Element 69 in our International Year of the Periodic Table series is thulium. A rare element, thulium is used in portable X-ray machines and also in some banknotes.

Thulium is, after promethium, the second-least abundant of the lanthanide elements. As a consequence, it’s more valuable than better known precious metals such as platinum and gold. It’s found in very small quantities in minerals with other lanthanide elements.

Despite its rarity, thulium does have some uses. One of thulium’s isotopes, thulium-170, can be used as a radioactive source in portable X-ray machines. These machines have a useful life of around a year. Thulium-170 has also been investigated as a potential treatment for some types of cancer.

Thulium, like europium, finds use in euro banknotes as an anticounterfeiting measure. Thulium compounds which fluoresce blue under UV light are used for this application.

Remember, you can keep track of all of the previous entries in this series on the site here, or on the Royal Society of Chemistry’s dedicated page.