Ultratrace analysis of transuranic actinides by laser-induced fluorescence

Ultratrace quantities of transuranic actinides are detected indirectly by their effect on the fluorescent emissions of a preselected fluorescent species. Transuranic actinides in a sample are coprecipitated with a host lattice material containing at least one preselected fluorescent species. The actinide either quenches or enhances the laser-induced fluorescence of the preselected fluorescent species. The degree of enhancement or quenching is quantitatively related to the concentration of actinide in the sample.

The invention relates generally to trace analysis of transuranic actinides 
and, more particularly, to detection of nanogram quantities of weakly 
fluorescing transuranic actinides by co-precipitating samples with 
preselected compounds and examining the laser-induced fluorescence of the 
resultant crystal. 
Actinide pollution poses a serious threat to public health. Because of 
radioactivity and toxicity, great care must be taken to contain and to 
monitor the spread of actinides whenever they are used in or occur as 
by-products of industrial processes. 
Trace analysis of actinides is crucial in a variety of industrial, 
ecological, and public health settings. These include monitoring of ground 
water near nuclear waste repositories and nuclear power plants, monitoring 
actinide mobility under a variety of environmental conditions, monitoring 
the leaching of actinides immobilized in special solid matrices in 
preparation for waste storage, and the monitoring of actinide recovery 
from oceanic and terrestrial mining operations. 
There are many methods of trace analysis that are applicable to the 
detection of actinides. Of these x-ray fluorescence analysis, neutron 
activation analysis, alpha spectroscopy, and optical fluorescence analysis 
are the most important and most sensitive. The advantages of x-ray 
fluorescence include direct analysis with little preparation, short 
analysis time (in minutes), and simple spectral interpretation. The main 
disadvantages of x-ray fluorescence are the safety hazards associated with 
an x-ray source and limited sensitivity. For example, direct analysis via 
x-ray fluorescence provides a sensitivity to uranium to only a 
parts-per-million range. DeKalb, et al., U.S. Pat. No. 3,936,633 teaches 
an x-ray fluorescence-based method of detecting lanthanides in a 
transition element host. 
Neutron activation is one of the most sensitive of modern analytical 
techniques. Its analytical method involves exciting the nuclei of trace 
elements by neutron bombardment. Then, either detection of the neutron 
energy spectrum from the sample via pulse height analysis is performed at 
the time of irradiation or examination of the energy spectrum of gamma 
photons emitted sometime after irradiation has taken place is performed. 
These energy spectra are very specific to the excited nuclei that create 
them. For uranium, the detection limit is in the parts per billion range 
at 0.1 ng/ml. Although relatively sensitive, the main disadvantage of 
neutron activation is the necessity for high neutron fluxes requiring 
large accelerators or cyclotrons. 
For the detection of actinides with significantly higher activities than 
uranium 238, alpha spectroscopy is by far the most sensitive technique. 
Detection limits for alpha counting analysis are typically 
femtocuries/liter or, for example, 10.sup.-2 parts per quadrillion for 
.sup.239 Pu. This is the sensitivity required to detect the plutonium 
background in the environment. However, such a detection limit is only 
obtainable with a minimum sample volume of 10 liters, a minimum counting 
period of three days, a signal-to-noise ratio of only 1:1, and thorough 
removal of interferring alpha emitters, such as .sup.241 Am, .sup.232 U, 
and .sup.249 Bk, before counting. 
Wright and his co-workers have pioneered the use of optical fluorescent 
analysis of trace quantities of selected ions, particularly lanthanide 
ions (Wright, U.S. Pat. No. 4,007,009 "Chemical Analysis of Ions 
Incorporated in Lattices Using Coherent Excitation Sources"; Miller, et 
al., "Spectroscopic System for the Study of Fluorescent Lanthanide Probe 
Ions in Solid", Anal. Chem., Vol. 49, pp. 1474-1482; Wright and Gustafson, 
"Ultratrace Inorganic Ion Determination by Laser Excited Fluorescence", 
Anal. Chem., Vol. 50, pp. 1147A-1160A; Johnston and Wright, "Trace 
Analysis of Nonfluorescent Ions by Associative Clustering with a 
Fluorescent Probe", Anal. Chem., Vol. 51, pp. 1774-1780). Their technique 
requires the coprecipitation of the unknown ion into some preselected host 
lattice (or the coprecipitation of the unknown ion and a preselected 
fluorescent probe ion, if the unknown ion is nonfluorescent). The spectrum 
of the fluorescent species in the precipitate is affected by the local 
structure of the crystal. Typically, the fluorescent species can occupy 
many different sites in the crystal, each having a different effect on its 
release or transfer of excitation energy. The fluorescent intensity from 
particular sites can be related to the concentration of the unknown ion in 
the original solution before coprecipitation. Fluorescent species at 
particular sites can be chosen for analysis by selectively exciting the 
species at a wavelength corresponding to the absorption frequency 
characteristic of the site. Sites must be chosen so that there is minimum 
energy transfer between adjacent species in the lattice, and host 
materials must be chosen which do not quench fluorescence. 
