Division of Nuclear Charge Deduced from X-Ray Measurements in the Spontaneous Fission ofCf252

The variation of nuclear charge with mass in the spontaneous fission of ${}_{}{}^{252}{}_{}{}^{}\mathrm{Cf}$ has been investigated by simultaneous measurement of the masses and characteristic $K$ x-ray energies associated with the fission fragments. The $K$ x rays were detected by a thin NaI(Tl) scintillator or by an argon-filled proportional counter in coincidence with a pair of solid-state detectors for the complementary fission fragments. The yield and energy of $K$ x rays emitted in the first centimeter (∼1 nsec) of fragment flight were determined as a function of fragment mass. The yield of $K$ x rays per fragment is a pronounced function of mass, rising from 0.04 in the region of mass (after neutron emission) below 98 ($N<\mathrm{}59\mathrm{}$) to a maximum of 0.13 near the center of the light group, falling to 0.05 in the mass region 131-135 ($Z\approx 50$, $N\approx 82$), rising to 0.15 near the center of the heavy group, and then increasing rapidly beyond mass 145 (region of deformed nuclei) to a peak of 1.0 around mass 154. The observed correlation of $K$ x-ray energies with fragment mass leads to a most probable nuclear-charge ($Z_P$) function in better agreement with the empirical rule of equal charge displacement (ECD) than with other postulates for charge division in nuclear fission.