IMPROVEMENT OF AM AND CM DETERMINATION IN SOIL SAMPLES
M. Pimpl,*1 R. H. Higgy 2
*1
2
Research Center Karlsruhe, Central Safety Department, Box 3640,
D-76021 Karlsruhe, GERMANY (e-mail: max.pimpl@hs.fzk.de)
National Centre for Nuclear Safety and Radiation Control, Atomic Energy Authority,
3 Ahmed El Zomor St., Nasr City, Cairo, EGYPT (e-mail: ruranda@yahoo.com)
The determination of Am and Cm in a radiochemical procedure for the sequential analysis of
Pu, Am, and Cm in soils was improved and optimized. This method uses only solvent extraction
and extraction chromatography for the separation and cleaning of Am and Cm from soils up to
50 g sample weight. After leaching with 8 M nitric acid, Pu is extracted with TOPO/
Cyclohexane. The remaining leaching solution is brought to pH 1 and Am and Cm are extracted
with TOPO/cyclohexane, too. After backextraction with 2 M nitric acid, Am and Cm are
separated from traces of matrix elements, especially Fe, on a TRU-column, afterwards on a
TEVA-column from lanthanides (TRU-resin and TEVA-resin of Eichrom Europe, SARL, Paris,
France). The clean Am/Cm-fraction is electroplated and measured by alpha spectrometry. The
detection limit of this optimized procedure with a counting time of 1000 min is 0.03 Bq per kg
soil at a 95 % confidence level.
INTRODUCTION
In the frame of environmental monitoring, very low activity levels of Am-241 have to
be determined. Am-241 present nowadays in the environment originates mainly from
atmospheric nuclear tests in the 1950´s and 1960´s. Although Am-241 is not produced
directly in nuclear explosions, it is produced from decay of Pu-241, a beta emitter with a
half life time of 14.4 a. The total ultimatively produced Am-241 will amount to 5.5 PBq.1
The highest Am-241 levels have to be expected in the northern hemisphere, with a
maximum of 25 Bqm-2 in the north temperate zone (40-50o), corresponding to 0.09 Bq/kg
of arable soil. As gamma-spectrometric measurements are not sensitive enough (minimum
detectable amount : 1-2 Bq/kg), such low specific activities of Am-241 have to be
determined by aid of alpha-spectrometry after radiochemical separation and purification.
Within the scope of decommissioning of nuclear installations, large amounts of more
or less radioactive contaminated wastes are produced. 98 % of these wastes can be
released directly or after decontamination from regulatory control for disposal, recycling or
reuse.2,3 As a criteria for the release, clearance levels have been established below which
the disposal, recycling and reuse of radioactive contaminated materials are released. As in
nearly all practical cases more than one radionuclide is involved, it has to be ensured that
the mixture is below the clearance level. A simple summation formula is used to prove this
criteria:
n
ci is the specific activity of radionuclide i
ci

1
cL,i is the clearance level of radionuclide i
c
i 1 L, i
n is the number of radionuclides in the mixture
1
As alpha-emitting radionuclides, due to their high radiotoxicity, have very low
clearance levels, it derives from this formular that it is necessary to determine their
specific activity in waste to be released as low as achievable in order to minimize the
clearance level for the mixture. As the same criteria for clearance are applied for the sites
after decommissioning of nuclear installations,4 soil samples have to be analysed for their
content of radionuclides, which have been handled or produced in the concerned
installation during operation.5 That implies that the same analytical procedures as in
environmental monitoring must be applied, using suitable chemical procedures for large
sample sizes and low-level counting techniques for the measurement.
A great number of different analytical procedures for the determination of Am and Cm
in environmental samples are reported in literature.6-25 Normally they are part of methods
for the determination of transuranic elements (Pu, Am, Cm) or actinide elements (Th, U,
Pu, Am, Cm) by alpha spectrometry after sequential radiochemical separation. After
ashing and leaching with mineral acids, in most of the procedures at first an anionexchange step is used to separate Th, U, and Pu from the leaching solution.6-9,12,16,19,26,27
Only few methods use an extraction procedure for this separation.10,11,21,22 Am and Cm
separation of the remaining leaching solution and subsequent radiochemical cleaning is
performed with ion-exchange in nitric acid–methanol medium,6-11,18 with extraction
procedures,13-15,17,19-22,24,25 or a combination of both.8,12,17,26,27 For the final alphaspectrometric measurement the cleaned Am/Cm-fraction is electroplatet6-15,18,22 or
coprecipitated with NdF3.16,19-21,24-27
Although all these procedures can be applied for soils, only few of them allow the
analysis of large sample sizes. For measuring the fallout level of Am-241 in soils, a
mimum mass of 10 g of soil has to be taken for analysis. More reliable results are
expected when increasing this amount to 50-100 g of soil. Procedures for Am and Cm
determination in such large samples are available,7-11 but as for the separation and cleaning
of the Am/Cm-fraction several ion-exchange steps are necessary, these methods are
extremely time consuming. Procedures using HDEHP (Bis(2-ethylhexyl)phosphoric acid)
for the extraction of Am and Cm and their separation from lanthanides are faster, but the
sample size is limited to a maximum of 10 g of soil.12-14 Another method is reported which
allows the determination of Am/Cm in soils up to 50 g sample size.19 This procedure uses
extraction chromatographic steps for the separation and cleaning of the Am/Cm-fraction.