The foregoing illustrates the limitations of the current technology. It 
would be advantageous to provide an alternative to available methods, 
particularly in regards to ultratrace analysis of weakly fluorescing 
transuranic actinides. 
SUMMARY OF THE INVENTION 
It is therefore an object of the invention to provide a rapid method for 
ultratrace analysis of transuranic actinides. 
Another object of the invention is to provide a rapid method for ultratrace 
analysis of transuranic actinides dissolved in natural and industrial 
fluids, such as ground water, seawater, ore leachings, extractions from 
soil samples, and the like. 
A further object of the invention is to provide a method for 
fluorescence-based ultratrace analysis of weakly fluorescing or 
nonfluorescing transuranic actinides. 
Still another object of the invention is to provide a rapid method for 
detecting plutonium or americium at nanogram-per-milliliter 
concentrations. 
Additional objects, advantages and novel features of the invention will be 
set forth in part in the description which follows, and in part will 
become apparent to those skilled in the art upon examination of the 
following or may be learned by practice of the invention. The objects and 
advantages of the invention may be realized and attained by means of the 
instrumentalities and combinations particularly pointed out in the 
appended claims. 
These and other objects are attained in accordance with the present 
invention wherein, generally, a preselected host material is prepared by 
coprecipitating host precursor materials with the actinide-containing 
sample to form a precipitate; the precipitate is then preferably calcined 
to form a calcined precipitate. Next, the calcined precipitate is 
illuminated by laser light of a preselected wavelength; and the 
fluorescence output of a fluorescent component species within the calcined 
precipitate (other than the actinide itself) is related to the 
concentration of actinide in the original sample. Alterations in the 
fluorescent output of the fluorescent component species is detected by 
comparing fluorescent output of the actinide containing sample with those 
of one or more standards each comprising identically prepared preselected 
host material with known concentrations of the actinide of interest. 
Suitable materials for the host lattice (the predominant composition of the 
preselected host material) include most ionic crystalline solids, but 
ionic crystalline solids with melting points above about 500.degree. C. 
are preferred, and such ionic crystalline solids which are insoluble or 
slightly soluble in water, which have high molecular mass components, and 
which are substantially transparent to the fluorescent signals and 
illumination beam are most preferred. 
Among these, terbium fluoride, TbF.sub.3, is particularly preferred because 
the component species, terbium, is itself strongly fluorescent and because 
there exists a high rate of excitation energy migration between terbium 
ions. As will be discussed more fully below, the latter property is of 
particular advantage in the detection of plutonium. Some of the other host 
lattice materials listed above are weakly or nonfluorescent at convenient 
wavelengths. Their use in accordance with the invention requires the 
incorporation of a fluorescent component species as an impurity within the 
crystal matrix. Such a fluorescent component species is chosen from among 
the lanthanides, transition metals, or uranium compounds, such as uranyl, 
or the like. 
The present invention is directed to problems associated with ultratrace 
analysis of weakly fluorescing transuranic actinides. It advantageously 
overcomes many of these problems by coprecipitating the transuranic 
actinides with host precursor materials to form a preselected host 
material in which the weakly fluorescing actinide acts as either a 
sensitizer or a quenching agent to a fluorescent component species within 
the preselected host material. In particular, the invention allows rapid 
ultratrace detection of plutonium ions at intra-host concentrations as low 
as 10 parts per million and of americium ions at intra-host concentrations 
as low as 1 part per million.

