2.6.6 - Strontium Isotopes (⁹⁰Sr/⁸⁹Sr and ⁹⁰Sr/⁹⁰Y) 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).

1. 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)

2. Add 12 mL of a high-performance LS cocktail (e.g. AquaLight+); mix well and remeasure in an open energy window (1-2000 keV)

3. 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.

1. 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)

2. Add 12 mL of a high-performance LS cocktail (e.g. AquaLight+); mix well and remeasure in an open energy window (1-2000 keV)

3. 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, 3^rd 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