Spontaneous fission of rutherfordium isotopes

We have found several new spontaneous fission (SF) activities and suggested assignments for some of them to rutherfordium (element 104) isotopes. Their half-lives and production cross sections have been measured by collecting recoils on a moving nickel or steel tape and transporting them past stationary mica track detectors at known velocities. The following tentative assignments are based on several cross bombardments and comparisons between experimental and calculated production cross sections: Rf256(9±2 ms), Rf257(3.8±0.8 s, 14±9% SF), Rf258(13±3 ms), Rf259(3.4±1.7 s, 9±3% SF), Rf260(21±1 ms), and Rf262(47±5 ms). Presently we are unable to assign several other new spontaneous fission activities with half-lives of ∼15 ms, ∼22 ms, ∼100 ms (