2.6.5 - ⁹⁰Sr/⁸⁹Sr by Cherenkov Counting

Due to the limitation of the Cherenkov Effect to higher β-energies, small amounts of high energetic radionuclides may be determined in presence of much higher amounts of low energy β-emitters, whereas conventional measurement of dual β-emitters in the same sample requires window setting and three quench correction curves.

Cherenkov counting therefore allows the rapid determination of both radioistrontium nuclides ⁸⁹Sr and ⁹⁰Sr in two single counting steps without time wasting ⁹⁰Y ingrowth. While both nuclides (⁹⁰Sr: E_βmax = 0.5 MeV, ⁸⁹Sr: E_βmax = 1.5 MeV) are accessible to scintillation measurement with window setting (see dual labelling, procedure 2.1.2.), only ⁸⁹Sr creates Cherenkov radiation in water with counting efficiencies of 40 to 60 % without chemical quenching (fig. 10). For ⁹⁰Sr, the Cherenkov light part is < 0.5 %, which allows its sufficient discrimination from ⁸⁹Sr in an excess of up to 20 fold.

Alternate procedures applying Cherenkov counting foresee the selective separation of ⁹⁰Y and its successive Cherenkov measurement, or ⁹⁰Sr separation e.g. by Sr SPEC columns, its elution with diluted HNO₃ followed by repeated Cerenkov counting of the ingrowing ⁹⁰Y. For a detailed survey see [L’Annunziata 2012] and [Vajda 2010].

The method described below can be improved by applying TDCR-Cherenkov counting to avoid the efficiency calibration procedure, when a LS counter with three identical photomultipliers is available (HIDEX 300SL) (see chapter 2.3.5.).

Materials and Equipment

• Gelating cocktail

• Plastic vial

Procedure

The separation of Sr/Y is either done by oxalate precipitation or on Eichrom/ TrisKem SPEC extraction chromatographic columns (see 2.3.2.).

Measuring Procedure (a):

• Sample measurement in aqueous solution (only ⁸⁹Sr)

• Sample measurement with scintillator (⁸⁹Sr and ⁹⁰Sr)

1. Measure 5 mL of the colorless aqueous sample in a plastic vial (energy window 0-15 keV)

2. Add 10 mL of a gelating cocktail; mix well and remeasure in an open energy

Measurements have to be done immediately after chemical separation in order to neglect ⁹⁰Y ingrowth (1 % after 1 hour).

The Cherenkov efficiency for ⁹⁰Sr with E_max = 546 keV is less than 1 %.

Measuring Procedure (b):

• Sample measurement in aqueous solution (only ⁸⁹Sr)

• Sample measurement after storage and determination of the ⁹⁰Sr activity from the ingrowth of ⁹⁰Y

1. Measure the colorless aqueous sample (up to 20 mL) in a plastic vial (energy window 0 to 15 keV)

2. Repeat the measurement after different times of storage

Note: The Cherenkov counting window is typically 0-15 keV! Plastic vials are preferable compared to glass vials because of their higher Cerenkov counting efficiency.

Evaluation

Procedure (a):

Determine the Cherenkov efficiencies for ⁸⁹Sr and the scintillation counting efficiencies for both Sr-isotopes. While Cherenkov counting of colorless samples is absent of chemical quenching, scintillation counting needs internal standardization or other appropriate quench calibration methods according to chapter 1.2 and 2.1.1.

The activities are calculated according to:

Procedure (b):

The activity ingrowth of the ⁹⁰Y and accordingly the ⁹⁰Sr concentration follows the exponential ingrowth (fig. 29).

and in equilibrium condition (> 25 d)

where

t = time difference between measurement after separation and after equilibration and

A₀(⁸⁹Sr) = Activity after separation

For more details in calculation see [BMU 2000].

Yield recovery for the chemical Sr/Y separation steps is determined by ICP-MS or ICP-OES through addition of 30 mg of Sr²⁺ carrier to the sample. The addition of a ⁸⁵Sr tracer interferes with LS counting and should be avoided, especially when TDCR Cherenkov Counting is applied.

This calculation method can also be applied for the analysis of dual mixtures of α- and β-emitting radionuclides, when E_βmax exceeds 0.5 MeV and the α-nuclide does not emit γ-rays exceeding about 450 keV (e.g. ³²P / ²⁴¹Am).

Lower Limit of Detection LLD: about 25 mBq per sample, but lower with preconcentration

L’Annunziata M.F. 2012: “Handbook of Radioactivity Analysis”, Chapter 15, 3^rd Edition 2012, Elsevier

BMU 2000: Messanleitungen für die Überwachung der Radioaktivität in der Umwelt und zur Erfassung radioaktiver Emissionen aus kerntechnischen Anlagen; Der Minister für Umwelt, Naturschutz und Reaktorsicherheit, Urban und Fischer, München, H-α-Gesamt-AWASS-01

BMU 2000: Messanleitungen Umweltradioaktivität H-Rn-222-TWASS-01, 4.1

Kalibrierung; see also Sr-analysis and others

Vajda N. and Kim C-K. 2010: Determination of Radiostrontium Isotopes – A review of analytical methodology; Appl. Radiat. Isot. 68 (2010) 2306-2326