DETAILED DESCRIPTION OF THE INVENTION 
In accordance with the present invention a method is provided for detecting 
trace quantities of transuranic actinides in solution. A sample suspected 
of containing a particular actinide is mixed with host precursor 
materials, specifically selected for the particular actinide. Conditions 
are maintained which allow coprecipitation of the host precursor materials 
and the actinide of interest. The resulting coprecipitate is calcined to 
remove organic impurities and to promote crystal rearrangements into more 
homogeneous lower energy configurations. Finally, the calcined 
coprecipitate is illuminated by laser light at a preselected wavelength 
corresponding to the absorption band of either a fluorescent component 
species of the preselected host material (e.g., as in the plutonium 
detection method described below wherein the actinide acts as a quenching 
agent) or the actinide itself (e.g., as in the americium detection method 
described below wherein the actinide acts as a sensitizing agent). The 
concentration of the actinide of interest in the sample is determined by 
examining the differences in fluorescent output between the calcined 
precipitate and identically prepared standards with known concentrations 
of the actinide (and, in some cases, with known concentrations of 
interfering impurities known to be present in the sample). 
The choice of the preselected host material is crucial to the invention. 
The invention is particularly directed to ultratrace analysis of weakly or 
nonfluorescing transuranic actinides. In accordance with the invention, 
this class of actinides is detected indirectly by fluorescence quenching 
or fluorescence sensitization of a fluorescent component species within 
the preselected host material. For sensitization or quenching to occur the 
fluorescent component species must possess near resonant energy levels to 
the actinide. Further preferences in the choice of host material include: 
(1) that at least one component species within the preselected host 
material have similar ionic radius and identical valence state to the 
actinide, thereby promoting selective precipitation of the actinide over 
other impurities during the coprecipitation step; and (2), in the case of 
detection by sensitization of a fluorescent component species, that the 
actinide have an absorption line (i) that is absent in the fluorescent 
component species, (ii) that is higher than the level at which resonant 
energy transfer takes place, and (iii) that is susceptible of 
non-radiative decay to the transfer level. 
A. Coprecipitation 
Coprecipitation is the process by which ions present in solution in small 
quantities can be taken out of solution via ion exchange or adsorption 
onto a component present in large quantities and precipitating out of 
solution. There are three kinetic processes which describe 
coprecipitation, transport through the solution, ion exchange with the 
surface of microcrystal and diffusion through the solid. 
The effect of these processes on the incorporation of an impurity, or 
microcomponent, into a coprecipitate is described in Chapter 3 of Walton, 
The Formation and Properties of Precipitates (John Wiley G. Sons, New 
York, 1967). Accordingly, Chapter 3 of this book is incorporated by 
reference as a guide to choosing host precursor materials which will 
promote co-precipitation of the actinide of interest. The distribution 
coefficient of a coprecipitating system is a measure of how readily an 
impurity ion (i.e., the microcomponent of the system, in this case an 
actinide) is incorporated into the macrocomponent (i.e., the preselected 
host material) of the coprecipitate. The larger the distribution 
coefficient of the microcomponent-macrocomponent system the more readily 
the microcomponent is incorporated into the coprecipitate. 
Large distribution coefficients result when the microcomponent ions are as 
similar as possible to the macrocomponent ions. Listing from the most 
important factors to the least important, the actinide and predetermined 
component species microcomponent ions should possess similar ionic radius, 
equal valence states and the same lattice structure as the host lattice 
components. The aim of these requirements is to reduce the excess free 
energy required to incorporate the coprecipitate into the lattice 
structure, thereby increasing the distribution coefficient. Less 
distortion is created in the lattice when exchange ions are of similar 
ionic radius. Similar lattice structure also reduces strains in the local 
crystal. Equal valences avoid the need for vacancies and interstitials 
required for charge compensation. Each of these exchange ion similarities 
keep the free energy of the crystal low and thus the distribution 
coefficient high. Preferably, host lattice materials are chosen for which 
the host lattice-actinide and host lattice-predetermined component species 
distribution coefficients are high. 
Another factor which affects the degree of incorporation of a 
microcomponent is the rate of precipitation. During very fast 
precipitation no equilibrium is established in the kinetic processes, and 
the incorporation process is controlled by diffusion of ions from 
solution. For similar ions with similar diffusion rates incorporation is 
not selective; neither enrichment nor depletion of competing 
microcomponent ions take place as could occur under very slow 
precipitation rates where near-equilibrium conditions exist. Consequently, 
the choice of host precursor materials and conditions that promote slow 
precipitation are preferred; however, slow precipitation is not crucial to 
the invention. 
