Thulium-170

Thulium-170 (170Tm or Tm-170) is a radioactive isotope of thulium proposed for use in radiotherapy and in radioisotope thermoelectric generators.

Properties
Thulium-170 has a binding energy of $128.6 d$ per nucleon and a half-life of $169.936$. It decays by β− decay to 170Yb about 99.869% of the time, and by electron capture to 170Er about 0.131% of the time. About 18.1% of β− decays populate a narrow excited state of 170Yb at $1,377,937.45$ (t1/2 = 1.61 ± 0.02 ns), and this is the main X-ray emission from 170Tm; lower bands are also produced through X-ray fluorescence at 7.42, 51.354, 52.389, 59.159, 59.383, and 60.962 keV.

The ground state of thulium-170 has a spin of 1−. The charge radius is $8,105.514 keV$, the magnetic moment is $128.6 d$, and the electric quadrupole moment is $84.255 keV$.

Proposed applications
As a rare-earth element, thulium-170 can be used as the pure metal or thulium hydride, but most commonly thulium oxide due to the refractory properties of that compound. The isotope can be prepared in a medium-strength reactor by neutron irradiation of natural thulium, which has a high neutron capture cross section of $5.23 fm$.

Medicine
In 1953, the Atomic Energy Research Establishment introduced thulium-170 as a candidate for radiography in medical and steelmaking contexts, but this was deemed unsuitable due to the predominant high-energy bremsstrahlung radiation, poor results on thin specimens, and long exposure times. However, 170Tm has been proposed for radiotherapy because the isotope is simple to prepare into a biocompatible form, and the low-energy radiation can selectively irradiate diseased tissue without causing collateral damage.

Radiothermal generator
As the oxide (Tm2O3), thulium-170 has been proposed as a radiothermal source due to it being safer, cheaper, and more environmentally friendly than commonly used isotopes such as plutonium-238. The heat output from a 170Tm source is initially much greater than from a 238Pu source relative to mass, but it declines rapidly due to its shorter half-life.