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 ).

–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

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

Bρm = Av/q

Uρe=Av2/2q

Bρm = Av/q

ΔE-E Z, A

A = 74

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.

Fragment Recoil Separator

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)

Fission velocity

Transmission T

kinetic energy

cross section

The transmission increases with the mass of the fission fragment

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

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.

One magnetic setting of

U on 1.25 g/cm2 Pb target

Isotopic

distribution of

each element

produced in

238U fission on

Pb target

Mass-distribution of U + p fragments

Fragment

projectiles

•Very asymetric

binary break-up

have been

observed

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

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.

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.

Chart of heavy fragments populated in 1A GeV U + p

• All processes

• Fission only

Overview of all fragments

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

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.

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.

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.

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.

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

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

Projections on proton and on neutron axes.

All fragments (black points)

• High energy

symmetric fission (red points)

• Low energy asymmetric fission ( blue points)

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

Fission velocity

Transmission T

kinetic energy

cross section

• 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.