A very wide range of ionic crystalline solids provides suitable host 
lattices. Ionic crystalline solids with melting points above about 
500.degree. C. are preferred. Representative of the materials which can be 
used to form the host lattice are SrSO.sub.4, PbSO.sub.4, Pb.sub.3 
(AsO.sub.4).sub.2, Ba.sub.3 (AsO.sub.4), Na.sub.2 SO.sub.4, NaCl, 
BaF.sub.2, CaF.sub.2, BaSO.sub.4, fluorides of trivalent lanthanides, and 
the like. Additional representitive materials are listed in Chapter 3 of 
Walton, The Formation and Properties of Precipitates (John Wiley and Sons, 
New York, 1967). Among these representitive materials, the most preferred 
are those which are insoluble or only slightly soluble in water, which are 
transparent or substantially transparent to the fluorescent signal and 
excitation beam, and which have relatively high molecular mass components. 
Examples of the most preferred materials include CaF.sub.2, LaF.sub.3, 
TbF.sub.3, GdF.sub.3, SrF.sub.2, PbCl.sub.2, and the like. Nonradiative 
decay via phonon production competes directly with fluorescence and energy 
transfer mechanisms which are basic to the invention. The most important 
parameter in phonon-assisted nonradiative decay rates is the energy gap 
from the excited ion state to the next lowest level. As the gap decreases 
the probability for nonradiative relaxation increases. The greater the 
number of phonons involved in the interaction necessary to bridge the 
energy gap, the less probable will be a nonradiative transition. 
Consequently, lattices with lower average phonon energies, which require 
more phonons to decay to the next lowest energy level, are less 
competitive with radiative decay mechanisms or energy transfer mechanisms 
than are lattices with high average phonon energies. Lattices with high 
molecular weight components vibrate at lower frequencies, and therefore 
have lower energy phonon spectra. Thus, such lattice materials are 
preferred over those with lower molecular mass components. 
B. Calcination 
After coprecipitation, the microcomponent (in this case an actinide ion) is 
trapped in the microcrystalline structure of the host lattice. Due to the 
heterogeneous nature of the coprecipitation, the actinide ions and 
fluorescent component species reside in a variety of sites. Each site 
possesses slightly different fluorescence characteristics resulting in a 
broad fluorescence spectrum, or in the case of selective excitation, a 
lower fluorescent yield per site. 
In addition to the actinides of interest and the fluorescent component 
species, other impurities in the water also coprecipitate out of solution. 
When these impurities are organic they cause a broad fluorescent 
background that tends to mask the fluorescence spectrum of the fluorescent 
component species. 
Both of these problems are eliminated by calcination of the precipitate. 
Heating the precipitate has several effects. First, temperatures above 
about 500.degree. C. will cause the organics to oxidize and then vaporize. 
Second, vacancies and interstitials become mobile at these elevated 
temperatures. The crystals then rearrange themselves into a more 
homogeneous lower energy configuration. Consequently, when cooled the 
precipitate possesses no interfering organic impurities and a smaller 
selection of sites for the actinide and the fluorescent component species. 
This leads to narrower more intense fluorescent spectrums which are also 
free of the broadband organic fluorescence background. Calcination 
temperature is, of course, limited by the melting point of the 
macrocomponent of the precipitate. Calcination for about 1-3 hours at 
between about 500.degree. C. and about 1000.degree. C. produces suitable 
results for CaF.sub.2 and TbF.sub.3. 
In some cases the actinide itself can become oxidized during calcination 
leading to a different species and therefore a different characteristic 
fluorescence spectrum. Uranium provides an excellent example of this 
phenomenon. In natural waters, uranium is found as uranyl, 
UO.sub.2.sup.++, complexed with various anions. It can be readily 
coprecipitated with calcium fluoride. The precipitate then contains a 
microcomponent consisting of uranyl ions. After calcination the 
precipitate gives a fluorescence spectrum characteristic of the uranate 
molecule, UO.sub.6.sup.-6. Additional oxygen from the air oxidized the 
UO.sub.2.sup.++ to UO.sub.6.sup.-6 during calcination. To avoid such 
oxidation calcination must be performed in vacuo. 
