which is
emitted by the plasma and which is fed to a spectrograph for
implementing the OES procedure. That kind of radiation is referred to
hereinafter as emission radiation. The second kind of radiation involves
that radiation which is radiated by the sample vapor cloud upon decay of
the excited kind of atom in the context of the LIF procedure. That kind of
radiation is referred to hereinafter as fluorescence radiation.
Depending on the structure of the apparatus for carrying out the
method according to the invention, it is possible firstly to implement OES
measurement of the emission radiation and only then, using the sample
vapor cloud generated for OES, to implement LIF measurement of the
fluorescence radiation, or it is possible for OES and LIF measurement to
be effected substantially simultaneously. If the measurement operations
are carried out in succession, it is particularly advantageous if excitation
for measurement of the fluorescence radiation is effected only when the
plasma previously generated for emission radiation measurement is
already substantially recombined.
If a laser diode is used for the LIF method, then this is
correspondingly referred to as diode laser-induced fluorescence
spectroscopy. This specific LIF method is referred to hereinafter as the
DILF method.
In accordance with the invention, there is proposed an apparatus
for isotope-selective measurement of chemical elements, in particular
5
CA 02347401 2001-03-05
radioactive elements, in materials, in particular radioactive materials,
comprising a first laser whose laser beam can be focused by means of a
first focusing unit on a sample to be analyzed, so that light-emitting
plasma is produced in the form of a sample vapor cloud for the purposes
of OES, and a radiation analysis unit in which the image of the radiation
emitted by the plasma can be formed by means of an imaging unit, which
is characterized in that there is provided a second laser whose laser beam
can be focused by means of a second focusing unit onto that space in
which the plasma is produced so that laser-induced fluorescence excitation
of the sample vapor cloud is possible.
Preferably a diode laser is selected for the second laser. The
apparatus according to the invention serves moreover in particular for
carrying out the method according to the invention.
The radiation analysis unit of the apparatus according to the
invention, besides serving for detection of the emission radiation, can also
serve for detection of the fluorescence radiation. A separate detection
device for detection of the fluorescence radiation in the context of the LIF
measurement procedure can admittedly be provided but is not absolutely
necessary. The radiation analysis unit is preferably a spectrograph but it
z0 is also possible to use other dispersive elements.
Advantageously, the optical axes of the first and second focusing
units are oriented in such a way that the laser beam of the first laser, that
is to say the laser for implementing OES measurement, impinges
substantially perpendicularly onto a substantially flat sample and the laser
beam of the second laser, that is to say the laser for implementing LIF
measurement, passes through the sample vapor cloud without in that case
impinging on the sample itself. Preferably, for that purpose, the optical
axis of the first focusing unit can extend vertically and that of the second
focusing unit can extend horizontally so that they are mutually
perpendicular. In that situation they are disposed substantially in the
same plane and are thus not inclined relative to each other.
6
CA 02347401 2001-03-05
If laser ablation takes place in a vacuum or under a reduced gas
pressure, the atoms retain their propagation direction after ablation.
There are no or only a few interatomic collisions which change the speed
(in terms of magnitude and/or direction). That means that the atoms at
the center of the rapidly propagating sample vapor cloud retain their
preferential direction, more specifically perpendicularly to the surface of
the sample. If now the atoms are excited by means of narrow-band laser
radiation perpendicularly to the propagation direction and the fluorescence
is additionally observed perpendicularly to the propagation direction, the
l0 Doppler broadening of spectral lines is quite considerably reduced. That
effect is of significance in particular in connection with the laser ablation
effect. In order to achieve the highest possible level of spectral resolution
the fluorescence must be measured from the center of the expanding
sample vapor cloud. That is achieved on the one hand by a closely
collimated diode laser beam and on the other hand by an operation
involving forming an image of the central region of the expanding sample
vapor cloud on (i) the photodetector (for example with apertures in front
of the detector), (ii) the entrance opening of a glass fiber, or (iii) the
entry
slit of a spectrograph. In the last case the slit side members of the entry
2o slit mask out the decentral fluorescence region.
With an improvement in the spectral resolution by virtue of a
reduction in the Doppler width isotope components can be better
separated. That means that the level of selectivity of optical isotope
measurement upon laser ablation is substantially increased. In addition it
is possible quite generally to separate isotope components which have
substantially smaller isotope shifts than uranium.
The reduction in Doppler broadening of spectral lines was observed
for the first time by the inventors of the present invention on Z35U. It was
possible to resolve the slight hyperfine splitting of the 682.88 nm line by
3o laser-induced fluorescence in the expanding ablation plasma.
