Effect of Chemical State on the Decay Constant ofU235m

Experimental investigation of the influence of chemical forms on the decay constant of ${\mathrm{U}}^{235m}$ has been performed. The ${\mathrm{U}}^{235m}$ atoms from a very thin ${\mathrm{Pu}}^{239}$ source were collected by the recoil method on platinum, carbon, and silicon plates in a pure argon atmosphere. Applying the differential method with two identical, specially designed 12-stage secondary electron multipliers, emission rates of the internal-conversion electrons of very weak energy from the isomer in different chemical states have been compared. Stability and sensitivity of the whole measuring system have been checked using ${\mathrm{C}}^{14}$ and metallic ${\mathrm{U}}^{235m}$ sources. We used metallic uranium (U) and uranium atoms slightly diffused into carbon (UC) or silicon (USi) base for the different chemical states. The results are: $\lambda \left(\mathrm{U}\right)-\lambda \left(\mathrm{UC}\right)=(3.18\mathrm{}\pm 0.50)\times {10}^{-3\mathrm{}}\lambda \left(\mathrm{U}\right)$, $\lambda \left(\mathrm{U}\right)-\lambda \left(\mathrm{USi}\right)=(2.21\mathrm{}\pm 0.36)\times {10}^{-3\mathrm{}}\lambda \left(\mathrm{U}\right)$, and $\lambda \left(\mathrm{USi}\right)-\lambda \left(\mathrm{UC}\right)=(0.97\mathrm{}\pm 0.43)\times {10}^{-3\mathrm{}}\lambda \left(\mathrm{USi}\right)$. Errors are standard deviations. We also determined the half-life of ${\mathrm{U}}^{235m}$ to be $T_{\frac{1}{2}}=26.05\mathrm{}\pm 0.05$ min by using a low-background $2\pi$ gas-flow counter. Furthermore, diffusion of the isomer into the base materials, carbon and silicon, was investigated in some detail.