DECAY PROPERTIES OF HEAVY MENDELEVIUM ISOTOPES

We investigated the nuclear properties of several mendelevium isotopes produced in the reactions of heavy ions with Es-254(g). We measured t he alpha particles, spontaneous fissions (SF), and photons emitted by counting samples resulting from chemical and/or mass separations. The Md-256 half-life is (78.1 +/- 1.8) min; it decays primarily by electro n capture (EC), but also by alpha emission (11 +/- 3)% of the time. Th e Md-256 ground state has J less-than-or-equal-to 2 (probably J(pi) = 1-), and a mass ''cess of (87.611 +/- 0.053) MeV. The Md-257 half-life is (5.523 +/- 0.050) h; it decays primarily by EC, but also by alpha emission (15.2 +/- 2.6)% of the time and by SF less than 1% of the tim e. The Md-257 mass excess is (88.989 +/- 0.003) MeV. The Md-258(g) (j( pi) = 8-) half-life is (51.50 +/- 0.29) d. It decays by alpha emission ; the sum of SF, EC, and beta- decay branches is less than 3 x 10(-3) %. The alpha decay of Md-258(g) populates Es-254(m) (0.60 +/- 0.08)% o f the time. The Md-(258) mass excess is (91.691 +/- 0.007) MeV. The ha lf-life of J(pi) = 1- Md-258(m) is (57.0 +/- 0.9) min. It decays by EC ; the branch for decay by alpha emission is less than 1.2%. The sum of SF and beta- decay branches is less than 30%. The half-life of Md-259 is (1.60 +/- 0.06) h; it decays primarily by SF. The alpha decay bran ch of Md-259 is less than 1.3%. From the Md-259 half-life we calculate d a SF hindrance factor associated with the 7/2-[514] proton configura tion of 3.6 x 10(6). From our data we proposed partial level schemes f or Es-252, Es-253, and Es-254.