C. Excitation Sources and Fluorescence Analysis 
A laser light source operating at a preselected wavelength is used to 
illuminate the calcined precipitate to excite the actinide or the 
fluorescent component species to a particular energy level. Whether the 
actinide quenches or sensitizes the fluorescent component species 
determines whether the excitation frequency (i.e., the preferred frequency 
of the laser) is preselected to excite the actinide or the fluorescent 
component species. If the actinide acts as a sensitizer, then a frequency 
is chosen to excite the actinide to a level above the near resonant energy 
level of the fluorescent component species, but close enough to the near 
resonant level so that non-radiative decay to that level is likely. If the 
actinide acts as a quenching agent to the fluorescent component species, 
then a frequency is chosen that corresponds to the near resonant energy 
level of the fluorescent component species. Use of a tunable dye laser is 
preferred in order to efficiently and selectively excite either the 
actinide or the fluorescent component species; however, use of a tunable 
dye laser is not crucial. 
FIG. 1 is a block diagram of one embodiment of apparatus for illuminating a 
sample and/or standard and for measuring the resulting fluorescent spectra 
in accordance with the practice of the invention. This apparatus allows 
measurement of both spectra and fluorescent lifetimes. 
For particular actinide-preselected host material combinations less 
complicated apparatus is adequate. For example, a filter fluorimeter alone 
may be sufficient for fluorescence analysis once an actinide-fluorescent 
component species system is chosen. 
In FIG. 1 output 4 from laser 2 is passed through interference filter 6, or 
alternatively through a Claassen prism iris combination, or the like, to 
filter out all plasma lines while allowing the chosen laser wavelength to 
pass. The laser beam is steered via aluminum coated mirrors, 14 and 16, to 
a sample illuminator 18, e.g., Spex model 1430 (Spex Corp., Metuchen, 
N.J.), or the like. There the laser light is focused to a 20 .mu.m-sized 
spot on the sample pellet. Collection optics 20 collect and direct the 
fluorescent light to double monochromator 22, e.g., Spex model 1402 double 
monochromator, or the like. Light from double monochromator 22 is directed 
to a photomultiplier tube 24, e.g., RCA model 31034a-02 (RCA Electronic 
Components, Harrison, N.J.), or the like. The photomultiplier tube is 
cooled, or the like, e.g., by a Products for Research model TE-104-RF 
thermo-electric cooler, in order to reduce the tube's dark current. Since 
the photomultiplier tube is a photon counting device, the signal is 
processed by fast preamplifier 26, and suitable post amplifier 28, 
discriminator 30, and rate meter 32, e.g., Ortec series 9300 (Ortec, Inc., 
Oak Ridge, Tenn.), or the like. The output of the rate meter either drives 
a chart recorder, e.g., Gould model 110 (Gold, Inc., Santa Clara, Calif.), 
or the like or is digitized by digital multimeter 36, e.g. Hewlett Packard 
model 3456 (Hewlett Packard, Palo Alto, Calif.), or the like. The output 
of the digital multimeter is stored in tabulated form in a microcomputer 
38, e.g., Tektronix model 4051 (Tektronix, Inc., Beaverton, Oreg.) in 
conjunction with a DEC LSI 11/23 (Digital Equipment Corp., Waltham, 
Mass.), or a similar microcomperized data acquisition system. When a 
spectrum is completed it is displayed by plotter 40. 
In order to make accurate determinations of fluorescent lifetimes and to 
increase the detection limits by discrimination against impurities 
possessing relatively fast fluorescence, a boxcar integrator 34 can be 
utilized, e.g., Princeton Applied Research model 162 (Princeton Applied 
Research, Princeton, N.J.), or the like. The boxcar replaces the rate 
meter in the signal processing system. 
The boxcar temporally discriminates by (1) receiving a trigger pulse from a 
repetitive signal; (2) waiting a specified amount of time; then (3) 
setting up a temporal window during which the boxcar integrates the 
signal. The output asymptotically approaches the average value of the 
input signal coincident with the temporal window. The cw laser light is 
pulsed by a mechanical chopper 8. The chopping rate is monitored by 
sampling a small portion of the beam via a beasmsplitter 10 and photodiode 
12. The photodiode output then acts as a trigger for boxcar 34. 
EXAMPLE I 
Plutonium Detection 
The concentration of plutonium ions, Pu.sup.+3, is determined indirectly by 
their quenching effect on terbium fluorescence. 
