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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.

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Presentation on theme: "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."— Presentation transcript:

1 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 A 1 and A 2 by emission of n and ɣ. Z = Z 1 + Z 2 After 235 U thermal neutron capture, the 236 U 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. 235 U-thermal fission (ILL) and fission of relativistic 238 U ions (GSI ).

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 235 U 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 238 U beams at A.GeV and at 1A.GeV by the FRS In-flight identification of bare fragments with recoils separators at β= ( ) Ions are emitted forwards --> High angular transmission. Thick targets 74

4 Bρ m = Av/q Uρ e =Av 2 /2q Bρ m = Av/q

5 ΔE-E  Z, A

6 A = 74

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8 In this experiment production yields by fission were measured for light nuclides down to Fission velocities and TKE. Odd-even effects 13 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.

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10 Fragment Recoil Separator

11 The beam intensity was between ( ) ions/s The 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

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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 MeV The fission occurs near the end of the de- excitation chain. On the H-target the mean fissioning nucleus is 220 Th 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

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

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

23 Total fission cross-sections σ / b Symmetric fission Asymmetric fission U / Pb U /p

24 Symmetric fission distributions 6.4 ± ± ± ± ± ± 0.30 U + p U + Pb σ z a.ch.u. a.m.u. a.ch.u.

25 Velocity of fission fragments and kinetic energies in U+p Measured velocities of FF agree with a fissioning element of 88 { "@context": "http://schema.org", "@type": "ImageObject", "contentUrl": "http://images.slideplayer.com/3423721/12/slides/slide_24.jpg", "name": "Velocity of fission fragments and kinetic energies in U+p Measured velocities of FF agree with a fissioning element of 88

26 Chart of heavy fragments populated in 1A GeV U + p All processes Fission only

27 Overview of all fragments

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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 (Z 0 = 88,90) into very asymmetric pairs z 1 /z 2 = 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 6.4 ± ± ± ± ± ± ± ± ± 0.30 U + p U + d U + Pb σ z a.ch.u. a.m.u. a.ch.u.

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.

37 238 U + 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 from very 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 are larger 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

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

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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

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50 Fission velocity Transmission T  kinetic energy  cross section

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52 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 A 1 and A 2 by emission of n and ɣ. Z = Z 1 + Z 2 After 235 U thermal neutron capture, the 236 U is excited in a collective deformed state, just above the barrier. On a PES, it overpasses the barrier towards saddle point, increasing its deformation and falls down to the scission point where it splits. The energy at scission can not be precisely defined because of the neutron and ɣ -emission and because 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.


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