Session 3WDB: Recent Developments in Fission II

Measurement of spontaneous fission of Fm isotopes

Spontaneous fission (SF) of neutron-rich Fm and transfermium nuclei exhibits various fascinating properties of fission. One of the highlights is the sudden change of fission-fragment mass distribution (FFMD) between ²⁵⁶Fm and ²⁵⁸Fm; the SF of ²⁵⁶Fm shows typical asymmetric mass distribution, while that of ²⁵⁸Fm shows very sharp symmetric one with total-kinetic energy (TKE) much higher than those of the asymmetric fission [1]. This high-TKE symmetric fission is observed only in neutron-rich Fm and Md isotopes, and is interpreted as the fission with compact configuration forming two spherical Sn fragments. Moreover, the SF of neutron-rich No, Lr, and Rf isotopes show another type of fission with low-TKE symmetric mass distribution [1]. This low-TKE symmetric fission competes or coexists with the high-TKE symmetric fission in neutron-rich Fm region. Recent progress in theoretical calculations has enabled to calculate the competition between the asymmetric and the high-TKE symmetric fission as well as their mass and TKE distributions. However, the experimental data to be compared are very limited. In particular, more precise and detailed mass and TKE distribution data are indispensable to understand the fission mechanism in this neutron-rich Fm region.

In the present work, we have measured fission-fragment mass and TKE distributions for the SF of ²⁵⁶Fm, ²⁵⁸Fm, ²⁵⁹Md, and ²⁵⁹Lr. These nuclei were produced in the multinucleon transfer reactions with the ¹⁸O beam and the ²⁵⁴Es target, and in the fusion-evaporation reaction of ²⁴⁸Cm(¹⁵N,4n)²⁵⁹Lr at the JAEA tandem accelerator facility. Reaction products were mass-separated with the on-line isotope separator, and were implanted into thin carbon foil to measure the kinetic energy of both the fission fragments with 4 pairs of Si detectors. Based on the deduced mass and TKE distribution data, we have obtained new insights into the competition and the coexistence of the high-TKE and low-TKE symmetric fissions as well as the asymmetric one.

This work was carried out with many collaborators at JAEA, Ibaraki Univ., Tokushima Univ., RCNP, Osaka Uiv., Tokyo Institute of Technology, Niigata Univ., Univ. of York, Kanazawa Univ., Kyushu Univ., Nagoya Univ., RIKEN, QST, and ORNL.

[1] E.K. Hulet et al., Phys. Rev. C 40, 770 (1989); M.R. Lane et al., Phys. Rev. C 53, 2893 (1996).   

Advances in Fission Product Yield Evaluation and Modeling

Throughout the past several years, there have been significant advances in modeling and experimental measurements for fission product yields. Facilities have been able to measure fission products with increasingly shorter half lives that can be used to constrain calculations that can consistently model both prompt and delayed fission observables. Here, we describe our modeling framework for calculating independent and cumulative fission products along with prompt and delayed neutron and gamma-ray multiplicities and energies. We discuss this work in the context of the larger multi-institute fission product yield evaluation project that combines the consistent modeling with calculations for the underlying fission fragment initial conditions, new measurements, and corrections to historic data.   

Measurement of prompt γ-ray spectrum for 235U(nth,f)

Prompt fission γ-ray spectra (PFGS) are important as they allow us to study the structure and de-excitation process of neutron-rich fission fragments (FFs). They are also required as nuclear data to design new types of reactors, such as the Generation-IV reators. For spontaneous fission of ²⁵²Cf, the PFGS show broad structures at around γ-ray energy of 15 MeV, which was explained by incorporating the giant dipole resonance γ-ray emission. For neutron-induced fissions (n_th,f), however, no data are available in the energy range higher than 7 MeV. With a goal to measure the PFGS for ²³⁵U(n_th,f) up to energies of 20 MeV, we have developed a new measurement system. The system consists of two position-sensitive multi-wired proportional counters (MWPCs) to detect both FFs in coincidence, and two large volume LaBr₃(Ce) scintillators to measure the γ-rays in fission. The measurement has been carried out at the PF1B cold-neutron facility of the Institut Laue Langevin (ILL), Grenoble, France. Throughout the 437-hour run the total number of recorded γ rays in coincidence with FFs was reached to 1.7x10⁹. The obtained PFGS reached about 20 MeV, high enough to reveal the structure associated with the GDR of the FFs, for the first time in (n_th,f). The resulting PFGS do not follow the linearly decreasing trend as a function of γ-ray energy in logarithmic scale, but reveal the bump structures at about 4 MeV, 6 MeV, and 15 MeV. In this contribution, we will discuss the origins of the bump structures in comparison with the statistical Hauser-Freshbach model calculation.