Cerenkov radiation results from a charged particle traversing a light transparent polar medium (e.g. water) with a velocity being higher than the phase velocity of light in this medium. This causes local electronic polarization of the dielectric molecules, which release electromagnetic radiation when returning to the ground state. For β-emitters in aqueous solution, a minimum energy of 262 keV is necessary. A reasonable efficiency is accessible for β-maximum energies exceeding 1 MeV (fig. 10). Cerenkov radiation does not require a scintillator. It can be detected in any medium (acidic or alkaline) and is not subject to chemical quenching. Further advantages of Cerenkov counting are the possibility to apply larger sample volumes and the absence of organic waste.
Drawbacks include a strong color quenching and lower counting efficiencies.
Due to the limitation of the Cerenkov Effect to higher β-energies, small amounts of high energetic radionuclides may be determined in presence of α- and much higher amounts of low energetic β-emitters. However, strong γ-emissions can contribute to the Cerenkov counting efficiency as being capable of producing Compton electrons above the threshold energy, if Eg is above 430 keV [Tayeb et al. 2014]. Consequently, the measured and calculated values for 40K and 210Pb/210Bi are above the average ones for pure β-emitters.
Cerenkov counting has recently been applied in combination with TDCR counting, because color quenching can automatically be corrected and no external calibration therefore is needed [Kossert 2010] (see also procedure 2.3.5.). Thus, Cerenkov radiation can be a suitable choice for discrimination between 89Sr and 90Sr, or for the determination via the higher energy β-emitting daughters, e.g. 90Y from 90Sr, or 210Bi from 210Pb, when the state of equilibrium is known.
Besides classical calibration procedures, the efficiencies for Cerenkov counting can also be calculated through TDCR and the known energy spectrum of Cerenkov electrons (Maxwell equations), e.g. for Y-90 through the following fit:
h (90Y) = 0.90 * (TDCRcorr)0.75 [Tayeb et al. 2014]
Possible interferences from accompanied γ-radiation (e.g. for 40K) have to be taken into account (fig. 10).
The TDCR correction for effects of geometry and color quenching even in yellow and brown foodgrade dyes has found to be effective for Cerenkov counting [Tayeb et al. 2014]. Plastic vials generally give higher counts and are preferable compared to glass vials.
In Radon containing water samples, Cerenkov radiation resulting from 214Bi with an Eβmax of 1.5 MeV (h = 40 %, Beckman LS 6000LL) and to a smaller extent from 214Pb with an Eβmax of 0.7 MeV can be used for its determination.
Figure 10: Cerenkov counting efficiency vs β-energy (calculated for TDCR [Tayeb et al. 2014] and measured by Beckman LS 6000LL [Möbius and Möbius 2008])
Kossert K. 2010: Activity standardization by means of new TDCR-Cerenkov counting technique; Applied Radiation and Isotopes 68 (2010) 1116-1120
Möbius S. and Möbius T. L. 2008: LSC-Handbuch, DGFS e.V. und Forschungszentrum Karlsruhe GmbH, Karlsruhe 2008
Tayeb M., Dai X., Corcoran F.C. and Kelly D.G. 2014: Evaluation of interferences on measurements of 90Sr/90Y by TDCR Cherenkov counting technique; J. Radioanal. Nucl. Chem. 300 (2014) 409/414