Due to the high selectivity of the extraction materials (TRU resin and TEVA resin of
Eichrom Industries Inc.) used for Am/Cm, the described method is much faster than the
ion-exchange methods having comparable results concerning recovery and qualitity of the
final alpha-spectra. The reported procedure was tested, optimized, and implemented in a
sequential analytical procedure for the determination of Pu, Am, and Cm in soils, replacing the time consuming anion-exchange in nitric acid–methanol medium used up to now.
EXPERIMENTAL
Reagents and Apparatus
All reagents used were analytical grade and were utilized as received. Only TOPO,
Trioctyl-phosphine-oxide (Fluka AG, FRG), was prooved for its extraction capability
before use. Pu-236 and Am-243 (Amersham Buchler, Braunschweig, FRG) were used as
2
tracers for chemical yield determination. TRU Resin (100-150 µm) and TEVA Resin (100150 µm) were used for the separation and cleaning of the Am/Cm-fraction (Eichrom
Europe, SARL, Paris, France). Alpha spectrometry was performed in an alpha
spectrometry system with passivated implanted planar silicon (PIPS) detectors, 300 mm²
active area (Alpha Analyst, Canberra Inc.,USA).
Pretreatment of the Samples
Known amounts of yield determinants, Pu-236 and Am-243, are added to 100 g of the
dry ground sample, and ashing at 550 oC overnight is performed. The residue of this dry
ashing is moistened with concentrated nitric acid, fumed off to dryness on a sand bath, and
ashed again at 550 oC. If traces of carbon remain in the ash, this procedure has to be
repeated until no carbon remains.
Leaching of actinides
The ash is weighed into a 1 liter Erlenmeyer flask, cautiously 8 M HNO3
(250 ml) is added, and the mixture is heated to boiling for 30 min under magnetic stirring.
NaNO2 (2,5 g) is added to the hot solution. After cooling the solution is filtered into a
600 ml beaker. The residue is retransferred into the Erlenmeyer flask and boiled once more
for 30 min with 8 M HNO3 while stirring. Again, NaNO2 is added to the hot solution.
After cooling the solution is filtered and the residue is discarded. The filtrates are
combined in the 600 ml beaker and filled to 500 ml with distilled water.
Separation of Plutonium
The sample solution is transferred into a 1,000 ml separation funnel and shaken for
15 min with 25 ml 0.2 M TOPO/Cyclohexane. After phase separation, the aqueous phase
is shaken once more with 25 ml 0.2 M TOPO/Cyclohexane for 15 min. The combined
organic phases are used for Pu determination.11,28,29
Separation of the Am/Cm-fraction
The aqueous phase after Pu-separation is given in a 1 liter beaker. Carefully the pH is
adjusted to 1.0-1.3 with NH3, 25%. This solution is transferred into a 1,000 ml separation
funnel and Am/Cm are extracted twice with 25 ml 0.5 M TOPO/Cyclohexane for 15 min
each. The aqueous phase is discarded, the organic phases are combined in a 250 ml
separation funnel and washed twice with 50 ml 0.1 M HNO3 each. Am and Cm are
backextracted twice with 25 ml 2 M HNO3 each. The organic phase is discarded, the
aqueous phase is washed with Chloroform twice, then evaporated to dryness. The residue
is dissolved in 20 ml 2 M HNO3, 300 mg ascorbic acid are added and dissolved by stirring.
Cleaning of the Am/Cm-fraction
Preparation and conditioning of the TRU-column: TRU resin is slurried in distilled
water. About 3 ml of the resin is given in a 20 cm long glass column with cock, inner
3
diameter 0.4 cm, whereby the resin is placed between two small plugs of glass wool. The
column is conditioned by passing 20 ml 2 M HNO3. The flow rate is adjusted to 1 ml/min
and kept constant during the following steps.