Terbium fluoride is formed and precipitated from water by adding terbium 
nitrate, Tb (NO.sub.3).sub.3, to ammonium fluoride, NH.sub.4 F. The 
reaction is described by the following equation. 
EQU Tb(NO.sub.3).sub.3 +3NH.sub.4 F.revreaction.3NH.sub.4 NO.sub.3 +TbF.sub.3 
The maximum TbF.sub.3 precipitation occurs when terbium ions and fluorine 
ions are present in stochiometric amounts, in this case, in the ratio of 3 
moles of fluorine for every mole of terbium. 
Plutonium has the unique ability to have all four common oxidation states 
coexist in the same solution. This is due to the tendancy of plutonium 
ions to disproportionate in the presence of water according to the 
following equation. 
EQU 3Pu.sup.+4 +2H.sub.2 O.revreaction.2Pu.sup.+3 +PuO.sub.2 +4H.sup.+ 
For example, a 10.sup.-2 M solution of Pu.sup.+4 in 0.344 M HNO.sub.3 
disproportionates into 12% Pu.sup.+3, 66% Pu.sup.+4, and 22% Pu.sup.+6. 
Due to such disproportionation, plutonium must be placed in a strong 
reducing or oxidizing solution in order to prepare it in a particular 
oxidation state. 
The Pu.sup.+3 form is ensured by mixing the sample with an appropriate 
amount of NaHSO.sub.3. Reduction of Pu.sup.+4 occurs according to the 
following equation. 
EQU 2Pu.sup.+4 +HSO.sub.3 +Na.sup.+ +H.sub.2 O.revreaction. 
EQU 2Pu.sup.+3 +SO.sub.4.sup.-2 +Na.sup.+ +3H.sup.+ 
As an example, the dominance of the Pu.sup.+3 form is ensured by mixing a 
5.times.10.sup.-3 M solution of plutonium with an equal volume of a 0.25 M 
solution of NaHSO.sub.3. 
In this example, the host precursor materials are Tb(NO.sub.3).sub.3, 
NH.sub.4 F, and NaHSO.sub.3. 
After terbium nitrate and ammonium fluoride are mixed with the 
actinide-containing sample, terbium fluoride starts to precipitate 
immediately and is finished within a period of about two hours. The 
solution is stirred and placed in a thick-walled centrifuge cone. It is 
centrifuged for about ten minutes and the supernatant is discarded. The 
precipitate is washed with about 10 ml of water and centrifuged for an 
additional 10 minutes. The supernatant is once again discarded and the 
precipitate is dried under a heat lamp at a temperature of 100.degree. C. 
for two hours. The precipitate is then removed from the centrifuge tube 
and placed in an agate mortar where it is ground to a fine powder. The 
powdered precipitate is then transferred to a quartz tube and a piece of 
wool is inserted in the neck of the tube to confine the powder. This tube 
is placed on a vacuum line where no oxygen can interact with the 
precipitate. A clamshell heater is then placed around the quartz tube. The 
heater is set at about 100.degree. C. for about one hour. After an hour, 
the temperature is increased slowly over a period of about one hour to 
about 800.degree. C. If the temperature is increased too rapidly, the 
powder begins to swirl violently and a portion of the sample can be lost. 
The precipitate is calcined for about three hours and then cooled 
thoroughly. After cooling, the calcined precipitate is pulverized using a 
pulverizing device such as the one sold under the tradename Wig-1-bug 
(Spex Corp. Metuchen, N.J.), or the like. 200 mg of pulverized calcined 
precipitate is placed in a pellet die and pressed for one minute at 10,000 
psi. This produces a sample pellet 1 cm in diameter and approximately 1 mm 
thick that is ready for fluorescence analysis. 
The finished pellet is placed between two pieces of quartz, one of which 
has a recessed space for the pellet. These are epoxied together and set 
aside for 24 hours. 
FIG. 2 shows the degree of terbium fluorescence quenching as a function of 
plutonium concentration. These results were obtained by exciting terbium 
fluorescence with 457.1 nm laser light. This wavelength, which is not 
crucial, is sufficient for exciting terbium into its .sup.5 D energy 
levels. It is crucial to this example that the preselected wavelength be 
chosen so that terbium is excited into its .sup.5 D energy levels. From 
these energy levels energy may be transferred to a near resonant energy 
level, the .sup.4 G.sub.7/2 level, in a nearby plutonium ion. 
