IAEA Regional Training Course on Sediment Core Dating
Techniques
RAF7/008 Project
Alpha Spectrometry
A. Laissaoui
Centre National de l’Energie des Sciences et des
Techniques Nucléaires
Rabat, 05 – 09 July 2010
IAEA Regional Training Course on Sediment Core Dating Techniques RAF7/008 Project
Alpha Spectrometry A. Laissaoui
Rabat, 05 – 09 July 2010
Alpha Spectrometry
A. Laissaoui
CNESTEN, Rabat – Morocco
Introdution
Alpha-emitting radioisotopes spontaneously produce alpha particles (or 4He nuclei) at characteristic
energies usually between about 4 and 6 MeV. Alpha particles are heavy charged particles. Because
they are large and slow, they lose energy readily in materials. A single sheet of paper stops them. Any
physical medium between the alpha emitting radionuclide and the active portion of the detector will
absorb some of the alpha particle energy. These attenuation characteristics, which manifest themselves
both within the sample and with any materials between the sample and the active detector volume,
cause a characteristic tailing in the alpha peak. The peaks tend to have an asymmetric shape rather
than the Gaussian shape typical in alpha spectroscopy. Specifically, the samples are counted in a
vacuum and the samples must be as thin as possible to avoid self absorption.
The alpha particle energies of many isotopes differ by as little as 10 to 20 keV. Because this is near the
resolution of the silicon detectors used in alpha spectrometers, such elements must be chemically
separated before analysis. Radiochemistry is intended to isolate specific elements in the sample to
minimize interferences between multiple alpha emitting nuclides. In order to account for the inevitable
loss of the sample during separation, a known quantity of a specific isotope or tracer is added to the
sample. The tracer is an isotope of the element under study - i.e., 232U for uranium. Since all isotopes
of an element behave chemically alike, the percent of tracer lost in the chemical processes is equal to
the percent of the sought radioisitopes lost, assuming the tracer is homogeneously mixed with the
sample and is brought into chemical equilibrium with the sample. In order to obtain the thinnest
sample possible, thereby minimizing self attenuation, samples must be properly mounted.
Electrodeposition, evaporation and micro precipitation of the sample are the most commonly used
mounting methods. After data is acquired, analysis software processes the spectrum and quantifies the
results for the isotopes of interest. Analysis can consist of simple count integration and efficiency
correction or can involve extensive background corrections, compensation for various chemical
process characteristics, processing of overlapping peaks, etc.
There are three principal steps to the preparation of an alpha spectroscopy sample preliminary treatment, chemical separation and counting source preparation. Preliminary
treatment is performed to homogenize the sample and to generally prepare it for subsequent
chemical processing. Different procedures are used for solid samples (e.g., soils and
sediments) and liquid samples. If tracer nuclides are to be added to the samples, this is
typically done early in the preliminary treatment phase. Chemical separation is used to isolate
the elements of interest. Typically, a raw sample is divided into subsamples - with each
subsample representing a specific element of interest. Techniques used for separation include
co-precipitation, liquid-liquid extraction, ion exchange and extraction chromatography. In
most cases, two or more of these techniques are combined. After separation, the next step is to
deposit the sample to produce a suitable source; In order to obtain the best possible resolution
with an alpha spectrometer, it is necessary to produce a thin, flat, uniform deposit. Ideally, the
source should have a layer of the alpha emitter with no foreign matter above this layer to
attenuate the alpha radiation. There are three principle methods of source mounting:
evaporation from an organic solvent, electrodeposition, and microprecipitation/filtration.
