Kern- und Teilchenphysik I
Lecture 6: Nuclear fission
Prof. Nico Serra
Dr. Patrick Owen, Dr. Silva Coutinho
http://www.physik.uzh.ch/de/lehre/PHY211/HS2016.html
Nuclear Fission and Fusion
We have seen that fission
is the fragmentation of a
heavy nucleus into 2
more stable components Usually requires to be
triggered by a slow
neutron For these nuclei it is energetically favourable (exothermic reaction) to
fuse into a single nucleus
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Nuclear and Particle Physics I
Nuclear Fission
Nuclides with Z>40 can in principle split, however the barrier is so
high that the tunnelling effect is very improbable, so in practice
only nuclides with very large A split.
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Nuclear and Particle Physics I
Nuclear Fission
Let’s consider an ✏ deformation of the sphere into an ellipsoid of axis a = R(1+✏)
and b = (1 ✏/2)
• The surface energy becomes Es = as A2/3 (1 + 25 ✏2 + ...)
• The Coulomb energy becomes Ec = ac Z 2 A
• The di↵erence in energy becomes
E=
1/3
(1
✏2
2/3
(2a
A
s
5
In order for the deformation to be favourable
implies:
Z2
2as
' 48
A
ac
1 2
5✏
+ ...)
ac Z 2 A
1/3
)
E should be negative, which
This happens for nuclei with Z > 114 and A > 270
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Nuclear and Particle Physics I
Induced Fission
If a large A material (e.g. 235U) breaks into smaller nuclei, close to the
iron binding energy peak, it releases energy
Spontaneous Fission is rare because it is suppressed by the tunnelling
of the Coulomb barrier
It is possible to perturbate the nucleus, such that it is in unstable state
and breaks, this is called induced fission
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Nuclear and Particle Physics I
Neutron Interaction
• Scattering: the neutron bounces o↵, with or without the same energy
• Activation: the neutron is absorbed and the resulting nuclide is radioactive
Inelastic Scattering:
Scattered neutron, E2
e le c tro n
– The nucleus emit a gamma ray
ne utro n
Incident neutron, E1
p ro to n
– Fission follows absorption
Gamma Photon, Eγ
E1 = Eγ + E2
Scattered neutron, E2
Elastic Scattering:
e le c tro n
ne utro n
Incident neutron, E1
p ro to n
a EA
E1 = EA + E2
Gamma Photon, Eγ
E γ ~ 7 MeV
Neutron Absorption:
e le c tro n
ne utro n
Incident thermal neutron, E
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p ro to n
Nuclear and Particle Physics I
Induced Fission
A neutron splits a
uranium nucleus,
releasing energy (quickly
turned to heat) and more
neutrons, which can
repeat the process.
Nico Serra
The energy appears
mostly in the kinetic
energy of the fission
products and in the beta
and gamma radiation.
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Nuclear and Particle Physics I
Outcome
• Energy is released (the quantity M (A, Z)⇤
nMn )
M (A1 , Z1 )
M (A2 , Z2 )
• One neutron triggers the reaction, 2 or 3 (on average) are produced and
can induce more fission
• Depending on the fission material, the shape and the mass the reaction
can be self-perpetuating (critical mass)
• Nuclear reactors are designed to have self-sustaining and controllable reaction
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Nuclear and Particle Physics I
Fission
• Only a few nuclides can fission
• Nuclides that can fission for any incoming neutron are called fissile
• The only naturally occurring fissile nuclide is
235
• Other fissile nuclides are 233 U , 239 P u and
naturally to any appreciable extent
P u, none of this is present
241
U
• Nuclides that can be induced to fission only by neutrons of energy higher
than a certain threshold are called fissionable, e.g. 238 U and 240 P u
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Nuclear and Particle Physics I
Energy by fission
Energy released per fission ~ 200 MeV [~ 3.2*10-11 J].
!  This is hundreds of thousands, or millions, of times
greater than energy produced by combustion, but still
only ~0.09% of mass energy of uranium nucleus!
!  The energy released appears mostly (85%) as kinetic
energy of the fission fragments, and in small part (15%)
as the kinetic energy of the neutrons and other particles.
!  The energy is quickly reduced to heat (random kinetic
energy) as the fission fragments are stopped by the
surrounding atoms.
!  The heat is used to make steam by boiling water,
!  The steams turns a turbine and generates electricity.
! 
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Nuclear and Particle Physics I