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Welcome to Mycore Scientific Inc How 210Pb Works for Dating Sediments Other Information Useful in Dating Sediments |
Measurement
of 210Pb
There are
three alternative methods of analyzing the concentration of 210Pb in
a sample. 1.
210Pb emits a low energy beta which is very difficult
to detect. However when it decays,
it also emits a 49kev gamma photon. The
gamma photon can be measured by gamma ray spectroscopy provided that the
detector is designed so that the low energy photon can penetrate into the active
volume of a germanium detector. 2.
The daughter
of 210Pb is 210Bi. 210Bi emits
a beta particle, which can be determined by radiochemical extraction of the Bi
and analyzing the sample on a low background proportional detector. The half-life of 210Bi is quite short
(a few days) so this analysis must be conducted promptly after separating the
Bi-210 from the sample. 3.
The grand
daughter of 210Pb is 210Po. 210Po
decays with a half-life of 138 days. It
emits an alpha particle. These
emissions can be detected by alpha spectrometry.
Alpha spectrometry can be combined with isotope dilution using 208Po
or 209Po to increase the accuracy of the analysis.
Since alpha spectroscopy also has a low background, a reasonable counting
efficiency (20 to 30%) and lower cost detection equipment, it is the most
frequent method of 210Pb analysis. This
assumption is not valid in many other systems (e.g. biota) where the isotopes of
210Pb and the uranium series are very mobile or a geo-chemical
process (such as different rates of bioaccumulation) fractionates the isotopes
and produces different behaviour Radiochemical
Separation of 210Po
For analysis
by alpha spectroscopy, 210Po must be separated from the sediment
matrix and separated from other alpha emitting isotopes.
The separation procedure consists of the following general steps. 1.
Dried,
ground samples are weighed into centrifuge tubes. 2.
A known
amount of 209Po is added to each sample. The
mass of the tracer is precisely determined by weight. 3.
Nitric and
Hydrochloric acids are added to the sample and the sample is heated at about 80
C for > 16 hours. 4.
The residual
siliceous solids are separated from the solution by centrifugation. 5.
The solution
is evaporated to dryness three times with the addition of small quantities of
HCl added after each drying. 6.
Ascorbic
acid is added to the dilute HCl solution. 7.
A small
silver plate is added to the solution with one side covered with adhesive glue.
8.
The Po
isotopes are electroplated onto silver disks. 9.
The disks
are cleaned and dried and stored for analysis by alpha spectroscopy. Measurement
of 210Po by Alpha Spectroscopy
210Po is measured by isotope dilution alpha
spectrometry.
Each
radioactive decay of 210Po emits an alpha particle that
has energy of 5.3MeV. The energy of
the 209Po alpha particle is about 5.1 MeV. Particles emitted at these two energies can be
identified using an alpha spectroscopy system.
The system consists of a surface barrier detector, preamplifier,
amplifier, mixer router, analog to digital converter, multi-channel analyzer,
and appropriate computer software. The samples
are counted for periods of 0.25 to 2 days depending upon the activity of 210Pb
in the samples. The activity of 210Po in the sample is
determined from the ratio of the total counts of 209Po to 210Po
and from the quantities of sediment and 209Po added to the
sample. Blanks and
standards are measured to verify the performance of all aspects of the
procedures and the instrumentation. The
209Po standard that is added to each sample also serves as an
excellent internal standard to monitor the quality of the analysis. Details are
presented in Flynn (1968) and Evans and Rigler (1980) with modifications
described in Cornett et al (1984) and Rowan et al (1995). |
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