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IAEA Regional Training Course on Sediment Core Dating Techniques RAF7/008 Project
Alpha Spectrometry A. Laissaoui
210
Pb Measurement in sediments
Pb, part of the primordial 238U decay chain, decays by  emission (Emax = 17 keV [85%]
and Emax = 61 keV [15%] followed by -radiation [E = 46 keV (5%]). The immediate 210Pb
decay product 210Bi is also a beta emitter with a much higher energy of beta radiation (Emax
= 1160 keV [100%]) than its parents. The resulting product 210Po is an alpha emitter (E =
5503 keV [100%]), which results from 210Pb according to the following process:
210
Lead-210 is a radioisotope that is a member of the 238U decay series. Uranium-238 has a halflife of 4.5x109 year and decays through several daughters to 226Ra, which has a half-life of
1600 year. Radium-226 in rocks, soil, and seawater decays to radon gas (222Rn, t1/2 = 3.8 day),
some of which escapes to the atmosphere. Radon gas undergoes decay through a series of
short-lived daughters to yield particulate 210Pb, which has a half-life of 22.3 year. This 210Pb
in the atmosphere can be delivered to the earth via rainfall (washout) or dry fallout (El-Daoushy
1988). Atmospheric 210Pb is ultimately delivered to waterbodies primarily by direct fallout,
but a fraction of the unsupported 210Pb that reaches marine systems may also be delivered via
transport from the watershed. After entering a waterbody, particle-reactive 210Pb adsorbs
quickly to materials in the water column and is deposited on the lake bottom. This 210Pb will
decay to 210Bi (t1/2 = 5.01 day) which, in turn, decays to 210Po that has a half-life of 138 day.
Decay of 210Po ultimately yields stable 206Pb. The stratigraphic distribution of unsupported
210
Pb activity must be estimated for sediment core dating. Presence of supported 210Pb in
sediments complicates the dating procedure. Atmospherically derived (unsupported or excess)
210
Pb must be distinguished from supported 210Pb, which is produced by decay-series
precursors within or attached to the sediment matrix. Supported 210Pb activity is assumed to
come from in situ 226Ra, with which it is generally believed to be in secular equilibrium. This
226
Ra in the sediments typically comes from local soil and bedrock particles, which are
delivered to ocean by erosion, or aeolian transport.
Unsupported 210Pb activity can be estimated by alpha counting in each stratigraphic sample
down to a depth where no measurable unsupported 210Pb remains. Supported 210Pb activity,
measured or approximated by an appropriate means, is subtracted from the total activity to
yield an estimate of unsupported 210Pb activity. Total 210Pb activity can be measured by alpha
counting of the granddaughter radionuclide, 210Po. This method has several advantages. The
high sensitivity of alpha detectors makes them suitable for counting samples with low
activity. Small amounts of sample can therefore be used, which permits high-resolution (i.e.,
small depth interval) counting, even in sediments with low bulk density. Nevertheless,
supported 210Pb activity is not measured directly with alpha counters. Instead, it is generally
estimated by the down-core, asymptotic total 210Pb activity (Fig. 1). Typically, it is assumed
that supported 210Pb activity has remained unchanged over the past 100–150 years. Less
commonly, 226Ra activity is measured by an independent analytical technique, allowing for
subtraction of supported 210Pb activity from the total 210Pb activity (i.e., 210Po) on a level-bylevel basis.
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IAEA Regional Training Course on Sediment Core Dating Techniques RAF7/008 Project
Alpha Spectrometry A. Laissaoui
Figure 1.- supported and unsupported 210Pb in the sediment.
In sediment core dating, unsupported 210Pb must be measured in each stratigraphic sample
down to a depth where no measurable unsupported lead remains. In alpha spectrometry, total
210
Pb activity is measured by alpha counting of the daughter radionuclide 210Po. Nevertheless,
two cases are to be considered:
1.- for old samples (held for almost two years), 210Pb is in secular equilibrium with 210Po.
Consequently, one determination of 210Po is sufficient to obtain the 210Pb activity.
2.- for freshly collected samples, the equilibrium is not achieved for the topmost layers. Thus,
two elapsed measurements of 210Po are required to obtain both the 210Pb and 210Po at the
sampling date.
The radiochemical procedure for the determination of 210Po in sediment is quite simple and is
based on the isolation of Po from the other elements by self deposition on silver disc. The
deposition is very specific and can be carried out in the presence of many other radionuclides.
