Yields and Half-Lives in the Mass-99 Fission-Product Chain

Half-lives, fission yields, and genetic relationships of Zr, Nb, and Mo members of the mass-99 fissionproduct chain were studied by radiochemical methods. Evidence was obtained for a ${\mathrm{Nb}}^{99}$ isomer of half-life 10±2 sec, which decays directly to ${\mathrm{Mo}}^{99}$ in at least 52% of its disintegrations. Attempts to observe gamma radiation from this isomer in rapidly separated niobium fission products were not successful. A half-life of 2.3±0.3 min was obtained for a known isomer of ${\mathrm{Nb}}^{99}$, in good agreement with reported values. The fraction of the mass-99 fission product chain which passes through the longer-lived isomer was found to be ≤ 0.36. The cumulative fractional chain yield of both isomers was determined to be 0.94±0.11, consistent with the predicted value of ∼0.99. Although this result is not sufficiently precise to be a stringent test of the proposed preference in low-energy uranium fission for a 50-42 proton split, it does indicate that the maximum effect would be sufficient to distort the "normal" charge-distribution curve by an amount equivalent to approximately one charge unit. No evidence was found for 35-sec ${\mathrm{Zr}}^{99}$, previously reported to be a fission product. If it occurs in fission, its cumulative fractional chain yield is ≤ 0.03, in contrast to the expected value of ∼ 0.80. An upper limit of 1.6 sec was estimated for the half-life of ${\mathrm{Zr}}^{99}$.