F. Caïtucoli

Fission fragment energy correlation measurements for the thermal neutron fission of 239Pu and 235U

Abstract Fission fragment mass and kinetic energy distributions and mass-versus-energy correlations have been measured for the thermal neutron induced fission of 239Pu and 235U. Unwanted events were eliminated by a coherence test based on time of flight. The global mass distribution for 239Pu shows much weaker fine structure than 235U. However, for events with high total kinetic energy this fine structure becomes more prominent and the yield of mass 135 dominates for 239Pu compared to mass 134 for 235U. Moreover, a prolate deformation for mass 102 in the 134 102 mass ratio for 235U is compatible with the data. The results show that EK(μH), hence EL and/or EH for 239Pu and 235U in the high mass yield regions have low energy tails produced in the act of fission. Furthermore, the data suggest that most of the proton pairs are broken after the effective determination of the structure of the nascent fragments and probably just before and/or in the act of scission itself.

Fission-fragment energy correlation measurements for 252Cf(sf) and structures in far-out asymmetric fission

Abstract Fragment-mass and kinetic-energy distribution measurements for 252 Cf(sf) have been extended to the far-out asymmetric low-yield fission region of M H = 190 compared with the existing data that go up to M H ≈ 168. The mass yield Y ( m ) shows a shoulder for M H ≈ 163 and an important enhancement for M H ≳ 176. The 〈 E K 〉( m ), σ E K ( m ) and, to a lesser extent, Y ( m ) distributions show strong and correlated amplitude fluctuations for M H ≳ 170. These data indicate strong and rapid changes in deformation of the nascent fragments and the coexistence of more than one deformed-shell stabilized scission configuration for the same mass splits. These structures and those present in the relatively high-yield regions are discussed in terms of the static macro-microscopic potential-energy calculations.

Fission-fragment energy correlation measurements for (nth, f) of 232U and 233U

Fission-fragment mass and kinetic energy distributions and their correlations have been measured for 232U and 233U. The results on these uranium isotopes and 235U are compared. The mass peak/valley ratio of 785 ± 68 for 232U is the highest of the three isotopes. The 〈EK〉(μH) distributions show significant differences. The dip ΔEK at symmetry is 16.2 ± 1.0 MeV (232U), 17.0 ± 1.0 MeV (233U) and 20.6 ± 1.1 MeV (235U). In the yields for high-kinetic-energy-selected events, the mass 134 dominates for 233U and 235U, but for 232U it is μH≈144, which dominates. This complete reversal of profiles can be understood in terms of fragment shells.

Experimental study of some important characteristics of the thermal neutron induced fission of 237Np

Abstract Fission fragment mass and kinetic energy distributions and their correlations have been studied for the thermal neutron induced fission of 237Np. The global mass distribution is rather smooth, apart from a weak shoulder at μH = 140–141. When low excitation events are selected, fine structures associated with the charge of the fragments are observed. Furthermore, there is a sudden increase in Ek for μH > 155, which is probably due to a spherical shell N = 50 in the light fragment and the corresponding deformed (but stable) heavy fragments with masses in the rare earth region. For the average (pre-neutron emission) total fragment kinetic energy, a value of 176.4 ± 0.6 MeV has been obtained, in agreement with the systematics. Also the prompt neutron emission curve v(m ∗ ) has been calculated, which shows the well-known saw-tooth shape. Finally, the energy distribution and the emission probability of the ternary α-particles have been determined.