In what follows, the procedure:
1.- weigh a known amount of dried sample in a glass beaker and add a weighed aliquot of
polonium tracer (208Po or 209Po).
2.- digest the sample using concentrated acids (HNO3 and HCl). If complete digestion is not
achieved, transfer the sample to a teflon beaker and digest with HF and/or HClO4.
3.- dissolve the residue in HCl and evaporate to near dryness, then dissolve the residue in
about 40 ml of HCl 0.5N and transfer the solution to a 100 ml beaker.
4.- rinse the original beaker with HCl 0.5N and transfer to the solution. The total volume
should be around 80 ml.
5.- place a silver disc coated on one side with heat-resistant tape in the bottom of the beaker.
6.- place the solution on a hot plate at a temperature about 80°C while stirring occasionally.
7.- after about 6 hours, remove the disc form the solution and rinse it with H2O and ethanol
and allow to dry. The Po source is ready to be counted in an alpha spectrometer.
Figure 2 shows a typical spectrum of polonium with 209Po as a tracer.
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IAEA Regional Training Course on Sediment Core Dating Techniques RAF7/008 Project
Alpha Spectrometry A. Laissaoui
Figure 2.- Alpha spectrum of polonium with 210Po as a tracer.
If a second self-deposition is to be done, the remaining solution should be kept for 3 – 6
months. But before storing the sample, Po must be removed. A second deposition is carried
out just after the first to remove Po, but not completely. Another way to remove the remaining
Po is the use of anion exchange resin in Cl- form. In 10M HCl medium, Po is strongly
adsorbed while Pb is not adsorbed onto the column. The procedure is as follows:
The solution obtained after selfdeposition of Po is slowly evaporated to near dryness with the
addition of small amounts of HNO3 and H2O2. The residue is dissolved in 20-30 ml of 10M
HCl. The solution is passed through the anion exchange resin Dowex 1X-8 of Cl- form to
completely remove the Po remaining after its selfdeposition. The column is then washed with
70 ml of HCl 10M. Under these conditions, Pb is not adsorbed onto the column, while Po is
strongly adsorbed. The solution is stored for 3 – 6 months to allow the ingrowth of 210Po from
210
Pb. Po is finally deposited following the same procedure described above. In this way, it is
possible to determine the original activities of both radionuclides in the sample at the
sampling date.
Activity of 210Po is calculated from the following expression:
A
I  B
t.R q ..M
(1)
Where:
I: the peak area in counts
B: Background in counts
T: the counting time in seconds
Rq: the chemical recovery
 : the detector efficiency
M: mass of the sample in kg
By assuming that the chemical recovery is the same for all isotopes of the same element, and
by applying equation 1 to the peak of the tracer, the activity of 210Po can be calculated from
the following expression:
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IAEA Regional Training Course on Sediment Core Dating Techniques RAF7/008 Project
Alpha Spectrometry A. Laissaoui
 Ie Be 
  
t
t b  At
Ae   e
.
 I t Bt  M
  
 tt tb 
(2)
Where the subindice e refers to the sample, b to the background and t to the tracer. At is the
specific activity of the tracer.
Figure 3 shows the origin of 210Po present in the sediment at T0, the sampling date, T1 the date
of the first autodeposition and T2 the date of the second autodeposition.
Figure 3.- Origin of 210Po in sediment at different dates.
By using Bateman equations, activities of 210Po and 210Pb at the sampling date are as follows:
APb(T0 ) 
APoT0  
APo(T2 )
 2 .e Pb .1
APoT1 
 APbT0 . 1 .e Po .1
  Po .1
e
(3)
(4)
Where 1 and 2 are
Pu and Am radiochemistry
Plutonium and americium are separated by sorption on ion-exchange columns and differential
elution with changing acid eluents. To solve interferences, intermediate purification steps
were required, involving ion-exchange and organic solvent extraction. In what follows, details
of the radiochemical procedure are done.
1.- weigh about 10 g of sediment to be analysed and add known amounts of tracers (242 or 236Pu
and 243Am).
