235 U-thermal fission (ILL) and fission of relativistic 238 U ions (GSI ).

1. 235U-thermal fission (ILL) and fission of relativistic 238U ions (GSI). The fission of a heavy fissile nucleus ( A, Z ) is the splitting of this nucleus into 2 fragments, called primary fragments A’1 and A’2. They are excited and de-excite to A1 and A2 by emission of n and ɣ. Z = Z1 + Z2 After 235U thermal neutron capture, the 236U is excited in a collective deformed state, just above the barrier. On a PES, it overpasses the barrier towards the saddle point, increasing its deformation and falls down to the scission point where it splits. The energy at scission cannot be precisely defined because of the neutron and ɣ-emission and since the elongation at scission does fluctuate. energy released at scission fluctuates over 15 MeV. Fission fragments are n-rich isotopes given the curvature of the stability valley fission fragments keep the n-excess.

2. The mass-distribution of fragments is asymmetric, guided by shell effects. • -The peak/valley ratio reflects the excitation of the fissioning nucleus. • In thermal fission of 235U its value is 800. • We have measured ONE of the two • fission fragments, identified A, Z and measured its velocity

3. Methods to separate fission fragments: • - Cumulative yields of long-lived isotopes by • off-line identification by chemistry • Identification (by β delayed γ-Spectroscopy) • ISOL techniques • - In flight separation by recoil spectrometers • LOHENGRIN • - Inverse kinematics at relativistic energy with 238U beams at 0.750 A.GeV and at 1A.GeV by the FRS • In-flight identification of bare fragments with recoils separators at β= (0.6 - 0.8) • Ions are emitted forwards --> High angular transmission. • Thick targets 74

4. Uρe=Av2/2q Bρm = Av/q Bρm = Av/q

5. ΔE-E  Z, A

6. A = 74

8. In this experiment production yields by fission were measured for light nuclides down to 10-6.Fission velocities and TKE. Odd-even effects13 new isotopes were identified, for 9 of them, the β-decay half lives were measured.Selecting ‘ cold ‘ fission events at the maximun of TKE, fragments are not exclusively even-even nuclides.

10. Fragment Recoil Separator

11. The beam intensity was between (2.105 -107) ions/sThe angular acceptance of the FRS is 15 mr The momentum acceptance Δp/p = 2%Separated fission fragments are identified in Z by measuring ΔE ( Z/ΔZ = 140 ) in mass number A by the time of flight (A/ ΔA = 250)

12. Fission velocity Transmission T  kinetic energy  cross section

15. The transmission increases with the mass of the fission fragment

16. U+Pb, U+Be and U+p compared

17. The fission of U on Pb occurs mainly via the collective excitation of the giant dipole resonance at 12 MeV On the Be-target the mean excitation energy of the U is evaluated to 20 MeVThe fission occurs near the end of the de-excitation chain. On the H-target the mean fissioning nucleus is 220Th excited at about 100 MeV as deduced from the mean value of A1, Z1 and from the fission fragment velocities.

18. One magnetic setting of U on 1.25 g/cm2 Pb target

20. Isotopic distribution of each element produced in 238U fission on Pb target

22. Mass-distribution of U + p fragments • Very asymetric binary break-up have been observed Fragment projectiles

23. Total fission cross-sections

24. Symmetric fission distributions <Z> a.ch.u. <A> a.m.u. σ z a.ch.u. U + p U + Pb 44.9 ± 0.10 42.9 ± 0.30 106.8 ± 0.25 101.0 ± 0.5 6.4 ± 0.2 6.9 ± 0.7

25. Velocity of fission fragments and kinetic energies in U+p • Measured velocities of FF agree with a fissioning element of 88<Z<92. • Curves are calculated assuming coulomb potential between the two fragments, conservation of momenta between the pair members and mean values of A for each element.

26. Chart of heavy fragmentspopulated in 1A GeV U + p • All processes • Fission only

27. Overview of all fragments

33. Conclusions In-flight fission of relativistic U has been studied for the first time with full identification of 1385 nuclides. Yields and velocities were measured. The properties of the fissioning sytems were studied in the 3 reactions U+Pb U+Be and U+p. New fragments were observed. 117 new nuclides were identified down to very small production cross sections of 0.5 nb

34. Conclusions • Isotopic cross sections of fission residues are all measured –down to100 μB- with a precision better than 20%. • Very heavy fission fragments are identified up to A = 184. • Fission of hot parent nuclei (Z0 = 88,90) into very asymmetric pairs z1/z2 = 0.1 – 0.4 are observed. • Fission velocities and kinetic energies are measured. • The yields of neutron-rich FF for 1 GeV.A U on p, important for radioactive beam facility, are available. • The properties of the fissioning sytems were studied in the 3 reactions U+Pb U+Be and U+p.

35. Symmetric fission distributions <Z> a.ch.u. <A> a.m.u. σ z a.ch.u. U + p U + d U + Pb 44.9 ± 0.10 43.7 ± 0.20 42.9 ± 0.30 106.8 ± 0.25 103.0 ± 0.2 101.0 ± 0.5 6.4 ± 0.2 7.7 ± 0.2 6.9 ± 0.7

36. Velocity distributions for heavy FF The three heavy isotope shapes are larger, due to fission. • The three light isotopes show a narrow peaks due to evaporation. • The intermediate isotope spectra indicate a superposition of FF and EVR.

37. 238U + p fragments were fully investigated The reaction is a model of 1 GeV p collision on a fissile target for technical applications.Complete nuclides distributions were obtained fromvery light fragments N (Z = 7) to very heavy ones up to W (Z = 74) The fission occurs along the de-exitation of the highly excited residus of the collision.

38. Width of velocity distributions • The width arelarger and constant for heavy isotopes. • When the neutron number N diminishes, the contribution of fission decreases. • There is no FF produced for Osmium Z = 76

39. Cross section distributions of the heavy FF • The contribution of Ti windows is only 3 % of the yields • Evaporation residues (in red) dominate for Z > 74

41. Projections on proton and on neutron axes. All fragments (black points) • High energy symmetric fission (red points) • Low energy asymmetric fission ( blue points)

46. Neutron excess of fragments • Large neutron-excesses come only from energy fission. • Heavy FF are neutron-deficients. • Very asymmetric fission are associated with a large number of emitted neutrons

50. Fission velocity Transmission T  kinetic energy  cross section