The Radioactive Decay of Tungsten 181

Using tungsten enriched in mass 180, neutron capture produces a strong activity in ${\mathrm{W}}^{181}$ with a half-life of about 140 days. Using scintillation crystal and photographic magnetic spectrometers, two gamma rays of energy 136.5 and 152.5 kev are found to occur. Neither of these had been previously observed and conversely none of the previously reported gamma energies are found to exist. The main decay of ${\mathrm{W}}^{181}$ takes place by $K$ capture directly to the ground state of ${\mathrm{Ta}}^{181}$. Coincidence measurements are made with some evidence that the gamma rays are in sequence.