1.6 - Cerenkov Counting

Cherenkov 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). Cherenkov 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 Cherenkov 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 Cherenkov 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 Cherenkov 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.

Cherenkov 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, Cherenkov 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 Cherenkov counting can also be calculated through TDCR and the known energy spectrum of Cherenkov electrons (Maxwell equations), e.g. for Y-90 through the following fit:

h (⁹⁰Y) = 0.90 _* (TDCR_corr)^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 Cherenkov counting [Tayeb et al. 2014]. Plastic vials generally give higher counts and are preferable compared to glass vials.

In Radon containing water samples, Cherenkov radiation resulting from ²¹⁴Bi with an E_βmax of 1.5 MeV (h = 40 %, Beckman LS 6000LL) and to a smaller extent from ²¹⁴Pb with an E_βmax of 0.7 MeV can be used for its determination.

Kossert K. 2010: Activity standardization by means of new TDCR-Cherenkov 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