Mass Symmetry in the Spontaneous Fission ofFm257

Abstract

ter all, the great success of the Talmi 6 approach shows the insensitivity of such transformations to configuration mixing.(3) Finally, the great sensitivity of the Ml rates to the small l± \ admixtures should remind us, once again, to exercise caution in the use of wave functions generated in a truncated space and serves to illustrate again that the important terms in the wave function are not always the largest but rather need to be determined anew for each specific operator. tWork performed in part under the auspices of the One of the significant aspects of spontaneous fission (SF) of the heavy nuclei is that the fragment mass distribution is strongly asymmetric for A ^ 254. From measurements of fragment kinetic energies for a series of nuclides ranging from 248 Cm up to 254 Cf and 254 Fm, Brandt et al. 1 note that the average mass of the heavy fragment remains relatively constant at A = 142 ± 1, while that of the light fragment increases. If this feature were to continue, a true symmetric distribution would not be reached until A approached 284. On the other hand, there has been some speculation that fission-fragment shell effects should accelerate the trend toward symmetry with the approach toward 264 Fm (=2 xl llSn Q2 ).Our measurement of the fragment kinetic energies from spontaneous fission of 257 Fm shows definite indications of the latter trend.The energy measurements were made with a pair of silicon surface-barrier detectors placed on opposite sides of a thin film on which the source was deposited. The amplified pulses were fed to a 512X512-channel two-parameter pulseheight analyzer; pulse heights of coincident events were stored on magnetic tape for subsequent computer processing.Two different sources of 257 Fm were employed.U.S.The first was isolated from debris from the "Hutch" nuclear explosion, 2 in which the mass-257 chain was produced by rapid multiple neutron capture in a uranium-thorium target. The second was produced by neutron capture in curium targets in the high-flux isotope reactor (HFIR) at Oak Ridge. Final purification of both preparations was accomplished by the standard ethanol-HC1 cation column elution for actinide-lanthanide separation, and by a hot alpha-hydroxyisobutyrate cation column elution for final isolation of fermium from other actinides. Purity of the sample was confirmed by alpha-pulse analysis. Solutions of the fermium were transferred to 35-to 40-Mg/cm 2 Zapon films and "freeze-dried" under vacuum to uniform spots 4 to 6 mm in diameter. Identical procedures were used to prepare 2B4 Cf sources (also from Hutch debris) and the 252 Cf standard sources used for energy calibration.The first (Hutch) source contained initially 0.8 SF/min and was measured over 32 weeks. A number of detectors and source distances were tried, and a total of 15 710 events were recorded. The second (HFIR) source started with 2.7 SF/ min and was measured over 75 days (17 951 events) in a constant source-detector geometry.The kinetic energy distribution of coincident fragments from spontaneous fissio...

β-delayed fission of180Tl

“…The symmetric split of 180 Hg would be expected to lead to two semimagic spherical nuclei 90 Zr, for which a compact scission configuration with a high total kinetic energy should result. This situation would be similar to the so-called "high-TKE" symmetric fission mode observed in the spontaneous fission of, e.g., 257,258 Fm, 258 No, 259,260 Md [48,54]. These nuclei demonstrate the phenomenon of bimodal fission with two distinctive groups of events with high and low TKE values.…”

“…4(a) of Ref. [48] for 257 Fm. In contrast, in the low-TKE mode, both fission fragments are believed to be strongly deformed, which results in an elongated scission shape with a lower TKE value.…”

“…This procedure relies on the knowledge of the average number of neutrons emitted as a function of fragment mass,ν(M), which was measured for, e.g., 257 Fm in Ref. [48] and which showed a staggering behavior in the range ofν ∼ 1-4. As no such data exist for the very neutron-deficient nuclei in the lead region, we used a simplified estimate, as described below.…”

“…In the literature, to estimate the energy loss caused by the emitted neutrons E i,ν in the fission of transuranium nuclei, a procedure introduced by Balagna et al [48], is used, see, e.g., Fig. 8 in Ref.…”

Applications of a global nuclear structure model to studies of the heaviest elements

“…However, in the 1970's evidence started to accumulate that there were rapid changes in fission properties in the heavy-fermium region. The first observation of the onset of symmetric fission at the end of the periodic system was a study [22] of 257 Fm fission. For 258 Fm the changes are even more fig.…”