7
CA 02347401 2001-03-05
It is particularly advantageous to provide a measurement head
which can be applied to the sample or which can be fitted over the sample
and which can be positioned as desired. If the surface area extent of the
sample to be analyzed is greater than the opening of the chamber of the
measurement head then the sample itself serves as a support or
supporting surface for the measurement head. The sample is then not in
the strict sense disposed within the chamber of the measurement head,
but it is only ensured that the resulting plasma or the sample vapor cloud
is within the measurement head. If the surface area extent of the sample
is smaller than the opening of the chamber of the measurement head then
the latter is inverted over the sample and requires some other support or
supporting surface. In that case it is not only the resulting plasma but
also the actual sample to be analyzed which are disposed within the
measurement head.
As the measurement head carries the first and second focusing
units and the imaging unit in a fixed arrangement relative to each other,
no adjustment operations on those optical components are required, in a
radioactive environment. The measurement head is small, mobile and
sufficiently robust to be able to reliably carry the optical components. It is
2o the only part of the apparatus according to the invention, in an
advantageous development thereof, that comes into contact with the
sample to be analyzed. Both plasma generation by laser ablation,
fluorescence production and also collection of the emission radiation which
is used for the analysis procedure takes place in the measurement head.
The measurement head which for example is freely placed on the sample
by means of a manipulator arm can be connected to the other
components of the measurement apparatus by way of optical fibers and
electric cables of any length. In the case of a radioactive sample those
other components are disposed completely outside the radioactive area in
3o a transportable unit.
8
CA 02347401 2001-03-05
The entire measurement apparatus can be of a transportable
nature. Flexible use in various environments outside laboratories is
possible. There is no need for chemical decomposition procedures or
other sample preparation operations prior to measurement. The total
material ablation by the first laser, the ablation laser, in a measurement
procedure, is less than 1 Ng.
The measurement head which is flexible in terms of handling
permits remote measurement of radioactive elements, in particular
uranium and plutonium, in radioactive materials. It is possible to
1o implement both investigations of extensive sample surfaces and also
random-type point measurement operations.
An embodiment of the present invention is described by way of
example hereinafter with reference to the accompanying drawings in
which
Figure 1 is a diagrammatic view of the apparatus according to the
invention with measurement units which are necessary for the
measurement operation and their networking,
Figure 2 is a diagrammatic view of a measurement head according
to the invention with its individual components,
zo Figures 3a/b show an example of fluorescence measurement of the
uranium isotope z38U with fluorescence excitation by frequency-modulated
diode laser radiation of the wavelength ~, = 682.880 - 0.01 nm, wherein
Figure 3a shows a comparison of the spectra with and without DLIF
measurement, wherein the measurement procedure was effected with a
z5 photodiode detection unit for detection of the fluorescence radiation and
an oscilloscope with a respective laser shot and the DLIF is amplitude-
modulated at 50 kHz, and
Figure 3b shows a frequency spectrum of the DLIF spectrum as
shown in Figure 3a after Fourier analysis.
9
CA 02347401 2001-03-05
Figure 1 diagrammatically illustrates the apparatus according to the
invention, while the details of the measurement head 17 are shown in
Figure 2.
The evacuatable measurement head 17 can be freely positioned and
preferably comprises aluminum. Its inside walls are matt black and it is of
an approximately hemispherical shape of a diameter of about 8 cm. Its
height is 5 cm. In the illustrated embodiment, the first and second
focusing units 19 and 29 which are integrated in the measurement head
wall 31 and the two imaging units 28 each comprise double-lens quartz
optics which are arranged in blackened metal sleeves, preferably
aluminum sleeves, of a diameter of 1 cm. They serve as focusing,
imaging and collimating units and are commercially available or can be
easily manufactured in-house. The measurement head 17 also has inlet
nozzles 20 for the argon gas which promotes plasma generation and
measurement.
Depending on the respective shape of the sample 23 the
measurement head 17 can be fitted directly onto the surface 18 with a
sealing ring 22, in particular a plastic sealing ring. The sample 23 can
equally be disposed in a sample holder which is of a simple design
2o configuration and which is adapted to the measurement head 17, for
example a smooth surface, onto which the measurement head 17 is set.
Adjustments of the optical system, that is to say the focusing units 19 and
29 and the two imaging units 28, are not necessary as they have already
been effected upon assembly of the measurement head 17.
In order to achieve the best possible ablation conditions and an
undisturbed plasma, argon gas is passed by way of the two inlet nozzles
20 into the measurement head 17 and the latter is evacuated by way of a
valve 21 to a pressure of between 1 and 100 haPa.
