Logo of the Physikalisch-Technische Bundesanstalt

TDCR Čerenkov Method - A New Method of Activity Determination

04.01.2010

In a dielectric, transparent medium, high-energy electrons from beta decay can generate Čerenkov light if their speed is higher than the phase velocity of light in this medium. With the aid of a liquid scintillation spectrometer, this effect can be used for activity determination. Up to now, applications have, however, been limited to relative procedures, i.e. reference solutions with known activity are needed for calibration. Yet, these methods offer numerous possibilities of application since low-energy electrons or heavy - and thus slow - alpha particles do not generate any Čerenkov light and, therefore, do not influence the measurements. Furthermore, aqueous solutions can be used directly, and Čerenkov measurements do not produce any mixed waste made of radioactive solution and organic liquids - as is the case for liquid scintillation counting, whose application is very successful.

At PTB, a new method has been developed which enables activity determinations by means of Čerenkov measurements whereby no reference solution is needed. The efficiency calculation is, thereby, based on a free parameter model - which is also the basis of two successful liquid scintillation methods, an efficiency tracing method and the triple-to-double coincidence ratio (TDCR) method [1]. In the case of the new method, the number of generated Čerenkov photons is derived from the Frank-Tamm formula [2]. Furthermore, the new method takes the fact into account that the emission of Čerenkov light takes place anisotropically. First measurements have been performed in a TDCR apparatus [3] with three secondary electron multipliers. The measured ratio of threefold to double coincidence counting rates makes it possible to determine the free parameter and, thus, the efficiency. The new method has been successfully applied for activity determinations of 32P, 89Sr, 90Y, 106Ru/106Rh and 204Tl.

Especially in the case of high-energy emitters, small relative uncertainties of clearly less than 1 % are yielded. The uncertainties are, thus, considerably larger than those obtained with the liquid scintillation techniques often used at PTB, but the accuracy achieved is still sufficient for numerous applications. The spectrum of potential applications encompasses measurements of radionuclides in nuclear medicine (e.g. 32P, 89Sr, 90Y) - but also applications in the field of environmental radioactivity, such as the determination of strontium isotopes or measurements of 210Pb (via 210Bi), are conceivable.

In the case of beta emitters with smaller end-point energies, such as 36Cl or 204Tl, the influence of so-called shape factors - which characterise the shape of the beta spectra - is very high. Since for these nuclides, the activity can be determined very precisely by means of liquid scintillation counting, the Čerenkov method can be used for the investigation and determination of shape factors.

The fundamentals of this method as well as the results of first test measurements will soon be submitted for publication [2]. A computer program which is to enable a simple application of the method is being developed. It is also planned to extend the application of the method to radionuclides with gamma radiation and complex decay schemes.

Literature

  1. Broda, R., Cassette, P., Kossert, K.:
    Radionuclide Metrology Using Liquid Scintillation Counting.
    In: Metrologia 44 (2007) pp. 36-52 (Special issue on radionuclide metrology).
  2. Kossert, K.:
    Primary activity standardization by means of Čerenkov counting.
    In preparation.
  3. Nähle, O., Kossert, K., Cassette, P.:
    Activity standardization of 3H with the new TDCR system at PTB.
    17th International Conference on Radionuclide Metrology and its Applications (ICRM), Bratislava, Slovakia, September 2009, submitted.