Neutron Sub atomic particle No charge and greater mass Free neutrons are unstable High penetrating power Essential in Fission process
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
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.
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
Neutron Energy DistributionRanges 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
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
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=Boltzmans 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
Thermal Neutron Speeds & EnergiesTemperature 0C Vm ( m/sec) Em (eV)20 2200 0.0252260 2964 0.0459537.8 3656 0.06991000 4580 0.1097
Interaction of Neutrons Neutrons are uncharged and can travel appreciable distances in matter without interacting Neutrons interact mostly by inelastic scattering or elastic scattering
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
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
Interaction of Neutrons .......E o , M, V → m....... (before collision).......E, M, V1 → m, ν 1 ....... (after collision)
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)
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
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 onecollision since the mass of neutrons and protonsare almost equal.