By way of optical waveguides 13 (wavelength 1064 nm, diameter
600 Vim), the plasma-generating radiation 26 of a pulsed Nd:YAG laser 2
(1064 nm, pulse 40 mJ max., 5-10 ns, with optical waveguide coupling-in)
CA 02347401 2001-03-05
is passed into the measurement head 17 and there focused by means of
the first focusing unit 19 onto the surface of the sample 23. The emission
radiation of the plasma 24 produced is passed by way of the two imaging
units 28 and two optical waveguides 14 (each being an optical waveguide
bundle with 200 ~m fibers, about 35 fibers) to a spectrograph 5 with a
time-resolving intensified CCD-detector unit (hereinafter referred to as the
ICCD-detector unit for the sake of brevity) as indicated at 6 and
measured. The ICCD-detector unit 6 has a resolution of at least 578 x
384 pixels and can be cooled by a Pettier element and a through-flow
cooler. The spectrograph 5 is a 0.5 m spectrograph with a wavelength
range of between 250 and 750 nm and of a resolution of 20 pm or better.
For measurement value acquisition, evaluation and for starting the
measurement procedure, the arrangement uses a fast Personal Computer
(PC) 1 which is connected by way of electric cables 15, preferably BNC-
cables, to the ICCD-controller 4 (intensified CCD-camera) and the Nd:YAG
laser 2.
Illumination of the ICCD-detector unit 6 is effected only after a
certain delay time (in the us-range) with respect to plasma formation and
with an illumination time of between about 20 and 50 ~s (light duration of
2o the plasma). For that purpose the ICCD-detector unit 6 is switched with
pulse and delay times of between 1 us and 1 ms by way of a pulse-delay
generator 3 connected to the ICCD-controller 4.
For the purposes of DLIF excitation, the narrow-band radiation 27
of the diode laser 7, with optical waveguide coupling-in and through-flow
cooler, is focused onto the sample vapor cloud through the second
focusing unit 29 which is integrated in the measurement head 17. The
radiation is passed to the measurement head 17 by way of optical
waveguides 13a of a diameter of 200 ~m or smaller. Wavelength
adjustment is effected by way of the diode laser driver 8. The diode laser
7 radiates continuously.
11
CA 02347401 2001-03-05
In accordance with the invention, DLIF measurement can be
effected in two different ways:
In accordance with the first method, the fluorescence radiation is
passed by way of optical waveguides 14 to the spectrograph 5 and
measured with the ICCD-detector unit 6. In order to avoid excessively
intensive background radiation due to emission that measurement
procedure is started at a time when the laser-generated plasma 24 has
extensively recombined, that is to say after about 50 ~s. In a particularly
advantageous manner, the ICCD-detector unit 6 can moreover be used to
control by way of stray light the wavelength of the diode laser radiation. A
specific wavelength measuring device is redundant as a result.
In accordance with the second method, measurement of the DLIF is
effected by way of a detection unit which is integrated in the
measurement head 17, preferably a photodiode detection unit 25. In this
case, detection can be effected at an earlier time, while troublesome
emission and stray radiation can be separated from the DLIF by
modulation methods. Frequency modulation of the diode laser radiation is
effected by way of the frequency or function generator 9. The associated
amplitude-modulated signal of the photodiode 32 can be further boosted
by a lock-in amplifier 11 and is recorded by a digital storage oscilloscope
10 with PC-interface and passed to the PC 1 for evaluation (for example
Fourier analysis). A higher level of sensitivity is to be expected with that
measurement procedure than when effecting measurement with the
ICCD-detector unit 6.
As indicated by the broken circular line in Figure 2, the photodiode
detection unit (PDDE) 25 can be integrated either in front of or behind the
plane of the drawing in Figure 2, in the wall 31 of the measurement head
17. It comprises a metal sleeve of a diameter of 2.5 cm and a length of
about 4 cm. Besides the photodiode 32 it contains a lens system 33 for
focusing the DLIF or fluorescence radiation onto the photodiode 32. For
the purposes of screening troublesome scattered and emission radiation, a
12
CA 02347401 2001-03-05
polarization or band pass filter 34 is additionally disposed between the
lens system 33 which in the illustrated embodiment comprises two lenses
and the photodiode 32. The spatial proximity of the PDDE 25 to the
plasma 24 and the large diameter afford a high level of collecting
efficiency. A further optical waveguide for radiation transport and thus
possible coupling-in and attenuation losses are avoided, thereby giving a
better level of detection efficiency than in the case of DLIF measurement
with the ICCD-detector unit 6. Furthermore, with the additional use of the
PDDE 25, it is possible for the OES and LIF to be measured at the same
to time or simultaneously.
Figures 3a and 3b show measurement of the diode laser-induced
fluorescence of the uranium isotope 238U with the photodiode 32. Figure
3a shows the original recording of the oscilloscope, wherein for pure
emission measurement the wavelength of the diode laser 7 was detuned
by 10 pm with respect to the transition (broken line). Figure 3b shows the
result after Fourier analysis. Lock-in amplification was waived in this
measurement procedure.
