The nuclide ${}_{}{}^{260}{}_{}{}^{}105$ has been produced in the ${}_{}{}^{249}{}_{}{}^{}\mathrm{Cf}({}_{}{}^{15}{}_{}{}^{}\mathrm{N},4n)$ reaction and its decay properties investigated. The $\alpha$-decay branch of 1.52 ± 0.13-sec ${}_{}{}^{260}{}_{}{}^{}105$ has been measured to be (90.4±0.6)% with spontaneous-fission decay the remainder [(9.6±0.6)%]. Measured $\alpha$-particle energies and intensities (% per decay) are 9.041±0.014 MeV (48±5), 9.074±0.014 MeV (25±3), and 9.120±0.017 MeV (17±3). Characteristic $L$ series x rays of element 103 were observed in coincidence with these $\alpha$-particle groups, thus providing elemental identification. The ${}_{}{}^{256}{}_{}{}^{}\mathrm{Lr}$ $\alpha$-decay daughter was observed in a direct time-correlated genetic linkage, thus providing the mass number assignment. The absence of characteristic $L$- or $K$-series x rays of element 104 in time coincidence with, but preceding, spontaneous-fission-decay events, has allowed an upper limit of 2.5% to be placed on the electron-capture decay branch of ${}_{}{}^{260}{}_{}{}^{}105$ to the elusive nuclide 0.1-sec ${}_{}{}^{260}{}_{}{}^{}104$. If ${}_{}{}^{260}{}_{}{}^{}104$ is a spontaneous-fission emitter with a half-life ≤ 100 μsec, as some predictions would indicate, then the electron-capture decay branch of ${}_{}{}^{260}{}_{}{}^{}105$ is μ0.2%.

RADIOACTIVITY ${}_{}{}^{260}{}_{}{}^{}105$ [from ${}_{}{}^{249}{}_{}{}^{}\mathrm{Cf}({}_{}{}^{15}{}_{}{}^{}\mathrm{N},4n)$ reaction]; measured $t_{\frac{1}{2}}$, $E_{\alpha }$, $I_{\alpha }$, $E_{\mathrm{SF}}$, $E_{\mathrm{SF}}$, $I_{\mathrm{EC}}$, $E_X$, $L_X$, $E_{\gamma }$, $I_{\gamma }$; $\alpha -X$ coin. and $\mathrm{SF}-X$ coin. $\alpha -\alpha$ time correlation; deduced ${}_{}{}^{256}{}_{}{}^{}\mathrm{Lr}$ levels, $J$, $K$, $\pi \alpha$ hindrance, SF hindrance, atomic number mass number. Enriched target.

• Received 9 May 1977

DOI:https://doi.org/10.1103/PhysRevC.16.1146