The sample solution is passed through the column, followed by washing with
10 ml 2 M HNO3 and then with 10 ml 9 M HCl. The Am/Cm-fraction is eluted with
20 ml 4 M HCl and evaporated to dryness. The residue is dissolved in 10 ml
2 M NH4SCN/0.1 M HCOOH.
Preparation and conditioning of the TEVA-column: TEVA resin is slurried in distilled
water. A glass column is prepared as described for the TRU resin. The column is
conditioned by passing 10 ml 2 M NH4SCN/0.1 M HCOOH. The flow rate is adjusted to
1 ml/min and kept constant during the following steps.
The Am/Cm-fraction in 2 M NH4SCN/0.1 M HCOOH obtained after the TRU column
is load on the TEVA column. The column is washed first with 10 ml
2 M NH4SCN/0.1 M HCOOH then with 10 ml 1 M NH4SCN/0.1 M HCOOH. The clean
Am/Cm-fraction is eluted with 20 ml 2 M HCl. The solution is evaporated to dryness,
fumed off with 10 ml Aqua Regia twice, then twice with 5 ml concentrated HCl.
Preparation of the Am/Cm-fraction for alpha spectrometry
The residue after cleaning was dissolved in 4 M HCL and (NH4)2C2O4, 4%, and
electrplatet as previously described.10,11,28-30 Alpha-spectra of high quality are achieved
with a resolution of 25-30 keV for the tracer peak Am-243.
RESULTS AND DISCUSSION
The radiochemical procedure presented here allows the sensitive determination of Pu,
Am, and Cm in soil samples up to 50 g. A schematic representation of the procedure is
given in Fig.1. To recognize the difference to the widely used ion-exchange method, this
method is included for comparison.
As actinides in soils are adsorbed on soil particles, leaching with strong mineral acids
like 8 M HNO3 is sufficient to bring them into solution.28 Even in presence of actinides
adsorbing substances like CaF2, BaSO4, MnO2, or Zr3(PO4)2 leaching is nearly
quantitative. The same was found in samples containing strong ion-exchangers like clayminerals, Al2O3, or Fe2O3. Only if highly annealed Pu is present in the sample material, a
two-stage boiling process with HNO3/HF and HNO3/Al(NO3)3 has to be applied to
dissolve Pu completely.
The leaching solution obtained contains major sample fractions as dissolved
substances. The first extraction step with 0.2 M TOPO/Cyclohexane seperates Pu nearly
quantitatively form the leaching solution, together with most of the U and Th. Pu is
backetxtracted with ascorbic acid in HCl, purified by coprecipitation with LaF3 and ionexchange, and finally electroplatet from HCl/oxalate solution.11,28,29
The remaining leaching solution is treated with NH3, 25%, until a pH of 1-1.3 is
adjusted. Am and Cm are extracted with 0.5 M TOPO/Cyclohexane whereas the most
important matrix elements Na, K, Mg, Ca, Al, and most of Fe are left in the aqueous
4
Ashed Sample
Add yield tracers
Leaching with 8 M HNO3
Add NaNO2
Filtrate
Dissolved Sample
Organic
Phase:
Extraction with 0.2 M TOPO/Cyclohexane
Pu-Determination
Aqueous Phase:
Am- and Cm-Determination
Adjust pH=1 with NH3
Extraction with 0.5 M TOPO/Cylohexane
Wash with 0.1 M HNO3
Backextraction with 2 M HNO3
Wash with CHCl3
Evaporate to dryness
ION-EXCHANGE METHOD
EXTRACTION METHOD
Dissolve in 9 M HCl
Dissolve in 2 M HNO3
Add 300 mg ascorbic acid
Purification with Cation- and
Anion-Exchange
Purification with TRU Resin
Wash with 9 M HCl
Collect passing solution and
washing solution
Evaporate to dryness
Dissolve in 1 M HNO3/CH3OH(93%)
Wash with 2 M HNO3
Wash with 9 M HCl
Elute with 4 M HCl
Evaporate to dryness
Dissolve in 2 M NH4SCN/
0.1 M HCOOH
Cleaning with Anion-Exchange
Cleaning with TEVA Resin
Wash with 1 M HNO3/CH3OH(86%)
Wash with 1 M HCl/0.5 M NH4SCN/
CH3OH(80%)
Elute with 1.5 M HCl/CH3OH(86%)
Evaporate to dryness
Fume off with HCl conc.