2.- proceed by wet leaching with HNO3 and H2O2. A total digestion is not necessary in this
case because Pu and Am are associated only with surface particles.
3.- Filtrate to separation to recover the aqueous phase containing Pu and Am by using a fast
filter.
4.- Add 50mmg of NaNO2 to adjust the oxidation state of Pu to Pu(IV) which is the unique
species retained by the resin. Heat and allow cooling to ambient temperature.
5.- Prepare an ion-exchange column with 10 cm of Dowex 1X-8 anion exchange resin and
Condition the column with 40 ml of 8M HNO3.
6.- Pass the sample through the column at a flow rate as slow as possible (1ml/min) and then
wash the column with 80 ml of HNO3. The effluent and washing are stored for the posterior
analysis of Am.
7.- wash the column with 60 ml of HCl 9M.
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IAEA Regional Training Course on Sediment Core Dating Techniques RAF7/008 Project
Alpha Spectrometry A. Laissaoui
8.- elute Pu with a solution composed of 80 ml of HCl 10M and 1.2 g of ammonium iodide.
NH4I is added to reduce the oxidation state of Pu(IV) to Pu(III).
9.- evaporate the elution to dryness by adding occasionally HNO3 and H2O2 until all I2 is
volatilised.
10.- add 20 ml of HNO3 8M and 50 mg of NaNO2. Heat and allow cooling to ambient
temperature.
11.- Prepare an ion-exchange column with 6 cm of Dowex 1X-8 anion exchange resin and
Condition the column with 20 ml of 8M HNO3.
12.- Pass the sample through the column at a flow rate as slow as possible (1ml/min) and then
wash the column with 30 ml of HNO3. The effluent and washing are stored for the posterior
analysis of Am.
13.- wash the column with 20 ml of HCl 9M.
14.- elute Pu with a solution composed of 60 ml of HCl 10M and 0.6 g of ammonium iodide.
15.- proceed by Electrodeposition following the procedure below.
The Am fraction from the sequential extraction after Pu is in a 500ml beaker.
Oxalate precipitation
 Evaporate on a hot plate until turbidity appears, the volume remain constant.
 Dissolve the salts formed with 300ml of deionized water and add 100mg of Calcium (Ca)
in the form CaCl2.
 Add 20g of oxalic acid previously diluted in hot water, then adjust the pH to 1.5 with
concentrated ammonia (NH4OH cc). a white precipitate appears.
 Filtrate the solution by gravity using a fast filter. Wash the filter and beaker with ammonia
oxalate 0.5%.
 Adjust the pH of the solution to 1 with concentrated HCl and ammonia cc.
 Add 100mg of CaCl2 and adjust the pH to 1.5 then add 15g of oxalic acid, a white
precipitate forms.
 Filtrate again and combine the two filters in a porcelain cresol.
 Ash using the following sequence of temperature: 100C for 1 hour, 200C for 12 hours
and finally 550C for 24 hours.
Double ion-exchange (clean-up).
 Dissolve the residue in ~50ml 10M HCl (or until total dissolution) and a few drops of
hydrogen peroxide. The residue is dissolved but solution is not transparent, proceed with
filtration.
NB: after the filtration, the solution is still obscure. A few drops of H2O2 are added,
the solution returns yellow but only for a while. The cause is the excess of Manganese in the
sample, the hydrogen peroxide was unable to stabilize the Mn in a low oxidation state. So
another reducing agent is used: NH2OH-HCl (Hydroxylammonium Chloride) 0.1g/ml. This
reagent is added drop by drop after the total elimination of peroxide from the solution by
evaporation. The solution returned yellow permanently what means that the Mn is well
stabilised.
 Prepare a double anion exchange column as follows: 2cm of AG1-X8 resin (hydrogen
form) in the bottom and 2cm of AG50W-X8 resin in the top (chloride form).
 Condition the column with 20ml of 10M HCl.
 Pass the solution through the column. Wash the beaker and the column with 10ml of 10M
HCl 3 times.
 Collect the effluent and washing in a beaker and evaporate to dryness.
