Millisecond Gamma Emitter from Protons on Tantalum

A search for gamma emitters in the millisecond range of half-life was conducted, using the 32-Mev Berkeley proton linear accelerator. Foil targets of eighteen common elements were bombarded, and the activity made in them was followed between beam pulses with a gated scintillation counter. The only activity found of measurable yield was from Ta, and it exhibited a half-life of 5.5±0.3 milliseconds. Using a gated nine-channel pulse-height analyzer with the NaI(Tl) crystal counter, the excitation curve of this activity was measured and its gamma-ray spectrum deduced. Gamma rays of 0.35 Mev and 0.22 Mev are indicated by the pulse spectrum, and no $\beta \text{'}\mathrm{s}$ are present commensurate with this half-life. Comparison of the excitation curve for this activity with those of ${\mathrm{Ta}}^{180}$ (8-hour) and ${\mathrm{W}}^{179}$ (30-min) leads to the tentative assignment of this decay to ${\mathrm{W}}^{180m}$.