2.6.6 - Strontium Isotopes (Sr-89/Sr-90 and Sr-90/Y-90) by TDCR Cherenkov Counting
(according to [Frenzel et al 2013], [L’Annunciata 2012] and HIDEX Application Note DOC 413-010)
Introduction
The recent progress in the TDCR technique facilitates a rapid method for the determination of Strontium isotopes by TDCR Cerenkov counting and subsequent LS counting [Olfert et al. 2014], [Frenzel et al. 2013], [Tayeb et al. 2014] (fig. 30). This method has recently been extended to the bioassay for 90Sr and is routinely used by IAEA for urine samples [Capote-Cuellar et al. 2015]. The main advantage is not requiring any calibration. The counting efficiency can be calculated from the TDCR values, obtained after the measurement by applying a simple linear equation. This is attributed to the fact that β-particles with energies below the Cerenkov threshold cannot be detected. Without correction, the activity of the sample would be under-estimated.
Another advantage of this rapid method is that there are no significant waiting times after sample preparation. The prompt availability of results allows the competent authorities and radiation protection experts a faster reaction after a nuclear incident.
This method can be applied for the determination 90Sr/90Y in routine analysis and 89Sr/90Sr analysis in case of a nuclear accident. The ratios of 89Sr/90Sr that occur after a nuclear incident can be determined for ratios of up to 170:1.
The counting efficiencies obtained with Hidex 300 SL for the TDCR-LSC measurement of 90Sr were above 97 %. The counting efficiencies for 89Sr and 90Y by TDCR-Cerenkov-Counting were higher than 60 %.
In general, the sample preparation is equivalent to the classical Cerenkov method (see 2.3.5). It is highly recommended to use Sr SPEC columns for the radiochemical separation of 90Sr to get reliable and reproducible results. The chemical yield should be determined with stable Sr to avoid interferences from 85Sr, if the user has access to an AAS or ICP-MS system.
Materials and Equipment
TDCR Liquid Scintillation Counter (Hidex 300, 600SL)
High-performance LS cocktail (e.g. AquaLight+)
Plastic LSC vials
Procedure for 90Sr / 90Y
The measurement described here can be carried out directly after the radiochemical separation without significant waiting times. Since the ingrowth of 90Y is determined by the first measurement, the second measurement has to follow directly. However, the measurements can also be carried out at any later time, e.g. two weeks after sample preparation. If the activities of 90Sr and 90Y are already in equilibrium, only one measurement is required (preferably 90Y via TDCR Cerenkov counting).
After Sr extraction from sample, measure 8 mL of the colorless liquid in a plastic vial, using the TDCR Cerenkov mode (energy window 1-25 keV)
Add 12 mL of a high-performance LS cocktail (e.g. AquaLight+); mix well and remeasure in an open energy window (1-2000 keV)
Subtract the result of the TDCR Cerenkov measurement from the result of the second measurement (with LS cocktail) to obtain the activity of 90Sr
Procedure for 89Sr/90Sr
The measurements have to be carried out directly after the radiochemical separation to avoid additional ingrowth of 90Y, which negatively affects the TDCR-Cerenkov and LSC measurements. In principle, the ingrowth of 90Y is negligible in this rapid method, if the measurements are performed directly after the sample preparation.
After Sr extraction from sample, measure 8 mL of the colorless liquid in a plastic vial, using the TDCR Cerenkov mode (energy window 1-25 keV)
Add 12 mL of a high-performance LS cocktail (e.g. AquaLight+); mix well and remeasure in an open energy window (1-2000 keV)
Subtract the result of the TDCR Cerenkov measurement from the result of the second measurement (with LS cocktail) to obtain the activity of 90Sr
Evaluation
The evaluation resembles to 2.3.5., however it is simplified through the direct use of the corrected TDCR value as efficiency parameter.
Evaluation for 90Sr/90Y
The efficiency for 90Y in H2O can be calculated according to this equation:
Evaluation for 89Sr/90Sr:
The efficiency for 89Sr in H2O can be calculated according to this equation:
Lower Limit of Detection LLD: about 25 mBq per sample (10 h counting time, R0 =50 cpm for standard model instrument), but depends on Sr extraction steps
L’Annunziata M.F. 2012: “Handbook of Radioactivity Analysis”, Chapter 15, 3rd Edition 2012, Elsevier
Capote-Cuellar A., Alonso Vazquez R. and Cruz Suarez R. 2015: Measurement of Tritium and Strontium in urine samples with TDCR LSC and Cerenkov method; IM 2015
Frenzel E., Kossert K., Oikari T., Otto R. and Wisser S. 2013: Neue Schnellmethode in der LSC-Messtechnik – Die Messung von 89Sr/90Sr und 90Sr/90Y mittels TDCR-Cerenkov-Zählung; Strahlenschutzpraxis 1/2013, pp. 26-32
Olfert S.M., Dai X. and Kramer-Tremblay S. 2014: Rapid determination of 90Sr/90Y in water samples by liquid scintillation and Cherenkov counting; J. Radioanal. Nucl. Chem. 300 (2014) 263-267
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
