Noble Gas Sample Re-Measurements at Labs – Xe-131m, Xe-133, Xe-133m, Xe-135

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Objective

Establishment of independent Noble Gas sample re-measurements at laboratories as one component of a comprehensive QA/QC program for IMS Noble Gas stations and support for the Noble Gas station certification process (with laboratory re-measurements).

High Activity Samples collected after the Fukushima Incident at JPX38

The main purpose of the radioxenon installations in the IMS is the detection of clandestine (underground) nuclear weapon tests. Therefore attention was given to develop high sensitivity instruments to detect the expected low activities. The radioactivity released during this event resulted in sample activities at the nearest station JPX38, Takasaki, which exceeded the dynamic range of the equipment.

Two samples from JPX38, which were collected shortly after the incident have been sent to BfS for analysis. At the laboratory the measurements were performed with small aliquots of the samples.

In the samples 3 radioxenon isotopes (Xe-133, Xe-133m and Xe-131m) were detected, however, only activity concentrations of Xe-133 and Xe-131 could be determined. The laboratory result indicate a Xe-133 concentration of 45 kBq/m3 in the highest activity sample, approximately 50 times higher than the station result.

The Xe-131m to Xe-133 ratio is 0.4%, which is in agreement which a power reactor in equilibrium.

Findings and Conclusions

Results show that laboratories are capable of providing an independent assessment of station results. Station improvements have already been triggered by laboratory re-measurements. The laboratory intercomparison showed agreement of results of most of the laboratories. It is expected to further improve the reliability of laboratory results by certification of laboratories for NG measurements, by further intercomparison exercises and by supply of Xenon standards for calibration.

For full scale operation of a QA/QC programme, the current number of available noble gas laboratories is insufficient.

Acknowledgements:
Mr. Marc Bean, CAL05, Radiation Protection Bureau, Health Canada
Mr. Derek Haas, USL16, Pacific Northwest National Laboratory
Mr. Thales Schröttner, ATL03, Seibersdorf Laboratories
Mr. Shilian Wang, CNL06, Beijing Radionuclide Laboratory

Background

Isotopes of interest: Xe-131m, Xe-133, Xe-133m, Xe-135

In general it is not expected to measure the same activities of the Xe isotopes and the same volume of stable Xe at the station and at the laboratories due to potential loss of gas or dead
volumes not accounted for by the NG systems. Therefore results of Xenon activity concentrations and isotope ratios from stations and laboratories are used for comparison since
they are independent of any gas losses. Isotope ratios cancel out differences in Xenon volumes and systematic uncertainties.

The measurement of Xenon activity concentrations in air is done in a two-step procedure. The sampled air volume has to be determined and the xenon activity in the sample is measured.
The measurement of the air volume of the sample is done by quantification of the stable Xenon volume in the counting cell. The concentration of stable Xenon in the atmosphere is constant at 87 ppb by volume, therefore the corresponding air volume of a sample is Vair = Vxe /0.087*10- 6, where Vxe is the stable Xe volume of the sample and Vair the corresponding air volume of a sample.

The activity concentration of a sample is:

The xenon volume measurement is done either with standalone gas chromatograph or with a Thermal Conductivity Detector.

Activity measurement at IMS Noble Gas stations is done either by high-resolution gammaspectrometry (SPALAX) or beta-gamma coincidence spectrometry (ARIX, SAUNA).

Status
So far 4 of the 16 IMS radionuclide laboratories have developed noble gas measurement capabilities and are actively participating in the PTS-organized pilot QA/QC program:

ATL03, Seibersdorf, Austria (low background high purity Germanium detector, high-resolution gamma-spectrometry)
CAL05, Ottawa, Canada (SAUNA Laboratory system, beta-gamma coincidence spectrometry)
CNL06, Beijing, China (SAUNA Laboratory system, beta-gamma coincidence spectrometry, as well as high purity Germanium detector, high-resolution gamma-spectrometry)
USL16, Richland, US (ARSA based laboratory system, beta-gamma coincidence spectrometry)

Additionally Bundesamt für Strahlenschutz (BfS), Freiburg, Germany, which has 40 years of experience in noble gas measurement participates in the program and has been supporting the
International Noble Gas Experiment since the beginning in 1999 (SAUNA Laboratory system, beta-gamma coincidence spectrometry; proportional counter).

Since 2007 257 Noble Gas samples from 26 IMS Noble Gas stations and one national station have been reanalyzed by 5 laboratories, including 38 Noble Gas samples in the aftermath of the Fukushima event. Two laboratory intercomparison exercises have been conducted.

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