The concentration of 100 parts per million by weight of .sup.242 Pu 
indicates that approximately one terbium ion in 15,000 has been 
substituted by a plutonium ion. Thus, on the average there are 25 
Tb.sup.+3 ions for every Pu.sup.+3 ion along any one of the crystalline 
axes. The onset of quenching of the terbium fluorescence at this 
concentration implies a significant energy migration among the Tb.sup.+3 
ions to allow the Pu.sup.+3 ions to accept the energy from either their 
nearest or next nearest neighbors, the largest separation for which the 
dipole--dipole interaction (the dominant Pu.sup.+3 -Tb.sup.+3 energy 
transfer mechanism) remains competitive with radiative processes. 
For the results in FIG. 2, 15 ml of 0.1 M Tb(NO.sub.3).sub.3 is added to a 
5 ml sample. This, in turn, is combined with 50 ml of 0.3 M NH.sub.4 F. 
For use with samples of unknown composition, The fluorescent output of the 
coprecipitate prepared with the unknown sample is compared to that of a 
standard, in this case an identically prepared sample with known Pu.sup.+3 
concentration. 
EXAMPLE II 
Americium Detection 
Concentrations of americium ion, Am.sup.+3, are determined indirectly by 
their sensitizing effect on terbium fluorescence. 
Sample preparation procedure follows that of the plutonium samples except 
that no reduction step is required since americium has no tendancy to 
disproportionate. 
A laser frequency is chosen to excite the Am.sup.+3 ions preferentially to 
their .sup.5 D levels. An energy transfer then takes place from the .sup.5 
D states of Am.sup.+3 ion to the near resonant .sup.5 D.sub.4 level of 
Tb.sup.+3 ions via electric dipole interaction. The Tb.sup.+3 ion then 
decays radiatively, thus enhancing or sensitizing the overall Tb.sup.+3 
ion fluorescence. 
FIG. 3 shows the degree of terbium fluorescence enhancement due to 
americium sensitization. Curve A is the fluorescent spectrum without 
americium; curve B is the fluorescent spectrum with americium. Here the 
concentration of Am.sup.+3 is 1 ppm, and the precipitate is excited by 
457.1 nm laser light. 
EXAMPLE III 
Americium Detection 
Here concentrations of americium ion, Am.sup.+3, are determined indirectly 
by their sensitizing effect on europium, which is present as an impurity 
in a calcium fluoride matrix. 
The calcium fluoride host lattice with europium, Eu.sup.+3, as an impurity 
(1-2%) is formed and precipitated from water by adding amnonium fluoride, 
NH.sub.4 F, to a solution of calcium nitrate, Ca(NO.sub.3).sub.2 which 
contains the unknown sample and europium ions, such that Eu.sup.+3 
comprise about 2% of the cations present (europium is added in the form of 
europium chloride, Eu Cl.sub.3, or europium nitrate, Eu(NO.sub.3).sub.3). 
For example, 15 ml of 0.1 M Ca (NO.sub.3).sub.2 and 0.002 M 
Eu(NO.sub.3).sub.3 is added to a 5 ml actinide-containing sample. This is 
then combined with 30 ml of 0.3 M NH.sub.4 F. Coprecipitation, calcination 
and preparation of the coprecipitate for fluorescent analysis proceeds as 
in the case for plutonium detection. The americium in the coprecipitate is 
preferentially excited to the .sup.5 D levels by 457.1 nm laser light. 
Energy transfer occurs between the .sup.5 D levels of americium to the 
.sup.5 D levels in europium. Americium concentration is determined by 
comparing the degree of europium fluorescence enhancement over an 
identically prepared standard containing the same concentration of 
europium. 
The descriptions of the foregoing examples of the invention has been 
presented for purposes of illustration and description. They are not 
intended to be exhaustive or to limit the invention to the precise form 
disclosed, and obviously many modifications and variations are possible in 
light of the above teaching. The examples were chosen and described in 
order to best explain the principles of the invention and its practical 
application to thereby enable others skilled in the art to best utilize 
the invention in various contexts and with various modifications as are 
suited to the particular use contemplated. It is intended that the scope 
of the invention be defined by the claims appended hereto.