(source: https://www.rsc.org/periodic-table/element/69/thulium)

When irradiated in a nuclear reactor, thulium produces an isotope that emits x-rays. This isotope is used to make a lightweight, portable x-ray machine for medical use: Portable Thulium X-Ray Unit With Separable Universal Mounting Means (1959). 

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All patent information has been obtained from Espacenet (European Patent Office).