Thulium-170 has a binding energy of 8105.5144(43) keV per nucleon and a half-life of 128.6±0.3 d. 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.[1] About 18.1% of β− decays populate a narrow excited state of 170Yb at 84.25474(8) keV (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.[2][3]
In 1953, the Atomic Energy Research Establishment introduced thulium-170 as a candidate for radiography in medical and steelmaking contexts,[7] but this was deemed unsuitable due to the predominant high-energy bremsstrahlung radiation, poor results on thin specimens, and long exposure times.[8] 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.[3][9]
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.[10][11] 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.[6]
^Alderman, Carol J. (1993). "Thulium heat sources for space power application". AIP Conference Proceedings. Vol. 271. pp. 1085–1091. doi:10.1063/1.43194.