Dissolve in HCl/(NH4)2C2O4
Wash with 2 M NH4SCN/
0.1 M HCOOH
Wash with 1 M NH4SCN/
0.1 M HCOOH
Elute with 2 M HCl
Evaporate to dryness
Fume off with Aqua Regia
Fume off with HCl conc.
Dissolve in HCl/(NH4)2C2O4
Electrodeposition and
Alpha-Spectrometry
Electrodeposition and
Alpha-Spectrometry
Fig. 1: Schematic representation of the analytical procedure.
5
solution. Am and Cm are backextracted with 2 M HNO3 and have to be purified from
traces of Fe and rare earth elements in order to obtain a carrier free preparation for alpha
spectrometric measurement.
In the ion-exchange procedure, a two-layer ion-exchange (cation-exchange directly
followed by anion-exchange) is applied to separate Fe and remaining traces of Pu and Th.
Tervalent actinides and lanthanides passes this column and they are strongly sorbed on a
second anion-exchange from nitric acid / methanolic media. Lanthanides are washed out
with methanolic NH4SCN/HCl-solution as thicyanato-complexes. The clean Am/Cmfraction is eluted with a HNO3/Methanole mixture. The preparation for alpha
spectrometric measurement can be performed by electroplating or micro-coprecipitation
with NdF3 or LaF3.
In the extraction procedure, ion-exchange is replaced by solid-phase extraction
chromatographic methods for Am/Cm separation and cleaning. In a first step TRU resin is
used to separate Am/Cm from Fe and traces of other alpha emitters, Pu, Th, U, as well as
their daughters. TRU resin consists of a bifunctional organophosphorous extractant, which
is dissolved in Tributylphoshate and coated onto an inert resin. The retention from nitric
acid in high concentrations is extremely high for tetravalent actinides (k~104-106) and
relatively high for tervalent ions like Am and Cm (k~102). Fe can be easily removed by
reduction to Fe(II) together with bivalent elements. The very low affinity of TRU material
for Am and Cm in chloride medium forms the basis for the selective stripping of Am and
Cm from the resin, together with lanthanides due to their chemical similarity.
In a second step TEVA resin was applied for the purification of the Am/Cm-fraction
from lanthanides. The extractant on the TEVA resin is an aliphatic quarterny amine
(Aliquat 336) which is coated on an inert resin. The Am/Cm-fraction in a
thiocyanate/formic acid-mixture is loaded on the column. Am and Cm are strongly sorbed
under these conditions, whereas the lanthanides are washed out as thicyanato-complexes.
The clean Am/Cm-fraction is eluted with 2 M HCl and the preparation for alpha
spectrometric measurement can be performed by electroplating or micro-coprecipitation
with NdF3 or LaF3.
The new extraction procedure was tested for soil and sediment samples. Up to a
sample size of 25 g of ash the recovery of Am-243-tracer is about 75 % (see Table 1). For
sample weights of 50 g the mean recovery decreases to about 60 %, and for sample sizes
of 100 g to about 30 %. Very probably the recovery can be increased by enlarging the
extraction columns, but this would automatically increase the volumes of the different
solutions and by this extend the time for analysis.
Table 1: Mean tracer recovery with the extraction method applied for different amounts
of ashed sample material
Material
Number of assays
performed
Amount of ash taken
for analysis in g
sediment
soil
soil
soil
soil
12
4
4
4
4
25
10
25
50
100
6
Mean recovery of
Am-243
in %
70  15
75  5
75  11
58  15
31  17
CONCLUSIONS
The procedure presented is suited for the sensitive and reliable determination of Am
and Cm in soil samples up to 50 g of sample weight. The time required for the Am/Cm
separation and cleaning is about one working day of 8 hours, which is much shorter than
with the ion-exchange procedure, which takes at least three working days. The achievable
lower detection limit on a 95 % confidence level is about 0.0015 Bq/sample (measuring
time 60,000 s, chemical yield 60 %, detector efficiency 18 %, background counts in the
peak area 5 counts in the measuring time), which means that the minumum detectable
activity of Am-241, Cm-242, and Cm-244 is about 0.03 Bq/kg, when 50 g of soil are used
for analysis. This is normally sufficient for environmental monitoring, as well as for
clearance measurements in samples from decommissioning projects of nuclear
installations.
