2.6.3 - Sr-90 and Pb-210 by Plastic Scintillator Using Microspheres

Introduction

Plastic Scintillators (PS) have been known since the development of the scintillation techniques. However, the capability of PS microspheres (PSm) for its application to routine determination of radionuclides has only been evaluated during the last two decades with satisfying results.

The scintillation mechanism in PSm is similar to that of classical LS, however, the energy resolution is less and the detection efficiency for weak β-emitters is lower due to particle and optical quenching.

PSm are solid solutions composed of one or two fluorescent solutes embedded in a polymeric aromatic solvent, typically polyvinyl toluene. For routine measurements PSm vials are prepared by a procedure similar to LS, taking into account a final homogenization step. After the measurement, the PSm and the radioactive aqueous sample can be segregated by simple filtration avoiding the production of mixed waste and recovering the PSm for reutilisation [Tarancón et al. 2002].

Although PSm are not yet available on the market, different synthesis methodologies have been published, see e.g. [Santiago et al. 2016] and can be provided by the team of the University of Barcelona.

PSresin materals are plastic scintillating microspheres with their surface coated with a selective extractant. Separation of the radionuclide of interest from interferences is performed in the column, which is afterwards directly measured in a scintillation counter without need of elution. The use of PSresins packaged in a 2 mL cartridge thus permits to join the advantages of solid phase extraction and scintillation counting in a single support (fig. 27).

Two procedures for radionuclide analysis in aqueous samples, as developed by the University of Barcelona group, have been chosen as example for the use of PSm (Sr/Pb and Tc, see chapter 2.3.14.).

In the case of the PSresin for the analysis of ⁹⁰Sr or ²¹⁰Pb, the plastic scintillating microspheres are coated with a solution of 4,4'(5')-di-t-butylcyclohexano 18-crown-6 in 1-octanol. This crown-ether retains Strontium and Lead at high nitric acid concentrations (6 M to 8 M) [Bagan 2011], [Lluch 2016]. 1 g of PSresin is packaged in a 2 mL SPE cartridge. Separation of the radionuclide of interest (⁹⁰Sr or ²¹⁰Pb) from interferences or daughter radionuclides formed is performed following the common procedures used in solid extraction chromatography. At the end, the cartridge with the PSresin and the ⁹⁰Sr or ²¹⁰Pb retained, are directly measured in a scintillation counter without need of elution or additional sample preparation.

Retention of Strontium and Lead in the PSresin depends on the amount of stable isotope in the sample. For 1 mg of Sr²⁺ or Pb²⁺ retention achieves is 90 % and for 5 mg it is around 60 %. The counting efficiency for ⁹⁰Sr is close to 90 %, whereas for ²¹⁰Pb is around 45 % due to the low energy β-particles emitted by ²¹⁰Pb.

Regarding the separation and counting conditions, two relevant issues should be taking into consideration:

• Sr/Pb PSresin cartridges must be measured immediately after separation to avoid ingrowth of daughters (⁹⁰Y and ²¹⁰Bi).

• LiNO₃ must be used in the last rinse step to avoid chemiluminiscence of the PSresin caused by HNO₃.

Although the method herein described is for water samples, the Sr/Pb PSresin has been used also for the fast analysis of Radiostrontium (⁸⁹Sr + ⁹⁰Sr) in milk samples [Sáez-Muňoz 2017].

Materials and Equipment

• 2 mL Sr/Pb PSresin cartridges (University of Barcelona)

• Sr²⁺ or Pb²⁺ carrier solution (water samples: 1 mg of Sr²⁺ or Pb²⁺ per sample)

• ⁹⁰Sr or ²¹⁰Pb standard solution

• 6 M HNO₃

• 6 M LiNO₃

Either separation can be performed in a vacuum box or using a peristaltic pump.

Procedure for ⁹⁰Sr

1. Sample 10 mL of water sample

2. Add concentrated HNO₃ to achieve a 6 M HNO₃ concentration

3. Spike the solution with 1 mg of a Strontium carrier solution

4. Add 2 mL of 6 M HNO₃ into the Sr/Pb PSresin cartridge for conditioning and allow draining (fig. 27)

5. Add the sample into the Sr/Pb PSresin cartridge

6. Rinse the Sr/Pb PSresin cartridge two times with 2 mL 6 M HNO₃

7. Rinse the Sr/Pb PSresin cartridge two times with 2 mL 6 M LiNO₃

8. Let the cartridges to drain for 5 minutes

9. Put the Sr/Pb PSresin cartridges in a 20 mL polyethylene vial and immediately start scintillation counting for 1 hour

10. Take an aliquot (e.g. 1 mL) of the wastes collected from the Sr/Pb PSresin cartridge to determine the chemical yield though ICP-OES

Evaluation

The ⁹⁰Sr or ²¹⁰Pb activity A is calculated from the net counting rate R_N, the counting efficiency ε, the chemical yield η and the sample volume V.

The counting efficiency ε can be obtained by running the same procedure with a sample of 10 mL with a known amount of ⁹⁰Sr or ²¹⁰Pb. Counting windows can be restricted to the area of the ⁹⁰Sr or ²¹⁰Pb signals (fig. 28).

(a) ^89/90Sr

(b) ²¹⁰Pb

Detection Limit (MDA): 10 mBq/L for ⁹⁰Sr and 40 mBq/L for ²¹⁰Pb

Bagan H., Tarancon A., Rauret G. and Garcia J.F. 2011: Radiostrontium separation and measurement in a single step using plastic scintillators plus selective extractants - Application to aqueous sample analysis; Anal. Chim. Acta 686 (2011) 50-56

Lluch E., Barrera J., Tarancón A., Bagán H. and García J.F. 2016: Analysis of ²¹⁰Pb in water samples with plastic scintillation resins; Anal. Chim. Acta 940 (2016) 38–45.

Sáez-Muñoz M., Bagán H., Tarancón A., Ortiz J., García J.F. and Martorell S. 2017: Rapid method for Radiostrontium determination in milk in emergency situations using PSresin; Submitted to Anal. Chim. Acta.

Santiago L.M., Tarancon A. and Garcia J.F. 2016: Influence of preparation parameters on the synthesis of plastic scintillation microspheres and evaluation of sample preparation; Advanced Powder Technology 27 (2016) 1309-1317

Tarancon A., Alonso E., Garcia J.F. and Rauret G. 2002: Plastic scintillation microspheres for radioactivity; Anal. Chim. Acta 471 (2002) 135-143