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).