REFERENCES
1. United Nations Scientific Committee on the Effects of Atomic Radiation. Ionizing
Radiation: Sources and Biological Effects. UNSCEAR 1982 Report, United Nations,
New York, 1982
2. German Commission on Radiological Protection (SSK): Clearance of Materials,
Buildings and Sites with Negligible Radioactivity from Practices Subject to
Reporting to Authorisation; Recommendation by the Commission on Radiological
Protection; passed at the 151st meeting on 12 February 1998
3. European Commission:Recommended radiological protection criteria for the
clearance of buildings and building rubble arising from the dismantling of nuclear
installations; draft of of a recommendation, to be published as Radiation Protection
No. 101, Luxembourg, March 1999
4. C. Graetz, E. Kugeler, I. Neuhaus, A. Nüsser, M. Schramke, R. Sefzig, S. Thierfeld,
”Clearance Levels for Nuclear Sites in Germany”, Proceedings of 2nd Intern.
Symposium Release of Radioactive Material from Regulatory Control, TÜV Nord
Akademie, Hamburg / Germany, 08-10 November 1999, pp. 84-94
5. U. HOEPPENER-KRAMAR, M. PIMPL, F. WILLMANN, J. Radioanal. Nucl.
Chem., 226 (1997) 99-103
6. E. HOLM, R. FUKAI, Talanta, 23 (1976) 853-855
7. E. HOLM, S. BALLESTRA, R. FUKAI, Talanta, 26 (1979) 791-794
8. E. HOLM, E. BALLESTRA, in ”Measurement of Radionuclides in Food and the
Environment”, Technical Report Series No. 295, IAEA, Vienna 1989, pp. 105-114
9. A. YAMOTO, J. Radioanal. Chem., 75 (1982) 265-273
10. M. AFSAR, H. SCHÜTTELKOPF, KfK-Report 4346 ,1988
11. M. PIMPL, H. SCHÜTTELKOPF, Report SLU-REK-61, 1986, pp. 69-76
12. M. H. HIATT, P.B. HAHN, Anal. Chem., 51 (1979) 295-298
13. C. W. SILL, K. W. PUPHAL, F. D. HINDMAN, Anal. Chem., 46 (1974) 1725-1737
14. R. P. BERNABEE, D. R. PERCIFAL, F. D. HINDMAN, Anal. Chem., 52 (1980)
2351-2353
15. R. P. BERNABEE, Health Physics, 44 (1983) 688-691
7
16. S. LESKINEN, J. K. MIETTINEN, T. JAAKOLA, J. Radioanal. Nucl. Chem.,
Articles, 115 (1987) 289-298
17. T. D. FILER, Anal. Chem., 46 (1974) 608-610
18. D. KNAB, Anal. Chem., 51 (1979) 1095-1097
19. A. BERNE, Americum Separation in Larger Soil Samples (up to 50 g), 41st Annual
Conference on Bioassay, Analytical & Environmental Radiochemistry (Eichrom
Workshop), Boston, MA, 1995
20. A. BERNE, Report EML-575, 1995
21. J. H. KAYE, R. S. STREBIN, R. D. ORR, J. Radioanal. Nucl. Chem., Articles,
194 (1995) 191-196
22. L. L. SMITH, J. S. CRAIN, J. S. YAEGER, E. P. HORWITZ, H. DIAMOND,
R. CHIARIZIA, J. Radioanal. Nucl. Chem., Articles, 194 (1995) 151-156
23. W. C. BURNETT, D. R. CORBETT, M. SCHULTZ, E. P. HORWITZ,
R. CHIARIZIA, M. DIETZ, A. THAKKAR, M. FERN, J. Radioanal. Nucl. Chem.,
226 (1997) 121-127
24 S. J. GOLDSTEIN, C. A. HENSLEY, C. E. ARMENTS, R. J. PETERS,
Anal. Chem., 69 (1997) 809-812
25. R. A. BOLL, G. K. SCHWEITZER, R. W. GARBER, J. Radioanal. Nucl. Chem.,
220 (1997) 201-206
26. J. MORENO, N. VAJDA, P. R. DANESI, J. J. LAROSA, E. ZEILER,
M. SINOJMERI, J. Radioanal. Nucl. Chem., 226 (1997) 279-284
27. J. MORENO, J. J. LAROSA, P. R. DANESI, K. BUMS, P. DEREGGE, N. VAIDA,
M. SINOJMERI, Radioactivity & Radiochemistry, 9 (1998) 35-44
28. H. Schüttelkopf, KfK-Report 3035, 1981
29. M. Pimpl, H. Schüttelkopf, Report SLU-REK-61, 1986, pp. 53-62
30. K. W. PUPHAL, D. R. OLSEN, Anal. Chem., 44 (1972) 284
8