2. Neutron
Sub atomic particle
No charge and greater mass
Free neutrons are unstable
High penetrating power
Essential in Fission process
3.
4. Neutron Source
Different sources of neutrons are:
Radioisotopes which undergo spontaneous fission
Radioisotopes which decay with alpha particles
Radioisotopes which decay with high energy photons
co-located with beryllium or deuterium
Plasma focus and plasma pinch devices
Light ion accelerators
5. Neutron Energies
Neutron energy indicates a free kinetic energy,
usually given in electron volts
En= mnV2/(2*gc) = 5.227*10-13 V2 ( eV)
Fission neutron energy range is (0.075……..17 ) MeV
Prompt neutron vs delayed neutron
Prompt neutrons have average energy of about 2 MeV.
7. Classification of Neutrons
• neutrons are classified according to their energy
• thermal neutrons have an energy of about
~ 0.025 eV
• epithermal neutrons, resonance neutrons, slow
neutrons have energies between 0.01 MeV and 0.1
MeV
• fast neutrons - 0.1 MeV and 20 MeV
• relativistic neutrons
8. Neutron Energy Distribution
Ranges
Fast neutrons > energy approximately 1 MeV
Slow neutrons > energy less than or equal 0.4 eV
Epithermal neutrons > energy 1 eV to 10 keV
Hot neutrons > energy about 0.2 eV
Thermal neutrons > energy of about 0.025 eV
Cold neutrons > energy 5x10−5 eV to 0.025 eV
Very cold neutrons > energy 3x10−7 eV to 5x10−5 eV
Ultra cold neutrons > energy less than 3x10−7 eV
Continuum region >energy 0.01 MeV to 25 MeV
Resonance region >energy 1 eV to 0.01 MeV
Low energy region > energy less than 1 eV
9. Thermal Neutrons
Slow neutron having energy 0.025 eV
Results from scattering
Moderator mediums are H , D , BeO, Be
Neutrons at a given temp posses wide range of
energies and corresponding speeds
Maxwell-Boltzman distribution:
2π
f (E ) = e − E / kT E 1/2
( πkT )3/2
10. Most Probable Neutron Velocities
• Most probable neutron velocity is given by
• Vm=(2kT/m)0.5=128.39 T0.5
v=most probable velocity of neutron (m/sec)
k=Boltzman's constant (1.3805*10-23 J/ K)
T=absolute temperature in degrees Kelvin ( K)
m =mass of neutron 1.66 x 10-24gram
• The energy corresponding to Vm is given by
Em= kT = 8.617*10-5 T (eV )
At T = 20 0C
Vm = 2200 m/sec
Em =0.025 eV
11. Thermal Neutron Speeds & Energies
Temperature 0C Vm ( m/sec) Em (eV)
20 2200 0.0252
260 2964 0.0459
537.8 3656 0.0699
1000 4580 0.1097
13. Interaction of Neutrons
Neutrons are uncharged and can travel
appreciable distances in matter without
interacting
Neutrons interact mostly by inelastic scattering
or elastic scattering
14. Inelastic Scattering
• A part of the kinetic energy that is transferred
to the target nucleus upon collision
• The nucleus becomes excited and a gamma
photon/photons are emitted
15. Elastic scattering
• Interaction between fast neutrons and low atomic
number Z
• For slowing down neutrons
• Responsible for neutron – slowing in reactors
• Fast neutrons lose K.E. As A result of scattering
collisions with nuclei which act as moderators
16.
17. Interaction of Neutrons
.......E o , M, V → m.......
(before collision)
.......E, M, V1 → m, ν 1 .......
(after collision)
18. Continued…
• total kinetic energy and momentum are
conserved and we have:
1
2
MV 2 = 1 MV12 + 1 mv 1
2 2
2
and MV = MV1 + mv 1
•Solving for V1 and substituting into:
(M - m)
V1 = V
(M + m)
19. Continued…
1
E 0 = MV for incident neutron
2
2
1
and E = MV12 we get
2
2
M−m
E = E0
M +m
• The energy transferred to target nucleus is:
M - m 2
E 0 - E = E 0 1 -
M +m
20. Continued…
1 2 1 2 4mME
Emax = MV -
2 2 MV1 = 2
(M + m )
• when: M = m; E = Emax
•for neutrons in a head on collision with hydrogen all
the kinetic energy can be transferred in one
collision since the mass of neutrons and protons
are almost equal.
