S288 Labcast 2 replay

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Subject Gen

Half life of metastable Ba137 & advice on TMA01 (recorded on Thursday 27 March 2014)

This OU Labcast is restricted to students taking one of the S288 Practical Science modules.


The Labcast (23 minutes)

An interactive S288 Labcast (recorded on Thursday 27 March 2014featuring "An experiment on the half-life of metastable  Ba137", presented by Nick Braithwaite can be seen by following this link to  'Labcast 2'. Note that the interactive features of the live Labcast are not available on the recording. During the Labcast Nick also talks about writing non technical reports like that in the second part of the S288 TMAs.

Radioactivity on the beach

The Labcast demonstrates the high radioactivity of a yellowish Cornish pebble with streaks of black pitchblende clearly visible. You can "handle" this same rock from the safe distance provided by the internet and a virtual microscope - just follow this link to the 'Carbis Bay pebble' and click on 'VIEW OBJECT' above and to the right of the pebble on that page.

The caesium/barium isotope generator

The Labcast experiment uses an isotope generator  which contains a sample of caesium-137. This is a radioactive metal which decays to barium-137 via β -decay. 95% of the caesium atoms decay to an excited state of barium-137 and the excited barium atoms then decay to their ground state by emitting γ-ray photons of energy  The remaining 5% of the caesium atoms decay directly to the ground state of barium-137 without emitting a γ-ray. The ground state of barium is stable (non-radioactive), so these are the only radioactive emissions occurring in the device. The electrons produced in this process are absorbed within the casing of the isotope generator, and only the γ-rays escape. These γ-ray photons can be detected by a scintillation counter or a Geiger counter located close to the generator. The radioactive scheme is illustrated in the 'Radioactivity schematic' linked below.

To measure the half-life of the excited state of barium-137 it is necessary to separate the barium nuclei from their parent caesium nuclei in the Cs/Ba isotope generator. The generator works on a principle called ‘ion exchange absorption’, whereby the caesium atoms remain bound in an insoluble compound, and the barium atoms are not. Therefore a small amount of water flushed through the generator removes some of the radioactive barium, leaving all the caesium atoms behind.

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