Fission fragment energy correlation measurements for the sub-barrier (nth, f) of 241Am and 243Am

Abstract Fission-fragment mass and kinetic-energy distributions and their correlations have been studied for the sub-barrier fission of the odd-odd 242 Am and 244 Am nuclei resulting from thermal neutron capture in 241 Am and 243 Am. Unwanted events were eliminated by a coherence test based on the time of flight. The 243 Am mass distribution is more asymmetric and shows structures at μ H ≈ 139 and 144, compared with that for 241 Am which is smooth and structureless. The structure at μ H ≈ 144 is caused by deformed neutron shells at N ≈ 88 in the heavy fragment and N ≧ 60 in the light fragment. While the 〈 E K 〉 at symmetry for 241 Am is ≈ 19 MeV higher than that for 235 U, it is shifted upwards only ≈ 6.5–10 MeV in the other mass regions. However, 〈 E K 〉 at symmetry for 243 Am is ≈ 6 MeV lower than for 241 Am and this decrease tapers down to ≈ 1 MeV for μ H > 135. These data show a decrease in the total fragment deformation for 241 Am at symmetry as predicted by calculations. However, the 243 Am data show a sudden change back to higher deformation.

Fission fragment energy correlation measurements for the thermal neutron sub-barrier fission of 231Pa

Abstract Fission fragment mass and kinetic energy distributions and mass-versus-energy correlations have been measured for the thermal neutron induced fission of 231 Pa. The global mass distribution and the mass distribution for selected kinetic energy events show fine structure with a mean period of ≈ 2.5 amu. The peak/valley ratio of the mass distribution is 2271 ± 854 compared to 554 ± 31 for 235 U. The dip ΔE K values at symmetric fission for 231 Pa and 235 U are 14.8 ± 4.2 and 20.6 ± 1.1 MeV respectively. The 〈 E K 〉( μ H ) for μ H ≈ 125–135 is flatter than for 235 U.

Mass and energy characteristics of the 241Pu(nth, f) fragments

Abstract Fission-fragment mass and kinetic energy distributions and their correlations have been studied for the thermal neutron induced fission of 241 Pu. The global mass distribution is smooth, apart from a small shoulder at μ H ≈ 144 probably due to the deformed shells N ≈ 88 and N ⩾ 60 in the heavy and the light fragment respectively. When low excitation events are selected, these structures become more pronounced. Furthermore, there is a sudden increase of 1–2 MeV in E (μ) for masses around 85 and above 155 which is probably associated with a spherical shell N = 50 in the light fragment. Finally, the prompt neutron emission curve v(m ∗ ) is calculated.

Structures in far-out asymmetric mass distributions in low energy fission

Abstract The far-out asymmetric mass distributions for 235 U(n th , f), 239 Pu(n th , f), 243 Am(n th , f) and 252 Cf(sf) have been determined. The structures in these data and those in the existing results on 238 U, 234 Np and 235 Np have been analysed in a systematic way in terms of the static scission-point model of Wilkins et al . The model explains well all the structures seen in very asymmetric fission.

Fission-fragment energy correlation measurements for the spontaneous fission of 244Cm

Abstract Fission-fragment mass and kinetic energy distributions and their correlations have been measured for the spontaneous fission of 244 Cm. About 3.54 × 10 5 fission events were collected. The global mass distribution shows shoulders at μ H ≈ 134 and μ H ≈ 143−144. The peak/valley ratio is 86. The dip ΔE K value at symmetry is 13.4 ± 1.5 MeV. In the yields for high-kinetic-energy selected events, the masses 139 and 144 dominate. The results of 244 Cm are compared with the other isotopes of Cm and discussed in terms of the structures present in the potential energy surface of the fissioning system caused by the structures in the nascent fragments.

Fission fragment energy correlation measurements for 229Th(nth,f)

Abstract Fission-fragment mass and kinetic energy distributions and their correlations have been measured for 229 Th. The results are compared with those for the neighbouring nuclei. The 229 Th yield data show a peak for the symmetric fission. The mass yield peak/valley ratio is 531 ± 49. The 〈 E K 〉( μ H ≈ 115−22) is flat and the dip ΔE K at symmetry is 18.0 ± 1.1 MeV. In the yields for high kinetic energy selected events, the mass 144 dominates. A complete systematic for the μ H ≈ 144 structure as a function of the fissioning nuclei is presented and discussed.