Dr. David Chettle, Professor, Department of Medical Physics and Applied Radiation Sciences.

Dr. Chettle’s group is developing the radioisotope ¹⁷⁰Tm as a low energy photon source for X-ray fluorescence measurements of mercury in human subjects. The attraction of this radioisotope is its emission of a 84.3 keV gamma ray which corresponds closely with K shell electron absorption edge of mercury (83.1 keV): this means that the ¹⁷⁰Tm gamma ray has almost the ideal energy to excite mercury K x-rays. As the mercury returns to the ground state, it emits radiation (it fluoresces) that can be used to quantify the amount of mercury present in the subject. Production of ¹⁷⁰Tm can be carried in MNR very effectively: the reaction is thermal neutron absorption by ¹⁶⁹Tm (100% abundant) which has a high neutron capture cross section of 105 barnes. The half life of ¹⁷⁰Tm is 129 days, which is convenient for using it as an x-ray fluorescence source. It also means that a thulium foil could be irradiated on several days, if necessary, to build up sufficient activity. However, there are difficulties. ¹⁷⁰Tm decays primarily by beta^- emission with a maximum energy of 0.968 MeV. These energetic beta^- particles have to be screened out so that they neither contribute significant dose to a human subject nor increase the background in the measured photon spectrum through bremsstrahlung emission. Also, the ¹⁷⁰Tm source emits characteristic x-rays from both Tm and Yb. Unless filtered out, these will also contribute dose to the subject and create a high count rate in the hyperpure germanium detector, making the detection of trace quantities of mercury more difficult. Lastly, thulium is not commonly available in extremely high purity. The most readily available samples (99.9% pure) were found to contain nearly 0.1 (0.4) % tantalum, which itself was activated and has a half life of 114.5 days. So, the main development work at present is concentrating on identifying higher purity thulium and designing beta^- shielding and photon filtration systems. As well as x-ray fluorescence, for which it may not have been used previously, ¹⁷⁰Tm has potential as a brachytherapy source and it has been used for this purpose in a few centres in other countries. It is also being used in industrial radiography as a portable source to measure thickness of low density materials.

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