[en] The new built Cosi Fan Tutte facility installed at the High Flux Reactor of the ILL in Grenoble was used to obtain correlated mass, kinetic energy and nuclear charge of the fission fragments from U235(nsub(th),f) and Th229(nsub(th),f). (author)

No abstract available

[en] The excitation function of the ²³¹Pa(n,f) reaction has been measured from 130 to 450 keV with a neutron energy resolution of 5 keV. This measurement confirmed the existence of two structures observed at Esub(n) = 200 keV and Esub(n) = 330 keV and put into evidence a narrow resonance at Esub(n) = 160 keV. The use of this 2 keV energy resolution has shown that the resonance width is <= 2 keV. The fission fragments angular distributions have been measured around these three structures with a 5 keV energy resolution. The excitation function and the angular distributions were simultaneously reproduced with a statistical model showing the competition between the different exit channels (neutron emission, γ emission, fission barrier at two maxima). This analysis shows that the narrow resonance at 160 keV presents all the characteristics of a pure vibration resonance (KJπ) = (33⁺). The 200 keV and 330 keV resonances are interpretated as correspondance to the vibrations Kπ = 0⁺ and Kπ = 0^-

[en] A method to obtain experimental angular distributions of fission fragments for (n,f) reactions is described. Fission fragments are detected in thin foils of polycarbonate plastic (Makrafol). The detectors are scanned by the spark method. Etching conditions and spark scanning conditions have been studied to obtain the best signal to noise ratio. This method is used for fission study of strong alpha-emitter actinides like ²³⁰Th and ²³¹Pa

[en] Fast neutrons up to 2.5 MeV have been used to induce light particle accompanied fission in ²³⁵U, ²³⁸U and ²³⁹Pu targets. The energy spectra and relative intensities of ³H and ⁴He have been measured with a ΔE-E telescope. Absolute particle yields were derived from the number of ⁴He particle per 1000 binary fissions of ²³⁵U and ²³⁹Pu by thermal neutrons measured under the same geometrical conditions. They observe that all the energy distributions are well reproduced by gaussian lineshape, the mean energies of which remain constant while varying the neutron incident energy. (For all nuclei, the ⁴He mean energies lie close to 16 MeV and the corresponding energies for ³H particle at about 8-9 MeV). The production rates of ³H and ⁴he in fast neutron fission of ²³⁵U decrease smoothly with the incident neutron energy up to En = 2.5 MeV. The decrease never exceeds 30% of the thermal neutron production rate. Conversely the ³H and ⁴He particle yields obtained in fast neutron fission of ²³⁹Pu are very close to the thermal neutron production rate. Therefore the corresponding particle yields for fast neutron induced fission of ²³⁸U remain constant between En = 1.5 MeV and En = 2.5 MeV. Some possible explanations for these observations are briefly and qualitatively explored

[en] Fragment-mass and kinetic energy distribution measurements for ²⁵²Cf(sf) have been extended to the far-out asymmetric low yield fission region up to Msub(H)=190 compared with the existing data that go up to Msub(H) approximately 168. The mass yield Y(M) shows an important enhancement for Msub(H)>=176. The < Esub(K)> (M), σEsub(K)(M) and, to a lesser extent, Y(M) distributions show strong and correlated amplitude fluctuations for Msub(H)>=170. These data indicate strong and rapid changes in deformation of the nascent fragments. These structures are discussed in terms of the static macro-microscopic potential energy calculations. Several spherical and deformed shells are involved to stabilize the light fragment and especially the proton spherical shell Z=28. So it is possible to observe very rich neutron nuclei like ₂₈⁷³Ni. Furthermore we select small excitation energy events (E*=Qsub(R) - Esub(KT)<=8 MeV). The distribution Y(M) for these nearly cold fragmentations is dominated by the strong neutron deformed shell N=88 and shows an important charge odd-even effect

No abstract available

[en] Important progress have been performed during the last years in nuclear fission; some results are presented to illustrate these progress

[en] The excitation function of the reaction ²³¹Pa(n,f) was measured from 130 to 450 keV with a neutron energy resolution of 5 keV. The measurements confirmed the existence of the two structures already observed around Esub(n)=200 keV and Esub(n)=330 keV and revealed a narrow resonance around Esub(n)=160 keV. The use of an energy resolution of only 2 keV showed that the width of this resonance is <=2 keV. The angular distributions of the fission fragments were also measured in the vicinity of these three structures with an energy resolution of 5 keV. The excitation function and the angular distributions were reproduced simultaneously with the aid of a statistical model allowing for the competition between the different exit channels (neutron emission, gamma emission, fission through a barrier with two peaks). This analysis shows that the narrow resonance at 160 keV has all the characteristics of a pure vibrational resonance (KJsup(π))=(33⁺). The resonances at 200 keV and 330 keV are interpreted as corresponding to the vibrational states Ksup(π)=0⁺ and Ksup(π)=0^-. (author)

[en] The new built Cosi Fan Tutte facility installed at the High Flux Reactor of the ILL in Grenoble was used to obtain correlated mass, kinetic energy and nuclear charge of the fission fragments from U235(n/sub th/,f) and Th229(n/sub th/,f)