Even-odd effects in prompt emission of spontaneously fissioning even-even Pu isotopes

The available experimental Y (A, TKE) data for Pu-236,Pu-238,Pu-240,Pu-242,Pu-244(SF) together with the Zp model prescription with appropriate parameters allows the investigation of even-odd effects in fragment distributions. The size of the global even-odd effect in Y(Z) is decreasing from Pu-244(SF) to Pu-236(SF) confirming the general observation of a decrease of the even-odd effect with the fissility parameter. Charge polarizations (Delta Z) and root-mean squares (rms) as a function of A of Pu236-244(SF) were obtained for the first time. In the asymmetric fission region both Delta Z (A) and rms(A) exhibit oscillations with a periodicity of about 5 mass units due to the even-odd effects. The total average charge deviations <Delta Z > (obtained by averaging Delta Z(A) over the experimental Y(A) distribution) are of about vertical bar 0.5 vertical bar for all studied Pu(SF) systems. The comparison of the calculated Delta Z(A) and rms(A) of Pu-240(SF) with those of Pu-239(n(th), f) reported by Wahl shows an in-phase oscillation with a higher amplitude in the case of Pu-240(SF), confirming the higher even-odd effect in the case of SF. As in the previously studied cases (U-233,U-235(n(th), f), Pu-239(n(th), f), Cf-252(SF)) the even-odd effects in the prompt emission of Pu236-244(SF) are mainly due to the Z even-odd effects in fragment distributions and charge polarizations and the N even-odd effects in the average neutron separation energies from fragments < Sn >. The size of the global N even-odd effect in < Sn > is decreasing with the fissility parameter, being higher for the Pu(SF) systems compared to the previously studied systems. The prompt neutron multiplicities as a function of Z, v(Z), exhibit sawtooth shapes with a visible staggering for asymmetric fragmentations. The size of the global Z even-odd effect in v(Z) exhibits a decreasing trend with increasing fissility. The average prompt neutron multiplicities as a function of TKE show an increase of the even-odd effect with increasing TKE, with global effect sizes close to each other (a decrease of the effect for heavier fissioning nuclei is not observed here). The amounts of the global even-odd effect in Y (Z) and of the N even-odd effect in < Sn > of Pu-240(SF) are larger compared to Pu-239(n(th), f). This fact affects the prompt emission leading to a lower Z even-odd effect in the prompt neutron multiplicity of Pu-240(SF) compared to Pu-239(n(th), f). (C) 2014 Elsevier B.V. All rights reserved.