Quantification of the measurement results is effected in the OES
spectrum by way of internal standardization with intensive lines of main
constituents of the sample 23 of known concentration. In the case of
isotope-selective DLIF measurements quantification is effected directly
from the levels of intensity of the signals.
In accordance with the invention the measurement head 17 shown
in Figure 2 is disposed for example in a glove box or so-called hot cell and
is thus radioactively contaminated. All other components of the
measuring system and the gas supply, that is to say all components
denoted by references 1 through 16, are disposed outside and are not
exposed to any radioactivity. Serving as the interface between the
outside and the inside are the optical waveguides 13, 13a and 14 which
are thus disposed partially within the radioactive area. No limits are set
on the length of the optical waveguides. In addition the measurement
13
CA 02347401 2001-03-05
head 17 is connected by way of two plastic hozes 16 (argon feed and
evacuation) and a BNC electric cable 15 to the photodiode 32, to the
measurement apparatus. The pre-vacuum pump 12 serves to generate
the vacuum, for pressures below 1 hPa and with a suction capacity of
about 5 m3/h.
For analysis of a new sample location the measurement head 17 is
fitted as a whole onto the appropriate sample location. The sample does
not have to be moved nor do any fresh adjustments have to be made.
The new measurement procedure can begin immediately.
l0 The dimensions of the measurement head 17 are variable and can
be varied according to the location of use. The approximate shape of a
hemisphere appears however to be suitable for all possible uses.
As however the spacing of the focusing units 19 and 29 and the
imaging units 28 from the plasma 24 should be at least 4 cm, the
dimensions of the inside diameter should correspond at a minimum to a
hemisphere of a 4 cm radius. If the spacing becomes less the optical
systems of the focusing and imaging units suffer from vapor deposition of
the ablated sample material and become opaque after a few 100
measurement procedures.
The height of the measurement head 17 is so selected that the
optical axes of all the lens systems present intersect about 1 cm above the
sample surface.
The lens systems permit 1:2 imaging of the plasma 24 onto the
optical waveguide end or onto the photodiode 32. Focusing of the two
laser beams corresponds to 1:1 imaging of the respective optical
waveguide end onto the plasma 24.
Finally the essential advantages of the described embodiment
should be set forth once again:
1. By virtue of the closed environment within the measurement head
17 the consumption of the argon which flows directly onto the
ablation location by way of inlet nozzles 20 is extremely minimized.
14
CA 02347401 2001-03-05
2. Only the measurement head 17 and a part of the optical
waveguides 13, 13a, 14 and the electric cables 15 and the plastic
hozes 16 come into contact with the radioactive environment.
3. Adjustment operations on the measurement head 17 in the
radioactive environment are not necessary.
4. The measurement head 17 can be placed virtually anywhere that
may be desired.
5. The chamber 30 of the measurement head 17 can be evacuated by
positioning with the sealing ring 22 on a smooth sample surface
l0 18/23, by way of the valve 21.
6. The analysis operation can generally be implemented without
previous mechanical or chemical preparation.
7. Fluorescence radiation or DLIF or LIF radiation can be measured
both with the ICCD-detector unit 6 and also with the photodiode
detection unit 25.
8. In the case of measurements with the photodiode detection unit 25
it is possible for DLIF and OES to be measured at the same time.
9. The photodiode detection unit 25 is a very compact unit with lens
system 33, band pass filter 34 and photodiode 32.
2o 10. By virtue of wavelength monitoring of the diode laser scatter light
with the spectrograph 5 it is possible to avoid using a further
wavelength measuring unit.
11. The overall structure in accordance with the embodiment by way of
example is transportable whereby only the measurement head 17
has to be renewed when displacing the measurement location.
CA 02347401 2001-03-05
List of references
1 Personal Computer
2 Nd:YAG laser
3 pulse-delay generator
4 ICCD controller
5 spectrograph
6 ICCD-detector unit
7 diode laser
8 diode laser driver
9 frequency or function generator
10 digital storage oscilloscope
11 lock-in amplifier
12 pre-vacuum pump
13 first optical waveguide
13a second optical waveguide
14 third optical waveguide
15 electric cable
16 plastic hozes
17 measurement head
18 smooth surface, support or sample to be
analyzed
19 first focusing unit
20 inlet nozzles
21 valve
22 sealing ring
23 sample to be analyzed
24 glowing plasma generated by Nd:YAG laser
ablation
25 photodiode detection unit
26 plasma-generating laser radiation
27 narrow-band laser radiation
28 imaging unit
29 second focusing unit
16
CA 02347401 2001-03-05
30 chamber
31 measurement head wall
32 photodiode
33 lens system
34 polarization or band pass filter.
17
