Lab Exercise - 210Po and 210Pb
Radium (226Ra) present in the ground continuously produces noble gas radon, part of which escapes into the atmosphere. As Rn decays, solid radioactive daughter nuclides are formed in the atmosphere and attached to aerosol particles. The most important of these is the relatively long-lived 210Pb (T1/2 = 22.3 a).
The daughters of radon will return to the ground surface with rain and dust and are taken up by plants. Transfer to animals and humans can take place through grazing and consumption of radionuclide-bearing plant or meat products. 210Pb is a beta-emitter, as well as its daughter nuclide 210Bi (T1/2 = 5 d). Instead, the daughter nuclide of 210Bi, 210Po (T1/2 = 138 d) is an alpha emitter (Eα= 5.3 MeV) and it can be easily separated and measured. In fresh vegetation samples there are both deposited 210Po and ingrown 210Po due to decay of 210Pb. In the samples stored for two years, the deposited 210Po has decayed completely and there is practically only 210Po formed from the decay of 210Pb. Thus the activity concentration measured for 210Po represents also the activity concentration of 210Pb. If the activity of 210Po initially in the plant is to be measured the determination should be done well before its decay and the result should be corrected with respect to both decay of 210Po and its in-growth from 210Pb. This, however, is possible only if the activity concentration of 210Pb is also measured.
The aim of this work is to determine the 210Po-concentration of vegetation. Due to volatility of polonium it is not possible to use hot temperature digestion for destroying organic matter but it is necessary to carry out HNO3–HCl digestion. The insoluble residue (e.g. silicates) is removed by filtration and excess acid is neutralized and solution is kept in contact with a silver disc for two hours. Polonium is more noble metal than silver and is electrodeposited on the surface of a silver disc. Hydrazine is added to reduce trivalent iron to divalent to prevent the blackening of silver due to precipitation of iron. To speed up the deposition the temperature is raised to +60 oC.
For determination of chemical yield, known amount of 209Po yield determinant tracer is added to the sample solution before the acid digestion. Alpha energy of 209Po is 4.88 MeV. When the activity measurement is carried out by alpha spectrometry, the peak area of both polonium isotopes (209Po and 210Po) are determined. The 210Po-concentration of the sample is calculated from the peak areas of 209Po and 210Po.