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.

The analysis of either the daughter or grand daughter isotopes of 210Pb requires that the analyst make an additional assumption that 210Pb is present in equal concentration with the daughter isotopes.   This assumption can be assured by storing the samples sealed in a container for about two years so that that the 210Pb isotopes cannot exchange and secular equilibrium is achieved.   At secular equilibrium the activity of the parent isotope is equivalent to that of the daughter isotope.

Sediment systems provide a natural storage system because Pb, Bi and Po are immobile or only mobilized together in these systems.  In all but the top few cm of most sediment cores the sediment is old enough to ensure that the concentration of Pb-210 will be equivalent to that of the shorter-lived daughter isotopes. Tests of the top few cm and of surficial sediment indicate that the Po and Pb concentrations are quite similar.  Therefore for 210Pb dating of sediments the analysis of the concentration of 210Po is equivalent to the measurement of 210Pb until the shorter lived isotope is radiochemically separated from the 210Pb.

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).