235 u-thermal fission (ill) and fission of relativistic 238 u ions (gsi )
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235 U-thermal fission (ILL) and fission of relativistic 238 U ions (GSI ). - PowerPoint PPT PresentationTRANSCRIPT
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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 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.
235U-thermal fission (ILL) and fission of relativistic
238U ions (GSI ).
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–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
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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
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Bρm = Av/q
Uρe=Av2/2q
Bρm = Av/q
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ΔE-E Z, A
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A = 74
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In this experiment production yields by fission were
measured for light nuclides down to 10-6.
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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Fragment Recoil Separator
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The beam intensity was between (2.105 -107) 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)
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Fission velocity
Transmission T
kinetic energy
cross section
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The transmission increases with the mass of the fission fragment
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U+Pb, U+Be and U+p compared
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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 220Th excited at about
100 MeV as deduced from the mean value
of A1, Z1 and from the fission fragment
velocities.
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One magnetic setting of
U on 1.25 g/cm2 Pb target
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Isotopic
distribution of
each element
produced in
238U fission on
Pb target
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Mass-distribution of U + p fragments
Fragment
projectiles
•Very asymetric
binary break-up
have been
observed
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Total fission cross-sections σ / b Symmetric
fission Asymmetric fission
U / Pb 1.4 +- 0.2 2.2 +- 0.2
U /p 1.53+-0.2 0.105 +- 0.01
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Symmetric fission distributions
6.4 ± 0.2 6.9 ± 0.7
106.8 ± 0.25 101.0 ± 0.5
44.9 ± 0.10 42.9 ± 0.30
U + p
U + Pb
σ z a.ch.u. <A> a.m.u.
<Z> a.ch.u.
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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.
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Chart of heavy fragments populated in 1A GeV U + p
• All processes
• Fission only
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Overview of all fragments
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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
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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.
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Symmetric fission distributions
6.4 ± 0.2 7.7 ± 0.2 6.9 ± 0.7
106.8 ± 0.25 103.0 ± 0.2 101.0 ± 0.5
44.9 ± 0.10 43.7 ± 0.20 42.9 ± 0.30
U + p
U + d
U + Pb
σ z a.ch.u. <A> a.m.u.
<Z> a.ch.u.
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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.
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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 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.
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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
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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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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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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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Fission velocity
Transmission T
kinetic energy
cross section
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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 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 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.