Nuclear mass and charge distributions in the cold region of the spontaneous fission of 252Cf

Abstract

The spontaneous fission of 252Cf was investigated experimentally in the cold-fission region. The fission-fragment mass- and nuclear-charge distributions were determined in total kinetic energy bins of 2 MeV width parallel to the average Qmax value as a function of mass up to (Qmax-TKE) equal to 15 MeV. In the evaluation bin with an average total kinetic energy of 11 MeV below the Qmax value, 220 nuclides were identified, 59 of them are extremely neutron-rich and lie outside the Karlsruhe Chart of Nuclides. The relative individual nuclide yields can be easily obtained from table 2 as a function of (Qmax-TKE). The fragment mass, nuclear charge and neutron number odd-even effects integrated over all mass splits of each (Qmax-TKE) bin were evaluated. There is no odd-even effect in the mass yield δΛ, except when (Qmax-TKE) becomes less than or equal to 3 MeV. The proton and the neutron odd-even effects δZ and δN integrated over all masses of one evaluation bin decrease linearly with (Qmax-TKE, however with different slopes. At (Qmax- TKE) = 9 MeV the neutron odd-even effect is about 8 times smaller than δZ. The proton odd-even effect as a function of fragment mass δZ(A) at constant (Qmax−TKE) shows an undulatory behaviour with about five fragment masses. This structure is still strong at the highest measured (Qmax − TKE) of 15 MeV, where δZ, the average, is largely reduced. The variance of the isobaric nuclear-charge distributions σz2(A) was found to be anti-correlated with δz(A). The structures in σz2(A) decrease with increasing (Qmax − TKE).The neutron odd-even effect δN(A) shows vehement changes with fragment mass from positive to negative values and from small to large amplitudes. It is concluded from the present results that δZ and δN cannot be interpreted as indicators of the intrinsic excitation energy at scission, that the upper limit for the intrinsic excitation energy in fission of 252Cf for all TKE values is 20 MeV, and that the structure of 5–6 mass units observed in many fission parameters of even-even fissioning systems stems from the shape of the surface of Q-values as a function of mass and nuclear charge.