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Alpha Spectrometry A. Laissaoui
DDCP extraction. (DiButyl-nn-diethyl carbamyl phosphate).
 Dissolve the residue in 20ml of 12M HNO3 and transfer the solution to a 250ml
separatory funnel (1). Wash the beaker with 10-15ml of 12M HNO3.
NB: before putting the sample in a 12M nitric medium, all Cl- must be removed. So
concentrated HNO3 is added until the coloured fumes disappear completely. The solution is
evaporated to dryness (gelatinous aspect) and put in 12M HNO3 (30-50ml).
 Add 1ml of DDCP and shake for 30s. let stand until the total separation of the two phases.
NB: the separation of the two phases is slow and not clear. Purple plumes are floating on
the organic phase. May be the Mn formed a complex with DDCP. The procedure is
followed.
 Transfer the aqueous phase to another separatory funnel (2) and let the DDCP phase in the
funnel (1).
 Add 1ml of DDCP to the funnel (2), shake 30s and let stand until the total separation of
the phases.
 Recover the aqueous phase in a beaker and save it.
 Combine the two organic phases from the two funnels (1) and (2) in the funnel (1). Add 6
ml of HNO3 12M to wash the organic phase. Shake 1mn and let the phase separate.
 Save the aqueous phase and add to the organic phase 10ml of toluene and 6ml of 2M
HNO3, shake 1mn and let stand. The Am passes to the aqueous phase (2M HNO3) which
is recovered in a 100ml beaker.
 Evaporate the 2M HNO3 fraction to dryness and the extraction is repeated.
 Evaporate the final 2M HNO3 fraction to dryness.
Ion exchange (rare earth separation)
 Prepare an ion-exchange column with 5cm of AG1-X4 anion exchange resin.
 Condition the column with 8 ml of 1M HNO3.
 Dissolve the sample with 10ml of 1M HNO3-93%CH3OH, add 1-2mg of NH4NO3 and
0.5cm3 of AG1-X4 resin. Move the beaker slightly and let stand overnight covered.
 Condition the column with 50ml of 1M HNO3-93%CH3OH.
 Pass the sample through the column at a flow rate as slow as possible.
 Wash beaker and column with 50ml of 1M HNO3-93%CH3OH.
 Rinse the column with 80ml of 0.1M HCl-0.5M NH4SCN-80%CH3OH to elute rare earth
elements.
 Wash the column with 20ml of 1M HNO3-93% CH3OH.
 Elute Am with 50ml of 1.5M HCl-86% CH3OH at a flow rate less than 1ml per minute.
 Evaporate the elution to dryness. Add 4ml of concentrated HNO3 and 2 drops of H2O2.
This step is repeated 3 times, the solution is clear.
Electrodeposition (Hallstadius)
 Evaporate the sample to dryness.
 Add 1ml of Na2SO4 (0.3M), evaporate to total dryness
 Add 0.3 ml of H2SO4, heat on a hot plate until the sample is dissolved.
 Add 4ml of H2O and 3 drops of thymol blue 0.4% indicator. The colour of the solution is
red.
 Adjust the pH to 2 with concentrated ammonia. The color changes to orange-yellow.
 Transfer the solution to an electrodeposition cell mounted with a stainless-steel disc.
Rinse the beaker two times with 1% H2SO4, transferring each rinse to the cell. Readjust
the pH to 2 – 2.2 with concentrated ammonia.
 Adjust the anode distance to the disc to about 3mm.
 Electrolyze at 1 Ampere for one hour.
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IAEA Regional Training Course on Sediment Core Dating Techniques RAF7/008 Project
Alpha Spectrometry A. Laissaoui
 At the end of the plating period, add about 1ml of concentrated ammonia, let the current
run for 1mn, remove the cell, and immediately transfer the solution back to the beaker.
Rinse the cell with 1% ammonia two or three times.
 Remove the disc. Rinse with 1% ammonia then acetone.
 Heat to fix the Am more solidly on the disc.
The sample is ready for the alpha counting.
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