Measurement of 90Sr in soil samples affected by the Fukushima Daiichi Nuclear Power Plant accident

Accurate and precise determination of 90Sr at femtogram level in IAEA proficiency test using Thermal Ionization Mass Spectrometry

A novel method for the determination of ultra-trace level ⁹⁰Sr has been recently developed applying thermal ionization mass spectrometry (TIMS). The method includes the chemical separation of Zr (isobaric interference of ⁹⁰Zr) from the samples followed by determination of ⁹⁰Sr/⁸⁸Sr abundance  sensitivity (2.1 × 10⁻¹⁰). The analytical performance of this method was assessed in the IAEA-TEL 2017-3 worldwide open proficiency test. For ⁹⁰Sr determination, tap water and milk powder samples were distributed amongst the participant laboratories with reference values of 11.2 ± 0.3 Bq kg⁻¹ (2.2 ± 0.1 fg g⁻¹) and 99.9 ± 5.0 Bq kg⁻¹ (19.5 ± 1.0 fg g⁻¹), respectively. The stable Sr concentrations were 39.4 ± 0.9 ng g⁻¹ and 2.5 ± 0.1 µg g⁻¹ while the ⁹⁰Sr/⁸⁸Sr isotope ratios were 6.47 ± 0.17 × 10⁻⁸ and 9.04 ± 0.45 × 10⁻⁹ in the tap water and milk powder samples, respectively. For TIMS measurement, 50 mL water and 1 g milk powder samples were taken for analysis. This TIMS method demonstrated an impressive accuracy (relative bias of 4.2% and −2.1%, respectively) and precision (relative combined uncertainty of 4.1% and 7.6%, respectively) when compared with radiometric techniques. For the first time in the history of inorganic mass-spectrometry, ⁹⁰Sr analysis using a TIMS instrument is confirmed by an independent proficiency test.

Method for 90Sr Analysis in Environmental Samples Using Thermal Ionization Mass Spectrometry with Daly Ion-Counting System

In this work, a new ⁹⁰Sr analysis method was developed using the Isotopx Ltd., Phoenix X62 thermal ionization mass spectrometer (TIMS). Excellent ion beam sensitivity was demonstrated with the detection of 1 mBq (0.2 fg) ⁹⁰Sr on a Daly ion-counting system. The abundance sensitivity for the ⁹⁰Sr/⁸⁸Sr ratio was 2.1 × 10^-10, and this could ensure measurement of 100 Bq·kg^-1 (19 fg·g^-1) ⁹⁰Sr in an environmental sample with 100 μg·g^-1 stable strontium concentration. For analytical method validation, ⁹⁰Sr was determined in two certified reference materials, for example, wild berry (IRMM-426) and freshwater lake sediment (NIST-4354), for the first time in the history of TIMS. This mass spectrometry method is faster than conventional radiometric techniques; however, interference from ⁹⁰Zr and peak tailing on the higher mass side from ⁸⁸Sr must be considered for a reliable ⁹⁰Sr determination.

Atmospheric Activity Concentration of 90Sr and 137Cs after the Fukushima Daiichi Nuclear Accident

On March 11, 2011, the Fukushima Daiichi Nuclear Power Plant (FDNPP) accident occurred and large amounts of radionuclides were discharged into the atmosphere. We have operated continuous aerosol samplings at four locations in Japan from the accident until the end of 2011. The activities of ⁹⁰Sr and ¹³⁷Cs in the aerosol samples were measured using low background liquid scintillation counters and high-purity germanium detectors, respectively. The atmospheric ⁹⁰Sr and ¹³⁷Cs concentrations decreased exponentially during 2011. The time variation of the ⁹⁰Sr/¹³⁷Cs ratio was obtained, and we found that the ratio rose from 1.2 × 10^-3 in March to 1.3 × 10^-1 in August 2011. One reason for the increase in the ⁹⁰Sr/¹³⁷Cs ratio could be the change in the primary emission source of activity at the FDNPP, which occurred near June 2011.

Strontium-90 activity concentration in soil samples from the exclusion zone of the Fukushima daiichi nuclear power plant

The radioactive fission product ⁹⁰Sr has a long biological half-life (˜18 y) in the human body. Due to its chemical similarity to calcium it accumulates in bones and irradiates the bone marrow, causing its high radio-toxicity. Assessing ⁹⁰Sr is therefore extremely important in case of a nuclear disaster. In this work 16 soil samples were collected from the exclusion zone (<30 km) of the earthquake-damaged Fukushima Daiichi nuclear power plant, to measure ⁹⁰Sr activity concentration using liquid scintillation counting. ¹³⁷Cs activity concentration was also measured with gamma-spectroscopy in order to investigate correlation with ⁹⁰Sr. The ⁹⁰Sr activity concentrations ranged from 3.0 ± 0.3 to 23.3 ± 1.5 Bq kg⁻¹ while the ¹³⁷Cs from 0.7 ± 0.1 to 110.8 ± 0.3 kBq kg⁻¹. The fact that radioactive contamination originated from the Fukushima nuclear accident was obvious due to the presence of ¹³⁴Cs. However, ⁹⁰Sr contamination was not confirmed in all samples although detectable amounts of ⁹⁰Sr can be expected in Japanese soils, as a background, stemming from global fallout due to the atmospheric nuclear weapon tests. Correlation analysis between ⁹⁰Sr and ¹³⁷Cs activity concentrations provides a potentially powerful tool to discriminate background ⁹⁰Sr level from its Fukushima contribution.